Section #14-2. Quality of electrical energy

The culprits of the deterioration in the quality of electrical energy

The properties of electrical energy, indicators and the most likely culprits for the deterioration in the quality of electrical energy are shown in Table 1:

Table 1. Properties of electrical energy, indicators and the most likely culprits for deterioration in quality your electrical energy.

electrical properties	KE indicator				Most likely
					the culprits of the deterioration of CE
					Energy supply
Voltage deviation	Steady voltage deviation
	δU y				organization
					Consumer with
Voltage fluctuations	Span of voltage change δU t
	Flicker dose P t				variable load
					Consumer with
Non-sinusoidality	Coefficient	distortion
	solidity curve		voltage Kv		non-linear load
	Coefficient of the nth harmonic
	voltage component K U(i)
					Consumer with unsymmetric
Asymmetry	Coefficient		asymmetries
three-phase system	stresses			reverse	load
stresses	sequences K 2U Coefficient
	voltage asymmetry at zero
	sequences K 0U
					Energy supply
Frequency deviation	Frequency deviation ∆f
					organization
					Energy supply
voltage dip	Voltage dip duration ∆t p
					organization
					Energy supply
voltage pulse	Impulse voltage U imp
					organization
					Energy supply
Temporary	Time factor
overvoltage	overvoltageK perU				organization

From the electrical networks of general-purpose power supply systems, power receivers for various purposes are fed, consider industrial power receivers.

The most characteristic types of electrical receivers, widely used in enterprises of various industries, are electric motors and electric lighting installations. Electrothermal installations are widely used, as well as

valve converters used to convert alternating current to direct current. Direct current in industrial enterprises is used to power DC motors, for electrolysis, in galvanic processes, for some types of welding, etc.

Electric lighting installations with incandescent, fluorescent, arc, mercury, sodium, xenon lamps are used in all enterprises for indoor and outdoor lighting, for urban lighting, etc. Incandescent lamps are characterized by nominal parameters: power consumption P nom , luminous flux

F nom, luminous efficiency η nom (equal to the ratio of the luminous flux emitted by the lamp to its power) and the average nominal service life T nom. These indicators largely depend on the voltage at the terminals of incandescent lamps. Voltage changes lead to corresponding changes in luminous flux and illumination, which ultimately affects labor productivity and human fatigue.

Valve converters, due to the specifics of their regulation, are consumers of reactive power (the power factor of valve converters in rolling mills ranges from 0.3 to 0.8), which causes significant voltage deviations in the supply network. Usually they have a system of automatic regulation of direct current by phase control. When the voltage in the network increases, the regulation angle automatically increases, and when the voltage decreases, it decreases. An increase in voltage by 1% leads to an increase in the reactive power consumption of the converter by about 1-1.4%, which leads to a deterioration in the power factor. Higher harmonics of voltage and current adversely affect electrical equipment, automation systems, relay protection, telemechanics and communications. Additional losses appear in electrical machines, transformers and networks, it becomes difficult to compensate for reactive power using capacitor banks, and the service life of the insulation of electrical machines is reduced. Non-sinusoidality factor

during the operation of thyristor converters of rolling mills, it can reach a value of more than 30% on the 10 kV side of the voltage supplying them; valve converters do not affect the voltage symmetry due to the symmetry of their loads.

Electric welding installations can cause disruption of normal working conditions for other electrical consumers. In particular, welding units, whose power currently reaches 1500 kW per unit, cause much larger voltage fluctuations in electrical networks than, for example, starting asynchronous motors with a squirrel-cage rotor. In addition, these voltage fluctuations occur for a long time and with a wide frequency range, including the most unpleasant range for electric lighting installations (about 10 Hz). AC electric arc and resistance welding installations are a single-phase non-uniform and non-sinusoidal load with a low power factor: 0.3 for arc welding and 0.7 for contact welding. Welding transformers and low power devices are connected to a 380/220 V network, more powerful ones - to a 6 - 10 kV network.

Electrothermal installations, depending on the heating method, are divided into groups: arc furnaces, direct and indirect resistance furnaces, electronic melting furnaces, vacuum, slag remelting, induction furnaces. This group of electrical consumers also has an adverse effect on the supply network, for example, arc furnaces, which can have a power of up to 10 MW, are currently being built as single-phase. This leads to a violation of the symmetry of currents and voltages. In addition, they lead to non-sinusoidal currents, and, consequently, voltages.

The main consumers of electricity in industrial enterprises are asynchronous electric motors. Voltage deviation from permissible norms affects the frequency of their rotation, the loss of active and reactive power (voltage reduction by 19%

nominal causes an increase in active power losses by 3%; voltage increase by 1% leads to an increase in reactive power consumption by 3%). The action of the asymmetric mode is qualitatively different compared to the symmetrical one. Of particular importance is the negative sequence voltage. The resistance of the negative sequence of electric motors is approximately equal to the resistance of the stalled motor and, therefore, 5÷8 times less than the positive sequence resistance. Therefore, even a small voltage unbalance causes significant negative sequence currents. Negative sequence currents are superimposed on positive sequence currents and cause additional heating of the stator and rotor (especially the massive parts of the rotor), which leads to accelerated aging of the insulation and a decrease in the available motor power. Thus, the service life of a fully loaded asynchronous motor operating at a voltage unbalance of 4% is reduced by 2 times.

Ways and means of improving the quality of electrical energy

Compliance of the PCE with the requirements of GOST is achieved by circuit solutions or the use of special technical means. The choice of these funds is made on the basis of a feasibility study, while the task is not to minimize damage, but to fulfill the requirements of GOST.

To improve all SCEs, it is advisable to connect power receivers with complicated operating modes to EES points with the highest short-circuit power values. When choosing a power supply scheme, enterprises take into account the limitation of short-circuit currents to the optimal level, taking into account the problem of increasing the SQE.

To reduce the effect on the "calm" load of valve electrical receivers and abruptly changing load, the connection of such receivers is carried out on separate sections of busbars of substations with transformers with split winding or with dual reactors.

Opportunities to improve each SCE.

1. Ways to reduce the range of frequency fluctuations:

1.1 increase in short circuit power at the point of connection of receivers with sharply variable and "calm" loads;

1.2 supply of sharply variable and "calm" loads through separate branches of the split windings of transformers.

2. Measures to maintain stress levels within acceptable limits:

2.1. Rational construction of solar power plant by using increased voltage for lines supplying the enterprise; use of deep inputs; optimal loading of transformers; justified use of conductors in distribution networks.

2.2. Use of jumpers for voltage up to 1 kV between workshops

2.3 Reducing the internal resistance of the SES of the enterprise by switching on the parallel operation of the GPP transformers, if the short-circuit currents do not exceed the permissible values ​​for the switching protection equipment.

2.4 Voltage regulation of generators of own power sources.

2.5 Using the adjustment capabilities of synchronous motors with automatic excitation control (ARV).

2.6 Installation of autotransformers and voltage regulation devices under load (OLTC) at power two-winding transformers.

2.7 The use of compensating devices.

3. Reduction of voltage fluctuation is achieved by using:

3.1 of dual reactors, the power of a sharply variable load that can be connected to one branch of the reactor is determined by

by expression	S r.n =			δUt			Where d U t	− voltage fluctuations
			u k.z.		50x in
			S n.t.		U n 2

on buses connected to one branch of the reactor during operation of a sharply variable load connected to another branch; u k.z. −

short circuit voltage of the transformer to which the dual reactor is connected; S n.t. − rated power of the transformer; x in - resistance of the reactor branch; U n −

rated mains voltage.

3.2 transformers with a split winding, the maximum power of a sharply variable load connected to one winding is determined by the formula S r.n = 0.8 S n.t. δ U t .

3.3 installation of high-speed static compensating devices.

4. Ways to deal with higher harmonics:

4.1 Increasing the number of rectifier phases.

4.2 Installation of filters or filter compensating devices.

5. Methods for dealing with asymmetry (not requiring the use of special devices):

5.1 Uniform distribution of single-phase loads by phases.

5.2 Connecting unbalanced loads to network sections with a larger short circuit power or increasing the short circuit power.

5.3 Allocation of unbalanced loads to individual transformers.

5.4 Using special techniques to eliminate unbalance: 5.4.1 Replacing transformers with a winding connection diagram Y-Y0

on transformers with a connection diagram ∆ - Y 0 (in networks up to

1 kV). In this case, the zero sequence currents, multiples of three, closing in the primary winding, balance the system, and the zero sequence resistance sharply

decreases.

5.4.2 Because networks of 6-10 kV are usually performed with an isolated neutral, then the reduction of asymmetric components is achieved by using capacitor banks (used for transverse compensation) included in an asymmetric or incomplete triangle. In this case, the distribution of the total power of the BC between the phases of the network is performed in such a way that the generated negative sequence current is close in value to the negative sequence current of the load.

5.4.3 An effective tool is the use of unregulated devices, for example, a single-phase load balancing device built on the basis of the Steinmetz circuit.

If Z n = R n , then

		balancing
comes				implementation
Q L = Q C =				where R n
active				power
Balancing scheme
loads.
single phase load
			R n + j ωL ,
Steinmetz				load
parallel
connect BC, which is on
				shown
dotted line.

In the text part of the power supply project, it is necessary to describe the power receivers indicating the category of power supply required for them and a description of the measures to ensure this category.

Requirements for the reliability of power supply.

All consumers of electrical energy are divided into 3 categories of power supply reliability in accordance with Ch. 1.2 PUE.

First category- in normal modes, they must be supplied with electricity from two independent mutually redundant power sources, and interruption of their power supply in the event of a power failure from one of the power sources can only be allowed for the period of automatic power restoration. (see also the first special category).

These categories of power supply are defined in the regulatory documents regarding each individual type of equipment or object (building, structure, mechanism). The technical conditions issued by the grid organization determine the category of power supply provided by the grid organization, for its part. Based on local regulatory documents, which define the category of reliability of a particular type of power receiver, a comparison is made. If the category of power supply according to specifications is lower than required by the regulatory documents, then it is necessary to provide for measures to ensure the required category by installing additional sources of electrical energy - batteries, diesel generators.

In connection with the replacement of GOST 13109-97 with GOST 32144-2013. Standards for the quality of electrical energy in general-purpose power supply systems and the introduction of GOST R 50571.5.52-2011 (IEC 60364-5-52:2009) Low-voltage electrical installations. Selection and installation of electrical equipment. the requirements for designers to voltage losses in electrical networks, as well as to the calculation of voltage losses, have changed.

Here is an example of a paragraph from the Explanatory Note:

Fire and burglar alarm devices, fire warning system, fire-fighting devices, VZU, emergency lighting are classified as Category I. Provided by ATS, UPS

To ensure the second category of reliability at the site, a quarantine facility is used single transformer substation with two cables entering the building from the transformer substation and DGU.

Power receivers of the first category in normal modes must be supplied with electricity from two independent mutually redundant power sources, and a break in their power supply in the event of a power failure from one of the power sources can only be allowed for the period of automatic power restoration. In this regard, emergency lighting fixtures are used with emergency power supplies. Also, emergency power units are built into the microclimate control panels and fire alarm devices and fire alarm systems.

MINISTRY OF SCIENCE AND EDUCATION OF UKRAINE

STATE HIGHER EDUCATIONAL INSTITUTION

DONETSK NATIONAL TECHNICAL UNIVERSITY

Research work

on the topic: "Quality of electricity"

Completed st.gr. ________________________ date signature Checked by ________________________ date signature

Donetsk, 2011

This work contains: 27 pages, 7 figures, 1 table, 6 sources. The object of the research work is: the quality of electricity in the power supply systems of Ukraine. The purpose of the work: to get acquainted with the factors affecting the quality of electricity, ways of its regulation; find out how automatic regulation of power quality is carried out; determine how the quality of electricity will affect its cost. In the work, power supply and power consumption systems of various designs are investigated, the main problems of these systems are identified, which can lead to a decrease in the quality of electricity. ELECTRICITY, POWER QUALITY, VOLTAGE ASYMMETRY, OVERVOLTAGE, AUTOMATED CONTROL, ELECTRICAL SYSTEM.

1. Power quality indicators………………………………………………4 1.1 Voltage fluctuation………………………………………………………6 1.2 Voltage fluctuation…… …………………………………………….8 1.2.1 Influence of voltage fluctuations on the operation of electrical equipment…………………………………………………………. ..8 1.2.2 Measures to reduce voltage fluctuations……………….9 1.3 Voltage unbalance…………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………….10 1.3.1 Effect of voltage unbalance on the operation of electrical equipment… ……………………………………………………… 11 1.3.2 Measures to reduce voltage unbalance………… 12 1.4 Voltage non-sinusoidality…………………………………… …..12 1.4.1 Influence of voltage non-sinusoidality on the operation of electrical equipment………………………………………………………….13 1.4.2 Measures to reduce voltage non-sinusoidality ..14 1.5 Frequency deviation 15 ………………………….........16 2. Automated power quality control…………..16 2.1 Basic requirements for models of electrical systems containing distributed mixed sources of voltage distortion…………..17 2.2 Methodology for determining the actual influence of the consumer on the PQ...19 3. Calculations for electricity depending on its quality……………….22 Literature…………………… ………………………………………………...26

1 POWER QUALITY INDICATORS

Electrical appliances and equipment are designed to operate in a specific electromagnetic environment. The electromagnetic environment is considered to be the power supply system and the electrical apparatus and equipment connected to it, connected inductively and creating, to some extent, interference that negatively affects each other's work. With the possibility of normal operation of the equipment in the existing electromagnetic environment, one speaks of the electromagnetic compatibility of technical means. Uniform requirements for the electromagnetic environment are fixed by standards, which allows you to create equipment and guarantee its performance in conditions that meet these requirements. The standards establish permissible levels of interference in the electrical network, which characterize the quality of electricity and are called power quality indicators (PQIs). With the evolutionary change in technology, the requirements for the electromagnetic environment also change, naturally in the direction of tightening. So our power quality standard, GOST 13109 of 1967, was revised in 1987 with the development of semiconductor technology, and revised in 1997 with the development of microprocessor technology. Indicators of the quality of electrical energy, methods for their assessment and norms are determined by the Interstate Standard: “Electric Energy. Compatibility of technical means is electromagnetic. Standards for the quality of electrical energy in general-purpose power supply systems” GOST 13109-97. Table 1.1 - Rationing of power quality indicators

	Name of PCE	Most likely cause
Voltage deviation
	steady state voltage deviation	consumer load schedule
Voltage fluctuations
	voltage swing	fluctuating load consumer
	flicker dose
Voltage unbalance in a three-phase system
	negative sequence voltage unbalance factor	consumer with unbalanced load
	zero sequence voltage unbalance factor
Non-sinusoidal voltage waveform
	voltage sine wave distortion factor	consumer with non-linear load
	coefficient of the nth harmonic voltage component
	frequency deviation	network features, climatic conditions or natural phenomena
	dip duration
	impulse voltage
	temporary overvoltage factor

Most of the phenomena that occur in electrical networks and degrade the quality of electrical energy occur due to the peculiarities of the joint operation of power consumers and the electrical network. Seven SCEs are mainly due to voltage losses (drops) in the section of the electrical network, from which neighboring consumers are powered. Voltage losses in the section of the electrical network (k) are determined by the expression: ΔU k \u003d (P k R k + Q k X k) / U nom Here, the active (R) and reactive (X) resistance of the k-th section of the network are practically constant , and the active (P) and reactive (Q) power flowing through the k-th section of the network are variable, and the nature of these changes affects the formation of electromagnetic interference:

With a slow change in the load in accordance with its schedule - voltage deviation; With a sharply changing nature of the load - voltage fluctuations; With an asymmetric load distribution over the phases of the electrical network - voltage asymmetry in a three-phase system; With a non-linear load - non-sinusoidal shape of the voltage curve.

In relation to these phenomena, consumers of electrical energy have the opportunity to influence its quality in one way or another. Everything else that worsens the quality of electrical energy depends on the characteristics of the network, climatic conditions or natural phenomena. Therefore, the consumer of electric energy does not have the opportunity to influence this, he can only protect his equipment with special means, for example, high-speed protection devices or uninterrupted power supply devices (UPS). 1.1 Voltage deviation. Voltage deviation - the difference between the actual voltage in the steady state operation of the power supply system and its nominal value. Voltage deviation at one point or another in the network occurs under the influence of load changes in accordance with its schedule.

Influence of voltage deviation on the operation of electrical equipment:

Technological installations:

With a decrease in voltage, the technological process deteriorates significantly, and its duration increases. Consequently, the cost of production increases. With an increase in voltage, the service life of the equipment decreases, and the likelihood of accidents increases. With significant voltage deviations, the technological process is disrupted.

Lighting:

The service life of lighting lamps is reduced, so at a voltage of 1.1 U nom, the service life of incandescent lamps is reduced by 4 times. At a voltage of 0.9 U nom, the luminous flux of incandescent lamps is reduced by 40% and fluorescent lamps by 15%. voltage value less than 0.9 U nom fluorescent lamps flicker, and at 0.8 U nom they simply do not light up.

Electric drive:

When the voltage at the terminals of the asynchronous motor is reduced by 15%, the torque is reduced by 25%. The engine may not start or stop.

With a decrease in voltage, the current consumed from the network increases, which leads to heating of the windings and a decrease in the life of the motor. With long-term operation at a voltage of 0.9 U nom, the service life of the motor is halved. With an increase in voltage by 1%, the reactive power consumed by the motor increases by 3 ... 7%. The efficiency of the drive and network is reduced.

The generalized load node of electrical networks (load on average) is:
- 10% of the specific load (for example, in Moscow this is the metro - ~ 11%);
-30% lighting and more;
- 60% asynchronous motors. Therefore, GOST 13109-97 sets the normal and maximum permissible values ​​of the steady-state voltage deviation at the terminals of electrical receivers within the limits, respectively, δUy norm = ± 5% and δUy prev = ± 10% of the rated network voltage. These requirements can be met in two ways: by reducing voltage losses and by regulating the voltage. ΔU = (P R + Q X) / U CPU (TC) Reducing voltage losses (ΔU) is achieved:

The choice of cross-section of the conductors of power lines (≡ R) according to the conditions of voltage losses. The use of longitudinal capacitive compensation of the reactive resistance of the line (X). However, this is dangerous due to an increase in short-circuit currents at X→0. Reactive power compensation (Q) to reduce its transmission through the mains, using capacitor banks and synchronous electric motors operating in overexcitation mode.

In addition to reducing voltage losses, reactive power compensation is an effective energy saving measure, providing a reduction in electricity losses in electrical networks.

Voltage regulationU:

In the power center, voltage regulation (U CPU) is carried out using transformers equipped with a device for automatic regulation of the transformation ratio depending on the magnitude of the load - regulation under load (OLTC). ~ 10% of transformers are equipped with such devices. The regulation range is ± 16% with a resolution of 1.78%. with disconnection from the network. Adjustment range ± 5% with a resolution of 2.5%.

Responsibility for maintaining tension within the limits established by GOST 13109-97, is assigned to the energy supply organization.

Indeed, the first (R) and second (X) methods are chosen when designing the network and cannot be changed in the future. The third (Q) and fifth (U TP) methods are good for regulating during seasonal changes in network load, but it is necessary to manage the operating modes of consumer compensating equipment centrally, depending on the operating mode of the entire network, that is, the power supply organization. The fourth way - voltage regulation in the power center (U CPU), allows the power supply organization to actively regulate the voltage in accordance with the load schedule of the network. GOST 13109-97 establishes the permissible values ​​​​of the steady-state voltage deviation at the terminals of the electrical receiver. And the limits of voltage change at the point of connection of the consumer should be calculated taking into account the voltage drop from this point to the power receiver and indicated in the power supply contract. 1.2 Voltage fluctuations Voltage fluctuations are rapidly changing voltage fluctuations lasting from half a cycle to several seconds. Voltage fluctuations occur under the influence of a rapidly changing network load. The sources of voltage fluctuations are powerful power consumers with a pulsed, sharply variable nature of active and reactive power consumption: arc and induction furnaces; electric welding machines; electric motors at startup.

The quality of electricity is required to be expressed in quantitative terms to evaluate the supply network. Providers are required to maintain compliance with GOSTs of such characteristics as voltage and frequency fluctuations. Depending on the connected consumers, the values ​​of the main indicators change, which, if there are significant deviations, can lead to failure of household appliances.

What affects the characteristics of the power supply network?

The quality of electricity depends on a huge number of factors that change performance beyond the limits established by regulations. So, the voltage may be too high due to an accident at the substation. Underestimated values ​​appear in the evening or in the summer season, when people return home and turn on TVs, electric stoves, split systems.

The quality of electricity according to GOSTs may vary slightly. In very poor supply networks, consumers have to use voltage stabilizers. Control over the characteristics is entrusted to Rospotrebnadzor, where you can contact in case of inconsistencies.

The quality of electricity can depend on the following factors:

• Daily fluctuations associated with uneven connection by consumers or with the influence of tides at sea stations.
• Changes in the air environment: humidity, ice formation on the supply wires.
• A change in the wind when the power is generated by wind turbines.
• The quality of the wiring, it wears out over time.

Why do we need the main characteristics of the supply network?

The quantitative value and deviation errors of the parameters are set according to GOST. The quality of electricity is spelled out in document 32144-2013. It was necessary to legalize these indicators because of the risk of fire of consumer devices, as well as disruption of the functioning of electrical appliances that are sensitive to voltage drops in installations. The latest devices are common in medical institutions, research centers, and military facilities.

Electricity was updated in 2013 in connection with the development of the energy sales market and the emergence of new electronic devices. Electricity as part of its supply should be considered as a product that meets certain criteria. In case of deviation of the established characteristics, administrative liability may be applied to providers. If, due to fluctuations in the incoming voltage, people have suffered or could have suffered, then criminal liability may already arise.

What happens to consumers when they deviate from normal eating patterns?

Power quality parameters affect the duration of the connected devices, often it becomes critical in production. The performance of the lines decreases, it increases. Thus, the torque on the motor shaft decreases when the values ​​of the mains indicators fall. The service life of lighting lamps is shortened, the luminous flux of the lamps becomes smaller or flickers, which affects the products produced in greenhouses. Significant influence is exerted on the processes of other biochemical reactions.

According to the laws of physics, a decrease in voltage with a constant load on the motor shaft leads to a rapid increase in current. This, in turn, leads to malfunctions of the safety switches. As a result, the insulation melts, in the best case they burn, in the worst case, the motor windings and electronics elements are irretrievably damaged. Under similar circumstances, the electric meter starts to rotate at a higher speed. The owner of the premises suffers losses.

Criteria for assessing the supply network

What does GOST contain? The quality of electricity is determined by the characteristics of three-phase networks and 50 Hz circuits common in everyday life:

• The steady value of the voltage deviation determines the value of the characteristic at which consumers can function without failure. The lower normal limit is set from 220 V to 209 V and the upper limit is 231 V.
• The range of change in the input voltage is the difference between the values ​​of the current and amplitude. Measurements are made for the cycle of the parameter difference.
• The dose of flicker is divided into short-term within 10 minutes and long-term, determined by 2 hours. Denotes the degree of susceptibility of the human eye to the flickering of light, which was caused by a fluctuation in the mains.
• The impulse voltage is described by the recovery time, which has a different value depending on the cause of the jump.
• Coefficients for assessing the quality of the supply network: by sinusoidal distortion, temporary overvoltage values, harmonic components, asymmetry in negative and zero sequences.
• The voltage dip interval is determined by the recovery period of the parameter set according to GOST.
• Deviation of the supply frequency leads to damage to electrical parts and conductors.

Fixed deviation of the input value

They try to make indicators of the quality of electricity corresponding to the established denominations prescribed in legislative acts. Attention is paid to the errors that arise when measuring U and f. If there are errors, then you can contact the supervisory authorities to hold the electricity supplier accountable.

The general requirements for power quality include the supply voltage deviation parameter, which is divided into two groups:

• Normal mode when the deviation is ±5%.
• The tolerance limit is set for fluctuations of ±10%. This will amount to a minimum threshold of 198 V and a maximum of 242 V for a 220 V network.

Voltage recovery should occur within a time interval of no more than two minutes.

The scope of the mains change

The power quality standards contain the supervision of such a parameter as the fluctuation of the voltage components. It sets the difference between the upper amplitude threshold and the lower one. Taking into account that the deviation tolerances of the parameter from the set value are within the limit of ±5%, the range of the limiting mode cannot exceed ±10%. The 220V power supply cannot fluctuate more or less than 22V, and 380V works normally within ±38V.

The resulting range of voltage fluctuations is calculated according to the following expression ΔU = U max -U min, in the standards, the results are indicated in% according to the calculations ΔU = ((U max -U min) / U nominal) * 100%.

Input value instability

The power quality system includes flicker measurements. This indicator is fixed by a special device - a flickermeter, which takes the amplitude-frequency characteristic. The results obtained are compared with the sensitivity curve of the visual organ.

GOST establishes the permissible limits for changing the dose of flicker:

• Short-term fluctuations indicator should not be higher than 1.38.
• Long-term changes must be within the parameter value of 1.0.

If we are talking about the upper limit of the indicator of the incandescent lamp circuit, then it is required that the result falls within the following limits:

• Short-term fluctuations - the indicator is set to 1.0.
• Continuous parameter changes - 0.74.

Perceptible differences

Power quality measurements include measurements of such a component as supply voltage pulses. It is explained by sharp drops and rises in electricity within the selected interval. The reasons for this phenomenon may be the simultaneous switching of a large number of consumers, the influence of electromagnetic interference due to thunderstorms.

Voltage recovery periods have been established that do not affect the operation of consumers:

• The causes of drops are a thunderstorm and other natural electromagnetic interference. The recovery period is no more than 15 µs.
• If the pulses appeared due to uneven switching of consumers, then the period is much larger and equal to 15 ms.

The largest number of accidents at substations occurs due to lightning strikes into the installation. The insulation of the conductors immediately suffers. The magnitude of the overvoltage can reach hundreds of kilovolts. Protective devices are provided for this, but sometimes they fail and residual potential is observed. At these times, a fault does not occur due to the strength of the insulation.

Duration of falling input value

The measured parameter is described as a voltage dip within ±0.1U nominal for an interval of several tens of milliseconds. For a 220 V network, a change in the indicator is allowed up to 22 V, if 380 V, then no more than 38 V. The recession depth is calculated according to the expression: ΔU n \u003d (U nominal −U min) / U nominal.

The duration of the recession is calculated according to the expression: Δt n \u003d t k −t n, here t k is the period when the voltage has already recovered, and t n is the starting point, the moment when the voltage drop occurred.

Power quality control obliges to take into account the frequency of failures, determined by the formula: Fn=(m(ΔU n ,Δt n)/M)*100%. Here:

• m(ΔU n ,Δt n) is defined as the number of recessions at a given time with depth ΔU n and duration Δt n .
• M - the total score of recessions during the selected period.

Why is the value of the recession needed?

The input value decay duration parameter is required to evaluate the reliability of the supply energy in quantitative terms. This indicator can be affected by the frequency of accidents at the substation due to the negligence of personnel, lightning. The result of the study of failures are forecasts for the degree of failure in the network under consideration.

Statistics allow drawing approximate conclusions about the stability of the supply. The electricity provider is provided with recommended data for carrying out preventive measures at installations.

Frequency deviation

Compliance with the frequency within certain limits refers to the necessary requirement of the consumer. With a decrease in the indicator by 1%, the losses are more than 2%. This is expressed in economic costs, reduced productivity of enterprises. For the average person, this results in higher amounts on electricity bills.

The speed of rotation of an asynchronous motor directly depends on the frequency of the supply network. Heating heating elements have lower performance when the parameter drops below 50 Hz. At overestimated values, damage to consumers or other mechanisms that are not designed for a high torque may occur.

Frequency deviation can affect the operation of the electronics. So on the TV screen, interference occurs when the indicator changes by ± 0.1 Hz. In addition to visual defects, the risk of failure of microelements increases. The method of combating deviations in the quality of electricity is the introduction of backup power units, which allow automatic restoration of voltage at set intervals.

Odds

For the normal operation of the supply network, the control of the following coefficients has been introduced:

• Non-sinusoidality of the voltage curve. The distortion of the sinusoid occurs due to powerful consumers: heating elements, convection ovens, welding machines. If this parameter deviates, the service life of the motor windings is reduced, the operation of relay automation is disrupted, and drive systems on thyristor control fail.
• A temporary overvoltage is a quantitative measure of the impulse change in an input quantity.
• The Nth harmonic is a characteristic of the sinusoidality obtained at the input of the voltage characteristic. Calculated values ​​are obtained from tabular data for each harmonic.
• It is important to take into account the asymmetry of the input value in the negative or zero sequence in order to exclude cases of uneven distribution of phases. Such conditions occur more often when the power supply is interrupted, connected according to the star or delta circuit.

Types of protection against unpredictable changes in the supply network

Improving the quality of electricity must be carried out within the time limits specified by law. But the consumer has the right to build the protection of his equipment using the following means:

• Power stabilizers ensure that the input value is maintained within the specified limits. Quality energy is achieved even with deviations of the input value by more than 35%.
• Sources are designed to maintain the consumer's performance for a specified period of time. The devices are powered by the stored energy in their own battery. In the event of a power outage, uninterruptible power supplies are able to keep the equipment of the entire office up and running for several hours.
• Surge protection devices operate on the principle of a relay. After exceeding the input value of the set limit, the circuit is opened.

All types of protection have to be combined to provide full confidence that expensive equipment will remain intact during an accident at a substation.

Quality of electrical energy

Introduction

electrical energy voltage

Electric energy as a commodity is used in all spheres of human life, has a set of specific properties and is directly involved in the creation of other types of products, affecting their quality. The concept of quality of electric energy (QE) differs from the concept of quality of other types of products. Each power receiver is designed to operate at certain parameters of electrical energy: nominal frequency, voltage, current, etc., therefore, for its normal operation, the required CE must be provided. Thus, the quality of electrical energy is determined by the totality of its characteristics, under which power receivers (EP) can operate normally and perform their functions.

CE at the place of production does not guarantee its quality at the place of consumption. The CE before and after switching on the EA at the point of its connection to the electrical network can be different. CE is also characterized by the term "electromagnetic compatibility". Electromagnetic compatibility is understood as the ability of an EA to function normally in its electromagnetic environment (in the electrical network to which it is connected), without creating unacceptable electromagnetic interference for other EAs operating in the same environment.

The problem of electromagnetic compatibility of industrial electric circuits with the supply network has arisen in connection with the widespread use of powerful valve converters, arc steel-smelting furnaces, welding installations, which, for all their economy and technological efficiency, have a negative impact on the CE.

Household EPs, as well as industrial ones, must also have electromagnetic compatibility with other EPs included in the common power grid, not reduce the efficiency of their work and not worsen the SCE.

CE in industry is assessed according to technical and economic indicators, which take into account damage due to damage to materials and equipment, disruption of the technological process, deterioration in the quality of manufactured products, and a decrease in labor productivity - the so-called technological damage. In addition, there is electromagnetic damage from low-quality electricity, which is characterized by an increase in electricity losses, failure of electrical equipment, disruption of automation, telemechanics, communications, electronic equipment, etc.

KE is closely related to the reliability of power supply, since the normal mode of power supply to consumers is such a mode in which consumers receive electricity uninterruptedly, in an amount agreed in advance with the power supply organization, and of a normalized quality. Article 542 of the Civil Code of the Russian Federation obliges to supply electricity, the quality of which meets the requirements of state standards and other mandatory rules or energy supply contracts.

In accordance with the Law of the Russian Federation "On the Protection of Consumer Rights" (Article 7) and the Decree of the Government of Russia dated August 13, 1997 No. 1013, electrical energy is subject to mandatory certification in terms of electricity quality indicators established by GOST 13109-97 "Quality standards for electrical energy in general purpose power supply systems. This means that each energy supplying organization, along with a license for the production, transmission and distribution of electricity, must obtain a certificate certifying that the quality of the energy it supplies meets the requirements of GOST 13109-97.

1. The main provisions of the state standard for the quality of electrical energy

GOST 13109-97 "Normals for the quality of electrical energy in general-purpose power supply systems" (hereinafter referred to as GOST) establishes indicators and norms for the quality of electricity in electrical networks of general-purpose power supply systems of three-phase and single-phase alternating current with a frequency of 50 Hz at points to which electrical networks are connected. owned by various consumers of electrical energy, or receivers of electrical energy (points of common connection). GOST 13109-97 is an interstate standard and has been in force in the Russian Federation since January 1, 1999.

The CE limits set by the standard are the EMC levels for conducted EMI in general power systems. Subject to the established PQ standards, the electromagnetic compatibility of electrical networks of power supply organizations and electrical networks of consumers of electrical energy or electric power is ensured.

The standard does not establish requirements for PQ in electrical networks for special purposes (contact, traction, communications), mobile installations (aircraft, trains, ships), etc.

Conductive electromagnetic interference in the power supply system - electromagnetic interference propagating through the elements of the electrical network.

General connection point - a point of a general-purpose electrical network, electrically closest to the networks of the considered consumer of electrical energy, to which the electrical networks of other consumers are connected or can be connected.

The standard does not establish CE norms for modes caused by force majeure (exceptional weather conditions, natural disasters, etc.).

GOST 13109-97 is the first standard in the field of PQ, which states that the established norms are to be included in the technical specifications for the connection of consumers and in power supply contracts.

In order to ensure the norms of the standard at the points of general connection, consumers who are the culprits for the deterioration of the KE are allowed to establish more stringent standards in the technical specifications for connection and in power supply contracts (with smaller ranges of changes in the corresponding KE indicators) than those established in the standard.

The norms of the standard should be applied in the design and operation of electrical networks, in establishing the levels of noise immunity of the EP and the levels of electromagnetic interference introduced by these receivers into the electrical network to which they are connected.

2. Indicators of the quality of electrical energy

The standard establishes the following power quality indicators (PQI):

Steady voltage deviation;

range of voltage change;

flicker dose;

coefficient of the n-th harmonic component of the voltage;

frequency deviation;

voltage dip duration;

impulse voltage;

temporary overvoltage coefficient.

When determining the values ​​of some SCEs, the standard introduces the following auxiliary parameters of electrical energy:

Interval between voltage changes;

voltage dip depth;

the frequency of occurrence of voltage dips;

pulse duration at the level of 0.5 of its amplitude;

duration of temporary overvoltage.

Part of the PQ characterizes the steady-state operating modes of the electrical equipment of the power supply organization and consumers of EE and gives a quantitative assessment of the features of the technological process of production, transmission, distribution and consumption of EE by CE. These SCEs include: the steady-state voltage deviation, the distortion factor of the sinusoidal curve of the voltage, the coefficient of the n-th harmonic component of the voltage, the coefficient of voltage asymmetry in the negative sequence, the coefficient of voltage asymmetry in the zero sequence, the frequency deviation, the range of voltage change.

The assessment of all SCEs related to voltage is made according to its current values.

To characterize the above indicators, the standard establishes numerical normal and maximum allowable values ​​of the SCE or norm.

Another part of the PQ characterizes short-term interference that occurs in the electrical network as a result of switching processes, thunderstorm atmospheric phenomena, the operation of protective equipment and automation, and in post-emergency modes. These include voltage dips and pulses, short-term overvoltages. For these SCEs, the standard does not establish acceptable numerical values. To quantify these SCEs, the amplitude, duration, frequency of their occurrence, and other characteristics established but not standardized by the standard should be measured. Statistical processing of these data makes it possible to calculate generalized indicators that characterize a particular electrical network in terms of the probability of short-term interference.

To assess the compliance of the SCE with the specified standards (with the exception of the duration of the voltage dip, impulse voltage and the coefficient of temporary overvoltage), the standard establishes a minimum calculation period of 24 hours.

Due to the random nature of the change in electrical loads, the requirement to comply with the PQ standards during all this time is practically unrealistic, therefore, the standard establishes the probability of exceeding the PQ standards. The measured SCEs should not go beyond the normally permissible values ​​with a probability of 0.95 for the estimated period of time established by the standard (this means that individual excesses of the normalized values ​​can be disregarded if their expected total duration is less than 5% for the specified period of time).

In other words, the CE according to the measured indicator complies with the requirements of the standard, if the total duration of the time for going beyond the normally permissible values ​​is no more than 5% of the set time period, i.e. 1 h 12 min, and for the maximum allowable values ​​- 0% of this period of time.

The standard indicates the likely culprits for the deterioration of the PQ. The frequency deviation is regulated by the power supply system and depends only on it. Separate EPs at industrial enterprises (and even more so at home) cannot influence this indicator, since their power is disproportionately small compared to the total power of generators of power plants of the energy system. Voltage fluctuations, asymmetry and non-sinusoidality of the voltage are caused mainly by the operation of individual powerful electric circuits at industrial enterprises, and only the magnitude of these SCEs depends on the power of the power supply system at the considered consumer connection point. Voltage deviations depend both on the level of voltage supplied by the power system to industrial enterprises, and on the operation of individual industrial EAs, especially those with a large consumption of reactive power. Therefore, PQ issues should be considered in direct connection with reactive power compensation issues. The duration of the voltage dip, the impulse voltage, the coefficient of temporary overvoltage, as already noted, are determined by the operating modes of the power system.

Table 2.1. the properties of electrical energy, their indicators characterizing and the most likely culprits for the deterioration of the CE are given.

Table 2.1. Properties of electrical energy, indicators and the most likely culprits for the deterioration of PQ

Properties of electrical energyPQ indicatorThe most likely culprits for the deterioration of PQVoltage deviationSustained voltage deviation Power supply organizationVoltage fluctuationsVoltage change range

Flicker dose Variable load consumerVoltage non-sinusoidal Voltage distortion factor

Coefficient of the nth harmonic component of the voltage Consumer with a non-linear load Unbalance of the three-phase voltage system Negative sequence voltage unbalance factor

Zero sequence voltage unbalance factor Unbalanced load consumer Frequency deviation Frequency deviation Power supply companyVoltage dipDuration of dip Power supply organizationImpulse voltageImpulse voltage Power supply companyTemporary overvoltageTemporary overvoltage factor Energy supply organization

The standard establishes the calculation methods and methods for determining the PQI and auxiliary parameters, the requirements for measurement errors and averaging intervals for the PQI, which should be implemented in PQ control devices when measuring indicators and processing them.

3. Characteristics of power quality indicators

Voltage deviation

Voltage deviations from nominal values ​​occur due to daily, seasonal and technological changes in the electrical load of consumers; changes in the power of compensating devices; voltage regulation by generators of power plants and substations of power systems; changes in the scheme and parameters of electrical networks.

The voltage deviation is determined by the difference between the effective U and the nominal voltage values, V:

The steady voltage deviation is, %:

where is the steady-state (effective) voltage value for the averaging interval (see clause 3.8).

In electric networks of single-phase current, the effective value of voltage is determined as the value of the voltage of the fundamental frequency without taking into account the higher harmonic components of the voltage, and in electric networks of three-phase current - as the effective value of the direct sequence voltage of the fundamental frequency.

The standard normalizes voltage deviations at the outputs of electrical energy receivers. Normally permissible and maximum permissible values ​​of the steady-state voltage deviation are respectively ±5 and ±10% of the nominal voltage value and at the points of common connection of consumers of electrical energy should be established in power supply contracts for hours of minimum and maximum loads in the power system, taking into account the need to comply with the standards for outputs of electrical energy receivers in accordance with regulatory documents.

Voltage fluctuations

Voltage fluctuations are caused by a sharp change in the load in the section of the electrical network under consideration, for example, by turning on an asynchronous motor with a high starting current ratio, technological installations with a fast-changing mode of operation, accompanied by shocks of active and reactive power - such as the drive of reversing rolling mills, arc steel-smelting furnaces, welding devices, etc.

Voltage fluctuations are characterized by two indicators:

dose of flicker.

The range of voltage change is calculated by the formula,%

where, are the values ​​of the extrema following one after another (or the extremum and the horizontal section) of the envelope of the rms voltage values, in accordance with Fig. 3.1.

Rice. 3.1. Voltage fluctuations

The frequency of repetition of voltage changes, (1/s, 1/min) is determined by the expression:

where m is the number of voltage changes over time T;

T is the measurement time interval taken equal to 10 min.

If two voltage changes occur with an interval of less than 30 ms, then they are considered as one.

The time interval between voltage changes is:

The assessment of the admissibility of the range of voltage changes (voltage fluctuations) is carried out using the curves of the dependence of the permissible range of oscillations on the frequency of repetition of voltage changes or the time interval between subsequent voltage changes.

The CE at the point of common connection with periodic voltage fluctuations having the shape of a meander (rectangular) (see Fig. 3.2) is considered to comply with the requirements of the standard if the measured value of the range of voltage changes does not exceed the values ​​determined from the curves of Fig. 3.2 for the corresponding frequency of repetition of voltage changes, or the interval between voltage changes.

Rice. 3.2. Voltage fluctuations of arbitrary shape (a) and meander-shaped (b)

The maximum allowable value of the sum of the steady-state voltage deviation δUU and the range of voltage changes δUt at the points of connection to electric networks with a voltage of 0.38 kV is ±10% of the rated voltage.

Flicker dose is a measure of a person's susceptibility to the effects of fluctuations in light output caused by fluctuations in mains voltage over a set period of time.

The standard establishes a short-term () and long-term flicker dose () (short-term is determined at an observation time interval of 10 minutes, long-term at an interval of 2 hours). The initial data for the calculation are flicker levels measured using a flickermeter - a device in which the sensitivity curve (amplitude-frequency characteristic) of the human eye is modeled. At present, the development of flickermeters for monitoring voltage fluctuations has begun in the Russian Federation.

The EC for the flicker dose complies with the requirements of the standard, if the short-term and long-term flicker doses, determined by measuring for 24 hours or by calculation, do not exceed the maximum allowable values: for a short-term flicker dose - 1.38 and for a long-term - 1.0 (with voltage fluctuations with a shape different from the meander).

The maximum permissible value for a short-term flicker dose at the points of general connection of electricity consumers with incandescent lamps in rooms where significant eye strain is required is 1.0, and for a long-term dose - 0.74, with voltage fluctuations with a shape different from the meander.

Non-sinusoidal voltage

In the process of generating, converting, distributing and consuming electricity, there are distortions in the shape of sinusoidal currents and voltages. The sources of distortion are synchronous generators of power plants, power transformers operating at elevated values ​​of magnetic induction in the core (with increased voltage at their terminals), AC-to-DC converters and EP with non-linear volt-ampere characteristics (or non-linear loads).

Distortions created by synchronous generators and power transformers are small and do not have a significant impact on the power supply system and on the operation of the ED. The main cause of distortions are valve converters, electric arc steel-smelting and ore-thermal furnaces, arc and contact welding installations, frequency converters, induction furnaces, a number of electronic technical equipment (television receivers, PC), gas discharge lamps, etc. Electronic electricity receivers and gas discharge lamps are created with their own operation, a low level of harmonic distortion at the output, but the total number of such EDs is large.

It is known from the course of mathematics that any non-sinusoidal function (for example, see Fig. 3.3) that satisfies the Dirichlet condition can be represented as the sum of a constant value and an infinite series of sinusoidal quantities with multiple frequencies. Such sinusoidal components are called harmonic components or harmonics. A sinusoidal component whose period is equal to the period of a non-sinusoidal periodic quantity is called the fundamental or first harmonic. The remaining components of the sinusoid with frequencies from the second to the nth are called higher harmonics.

Rice. 3.3. Non-sinusoidal voltage

Voltage non-sinusoidality is characterized by the following indicators:

· the distortion factor of the sinusoidality of the voltage curve;

· coefficient of the n-th harmonic component of the voltage.

The distortion factor of the sinusoidality of the voltage curve is determined by the expression, %

where - the effective value of the n-th harmonic component of the voltage, V; - the order of the harmonic component of the voltage, - the order of the last of the harmonic components of the voltage taken into account, the standard sets N = 40;

The effective value of the voltage of the fundamental frequency, V.

It is allowed to determine by expression, %

where is the rated voltage of the network, V.

The coefficient of the n-th harmonic component of the voltage is, %

It is allowed to calculate by expression, %SRC= "publ_image/Image48.gif" align= "top"> (3.10)

For the calculation, it is necessary to determine the voltage level of the individual harmonics generated by the non-linear load.

The phase voltage of the harmonic at the design point of the network is found from the expression:

where is the effective value of the phase current of the nth harmonic;

Non-linear load voltage (if the calculated point coincides with the non-linear load connection point, then =);

Rated mains voltage;

Short circuit power at the point of connection of a non-linear load.

For the calculation, it is necessary to first determine the current of the corresponding harmonic, which depends not only on electrical parameters, but also on the type of non-linear load.

Normally permissible and maximum permissible values ​​at the point of common connection to electrical networks with different rated voltages are given in Table 3.1.

Table 3.1. Values ​​of the distortion factor of the sinusoidal voltage curve

Normally admissible values ​​at , kVMaximum permissible values ​​at , kV0.386 -2035110-3300.386 -2035110-3308.05.04.02.012.08.06.03.0

Voltage unbalance

The most common sources of voltage asymmetry in three-phase power supply systems are such consumers of electricity, the symmetrical multi-phase execution of which is either impossible or impractical for technical and economic reasons. Such installations include induction and electric arc furnaces, traction loads of railways, made on alternating current, electric welding units, special single-phase loads, lighting installations.

Asymmetric voltage modes in electrical networks also occur in emergency situations - with a phase failure or asymmetric short circuits.

Voltage asymmetry is characterized by the presence in a three-phase electrical network of negative or zero sequence voltages, which are much smaller in magnitude than the corresponding components of the direct (main) sequence voltage.

The asymmetry of the three-phase voltage system occurs as a result of the imposition of a direct sequence of voltages of the negative sequence system on the system, which leads to changes in the absolute values ​​of phase and interphase voltages (Fig. 3.4.).

Rice. 3.4. Vector diagram of positive and negative sequence voltages

In addition to the unbalance caused by the voltage of the negative sequence system, there can be unbalance from the superimposition of the zero sequence system voltages on the positive sequence system. As a result of the displacement of the neutral of a three-phase system, asymmetry of phase voltages occurs while maintaining a symmetrical system of phase-to-phase voltages (Fig. 3.5.).

Rice. 3.5. Vector diagram of positive and zero sequence voltages

Voltage asymmetry is characterized by the following indicators:

· voltage unbalance factor in reverse order;

· coefficient of voltage asymmetry in the zero sequence.

The coefficient of voltage asymmetry in the reverse sequence is, %

where is the effective value of the negative sequence voltage of the fundamental frequency of the three-phase voltage system, V;

The effective value of the positive sequence voltage of the fundamental frequency, V.

It is allowed to calculate by expression,%:

where is the nominal value of the phase-to-phase voltage of the network, V.

The coefficient of voltage asymmetry in the zero sequence is, %:

where is the effective value of the voltage of the zero sequence of the main frequency of the three-phase voltage system, V.

It is allowed to calculate by the formula,%

where is the nominal value of the phase voltage, V.

The measurement of the voltage unbalance factor in the zero sequence is carried out in a four-wire network.

The relative error in determining and according to formulas (3.15) and (3.16) is numerically equal to the value of voltage deviations from.

Normally permissible and maximum permissible values ​​of the voltage asymmetry coefficient in reverse sequence at the point of common connection to electric networks are 2.0 and 4.0%.

The normalized values ​​of the zero sequence voltage asymmetry coefficient at the point of common connection to four-wire electrical networks with a rated voltage of 0.38 kV are also equal to 2.0 and 4.0%.

Frequency deviations

Frequency deviation - the difference between the actual and nominal values ​​of frequency, Hz

The standard sets the normal and maximum permissible values ​​for the frequency deviation equal to ± 0.2 Hz and ± 0.4 Hz, respectively.

voltage dip

Voltage dips include a sudden significant change in voltage at a point in the electrical network below the level of 0.9, followed by the restoration of voltage to its original level or close to it after a period of time from ten milliseconds to several tens of seconds (Fig. 3.6).

Rice. 3.6. voltage dip

The characteristic of the voltage dip is its duration - equal to:

where and are the initial and final moments of the voltage dip.

The voltage dip is also characterized by the voltage dip depth - the difference between the nominal voltage value and the minimum effective voltage value, expressed in units of voltage or as a percentage of its nominal value. The voltage dip is calculated by the expressions

The maximum allowable value of the voltage dip duration in electrical networks with voltage up to 20 kV inclusive is 30 s. The duration of the automatically eliminated voltage dip at any point of connection to the electrical networks is determined by the time delays of relay protection and automation.

Voltage impulse and temporary overvoltage

Distortion of the shape of the supply voltage curve can occur due to the appearance of high-frequency pulses during switching in the network, the operation of arresters, etc. Voltage pulse - a sharp change in voltage at a point in the electrical network, followed by the restoration of voltage to its original level or close to it. The magnitude of voltage distortion in this case is characterized by an indicator of impulse voltage (Fig. 3.7).

Rice. 3.7. Pulse voltage parameters

The impulse voltage in relative units is:

where is the value of the impulse voltage, V.

The amplitude of the pulse is the maximum instantaneous value of the voltage pulse. The pulse duration is the time interval between the initial moment of the voltage pulse and the moment the instantaneous value of the voltage is restored to its original level or close to it.

The indicator - impulse voltage is not standardized by the standard.

Temporary overvoltage - an increase in voltage at a point in the electrical network above 1.1 for more than 10 ms, occurring in power supply systems during switching or short circuits (Fig. 3.8.).

Rice. 3.8. Temporary overvoltage

A temporary overvoltage is characterized by a temporary overvoltage coefficient (): this is a value equal to the ratio of the maximum value of the envelope of the amplitude values ​​of the voltage during the existence of a temporary overvoltage to the amplitude of the rated voltage of the network.

The duration of a temporary overvoltage is the time interval between the initial moment of the occurrence of a temporary overvoltage and the moment of its disappearance.

The coefficient of temporary overvoltage is also not standardized by the standard.

The values ​​of the coefficient of temporary overvoltage at the points of connection of the general-purpose electrical network, depending on the duration of temporary overvoltages, do not exceed the values ​​given in Table 3.3.

Table 3.3. Dependence of the coefficient of temporary overvoltage on the duration of overvoltage

Temporary overvoltage duration, sUp to 1Up to 20Up to 60Temporary overvoltage coefficient, p.u.1.471.311.15

On average, about 30 temporary overvoltages are possible at the connection point per year.

When the neutral conductor breaks in three-phase electrical networks with voltages up to 1 kV, operating with a solidly grounded neutral, temporary overvoltages occur between the phase and the ground. The level of such overvoltages with a significant asymmetry of phase loads can reach the values ​​of the phase-to-phase voltage, and the duration is several hours.

Statistical assessment of power quality indicators

Changes in the parameters of the electrical network, power and the nature of the load over time are the main reason for the change in the SCE. Thus, the SCE - the steady voltage deviation, the coefficients characterizing the non-sinusoidality and asymmetry of the voltage, the frequency deviation, the amplitude of the voltage change, etc. - are random quantities and their measurement and processing should be based on probabilistic-statistical methods. Therefore, as already noted, the standard establishes the SQE norms and stipulates the need to fulfill them within 95% of the time of each day (for normally acceptable values).

The most complete characterization of random variables is given by the laws of their distribution, which make it possible to find the probabilities of the occurrence of certain values ​​of the SCE. We will explain the use of probabilistic-statistical methods using the example of estimating voltage deviations.

Operating experience shows the presence of daily, weekly and longer cycles of voltage deviations in time. Statistical data confirm that the distribution law of voltage deviations in electrical networks can be most accurately described using the normal distribution law, which is used in the practice of PQ control.

The analytical description of the normal law is carried out using two parameters: the mathematical expectation of a random variable and the standard deviation from the mean. The equation for the distribution curve of voltage deviations from the nominal, corresponding to the normal distribution law, has the form:

Expression (3.25) is written for a continuous process of changing a random variable. To simplify the CE control devices, continuous random variables, which are the CEs, are replaced during control by discrete sequences of their values.

The most convenient form of presenting information about changes in a random variable is a histogram. Histogram - a graphical representation of the statistical series of the indicator under study, the change of which is random (Fig. 3.9.). In this case, the entire range of voltage deviations is divided into intervals of equal width (for example, 1.25%). Each interval is given a name - the value of voltage deviations corresponding to the middle of the interval, and the probability (frequency) of voltage deviations falling into this interval is found

where is the number of hits in the i-th interval;

The total number of measurements.

Rice. 3.9. Voltage deviation histogram

Based on the histogram, the answer is given: what is the quality of electricity at the control point. Such an assessment is made by the sum of the values ​​of falling into intervals that fit into the allowable range of voltage deviations. With the help of the histogram, the probability of voltage deviations beyond the normally permissible values ​​is also found. This allows you to judge the reasons for the low quality of voltage in the electrical network and choose measures to improve it.

Numerical characteristics and determined from the histogram are widely used to assess the voltage quality.

Mathematical expectation determines the average level of voltage deviations at the considered point of the network for a controlled period of time

where k is the number of histogram bins.

Dissipation of voltage deviations is characterized by dispersion. It is equal to the mathematical expectation of the squared deviations of a random variable from its mean value and is determined from the expression

The parameter is the standard deviation and characterizes the scatter of the histogram, i.e. spread of voltage deviations around the mathematical expectation. For most histograms of voltage deviations, the integral probability of falling into range 4 is 0.95. This means that in order to meet the requirements of the standard, the measured value must not exceed 1/4 of the width of the allowable range. So, if the allowable voltage deviation range, then it is necessary that it does not exceed 2.5%.

The standard establishes methods and techniques for determining the SCE and auxiliary parameters that implement the provisions of mathematical statistics and probability theory. For the measured discrete values ​​of the SQI, averaging intervals are set, presented in Table 3.4.

Table 3.4. Intervals for averaging the results of measurements of KE indicators

KE indexAveraging interval, sSteady voltage deviation Span of voltage change Flicker dose Coefficient of distortion of the sinusoidality of the voltage curve Coefficient of the n-th harmonic component of the voltage Coefficient of voltage asymmetry in the negative sequence Coefficient of voltage asymmetry in the zero sequence Frequency deviation Duration of the voltage dip Impulse voltage Temporary overvoltage factor60 - - 3 3 3 3 20 - - -

For the averaging intervals of various SQIs, the standard sets the number of observations (N) and, using the methodology set out in the standard, one or another SQI is determined. For example, calculate the value of the average voltage in volts, as a result of averaging N voltage observations over a time interval of 1 min using the formula:

where is the voltage value in the i-th observation, V.

The number of observations for 1 min in accordance with the standard should be at least 18. The value of the steady-state voltage deviation is calculated using the formula,%

The values ​​of the SCE accumulated over the minimum billing period are processed by the methods of mathematical statistics and the probabilities of their compliance with the norms of the standard are determined.

The methods for determining the SQE established by the standard are implemented in the hardware controls for the QE. The form of presentation of measurement processing results must also meet the requirements of the standard.

Table 3.5 summarizes the SCE standards.

Table 3.5. Electricity quality standards

EC indicator, units measurementsNorms KENNormally allowablemaximum allowableSteady voltage deviation, % ± 5± 10Voltage change range, % Curves 1,2 in fig. 3.2Dose of flicker, refers. unit: Short-term

Long -

1.0; 0.74 Voltage curve sinusoidal distortion factor, %According to the table

1According to the table

3.1 Coefficient of the n-th harmonic component of the voltage,% According to the table

2According to the table

3.2 Negative sequence voltage unbalance factor , %24Voltage unbalance factor by zero sequence , %24Frequency deviation , Hz± 0.2± 0.4Voltage dip duration , s-30Impulse voltage , kV--Coefficient of temporary overvoltage , refers. units:--

4. Influence of power quality on the operation of power receivers

Deviations of the PQI from the normalized values ​​worsen the operating conditions of electrical equipment of power supply organizations and consumers of electricity, can lead to significant losses both in industry and in the domestic sector, cause, as already noted, technological and electromagnetic damage.

Typical types of electrical receivers

From the electrical networks of general-purpose power supply systems, EPs for various purposes are fed, consider industrial and household EPs.

The most characteristic types of ED, widely used in enterprises of various industries, are electric motors and electric lighting installations. Electrothermal installations, as well as valve converters, which are used to convert alternating current to direct current, are widely used. Direct current in industrial enterprises is used to power DC motors, for electrolysis, in galvanic processes, for some types of welding, etc.

Electric motors are used in drives of various production mechanisms. In installations that do not require speed control during operation, AC electric drives are used: asynchronous and synchronous electric motors.

The most economical field of application of asynchronous and synchronous electric motors, depending on the voltage, has been established. At voltages up to 1 kV and power up to 100 kW, it is more economical to use asynchronous motors, and over 100 kW - synchronous motors, at voltages up to 6 kV and power up to 300 kW - asynchronous motors, and above 300 kW - synchronous motors, at a voltage of 10 kV and power up to 400 kW - asynchronous motors, above 400 kW - synchronous.

The wide distribution of asynchronous motors is due to their simplicity in design and operation and relatively low cost.

Synchronous motors have a number of advantages over asynchronous motors: they are usually used as reactive power sources, their torque is less dependent on the terminal voltage, and in many cases they are more efficient. At the same time, synchronous motors are more expensive and difficult to manufacture and operate.

Electric lighting installations with incandescent, fluorescent, arc, mercury, sodium, xenon lamps are used in all enterprises for indoor and outdoor lighting, for urban lighting, etc.

AC electric arc and resistance welding installations are a single-phase non-uniform and non-sinusoidal load with a low power factor: 0.3 for arc welding and 0.7 for contact welding. Welding transformers and low power devices are connected to a 380/220 V network, more powerful ones - to a 6 - 10 kV network.

Valve converters, due to the specifics of their regulation, are consumers of reactive power (the power factor of valve converters of rolling mills ranges from 0.3 to 0.8), which causes significant voltage deviations in the supply network; the non-sinusoidality coefficient during the operation of thyristor converters of rolling mills can reach a value of more than 30% on the 10 kV side of their supply voltage; valve converters do not affect the voltage symmetry due to the symmetry of their loads.

Electric welding installations can cause disruption of normal working conditions for other EAs. In particular, welding units, whose power currently reaches 1500 kW per unit, cause much larger voltage fluctuations in electrical networks than, for example, starting asynchronous motors with a squirrel-cage rotor. In addition, these voltage fluctuations occur for a long time and with a wide frequency range, including the most unpleasant range for electric lighting installations (about 10 Hz).

Electrothermal installations, depending on the heating method, are divided into groups: arc furnaces, direct and indirect resistance furnaces, electronic melting furnaces, vacuum, slag remelting, induction furnaces. This group of EP also has an adverse effect on the supply network, for example, arc furnaces, which can have a power of up to 10 MW, are currently being built as single-phase. This leads to a violation of the symmetry of currents and voltages (the latter occurs due to voltage drops on the network resistances from currents of different sequences). In addition, arc furnaces, as well as valve installations, are non-linear EFs with low inertia. Therefore, they lead to non-sinusoidal currents, and, consequently, voltages.

The modern electrical load of an apartment (cottage) is characterized by a wide range of household EPs, which, according to their purpose and influence on the electrical network, can be divided into the following groups: passive consumers of active power (incandescent lamps, heating elements of irons, stoves, heaters); EP with asynchronous motors operating in a three-phase mode (drive of elevators, pumps - in the water supply and heating system, etc.); EP with asynchronous motors operating in a single-phase mode (drive of compressors of refrigerators, washing machines, etc.); EP with collector motors (drive of vacuum cleaners, electric drills, etc.); AC and DC welding units (for repair work in the workshop, etc.); rectifying devices (for charging batteries, etc.); radio-electronic equipment (TVs, computer equipment, etc.); high-frequency installations (microwave ovens, etc.); fluorescent lighting lamps.

The impact of each individual household EP is insignificant, while the totality of EP connected to the 0.4 kV buses of a transformer substation has a significant impact on the supply network.

Influence of voltage deviations

Voltage deviations have a significant impact on the operation of induction motors (IM), which are the most common receivers of electricity in industry.

Rice. 4.1. Mechanical characteristics of the motor at rated (M1) and reduced (M2) voltages

When the voltage changes, the mechanical characteristic of the AM changes - the dependence of its torque M on slip s or rotational speed (Fig. 4.1). With sufficient accuracy, we can assume that the torque of the motor is proportional to the square of the voltage at its terminals. With a decrease in voltage, the torque and speed of the motor rotor decreases, as its slip increases. The decrease in the rotational speed also depends on the law of change in the moment of resistance Mc (Mc is taken constant in Fig. 4.1) and on the engine load. The dependence of the motor rotor speed on voltage can be expressed:

where - synchronous speed;

Engine load factor;

Nominal voltage and slip values, respectively.

From formula (4.1) it can be seen that at low engine loads, the rotor speed will be greater than the rated speed (at rated engine load). In such cases, voltage drops do not lead to a decrease in the productivity of technological equipment, since the reduction in the engine speed below the nominal does not occur.

For motors running at full load, lowering the voltage results in a reduction in speed. If the performance of mechanisms depends on the engine speed, then it is recommended to maintain a voltage at the outputs of such engines that is not lower than the nominal voltage. With a significant decrease in the voltage at the outputs of motors operating at full load, the moment of resistance of the mechanism may exceed the torque, which leads to a “tipping” of the motor, i.e. to stop him. To avoid damage, the motor must be disconnected from the mains.

Reducing the voltage worsens the conditions for starting the engine, as this reduces its starting torque.

Of practical interest is the dependence of the active and reactive power consumed by the motor on the voltage at its terminals.

In the event of a decrease in the voltage at the motor terminals, the magnetizing reactive power decreases (by 2 - 3% with a decrease in voltage by 1%), with the same power consumption, the motor current increases, which causes overheating of the insulation.

If the motor runs for a long time at reduced voltage, then due to the accelerated wear of the insulation, the service life of the motor is reduced. Approximately the service life of insulation T can be determined by the formula:

where is the service life of the motor insulation at rated voltage and rated load; is a coefficient depending on the value and sign of the voltage deviation, as well as on the load factor of the motor and equal to:

at - 0.2< <0; (4.3);

at 0.2 ≥ > 0; (4.4)

Therefore, from the point of view of engine heating, negative voltage deviations within the considered limits are more dangerous.

A decrease in voltage also leads to a noticeable increase in reactive power lost in the leakage reactances of lines, transformers and AM.

An increase in voltage at the motor terminals leads to an increase in the reactive power consumed by them. At the same time, the specific consumption of reactive power increases with a decrease in the engine load factor. On average, for each percentage increase in voltage, the consumed reactive power increases by 3% or more (mainly due to an increase in the no-load current of the motor), which in turn leads to an increase in active power losses in the electrical network elements.

Incandescent lamps are characterized by nominal parameters: power consumption, luminous flux, luminous efficiency (equal to the ratio of the luminous flux emitted by the lamp to its power) and the average nominal service life. These indicators largely depend on the voltage at the terminals of incandescent lamps. With voltage deviations of 10%, these characteristics can be approximately described by the following empirical formulas:

Rice. 4.2. Dependences of the characteristics of incandescent lamps on voltage: 1 - power consumption, 2 - luminous flux, 3 - luminous efficiency, 4 - service life

From the curves in Figs. 4.2. it can be seen that with decreasing voltage, the luminous flux decreases most noticeably. When the voltage rises above the nominal value, the luminous flux F, the lamp power P and the light output h increase, but the service life of the lamps T sharply decreases and as a result they quickly burn out. At the same time, there is an overspending of electricity.

Voltage changes lead to corresponding changes in luminous flux and illumination, which ultimately affects labor productivity and human fatigue.

Fluorescent lamps are less sensitive to voltage fluctuations. With an increase in voltage, the power consumption and luminous flux increase, and with a decrease, they decrease, but not to the same extent as with incandescent lamps. At low voltage, the ignition conditions of fluorescent lamps worsen, so their service life, determined by the spraying of the oxide coating of the electrodes, is reduced both with negative and positive voltage deviations.

With voltage deviations of 10%, the service life of fluorescent lamps is reduced by an average of 20 - 25%. A significant disadvantage of fluorescent lamps is their consumption of reactive power, which increases with increasing voltage supplied to them.

Valve converters usually have an automatic DC control system by phase control. When the voltage in the network increases, the regulation angle automatically increases, and when the voltage decreases, it decreases. A 1% increase in voltage leads to an increase in the reactive power consumption of the converter by about 1-1.4%, which leads to a deterioration in the power factor. At the same time, other characteristics of valve converters improve with increasing voltage, and therefore it is beneficial to increase the voltage at their terminals within acceptable values.

Electric ovens are sensitive to voltage fluctuations. Reducing the voltage of electric arc furnaces, for example, by 7% leads to a lengthening of the steel melting process by 1.5 times. Increasing the voltage above 5% leads to excessive consumption of electricity.

Voltage deviations adversely affect the operation of electric welding machines: for example, for spot welding machines, a 15% change in voltage results in a 100% defective product.

Effect of voltage fluctuations

Lighting devices, especially incandescent lamps and electronic equipment, are among the EAs that are extremely sensitive to voltage fluctuations:

The standard defines the effect of voltage fluctuations on lighting installations that affect human vision. Flashing light sources (flicker effect) causes an unpleasant psychological effect, fatigue of vision and the body as a whole. This leads to a decrease in labor productivity, and in some cases to injuries.

The strongest impact on the human eye is caused by flashes with a frequency of 3-10 Hz, therefore, the allowable voltage fluctuations in this range are minimal - less than 0.5%.

With the same voltage fluctuations, the negative effect of incandescent lamps is manifested to a much greater extent than gas-discharge lamps. Voltage fluctuations greater than 10% can cause the discharge lamps to go out. Depending on the type of lamps, they are ignited after a few seconds and even minutes.

Voltage fluctuations disrupt normal operation and reduce the life of electronic equipment: radios, televisions, telephone and telegraph communications, computer equipment, x-ray installations, radio stations, television stations, etc.

With significant voltage fluctuations (more than 15%), the conditions for the normal operation of electric motors may be violated, the contacts of magnetic starters may fall off with a corresponding shutdown of running motors.

Voltage fluctuations with a range of 10 - 15% can lead to failure of capacitor banks, as well as valve converters.

The influence of voltage fluctuations on individual power receivers has not yet been studied enough. This complicates the technical and economic analysis in the design and operation of power supply systems with sharply variable loads.

Influence of voltage unbalance

Voltage asymmetry, as already noted, is most often caused by the presence of an asymmetric load. Asymmetric load currents flowing through the elements of the power supply system cause asymmetric voltage drops in them. As a result, an asymmetric voltage system appears on the outputs of the EA. Voltage deviations at the EA of the overloaded phase may exceed the normally permissible values, while voltage deviations at the EA of other phases will be within the normalized limits. In addition to the deterioration of the voltage regime of the EP in the asymmetric mode, the operating conditions of both the EP themselves and all network elements deteriorate significantly, the reliability of the operation of electrical equipment and the power supply system as a whole decreases.

The action of the asymmetric mode is qualitatively different compared to the symmetrical one for such common three-phase EDs as asynchronous motors. Of particular importance to them is the negative sequence voltage. The resistance of the negative sequence of electric motors is approximately equal to the resistance of a stalled motor and, therefore, 5 to 8 times less than the positive sequence resistance. Therefore, even a small voltage unbalance causes significant negative sequence currents. Negative sequence currents are superimposed on positive sequence currents and cause additional heating of the stator and rotor (especially the massive parts of the rotor), which leads to accelerated aging of the insulation and a decrease in the available motor power (decrease in motor efficiency). Thus, the service life of a fully loaded asynchronous motor operating at a voltage unbalance of 4% is reduced by 2 times. With a voltage unbalance of 5%, the available motor power is reduced by 5 - 10%.

With asymmetry of mains voltages in synchronous machines, along with the occurrence of additional losses of active power and heating of the stator and rotor, dangerous vibrations can occur as a result of the appearance of sign-alternating torques and tangential forces pulsing at a double mains frequency. With significant asymmetry, vibration can be dangerous, and especially with insufficient strength and the presence of defects in welded joints. With current asymmetry not exceeding 30%, dangerous overvoltages in structural elements, as a rule, do not occur.

The rules for the technical operation of electrical networks and stations in the Russian Federation indicate that “long-term operation of generators and synchronous compensators with unequal phase currents is allowed if the current difference does not exceed 10% of the rated stator current for turbogenerators and 20% for hydrogenerators. In this case, the currents in the phases must not exceed the nominal values. If these conditions are not met, then special measures must be taken to reduce the asymmetry.”

In the case of reverse and zero sequence currents, the total currents in the individual phases of the network elements increase, which leads to an increase in active power losses and may be unacceptable from the point of view of heating. Zero sequence currents flow constantly through the ground electrodes. This additionally dries up and increases the resistance of grounding devices. This may be unacceptable from the point of view of the operation of relay protection, as well as due to the increased impact on low-frequency communication installations and railway blocking devices.

Voltage asymmetry significantly worsens the operating modes of multi-phase valve rectifiers: the ripple of the rectified voltage increases significantly, the operating conditions of the pulse-phase control system of thyristor converters worsen.

Capacitor units with voltage asymmetry are unevenly loaded with reactive power in phases, which makes it impossible to fully use the installed capacitor power. In addition, capacitor units in this case reinforce the already existing asymmetry, since the output of reactive power to the network in the phase with the lowest voltage will be less than in the other phases (proportional to the square of the voltage on the capacitor unit).

Voltage asymmetry also significantly affects single-phase EAs, if the phase voltages are unequal, then, for example, incandescent lamps connected to a phase with a higher voltage have a greater luminous flux, but a significantly shorter service life compared to lamps connected to a phase with a lower voltage . Voltage asymmetry complicates the operation of relay protection, leads to errors in the operation of electricity meters, etc.

Influence of non-sinusoidal voltage

EA with non-linear current-voltage characteristics consume non-sinusoidal currents from the network when a sinusoidal voltage is applied to their terminals. The currents of higher harmonics, passing through the elements of the network, create voltage drops in the resistances of these elements and, superimposed on the main voltage sinusoid, lead to distortions in the shape of the voltage curve in the nodes of the electrical network. In this regard, an EP with a nonlinear current-voltage characteristic is often called a source of higher harmonics.

The most serious CE violations in the electrical network occur during the operation of powerful controlled valve converters. In this case, the order of the higher harmonic components of current and voltage in the network is determined by the formula

where m is the number of rectification phases; is a sequential series of natural numbers (0,1,2…).

Depending on the rectification circuit, valve converters generate the following current harmonics into the network: with a 6-phase circuit - up to the 19th order; with a 12-phase circuit - up to the 25th order inclusive.

The distortion factor of the sinusoidality of the voltage curve in networks with electric arc steel-smelting and ore-thermal furnaces is determined mainly by the 2nd, 3rd, 4th, 5th, 7th harmonics.

The coefficient of distortion of the sinusoidality of the voltage curve of arc and resistance welding installations is mainly determined by the 5th, 7th, 11th, 13th harmonics.

The currents of the 3rd and 5th harmonics of discharge lamps are 10 and 3% of the current of the 1st harmonic. These currents coincide in phase in the corresponding linear wires of the network and, adding up in the neutral wire of the 380/220 V network, determine the current in it, which is almost equal to the current in the phase wire. Other harmonics for gas-discharge lamps can be neglected.

Studies of the magnetizing current curve of transformers connected to a sinusoidal voltage network showed that with a three-rod core and U / U winding connections; and /U; in the electrical network there are all odd harmonics, including harmonics that are multiples of three. Harmonics that are multiples of three are due to the asymmetry of the magnetizing currents in phases:

The effective value of the magnetizing current of the transformer:

Magnetizing currents form systems of positive and negative sequence currents, which are the same in absolute value for harmonics that are multiples of three. For other odd harmonics, negative sequence currents are about 0.25 positive sequence currents.

If a non-sinusoidal voltage is applied to the inputs of transformers, additional components of the higher harmonics of the current appear. GPP transformers give the 5th harmonic of a small magnitude.

In general, non-sinusoidal modes have the same disadvantages as asymmetric ones.

Higher harmonics of current and voltage cause additional losses of active power in all elements of the power supply system: in power lines, transformers, electrical machines, static capacitors, since the resistance of these elements depends on the frequency.

So, for example, the capacitance of capacitors installed in order to compensate for reactive power decreases with increasing frequency of the input voltage. Therefore, if there are higher harmonics in the mains voltage, then the resistance of the capacitors at these harmonics is much lower than at a frequency of 50 Hz. Because of this, in capacitors intended for reactive power compensation, even small higher harmonic voltages can cause significant harmonic currents. In enterprises with a large proportion of non-linear loads, capacitor banks do not work well. They are either switched off by overcurrent protection or fail in a short time due to swelling of the cans (or accelerated aging of the insulation). There are cases when, at enterprises with a developed cable network with a voltage of 6-10 kV, capacitor banks find themselves in the current resonance mode (or close to this mode) at the frequency of any of the harmonics, which leads to a dangerous current overload.

Higher harmonics cause:

· accelerated aging of the insulation of electrical machines, transformers, cables;

· deterioration of the power factor of the ED;

· deterioration or disruption of the operation of automation devices, telemechanics, computer equipment and other devices with electronic elements;

· measurement errors of induction electricity meters, which lead to incomplete accounting of consumed electricity;

· malfunction of the valve converters themselves at a high level of higher harmonic components.

· The presence of higher harmonics adversely affects the operation of not only consumer electrical equipment, but also electronic devices in power systems.

· For some installations (a system of pulse-phase control of valve converters, complete automation devices, etc.), the permissible values ​​of individual current (voltage) harmonics are indicated by the manufacturer in the product passport.

· The voltage curve supplied to the EP should not contain higher harmonics in the steady state of the power grid. It should be emphasized that under the operating conditions of the EA, the voltage non-sinusoidality manifests itself in conjunction with the actions of other influencing factors and therefore it is necessary to consider the entire set of factors together.

Influence of frequency deviation

The stringent requirements of the standard for deviations in the frequency of the supply voltage are due to the significant influence of frequency on the operating modes of electrical equipment, the course of technological processes of production and, as a result, the technical and economic performance of industrial enterprises.

The electromagnetic component of the damage is due to an increase in active power losses in electrical networks and an increase in active and reactive power consumption. It is known that reducing the frequency by 1% increases losses in electrical networks by 2%.

The technological component of the damage is caused mainly by the underproduction by industrial enterprises of their products and the cost of additional time for the enterprise to complete the task. According to expert estimates, the value of technological damage is an order of magnitude higher than electromagnetic damage.

An analysis of the work of enterprises with a continuous production cycle showed that most of the main technological lines are equipped with mechanisms with constant and fan resistance torques, and asynchronous motors serve as their drives. The frequency of rotation of the motor rotors is proportional to the change in the frequency of the network, and the performance of production lines depends on the engine speed.

The degree of influence of frequency on the performance of a number of mechanisms can be expressed in terms of the active power they consume:

where a - coefficient of proportionality, depending on the type of mechanism; - network frequency; - exponent.

Depending on the values ​​of the exponent n, EP can be divided into the following groups:

1.mechanisms with a constant moment of resistance - piston pumps, compressors, machine tools, etc.; for them n=1;

2.mechanisms with a fan moment of resistance - centrifugal pumps, fans, smoke exhausters, etc.; for them n=3; at TPPs, KPPs, NPPs, these are usually the motors of feed water pumps, circulation pumps, smoke fans, oil pumps, etc.

.mechanisms for which n = 3.5-4 are centrifugal pumps operating with a large static head (back pressure), for example, boiler feed pumps.

EPs of the 2nd and 3rd groups, which are most affected by frequency, have adjustment capabilities, due to which the power they consume from the network remains practically unchanged.

The motors of auxiliary needs of power plants are most sensitive to frequency reduction. The decrease in frequency leads to a decrease in their productivity, which is accompanied by a decrease in the available power of generators and a further shortage of active power and a decrease in frequency (there is a frequency avalanche).

Such electronic devices as incandescent lamps, resistance furnaces, electric arc furnaces practically do not react to frequency changes.

Frequency deviations adversely affect the operation of electronic equipment: a frequency deviation of more than +0.1 Hz leads to brightness and geometric background distortions of the television image, frequency changes from 49.9 to 49.5 Hz entail an almost fourfold increase in the allowable range of the television signal to the background interference. Changing the frequency to 49.5 Hz requires a significant tightening of the requirements for the signal / background noise ratio in all parts of the television path - from the equipment of the hardware-studio complex to the television receiver, the implementation of which is associated with significant material costs.

In addition, the reduced frequency in the electrical network also affects the service life of equipment containing elements with steel (electric motors, transformers, reactors with a steel magnetic circuit), due to an increase in the magnetization current in such devices and additional heating of steel cores.

To prevent system-wide accidents caused by frequency reduction, special devices for automatic frequency unloading (AFD) are provided, which cut off some of the less responsible consumers. After the power shortage has been eliminated, for example, after switching on the backup sources, special frequency automatic reclosing devices (CHAR) turn on the disconnected consumers and the normal operation of the system is restored.

Maintaining a normal frequency that meets the requirements of the standard is a technical, not a scientific problem, the main way to solve it is to introduce generating capacities in order to create power reserves in the networks of power supply organizations.

Influence of electromagnetic interference

In general-purpose power supply systems, electronic and microelectronic control systems, microprocessors and computers are widely used, which led to a decrease in the level of noise immunity of EP control systems and a sharp increase in the number of their failures. The main cause of failures is the impact of electromagnetic transient interference that occurs during electromagnetic transients both in power grids, and in urban and industrial electrical networks. The duration of transient processes ranges from several periods of industrial frequency current to several seconds, and the effective interference frequency band can reach tens of megahertz.

Electromagnetic crosstalk, accompanied by voltage dips, occurs mainly during single-phase short circuits of overhead lines due to insulation flashing. These damages either self-destruct or are eliminated during a short-term disconnection followed by automatic reclosing (AR). In addition, the cause of voltage dips are phase-to-phase short circuits resulting from atmospheric phenomena, as well as disconnection of supply lines and capacitors. The number of voltage dips with a depth of up to 20% reaches 55 - 60% in distribution networks. Over 60% of machine shutdowns are due to voltage dips with a depth of more than 20%.

The cause of electromagnetic transient interference in general-purpose power supply systems can be overvoltages that occur during single-phase earth faults, when switching banks of capacitors and resonant filters, when disconnecting unloaded cable lines and transformers, while switching contacts of switches and other switching equipment, in open-phase modes operation of the electrical network due to various reasons leading to ferroresonant phenomena. The susceptibility of electronic equipment and computers to surges depends both on the frequency response of the ED and on the frequency response of electromagnetic interference.

An increase in the power of power systems and the number of overhead lines used to improve the reliability of power supply to industrial enterprises leads to a decrease in the reliability of the functioning of complex electronic control systems and an increase in the number of failures of noise-sensitive ED.

As already noted, with the values ​​of all SCEs for voltage different from the normalized ones, accelerated aging of the insulation of electrical equipment occurs, as a result, the intensity of failure flows increases over time. So, if the mains voltage curve is not sinusoidal, even with the resonant tuning of the arc extinguishers, the current of higher harmonics passes through the place of the earth fault, and the cable may burn at the place of the first damage. In this case, as experience shows, two or more accidents due to overvoltages can occur simultaneously.

At low PQ, there is an interdependence of element failures, for example, when the negative effect of non-linear, asymmetric and shock loads is compensated with the help of appropriate corrective devices when one or another device is turned off. Thus, the failure of a high-speed static compensator causes asymmetry, voltage fluctuations and harmonics, which were previously compensated, which, in turn, is fraught with the occurrence of false alarms of relay protection, emergency failure of some types of electrical equipment and other similar negative consequences. Failures in the channels of information transmission through power circuits in the presence of harmonics lead to the submission of incorrect commands to control the switching equipment. Thus, PQ significantly affects the reliability of power supply, since the accident rate in networks with low PQ is higher than in the case when PQ is within acceptable limits.

5. Quality control of electrical energy

.1 Main tasks and types of power quality control

The main tasks of PQ control are:

Verification of compliance with the requirements of the standard in terms of operational control of the PQ in general-purpose electrical networks;

Checking the compliance of the actual values ​​​​of the SQI at the network interface in terms of balance sheet belonging to the values ​​\u200b\u200bfixed in the power supply agreement

Development of technical conditions for the connection of the consumer in terms of CE;

Verification of the fulfillment of contractual conditions in terms of PQ with the determination of the allowable calculated and actual contributions of the consumer to the deterioration of PQ;

Development of technical and organizational measures to ensure CE;

Determination of discounts (surcharges) to EE tariffs for its quality;

Electrical energy certification;

Search for the "culprit" of the distortions of the SCE.

Depending on the goals to be solved during the control and analysis of CE, measurements of the CE can take four forms:

· diagnostic control;

· inspection control;

· operational control;

· commercial accounting.

PQ diagnostic control - the main purpose of diagnostic control at the interface between the electrical networks of the consumer and the power supply organization is to detect the "culprit" of the deterioration of the PQ, determine the permissible contribution to the violation of the requirements of the standard for each SCE, include them in the power supply contract, normalize the PQ.

Diagnostic control should be carried out when issuing and verifying the fulfillment of technical conditions for connecting a consumer to an electrical network, when monitoring contractual conditions for electricity supply, and also in cases where it is necessary to determine the share contribution to the deterioration of the PQ of a group of consumers connected to a common power center. Diagnostic control should be periodic and include short-term (no more than one week) measurements of PQE. During diagnostic control, both normalized and non-normalized SCEs are measured, as well as currents and their harmonic and symmetrical components and their corresponding power flows.

If the results of the PQ diagnostic control confirm the “guilty” of the consumer in violating the PQ standards, then the main task of the power supply organization together with the consumer is to develop and evaluate the possibilities and timing of measures to normalize the PQ. For the period prior to the implementation of these measures, on-line control and commercial accounting of PQ should be applied at the interface between the electrical networks of the consumer and the power supply organization.

At the next stages of PQ diagnostic measurements, the control points should be the buses of regional substations, to which the cable lines of consumers are connected. These points are also of interest for monitoring the correct operation of transformer on-load tap-changers, for collecting statistics and fixing voltage dips and temporary overvoltages in the electrical network. Thus, the operation of already existing means of ensuring CE is controlled: synchronous compensators, banks of static capacitors and transformers with on-load tap-changers that provide specified ranges of voltage deviations, as well as the operation of protection and automation equipment in the electrical network.

PQ inspection control is carried out by certification bodies to obtain information on the state of certified electricity in the electric networks of the power supply organization, on compliance with the conditions and rules for applying the certificate, in order to confirm that the PQ continues to comply with the established requirements during the validity period of the certificate.

Operational control of the PQ is necessary under operating conditions at the points of the electrical network where voltage distortions exist and cannot be eliminated in the short term. Operational control is necessary at the points of connection of traction substations of railway and urban electrified transport, substations of enterprises with EP with non-linear characteristics. The results of operational control should be sent via communication channels to the control points of the electrical network of the power supply organization and the power supply system of the industrial enterprise.

Commercial metering of the PCE - should be carried out at the interface between the electrical networks of the consumer and the energy supply organization, and based on its results, discounts (surcharges) on electricity tariffs for its quality are determined.

The legal and methodological basis for ensuring the commercial accounting of CE in electrical networks is the Civil Code of the Russian Federation (CC RF), part 2, GOST 13109 - 97, Instructions on the procedure for payments for electrical and thermal energy (No. 449 of December 28, 1993 of the Ministry of Justice of the Russian Federation ).

Commercial metering of the PV should be continuously carried out at the metering points of the consumed electricity as a means of economic impact on the culprit of the deterioration of the PV. For these purposes, devices should be used that combine the functions of electricity metering and measuring its quality. The presence in one device of the functions of electricity metering and PQE control will allow to combine operational control and commercial metering of PQ, while common communication channels and means of processing, displaying and documenting ASKUE information can be used.

PQ commercial metering devices should register the relative time of exceeding the normal and maximum allowable SQI values ​​at the electricity control point for the billing period, which determine the premiums to tariffs for those responsible for the deterioration of PQ.

.2 Standard requirements for power quality control

Control over compliance with the requirements of the standard by power supply organizations and consumers of electrical energy should be carried out by supervisory authorities and accredited testing laboratories for PQ.

PQ control at the points of general connection of consumers of electrical energy to general-purpose systems is carried out by energy supply organizations (control points are selected in accordance with regulatory documents). Frequency of measurements of SQE:

for a steady voltage deviation - at least twice a year, depending on seasonal changes in loads in the distribution network of the power center, and if there is automatic counter voltage regulation in the power center, at least once a year;

for the rest of the PCE - at least once every two years, with the network scheme and its elements unchanged and a slight change in the nature of the consumer's electrical loads that worsen the PQ.

Consumers of electricity that worsen the PQ should carry out control at the points of their own networks, closest to the points of common connection of these networks to the general-purpose electrical network, as well as at the terminals of electrical energy receivers that distort the PQ.

The frequency of PQ control is established by the consumer of electric energy in agreement with the energy supply organization.

The control of the CE supplied by traction substations of alternating current to electric networks with a voltage of 6 - 35 kV should be carried out:

· for electrical networks 6 - 35 kV, administered by power systems, at the points of connection of these networks to traction substations;

· for electrical networks 6 - 35 kV, not under the authority of power systems, at points selected by agreement between traction substations and consumers of electricity, and for newly built and reconstructed (with the replacement of transformers) traction substations - at the points of connection of consumers of electrical energy to these networks.

5.3 Discounts and surcharges on the electricity quality tariff

In paragraph 1 of Art. 542 part 2 of the Civil Code of the Russian Federation establishes: "the quality of the energy supplied by the energy supply organization must comply with the requirements established by state standards and other mandatory rules, or provided for by the energy supply agreement."

In order to ensure the norms of the standard at the points of common connection, it is allowed to establish in power supply contracts with consumers - the "culprits" of the deterioration of the PQ, more stringent standards (with smaller ranges of changes in the corresponding PQ indicators) than those established in the standard, which consumers are required to maintain at the interface of the balance belonging of electrical networks.

In case of violation by the energy supply organization of the requirements for CE, the subscriber has the right to prove the amount of damage and recover it from the energy supply organization in accordance with the rules of Art. 547 of the Civil Code of the Russian Federation. At the same time, given that the subscriber still used energy of inadequate quality, he must pay for it, but at a proportionately reduced price (clause 2, article 542 of the Civil Code of the Russian Federation).

Obviously, violations can be mutual and for different SCEs. The party responsible for the reduction of the PQ is determined in accordance with the Rules for the Application of Discounts and Surcharges to Electricity Quality Tariffs.

The instruction on the procedure for payments for electricity and heat in section 4 "Discounts (surcharges) to the tariff for electricity quality" establishes penalties for the culprit of the deterioration of the PQ.

The mechanism of penalties established by the Instruction does not apply to all SCEs, but to those numerical values, the norms of which are in the standard:

steady voltage deviation;

the distortion factor of the sinusoidality of the voltage curve;

voltage unbalance factor in reverse sequence;

coefficient of voltage asymmetry in the zero sequence;

frequency deviation;

range of voltage change.

Of the listed SCEs, the distortion coefficient of the sinusoidality of the voltage curve and the coefficients of the harmonic components of the voltage reflect the same phenomenon - non-sinusoidality. Moreover, it reflects all the harmonics in total, and - each of the 40 harmonics separately. Therefore, the Instructions apply discounts (surcharges) for the total impact (coefficient), in addition, it must be taken into account that discounts (surcharges) for individual SCEs are added up. Therefore, the indicator is not included in the Instruction. The duration of the voltage dip is not included in the discounts (surcharges), since the amount of sanctions for the listed SCEs depends on the total duration of the supply of low-quality electrical energy per month, and in terms of voltage dips, the duration of one dip is normalized without rationing their number.

Discounts (surcharges) for the quality of electrical energy are applied in settlements with all consumers.

The value of the discount (surcharge) depends on:

from the number of SCEs for which there is a violation of the norms of the standard at the point of electricity metering during the billing period;

from the relative time of exceeding the normal and maximum allowable values ​​of the SQI at the point of electricity control during the billing period.

The specific value of the discount (surcharge), depending on the degree of violation of these factors, can be from 0.2 to 10% of the electricity tariff.

Payment under the tariff with a discount (surcharge) for CE is made for the entire volume of electric energy supplied (consumed) during the billing period. If the energy supply organization is guilty of the violation, the penalty is implemented in the form of a discount from the tariff, if the consumer is guilty, in the form of a surcharge.

For unacceptable voltage and frequency deviations, the unilateral responsibility of the power supply organization is provided. For voltage deviation, the power supply organization is responsible to the consumer if the subscriber does not exceed the technical limits of consumption and generation of reactive power.

Responsibility for violation of the norms for the remaining four SCEs rests with the culprit of the deterioration of the CE. The culprit is determined on the basis of a comparison of the allowable contribution included in the contract to the value of the considered SCE at the control point with the actual contribution determined by measurements.

Literature

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