Requirements for power supply reliability and power quality. Cottage power supply project Explanatory note (continued) Section quality of electrical energy
Power quality
This section of the project was developed on the basis of the “Information Letter - Instruction IP-22/99” and in accordance with the Russian Law “On the Protection of Consumer Rights” (Article 7) and the Russian Government Decree of August 13, 1997 No. 1013 Electric energy is a commodity and is subject to mandatory certification according to quality indicators established by GOST 131-9-97 “Quality standards for electrical energy in general-purpose power supply systems.”The quality of electricity in accordance with the “Rules for Certification of Electrical Energy” must meet 6 main points:
1 - steady voltage deviation;
2- frequency deviation;
3 - distortion factor of the sinusoidal voltage waveform;
4 - coefficient of the nth harmonic component of voltage;
5 - negative sequence voltage asymmetry coefficient;
6 - zero sequence voltage asymmetry coefficient.
Voltage deviation characterized by an indicator of steady-state voltage deviation, for which the following standards are established:
The normally permissible and maximum permissible values of the steady-state voltage deviation at the terminals of electrical energy receivers are equal to 5% and 10%, respectively, of the rated voltage of the electrical network.
Normally permissible and maximum permissible values of steady-state voltage deviation at points of common connection of electrical energy consumers to electrical networks with a voltage of 0.38 kV or more must be established in contracts for the use of electrical energy between the energy supplying organization and the consumer.
Voltage frequency deviation characterized by a deviation indicator for which the following standards are established:
Normally permissible and maximum permissible values of frequency deviation are 0.2 and 0.4 Hz, respectively.
Sine distortion factor The voltage curve for normal mode is -8% for 0.38 kV, -5% for 6-10 kV, the maximum permissible values are 12% and 8%, respectively.
Coefficient of nth harmonic component voltages at points of common connection to electrical networks with different rated voltages are given in table 2 GOST 13109-97.
Voltage asymmetry characterized by the following indicators:
Negative sequence voltage asymmetry factor;
zero-sequence voltage asymmetry coefficient.
The normally permissible and maximum permissible values of the negative sequence voltage asymmetry coefficient at points of common connection to electrical networks are 2.0 and 4.0%, respectively.
2.1. Electricity quality indicators and their standardization
For a long time, the development of the energy sector in our country was accompanied by underestimation and often ignorance of the problems of the quality of electrical energy, which led to massive agitation of electromagnetic compatibility of electrical networks, consumers and power systems. Electromagnetic compatibility is defined as the ability of an electrical device to function satisfactorily in the electromagnetic environment to which other devices also belong. The quality of electrical energy is deteriorating from year to year, while demands for its improvement are increasing. Now there is a difficult situation when there are many technological processes, for example, biotechnology, automatic lines, computing, vacuum, microprocessor technology, telemechanics, electrical measuring systems, etc. Given the current quality of electrical energy, they cannot operate reliably (without disruptions).
After all, the time has come when electrical energy (EE) must be considered as a commodity, which, under any management system, is characterized by certain (specific) indicators, the list and values of which determine its consumer quality.
Power quality (QE) there is a corresponding set of its parameters that describe the features of the EE transmission process for its use under normal operating conditions, determine the continuity of power supply (the absence of long or short-term interruptions in power supply) and characterize the supply voltage (magnitude, asymmetry, frequency, waveform). Before this definition, two more remarks need to be added.
Firstly: KE is generally expressed by the degree of consumer satisfaction with power supply conditions, which is important from a practical point of view.
Secondly: KE depends not only on the power supply conditions, but also on the characteristics of the electrical equipment that is used (its criticality to electromagnetic obstacles (EMI), as well as the ability to generate them) and operating practices. The last remark determines the fact that responsibility for KE should be borne not only by supplying organizations, but also by consumers of electricity and manufacturers of electrical equipment.
The International Electrotechnical Commission (IEC) develops and approves KE standards of three types: defining ones, which contain a description of the electromagnetic environment, terminology, instructions for limiting the equal generation of EMF and for measuring and testing means for determining power quality indicators (PQE), recommendations for the manufacture of electrical equipment; general standards that provide permissible levels of EMF that are generated or their permissible levels in electrical networks for domestic or industrial purposes; detailed (subject) standards, which contain requirements for individual products and are attached from the point of view of KE.
The main organization in Europe that coordinates work regarding standardization in electrical engineering, electronics and related fields of knowledge is the MEK. It is also necessary to name such international organizations as the Committee on Large Electrical Systems and the Union of EE Manufacturers and Distributors. An influential regional organization that deals with normalization in the field of CE for the countries of the European Union (EU) is CENELEC. There are also a number of international professional organizations and national committees that develop national standards for EC, usually based on IEC standards. The adoption of norms occurs mainly by the method of expert assessments, by voting.
Normalization of PKE values is one of the main issues of the KE problem. The PKE system is formed by quantitative characteristics of slow (deviation) and fast (oscillation) changes in the effective voltage value, its shape and symmetry in a three-phase system, as well as changes in frequency. Enterprise energy services personnel cannot influence the frequency level in the network. The exception is cases of power supply from autonomous sources, which are relatively rare in practice. Therefore, in the following, only issues that relate to voltage control units are considered.
The principles of voltage standardization of PKE are based on technical and economic prerequisites and are as follows:
Voltage PKEs have an energy value, that is, they characterize the power (energy) distortion of the voltage curve, the degree of negative effect of this energy on electrical equipment, and the efficiency of technological processes is compared with the values of the specified PKE distortions;
The maximum permissible PKE values are selected based on technical and economic considerations;
PKE are normalized with a given reliability over a certain time interval to obtain specific values that allow comparison.
The PKE system, which is based on these premises, can be used starting from design work. It makes it possible to implement mass metrological support for KE monitoring using relatively simple and inexpensive instruments, as well as to implement measures and technical means for KE normalization.
In Ukraine, on January 1, 2000, the interstate standard GOST 13109-97 “Electric energy quality standards in general-purpose power supply systems” came into force. The standard establishes indicators and standards of KE in electrical networks of general purpose power supply systems of replaceable three-phase and single-phase current with a frequency of 50 Hz in nodes to which electrical networks are connected, which are owned by different EE consumers, or EE receivers (at common connection nodes). Subject to compliance with these standards, electromagnetic compatibility of electrical networks of general purpose power supply systems and electrical networks of EE consumers (EE receivers) is ensured.
The standards established by this standard are mandatory in all operating modes of general purpose power supply systems, except for modes that are determined by the following:
Exceptional weather conditions and natural disasters (hurricane, flood, earthquake, etc.);
Unforeseen situations that are caused by the actions of a party that is not the energy supply organization and consumer (fire, explosion, military action, etc.);
Conditions that are regulated by government authorities, as well as those related to the elimination of consequences caused by exceptional weather conditions and unforeseen circumstances.
The norms established by this standard are subject to inclusion in the technical specifications for the connection of EE consumers and in contracts for the use of EE between electricity suppliers and consumers. According to GOST 13109-97, the KE indicators are:
Stable voltage deviation dU y;
Voltage swing dUt;
Pt flicker dose;
Voltage curve sinusoidal distortion factor KU;
Coefficient of the nth harmonic component of voltage KU (n);
Negative sequence voltage asymmetry coefficient K 2U ;
Zero sequence voltage asymmetry coefficient K 0U ;
Frequency deviation (f;
Voltage dip duration Dtn;
Pulse voltage U imp;
Temporary overvoltage factor K perU.
It should be noted that two types of norms on KE are considered - normally permissible and maximum permissible. The assessment of compliance of the PKE with the specified standards is carried out during the billing period, which is equal to 24 hours.
Most of the phenomena that are observed in electrical networks and deteriorate the quality of electrical energy occur due to the peculiarities of the general operation of electrical receivers and the electrical network, and their electromagnetic compatibility. Seven PKEs are mainly caused by voltage losses (drops) in the section of the electrical network from which consumers are powered.
Voltage loss in a section of the electrical network is determined by the expression:
The active (R) and reactive (X) resistance of the network sections indicated here are assumed to be constant, and the active (P) and reactive (Q) powers that are transmitted through the network section are replaceable. The nature of these changes, moreover, can be different, which prompts different definitions of voltage loss:
When the load changes slowly according to its schedule - voltage deviation;
With a sharply changing nature of the load - voltage fluctuation;
When the load is distributed asymmetrically across the phases of the electrical network - voltage unbalance in a three-phase system;
For nonlinear load – non-sinusoidal load curve shape.
From those phenomena that the consumer of electrical energy cannot influence, he can only protect his equipment with special means, for example, high-speed protection devices or guaranteed power devices.
Responsibility for maintaining voltage within the limits established by GOST 13109-97 rests with the energy supply organization.
Voltage deviation (VV) – discrepancy between the actual voltage in a stable operating mode of the power supply system and its nominal value. The specified deviation is characterized by the indicator of stable VN dU y.
Voltage deviation at one or another point in the network occurs, as already noted, under the influence of a slow load change according to its schedule.
GOST 13109 – 97 sets permissible values of constant voltage deviation on the terminals of the electrical receiver. And the limits of voltage change at the point of consumer connection must be determined taking into account the voltage drop from the specified point to the power receiver and specified in the energy supply contract.
Voltage fluctuations (VF) are voltage deviations that occur in the interval from half a cycle to several seconds.
The sources of voltage fluctuations are powerful electrical receivers with a pulsed, sharply changing nature of active and reactive energy consumption: arc and induction furnaces; electric welding devices; electric motors in starting modes, etc. CN is characterized by the following indicators:
Range of voltage changes dUt;
Flicker dose Pt.
Flicker – This is a person’s subjective perception of fluctuations in the luminous flux of artificial lighting sources, which are caused by voltage fluctuations in the electrical network that powers these sources.
Flicker dose – a measure of a person’s susceptibility to the effects of flicker over a specified period of time. Flicker perception time - the minimum period of time for a person’s subjective perception of flicker caused by voltage fluctuations of a certain shape.
The short-term flicker dose is determined over an observation time interval that does not exceed 10 minutes. The long-term dose of flicker is determined over an observation time interval of 2 hours.
Voltage non-sinusoidality is a distortion of the sinusoidal shape of the voltage curve.
Electrical receivers with a nonlinear current-voltage characteristic consume current whose curve shape differs from sinusoidal. And the flow of such current through the elements of the electrical network creates a voltage drop across them that is different from a sinusoidal one. This is the reason for the bending of the sinusoidal shape of the voltage curve.
Figure 2.1. Non-sinusoidal voltage
The sinusoidal voltage is characterized by the following indicators:
The coefficient of curvature of the sinusoidal voltage curve K U;
Coefficient of the nth harmonic component of voltage K U (n).
Voltage asymmetry - asymmetry of a three-phase voltage system.
Voltage asymmetry occurs only in a three-phase network under the influence of uneven distribution of loads across its phases. GOST 13109-97 indicates a consumer with an asymmetric load as a reliable source of the culprit for voltage asymmetry.
The sources of voltage asymmetry are: arc steel-smelting furnaces, alternating current traction substations, electrical power supply machines, single-phase electrothermal installations and other single-phase, two-phase and asymmetrical three-phase consumers of electricity, in particular for domestic purposes.
So the total load of individual enterprises contains 85...90% of the asymmetric load. And the zero-sequence voltage asymmetry coefficient (K 0U) of one 9th surface house can be 20%, which on the busbars of a transformer substation (point of common connection) can exceed the permissible 2%.
Figure 2.2. Voltage asymmetry
Voltage asymmetry is characterized by the following indicators:
Negative sequence voltage asymmetry coefficient K 2U;
Zero sequence voltage asymmetry coefficient K 0U.
Frequency deviation is the deviation of the actual frequency of the replacement voltage (f fact) from the nominal value (f nom) in the constant operating mode of the power supply system.
The frequency deviation of the alternating current voltage in electrical networks is characterized by the frequency deviation indicator (f.
Voltage dip is a sudden and significant decrease in voltage (less than 90% U nom) lasting from several periods to several tens of seconds with further voltage recovery.
The causes of voltage dips are the activation of automatic protection means during the disconnection of lightning overvoltages, short circuit currents (short circuit), as well as during erroneous activation of the protection or as a result of erroneous actions of operating personnel.
GOST 13109-97 does not standardize voltage dips; it limits its duration to 30 seconds. True, voltage dips lasting 30 seconds practically never happen - the voltage is not restored.
The voltage dip is characterized by the duration of the voltage dip Dtn. .
Voltage pulse - a sharp increase in voltage lasting less than 10 milliseconds.
Pulse overvoltages occur during thunderstorms and when switching equipment (transformers, motors, capacitors, cables), in particular when switching off short-circuit currents. The magnitude of the overvoltage pulse depends on many conditions, but is always significant and can reach many hundreds of thousands of volts.
GOST 13109-97 provides reference values of surge overvoltage during switching for different types of networks.
Fig.2.3. Voltage pulse
The voltage pulse is characterized by the pulse voltage indicator U imp.
Temporary overvoltage is a sudden and significant increase in voltage (more than 110% U nom) lasting more than 10 milliseconds.
Temporary overvoltages occur during equipment switching (switching, short-term) and during short circuits to ground (long-term).
Switching overvoltages occur when long high-voltage power lines are unloaded. Long-term overvoltages occur in networks with a compensated neutral, four-wire networks when the neutral wire is broken, and in networks with an isolated neutral during a single-phase short circuit to ground (in 6-10-35 kV networks, continuous operation is allowed in this mode). In these cases, the voltage of the undamaged phases relative to earth (phase voltage) can increase to the value of the interphase (line) voltage.
Temporary overvoltage is characterized by the temporary overvoltage coefficient K per.U.
The standards for the given PKE are presented in Table 2.1. If the change in VN and frequency deviation is random, then the requirements of GOST 13109-97 apply to those of them that during the calculation period have an integral reliability of at least 95%.
Table 2.1. – Norms of KE indicators and possible reasons for their decrease
| Symbol |
KE indicator, unit of measurement |
KE standards
GOST 13109-97 |
More probable reason | |
|
normally acceptable |
maximum permissible |
|||
| Voltage deviation | ||||
| δuy | Sustainable VN, % | ±5 | ±10 | |
| Voltage fluctuation | ||||
| δut | Voltage change range, % | - | curves 1.2 in Fig. 2.1 | |
| Flicker dose, visible. od.: short-term long lasting |
||||
| Voltage sinusoidality | ||||
| Ku | Voltage sinusoidal curvature coefficient, % | according to table 2.1.2 | according to table 2.1.2 | |
| Ku(n) | Coefficient of the nth harmonic component of voltage, % | according to table 2.1.3 | according to table 2.1.3 | |
| Voltage asymmetry in a three-phase system | ||||
| K 2 u | Negative sequence voltage asymmetry coefficient, % | 2 | 4 | |
| K 0 u | Zero sequence voltage asymmetry factor, % | 2 | 4 | |
| Other | ||||
| Df | Frequency deviation, Hz | ±0.2 | ||
MINISTRY OF SCIENCE AND EDUCATION OF UKRAINE
STATE HIGHER EDUCATIONAL INSTITUTION
DONETSK NATIONAL TECHNICAL UNIVERSITY
Research work
on the topic: “Power quality”
Completed st.gr. _________________________________ date signature Checked ________________________ 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 power supply systems of Ukraine. Purpose of the work: to become familiar with the factors affecting the quality of electricity and methods of regulating it; find out how automatic regulation of power quality is carried out; determine how the quality of electricity will affect its cost. The work examined power supply and power consumption systems of various designs and identified the main problems of these systems, which can lead to a decrease in the quality of power. ELECTRICITY, ELECTRIC POWER QUALITY, VOLTAGE UNSYMMETRY, OVERVOLTAGE, AUTOMATED CONTROL, ELECTRICAL SYSTEM.
1. Power quality indicators…………………………………………4 1.1 Voltage deviation……………………………………………………………6 1.2 Voltage fluctuations…… …………………………………………….8 1.2.1 The influence of voltage fluctuations on the operation of electrical equipment………………………………………………………. ..8 1.2.2 Measures to reduce voltage fluctuations…………….9 1.3 Voltage asymmetry……………………………………………10 1.3.1 The influence of voltage asymmetry on the operation of electrical equipment… ……………………………………………………11 1.3.2 Measures to reduce voltage asymmetry…………12 1.4 Voltage non-sinusoidality………………………………… …..12 1.4.1 The influence of non-sinusoidal voltage on the operation of electrical equipment…………………………………………………….13 1.4.2 Measures to reduce non-sinusoidal voltage..14 1.5 Frequency deviation …………………………………………………….15 1.6 Temporary overvoltage………………………………………………………15 1.7 Pulse overvoltage………… ………………………….....16 2. Automated control of power quality…………..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 energy efficiency...19 3. Payments for electricity depending on its quality……………….22 Literature…………………………………………………………… …………………………...26
1 ELECTRIC 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 devices and equipment connected to it, connected inductively and creating interference to one degree or another, negatively affecting each other’s operation. If it is possible for equipment to operate normally in the existing electromagnetic environment, they speak of electromagnetic compatibility of technical equipment. Unified requirements for the electromagnetic environment are established by standards, which makes it possible to create equipment and guarantee its performance in conditions that meet these requirements. The standards establish acceptable levels of interference in the electrical network, which characterize the quality of electricity and are called power quality indicators (PQI). With the evolutionary change in technology, the requirements for the electromagnetic environment also change, naturally in the direction of tightening. So our standard for power quality, GOST 13109 from 1967, was revised in 1987 with the development of semiconductor technology, and revised in 1997 with the development of microprocessor technology. The quality indicators of electrical energy, methods of their assessment and standards are determined by the Interstate Standard: “Electric Energy. Electromagnetic compatibility of technical equipment. Standards for the quality of electrical energy in general-purpose power supply systems" GOST 13109-97. Table 1.1 – Standardization of power quality indicators
Name of PKE | Most likely cause |
|
Voltage deviation |
||
steady voltage deviation | consumer load schedule |
|
Voltage fluctuations |
||
voltage range | consumer with rapidly changing load |
|
flicker dose |
||
Voltage asymmetry in a three-phase system |
||
negative sequence voltage asymmetry factor | consumer with asymmetric load |
|
zero sequence voltage asymmetry coefficient |
||
Non-sinusoidal voltage waveform |
||
voltage waveform distortion factor | consumer with nonlinear load |
|
coefficient of the nth harmonic component of voltage |
||
frequency deviation | characteristics of the network, climatic conditions or natural phenomena |
|
voltage dip duration |
||
impulse voltage |
||
temporary overvoltage factor |
||
- With a slow change in the load in accordance with its schedule, there is a voltage deviation; With a sharply changing nature of the load, there are voltage fluctuations; With an asymmetrical distribution of the load across the phases of the electrical network, there is voltage asymmetry in a three-phase system; With a nonlinear load, there is a non-sinusoidal shape of the voltage curve.
The influence of voltage deviation on the operation of electrical equipment:
- Technological installations:
- When the voltage decreases, the technological process deteriorates significantly and its duration increases. Consequently, the cost of production increases. When the voltage increases, the service life of the equipment decreases and the likelihood of accidents increases. When significant voltage deviations occur, the technological process fails.
- Lighting:
- The service life of lighting lamps is reduced, so at a voltage value of 1.1 U nom, the service life of incandescent lamps is reduced by 4 times. At a voltage value of 0.9 U nom, the luminous flux of incandescent lamps is reduced by 40% and fluorescent lamps by 15%. When the voltage is 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 an asynchronous electric motor decreases by 15%, the torque decreases by 25%. The engine may not start or may stall.
- When the voltage decreases, the current consumed from the network increases, which leads to heating of the windings and a decrease in the service life of the motor. During long-term operation at a voltage of 0.9 U, the nominal service life of the motor is reduced by half. 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.
- 10% of specific load (for example, in Moscow this is the metro - ~ 11%);
-30% lighting, etc.;
- 60% asynchronous electric motors. Therefore, GOST 13109-97 establishes normal and maximum permissible values of steady-state voltage deviation at the terminals of electrical receivers within the limits, respectively, δUy nor = ± 5% and δUy pre = ± 10% of the rated network voltage. These requirements can be met in two ways: reducing voltage losses and regulating voltage. ΔU = (P R + Q X) / U CPU (TP) Reducing voltage losses (ΔU) is achieved:
- Selecting the cross-section of power line conductors (≡ R) according to the conditions of voltage loss. Using longitudinal capacitive compensation of line reactance (X). However, this is dangerous due to an increase in short-circuit currents at X→0. Compensation of reactive power (Q) to reduce its transmission through electrical networks, using capacitor units and synchronous electric motors operating in overexcitation mode.
Voltage regulation:
- 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 load size - on-load regulation (OLTC). ~10% of transformers are equipped with such devices. The regulation range is ± 16% with a discreteness of 1.78%. The voltage can be regulated at intermediate transformer substations (U TS) using transformers equipped with a device for switching taps on windings with different transformation ratios - switching without excitation (PBV), i.e. with disconnection from the network. Control range ± 5% with 2.5% resolution.
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 selected when designing the network and cannot be changed later. The third (Q) and fifth (U TP) methods are good for regulating seasonal changes in the network load, but it is necessary to control the operating modes of the compensating equipment of consumers centrally, depending on the operating mode of the entire network, that is, the energy supply organization. The fourth method - voltage regulation in the power center (U CPU), allows the energy supply organization to quickly regulate the voltage in accordance with the network load schedule. GOST 13109-97 establishes the permissible values of steady-state voltage deviation at the terminals of the electrical receiver. And the limits of voltage change at the point of consumer connection must be calculated taking into account the voltage drop from this point to the power receiver and specified in the energy supply contract. 1.2 Voltage fluctuations Voltage fluctuations are rapidly changing voltage deviations lasting from half a cycle to several seconds. Voltage fluctuations occur under the influence of rapidly changing network load. Sources of voltage fluctuations are powerful electrical receivers with a pulsed, sharply variable nature of active and reactive power consumption: arc and induction furnaces; electric welding machines; electric motors at start-up.According to GOST 23875-88, the quality of electrical energy is understood as the degree of compliance of electrical energy parameters with their established values.
A parameter is understood as a quantity that quantitatively characterizes any property of electrical energy (for example, voltage, frequency, voltage curve shape, etc.).
The difference between the current value of the electrical energy parameter and its nominal or basic value is called the deviation of the electrical energy parameter. The basic value of the parameter can be taken as the operating average, calculated value, limit value, or stipulated by the power supply contract.
Steady-state voltage (frequency) deviation is the voltage (frequency) deviation in the steady-state operating mode of the power supply system.
Voltage deviation is estimated as a percentage
Voltage fluctuations are a series of single changes in voltage over time. Voltage fluctuations are characterized by the magnitude of the voltage change and the flicker dose.
The range of voltage fluctuations is a value equal to the difference between the highest and lowest voltage values over a certain time interval in the steady state operation of a source, electrical energy converter or power supply system
Flicker is a person’s subjective perception of fluctuations in the luminous flux of artificial lighting sources caused by voltage fluctuations in the electrical network.
Flicker dose is a measure of a person’s susceptibility to the effects of flicker over a specified period of time.
Overvoltage in the power supply system refers to the excess of voltage above the highest operating voltage established for a given electrical equipment. Temporary overvoltage means an increase in voltage at a point in the electrical network above 1.1 U HOM , lasting more than 10 ms, occurring in power supply systems during switching
and short circuits.
A voltage pulse is a sudden change in voltage at a point in an electrical network, followed by restoration to the original or close to it level in a period of time of up to several milliseconds.
Voltage sag means a sudden significant drop in voltage (below 0.9 U NOM) in the power supply system with its subsequent restoration after a period of time from ten milliseconds to several tens of seconds.
According to GOST 13109-97, the normally permissible and maximum permissible values of the steady-state voltage deviation at the terminals of electrical energy receivers are equal to +5% and +10%, respectively, of the rated voltage of the electrical network.
The limits of permissible voltage swings depend on the frequency of repetition of voltage fluctuations per minute and for voltage fluctuations that have a meander shape, they vary from fractions of a percent to 10% of the nominal value.
Normally permissible and maximum permissible frequency deviation values are +0.2 and +0.4 Hz, respectively.
The voltage dip is characterized by the duration of the voltage dip. The maximum permissible value of the duration of a voltage dip in electrical networks up to 20 kV inclusive is 30 s.
Rice. 3.1 illustrates some of the above definitions.
Distortion of the shape of the alternating voltage (current) curve - the difference in the shape of the alternating voltage (current) curve from the required one.
The coefficient of the shape of the alternating voltage (current) curve is a value equal to the ratio of the effective value of the periodic voltage (current) to its average value (for half a period).
For sine wave 
.
The amplitude coefficient of the alternating voltage (current) curve is a value equal to the ratio of the maximum absolute value of the voltage (current) over the period to the effective value of the periodic voltage (current). (For sinusoid 
).
The sinusoidal distortion factor of the voltage (current) curve is one of the main indicators of power quality, equal to the ratio of the effective value of the sum of higher harmonic components to the effective value of the main component of the alternating voltage (current):
% ,
Where n- serial number of the harmonic component of voltage. The second indicator of non-sinusoidality is the coefficient n th harmonic component of voltage:
,
%.
The normally permissible and maximum permissible values of the voltage curve sinusoidal distortion coefficient are, respectively, at the points of connection to electrical networks:
With U NOM = 0.38 kV 8 and 12%, s U NOM = 6 -20 kV 5 and 8%, s U NOM = 35 kV 4 and 6% , With U NOM= 110 - 330 kV 2 and 3%. .
To characterize voltage asymmetry, asymmetry coefficients for negative and zero sequences are used.
The negative sequence unbalance factor is given for phase-to-phase voltages, the geometric sum of which is always zero. It is equal to the ratio, %,
,
% ,
Where U 2 , U 1 - negative and positive sequence components when decomposed using the method of symmetrical components of the phase-to-phase voltage system.
The zero sequence asymmetry coefficient is defined as
,
% .
It is equal to the percentage ratio of the components of the zero and positive sequences when decomposed using the method of symmetrical components of the phase voltage system. Moreover, it is known that the ratio U 1 And U 1 F for connected systems of phase and phase-to-phase voltages has a simple form:
U 1
=
U 1 F .
Normally permissible and maximum permissible values of the negative sequence voltage asymmetry coefficient at points of common connection to electrical networks are equal to 2 and 4%, respectively.
Normally permissible and maximum permissible values of the zero-sequence asymmetry coefficient at points of common connection to four-wire electrical networks with a rated voltage of 0.38 kV are equal to 2 and 4%, respectively.
The positive and zero sequence components can be introduced using a linear transformation based on a matrix equation:
,
Where 
,
;
;
A 3
= 1;
A 4 = A; 1+ a + a 2 = 0.
Here 
And 
symbol for column vectors of phase voltages and voltages included in symmetrical systems of zero, direct and negative sequences, i.e.
= 
= 
.
This means that systems of phase quantities can be composed of systems of zero ( 
,
,
),
straight line as coinciding with the basic order of phase alternation ( 
,A 2
,A
)
and reverse sequences ( 
,
A
,
A 2 
).
The phase alternation shown in Fig. 1 is taken as the main one. 3.2. The arrow indicates that after reaching a positive maximum voltage in phase A, a positive maximum must occur in phase B, and then in phase C. The order of phase voltages in the column vector of phase voltages corresponds to the basic order of phase alternation.
SECTION 9. Power Quality
GROUNDING OF CABLE SCREENS
Cable shield connections in the form of a “pigtail” cannot be recommended for ensuring the EMC of cable lines, with the exception of low-frequency applications, in any case the length of the “pigtail” should not exceed 30 mm. To ground CL screens, it is recommended to use special clamps or connectors.
The basic rule is that the screens of control and power cables should be grounded at both ends. This reduces common mode interference. Special cases are double shielding of cables, grounding through a capacitor or surge protection device. Through the use of capacitors, the coupling between low and high frequency currents is achieved.
The use of twisted pairs significantly reduces induced interference;
Coaxial cables, despite their use for carrying high-frequency signals, are not very good for lower-mid frequencies;
Screens in the form of a braid on the outer surface of the cable are superior in electrical parameters to screens in the form of a spirally wound foil;
Braid and foil are better, the thicker the wire or foil material;
Longitudinal installation of foil is better than spiral installation, but it is difficult to bend;
External screen in the form of braid and foil or double braid is much better than a single screen;
Individual twisted pairs in a common shielded cable may require individual shields to prevent capacitive interference between signal conductors;
Multilayer screens with insulation between the screen layers are better than those without insulation.
Conclusions on the section
Design solutions for ensuring EMC of high-voltage substations include: development of layout solutions, design of the substation grounding device, development of cable ducts and lightning protection systems, design of an operational direct current system and an alternating current power supply system.
Electrical energy quality indicators (EQI), methods for their assessment and standards are determined by the Interstate Standard: “Electric Energy. Electromagnetic compatibility of technical equipment. Standards for the quality of electrical energy in general purpose power supply systems" GOST 54149-2010.
The EC limits established by this standard are electromagnetic compatibility levels for conducted electromagnetic interference in general purpose power supply systems. Subject to compliance with these standards, electromagnetic compatibility of general-purpose electrical power supply networks and electrical networks of electrical power consumers (electric power receivers) is ensured.
The standards established by this standard are subject to inclusion in the technical specifications for connecting electrical energy consumers and in contracts for the use of electrical energy between electricity supply organizations and electrical energy consumers.
In addition to the EMC requirements in connection with the issuance of Russian Government Decree No. 1013 of August 13, 1997 on the inclusion of electrical energy in the list of goods subject to mandatory certification, EC must also be observed from the point of view of the Russian Federation Law “On the Protection of Consumer Rights”. In light of this government decree, a joint decision was made by the State Standard of Russia and the Ministry of Fuel and Energy of the Russian Federation “On the procedure for introducing mandatory certification of electrical energy” dated 03/03/1998, and also a “Temporary procedure for certification of electrical energy” was introduced.
