Cottage power supply project Explanatory note (continued). Requirements for reliability of power supply and quality of electricity Section quality of electrical energy

Electrical energy quality

Introduction

electrical energy voltage

Electrical energy as a commodity is used in all spheres of human activity, has a set of specific properties and is directly involved in the creation of other types of products, affecting their quality. The concept of electrical energy quality (EQ) differs from the concept of quality of other types of products. Each electrical receiver is designed to operate under certain parameters of electrical energy: rated 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 electrical receivers (ER) can operate normally and perform their intended functions.

CE at the point of production does not guarantee its quality at the point of consumption. The EC before and after turning on the electric power supply at the point of its connection to the electrical network may be different. CE is also characterized by the term “electromagnetic compatibility”. Electromagnetic compatibility is understood as the ability of an electronic device to function normally in its electromagnetic environment (in the electrical network to which it is connected), without creating unacceptable electromagnetic interference for other electronic devices operating in the same environment.

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

Household EDs, like industrial ones, must also have electromagnetic compatibility with other EDs included in the general power grid, not reduce their operating efficiency and not degrade the PCE.

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 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.

CE is closely related to the reliability of power supply, since the normal mode of power supply to consumers is one in which consumers receive electricity uninterruptedly, in a quantity previously agreed upon with the energy supply organization, and of standardized quality. Article 542 of the Civil Code of the Russian Federation obliges the supply of 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, electric energy is subject to mandatory certification according to the indicators of electricity quality established by GOST 13109-97 “Electric energy quality standards 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 receive a certificate certifying that the quality of the energy it supplies meets the requirements of GOST 13109-97.

1. Basic provisions of the state standard for the quality of electrical energy

GOST 13109-97 “Standards for the quality of electrical energy in general-purpose power supply systems” (hereinafter referred to as GOST) establishes indicators and standards for the quality of electricity in electrical networks of general-purpose power supply systems of alternating three-phase and single-phase current with a frequency of 50 Hz at points to which electrical networks located 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 established by the standard are electromagnetic compatibility levels for conducted electromagnetic interference in general purpose power supply systems. Subject to compliance with the established CE standards, electromagnetic compatibility of electrical networks of energy supply organizations and electrical networks of consumers of electrical energy or electric power is ensured.

The standard does not establish requirements for CE in special-purpose electrical networks (contact, traction, communications), mobile installations (airplanes, trains, ships), etc.

Conducted electromagnetic interference in the power supply system is electromagnetic interference propagating through the elements of the electrical network.

Point of general connection - a point in a general-purpose electrical network that is electrically closest to the networks of the electrical energy consumer in question, to which the electrical networks of other consumers are connected or may be connected.

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

GOST 13109-97 is the first standard in the field of energy efficiency, which states that the established standards are subject to inclusion in the technical conditions for connecting consumers and in energy supply contracts.

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

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

2. Electric energy quality indicators

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

Steady voltage deviation;

voltage change range;

flicker dose;

coefficient of the nth harmonic component of voltage;

frequency deviation;

duration of voltage dip;

impulse voltage;

temporary overvoltage factor.

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

Interval between voltage changes;

voltage dip depth;

frequency of voltage dips;

pulse duration at the level of 0.5 of its amplitude;

duration of temporary overvoltage.

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

All PCEs related to voltage are assessed based on their current values.

To characterize the above indicators, the standard establishes numerical normal and maximum permissible values ​​of PKE or norms.

Another part of the PKE 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 PKEs, the standard does not establish acceptable numerical values. To quantify these PCEs, the amplitude, duration, frequency of their occurrence and other characteristics established but not standardized by the standard must be measured. Statistical processing of this data makes it possible to calculate generalized indicators that characterize a specific electrical network in terms of the likelihood of short-term interference.

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

Due to the random nature of changes in electrical loads, the requirement to comply with CE standards during this entire time is practically unrealistic, therefore the standard establishes the probability of exceeding CE standards. The measured PCEs should not go beyond the normally permissible values ​​with a probability of 0.95 for the calculated period of time established by the standard (this means that individual exceedances of standardized values ​​can be ignored if their expected total duration is less than 5% over the established period of time).

In other words, the CE according to the measured indicator meets the requirements of the standard if the total duration of time exceeding the normally permissible values ​​is no more than 5% of the established period of time, i.e. 1 hour 12 minutes, and beyond the maximum permissible values ​​- 0% of this period of time.

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

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

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

Properties of electrical energy EC indicator The most likely culprits for EC deterioration Voltage deviation Steady-state voltage deviation Energy supply organization Voltage fluctuations Voltage change range

Flicker dose Consumer with variable load Non-sinusoidal voltage Distortion coefficient of sinusoidal voltage curve

Coefficient of the nth harmonic component of voltage Consumer with a nonlinear load Asymmetry of a three-phase voltage system Negative sequence voltage asymmetry coefficient

Zero sequence voltage asymmetry factor Consumer with asymmetric loadFrequency deviationFrequency deviation Energy supply organization Voltage dip Duration of voltage dip Energy supply organizationVoltage impulseImpulse voltage Energy supply organization Temporary overvoltage Temporary overvoltage coefficient Energy supply organization

The standard establishes methods of calculation and methods for determining PCE and auxiliary parameters, requirements for measurement errors and averaging intervals of PCE, which must be implemented in CE monitoring devices when measuring indicators and their processing.

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 at substations of power systems; changes in the layout 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-state voltage deviation is equal, %:

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

In single-phase electrical networks, the effective value of voltage is defined as the value of the fundamental frequency voltage without taking into account the higher harmonic components of the voltage, and in three-phase electrical networks - as the effective value of the positive sequence voltage of the fundamental frequency.

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

Voltage fluctuations

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

Voltage fluctuations are characterized by two indicators:

dose of flicker.

The range of voltage changes is calculated using the formula, %

where, are the values ​​of the extrema that follow 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 during time T;

T - measurement time interval, taken equal to 10 minutes.

If two voltage changes occur less than 30 ms apart, they are treated as one.

The time interval between voltage changes is:

Assessment of the permissibility of voltage change ranges (voltage fluctuations) is carried out using curves of the dependence of permissible fluctuation ranges on the frequency of repetitions of voltage changes or the time interval between subsequent voltage changes.

CE at the point of common connection with periodic voltage fluctuations having a meander (rectangular) shape (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 in 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 permissible value of the sum of the steady-state voltage deviation δUУ and the range of voltage changes δUt at points of connection to electrical networks with a voltage of 0.38 kV is equal to ±10% of the rated voltage.

Flicker dose is a measure of a person's susceptibility to the effects of fluctuations in luminous flux caused by voltage fluctuations in the supply network over a specified period of time.

The standard establishes short-term () and long-term doses of flicker () (short-term is determined over an observation time interval of 10 minutes, long-term over an interval of 2 hours). The initial data for the calculation are flicker levels measured using a flicker meter - a device in which the sensitivity curve (amplitude-frequency response) of the human organ of vision is simulated. Currently, the development of flicker meters to monitor voltage fluctuations has begun in the Russian Federation.

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

The maximum permissible value for a short-term flicker dose at points of common connection of electricity consumers with incandescent lamps in rooms where significant visual strain is required is 1.0, and for a long-term flicker dose it is 0.74, with voltage fluctuations with a shape other than a meander.

Non-sinusoidal voltage

In the process of generating, converting, distributing and consuming electricity, distortions in the shape of sinusoidal currents and voltages occur. Sources of distortion are synchronous generators of power plants, power transformers operating at increased values ​​of magnetic induction in the core (at increased voltage at their terminals), AC-to-DC converting devices and electric drives with nonlinear current-voltage characteristics (or nonlinear loads).

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

It is known from a mathematics course 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 values ​​with multiple frequencies. Such sinusoidal components are called harmonic components or harmonics. The sinusoidal component, the period of which is equal to the period of the 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

Non-sinusoidal voltage is characterized by the following indicators:

· distortion factor of the sinusoidal voltage curve;

· coefficient of the nth harmonic component of voltage.

The voltage curve sinusoidal distortion coefficient is determined by the expression, %

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

Effective value of the fundamental frequency voltage, V.

It is allowed to determine by expression, %

where is the rated network voltage, V.

The coefficient of the nth harmonic component of voltage is equal, %

Can be calculated using the expression, %SRC= “publ_image/Image48.gif” align= “top”> (3.10)

To calculate, it is necessary to determine the voltage level of individual harmonics generated by the nonlinear load.

The harmonic phase voltage 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;

Voltage of nonlinear load (if the design point coincides with the point of connection of the nonlinear load, then =);

Rated mains voltage;

Short circuit power at the point of connection of a nonlinear load.

For the calculation, it is necessary to first determine the current of the corresponding harmonic, which depends not only on the electrical parameters, but also on the type of nonlinear 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. Voltage curve sinusoidal distortion coefficient values

Normally acceptable values ​​for , 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 electricity consumers, the symmetrical multiphase design of which is either impossible or impractical for technical and economic reasons. Such installations include induction and electric arc furnaces, railway traction loads running on alternating current, electric welding units, special single-phase loads, and lighting installations.

Asymmetrical voltage modes in electrical networks also occur in emergency situations - during phase failure or asymmetrical short circuits.

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

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

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

In addition to the asymmetry caused by the voltage of the negative sequence system, asymmetry can arise from the superposition of voltages of the zero sequence system 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:

· negative sequence voltage asymmetry coefficient;

· zero-sequence voltage asymmetry coefficient.

The negative sequence voltage asymmetry coefficient is equal, %

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

RMS value of positive sequence voltage of fundamental frequency, V.

It is allowed to calculate using the expression, %:

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

The zero-sequence voltage asymmetry coefficient is equal, %:

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

Can be calculated using the formula, %

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

Measurement of the zero-sequence voltage asymmetry coefficient is carried out in a four-wire network.

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

The normally permissible and maximum permissible values ​​of the negative sequence voltage asymmetry coefficient at the point of common connection to electrical 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 frequency values, Hz

The standard establishes normal and maximum permissible values ​​of 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 a restoration of the voltage to the original or close to it level after a period of time from ten milliseconds to several tens of seconds (Fig. 3.6).

Rice. 3.6. Voltage dip

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

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

Voltage surge and temporary overvoltage

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

Rice. 3.7. Pulse voltage parameters

The pulse voltage in relative units is equal to:

where is the value of the pulse voltage, V.

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

Indicator - pulse voltage is not standardized by the standard.

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

Rice. 3.8. Temporary overvoltage

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

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

The temporary overvoltage factor is also not standardized by the standard.

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

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

Duration of temporary overvoltage, s Up to 1 Up to 20 Up to 60 Temporary 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 significant asymmetry of phase loads can reach the values ​​of phase-to-phase voltage and last for 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 changes in the PCE. Thus, PCE - steady-state voltage deviation, coefficients characterizing the non-sinusoidality and asymmetry of voltages, frequency deviation, voltage change range, etc. - are random quantities and their measurements and processing should be based on probabilistic-statistical methods. Therefore, as already noted, the standard establishes PCE norms and stipulates the need to comply with them within 95% of the time of each day (for normally acceptable values).

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

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

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 average. The equation for the distribution curve of voltage deviations from the nominal value, 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 CE monitoring devices, continuous random variables, which are PCEs, 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. A histogram is 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;

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. This assessment is made based on the sum of the values ​​that fall within the intervals that fall within the permissible range of voltage deviations. Using the histogram, the probability of voltage deviations beyond the normally permissible values ​​is also found. This allows us to judge the reasons for the low quality of voltage in the electrical network and select measures to improve it.

To assess the quality of voltage, numerical characteristics and determined from a histogram are widely used.

The mathematical expectation determines the average level of voltage deviations at the considered network point over a controlled period of time

where k is the number of histogram intervals.

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

The parameter is the standard deviation and characterizes the scattering of the histogram, i.e. spread of voltage deviations around the mathematical expectation. For most voltage deviation histograms, the cumulative probability of being in range 4 is 0.95. This means that to satisfy the requirements of the standard, the measured value should not exceed 1/4 of the width of the permissible range. So, if the permissible range of voltage deviation is acceptable, then it is necessary that it does not exceed 2.5%.

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

Table 3.4. Intervals for averaging measurement results of CE indicators

KE indicator Averaging interval, s Steady-state voltage deviation Range of voltage changes Flicker dose Distortion coefficient of the sinusoidal voltage curve Coefficient of the n-th harmonic component of voltage Negative sequence voltage asymmetry coefficient Zero-sequence voltage asymmetry coefficient Frequency deviation Duration of voltage dip Impulse voltage Temporary overvoltage coefficient60 - - 3 3 3 3 20 - - -

For averaging intervals of various PKEs, the standard establishes the number of observations (N) and, using the methodology set out in the standard, one or another PKE 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 minute using the formula:

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

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

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

The methods for determining the PCE established by the standard are implemented in the hardware for monitoring the PCE. The form for presenting measurement processing results must also meet the requirements of the standard.

Table 3.5 provides summary data on PKE standards.

Table 3.5. Electrical energy quality standards

CE indicator, units. measurementsKEN normsNormally permissiblemaximum permissibleSteady voltage deviation, %± 5± 10Voltage change range, %Curves 1,2 in Fig. 3.2 Flicker dose, relative. units: Short-term

Long-term -

1.0; 0.74 Distortion coefficient of the sinusoidal voltage curve, % According to the table

1According to the table

3.1 Coefficient of the nth harmonic component of voltage, % According to the table

2According to the table

3.2 Negative sequence voltage asymmetry factor , %24 Voltage asymmetry coefficient for zero sequence , %24 Frequency deviation , Hz ± 0.2 ± 0.4 Voltage dip duration , s-30Pulse voltage , kV--Temporary overvoltage factor , relates. units:--

4. The influence of power quality on the operation of electrical receivers

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

Typical types of electrical receivers

EDs for various purposes are powered from the electrical networks of general-purpose power supply systems; let’s consider industrial and household EDs.

The most typical types of electric motors, widely used at enterprises in various industries, are electric motors and electric lighting installations. Electrothermal installations, as well as valve converters used to convert alternating current into 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 area of ​​application of asynchronous and synchronous electric motors has been established depending on the voltage. 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, at voltages up to 6 kV and power up to 300 kW - asynchronous motors, and above 300 kW - synchronous, at voltages up to 10 kV and power up to 400 kW - asynchronous motors, above 400 kW - synchronous.

The wide spread 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 sources of reactive power, their torque is less dependent on the terminal voltage, and in many cases they have higher efficiency. At the same time, synchronous motors are more expensive and complex to manufacture and operate.

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

AC electric welding installations for arc and resistance welding represent a single-phase uneven 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.

Due to the specific nature of their regulation, valve converters 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; The non-sinusoidal coefficient during operation of thyristor converters in rolling mills 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 to normal operating conditions for other electrical equipment. In particular, welding units, the power of which currently reaches 1500 kW per unit, cause significantly greater voltage fluctuations in electrical networks than, for example, the start-up of asynchronous motors with a squirrel-cage rotor. In addition, these voltage fluctuations occur over a long period of time and over a wide range of frequencies, including in the most unpleasant range for electric lighting installations (about 10 Hz).

Electrothermal installations, depending on the heating method, are divided into groups: arc furnaces, resistance furnaces of direct and indirect action, electronic melting furnaces, vacuum, slag remelting, induction furnaces. This group of electric power plants also has an adverse effect on the power supply network, for example, arc furnaces, which can have a power of up to 10 MW, are currently constructed as single-phase. This leads to a violation of the symmetry of currents and voltages (the latter occurs due to voltage drops at the network resistances from currents of different sequences). In addition, arc furnaces, like valve units, are nonlinear electric generators 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 electric power supplies, 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); Electric drives with asynchronous motors operating in three-phase mode (drive elevators, pumps in water supply and heating systems, etc.); Electric drives with asynchronous motors operating in single-phase mode (drive compressors for refrigerators, washing machines, etc.); ED with commutator motors (drive of vacuum cleaners, electric drills, etc.); AC and DC welding units (for repair work in the workshop, etc.); rectifier 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 electric power supply is insignificant, but the totality of electric power supply units connected to the 0.4 kV buses of a transformer substation has a significant impact on the supply network.

Impact of voltage deviations

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

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

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

where is the synchronous rotation speed;

Engine load factor;

Rated stress and slip values ​​respectively.

From formula (4.1) it is clear 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 process equipment, since the engine speed does not decrease below the rated speed.

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

A decrease in voltage also worsens the conditions for starting the engine, since 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.

If the voltage at the motor terminals decreases, the reactive magnetizing power decreases (by 2 - 3% when the voltage decreases by 1%), with the same power consumption, the motor current increases, which causes insulation overheating.

If the motor operates for a long time at low voltage, then due to accelerated wear of the insulation, the service life of the motor is reduced. The approximate service life of the 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 that depends on the value and sign of the voltage deviation, as well as on the motor load factor and is 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 are more dangerous within the considered limits.

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

An increase in voltage at the motor terminals leads to an increase in the reactive power they consume. At the same time, the specific consumption of reactive power increases with a decrease in the engine load factor. On average, for every percent 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 elements of the electrical network.

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

From the curves in Fig. 4.2. It can be seen that as the voltage decreases, the luminous flux drops most noticeably. When the voltage increases above the rated voltage, the luminous flux F, lamp power P and luminous efficiency h increase, but the service life of lamps T sharply decreases and as a result they quickly burn out. At the same time, there is also excessive consumption of electricity.

Changes in voltage 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. As the voltage increases, the power consumption and luminous flux increase, and when the voltage decreases, they decrease, but not to the same extent as incandescent lamps. At low voltage, the ignition conditions for fluorescent lamps worsen, so their service life, determined by the sputtering 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 on average by 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 current control system using 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 of 1% leads to an increase in the reactive power consumption of the converter by approximately 1-1.4%, which leads to a deterioration in the power factor. At the same time, other indicators 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 energy consumption.

Voltage deviations negatively affect the operation of electric welding machines: for example, for spot welding machines, when the voltage changes by 15%, 100% defective products are obtained.

Effect of Voltage Fluctuations

Electrical devices that are extremely sensitive to voltage fluctuations include lighting devices, especially incandescent lamps and electronic equipment:

The standard defines the impact 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.

Blinking with a frequency of 3 - 10 Hz has the strongest impact on the human eye, so permissible 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 that of gas-discharge lamps. Voltage fluctuations greater than 10% may cause discharge lamps to go out. Depending on the type of lamp, their ignition occurs in a few seconds or even minutes.

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

If there are significant voltage fluctuations (more than 15%), the conditions for normal operation of electric motors may be disrupted, and the contacts of magnetic starters may fall off with a corresponding shutdown of operating motors.

Voltage fluctuations with a swing 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 sufficiently studied. This complicates technical and economic analysis when designing and operating power supply systems with sharply variable loads.

Influence of voltage asymmetry

Voltage asymmetry, as already noted, is most often caused by the presence of an asymmetrical load. Asymmetrical load currents flowing through the elements of the power supply system cause asymmetrical voltage drops in them. As a result, an asymmetrical voltage system appears at the ED terminals. Voltage deviations in the ED of the overloaded phase may exceed normally permissible values, while voltage deviations in the ED of other phases will be within normalized limits. In addition to the deterioration of the voltage conditions of the electric power supply in an asymmetrical mode, the operating conditions of both the electric power supply itself and all network elements are significantly worsened, and the reliability of the electrical equipment and the power supply system as a whole is reduced.

The effect of an asymmetrical mode is qualitatively different compared to a symmetrical one for such common three-phase electric motors as asynchronous motors. Of particular importance for them is the negative sequence voltage. The negative sequence resistance of electric motors is approximately equal to the resistance of a stalled motor and, therefore, is 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 (reduction in motor efficiency). Thus, the service life of a fully loaded asynchronous motor operating at a voltage asymmetry of 4% is reduced by 2 times. With a voltage unbalance of 5%, the available motor power is reduced by 5 - 10%.

If the network voltage is unbalanced in synchronous machines, along with the occurrence of additional active power losses and heating of the stator and rotor, dangerous vibrations can occur as a result of the appearance of alternating torques and tangential forces pulsating at double the network frequency. With significant asymmetry, vibration can be dangerous, and especially if there is insufficient strength and the presence of defects in welded joints. When current asymmetry does not exceed 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 should not exceed the rated values. If these conditions are not met, then special measures must be taken to reduce asymmetry.”

In the case of negative and zero sequence currents, the total currents in individual phases of the network elements increase, which leads to an increase in active power losses and may be unacceptable from a heating point of view. Zero sequence currents flow constantly through the ground electrodes. This further dries and increases the resistance of the 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 interlocking devices.

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

Capacitor installations with voltage unbalance are unevenly loaded with reactive power across phases, which makes it impossible to fully utilize the installed capacitor power. In addition, capacitor installations in this case increase the already existing asymmetry, since the supply of reactive power to the network in the phase with the lowest voltage will be less than in other phases (proportional to the square of the voltage at the capacitor installation).

Voltage asymmetry also significantly affects single-phase EDs; 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

EDs with nonlinear current-voltage characteristics consume non-sinusoidal currents from the network when a sinusoidal voltage is applied to their terminals. Currents of higher harmonics, passing through network elements, 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 at the nodes of the electrical network. In this regard, EPs with a nonlinear current-voltage characteristic are often called sources of higher harmonics.

The most serious violations of FE 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 coefficient of the sinusoidal voltage curve in networks with electric arc steel-smelting and ore-smelting furnaces is determined mainly by the 2nd, 3rd, 4th, 5th, and 7th harmonics.

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

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

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

Effective value of the magnetizing current of the transformer:

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

If a non-sinusoidal voltage is supplied to the transformer inputs, additional components of higher current harmonics arise. GPP transformers produce a small 5th harmonic.

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

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

For example, the capacitance of capacitors installed to compensate for reactive power decreases with increasing frequency of the supplied voltage. Therefore, if there are higher harmonics in the supply voltage, then the resistance of the capacitors at these harmonics turns out to be significantly lower than at a frequency of 50 Hz. Because of this, in capacitors designed to compensate for reactive power, even small harmonic voltages can cause significant harmonic currents. In enterprises with a large proportion of nonlinear loads, capacitor banks perform poorly. They are either turned off by overcurrent protection or fail in a short time due to swelling of the cans (or accelerated aging of the insulation). There are known cases when, at enterprises with a developed cable network with a voltage of 6-10 kV, capacitor banks find themselves in 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 insulation of electrical machines, transformers, cables;

· deterioration of the electric power factor;

· 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;

· disruption of the operation 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 (pulse-phase control system 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 electric drive should not contain higher harmonics in the steady state of the electrical network. It should be emphasized that under the operating conditions of the electric power supply, the non-sinusoidal voltage appears together with the actions of other influencing factors and therefore it is necessary to consider the entire set of factors together.

Effect of frequency deviation

The strict 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 production processes and, as a consequence, the technical and economic indicators of the operation 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 the consumption of active and reactive power. It is known that a decrease in frequency by 1% increases losses in electrical networks by 2%.

The technological component of the damage is caused mainly by the underproduction of their products by industrial enterprises and the cost of additional operating time of 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 operation of enterprises with a continuous production cycle showed that most of the main production lines are equipped with mechanisms with constant and fan resistance torques, and their drives are asynchronous motors. The rotation speed of the motor rotors is proportional to the change in the network frequency, and the productivity of the technological lines depends on the engine speed.

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

where a is the proportionality coefficient, depending on the type of mechanism; is the network frequency; is the 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, metal-cutting machines, etc.; for them n=1;

2.mechanisms with fan moment of resistance - centrifugal pumps, fans, smoke exhausters, etc.; for them n=3; at thermal power plants, thermal power plants, and nuclear power plants, these are usually the engines of feedwater pumps, circulation pumps, smoke fans, oil pumps, etc.

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

EDs of the 2nd and 3rd groups, which are most susceptible to the influence of frequency, have adjustment capabilities, thanks to which the power they consume from the network remains practically unchanged.

The most sensitive to frequency reduction are auxiliary motors of power plants. A 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 (a frequency avalanche occurs).

EDs such as incandescent lamps, resistance furnaces, and electric arc furnaces practically do not respond to frequency changes.

Frequency deviations negatively affect the operation of electronic equipment: frequency deviations of more than +0.1 Hz lead 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 permissible range of the television signal to the background hindrance. Changing the frequency to 49.5 Hz requires a significant tightening of the requirements for the signal/background interference ratio in all parts of the television path - from the equipment of the studio complex to the television receiver, the implementation of which is associated with significant material costs.

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

To prevent system-wide accidents caused by a decrease in frequency, special automatic frequency unloading (AFD) devices are provided, which disconnect some of the less critical consumers. After eliminating the power shortage, for example, after switching on backup sources, special frequency automatic restart devices (FACR) turn on disconnected consumers and 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 solution to which is the introduction of generating capacities in order to create power reserves in the networks of energy supply organizations.

Effect of electromagnetic interference

In general-purpose power supply systems, electronic and microelectronic control systems, microprocessors and computers have found widespread use, which has led to a decrease in the level of noise immunity of electrical 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 transient processes both in power system networks 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 frequency band of interference can reach tens of megahertz.

Electromagnetic transient interference, accompanied by voltage dips, occurs mainly during single-phase short circuits of overhead lines due to insulation overlap. These faults either self-destruct or are eliminated by a short-term shutdown followed by automatic reclosure (AR). In addition, the cause of voltage dips is 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 occur 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 ground faults, during switching of capacitor banks and resonant filters, when disconnecting unloaded cable lines and transformers, during simultaneous switching of contacts of switches and other switching equipment, in open-phase modes operation of the electrical network due to various reasons leading to ferroresonance phenomena. The susceptibility of electronic equipment and computers to overvoltages depends on both the frequency response of the electronic device and the frequency response of electromagnetic interference.

An increase in the power of power systems and the number of overhead lines used to increase 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 electronic devices.

As already noted, at values ​​of all voltage PCEs different from the standardized ones, accelerated aging of electrical equipment insulation occurs, as a result, the intensity of failure flows increases over time. Thus, if the network voltage curve is non-sinusoidal, even with the resonant setting of the arc extinguishing devices, a current of higher harmonics passes through the ground fault, and burning of the cable may occur at the site of the first fault. In this case, as operating experience shows, two or more accidents due to overvoltage may occur simultaneously.

At low FE, there is an interdependence of element failures, for example, when the negative influence of nonlinear, asymmetrical and shock loads is compensated with the help of appropriate corrective devices when a particular device is turned off. Thus, the failure of a high-speed static compensator causes the appearance of voltage asymmetry, oscillations and harmonics, which were previously compensated, which, in turn, is fraught with the occurrence of false alarms of relay protection, emergency failure of certain types of electrical equipment and other similar negative consequences. Failures in the channels for transmitting information along power circuits in the presence of harmonics lead to the submission of incorrect commands to control switching equipment. Thus, CE significantly affects the reliability of power supply, since the accident rate in networks with low CE is higher than in the case when PCE are within acceptable limits.

5. Electrical energy quality control

.1 Main tasks and types of power quality control

The main objectives of FE control are:

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

Checking the compliance of the actual PCE values ​​at the network interface according to the balance sheet with the values ​​recorded in the energy supply contract;

Development of technical conditions for consumer connection in terms of energy supply;

Checking the fulfillment of contractual terms in terms of CE with the determination of the permissible calculated and actual contributions of the consumer to the deterioration of CE;

Development of technical and organizational measures to ensure CE;

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

Electrical energy certification;

Search for the “culprit” of PCE distortions.

Depending on the goals solved when monitoring and analyzing CE, PCE measurements can take four forms:

· diagnostic control;

· inspection control;

· operational control;

· commercial accounting.

Diagnostic control of CE - the main goal of diagnostic control at the interface between the electrical networks of the consumer and the energy supply organization is to detect the “culprit” for the deterioration of CE, determine the acceptable contribution to the violation of the standard requirements for each PKE, include them in the energy supply contract, and normalize the CE.

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

If the results of diagnostic monitoring of the energy efficiency confirm the consumer’s “guilty” of violating the energy efficiency standards, then the main task of the energy supply organization together with the consumer is to develop and evaluate the possibilities and timing of measures to normalize the energy efficiency. For the period before the implementation of these measures, operational control and commercial metering of energy efficiency must be applied at the interface between the electrical networks of the consumer and the energy supplying organization.

At the next stages of diagnostic measurements of CE, 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 on-load tap-changer devices of transformers, for collecting statistics and recording voltage dips and temporary overvoltages in the electrical network. This controls the operation of existing means of providing CE: 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.

Inspection control of CE is carried out by certification bodies to obtain information about the state of certified electricity in the electrical networks of the energy supplying organization, on compliance with the conditions and rules of application of the certificate, in order to confirm that CE continues to meet the established requirements during the validity period of the certificate.

Operational monitoring of CE is necessary under operating conditions at points in the electrical network where voltage distortions exist and cannot be eliminated in the near future. Operational control is necessary at the connection points of traction substations of railway and urban electrified transport, substations of enterprises with electrical equipment with nonlinear characteristics. The results of operational control should be sent via communication channels to control centers of the electrical network of the energy supplying organization and the power supply system of an industrial enterprise.

Commercial metering of PKE must 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) to electricity tariffs for its quality are determined.

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

Commercial metering of energy efficiency must be continuously carried out at points of metering of consumed electricity as a means of economic influence on the culprit of energy efficiency deterioration. 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 PKE control will make it possible to combine operational control and commercial accounting of KE, while common communication channels and means for processing, displaying and documenting AMR information can be used.

Commercial metering devices for energy efficiency must record the relative time of exceeding the normal and maximum permissible values ​​of energy efficiency at the electricity control point for the billing period, which determine the tariff surcharges for those responsible for the deterioration of energy efficiency.

.2 Standard requirements for power quality control

Monitoring compliance with the requirements of the standard by energy supply organizations and consumers of electrical energy should be carried out by supervisory authorities and accredited CE testing laboratories.

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

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 in the presence of automatic counter-voltage regulation in the power center, at least once a year;

for other PKE - at least once every two years, provided that the network diagram and its elements remain unchanged and there is a slight change in the nature of the consumer’s electrical loads, which worsens the KE.

Electricity consumers who deteriorate the EC must carry out monitoring at the points of their own networks closest to the points of general connection of these networks to the general-purpose electrical network, as well as at the terminals of electrical energy receivers that distort the EC.

The frequency of EC monitoring is set by the consumer of electrical energy in agreement with the energy supply organization.

Monitoring of EC released by AC traction substations into electrical networks with a voltage of 6 - 35 kV should be carried out:

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

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

5.3 Discounts and surcharges to the tariff for the quality of electricity

In paragraph 1 of Art. 542 part 2 of the Civil Code of the Russian Federation establishes: “the quality of 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.”

To ensure the norms of the standard at points of general connection, it is allowed to establish in power supply contracts with consumers - the “culprits” of the deterioration of energy efficiency, more stringent standards (with smaller ranges of change in the corresponding indicators of energy efficiency) than those established in the standard, which consumers are obliged to maintain at the boundary of the balance sheet of electrical networks.

In case of violation by the energy supplying organization of the requirements for CE, the subscriber has the right to prove the amount of damage and recover it from the energy supplying organization according to the rules of Art. 547 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 commensurately reduced price (Clause 2, Article 542 of the Civil Code of the Russian Federation).

Obviously, violations can be mutual and according to different PCEs. The party guilty of reducing the efficiency factor is determined in accordance with the Rules for the application of discounts and surcharges to tariffs for the quality of electricity.

The instructions on the procedure for payments for electric and thermal energy in section 4 “Discounts (surcharges) to the tariff for the quality of electricity” establish penalties for the culprit of the deterioration of the energy efficiency.

The mechanism of penalties established by the Instructions does not apply to all PCEs, but to those numerical values ​​whose norms are in the standard:

steady voltage deviation;

voltage waveform sinusoidal distortion factor;

negative sequence voltage asymmetry coefficient;

zero sequence voltage asymmetry coefficient;

frequency deviation;

voltage change range.

Of the listed PCEs, 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) based on the total impact (coefficient); in addition, it must be taken into account that discounts (surcharges) for individual PCEs add up. Therefore, the indicator is not included in the Instructions. The duration of a voltage dip is not included in discounts (surcharges), since the volume of sanctions for the listed PKE depends on the total duration of supply of electrical energy of reduced quality per month, and in terms of voltage dips, the duration of one dip is normalized without standardizing their quantity.

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

The value of the discount (surcharge) depends on:

on the number of PKE for which a violation of the standard occurs at the point of metering of electrical energy during the billing period;

on the relative time of exceeding the normal and maximum permissible PCE values ​​at the electricity control point 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 at a tariff with a discount (surcharge) for EC is made for the entire volume of electrical energy supplied (consumed) during the billing period. If the energy supplying 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 deviations in voltage and frequency, the energy supply organization is subject to unilateral responsibility. For voltage deviations, the energy supplying organization is responsible to the consumer if the subscriber does not exceed the technical limits for consumption and generation of reactive power.

Responsibility for violating the standards for the four remaining PKEs rests with the person responsible for the deterioration of the KE. The culprit is determined based on a comparison of the allowable contribution included in the contract to the value of the considered PKE at the control point with the actual contribution determined by measurements.

Literature

1.GOST 13109-97 “Electric energy quality standards in general-purpose power supply systems.”

Guidelines for monitoring and analyzing the quality of electricity in general purpose power supply systems (RD 34.15.501 - 88).

Zhezhelenko I.V. Electric power quality indicators and their control at industrial enterprises. M.: Energoatomizdat, 1986. 168 p.

Ivanov V.S., Sokolov V.I. Consumption modes and quality of electricity in power supply systems of industrial enterprises. M.: Energoatomizdat, 1987. 336 p.

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Rules for the application of discounts and surcharges to tariffs for the quality of electricity (approved by Glavgosenergonadzor on May 14, 1991).

Petrov V.M., Shcherbakov E.F., Petrova M.V. On the influence of household electrical receivers on the operation of related electrical devices. Industrial Energy, 1998, No. 4.

Levin M.S., Muradyan A.E., Syrykh N.N. Quality of electricity in rural networks. M.: Energy, 1975. 224 p.

Kudrin B.I., Prokopchik V.V. Power supply for industrial enterprises. Minsk: Higher School, 1988. 357 p.

Instructions on the procedure for payments for electrical and thermal energy (registration No. 449 of December 28, 1993 of the Ministry of Justice of the Russian Federation).

Golovkin P.I. Energy system and consumers of electrical energy M.: Energia, 1973. 168 p.

Mozgalev V.S., Bogdanov V.A., Kartashev I.I., Ponomarenko I.S., Syromyatnikov S.Yu. Assessing the effectiveness of power quality control in EPS. Electric stations, 1999, No. 1.

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.

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

Requirements for reliability of power supply.

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

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

These categories of power supply are defined in regulatory documents regarding each individual type of equipment or facility (building, structure, mechanism). The technical conditions issued by the network organization determine the category of power supply that the network organization, for its part, provides. A comparison is made based on local regulatory documents that define the reliability category of a particular type of electrical receiver. If the category of power supply according to technical specifications is lower than required in regulatory documents, then it is necessary to take 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. Electrical energy quality standards 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 usual requirements for designers regarding voltage losses in electrical networks, as well as for calculating voltage losses, have changed.

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

Fire alarm devices, fire warning systems, fire extinguishing devices, emergency lighting devices, and emergency lighting are classified in category I. Provided by ATS device, UPS

To ensure the second category of reliability at the site, a quarantine facility is used single-transformer substation with input into the building of two cables from the transformer substation and the diesel generator set.

Electric receivers of the first category in normal modes must be provided with electricity from two independent, mutually redundant power sources, and an interruption in their power supply in the event of a power failure from one of the power sources can be allowed only for the duration of automatic power restoration. In this regard, emergency lighting fixtures are used with emergency power units. Emergency power units are also built into microclimate control panels and fire alarm devices and fire warning systems.

The quality of electricity needs to be expressed in quantitative terms to evaluate the supply network. Providers are required to maintain compliance with GOST characteristics such as voltage and frequency fluctuations. Depending on the connected consumers, the values ​​of the main indicators change, which, if their deviations are significant, 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 the indicators beyond the limits established by regulations. Thus, the voltage may be too high due to an accident at the substation. Low values ​​appear in the evening or in the summer season, when people return home and turn on televisions, electric stoves, and 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, which can be contacted if inconsistencies arise.

Power quality may 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 power wires.
• Changes in the wind when power is generated by wind turbines.
• The quality of the wiring will wear out over time.

Why are the main characteristics of the power supply network needed?

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

Electricity was updated in 2013 due to the development of the energy market and the emergence of new electronic devices. Electricity, as part of its supply, should be considered as a product that meets certain criteria. If the established characteristics are deviated, administrative liability may be applied to providers. If, due to fluctuations in incoming voltage, people were or could be injured, then criminal liability may arise.

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

Power quality parameters affect the operating time of connected devices; this often becomes critical in production. The productivity of the lines decreases and increases. Thus, the torque on the motor shaft decreases when the values ​​of the supply network indicators drop. The service life of lighting lamps is shortened, the luminous flux of lamps becomes smaller or flickers, which affects the products produced in greenhouses. The processes of other biochemical reactions have a significant impact.

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; at best, they burn; at worst, the motor windings and electronic elements deteriorate irrevocably. Under similar circumstances, the electric meter begins 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 common household circuits with a frequency of 50 Hz:

• 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 normal limit to 231 V.
• The range of change in input voltage is the difference between the effective and amplitude values. Measurements are made per cycle of the parameter difference.
• The flicker dose is divided into short-term, within 10 minutes, and long-term, defined as 2 hours. Indicates the degree of susceptibility of the human eye to flickering light caused by power supply fluctuations.
• The pulse voltage is described by the recovery time, which has different values ​​depending on the cause of the surge.
• Coefficients for assessing the quality of the supply network: sinusoidal distortion, temporary overvoltage values, harmonic components, reverse and zero sequence asymmetry.
• The voltage dip interval is determined by the recovery period of the parameter established in accordance with GOST.
• Deviation of the supply frequency leads to damage to electrical parts and conductors.

Fixed input deviation

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

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 permissible operating limit is set for fluctuations of ±10%. For a 220 V network, this will be a minimum threshold of 198 V and a maximum of 242 V.

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

Range of supply network changes

Electricity quality standards contain oversight of such a parameter as fluctuations in voltage components. It sets the difference between the upper amplitude threshold and the lower one. Considering that the tolerances for deviation of the parameter from the set value are within the limit of ±5%, the range of the limit mode cannot exceed ±10%. The 220 V supply network cannot fluctuate more or less than 22 V, and 380 V works normally within ±38 V.

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

Input instability

The power quality system includes flicker dose measurements. This indicator is recorded by a special device - a flicker meter, which records the amplitude-frequency response. The results obtained are compared with the sensitivity curve of the visual organ.

GOST establishes permissible limits for changing the flicker dose:

• Short-term fluctuations the 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 incandescent lamp circuit indicator, then it is required that the result falls within the following limits:

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

Tangible changes

Power quality measurements involve 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 reasons for the differences are thunderstorms 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 longer and equal to 15 ms.

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

Input decay time

The measured parameter is described as a voltage dip that falls within the limits of ±0.1U nominal over 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 depth of decline is calculated according to the expression: ΔU n =(U nominal −U min)/U nominal.

The duration of the decline is calculated according to the expression: Δt n =t k −t n, here t k is the period when the voltage has already been restored, and t n is the starting point, the moment when the voltage drop occurred.

Power quality control requires taking 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 declines at a set time with depth ΔU n and duration Δt n.
• M is the total count of declines during the selected period.

Why is the decay value needed?

The parameter, the duration of the decay of the input value, is required to assess the reliability of the supplied energy in quantitative terms. This indicator may be influenced by the frequency of accidents at the substation due to personnel negligence and lightning. The result of the failure study is predictions of the degree of failure in the network under consideration.

Statistics allow us to draw 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

Maintaining the frequency within certain limits is a necessary requirement of the consumer. If the indicator decreases by 1%, the losses are more than 2%. This is expressed in economic costs and reduced productivity of enterprises. For the average person, this results in higher amounts on their electricity bills.

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

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

Odds

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

• Non-sinusoidal voltage curve. Distortion of the sine wave occurs due to powerful consumers: heating elements, convection ovens, welding machines. If this parameter deviates, the service life of motor windings is reduced, the operation of relay automation is disrupted, and thyristor-controlled drive systems fail.
• Temporary overvoltage is a quantitative assessment of a pulse change in an input quantity.
• The Nth harmonic is the sinusoidal characteristic of the voltage characteristic obtained at the input. The calculated values ​​are obtained from tabular data for each harmonic.
• It is important to take into account the asymmetry of the input quantity in reverse or zero sequence to eliminate cases of uneven phase distribution. Such conditions occur more often when the power supply network connected according to a star or delta circuit is broken.

Types of protection against unpredictable changes in the power supply

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

• Power stabilizers guarantee that the input value is maintained within the specified limits. Quality energy is achieved even with input value deviations of more than 35%.
• The sources are designed to maintain the consumer's performance for a specified period of time. The devices are powered by the accumulated energy in their own battery. In the event of a power outage, uninterruptible power supplies are capable of maintaining the functionality of the equipment of an entire office for several hours.
• Surge protection devices operate on the relay principle. After the input value exceeds the set limit, the circuit opens.

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