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CALCULATION OF ELECTRIC ENERGY QUALITY INDICATORS
Guidelines
For independent work students
IVANOVO 2010
Compiled by: O. A. Bushueva
E. V. TYUTIKOVA
Editor M. I. Sokolov
The guidelines are intended for students of specialties 140205, 140211, and can also be useful to students of other specialties studying the discipline “Electrical supply”.
Approved by the cycle methodological commission of the EEF.
Reviewer
Department of Electrical Systems, State Educational Institution of Higher Professional Education "Ivanovo State Energy University named after. IN AND. Lenin"
CALCULATION OF ELECTRIC ENERGY QUALITY INDICATORS
Guidelines
for independent work of students
Compiled by: Bushueva Olga Aleksandrovna
Tyutikova Ekaterina Vladimirovna
Editor N. B. Mikhaleva
Signed for printing Format 60x84 1/16
Printing is flat. Conditional oven l. 2.09. Circulation 200 copies. Order
GOUVPO "Ivanovo State Energy University named after V.I. Lenin"
153003, Ivanovo, st. Rabfakovskaya, 34
Printed in UIUNL ISUE
Introduction 4
Power quality problem 4
Characteristics of the main quality indicators
Electricity 5
Calculation of voltage deviations and
Its admissibility rating is 8
Assessment of the permissibility of voltage fluctuations 10
Non-sinusoidal voltage. Calculation
Sinusoidal curve distortion coefficient
Voltage and assessment of its admissibility 15
Voltage imbalance. Calculation of coefficient
Negative sequence voltage asymmetries
And an assessment of its admissibility. 20
Tasks for independent decision 25
8. Control questions 35
Bibliography 37
Introduction
The purpose of these guidelines is to acquire the necessary theoretical knowledge and practical skills for calculating indicators of the quality of electrical energy performed when studying the topic “Quality of Electrical Energy”. This topic is studied by students of specialty 140205 in the discipline “Special Issues of Energy Systems,” and by students of specialty 140211 in the discipline “Power Supply Systems.”
The guidelines contain the necessary theoretical material and calculations that are carried out during the design and operation of electrical networks for various purposes.
The instructions will help students prepare for practical classes, as well as for the interdisciplinary final exam in their specialty.
Power quality problem
The widespread use in industry of powerful nonlinear, asymmetrical and sharply changing loads that can significantly distort the basic characteristics of electrical energy raises the problem of electromagnetic compatibility of electrical equipment and electrical networks. Electromagnetic compatibility refers to the ability of electrical energy consumers to function normally and not introduce unacceptable distortions into the electrical network that impede the work of other consumers.
With poor electromagnetic compatibility, first of all, the quality of electrical energy decreases. Power supply systems and power receivers are designed in such a way that the best functioning is achieved when they are powered from a single-phase or symmetrical three-phase system with a voltage of a given amplitude and sinusoidal shape with a frequency of 50 Hz. However, in real electrical networks, as a result of various electromagnetic interference, deviations from these ideal parameters occur, which leads to deterioration in the operation of electricity consumer installations, manifested in technical and economic damage.
Reduced quality of electricity has a negative impact on both the operation of individual electrical receivers and normal functioning energy system as a whole. When the quality of electricity in electrical networks decreases, the following occur: negative consequences:
- increase in electricity losses in all elements of the electrical network;
- overheating of rotating machines, accelerated aging of insulation, reduced service life or failure of electrical equipment;
- growth in electricity consumption and the required power of electrical equipment;
- malfunction and false alarms of relay protection and automation devices;
- interference in the operation of television and radio equipment, failures electronic systems management and computer technology;
- bad influence on the communication line and automatic blocking devices on railways Oh;
- deterioration in business performance industrial enterprises etc.
The presence of electrical connections between power systems significantly expands the area of negative influence of reduced power quality, thereby exacerbating the problem of electromagnetic compatibility. There is a need to evaluate and control the quality of electricity not only at the given point of connection of the consumer to the energy supply organization, but also at various remote points of the electrical network.
Inattention to the quality of electricity during the operation of electrical networks leads to a progressive breakdown of the power supply to consumers and disruptions in the operation of electrical receivers. Therefore, studying the issues of assessing the quality of electricity at various points of the electrical network is an important task in the training of electrical engineers.
The bulk of electrical and electronic equipment provides operation from a power source with certain characteristics that determine the minimum and maximum limits for rms voltage and frequency.
The user, in turn, hopes that the power will always be continuous and within the limits of error. The supplier does not guarantee this, and at a reasonable price this is almost impossible to achieve.
The quality of the power supply cannot be assessed in advance, because it passes through several transformers, many kilometers of power lines, and is mixed with the output of other generators.
The concept of "good power quality" can be used to describe a continuously supplied power supply within the voltage and frequency tolerances and a pure sine wave.
"Poor power quality" describes power that deviates from the norm; whether the deviation plays an important role depends on the purpose of the installation, the design of the equipment and the installation.
The most important reasons Bad quality electricity are divided into two categories described below.
- Power system quality issues
- Installation and loading issues
There are no clear boundaries between these two categories, because Interference caused to equipment at one site may cause failure or damage to equipment at another site. For example, a heavy load from arc furnaces in a factory or small household can cause a voltage dip for several neighboring users when turned on.
The result may be a complete shutdown of the computer network, causing a larger outage than the user expects.
1. Power system quality problems
Power failure
A complete power outage lasting more than a minute caused by power generation or distribution, a substation breakdown, a power line down, or load sharing during system overload. The consequence is a complete shutdown of the substation.
Examples of power outages
Temporary interruption
A power outage lasting less than a minute, usually caused by an automatic restart, restoring power supply after temporary interruptions. Computers and communications equipment shut down and data is lost. Restarting may take several minutes, and data recovery may take longer.
Transients
Sudden surges in voltage superimposed on the power supply voltage. Can be caused by several factors, including residual effects from lightning strikes, the inclusion of reactive power compensation capacitors, and the inclusion of inductive loads.
Voltage pulse oscillogram
Undervoltage or overvoltage
Long-term sharp exceeding of design parameters caused by breakdown of tap switches. When deliberately reducing the voltage to reduce the load, it can cause unstable operation of equipment, including computer reboots, failure solenoid valves and overheating of engines with a squirrel cage. Overvoltage can cause permanent damage to various electrical and electronic equipment.
Voltage sags or surges
Short-term voltage fluctuations that go beyond normal limits and are caused by turning on or off powerful loads, such as large motors. In extreme conditions, voltage dips can cause equipment to shut down, and voltage surges can cause breakdowns.
Voltage asymmetry
Asymmetry of phase voltage of three-phase power supply due to differential load of phases, which results in the appearance of circulating current (and overheating) of transformers, as well as reduced operating efficiency three-phase motors.
Flicker
Periodic fluctuations in power supply caused by changes cyclic load, for example, from the operation of a drive system with switching cycles. The result is flickering lighting.
Harmonic vibrations
Voltage variation caused by non-linear loads. The result is overheating due to increased swirl and hysteresis losses in transformers, overheating and reduced torque in motors, as well as overheating in neutral wires and capacitors for reactive power compensation.
Distorted signal depicted as a Fourier series
Some defects, such as interruptions and flickers, are immediately noticeable by the user, while others manifest themselves through their impact on equipment and substations. Equipment resilience to failures can be improved in several ways.
Although these problems are classified as power problems, they may be caused by problems at the user's site.
2. Installation and load related problems
There are three main problems with installations:
- Ground leakage currents
- Voltage sags and surges
Harmonic currents
Harmonic currents arise from the increasing prevalence of non-linear loads in use and cause problems in wiring, transformers and motors. The use of harmonic currents from the power supply introduces distortion into the voltage waveform, which, if not checked, can cause problems for other users of the power supply. Thus, a framework is established for the permitted amplitude of the fundamental harmonics.
Ground leakage currents
Ground leakage currents arise from the most modern electronic equipment.
For individual units the current is quite small, often less than 3.5mA, but as for computers, for example, the current can be quite large. In addition, there is a significant high-frequency component in the leakage current resulting from transient filtering in power units.
Basic grounding systems were designed to function as protective grounding(ie, to provide a low-impedance path for short-circuit current, to provide overcurrent protection), rather than to cope with constant leakage currents, especially at high frequency. High sensitivity to noise from modern computers and communication systems equipment imposed Additional requirements to the grounding system.
Voltage sags and surges
Basic defects from voltage deviations are attributed to power supply problems, but this is not always the cause.
Switching heavy loads, such as large motors and arc furnaces, causes voltage sags and, if the load is inductive, transient electrical overvoltages. Dips can last several seconds while the equipment picks up speed, causing problems for voltage-sensitive equipment. Such transient overvoltages can cause damage to electronic equipment and, through inductive coupling to the data line, errors in data processing in computers and communications systems.
In circumstances where power factor compensation capacitors are used, resonance with the inductive reactance of the power supply may occur, causing the capacitors to fail.
Practical solutions include separating one power system from the other and using wires with good cross-sectional area.
Certain solutions may be useful when the source of the problem is not under the client's control.
In any case, using preventive measures in practice is the best choice.
PROPERTIES OF ELECTRIC ENERGY, INDICATORS AND MOST LIKELY culprits for deterioration in the quality of electric energy
1 (Appendix A GOST).
Properties of electricity | CE indicator | The most likely culprits for the deterioration of FE |
Voltage deviation | Steady-state voltage deviation δ Uy | Energy supply organization |
Voltage fluctuations | a) Voltage change range δ Ut b) Flicker dose Pt | Consumer with variable load |
Non-sinusoidal voltage | a) Distortion factor of the sinusoidal voltage curve KU b) Coefficient n-th harmonic component of voltage КU(n) | Consumer with nonlinear load |
Unbalance of three-phase voltage system | a) Negative sequence voltage asymmetry factor K 2U b) Zero sequence voltage asymmetry factor K 0U | Consumer with asymmetric load |
Frequency deviation | Frequency deviation Δ f | Energy supply organization |
Voltage dip | Voltage dip duration Δ t P | Energy supply organization |
Voltage pulse | Pulse voltage U imp | Energy supply organization |
Temporary overvoltage | Temporary overvoltage factor K lane U | Energy supply organization |
ELECTRIC ENERGY QUALITY INDICATORS
AND THEIR INFLUENCE ON THE OPERATION OF ELECTRICAL INSTALLATIONS.
1. Steady-state voltage deviation
Steady voltage deviation: normally permissible δUy (%) ±5 maximum permissible δUy (%) ±10
Deviation of voltage from nominal values occurs due to daily, seasonal and technological changes in the consumer’s electrical load, namely: 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.
undervoltage- deterioration in starting, increase in electric motor currents, which entails heating of the windings, insulation failure and reduced motor service life;
Overload of adjustable rectifiers, converters and stabilizers;
overvoltage- excessive consumption of electricity, increased reactive power of motors, phase-controlled rectifiers, breakdown of controlled rectifiers, converters and stabilizers.
The reasons for discrepancies in the steady-state voltage deviation may be:
– incorrectly selected transformation ratio of the transformer 6–10/0.4 kV or not
timely seasonal switching of taps of these transformers;
– significant asymmetry of phase loads in 0.4 kV networks;
– significant voltage losses in the distribution network exceeding the limit values;
– absence of transformers with on-load voltage regulation (OLV) in the power center (CP);
– absence of an automatic voltage regulator (AVR) in the CPU or its non-use;
– incorrect operation of the AVR or incorrectly selected voltage regulation law in the CPU;
– heterogeneity of loads of 6–10 kV distribution lines and incompatibility of requirements
consumers of the entire distribution network on CPU buses;
– incorrect settings of control devices on generators that increase
transformers and coupling autotransformers, lack or insufficient use
special devices in intersystem lines and power supply networks of power systems that regulate
reactive power (synchronous compensators, batteries of static compensators and
shunt reactors);
– consumer exceeding the power allowed to him or violating contractual
conditions with ESO for use special means regulating reactive power
(static capacitor banks, synchronous motors);
– reduced capacity of supply networks, etc.
2. Voltage fluctuations.
Voltage fluctuations are characterized by the following indicators:
- voltage change range;
- a dose of flicker.
The maximum permissible value of the sum of voltage deviation and voltage swing in electrical networks 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 vibrations luminous flux caused by voltage fluctuations in the network over a certain period of time.
GOST establishes two flicker dose characteristics: short-term (observation time 10 minutes) and long-term (2 hours).
Voltage fluctuations are caused by a sharp change in the load on the section of the electrical network under consideration, for example, by turning on 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 (drives of reversible rolling mills, arc steel-smelting furnaces, welders etc.). The propagation of voltage fluctuations towards the power supply system occurs with attenuation of the fluctuations in amplitude. Moreover, the more powerful the power supply system, the greater the attenuation coefficient.
Compensation is carried out by using high-speed reactive power sources that can compensate for changes in reactive power. To reduce the influence of abruptly changing loads on sensitive electrical receivers, a separation method is used, in which the abruptly variable and sensitive to voltage fluctuations loads are connected to different transformers.
Electrical receivers that are extremely sensitive to voltage fluctuations include: lighting, especially incandescent lamps and electronic equipment. Voltage fluctuations cause incandescent lamps to blink (flicker effect), which creates an unpleasant psychological effect in humans, visual fatigue, decreased productivity, and injuries. If there are significant voltage fluctuations, the conditions may be violated normal operation electric motors, the contacts of magnetic starters may fall off with a corresponding shutdown of running motors, voltage phase fluctuations cause vibration of the electric motors.
3. Non-sinusoidal voltage
Non-sinusoidal voltage is characterized by the following indicators:
- distortion factor of the sinusoidal voltage curve;
- coefficientn-th harmonic component of voltage.
The main cause of distortion is the use of nonlinear electrical receivers, such as: valve converters, electric arc and steel-smelting furnaces, arc and contact welding installations, frequency converters, induction furnaces, a number of electronic technical equipment (TVs, computers), gas-discharge lamps and others. Electronic receivers and gas-discharge lamps create a low level of distortion during operation, but since there are many such electrical receivers, their overall influence is large. During operation, these devices consume fundamental frequency energy, which is spent not only on useful work and covering losses, but also for the formation of a flow of higher harmonics, which is “thrown out” into the external network.
Influence:
growth of losses in electric machines ah, vibrations, disruption of automatic protection, increased errors in measuring equipment;
Non-sinusoidal voltage fronts affect the insulation of cable power lines - single-phase short circuits to ground become more frequent. Similar to a cable, capacitors break through.
Methods for reducing non-sinusoidal voltage can be divided into three groups:
Circuit solutions: allocation of nonlinear loads to a separate bus system, grouping of valve converters according to a phase multiplication scheme, connection of a nonlinear load to a system with a higher short circuit power (Ss);
The use of equipment characterized by a reduced level of generation of higher harmonics, for example, “non-saturable” transformers and multiphase valve converters;
The use of filter devices: parallel narrow-band resonant filters, filter-compensating and filter-balancing devices (PKU and FSU).
4. Voltage asymmetry
Voltage asymmetry is characterized by the following indicators:
- negative sequence voltage asymmetry coefficient;
- zero-sequence voltage asymmetry coefficient.
Sources of voltage and current asymmetry include the following:
Non-transposed power lines and unevenly connected single-phase household loads, creating systematic voltage asymmetry;
Domestic loads switching on at different times in phases, etc., creating random voltage asymmetry.
Consumers of electrical energy whose symmetrical multiphase design is either impossible or impractical for technical and economic reasons. Such installations include induction and arc electric furnaces, electric welding units, special single-phase loads, lighting installations, etc.
Impact: additional heating of electric motors, increase in total losses, overheating of neutral conductors, possibility of fire, increase in resistance of grounding devices, increase in ripple of rectified voltages, violation of control of thyristor converters, poor-quality compensation of reactors. power by capacitor units.
Asymmetrical voltage modes in electrical networks also occur in emergency situations due to phase loss, operating zero or asymmetrical short circuits.
Unlike the direct sequence, in the reverse sequence there is a reverse alternation of phases; Accordingly, if the permissible value is exceeded, this component will prevent the motors from rotating in a given direction, reducing its efficiency. The negative sequence includes harmonics with numbers 3n+2, where n varies from 0 to 12 (for the device). At long work with a negative sequence asymmetry coefficient K2U=2-4%, the service life of the electric machine is reduced by 10-15%, and if it operates at rated load, the service life is halved.
In the zero sequence there is no phase alternation; all phases have the same initial phase. If the permissible value is exceeded, this component will create an increased current in the neutral wire. The zero sequence includes harmonics with numbers divisible by 3.
5. Frequency deviation.
Normally permissible frequency deviation Δf (Hz) ±0.2 Maximum permissible frequency deviation Δf (Hz) ±0.4
Frequency deviation is the difference between the actual and nominal frequency values:
decreased performance of electric drives, reduced service life of electrical machines, distortion of television images.
6. Voltage dip.
The characteristic of a voltage dip is its duration and depth of the dip.
The maximum permissible value for the duration of a voltage dip in electrical networks up to 20 kV inclusive is 30 seconds.
Voltage dip - a sudden drop in voltage at a point in the electrical network below 0.9 U nom , 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.
The duration of a voltage dip is the time interval between the initial moment of the voltage dip and the moment the voltage is restored to the original or close to it level.
The reason is electromagnetic transient processes during short circuits, switching electrical equipment, and a break in the neutral wire.
shutdown of equipment during failures, failure under worsening operating conditions.
7. Pulse voltage.
Voltage pulse - a sharp change in voltage at a point in the electrical network, followed by a restoration of the voltage to the original or close to it level over a period of time of up to several milliseconds;
Pulse amplitude - 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.
The magnitude of the voltage distortion is characterized by the pulse voltage indicator in volts, kilovolts and the pulse rise time of no more than 5 ms. The magnitude of the pulse voltage is not standardized by the standard, but according to statistics for lightning and switching pulses, the voltage magnitude with a duration of 0.5 amplitude (μs) can reach: in a network of 0.38 kV - 4.5 kV; in the network 6 kV - 27 kV; in the network 35 kV - 148 kV.
8. Temporary overvoltage.
Temporary overvoltage - an increase in voltage at a point in the electrical network above 1.1 U nom lasting more than 10 ms, occurring in power supply systems during switching or short circuits.
Temporary overvoltage coefficient is a value equal to the ratio of the maximum value of the amplitude envelope: voltage values during the existence of a temporary overvoltage to the amplitude of the nominal network voltage.
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MINISTRY OF EDUCATION AND SCIENCE OF RUSSIA
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higher professional education
"Samara State Technical University"
in Syzran
Department of EPP
“Causes of deterioration in power quality”
Completed:
student gr. EVB-481
Kashaeva D.V.
Checked:
senior teacher
Alekseeva I.Yu.
Introduction
1. Standardization of power quality
2. Reasons for deterioration in power quality
Bibliography
Introduction
One approach to diagnosing faults related to power quality is to check at a point that is located as close as possible to the consumer experiencing the problem. This consumer is usually an electronic device that is sensitive to power quality and experiences some problems. A possible cause is poor power quality, but part of your job is to separate that cause from others. possible reasons(hardware malfunction, software failure, etc.) Like a detective, you need to start by examining the “crime scene.” An approach like upstream checking can be time consuming. It is based on being attentive and taking measurements of key parameters.
An alternative method is to move from the building's electrical system entry to the fault point using a three-phase test instrument. This approach is most effective if the cause of the fault is in the power supply network.
However, based on numerous audits, it has been concluded that the causes of the vast majority of power quality problems are located in plants (buildings). Usually, best quality electricity is observed at the entrance to the building's electrical system (at the point of connection to the public power supply networks). As it moves through the distribution system, the quality of power gradually decreases. This is due to problems that originate from consumers located in the building. Another salient fact is that 75% of all power quality problems are related to wiring and grounding!
For this reason, many power quality agencies believe that the troubleshooting process should begin with the building's electrical system and then, if necessary, use monitoring instruments at the utility connection point. Below is a troubleshooting procedure based on a bottom-up approach to help you get the job done.
1. Standardization of power quality
Standards for electricity quality indicators are established by the current GOST 13109-97 “Electric energy quality standards in power supply systems general purpose". It establishes indicators and standards of energy efficiency 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 owned by various consumers of electrical energy or receivers of electrical energy (points of general connection) are connected.
The EEC limits established by the standard are electromagnetic compatibility levels for electromagnetic interference in general purpose power supply systems. Subject to compliance with the established EEC standards, electromagnetic compatibility of electrical networks of energy supply organizations and electrical networks of electrical energy consumers is ensured. According to the indicators regulated by this standard, electrical energy is subject to mandatory certification.
The standard establishes the following power quality indicators (PQEE):
- steady-state voltage deviation;
- voltage change range;
- flicker dose;
- distortion factor of the sinusoidal voltage curve;
- coefficient of the nth harmonic component of voltage;
- negative sequence voltage asymmetry coefficient;
- zero-sequence voltage asymmetry coefficient;
- frequency deviation;
- duration of voltage dip;
- impulse voltage;
- temporary overvoltage coefficient.
In this work, the goal was to improve the sinusoidality of the voltage, therefore, in the future, the quality of electricity is assessed by two indicators of the energy efficiency factor, characterizing the degree of deviation of the voltage shape from the sinusoid:
- distortion coefficient of the sinusoidal voltage curve;
- coefficient of the nth harmonic component of voltage.
These indicators are defined as values averaged over 3 s.
Determination of indicators characterizing the sinusoidal voltage is carried out as follows. The voltage sinusoidal distortion coefficient is determined by the formula
, (1.1)
where is the value of the nth harmonic component of the voltage; - value of the first (fundamental) voltage harmonic.
The harmonic values are normalized to. GOST 13109-97 defines that the quality of electricity in terms of the distortion coefficient of the sinusoidal voltage curve and the coefficient nth harmonic The voltage component at the point of common connection is considered to comply with the requirements of the standard if the highest of all distortion coefficient values measured within 24 hours does not exceed the maximum permissible value. Also, the value of the distortion coefficient corresponding to a probability of 95% over a specified period of time should not exceed the normally acceptable value.
In table 1.1 gives the normally permissible and maximum permissible values of the voltage sinusoidal distortion coefficient for networks of various voltage classes.
Table 1.1 Electricity quality standards based on the distortion factor of the sinusoidal voltage curve
The coefficient of the nth harmonic component of the voltage is found by the expression
. (1.2)
Normally permissible values of the coefficients of the nth harmonic component of voltage are given in table. 1.2.
Table 1.2 Normally acceptable coefficient valuesnthharmonic component of voltage
|
Odd harmonics, not multiple of 3, at, kV |
Odd harmonics, multiples of 3, at, kV |
Even harmonics, multiples of 3, at, kV |
|||||||||||||
|
0.2+ +1.3HH25/n |
0.2+ +0.8HH25/n |
0.2+ +0.6HH25/n |
0.2+ +0.2HH25/n |
The maximum permissible values of the coefficients of the harmonic components of the voltage are taken to be 1.5 times higher than the normally permissible values indicated in the table. 1.2.
Currently, there is no legitimate document establishing a methodology for calculating the permissible influence of a consumer on the EEC and a procedure for assessing compliance with established requirements. Until 2001, in Russia there were “Rules for connecting a consumer to a general purpose network under the conditions of influence on the quality of electricity”, as well as “Rules for the application of discounts and surcharges to tariffs for the quality of electricity” (approved by Glavgosenergonadzor on May 14, 1991), according to which when In case of deviation from the standard values of PKEE due to the fault of the consumer, the electricity supply organization may impose penalties in the amount of up to 10% of the tariff for consumed electrical energy for each violated indicator.
In relation to an average traction substation with a processing capacity of up to 30 million kWh per year, the surcharge for violation of standards for only one PKE could be about 1.8 million rubles in 2001 prices. in year. Such sanctions significantly affect the economic state of traction power supply systems and justify significant costs for improving the quality of electricity in their networks.
However, in 2001 the above rules were repealed as regulations, contrary to the Civil Code of the Russian Federation. Currently, the requirements for EEC for indicators characterizing the voltage form are established in the form of an obligation of the energy supply organization to maintain the values of EEC at the point of power quality control in accordance with GOST 13109-97, provided that the consumer does not exceed the permissible influence of it established in the technical specifications or in the power supply contract electrical installations to the values of PEE at this point. In other words, sanctions for violation of EEC due to the fault of the consumer must be specifically stipulated in the electricity supply contract. However, if the system of surcharges is specified in the contract, then, according to JSC Russian Railways, the damage from failure to comply with the requirements of GOST 13109-97 for only two indicators of electricity quality can annually amount to about 1.2-1.4 billion rubles. along the Russian railway network.
Let us consider in more detail the reasons for the deterioration of the sinusoidal voltage waveform in the traction network of AC railways.
2. Reasons for deterioration in power quality
In terms of voltage quality for the normal operation of electrical equipment connected to the network alternating current, the ideal is a perfectly sinusoidal shape of the supply voltage. However, in modern enterprises, loads, volt- and current-voltage characteristics, which are nonlinear in nature (nonlinear loads), have become widespread. Connecting such consumers, which contain nonlinear elements, often leads to a deviation of the voltage waveform from a sinusoid.
These consumers include various types of valve converters (mainly thyristor), arc and contact electric welding installations, gas-discharge lamps, electric arc steel-smelting and ore-smelting furnaces, power magnetic amplifiers and transformers. These loads consume current from the network, the curve of which turns out to be non-sinusoidal, and in many cases non-periodic, resulting in non-linear distortions of the voltage curve, i.e. non-sinusoidal modes.
It is worth noting that only nonlinear inertia-free resistances are generators of higher harmonics of current and voltage. Inertial elements, i.e. elements whose nonlinearity of current-voltage characteristics is due to slowly occurring processes (mainly thermal), do not introduce distortions into the sinusoidality of the voltage waveform.
The main reason for the distortion of the sinusoidal voltage waveform in AC power supply systems with an industrial frequency of 50 Hz is the presence in the system of various types of nonlinear inertia-free resistances, such as semiconductor devices, coils with ferromagnetic cores and other elements.
On railways electrified with alternating current, a special part of the nonlinear elements consists of semiconductor devices: diodes and thyristors. These elements are actively used in rectifier-inverter converters of electric locomotives, in which conversion occurs in AC traction mode contact network into rectified current to power traction motors (rectification), as well as reverse conversion in recuperation mode (inversion) of the electrical energy of traction motors, operating in this case as a generator.
The main distortion of the alternating voltage shape when supplying the consumer with a pulsating current obtained during the rectification process exists due to the occurrence of natural commutation of the current of the rectifier thyristors, which occurs at the beginning of each half-cycle of the alternating voltage. Moreover, the larger the load, the more powerful the consumer, the greater the degree of this distortion can be obtained at the point of connection to the power supply system network.
The physical essence of alternating voltage sinusoidal distortion lies in the occurrence of a short-circuit mode of the alternating current circuit (windings of the power transformer of an electric locomotive) during the current switching intervals of the thyristor arms of the rectifier, as a result of which a dip in the sinusoidal voltage curve occurs at these intervals. These dips distort the shape of the voltage curve and lead to the appearance of higher harmonic components in the frequency spectrum of the voltage. The odd (3rd, 5th, 7th and 9th) harmonics have the greatest amplitude during electric locomotive operation.
The deviation of the alternating voltage shape from a sinusoid is one of the main parameters characterizing the quality of electrical energy in a traction power supply system. The importance of this parameter is determined by the fact that voltage distortions in the contact network affect both the operational characteristics of electric locomotives and the traction power supply system. Thus, higher voltage harmonics generated by an electric locomotive lead to additional losses in the windings of the auxiliary machines of the electric locomotive. In a power transformer, voltage harmonics cause an increase in steel losses due to hysteresis, as well as an increase in copper losses in the windings. This reduces the service life of the insulation and also increases the cost of electricity for traction of trains.
The influence of non-sinusoidal voltage on induction and electronic metering devices for electricity consumed by an electric locomotive leads to a significant increase in the error in the measurement results of these devices. Harmonics can also disrupt the operation of protective relays or degrade their performance.
Increased values of the coefficient in the traction network are determined not only by the use of semiconductor devices in the power circuits of an electric locomotive, which generate harmonics in the frequency range from 150 to 1000 Hz, but also by transient processes in the “electric locomotive - contact network” system, which results in high-frequency voltage fluctuations on the pantograph of an electric locomotive with frequencies of 750-1950 Hz.
Voltage fluctuations on the current collector are caused by the transition of the electric locomotive rectifier from the conduction mode to the switching mode at the moment of supplying control pulses to the thyristors (commutation oscillations) and the reverse transition after the end of the switching process (post-commutation oscillations). Moreover, their amplitude when the electric locomotive is positioned closer to the middle of the feeder zone can be significant. The frequency of these voltage fluctuations is determined by the ratio of the inductance of the AC circuit of the electric locomotive and the capacitance of the contact network relative to the ground.
Free switching and post-commutation voltage fluctuations formed on the current collector are transformed to the secondary voltage side of the electric locomotive, where they create overvoltages on the thyristor arms of the converter. Since voltage fluctuations are repeated every half-cycle of the supply voltage, this periodicity limits the valve strength of the rectifier thyristors and, as a result, contributes to their rapid failure. In addition, these fluctuations appear in the rectified voltage curve, affecting the electromagnetic processes occurring in the rectified current circuit. electric current collector voltage
Switching and post-commutation oscillations contribute to the appearance in the frequency spectrum of the contact network voltage of harmonics corresponding to the frequencies of these oscillations. In other words, voltage fluctuations caused by the beginning and end of the process of switching the current of the thyristors of an electric locomotive reduce the quality of electricity in the contact network.
The range of questions devoted to the problem of higher harmonics in electrical networks is as follows:
- assessment of electromagnetic compatibility of sources of higher harmonics and other loads, i.e., the influence of harmonics on electrical installations;
- assessment of the resulting economic damage;
- quantitative assessment of higher current harmonics generated by various nonlinear loads;
- predicting the values of higher harmonics of current and voltage, as well as reducing the level of harmonic components.
Conclusion
Electrical energy as a commodity is used in almost all processes associated with human activity. Having specific properties, electricity is directly involved in the creation of other types of products, affecting their quality. The concept of electrical energy quality (EQE) 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 EEC must be provided.
Thus, the quality of electrical energy is determined by the totality of its characteristics, under which electrical receivers can operate normally and perform their intended functions.
Power quality is often characterized by the term “electromagnetic compatibility”. Electromagnetic compatibility is understood as the ability of electrical receivers to function normally in its electromagnetic environment, that is, in the electrical network to which it is connected, without creating unacceptable electromagnetic interference for other receivers operating in the same environment.
The problem of electromagnetic compatibility of industrial consumers with the power supply network arose in connection with the widespread use of devices that, despite their cost-effectiveness and technological efficiency, have a negative impact on the energy efficiency factor. Household consumers, like industrial ones, must also have electromagnetic compatibility with other consumers included in the general power grid, not reduce the efficiency of their operation and not deteriorate the CEE indicators.
CEE in industry is assessed according to technical and economic indicators, which take into account damage resulting from 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 electrical equipment, disruption of automation, telemechanics, communications, electronic equipment, etc.
The quality of electricity is 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 quantities previously agreed upon with the energy supply organization, and of standardized quality.
Bibliography
1. RULES FOR ELECTRICAL INSTALLATIONS (PUE) (Seventh edition, revised and expanded, as amended) 2015
2. Resources of a specialized electric power Internet site forca.ru
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In accordance with GOST 13109-87, there are main and additional indicators of power quality.
To the main indicators of power quality, which determine the properties of electrical energy that characterize its quality, include:
1) voltage deviation (δ U, %);
2) voltage change range (δ U t,%));
3) dose of voltage fluctuations (ψ,%);
4) coefficient of non-sinusoidality of the voltage curve (k nsU,%);
5) coefficient of the nth harmonic component of the voltage of odd (even) order (kU(n), %);
6) negative voltage sequence coefficient (k 2U,%);
7) zero sequence voltage coefficient (k 0U, %);
8) duration of voltage dip (Δ t pr, s);
9) pulse voltage (U imp, V, kV);
10) frequency deviation (Δ f, Hz).
Additional power quality indicators, which are forms of recording the main indicators of power quality and are used in other regulatory and technical documents:
1) voltage amplitude modulation coefficient (k modes);
2) unbalance coefficient of interphase voltages (k sky.m);
3) phase voltage unbalance coefficient (k neb.f).
Let us note the acceptable values of the above-mentioned power quality indicators, expressions for their determination and areas of application. During 95% of the day (22.8 hours), power quality indicators should not go beyond the normal permissible values, and during the entire time, including emergency modes, they should be within the maximum permissible values.
Electric power quality control at characteristic points of electrical networks is carried out by the personnel of the electrical network enterprise. In this case, the duration of measuring the power quality indicator should be at least 24 hours.
Voltage deviations
Voltage deviation is one of the most important indicators quality of electricity. The voltage deviation is found by the formula
δ U t = ((U(t) - Un)/Un) x 100%
Where U(t) - the effective value of the direct sequence voltage of the fundamental frequency, or simply the effective value of the voltage (with a non-sinusoidal coefficient less than or equal to 5%), at time t, kV; Un - rated voltage, kV.
Magnitude U t = 1/3 (U AB(1) + U BC(1) + U AC(1)), where U AB(1), U BC(1), U AC(1) are the effective values of the main phase-to-phase voltages frequencies.
Due to changes in loads over time, changes in voltage levels and other factors, the magnitude of the voltage drop in network elements and, consequently, the voltage level U t changes. As a result, it turns out that at different points of the network at the same time, and at one point at different times, voltage deviations are different.
Normal operation of electrical receivers with voltages up to 1 kV is ensured provided that the voltage deviations at their input are equal to ±5% (normal value) and ±10% (maximum value). In networks with a voltage of 6 - 20 kV, the maximum voltage deviation is set to ±10%.
The power consumed by incandescent lamps is directly proportional to the supplied voltage to the power of 1.58, the luminous efficiency of the lamps is to the power of 2.0, the luminous flux is to the power of 3.61, and the service life of the lamps is to the power of 13.57. The operation of fluorescent lamps depends less on voltage deviations. So their service life changes by 4% with a voltage deviation of 1%.
A decrease in illumination of workplaces occurs with a decrease in voltage, which leads to a decrease in the productivity of workers and a deterioration in their vision. With large voltage drops, fluorescent lamps do not light up or blink, which leads to a reduction in their service life. As the voltage increases, the service life of incandescent lamps decreases sharply.
The rotation speed of asynchronous electric motors and, consequently, their performance, as well as the reactive power consumption, depend on the voltage level. The latter is reflected in the magnitude of voltage and power losses in sections of the network.
A decrease in voltage leads to an increase in the duration of the technological process in electrothermal and electrolysis installations, as well as to the impossibility of stable reception of television broadcasts in public networks. In the latter case, so-called voltage stabilizers are used, which themselves consume significant reactive power and have power losses in the steel. Scarce transformer steel is consumed in their production.
To ensure the required bus voltage low voltage all TPs use the so-called counter voltage regulation at the nutrition center. Here, in maximum load mode, the maximum permissible voltage on the CPU buses is maintained, and in minimum load mode, the minimum voltage is maintained.
In this case, the so-called local voltage regulation at each transformer point by setting the distribution transformer tap switch to the appropriate position. In combination with centralized (in the CPU) and specified local voltage regulation, regulated and unregulated capacitor units are used, also related to local voltage regulation.
Voltage range
The voltage change range is the difference between the amplitude or effective voltage values before and after a single voltage change and is determined by the formula
δ Ut = ((U i - U i+1)/√2U n) x 100%
Where U i and U i+1- values of successive extrema or extremum and horizontal section of the envelope of amplitude voltage values.
Voltage change ranges include single voltage changes of any shape with a repetition rate from twice per minute (1/30 Hz) to once per hour, having an average voltage change rate of more than 0.1% per second (for incandescent lamps) and 0. 2% per second for other receivers.
Rapid changes in voltage are caused by the shock operation of the engines of metallurgical rolling mills, traction installations of railways, meadow steel-smelting furnaces, welding equipment, as well as frequent starts of powerful short-circuited asynchronous electric motors, when their starting reactive power is several percent of the short-circuit power.
The number of voltage changes per unit time, i.e. the frequency of voltage changes, is found by the formula F = m/T, where m is the number of voltage changes during time T, T is the total time of observation of voltage swings.
The basic requirements for voltage fluctuations are determined by considerations of protecting human vision. It has been established that the greatest sensitivity of the eye to light flickering is in the frequency region of 8.7 Hz. Therefore, for incandescent lamps that provide working lighting with significant visual strain, the voltage swing is allowed no more than 0.3%, for pumping lamps in everyday life - 0.4%, for fluorescent lamps and other electrical receivers - 0.6.
The permissible vibration ranges are shown in Fig. 1.
Rice. 1. Permissible ranges of voltage fluctuations: 1 - working lighting with incandescent lamps with high visual strain, 2 - household incandescent lamps, 3 - fluorescent lamps
Region I corresponds to the operation of pumps and household appliances, II - cranes, lifts, III - arc furnaces, manual resistance welding, IV - work piston compressors and automatic contact welding.
To reduce the magnitude of voltage changes in the lighting network, separate power supply of the lighting network receivers and the power load from different power transformers, longitudinal capacitive compensation of the supply network, as well as synchronous electric motors and artificial sources of reactive power (reactors or capacitor banks, the current of which is formed using controlled valves) are used to obtain the required reactive power).
Dose of voltage fluctuations
The dose of voltage fluctuations is identical to the range of voltage changes and is introduced into existing electrical networks as they are equipped with appropriate devices. When using the indicator “dose of voltage fluctuations,” an assessment of the admissibility of the range of voltage changes may not be made, since the indicators under consideration are interchangeable.
The dose of voltage fluctuations is also an integral characteristic of voltage fluctuations that cause irritation in a person that accumulates over a set period of time due to flashing lights in the frequency range from 0.5 to 0.25 Hz.
The permissible maximum dose value of voltage fluctuations (ψ, (%) 2) in the electrical network to which lighting installations are connected should not exceed: 0.018 - with incandescent lamps in rooms where significant visual strain is required; 0.034 - with incandescent lamps in all other rooms; 0.079 - with fluorescent lamps.
Voltage curve non-sinusoidal coefficient
When working in a network of powerful rectifier and converter units, as well as arc furnaces and welding units, i.e., nonlinear elements, distortion of the current and voltage curves occurs. Non-sinusoidal current and voltage curves represent harmonic oscillations having different frequencies (industrial frequency is the lowest harmonic, all others in relation to it are higher harmonics).
Higher harmonics in the power supply system cause additional energy losses, shorten the service life of cosine capacitor banks, electric motors and transformers, lead to difficulties in setting up relay protection and signaling, as well as operating electric drives with thyristor control, etc.
where N is the order of the last of the harmonic components taken into account, Un is the effective value of the nth (n = 2, ... N) harmonic component of the voltage, kV.
Normal and maximum permissible values of k nU should not respectively exceed: in an electrical network with voltage up to 1 kV - 5 and 10%, in an electrical network of 6 - 20 kV - 4 and 8%, in an electrical network of 35 kV - 3 and 6%, in electric network 110 kV and above 2 and 4%.
To reduce higher harmonics, power filters are used, which are a series connection of inductive and capacitive reactances tuned to resonance at a specific harmonic. In order to eliminate low-frequency harmonics, converter units with a large number of phases are used.
Coefficient of the nth harmonic component of the voltage of odd (even) order
The coefficient of the nth harmonic component of the voltage of the odd (even) order is the ratio of the effective value of the nth harmonic component of the voltage to the effective value of the voltage of the fundamental frequency, i.e. kU(n) = (Un /U n) x 100%
The value of the coefficient kU(n) determines the spectrum of n harmonic components, for the suppression of which the corresponding power filters must be designed.
Normal and maximum permissible values should not respectively exceed: in an electrical network with voltage up to 1 kV - 3 and 6%, in an electrical network of 6 - 20 kV 2.5 and 5%, in an electrical network of 35 kV - 2 and 4%, in an electrical network networks 110 kV and above 1 and 2%.
Voltage asymmetry
Voltage asymmetry occurs due to the load of single-phase electrical receivers. Since distribution networks with voltages above 1 kV operate with an isolated or compensated neutral, this is caused by the appearance of a negative sequence voltage. Asymmetry manifests itself in the form of inequality and is characterized by negative voltage sequence factor:
k 2U = (U 2(1) /U n) x 100%,
Where U 2(1) - effective value of the negative sequence voltage of the fundamental frequency of a three-phase voltage system, kV. The value of U 2(1) can be obtained by measuring three voltages of the fundamental frequency, i.e. U A (1) , U B (1) , U C (1) . Then
Where y A, y B and y C - conductivity of phases A, B and C of the receiver.
In networks with voltages above 1 kV, voltage asymmetry manifests itself mainly due to single-phase electrothermal installations (indirect arc furnaces, resistance furnaces, induction channel furnaces, electroslag remelting installations, etc.
The presence of negative sequence voltage leads to additional heating of the excitation windings of synchronous generators and an increase in their vibration, to additional heating of electric motors and a sharp reduction in the service life of their insulation, a decrease in reactive power generated by power capacitors, additional heating of lines and transformers? increase in the number of false alarms of relay protection, etc.
At the terminals of a symmetrical electrical receiver, the normally permissible asymmetry coefficient is 2%, and the maximum permissible is 4%.
The influence of asymmetry is significantly reduced when single-phase electrical receivers are powered from separate transformers, as well as when using controlled and uncontrolled baluns that compensate for the equivalent negative sequence current consumed by single-phase loads.
In four-wire networks with voltages up to 1 kV, the asymmetry caused by single-phase receivers connected to phase voltages is accompanied by the passage of current in the neutral wire and, consequently, the appearance of a zero-sequence voltage.
Zero sequence voltage factor k 0U = (U 0(1) /U n.f.) x 100%,
Where U0(1) - effective value of the zero-sequence voltage of the fundamental frequency, kV; U n.f. - rated value of phase voltage, kV.
Magnitude U 0(1) is determined by measuring three phase voltages of the fundamental frequency, i.e.
where y A, y B, y C, y O - conductivity of phases A, B, C of the receiver and conductivity of the neutral wire; U A (1), U B(1), U C(1) - effective values of phase voltages.
The permissible value of U 0 (1) is limited by the requirements for voltage deviation, which are satisfied by a zero sequence coefficient equal to 2% as the normal level and 4% as the maximum level.
A reduction in value can be achieved by rational distribution of single-phase load between phases, as well as by increasing the cross-section of the neutral wire to the cross-section of the phase wires and using transformers in the distribution network with a star-zigzag connection group.
Voltage dip and intensity of voltage dips
Voltage dip- this is a sudden significant decrease in voltage at a point in the electrical network, followed by a restoration of the voltage to the original or close to it level after a period of time from several periods to several tens of seconds.
Voltage dip durationΔ t pr - the time interval between the initial moment of voltage failure and the moment of voltage restoration to the original or close to it level (Fig. 2), i.e. Δ t pr = t sun - t start
Rice. 2. Duration and depth of voltage dip
Δ value t pr ranges from several periods to several tens of seconds. The voltage dip is characterized by the intensity and depth of the dip δ Upr, which is the difference between the nominal voltage value and the minimum effective voltage value Umin during the voltage dip, and is expressed as a percentage of the nominal voltage value or in absolute units.
Magnitude δ Upr is determined in the following way:
δUpr = ((Un - Umin ) / Un) x 100% or δUpr = Un - Umin
Voltage sags intensity m * represents the frequency of occurrence of voltage dips of a certain depth and duration in the network, i.e. m * = (m(δUpr, Δ t pr)/ M) x 100%, where m(δUpr, Δ t etc)- number of voltage dips with depth δUpr and duration Δt etc for time T; M is the total number of voltage dips during time T.
Some types of electrical receivers (computers) are sensitive to voltage dips, which in most cases occur during short circuits in the network, therefore, power supply projects for such receivers should include measures to reduce the duration, intensity and depth of voltage dips. GOST does not indicate acceptable values for the duration of voltage dips.
This is a sudden change in voltage followed by a recovery of the voltage to normal levels over a period of time ranging from a few microseconds to 10 milliseconds. It represents the maximum instantaneous value of the pulse voltage U imp (Fig. 3).
Rice. 3. Pulse voltage
The pulse voltage is characterized by the pulse amplitude U" pulse, which is the difference between the voltage pulse and the instantaneous voltage value of the fundamental frequency corresponding to the moment the pulse begins. Pulse duration t pulse is the time interval between the initial moment of the voltage pulse and the moment the instantaneous voltage value is restored to the normal level. Can The pulse duration t imp0.5 can be calculated based on the 0.5 level of its amplitude (see Fig. 3).
Pulse voltage is determined in relative units using the formula Δ U imp = U imp/(√2U n)
Electrical receivers such as computers, power electronics, etc. are also sensitive to voltage pulses. Pulse voltages appear as a result of switching in the electrical network. Measures to reduce surge voltages should be taken into account when developing specific power supply projects. GOST does not indicate permissible values of pulse voltages.
Frequency deviations
Frequency changes are caused by changes in the total load and the characteristics of the turbine speed controllers. Large frequency deviations result from slow, regular load changes with insufficient active power reserves.
Voltage frequency, unlike other phenomena that worsen the quality of electricity, is a system-wide parameter: all generators connected to one system generate electricity at a voltage of the same frequency - 50 Hz.
According to Kirchhoff's first law, there is always a strict balance between power generation and power generation. Therefore, any change in load power causes a change in frequency, which leads to a change in the active power output of generators, for which purpose the turbine-generator units are equipped with devices that allow regulating the flow of energy into the turbine depending on frequency changes in the electrical system.
With a certain increase in load, it turns out that the power of the turbine-generator units is exhausted. If the load continues to increase, balance is established at a lower frequency - a frequency deviation occurs. In this case, there is a shortage of active power to maintain the rated frequency.
The frequency deviation Δ f from the nominal value f n is determined by the formula Δ f = f - f n, where f is the current frequency value in the system.
Frequency changes exceeding 0.2 Hz significantly affect the technical and economic performance of electrical receivers, therefore the normally permissible value of frequency deviation is ±0.2 Hz, and the maximum permissible value of frequency deviation is ±0.4 Hz. In post-emergency modes, a frequency deviation from +0.5 Hz to - 1 Hz is allowed for no more than 90 hours per year.
Deviation of frequency from the nominal one leads to an increase in energy losses in the network, as well as to a decrease in the productivity of process equipment.
Voltage amplitude modulation coefficient and unbalance coefficient of phase-to-phase and phase-to-phase voltages
Voltage amplitude modulation ratio characterizes voltage fluctuations and is equal to the ratio of the half-difference of the largest and smallest amplitudes of the modulated voltage, taken over a certain time interval, to the nominal or base value voltage, i.e.
k mod = (U nb - U nm)/(2√2 U n),
Where U nb and U nm are the largest and smallest amplitudes of the modulated voltage, respectively.
Phase-to-phase voltage unbalance factor k neb.mf characterizes the unbalance of phase-to-phase voltages and is equal to the ratio of the range of unbalance of phase-to-phase voltages to the nominal voltage value:
k sky.mf = ((U nb - U nm)/U n) x 100%
Where U nb and U nm are the highest and lowest effective values of the three phase-to-phase voltages.
The phase voltage unbalance factor k neb.f characterizes the asymmetry of the phase voltages and is equal to the ratio of the phase voltage unbalance range to the rated value of the phase voltage:
k neb.f = ((U nb.f - U nm.f)/U n.f) x 100%,
Where U nb and U nm are the highest and lowest effective values of the three phase voltages, U n.f is the nominal value of the phase voltage.