Anton Dyachkov: “The more complex, the more interesting. There are never too many projects
Electrical energy is one of the most common commodities in the buying and selling processes. At the same time, electrical energy has special properties:
Coincidence in time of the processes of production, transmission, distribution and consumption;
Dependence of quality characteristics electrical energy not only from production, transmission and distribution processes, but also from consumption processes.
That is, electricity is one of the few goods whose quality can directly depend on the consumer. However, electricity as a product is subject to the relevant requirements of the Civil Code of the Russian Federation, the Federal Law “On the Protection of Consumer Rights”, etc. Electric energy quality standards are determined by the interstate standard, governing documents, although a number of properties of electrical energy can directly create threats to the safety of life, health, and people (Table 4.1). Therefore, it is advisable to regulate power quality standards by special technical regulations at the level of federal law.
Table 4.1.
Consumer damage in case of violation of power quality standards
| Properties of electricity | Type of damage |
| Frequency deviation | Underproduction and defective products |
| Voltage deviation | Underproduction and defective products, reduced service life of electrical equipment, additional losses of power and energy |
| Voltage dip | Failure of electronic equipment, product defects, threat to human life safety |
| Voltage pulse | Equipment failure, threat to life safety and human health |
| Temporary overvoltage | Equipment failure |
| Unbalance of a three-phase voltage system in a 4-wire network - in a 3-wire network | Additional losses of power and energy, inability to use equipment. Additional power and energy losses, reduced service life and equipment failure |
| Non-sinusoidal voltage | Additional power and energy losses, reduced service life of electrical equipment, operational failure and equipment failure |
| Voltage fluctuations | Adverse effects on human vision, operational failure and equipment failure |
There are other reasons for the increased status of power quality standards. Some of them:
Electricity quality standards are mandatory for compliance in all operating modes of general purpose power supply systems, with the exception of modes caused by force majeure.
GOST 13109-97 standards are subject to inclusion in the technical conditions (TU) for connection and in energy supply contracts.
Requirements for the quality of electricity in technical specifications and energy supply contracts for consumers who are the source of deterioration in the quality of electricity may be more stringent than the standards of GOST 13109-97.
Electric power quality standards must be applied when designing and operating electrical networks, establishing levels of noise immunity and noise emission of technical equipment.
The power quality standards established by GOST 13109-97 are mandatory for power supply systems for electricity consumers if there are no industry regulations for these systems.
4.2. The influence of power quality on the operation of consumers, energy and resource costs
In practice, deviations in the parameters of electrical energy supplied to consumers from the required standardized values are observed. These deviations negatively affect the work of consumers and lead to unproductive losses of energy and material resources. The reasons for deterioration in power quality may be:
short circuits in the distribution network;
accidents in the electrical network;
uneven distribution of consumer load across individual phases;
activation of protective equipment and automation;
electromagnetic and network disturbances (transient processes) associated with the switching on, switching off and operation of powerful electricity consumers, etc.
Indicators of the quality of electrical energy are associated with changes in voltage, as well as with the conditions for providing loads in three-phase network and must comply with the requirements of GOST 13109-97 (2002).
Let's consider the impact of some quality indicators on the work of consumers.
Voltage deviation from the nominal value. Voltage deviations from the nominal value occur due to daily, seasonal and technological changes in the electrical load of consumers, changes in the power of compensating devices, voltage regulation at the terminals of power plant generators and transformers at power system substations, as well as changes in the circuits and parameters of electrical networks.
In accordance with GOST 13109-97 (2002), normal and maximum permissible voltage deviations are established at the terminals of electrical energy receivers, which amount to ±5 and ±10% of the nominal voltage value.
First of all, consumers are affected by a steady voltage deviation. When the voltage decreases relative to its rated value, a decrease occurs luminous flux from incandescent lamps, the illumination in the room and in the workplace is reduced. Thus, a decrease in voltage by 10% leads to a decrease in illumination work surface on average by 40%, which causes a decrease in labor productivity and increased staff fatigue. Increasing the voltage for incandescent lamps by 10% also leads to a reduction in their service life and causes excessive illumination of work surfaces, which adversely affects the perception of information from monitors and digital devices. Gas-discharge fluorescent lamps within the specified range of voltage changes do not change the light output so significantly, but an increase in voltage by 10-15% leads to a sharp decrease in their service life, and a decrease in voltage by 20% causes lamp ignition failures.
Deviation of voltage from the nominal value leads to a change in the technical parameters of the electric drive. Input voltage reduction asynchronous motors promotes change in such mechanical characteristics, as electromagnetic torque, rotation frequency (slip). At the same time, the performance of the mechanism decreases, and when the voltage drops to a level where the mechanical torque on the motor shaft exceeds the electromagnetic torque, starting the engine becomes impossible. It has been established that when the voltage decreases by 15% of the nominal value, the electromagnetic torque of an asynchronous motor decreases to 72%, and in the event of voltage dips, the motor can stop altogether. When the voltage at the input of the electric motor decreases with the same power consumption, the current consumption increases and additional heating of the motor windings occurs, which leads to a reduction in its service life. When the engine operates at a voltage of 0.9 rated value, its service life is reduced by almost half.
An increase in voltage at the electric motor input causes an increase in reactive power consumption. On average, for every percentage increase in voltage, reactive power consumption increases by 3% for motors with a power of 20-100 kW and by 5-7% for motors of lower power.
The use of electrical energy in electrothermal installations with voltage deviations changes the technological process and the cost of manufactured products. Heat generation in electrothermal systems is proportional to the applied voltage to the second power, so with a voltage deviation of even 5%, performance can change by 10-20%.
The operation of electrolysis plants at reduced voltage is associated with a decrease in their productivity, additional consumption of electrode systems, an increase in specific energy consumption and the cost of products obtained during the electrolysis process.
A decrease in voltage by 5% of the nominal value leads, for example, to an 8% reduction in output in the production of chlorine and caustic soda. Voltage increase more than 1.05 U nom causes unacceptable overheating of the electrolyzer baths.
Voltage fluctuations. Voltage fluctuations occur due to a sharp variable change in load on a section of the electrical network, for example, due to the inclusion of an asynchronous motor with a high starting current ratio, technological installations with a rapidly alternating operating mode, accompanied by surges in active and reactive power, such as the drive of reversible rolling mills, arc steel-smelting furnaces, welders and so on.
Voltage fluctuations are often reflected in light sources. The human eye begins to perceive fluctuations in light output caused by voltage fluctuations. Fluctuations in network voltage negatively affect the visual perception of objects, graphic and text information. In this case, the occurrence of flicker effects (light flickering) depends on the limits of voltage change and oscillation frequency, which is associated with deterioration of working conditions, decreased productivity and fatigue of workers.
Voltage fluctuations negatively affect the operation of high-frequency converters, synchronous motors, and the quality of operation of induction heating devices. When network voltage changes, defective products can be produced in the textile and paper industries. Fluctuations in the frequency of the motors of winding and broaching devices lead to breaks of threads and paper, and to the production of products of different thicknesses.
Voltage fluctuations can cause protective and automatic control systems to malfunction. When the voltage changes and fluctuates over 15%, magnetic starters can be turned off.
Deviation of the AC voltage frequency from the nominal value. One of the most important parameters electrical system providing generation and consumption of electricity alternating current, is the stability of the network frequency. The frequency of alternating voltage in the electrical system is determined by the rotation speed of generators in power plants. If there is no balance in the production and consumption of electricity, the generators begin to rotate at a different frequency, which is reflected in the network frequency. Thus, the network frequency deviation is a system-wide indicator characterizing the power balance in the system. To compensate for changes in frequency and voltage at network nodes, the system must have a reserve of active and reactive powers, as well as control devices that allow maintaining deviations of operating parameters within the normalized values. Deviations in network frequency often serve as a signal to increase electricity production by generating stations and to shed part of the load during overloads and in case of accidents with short circuits in the system. Frequency normalization can be achieved as a result of strict adherence to the balance of generated and consumed power, excluding emergency situations and unauthorized switching at power plants and substations.
When the frequency changes, the power of metal-cutting machines, fans, centrifugal pumps. Reducing the frequency often leads to changes in equipment performance, and often to a deterioration in the quality of products.
Voltage asymmetry in a three-phase system with uneven load distribution across phases. Voltage asymmetry is caused by the presence of powerful single-phase loads, uneven load distribution between phases, and a break in one of the phase wires.
Uneven values of voltage and current in phases usually indicate an uneven distribution of consumer loads across individual phases.
Asymmetrical values of phase voltages lead to additional losses in electrical networks. At the same time, the service life of asynchronous motors is significantly reduced due to additional thermal heating, in this case it is advisable to choose engines with a higher rated power than the required one.
The asymmetry of phase voltages in AC electrical machines is equivalent to the appearance of magnetic fields, the magnetic induction vectors of which rotate in the opposite direction with double the synchronous frequency, which can disrupt technological processes.
If the voltage of the network through which synchronous motors are powered is unbalanced, dangerous vibrations may additionally occur. With significant asymmetry of the phase voltage, vibrations can be so significant that there is a danger of destruction of the foundations on which the motors are installed and damage to welded joints.
Phase voltage asymmetry has a noticeable impact on the operation of power transformers, causing a reduction in their service life. An analysis of the operation of three-phase power transformers showed that at a rated load and a current asymmetry coefficient of 10%, the service life of transformer insulation is reduced by 16%.
Non-sinusoidal voltage curve under nonlinear load. The non-sinusoidality of the voltage curve is equivalent to the occurrence of higher harmonic components in the supply voltage. Most often, the appearance of higher harmonics is associated with the connection of equipment with a nonlinear dependence of the load resistance. Such equipment includes converting devices (rectifiers, converters, stabilizers), gas-discharge devices (fluorescent lamps), installations with current interruption in the technological process (electric welding, arc furnaces, etc.).
The non-sinusoidal voltage curve affects all consumer groups. This is caused by additional heating of the elements of electrical receivers from higher harmonics. Higher harmonics cause additional power losses in motors, transformers, as well as heat losses in insulation, power cables and systems that use electrical capacitors, worsen the operating conditions of capacitor banks of reactive power compensation devices. With a non-sinusoidal voltage curve, accelerated aging of the insulation of electrical machines, transformers, capacitors and cables occurs as a result of irreversible physical and chemical processes occurring under the influence of high-frequency fields, increased heating of the current-carrying parts of the cores and insulation.
Thus, a decrease in the quality of electricity leads to deterioration of working conditions, a decrease in production volumes, loss of resources due to deterioration in product quality and a decrease in the service life of equipment, as well as additional costs of electrical energy.
Power quality indicators can be determined using special devices. As a result of analyzing the readings of these devices, in some cases it is possible to identify the culprits for the deterioration in the quality of electricity, which may be the energy supply organization, consumers with variable, nonlinear or asymmetric load.
Currently, there are devices to improve the quality of electricity. It is possible to reduce the influence of higher harmonics on the supply voltage using special active filters that suppress higher harmonics. To distribute the load evenly, balancing devices are used, which include capacitive and inductive elements.
4.3. Checking the quality of power installations
As shown above, the condition of the power plant elements and power supply systems often depends on the quality of operation. industrial production, and the quality of life of the population. The quality of energy supply directly affects the efficiency, reliability and safety of energy consumers.
The task of energy quality audit– obtain evidence of the actual values of output parameters (consumer properties) of a power plant, energy carrier, energy equipment and check the compliance of these parameters with the reasonable needs of industrial and household consumers, design and technical documentation, established standards and regulations, as well as the current level of technological development.
Basic information about technical specifications electrical equipment is contained in their technical data sheets. In addition, standards require equipment manufacturers to apply nominal operating parameters to its surface.
The performance characteristics of equipment required by consumers can usually be gleaned from the design and operational documentation for the facility in which the equipment is installed.
The same applies to energy supply systems in general, for which there should also be a specialized document: power supply diagram.
Unfortunately, it often happens that it is not possible to find the necessary documentation, the equipment markings are painted over, and the requirements on the basis of which the power plant design was developed do not correspond to modern ones.
The quality of the energy carrier is fixed in energy supply contracts and, as a rule, must be confirmed by a certificate or guaranteed by the supplier.
However, both in our country are still in the initial stages of development, and in contractual practice it is customary to limit ourselves to indicating only the energy characteristics of the energy carrier.
Therefore, today one of the main sources of audit evidence for quality characteristics operation of power plants are logs of operational accounting and control measurements performed by the auditor himself.
Let's look at the features of an energy quality audit using the example of power supply systems.
Quality of electrical energy, as is known, is determined by its suitability for ensuring normal functioning technical means (electrical, electronic, radio-electronic and others) of consumers of electrical energy.
Let us emphasize once again that the peculiarity of electrical energy as a product, in particular, lies in the continuity and simultaneity of the processes of production and consumption, as a result of which a distorting effect on the quality of energy can be exerted both by the consumer’s electrical receivers and introduced from the outside in the form of constructive electromagnetic interference propagated via the general electrical network. At the same time, sources of distortion in the quality of electrical energy can be both one’s own power receivers and power receivers of other consumers, as well as electrical equipment of power stations and networks. In terms of terms and definitions of electrical energy quality parameters, the energy auditor should be guided by GOST 23875-88.
The quality of electrical energy (QE) has a significant impact on the reliability and efficiency of electrical equipment. Deterioration of CE can lead to property damage for consumers (failure electrical equipment), disruption of the operation of automation devices, telemechanics, communications, electronic equipment, increased power losses, unregulated changes in the technological process, decreased quality of products, labor productivity, etc. In some cases, CE can affect the safety of life and health of people.
Often, due to unsatisfactory CE, investment in modern technologies And industrial equipment, demanding on power supply parameters.
In many respects, the current situation with CE in electric networks is explained by the fact that for a long time the Russian electric power industry developed along an extensive path. First of all, the tasks of providing electricity to the growing needs of industry, agriculture and public services in the country, increasing the reliability of power supply, etc. were solved.
At this stage of development of the electric power industry, the provision of energy supply supplied to consumers was not considered by energy supply organizations as one of the main tasks in relations with them.
In this regard, energy supply organizations did not pay due attention to the creation of a power supply management system sold to consumers, including the creation of an organizational structure, the development of internal documents, the organization of a system for monitoring and analysis of energy efficiency, etc. Energy supply issues were not addressed in energy supply contracts and technical specifications for connection of consumers.
Currently, the demand for CE audit is constantly increasing. Electricity consumers, both legal and individuals, do not want to put up with a situation where energy supply organizations do not ensure the quality of supplied energy.
In this regard, the task of an energy quality audit is not only to establish the degree of compliance of the parameters of the energy carrier or energy equipment with the established requirements, but also to develop a set of measures to ensure the stability of maintaining the required quality indicators and their protection from possible distortion.
A qualified audit of the electrical energy quality management system will allow energy supply organizations to improve the quality of supplied energy, reduce losses from claims from consumers, increase the reliability of power supply and the stability of revenue.
The quality system of an energy supplying organization is understood as the totality of the organizational structure, methods, processes and resources of an energy supplying organization, which is necessary for the administrative management of ensuring the quality of supplied electrical energy.
Audits are carried out by monitoring the production of electrical energy and/or the quality system, as well as examining protocols for periodic or continuous monitoring of CE.
Electrical energy quality control involves assessing the compliance of indicators with established standards and identifying the party responsible for the deterioration of these indicators.
Standards for the quality of electrical energy in general-purpose power supply systems are established for the following CE indicators:
Frequency deviation;
Steady-state voltage deviation;
Distortion factor of the sinusoidal voltage waveform;
Coefficient of the nth harmonic component of voltage;
Negative sequence voltage asymmetry factor;
Zero sequence voltage asymmetry factor.
The first two indicators are the most critical for electricity consumers, therefore, taking into account only these two indicators, the most widespread procedure for mandatory certification of electrical energy has been established.
Determining the quality indicators of electrical energy is a non-trivial task.
Most processes in electrical networks are fast-flowing, all standardized indicators of the quality of electrical energy cannot be directly measured at once - they must be calculated, and the final conclusion can only be given by statistically processed results.
Therefore, to determine FE indicators, it is necessary to perform a large volume of measurements at high speed and simultaneous mathematical and statistical processing of the values of these parameters. Moreover, the largest flow of measurements is necessary to determine the non-sinusoidal voltage. To determine all harmonics up to the 40th inclusive and within permissible errors, it is necessary to measure the instantaneous values of three phase-to-phase voltages 256 times per period (3·256·50=38400 per second). And to determine the guilty party, the instantaneous values of phase currents and the phase shift between voltage and current are simultaneously measured; only in this case is it possible to determine from which side and what magnitude this or that interference was introduced. The most complex mathematics is involved in estimating voltage fluctuations. GOST 13109-97 normalizes these phenomena for a meander (rectangular) shape envelope, and in the network voltage fluctuations are random.
Here it is necessary to point out the most common reasons that worsen CE indicators:
Distance of the consumer from the food center;
Small cross-section of wires in high-voltage external networks through which electricity is supplied to the consumer;
Poor quality electrical connections in the consumer’s internal network;
Exceeding by consumers the power of electrical receivers agreed with the power supply organization;
Unauthorized connection of subscribers not registered with the power supply organization;
The use by consumers of electricity receivers with sharply variable loads and switching power supplies;
Transient processes in electrical networks due to short circuits, lightning strikes on network elements, actions of relay protection and automation systems, switching of various electrical equipment, breaks of the neutral wire in 0.4 kV networks;
Erroneous actions of personnel and false alarms of protective equipment and automation;
Lack or insufficiency of centralized voltage regulation and reactive power compensation means.
When expressing an opinion on ways to improve CE, it is advisable for the auditor to consider the effectiveness of the following technical measures:
1. carrying out a phased reconstruction in the most remote sections of the 6-10/0.4 kV distribution network, where the voltage level is unacceptably low;
2. increasing the cross-section of power lines;
3. connection to a more powerful energy supply system;
4. organization of work to identify subscribers who have unauthorizedly connected to the power grid;
5. periodic rephasing of loads;
6. power supply of powerful distorting loads from a separate bus system;
7. implementation of automated systems for commercial electricity metering with control of energy efficiency or automated control systems of energy efficiency;
8. performing seasonal switching of consumers at transformer substations;
9. Application of VFDs or devices soft start electrical receivers with high starting currents;
10.use of capacitor units to compensate reactive power in the distribution network;
In addition, it is important to express an opinion on energy supply contracts regarding a clear distribution of responsibilities of the parties for unacceptable deviations of indicators from established standards.
Note: Impact issues on various components environment and applicability, as well as economic aspects are discussed in section 3.6.7
The problem of ensuring the quality of electrical energy (EQ) in Russian electrical power systems has always received great attention. Many methods have been developed for drawing up general equivalent circuits for power supply systems with non-sinusoidal and asymmetrical loads, taking into account the mutual influence of energy consumers.
Currently, there is no practical solution to this problem due to the lack of control levers at the legislative level. Until now, the country has not approved regulations on the quality of electrical energy. Certification of electrical energy in Russia according to two indicators (steady voltage deviation and frequency deviation) is not able to solve the problem of ensuring quality in power supply networks, even to a small extent. In many ways, this is a forced and costly measure for network organizations, and non-payments by subscribers further complicate this task.
At the same time, it is now possible to take a significant step towards ensuring the required level of CE of network power systems (SES), while spending insignificant amounts of money on the part of network companies. We are talking about a gradual transition to the principles of economic interest of all parties in providing the required energy efficiency, which is determined by the degree of distortion of the voltage of the electrical network due to interference introduced by both the energy supply organization and consumers.
The key points here are:
Practical introduction of contractual obligations on the division of mutual responsibility for energy efficiency between electricity suppliers and consumers;
Development of a system of measures of economic incentives or punishment depending on the impact of the SES subject on the CE in the network;
Development of technical measuring instruments and their serial production, which will allow instrumental implementation of the adopted economic measures;
- introduction of mandatory certification of all newly connected and reconstructed consumers and power plants according to the permissible contribution (emission) to voltage distortion.
To ensure the quality of electricity in power supply systems, it is necessary to solve the main problems:
1. It is necessary to develop and officially approve a method for determining the culprit of PKE distortions
2. Ensure the use of electricity metering tools while simultaneously continuously monitoring its quality.
The currently adopted system of discounts and allowances is essentially incentive and, as far as we know, has not yet been applied in practice. One of the main reasons here is that currently there are no instruments that would measure power quality indicators (PQI) over sufficiently long time intervals (at least a month) while simultaneously taking into account the consumed electricity and identifying the culprit of the introduced distortions. A key role in this issue should be played by the widespread use of an electric energy meter, which pays for consumed (supplied) electricity depending on its quality indicators. Such a device must have high accuracy(class 0.5) and simultaneously measure active and reactive powers (including distortion powers) in all quadrants.
The quality of electricity is manifested through the quality of operation of each electrical receiver. Modern electrical appliances, including household ones, are necessarily equipped with a stabilizing power supply (refrigerator, air conditioner, washing machine, dishwasher, computer and TV), they are designed to stabilize the quality indicators of electrical energy in order to maximize the service life of the device itself. But while forming suitable indicators for powering devices, they inevitably spoil the current and voltage curves in the 220V network due to their generation of higher harmonics. This happens even in mode idle move when the TV is plugged in but does not work.
The generated harmonics have a stimulating effect on electricity meters; they “accelerate” the meter, forcing it to work within the limits of its error, but in the range of overestimating values.
Why is it so important for the consumer to pay attention to the accuracy class? Which meter error is more profitable to choose?
When compared, the difference in error readings between accuracy classes 0.5 and 1.0 of the electric meter is 3.0%. The annual overpayment for errors in electrical energy measurements will be about 30% of the cost of the meter; such a purchase will fully pay for itself in three years.
Taking into account the constant increase in the cost of electrical energy, using a meter with an accuracy class of 0.5 will allow you to accurately account for consumption and save your budget.
The best solution for a consumer to replace an electronic device for metering electrical energy would be a meter with an accuracy class of 0.5.
LITERATURE
1. GOST 13109-97. Electric Energy. Electromagnetic compatibility of technical equipment. Standards for the quality of electrical energy in general-purpose power supply systems. – Minsk: Interstate. Council for Standardization, Metrology and Certification, 1998.
2. RD 153-34.0-15.502-2002. Guidelines on control and analysis of the quality of electrical energy in general-purpose power supply systems. – M.: Energoservis, 2002.
3. G.S. Kudryashev, A.N. Tretyakov, O.N. Shpak, Rakhmet Halymiin // Current problems of operation of the machine and tractor fleet, technical service, energy and environmental safety in the agro-industrial complex. – Irkutsk: IrGSHA, 2007.
UDC 621.311
The standard energy supply contract specifies the supplier's obligations in detail. One of them concerns power quality indicators. It will be useful to find out what exactly is meant by this term, what indicators are being discussed, and also obtain information about current regulatory documents. This information will allow you to competently file a claim against the supplier if the quality of electricity does not meet the established requirements of the GOST standard.
What is power quality?
For each type of electrical network, certain characteristics (quality parameters) are established. The correspondence between them and actual values determines the quality of electrical energy.
Changes in the PCE may occur due to losses of electricity during transmission over a distance, an increase in the consumed load, electromagnetic phenomena, etc.
To assess the quality of electricity, measurements of the main indicators of CE are carried out. They are described in detail in the standards of GOST 13109-97, as well as in its new edition 13109 99, we present excerpts from brief description each indicator.
Main indicators of power quality
Since perfect compliance with the nominal parameters cannot be achieved, deviations are provided for in the standardization of indicators. They can be permissible and extremely permissible. Below are the main quality indicators and the acceptable standards for each of them.
Voltage deviation
This indicator is determined using a special coefficient that characterizes established deviations in relation to the nominal ones. The following formula is used for the calculation: δU set = 100% * (U t - U n)/ Un, where U t is the current indicator, U n is the nominal value. Quality indicators are measured at power receivers. An oscillogram of this process is presented below.
Rice. 1. Steady-state deviation and voltage fluctuationsSuch quality deviations are typical when there are significant changes in load or large losses during the transmission of electricity. Indicators are considered acceptable when U mouth is no more than 5.0%, the maximum permissible is 10.0%.
Voltage fluctuations
This parameter characterizes temporary deviations in the amplitude of electric current oscillations. An oscillogram of the process is presented in Figure 1. This is a component parameter of power quality, since to characterize voltage fluctuations it is necessary to take into account:
- the scope of change;
- dose of fluctuations (repetition frequency);
- duration of deviations.
The first two points require a little clarification.
Voltage change range.
This power quality parameter is described by the difference between the maximum and minimum deviations. The swing coefficient is determined by the following formula: (U Pmax - U Pmin)/U nom, where U Pmax is the maximum swing value, U Pmin is the minimum, U nom is the nominal value. The permissible value for the swing factor is no more than 10%.
Dose of voltage fluctuations.
This criterion serves to describe the frequency with which deviations occur. It should be taken into account that if the time period between fluctuations is less than 30.0 milliseconds, then they must be considered as one deviation.
For the calculation, the following expression is used: F repeat = m/T, while m determines the number of changes over a certain time period of measurements - T, equal to 10 minutes. The norms of this indicator are directly related to the flicker dose; it will be described below.
Frequency deviation
On general purpose systems this parameter is set to 50.0 Hz. The standard allows for an increase or decrease in frequency by 2.0% or 4.0% (permissible and limit values, respectively). Exceeding the permissible frequency deviations leads to failure of pulsed power supplies and malfunctions of electric generators.
Flicker dose
This parameter describes the effect on a person produced by the flickering of light sources due to changes in the amplitude of the electric current. Measurements are made using special instruments that determine permissible flicker.
Temporary overvoltage factor
This characteristic determines how much the current amplitude is above the maximum permissible threshold. Such deviations are typical during short circuits or switching processes. The random nature of the deviations does not allow the indicator to be normalized, but the collected statistics are used to determine the quality of electricity in a single-phase or three-phase network.
Voltage dip
This parameter means a significant decrease in amplitude (more than 10.0% of the nominal), followed by recovery. The cause of voltage dips may be a short circuit or a sharp increase in load.
The characteristics for this power quality indicator are described by the following components:
- The depth of the voltage “sag”, in some cases it can tend to zero.
- The number of deviations over a certain period of time.
- Duration.
The latter requires some explanation.
Duration of voltage dip.
By this criterion one can judge both the quality and reliability of the power supply. A sag of minimal duration may not cause any malfunction of electrical or electronic devices. If the duration is several seconds, there is a high probability of shutting down equipment with electrical or electronic control circuits. In addition, the reactive component of electric motors increases, which leads to a decrease in power factor.
Due to the random nature of the phenomenon, its normalization is not provided.
Pulse voltage
Manifests itself in the form of a short-term (up to 10 milliseconds) increase in the amplitude of electricity. Such a sharp jump can be caused by switching processes or lightning discharges. Since such network states are random in nature, pulse normalization is not provided.
The following characteristics are used to describe high-frequency pulses:
- Maximum amplitude parameter. In networks up to 1 kV, with a direct lightning strike, the amplitude of the surge can reach 6 kV.
- Duration. The duration of a high-amplitude (thunderstorm) pulse, as a rule, does not exceed several milliseconds.
Voltage asymmetry in a three-phase system
Such a clear deterioration in the quality of electricity can be caused by an incorrectly distributed load between the phases of one circuit, a short circuit to the ground, a broken neutral, or the connection of a consumer with an asymmetric load.
In this regard, a requirement has been established that the load difference between the phases of one circuit should not be more than 30.0% within one electrical panel and 15.0% at the starting point of the supply line.
To determine the asymmetry indicators, the zero and negative sequence coefficients are used. The first is calculated by the formula: K np = 100% * U np / U nom, the second: Kop = 100% * U op / U nom, where U np is the amplitude of the zero sequence, U op is the reverse.
According to the established standards for voltage regulation in networks up to 1 kV, the values of U np and U op should be no more than 2% and 4% (permissible and limit values).
Non-sinusoidal voltage waveform
Fig 5. High order harmonics The reason for this deviation is the connection to a consumer network with a nonlinear current-voltage characteristic. Typical example– converter based on thyristors.
Rice. 6. Third order harmonic To describe this deviation from quality indicators, the sinusoidal distortion coefficient is used, which is determined by the formula Kи = ⎷ ∑U N 2 / U nom * 100%, where U is the harmonic amplitude.
Acceptable and maximum permissible standards characterizing high-quality or low-quality electricity for various networks are given in the table below.
How to check and measure the quality of electrical energy?
Before proceeding with measurements that determine the quality of the electrical network, it should be taken into account that the PKE must be recorded by representatives of the electricity supplier. Based on the results of the inspection, a report is drawn up, on the basis of which a claim can be made.
To check all characteristics of electricity for compliance with the requirements of GOST 53144-2013, GOST R 54149-2010 and other regulatory documents, special measuring equipment will be required. But some of the main indicators can be measured using a conventional multimeter or the discrepancy can be determined by indirect signs.
How to independently identify a decrease in power quality?
We list the indicators that can be checked using a multimeter in AC voltage measurement mode:
- Established deviation.
- Overvoltage (including phase imbalance).
- Dips.
The second and third points are rather arbitrary; the duration of the distortion may not be sufficient for the device to respond, and voltage drops will be difficult to distinguish from overvoltages and sags.
Indirect methods for determining the quality of electricity include analysis of the state of the network based on the operation of a lamp with an incandescent filament. A glow that is too bright will indicate increased voltage, a dim light will indicate a “subsidence”, blinking will indicate changes.
Uncharacteristic operation of electrical equipment also indicates insufficient quality of electricity. For example, a refrigerator compressor is constantly functioning, unstable operation of electronics, spontaneous shutdown of household appliances, all this indicates insufficient voltage in the household network. Excess voltage will trigger the protection relay, if installed.
Without considering the inevitable transient processes shown in Fig. 10.7, we note that a long-term increase or decrease in the supply network leads to a reduction in the service life of motors and power supplies. A reduction is less desirable due to the significant increase in current consumption, disruption and failure of electronics and computer equipment. A complete loss of supply voltage has a negative impact. Short-term surges and dips are caused by transient processes in the electrical system, accompanied by high-frequency interference that leads to failure of electronic equipment. A surge can lead to failure of the consumer if the switching and especially protective equipment does not meet the requirements for speed and selectivity.
What affects the quality of power supply
Negative influence for power electrical equipment and measuring instruments cause long-term distortion of the voltage curve, especially voltage distortion having a “jaggy” character, caused by switching power thyristors and diodes in powerful distortion sources. The most dangerous are distortions of the curve crossing zero. These distortions can cause additional switching of diodes of low-power power supplies, accelerated aging of capacitors, failure of computers and printers and other equipment.
The quality problem in domestic electrical networks is very specific. In all industrial developed countries Connecting powerful nonlinear loads that distort the shape of current curves and the electrical network is allowed only if the requirements for ensuring power quality are met and the appropriate corrective devices are available. In this case, the total power of the newly introduced nonlinear load should not exceed 3...5% of the power of the entire load of the energy company. A different picture is observed in our country, where such consumers connect quite chaotically.
Issue technical specifications accession is largely formal due to the lack of clear methods and mass certified instruments that record “who is to blame.” At the same time, the industry practically did not produce the necessary filter-compensating, balancing, multifunctional optimizing devices, etc.
As a result, Russia's electrical networks were oversaturated with distorting equipment.
In some regions, complexes of electrical networks of power systems and consumer distribution networks were formed, unique in their power and degree of distortion of current curves, which significantly aggravated the problem of supplying consumers with high-quality electricity.
To determine the compliance of the values of measured power quality indicators with the standards of the standard, with the exception of the duration of a voltage dip, pulse voltage, and temporary overvoltage coefficient, a minimum measurement time interval of 24 hours is established, corresponding to the billing period. The total duration of PKE measurements should be selected taking into account the mandatory inclusion of workdays and weekends typical for the PKE measured. The recommended total measurement duration is 7 days. A comparison of the PCE with the norms of the standard must be made for each day of the total duration of measurements separately for each PCE. In addition, PCE measurements should be carried out at the request of the energy supply organization or consumer, as well as before and after connecting a new consumer.
Methods for improving power quality
There are three main groups methods for improving power quality:
- rationalization of power supply, which consists, in particular, of increasing network power and supplying non-linear consumers with increased voltage;
- improving the structure of the 1UR, for example, ensuring the nominal load of motors, using multiphase rectification circuits, including corrective devices in the consumer composition;
- the use of quality correction devices - regulators of one or more power quality indicators or related power consumption parameters.
Economically, the third group is the most preferable, since changing the structure of the network and consumers leads to significant costs.
The design of new consumer networks must be carried out taking into account modern quality requirements, focusing on the development of power quality regulators various types. A targeted impact on changing one type of distortion causes an indirect impact on other types of distortions. For example, compensating for voltage fluctuations causes a reduction in harmonic levels and results in a change in voltage deviations.
Deviations are slow and are caused either by changes in the level in the power center, or by losses in network elements (Fig. 10.8). the deviation requirements for the latest power receivers are not met due to significant losses in the cable line and on the power buses. total losses l/c.p, %, are determined by the expression:
Analyzing the diagram (see Fig. 10.8), we can conclude that the requirements for deviations can be met through regulation in the power center (gpp, rp) and by reducing losses in network elements.
Regulation is implemented by changing the transformation ratio of the supply transformer. For this purpose, transformers are equipped with means of regulation under load (RPN) or have the ability to switch taps of control branches without excitation (pbv), i.e., disconnecting them from the network while switching branches. Transformers with on-load tap-changer allow regulation in the range from ±10 to ±15% with a resolution of 1.25…2.50%. transformers with pbv usually have an adjustment range of ±5%.
Reducing losses in supply lines or cables can be achieved by reducing active and (or) reactance. Reducing resistance is achieved by increasing the cross-section of the wires or using longitudinal compensation devices (LPDs).
Longitudinal capacitive compensation of line parameters consists of sequential connection of capacitors in the line section, due to which its reactance decreases: Х'л= XL ХC< Хл.
Fluctuations in the power supply system of an industrial enterprise are caused by surges in reactive power of loads. Unlike deviations, fluctuations occur much faster. Oscillation repetition frequencies reach 10...15 Hz at reactive power surge rates of up to tens and even hundreds of megavars per second. Voltage fluctuation range
From expression (10.33) it follows that in order to reduce bU, it is necessary to reduce Xcs or load reactive power surges QH, to reduce which fast-acting reactive power sources must be used that can provide reactive power surge rates commensurate with the nature of the load change. In this case, the condition is satisfied
Connecting the IRM leads to a decrease in the amplitudes of the resulting reactive power oscillations, but increases their equivalent frequency. If the response is insufficient, the use of IRM can even lead to a worsening of the situation.
To reduce the influence of sharply variable loads on sensitive electrical receivers, a method of load sharing is used, in which dual reactors, three-winding transformers, with split windings, or powering loads from different transformers are most often used. The effect of using a dual reactor is based on the fact that the mutual induction coefficient between the windings of a dual reactor is not equal to zero, and the voltage drop, which decreases by 50...60% due to the magnetic coupling of the reactor windings, in each section is determined by the formulas:
where Km is the coefficient of mutual induction between the windings of the reactor sections; XL is the inductive reactance of the reactor winding section.
Transformers with a split winding allow you to connect a sharply variable load (a source of distortion) to one branch of the lower winding, and a stable one to the other. The relationship between changes in the windings is determined by the expression
Reducing voltage asymmetry is achieved by reducing the network resistance to negative and zero sequence currents and reducing the values of the currents themselves. Considering that the resistance of the external network (transformers, cables, lines) is the same for positive and negative sequences, it is possible to reduce these resistances only by connecting an asymmetric load to a separate transformer.
The main source of asymmetry is single-phase loads. When the ratio between the short circuit power in the SK 3 network node to the single-phase load power is greater than 50, the negative sequence coefficient usually does not exceed 2%, which meets the requirements of GOST.
The asymmetry can be reduced by increasing SK3 at the load terminals. This is achieved, for example, by connecting powerful single-phase loads through its own transformer to 110 - 220 kV buses. Reducing systematic asymmetry in low-voltage networks is carried out by rationally distributing single-phase loads between phases so that the resistances of these loads are approximately equal to each other. If asymmetry cannot be reduced using circuit solutions, then special devices are used.
As such balancing devices, asymmetrical connection of capacitor banks (Fig. 10.9, a) or special balancing circuits (Fig. 10.9, b) of single-phase loads are used.
If the asymmetry changes according to a probabilistic law, then to reduce it, automatic baluns are used, in circuits in which capacitors and reactors are assembled from several small parallel groups and connected depending on the change in current or reverse sequence (disadvantage - additional losses in the reactors). A number of devices are based on the use of transformers, for example transformers with rotating magnetic field, representing an asymmetric load, or transformers that allow phase-by-phase voltage regulation.
How to reduce non-sinusoidal voltage
Non-sinusoidal reduction is achieved:
- circuit solutions: allocation of nonlinear loads to a separate bus system; dispersing loads across various power units with connecting electric motors in parallel to them; grouping of converters according to the phase multiplication scheme; connecting the load to a system with higher power SK 3;
- using filter devices: connecting narrow-band resonant filters in parallel with the load; turning on filter compensating devices; use of filter balancing devices; the use of IRM containing filter compensating devices;
- the use of special equipment characterized by a reduced level of generation of higher harmonics: the use of “non-saturating” transformers; the use of multiphase converters with improved energy performance.
Development modern base power electronics and high-frequency modulation methods led to the creation of devices that improve the quality of electricity - active filters, divided into serial and parallel, current and voltage sources. This resulted in four basic circuits (Figure 10.10).
Inductance is used as an energy storage device in the current source converter, and capacitance is used in the voltage source converter. The equivalent circuit of the power resonant filter is shown in Fig. 10.11.
The filter resistance Z at frequency с is equal to When XL = Xc at frequency с a voltage resonance occurs, meaning that the filter resistance for the harmonic component with frequency с is equal to 0.
In this case, harmonic components with frequency co will be absorbed by the filter and will not penetrate the network. The principle of constructing resonant filters is based on this phenomenon.
In networks with nonlinear loads, as a rule, harmonics of the canonical series arise, the order number of which is v = 3, 5, 7,... The levels of harmonics with such an order number usually decrease with increasing frequency. Therefore, in practice, chains of parallel-connected filters tuned to the 3rd, 5th, 7th and 11th harmonics are used. Such devices are called narrowband resonant filters. If XL and Xc are the resistance of the reactor and capacitor bank at the fundamental frequency, then, using expression (10.38), we obtain
A filter, which, in addition to filtering harmonics, will generate reactive power and compensate for power and voltage losses in the network, is called a compensating filter (FKU).
If a device, in addition to filtering higher harmonics, performs voltage balancing functions, then such a device is called a filament balun (FSU). Structurally, FSUs are an asymmetrical filter connected to a linear network. The choice of line voltages to which the FSU filter circuits are connected, as well as the power ratio of the capacitors* included in the filter phases, are determined by the voltage balancing conditions.
Thus, devices such as PKU and FSU act simultaneously on several indicators (non-sinusoidality, asymmetry, voltage deviations). Such devices for improving the quality of electrical energy are called multifunctional optimizing devices (Fig. 10.12). The feasibility of their development lies in the fact that sharply variable loads such as chipboard cause simultaneous distortion in a number of indicators, which required a comprehensive solution to the problem.
The category of such devices includes high-speed static reactive power sources. According to the principle of reactive power regulation, they can be divided into IRM of direct and indirect compensation. Such devices, having high performance, reduce voltage fluctuations. Phase-by-phase regulation and the presence of filters ensure balancing and reduction of higher harmonic levels.
When developing a strategy for improving the quality of electricity in electrical networks and ensuring conditions for electromagnetic compatibility, it should be taken into account that significant material resources and a sufficiently long period of time are required to correct the situation. The development of a full range of measures requires a technical and economic assessment of the consequences of reduced quality, which is difficult due to the following circumstances:
- the impact of electricity quality on the quality and quantity of products, as well as on the service life of electrical receivers, is integral; changes in most quality indicators over time are stochastic due to their dependence on the operating modes of a large number of electrical receivers;
- the consequences of reduced power quality often manifest themselves in the final product, the qualitative and quantitative characteristics of which are also affected by other factors;
- lack of reporting data that allows establishing cause-and-effect relationships between real quality indicators, on the one hand, and the operation of electrical equipment and the quality of products, on the other;
- poor equipment of domestic electrical networks with means of measuring power quality indicators.
However, to ensure the required GOST 13109 - 97 indicators, it is necessary to carry out a set of organizational and technical measures aimed at identifying the causes and sources of violations and consisting in individual and centralized suppression of interference, ensuring increased noise immunity of electrical receivers sensitive to distortion.
An increase in the number and installed power of electrical receivers with nonlinear and asymmetrical loads, the emergence of new electrical installations have made distorted modes a characteristic and integral feature of the work modern system electricity supply In this case, violation of GOST 13109-97 is possible both on the part of the energy supply organization (steady voltage deviation, frequency deviation; duration of voltage dip; pulse voltage; temporary overvoltage coefficient), and on the part of consumers.
A consumer with a variable load can violate the standard in terms of the amplitude of change 8 U, and the flicker dose with nonlinear - according to the sinusoidal distortion coefficient of the Ki curve and the harmonic component coefficient with the asymmetric - according to the voltage asymmetry coefficient according to the negative sequence and the voltage asymmetry coefficient according to the zero sequence Koi.
Frequency deviation indicators depend on the balance of active and reactive powers in the power system, so maintaining them is the responsibility of energy supply organizations whose networks are responsible for voltage dips, pulses and short-term overvoltages. Failure—an inevitable occurrence for anyone’s network—leads to immediate consequences, the more significant the greater their depth and duration.
The reason that causes non-sinusoidality, asymmetry, fluctuations and voltage deviations is one or another type of electrical receiver, determined by the technological process (production). Deviation causes a change in the load of any production. Enterprises with powerful welding devices also generate fluctuations and voltage asymmetry; arc steel-smelting furnaces are also non-sinusoidal; electrolysis of non-ferrous metallurgy - fluctuations, non-sinusoidality; single-phase load - asymmetry; traction substations - non-sinusoidality and voltage asymmetry.
We looked at distortions in steady-state operating conditions. But there are industrial sources of voltage distortion that create interference in startup modes or during regulation. Higher harmonics are generated during starting and braking of AC electric motors with variable speed, converters during regenerative braking. Transformers cause short-term overvoltages when switched on and off.
A consumer can be a source of distortion for several PCEs. The number and location of sources in the circuit are known approximately, and the level of distortion they introduce is practically unknown. Distorting currents spread through networks depending on the network design, its frequency characteristics, etc. The currents are summed up at the nodes, so the distortion is determined by the action of several culprits.
If we consider all the points (nodes) where the PCE should be maintained (and checked), then there is an object with cenological properties. But the existing theory for calculating PCE is based on the normal distribution. The current situation is similar to the situation with the calculation of electrical loads in the 50-60s. XX century, when it was believed that the probabilistic Gaussian approach would solve the problem of loads. It is obvious that there is a large area of theory and practice, most important in the use of electricity, that requires new ideas.
To meet the quality requirements of consumers, the voltage values at each point of the electrical network must be within certain acceptable limits. Practically without special control devices, the permissible voltage regime can be ensured only when the total losses are small. This can only happen in networks of short length and with a small number of intermediate transformations.
In electrical distribution networks, deviations are usually determined for characteristic points - the most sensitive consumers to deviations and the most distant from them. transformer substations connection points for electrical receivers. At a fixed point in time, for any point in the radial network, the value bU is determined by the expression
Voltage swings following each other create oscillations of 5Ut. Oscillations are normalized according to the degree of impact on human vision. Process visual perception oscillations (flicker) starts from the upper limit of the oscillation frequency of about 35 Hz with changes of less than 10%. The most irritating effect of flashing light occurs in humans at a frequency of 8.8 Hz, with a certain swing value U. The duration of exposure to vibrations is 10 minutes. From a flicker point of view, incandescent lamps are the loads most sensitive to the magnitude of voltage changes.
Sources of fluctuations in modern electrical systems are powerful electrical receivers characterized by a pulsed, sharply variable nature of consumption of active and reactive power. They are characterized by: power supply from busbars with a voltage of 35 - 220 kV; significant changes in consumed active P and reactive power Q (which can exceed 1.5 times) at high speed during the day; presence of nonlinear elements in current collectors.
Such electrical receivers include, in priority order according to the degree of impact on this PKE: arc steel-smelting furnaces; ore thermal furnaces; high-power electric motors (in particular, rolling mills); induction furnaces; resistance welding machines; converters for electrolysis plants; synchronous motors; drives of pumps and compressors in distribution networks. Thus, when the DSP100 furnace was operating at a voltage of 35 kV, the bU value in the network was (4.3...8.2)% at cos
0.1…0.3 during the period of metal melting and cos cp = 0.70…0.77 in other modes. In this case, the oscillation turned out to be equal to 8.3 Hz.
The instability of oscillations is largely determined by the variability of reactive power consumption, therefore, by analyzing its process of change, one can obtain fairly reliable information about the nature of oscillations in the electrical network under study.
In electrical systems, oscillations propagate in the direction of the low buses with virtually no attenuation, and towards the high buses with attenuation in amplitude. This effect manifests itself depending on the value of SK 3 of the system. When oscillations propagate in any direction, their frequency spectrum is preserved, and the attenuation or amplification coefficient K is determined by the relation
where Skz is the short-circuit power of the transformation stage; St nom - rated power of the transformer; Ek - transformer short circuit.
Sources of harmonic distortions are mainly loads with nonlinear characteristics: arc steel-smelting furnaces; valve converters; transformers with nonlinear current-voltage characteristics; frequency converters; induction furnaces; rotating electric cars, powered through valve converters; television receivers; fluorescent lamps; mercury lamps. The last three sources create a low level of harmonic distortion at the output during their operation, but their total amount is large. The effect of superimposed distortion leads to significant levels of distortion, even in high voltage networks. Thus, the value of harmonic distortion of CTSHIA in 230 kV networks due to the operation of television receivers can reach 1%. So far, in power supply nodes of industrial enterprises, the values of the distortion coefficient of the sinusoidal curve Ki and the coefficient of the harmonic component exceed GOST standards (Table 10.4).
The propagation of current harmonics through the network also depends on the circuit parameters and network configuration. When current harmonics propagate from a source in the direction of a higher network, a decrease in the amplitudes of the harmonic components occurs, usually caused by an increase in the value of SK3 of the system. If harmonics propagate towards the networks low voltage, then the harmonic attenuation is weaker. Voltage asymmetry has a significant impact on the operation of electrical equipment, primarily electric motors and power transformers. With a negative voltage sequence coefficient equal to
nom 4%, the service life of electric motors is reduced by approximately