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Abstract

Содержание

Introduction

With the development of electrical equipment at industrial enterprises, the introduction of electric receivers with non-linear current-voltage characteristics has increased. In this regard, the nonsinusoidal voltage understand the distortion of the sinusoidal voltage curve.

When designing power supply, it is necessary to provide measures for the normalization of the modes of electrical networks supplying power consumers, whose operation adversely affects the quality of electricity. Solving the problem of power quality is one of the most difficult tasks to accomplish when designing power supply. It is necessary to ensure proper quality of electricity, established by the relevant standards, rules of design and operation.

1. Effect of nonsinusoidal voltage

In three-phase networks, the quality of electricity is characterized by voltage deviations, voltage and frequency fluctuations, the nonsinusoidal voltage waveform, as well as voltage unbalance and neutral displacement. Electricity quality indicators must comply with the requirements of [1], which regulates the quality standards and allowable deviations. Measures to ensure the quality of electricity, given in [1], should be addressed comprehensively in the design of power supply. They should be based on a rational technology and production modes, the correct choice of types and parameters of electrical equipment and on the optimal solution of the power supply system as a whole, taking into account energy and technological factors.

It is necessary to envisage and work out the mandatory recommendations and measures for the implementation of the necessary daily monitoring of electric power quality indicators and the necessary and timely switching at different operating modes during operation. In this regard, the projects should provide devices and devices necessary for monitoring the quality of electricity in [1]. The electricity industry must ensure the supply of electrical equipment that does not degrade the quality of electricity in power supply systems.

Compliance with the indicators [1] contributes to an increase in production, improvement of its quality and overall profitability of production. Industrial enterprises are obliged to take measures to ensure that such indicators of power quality as the non-sinusoidal voltage waveform, voltage fluctuations, asymmetry are within the normalized values, since the deterioration of these indicators is caused by the operation of certain types of electrical receivers and is practically independent of the power system [2].

The problem of providing sinusoidal voltage and current in the power supply networks of power supply systems and power supply networks arose in connection with the use of high-power electrical receivers with non-linear volt-ampere characteristics, such as static converters, steel-making arc and induction furnaces, transformers, synchronous motors, welding units. Currently, the problem of the emergence of higher harmonics is one of the important parts of the general problem of electromagnetic compatibility of electricity receivers with the power supply network.

In electrical networks, distortion of the voltage curve leads to the following negative consequences:

• deterioration, and sometimes disruptions in the operation of electricity receivers, including those that create non-sinusoidality in electrical networks;

• computers fail;

• acceleration of aging of insulation of electrical machines, devices and cables, which leads to a decrease in the reliability and service life of electrical equipment;

• the accuracy of electrical measurements deteriorates;

• there are violations in the operation of automation, remote control and relay protection;

• it is difficult, and in some cases it becomes impossible to use power circuits as channels for information transfer;

• the use of capacitor batteries is limited due to overloading them with high harmonic currents and the occurrence of resonant phenomena.

2. How dangerous are higher harmonics?

The effects caused by the manifestation of higher harmonics can be divided by the duration of the impact on the instantaneous and long-lasting. It is customary to attribute to instantaneous: distortion of the form of the supply voltage, voltage drop on the distribution network, effects from harmonics, including resonance at the frequency of harmonics, harmful crosstalk on the data transmission network, noise in the acoustic range, machine vibration. Long-term problems include: excessive heat losses in generators and transformers, overheating of capacitors and distribution networks (conductors).

Let us consider in more detail the influence of voltage and current harmonics on the insulation of electrical machines and capacitors, as well as on automation devices. The distortion of the voltage waveform has a negative effect on the occurrence and course of ionization processes in the insulation of electrical machines and transformers. During the course of gas inclusions in isolation, ionization occurs, the essence of which consists in the formation of space charges and their subsequent neutralization. Neutralization of charges is associated with energy dissipation, the result is a mechanical, electrical and chemical effect on the surrounding dielectric. In connection with this, local defects in insulation develop, which leads to an increase in dielectric losses and to a shortened service life.

In transformers, voltage harmonics cause an increase in hysteresis losses, losses due to eddy currents in steel, and losses in windings. In addition, the service life of the insulation is reduced. The increase in losses in the windings is most important in the case of a converter transformer, since the presence of a filter, usually connected to the AC side, does not reduce the harmonics of the current in the transformer. In addition, there may be local overheating of the transformer tank. The negative aspect of the effect of harmonics on high-power transformers is the circulation of three times the zero-sequence current in delta-connected windings. This can lead to overloading.

In motors, the harmonics of voltage and current lead to additional losses in the rotor windings, in the stator circuits, as well as in the steel of the rotor and stator. Due to eddy currents and the surface effect, losses in the stator and rotor conductors are greater than those determined by ohmic resistance. Leakage currents caused by harmonics in the end zones of the stator and the rotor also cause additional losses. Additional losses are one of the negative phenomena caused by harmonics in rotating machines. They lead to an increase in the overall temperature of the machine and to local overheating, most likely in the rotor, which can lead to serious consequences. It should also be noted that under certain conditions of harmonic overlap, mechanical rotor vibration may occur [3].

In cable lines, voltage harmonics increase the effect on the dielectric in proportion to the increase in the maximum amplitude value. This, in turn, increases the number of cable damage and the cost of repairs.

In batteries, current harmonic capacitors also lead to additional energy losses. In this regard, there is an additional heating of the condenser, which can lead to the exit of the last failure. Condenser may also be damaged if harmonic resonances occur in the network.

Harmonics can interfere with protection devices or degrade their performance. The nature of the violation depends on the principle of operation of the device. The most common are false alarms, which are likely in the operation of protection systems based on resistance measurements. The effect of harmonics on induction power measurement and energy metering devices leads to a deterioration in the accuracy of their measurement results.

It should also be noted the effect of harmonics in power circuits on signals in communication lines (in particular, in telephone lines). Low noise level leads to a certain discomfort, with its increase, part of the transmitted information is lost, and in exceptional cases the connection becomes impossible. In this regard, in any technological changes in power supply systems and communication systems, it is necessary to consider the effect of power lines on telephone lines.

Fluorescent and mercury lamps. The ballast devices of these lamps sometimes contain capacitors and under certain conditions resonance may occur, leading to lamp failure.

The effect of higher harmonics on converting equipment. The cuts on the voltage sine wave that occur during the switching of the valves can affect the synchronization of other similar equipment or devices that are controlled at the moment of the transition of the voltage curve of zero value.

3. Normalization of nonsinusoidal voltage

The normal operation of electrical equipment depends on the quality of the electricity supply system. The mutual influence of electrical equipment and power supply system is called electromagnetic compatibility.

Industrial enterprises use devices with non-linear current-voltage characteristics. A characteristic feature of these devices is their use of non-sinusoidal currents from the network when they are connected to the terminals of sinusoidal voltage. Non-sinusoidal current curves can be considered as complex harmonic oscillations consisting of a set of simple harmonic oscillations of various frequencies. In this case, the periodic function of changing nonsinusoidal currents can be decomposed into a Fourier series:

formula

where v – harmonic number;

av, bv – Fourier row coefficients;

n – the number of the last harmonics to be considered.

When v=1, the first harmonic or the main harmonic (with a frequency of 50 Hz) is determined, the other members of the series are called the highest harmonics.

The currents of higher harmonics, passing through the network elements, cause a voltage drop in the resistances of these elements, which, superimposed on the main sinusoid voltage, lead to a distortion of the voltage curve.

The quality standards set by the standard [4] are electromagnetic compatibility levels for electromagnetic interference in general-purpose power systems. At observance of the specified norms, electromagnetic compatibility of electrical networks of general-purpose power supply systems and electrical networks of consumers of electricity (receivers of electricity) is ensured.

Nonsinusoidal voltage in all standards is estimated by KU coefficients of voltage sinusoidal distortion. Nonsinusoidal voltage is characterized by such indicators as [5]:

• coefficient of the n-th harmonic component of voltage:

formula

• voltage curve distortion factor:

formula

The averaging interval the number N of observations must be equal to at least 9.

Normally permissible and maximum permissible values of the sinusoidal distortion coefficient of the voltage curve at the points of common connection to electric networks with different nominal voltage are given in [2].

Normally admissible values of the n-th harmonic component of the voltage at the points of common connection to electric networks with different nominal voltages Uн are given in [2].

4. Reducing nonsinusoidal voltage and currents

In cases where the values of currents or voltages of higher harmonics are more acceptable, it is necessary to envisage a number of measures to reduce the non-sinusoidal voltages and currents. The feasibility of measures to reduce non-sinusoidality may also be due to the improvement of technical and economic indicators of the work of elements of electrical networks [6]. Reducing non-sinusoidality can be done in one of the following ways:

• reducing the nonlinearity of the source of interference;

• transfer of interference sources to a separate busbar section or to a higher voltage;

• using active and passive filters.

Reducing the higher harmonics generated by the transducers can be achieved by increasing the number of rectification phases in the converter installations (usually up to 12) or by applying special transducer circuits and laws controlling them to improve the shape of their primary, network, current.

The rational construction of the network circuit in terms of reducing non-sinusoidality consists in supplying non-linear loads from individual lines or transformers or connecting them to individual windings of three-winding transformers.

Using filters is a common way to reduce higher harmonics. The harmonic filter is a series-connected reactor and a capacitor battery (Fig. 1).

High harmonic filter scheme

Figure 1 – High harmonic filter scheme

where Rc – network impedance;

xl, xc – resistance of the reactor and capacitor battery filter.

The parameters of the reactor and capacitor bank are selected so that their resulting resistance for a certain harmonic frequency is zero. In general, for each harmonic you need your own filter. The filter forms a branch with very low resistance, parallel to the electrical network, shunts it at a frequency of a given harmonic and, accordingly, reduces the voltage of this harmonic [7]. Such filters can be connected both at the places of generation of higher harmonics (at valve installations), and at network nodes with an unacceptable level of current harmonics or at current resonance.

Conclusion

Battery capacitors used in filters, it is advisable to simultaneously use to compensate for reactive power. It is economically feasible to use such multifunctional devices designed not only to reduce non-sinusoidality, but also to compensate for reactive power. Such installations are often called filter-compensating [8]. Filters higher harmonic voltage components

Filters higher harmonic voltage components

Figure 2 – Filters higher harmonic voltage components
(animation: 7 frames, always repeated, 61 kilobytes)

High harmonic filters improve power factor, significantly reducing the level of higher harmonics. Reducing losses caused by the processes of transmission and distribution of electricity, improve the quality indicators of electricity, increase the reliability of the energy system of the object. Due to these performance characteristics, the use of a harmonic filter gives a visible economic effect, which consists of the following factors:

• reduced equipment maintenance costs;

• electricity consumption decreases;

• quality and reliability of power supply is improved;

• minimizes the risk of penalties provided for an insufficiently high power factor.

The effect of nonsinusoidality negatively affects the operation of power electrical equipment, protection systems, automation, telemechanics and communications. As a result of exposure to voltage harmonics, there are economic damages caused by the deterioration of energy performance, reduced reliability of electrical networks and reduced service life of electrical equipment. Therefore, it is important to use means to combat non-sinusoidal stress.

Reducing the nonsinusoidal voltage contributes to increasing the profitability of production, reduces energy losses, which means it is economically profitable in terms of costs, especially when the number of consumers is constantly increasing.

References

  1. ГОСТ 32144-2013. Электрическая энергия. Совместимость технических средств электромагнитная. Нормы качества электрической энергии в системах электроснабжения общего назначения. М.: Стандартинформ, 2014 – 20 с.
  2. Ермилов А. А. Основы электроснабжения промышленных предприятий. М.: Энергоатомиздат, 1983.
  3. Жежеленко И. В. Высшие гармоники в системах электроснабжения промпредприятий. М.: Энергоатомиздат, 2000.
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  5. Жежеленко И. В. Высшие гармоники в системах электроснабжения промпредприятий. М.: Энергоатомиздат, 2000.
  6. Князевский Б. А., Липкин Б. Ю. Электроснабжение промышленных предприятий. М.: Высшая школа, 1979. – 431 с.
  7. Аррилага Д. М. Гармоники в электрических системах. М.: Энергоатомиздат, 1990. – 215 с.
  8. Железко Ю. С. Компенсация реактивной мощности и повышение качества электроэнергии. М.: Энергоатомиздат, 1985.