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Kamalov Tulyagan Sirajiddinovich, doctor, of technical sciences, professor, head of department Scientific and Technical Center,
Uzbekistan, Tashkent Shayumova Zamira Mustafayevna, the basic doctoral student E-mail: [email protected]
THE IMPACT OF HIGHER HARMONICS ON THE WORK OF ELECTRIC EQUIPMENT AND THEIR CLASSIFICATION
Abstract: The article deals with the influence of higher harmonics on the choice of electric motor power. And also the possibility of using the advantage of using frequency converters, which allow saving energy, is presented, which is actual for today. The above theoretical concepts are justified by the influence of the non-sinusoidal stresses and currents on the operation of electrical equipment and caused by the manifestation of higher harmonics.
Keywords: Higher harmonics, motor, current, power, frequency, Converter, motor efficiency, rectifier.
At the present time, due to the increase in energy consumption - the influence of higher harmonics on the choice of engine power is one of the important problems of the modern day in the electrical industry.Harmonics are sinusoidal waves that are summed up with a fundamental frequency of 50 Hz, that is, the first harmonic of 50 Hz, the fifth harmonic of -250 Hz [3]. Because of the harmonics caused by non-linear loads, this is one of the main problems that must be solved. Nonsi-nusoidality is due to the presence of static converters, then -synchronous generators, transformers, motors and other nonlinear loads.
The emergence of higher harmonics is associated with a wide application of semiconductor converters in power supply systems of enterprises of various industries. A number of papers [3-13] have been devoted to the study of the composition of higher harmonics of current and voltage and their effect on the quality of electrical energy.
Harmonics in the composition of current, voltage or both current and voltage affect the quality of electrical energy. Bad indicators of the quality of electrical energy lead to many negative consequences. The most unpleasant for industrial consumers: reduced efficiency and increased power consumption, overheating of cables, electric motors and transformers; reduction of reliability of the power supply system; a decrease in the efficiency of production and an increase in the specific energy intensity of a unit of final output; reduction of the service life of electrical equipment; damage to sensitive equipment; tripping of circuit breakers; fuse fuse; premature wear of equipment; overheating and failure of capacitors; the appearance of strong currents in neutral wires; occurrence of resonance in the network; failure to connect to power supply networks in case of too high level of harmonics; shutdown and downtime of process equipment caused by accidents
and switching in external networks; direct losses associated with the under-release of the final product; indirect losses due to possible repair work of mechanical equipment, as well as maintenance costs [1-37].
For example, it is known that with an acceptable voltage asymmetry of 2% and a non-sinusoidal 5%, the service life of asynchronous motors is reduced by 21%, synchronous by 32%, transformers by 8%. It is noted that when the asynchronous motor is operating under non-sinusoidal voltage conditions, its power factor and the torque on the shaft are slightly reduced. If the amplitudes of the 5th and 7th harmonic voltages are respectively 20 and 15% of the amplitude of the 1st harmonic, then the motor power factor is reduced by 2.6% in comparison with its value at a sinusoidal voltage [3-15].
Therefore, it is necessary to consider a number of several methods to improve the term of electrical devices and the quality of the generated electricity:
1. Make classification of harmonics;
2. Consider sources of harmonics;
3. What equipment is the source of higher harmonics.
Causes from higher harmonics there are additional losses
in electrical machines, transformers and networks, it is difficult to compensate for reactive power with the help of capacitor banks; the service life of insulation of electric machines and apparatus is shortened.
Voltage and current harmonics lead to additional losses in the stator windings, in the rotor circuits, as well as in the stator and rotor steels. Leakage currents caused by harmonics in the end zones of the stator and rotor lead to additional losses. The average distribution of losses from higher harmonics is characterized by the following: stator windings 14%; rotor chains 41%; end zones 19%; asymmetric pulsations 26%. For an average asynchronous motor with a specific weight of the
THE IMPACT OF HIGHER HARMONICS ON THE WORK OF ELECTRIC EQUIPMENT AND THEIR CLASSIFICATION
5th harmonic of the voltage equal to 20% of the main, the torque due to the 5th harmonic does not exceed 0.1% of the torque developed at the industrial frequency [3-15].
The influence of higher harmonics on electrical installations causes malfunctions, overheating of electric receivers; In particular, the influence of the harmonic on devices without semiconductor elements is significantly reduced, it reduces the sinusoidal distortion, confuses synchronicity, interference in the data transmission networks, and accordingly affects the efficiency of the engine, transmission and conversion device.
When the voltage exceeds the nominal value by 3-5%, the level of the harmonics of the magnetizing current increases by a factor of 1.5-2. With a large installed capacity of transform-
ers of guild substations, this can lead to a marked increase in the voltage of the higher harmonics in the network [3-40].
The main sources of higher harmonics are: electric drives with semiconductor converters, rotating electrical machines, saturated magnetic circuits, rectifiers and frequency converters of all types, powerful single-phase receivers, electric welding installations, lighting systems using economical lamps [1-37]. (Fig. 1). A synchronous generator operating in an electrical network of an enterprise can be considered simultaneously as a source of higher-order electromagnetic harmonics due to the features of the magnetic circuit of the machine and as a consumer of higher harmonics of current generated by other harmonics sources.
Figure 1. The source of higher harmonics (classification of harmonics)
The source of the superimposed current for a low-power converter is usually a generator of low-frequency oscillations with an amplifier; Synchronous generators are used for highpower converters. When the converter is operating only in rectifier mode, the current source can be replaced by, since during inverting, the current source must operate in the generator mode. The use of a special current source or resistor is
associated with additional losses, reaching 10% of the energy consumed by the converter [3-103].
The effects caused by the manifestation of higher harmonics can be divided by the duration of the effect on instantaneous and long-term effects.
To instantaneous it is customary to include: - Distortion of the form of supply voltage;
- Voltage drop on the distribution network;
- Effects from harmonics, including resonance at the frequency of harmonics;
- Harmful interference on the data network;
- Noise in the acoustic range, vibration of machines.
Long-term problems include:
- Heating and additional losses in transformers and electric machines;
- heating of capacitors;
- Heating of distribution network cables
It should be especially noted that even low levels of higher harmonics can cause disturbances in operating modes for some types of control, protective and measuring equipment due to distortions in the shape of the curve of the measured voltages and currents on the secondary windings of the measuring transformers. As a result, higher harmonics lead to false alarms on protective transformers at transformer substation.
The influence of higher harmonics on rotating machines is largely identical to the effect of unbalance of voltages and currents. In addition, the presence in the current curve of the stator of higher harmonics leads to the appearance in the gap of non-synchronous magnetic fields moving relative to the rotor. In this case, higher harmonics of the 5th and 11th orders create negative sequence fields rotating relative to the rotor in the opposite direction, and the components of the 7th and 13th orders create direct sequence fields [6]. However, since the frequency of their rotation is higher than the rotor speed with a harmonic order multiplicity, the fields of both sequences induce higher-frequency currents in the rotor circuits that flow in the upper layers of the massive parts of the rotor and, closing at its ends, cause local overheating. Harmonics have an impact on the accu-
racy of measuring the reactive power, which has for the case of sinusoidal currents and voltages, and affects the measurement of the power factor. To improve the shape of the input current or to compensate for reactive power, active filters are used that are connected in parallel to the mains.
The use of filters is currently a common way to reduce the level of higher harmonics. When installing filters, the problem of reactive power compensation is also partially or completely solved, since the capacitor banks included in the filters are sources of reactive power [3-131]. The functionality of active filters is much more passive. For example, in the active filter one or several harmonics can be programmed, which must be suppressed, it is possible to work in the compensation mode, etc. Active filters are simply necessary for those enterprises in which all methods of suppressing the higher harmonics failed to produce the expected result.
Conclusions
The problem of higher harmonics is topical and requires attention by analyzing electrical metrics and affecting the quality of electrical energy. To determine the more accurate value of the influence of higher harmonics, it is necessary to use the true definition of the effective value. The increase in losses is due to the appearance of higher harmonics. Therefore, it is necessary to evaluate the sources of higher harmonics, powered by energy converters and the efficiency of not only the system as a whole, but also of the electric motor. One of the promising ways to reduce nonsinusoidality in electrical networks is to improve the shape of the network current curve of the converters. This can be achieved by compensating the higher harmonics of the magnetic flux of the converter transformer.
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