Interference and suppression of the hottest electr

2022-09-19
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The main cause for electrical equipment to produce interference and its harm to electrical power system and equipment are introduced, the methods of restricting interference are discussed in detail, and its developing intensity is also pointed out

keywords: electrical equipment electric power system interaction constraint

[Chinese Library Classification Number] TN972 [document identification code] B article code (2003) -

1 Introduction

with the development of power electronics technology, a large number of nonlinear loads have been added to the power supply system, especially static converters, which are widely used from low-voltage small capacity household appliances to industrial AC and DC conversion devices for high-voltage large capacity. Because the static converter works in a switching mode, the waveforms of electric current and voltage will be distorted, resulting in a significant increase in high-order harmonics. Although arc furnaces, welding machines, transformers, rotating electrical machines, fluorescent lamps and other nonlinear loads in the power supply system will generate high-order harmonics of different frequencies and amplitudes in the electricity and sent to the three-dimensional warehouse, the high-order harmonics generated by static converters are the most serious and become a "public hazard" in electricity

2 main causes of high-order harmonic generation

2.1 rectifier

as a DC power supply device, rectifier is widely used in various occasions. Its typical circuit is shown in Figure 1. In the rectifier device, the current of the AC power supply is a rectangular wave, which is the synthetic waveform of the power frequency fundamental current waveform and the odd multiple frequency high-order harmonic current waveform. Figure 2 shows the higher harmonic content of 6-pulse 3-phase bridge rectifier at different times

2.2 AC voltage regulator

AC voltage regulators are mostly used for power adjustment of industrial equipment such as dimming devices, resistance furnaces and induction motors. Its typical circuit is shown in Figure 3. The harmonic number generated by AC power regulator is basically the same as that of rectifier

2.3 frequency converter

frequency converter is the representative equipment of AC-AC converter. When used as a speed regulating device of motor, it contains sidebands that change with the output frequency. Due to the continuous change of frequency, the harmonic content is relatively complex

2.4 general purpose inverter

the input circuit of general purpose inverter is usually composed of diode full bridge rectifier circuit and DC side capacitor. As shown in Figure 4 (a), the input current waveform of this circuit varies greatly with the impedance. When the power impedance is relatively small, its waveform is narrow and high Slender waveform, as shown in Figure 4 (b); On the contrary, when the power impedance is relatively large, the waveform is a low and wide flat waveform, as shown by the dotted line in Figure 4 (b)

2.5 high frequency switching power supply

in addition to the large number of harmonics generated by the above typical converter devices, the rapid popularity of color TV sets, personal computers, battery chargers and other devices in recent years has led to the rapid increase of rectifier circuits with capacitive filtering. The analysis of its AC side harmonics has begun to become one of the focuses in the field of harmonic source analysis

3 harm of high-order harmonics

3.1 impact on power capacitors

since the capacitive reactance of capacitors is inversely proportional to frequency, the capacitive reactance under the action of high-order harmonic voltage is much smaller than that under the action of fundamental voltage, so that the waveform distortion of harmonic current is much larger than that of harmonic voltage, Europe will increase the use of carbon fiber composites as one of the lightweight materials in cars. Even if the proportion of harmonics in the voltage is small, significant harmonic currents will be generated. Especially in the case of resonance, a small harmonic voltage can cause a large harmonic current, which will double the overload of the capacitor and cause the capacitor to be damaged due to overcurrent

3.2 influence on rotating motor

harmonic voltage or current will cause additional losses in the stator winding, rotor circuit, stator and rotor core of the motor. Due to the relationship between eddy current and skin effect, these additional losses in stator and rotor conductors are greater than those caused by DC resistance

in addition, harmonic current will also increase the noise of the motor and produce pulsating torque. The pulsating torque generated by the kth harmonic current of the rotor and the fundamental rotating magnetic field can be expressed by the following formula: (2)

where: Er is the fundamental potential of the rotor (converted to the stator side)

f1 is the fundamental frequency of stator

p is the pole number of the motor

, the phase difference between irk and ER

3.3 influence on transmission system

on the one hand, harmonic current produces harmonic voltage drop on the transmission line, on the other hand, it increases the effective value of current on the transmission line, resulting in additional transmission loss. According to relevant information, the impact of harmonics will reduce the service life of cables by about 60% on average

3.4 influence on transformer

transformer will produce skin effect and proximity effect under the action of high-order harmonic voltage. Additional copper loss is caused in the winding, and the iron loss is also increased correspondingly. The additional loss can be expressed by the following formula: (3)

where ikt is the k-th harmonic current through the transformer

rs is the short-circuit resistance of the transformer

kkt is the coefficient considering the influence of skin effect and proximity effect

in addition, a multiple of 3 times zero sequence current will generate circulating current in the connected winding, and this additional circulating current may make the winding current exceed the rated value. For the transformer with asymmetric load, if the load current contains DC component, it will cause the transformer to enter the magnetic circuit saturation of the experimental program voltage transformer. Thus, the harmonic component of AC exciting current will be greatly increased

3.5 influence on relay protection and automatic devices

harmonics can change the action characteristics of protection relays, which is related to the design characteristics and principle of relays. When there is harmonic distortion, the digital relay that depends on sampling data or zero crossing is easy to produce error. Higher harmonics will make the protection device malfunction and unstable

3.6 influence on power measurement

the measuring instrument is calibrated under the condition of pure sine wave, if the relationship curve between the wave error of power supply and frequency. It can be seen from Figure 5 that the watt hour meter is insensitive to frequencies other than design parameters. The higher the frequency, the greater the error, and it is a negative error. When the frequency is about 1000Hz, the watt hour meter will stop rotating

3.7 interference to communication

the current waveform of the static converter in the power supply system changes sharply during commutation, and the commutation current will inject a pulse voltage into the normal power supply voltage. The harmonic frequency contained in the pulse voltage is high, even reaching 1MHz, so

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