Talking about the Harm of Power Grid

Do you know how much harm harmonics pose to the grid? With the increasing pollution of harmonics on the power grid, people pay more and more attention to the harmonics of the power grid. Harmonic control is also imperative. This paper will deal with the causes of harmonics of the power grid, the harm of harmonics, etc. Analyze.

First, the basic concept of harmonics

The harmonics of the power supply system are defined as the Fourier series decomposition of the periodic non-sinusoidal electric energy. In addition to obtaining the same component as the fundamental frequency of the power grid, a series of components larger than the fundamental frequency of the power grid are obtained. This part of the electric quantity is called harmonic. The ratio of the harmonic frequency to the fundamental frequency (n = fn / f1) is called the harmonic order. Non-integer harmonics are sometimes found in the power grid, called non-harmonics or fractional harmonics.

For the 50Hz power supply used in China, the fundamental wave is 50Hz, the 3rd harmonic is 150Hz, the 5th harmonic is 250Hz, and so on.

Schematic diagram of 5th and 7th harmonics of fundamental wave superposition

Second, the cause of grid harmonics

The root cause of higher harmonic generation is due to the nonlinear characteristics of certain devices and loads in the power system, that is, the waveform distortion caused by the relationship between the applied voltage and the generated current is not linear (proportional).

The grid harmonics come from three aspects:

(1) The power quality is not high and the harmonics are generated;

Due to the problem of the manufacturing process of the generator, the magnetic induction intensity distribution on the armature surface is slightly deviated from the sine wave, and therefore, the induced electromotive force generated is slightly deviated from the sinusoidal electromotive force, that is, the generated current slightly deviates from the sinusoidal current. Of course, when several such power supplies are connected to the grid, the current of the main power supply will also deviate from the sine wave. When the generator emits a harmonic potential, harmonic potential is also generated. The harmonic potential depends on the structure and working conditions of the generator itself, and is basically independent of the external impedance. Therefore, it can be regarded as a harmonic constant voltage source, but its value is small.

(2) Harmonics generated by the transmission and distribution system;

The nonlinear components present in the power supply system are another source of harmonics. These nonlinear components mainly include transformer excitation branches, thyristor control components of AC/DC converter stations, thyristor-controlled capacitors, and reactor groups.

(3) Harmonics generated by electrical equipment.

Harmonics are generated due to the nonlinearity of the electrical equipment. When the current flows through the linear load, the current on the load is linear with the applied voltage. When the current flows through the nonlinear load, the current on the load is a non-sinusoidal wave, that is, harmonics are generated. Common non-linear loads such as: rectifiers, switching power supplies, inverters, computers, UPS, fluorescent lights, microwave ovens, televisions, etc.

Third, the harm of harmonics to the power grid

1) System angle, harmonics will lead to some abnormal phenomena: First, the ultra-high voltage long line, if the harmonic current is large, the submerged power arc will be delayed, the single-phase reclosing may fail, expand the accident, the arc suppression coil This problem also exists in the grounded system. Second, when the harmonic component is large, it may cause protection misoperation or rejection. For example, the zero-order third harmonic is too large, which may cause the grounding protection to malfunction. The third is the measurement and measurement error. Especially for the meter for zero-crossing detection phase, it is more serious.

2) Harmonics cause additional loss of equipment and reduce efficiency. Especially for the impact of the capacitor bank, as the frequency increases, the dielectric loss will increase significantly; for the transmission line, due to the high harmonic frequency and skin effect, the line resistance will increase, thus causing the accessory line loss; At the same time, transformers and motors, etc., will cause some additional copper and iron losses, resulting in local overheating.

3) Accelerate insulation aging and greatly shorten equipment life. Under the action of harmonics, the physical process of insulation aging is obviously intensified, which is very harmful to cables and capacitors.

4) Local series or parallel resonance may be generated and the harmonic level is amplified. As a result, the devices in the harmonic branch are damaged by overvoltage or overcurrent.

5) Harmonics interference with the communication system. If the harmonic frequency is close to the carrier frequency, the power line carrier communication and the telecontrol device signal transmission will be interfered to some extent. In addition, electromagnetic, electrostatic and conductive coupling paths will also interfere with the parallel laying communication lines.

Fourth, the national standard requirements for harmonics

1. Harmonic voltage limit "Power quality, utility grid harmonics" GB/T14549-93 Utility grid harmonic voltage (phase voltage)

2. The allowable value of the harmonic current injected into the common link

V. Summary

As can be seen from the above, the harm caused by the harmonics of the power grid is huge, and even serious grid accidents will occur. The national standards have strict requirements on the voltage and current harmonics injected into the power grid. Therefore, the control of the harmonics of the power grid is bound to be Row. The power quality analyzer independently developed by Guangzhou Zhiyuan Electronics Co., Ltd. can monitor voltage and current unbalance/harmonic content analysis (parity and harmonic distortion variability analysis), measure harmonic content, perform effective analysis, and find faults in time. Hidden dangers, advance management and rectification, reduce the harm caused by harmonics of power grids, improve the power supply quality of enterprise equipment, improve the reliability of equipment, and reduce the economic losses caused by equipment malfunction.
(The manuscript is provided by ZLG Zhiyuan Electronics)

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