The hazards of harmonics in the power system are numerous and summarized in the following aspects:
1. Harm to power distribution lines
(1) Impact on the stable operation of the line
The power lines and power transformers in the power distribution system are generally detected and protected by electromagnetic relays, inductive relays or transistor relays, which ensure the safety of the lines and equipment in the event of a fault. However, due to electromagnetic relays and inductive relays, when the content below 10% is up to 40%, it causes the relay protection to malfunction. Therefore, it cannot fully and effectively protect against harmonics. Transistor relay has many advantages, but due to the use of a rectifier sampling circuit, it is susceptible to harmonics, resulting in misoperation or refusal. In this way, harmonics will seriously threaten the stable and safe operation of power supply and distribution systems.
(2) Affecting the quality of the power grid
Harmonics in the power system can distort the voltage and current waveforms of the power grid. For example, frequency converters, hot-dip galvanizing, and intermediate frequency smelting in industrial power distribution systems generate a large number of odd harmonics. In addition, harmonic voltage and harmonic current of the same frequency need to generate active power and reactive power of the same harmonic, thereby reducing the grid voltage and wasting the capacity of the grid.
2. Harm to power equipment
(1) Hazards to Power Capacitors
When there is a harmonic in the grid, the voltage at the terminal increases after the capacitor is inserted, and the current through the capacitor increases more, causing the capacitor to lose power. For film-membrane composite dielectric capacitors, the loss power when harmonics are allowed is 1.38 times the power without harmonics, and the loss power when harmonics are allowed for all-film capacitors is 1.43 times when there is no harmonics, but if Higher harmonic content, beyond the capacitor allowable conditions, will make the capacitor over-current and overload, the power loss exceeds the above value, so that the capacitor abnormal heat, the insulation medium under the effect of the electric field and temperature will accelerate aging. Especially when the capacitor is put into the power grid where the voltage has been distorted, the harmonic of the power grid may also be exacerbated, that is, the phenomenon of harmonic expansion occurs. In addition, the presence of harmonics tends to make the voltage appear sharp. The steep-peak voltage wave easily induces partial discharge in the medium. Due to the large voltage change rate and partial discharge intensity, it can accelerate the aging of the insulating medium, thus shortening it. The service life of the capacitor. In general, for every 10% increase in voltage, the life of the capacitor is reduced by about 1/2. In addition, in the case of severe harmonics, the capacitor will bulge, breakdown or explosion.
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