Overview of harmonic filter

Harmonic
1. In the power grid, there are a large number of nonlinear loads, causing the grid current waveform to no longer be a sine wave. This non-sine wave can be decomposed by Fourier series to get the power frequency fundamental current and harmonic current. Harmonic currents produce harmonic voltages of the same frequency on the grid impedance, and are superimposed on the fundamental sinusoidal voltage to cause distortion of the grid voltage. Distorted grid voltage is applied to all electrical equipment ends, which will harm the normal operation of these equipment. Because electrical equipment is designed and manufactured according to sinusoidal voltage. When the voltage is distorted, the electrical equipment will heat up or the torque will be unstable, or even be damaged. Harmonic interference has become a major "public hazard" in the current power system that affects power quality. Therefore, the power sector requires users to take certain measures to suppress the harmonics generated by equipment connected to the power grid.
2. In order to ensure the normal operation of electrical equipment and suppress harmonic interference, my country formulated the national standard "Power Quality Public Grid Harmonics" in 1993, which stipulated the allowable value of grid harmonics.

Harmonic source
1. Iron core equipment with ferromagnetic saturation characteristics, such as transformers;
2.Equipmentwith strong nonlinearity and arc phenomenon, such as gas discharge lamps, AC arc welding machines and steel-making electric arc furnaces, etc.;
3. Power supply equipment based on power electronic components such as various power converter equipment (rectifiers, inverters and frequency converters), phase-controlled speed and voltage regulation devices, large-capacity power thyristor controllable switch equipment, etc.

Resonance caused by harmonics
1. At the power frequency, the capacitive reactance of the capacitor is much larger than the inductive reactance of the system. There will be no resonance. But for harmonic frequencies, if the inductance of the system is greatly increased and the capacitive reactance is greatly reduced, parallel resonance or series resonance may occur. This kind of resonance will magnify the harmonic current by several times or even dozens of times, which will pose a great threat to the system and capacitors.
2. The result of parallel resonance will amplify the harmonic current, causing the capacitor to over-current and malfunction;
3. The result of series resonance will cause the capacitor to produce over-voltage and malfunction.

Harmonic control method
1. For power grids with harmonic sources, reactive power compensation and harmonic control are usually combined, that is, a reactor is connected in series in the capacitor circuit to form a detuned filter circuit (non-tuned) or a pure filter circuit ( Tuned).
2. The detuning filter circuit is mainly used to prevent the harmonic overload of the capacitor, and the filtering effect is not large. It is usually installed in the power grid with small harmonic current.
3. When the harmonic current exceeds the specified allowable value, or although the harmonic current does not exceed the limit, but interferes with the power grid, a tuned filter circuit should be set. The filter circuit has the function of absorbing harmonic currents and compensating fundamental reactive power.

The impact of harmonics on power system components
1. The components in the public grid produce additional harmonic losses, which reduces the efficiency of power generation and transmission equipment. When a large amount of third harmonic current flows through the neutral line of the transformer, it will cause The wiring is overheated and even a fire occurs.
2. Affect the normal operation of various electrical equipment. Harmonics cause electrical equipment to overheat and generate vibration and noise, and cause insulation aging, shortening its service life, and even malfunction or burn.
3. Harmonics can also cause parallel resonance or series resonance in the power system to amplify the harmonic content and cause damage to capacitors and other equipment.
4. Harmonics cause malfunctions of relay protection and automatic devices, causing confusion in electric energy measurement.
5. Harmonics cause serious interference to communication equipment and electronic equipment, affecting the use of enterprise precision instruments, especially foreign imported equipment.