How to select harmonic control devices based on load?

We all know that the targeted solution to power interference problems is to take measures at the harmonic source. So how to install harmonic control devices for harmonic source equipment?

Generally, passive filters using LC notch circuits are not suitable for harmonic suppression in equipment. This type of filter is connected in parallel on the line, providing a low impedance path for harmonic currents.

This principle of filtering cannot be used for the power input of devices, mainly due to the following issues:

1： Uncertain filtering effect:

When combined with a frequency converter, this passive filter cannot guarantee that the harmonic current emission of the transmission system meets specific requirements, as the actual effect of the filter is closely related to the impedance of the power grid;

The magnitude of the harmonic current flowing into the filter depends on the ratio of the grid impedance to the impedance ZF of the filter. The larger this ratio, the better the filtering effect of the filter.

When the impedance ZF of the filter is constant, the larger the impedance Z0 of the line, the better the filtering effect. If the impedance of the circuit is constant, the smaller the impedance ZF of the filter, the better the filtering effect.

Analysis of the filtering effect of notch filters

Therefore, the effectiveness of notch filters is closely related to the impedance of the power grid. Figure 3 shows the filter performance under different grid impedances.

The influence of different grid impedances on the filtering effect of notch filters

2： Absorb upstream harmonic current:

The notch filter provides a low impedance path for specific harmonic currents, so not only can the harmonic currents generated by the frequency converter be bypassed, but also the harmonic currents from upstream can be bypassed, which can easily cause filter overload and even damage.

3： Excessive capacitive reactive power generation:

Due to the use of larger capacitors, notch filters will emit larger capacitive reactive power. This is a good thing for traditional industrial power grids, as it can compensate for reactive power while filtering. However, the frequency converter itself does not require capacitive reactive power, and excessive capacitive reactive power emitted by the filter can make the transmission system a capacitive load, causing adverse effects on the power grid.

4： Possible resonance with the system:

The notch circuit exhibits capacitance or inductance at frequencies outside the tuning frequency, which is highly likely to resonate with the system's capacitance or inductance, causing system instability. In severe cases, it may even damage the system.

Therefore, harmonic filters used for frequency converters must be specially designed to avoid various defects mentioned above. In recent years, with the widespread application of frequency converters and the increasingly serious problems caused by harmonics generated by frequency converters, people have conducted in-depth research on the mechanism of harmonic generation in frequency converters and developed suitable harmonic control devices.