Currently, various harmonic sources exist in power systems. Harmonic sources refer to electrical equipment that injects harmonic current into the power grid or generates harmonic voltage within it. The most common harmonic sources are devices such as converters. Harmonic generation typically occurs in two ways: first, on the input side of the frequency converter, where rectifier circuits on the power supply side generate harmonics due to their nonlinearity; second, on the output side of the frequency converter, where voltage-type circuits produce rectangular waves of output voltage, and current-type circuits produce rectangular waves of output current. These rectangular waves contain numerous harmonics. Therefore, some have concluded that the harmonics present in the power grid mainly stem from the presence of various nonlinear components within the power system.
To reduce the impact of harmonics generated by frequency converters on the power grid, which is causing increasing harmonic pollution, frequency converter manufacturers, including Shenzhen Anbangxin Frequency Converter Manufacturer, place great emphasis on harmonic mitigation. In Anbangxin's brand frequency converter series, the harmonic rate is consistently controlled within national standards, below 5%. Currently, Anbangxin employs two methods for harmonic mitigation: first, installing AC and DC reactors; and second, installing filters. These two methods are currently the most common and effective harmonic mitigation approaches.
The principles of these two types of harmonic mitigation methods will be analyzed and introduced below.
1. Install AC and DC reactors. When the frequency converter is used on a distribution transformer with a capacity greater than 500 kVA, and the transformer capacity is more than 10 times the frequency converter capacity, an AC reactor should be installed on the input side of the frequency converter. When the three-phase output voltage of the distribution transformer is unbalanced and the imbalance rate is greater than 3%, an AC reactor is also required.
2. Install a passive filter: Install the passive filter on the AC side of the frequency converter. The passive filter consists of L, C, and R elements forming a harmonic resonance circuit. When the harmonic frequency of the lc circuit is the same as the frequency of a certain higher harmonic current, it can prevent the higher harmonic from flowing into the power grid.
3. Install an active filter: An active filter detects high-order harmonics in the current and, based on the detection results, inputs a current with the opposite phase to the high-order harmonic components, achieving real-time compensation for harmonic currents. Compared to passive filters, it offers high controllability and fast response, and is multi-functional. Furthermore, it eliminates the risk of resonance with the system impedance.
