Abstract: Nowadays , there are various harmonic sources in the power system. A harmonic source is an electrical device that injects harmonic current into the power grid or generates harmonic voltage in the power grid. The most common harmonic sources are devices such as converters.
Harmonics are generally generated in two ways. One is generated on the input side of the frequency converter, where the nonlinearity of the rectifier circuit on the power supply side generates harmonics. The other is generated on the output side of the frequency converter, where the output voltage of a voltage-source circuit is a rectangular wave, and the output current of a current-source circuit is a rectangular wave. Rectangular waves contain a significant number of harmonics. Therefore, some have concluded that the harmonics present in the power grid are mainly due to the presence of various nonlinear components in 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.
