According to the data of some developed countries, China's transmission line loss rate is much higher than that of the United States. The utilization rate of electric power in developed countries is about 11%, but it is far behind that in developed countries. According to the rough statistics of relevant departments, the economic loss caused by the non-standard power quality in China has exceeded 150 billion yuan, while that in the United States is only about 26 billion yuan. This has caused great pollution to China's power quality.

In the problem of harmonic source, according to relevant statistics, rectifier users account for about 90% of the harmonic source. Among them, smelting, steel rolling and other enterprises are large users of electricity, and most of these enterprises' harmonics are generated by the frequency conversion rectification of medium frequency furnace device. The load impact and nonlinearity caused by medium frequency furnace will have the following adverse effects:

1. Voltage and current distortion of public power grid

2. Fluctuation and flicker of grid voltage.

3. Increase of reactive power

4. Three phase unbalance of power grid

5. Current harmonic content exceeds the standard, etc

Most of the equipment of medium frequency furnace has low power factor and, and a large number of harmonic currents are generated in the production process and injected into the distribution network, resulting in relay protection misoperation and affecting the normal operation of the control device, which poses a serious threat to the normal production of the enterprise. Therefore, it is very necessary to suppress the harmonic of medium frequency furnace for smelting and rolling.