Analysis of the reasons why the fuse of reactive power compensation cabinet frequently burns out after using frequency converter

    某小区的供热站，一台250kW的鼓风机向锅炉内吹煤粉。原来使用节风门调整鼓风机的风量，从而控制燃烧量。现在采取了变频器控制鼓风机，通过鼓风机的转速快慢来控制燃烧量。改造后，节电效果显著，但是却发现无功补偿装置中的保险频繁烧毁。希望解决这个问题。

　　This is because the inverter generates strong harmonic current emissions when it is working.

　　The harmonic current emitted by the inverter is injected into the system, which will generate a large current on the capacitor. There are two reasons for this.

　　First, the frequency of harmonic current is higher, and the compensation capacitor has a smaller impedance to high-frequency current, so it will generate a larger current.

　　Second, harmonic currents can resonate in the system, causing them to be amplified, which is usually the main cause of the problem.

　　In the power system, the transformer and the reactive power compensation capacitor form an LC parallel circuit, as shown in Figure 1. This circuit has an inherent resonant frequency f0. When f0 is the same as the frequency of the harmonic, the harmonic current will be generated in the loop formed by LC, which is generally 10 to 20 times the harmonic source current, which is very harmful to the system and capacitor.

　　The burning of the fuse on the compensation capacitor branch just proves that it has played a protective role, otherwise it will lead to more serious consequences, such as capacitor explosion, transformer overheating, etc.

　　When the above problem occurs, there are two ways to solve it.

　　The first method is to connect the reactance L1 in series with the capacitor and make the resonance point of the series circuit formed by L1 and C lower than the frequency of the lowest harmonic current, in this case, lower than the 5th harmonic. The resonant frequency of the loop formed by the reactance (L+L1) and the compensation capacitor C must be lower than the frequency of the harmonic source, so no resonance occurs.

　　The second method is to reduce the harmonic current of the harmonic source. The specific method is to install a HTHF harmonic filter at the power input end of the inverter.

　　In this case, the second method was adopted. After taking these measures, the reactive power compensation cabinet worked normally and the fuse did not burn out again.

　　Through this case, we know that when enterprises carry out inverter energy-saving transformation on motors, they cannot ignore the harm caused by inverter harmonic current. How to avoid the risks brought by harmonic current? As long as the harmonic current limit requirements are put forward for the energy-saving cabinet during the scheme design. (The limit requirements for the harmonic current of the energy-saving cabinet can adopt the GB17625 standard or the IEEE519 standard. According to the survey, European and American companies adopt more IEEE519 standards, the main content of which is to stipulate that the total harmonic current distortion rate THID < 8%.

　　As long as preventive measures are taken in the design of the power saving plan, the risks caused by harmonic currents can be completely avoided.