Many inverters are equipped with reactors on the input side, in order to reduce the harmonic current emission, on the other hand, to improve the anti-surge interference characteristics of the inverter.
The rectifier circuit of the inverter is equivalent to a harmonic voltage source. We know that when the voltage source is large, the output current will be small. Therefore, installing a reactor at the power input of the inverter can reduce the emission of harmonic current.
Installing a reactor at the power input of the inverter can effectively suppress the harmonic current of the inverter, but it does not meet the requirements of any harmonic standard.
The degree of suppression of the harmonic current of the inverter by the reactor is related to the reactance of the reactor. The reactance of the reactor is the ratio of the voltage drop across the reactor to the rated voltage when the rated current flows through the reactor. For example, a reactor with a rated voltage of 380V (rms), a rated operating current of 100A, and a reactance of 5% means that if a current of 100A flows through the reactor, the voltage drop across it is 380x0.05 = 19V ( Valid value).
The harmonic current after installing the reactor in front of the inverter is shown in Table 1. Table 1 also shows the limit value of the GB17625 standard. It can be seen from the table that the use of reactors cannot meet the requirements of the GB17625 standard. The best effect that can be achieved with a reactor is THID < 35%. For better results, use a harmonic filter.
Table 1 Harmonic current when different reactors are installed in front of the inverter
Harmonic number
GB17625
Claim
Harmonic distortion rate corresponding to different series reactors
3%
4%
5%
6%
7%
5
10.7%
40%
34%
32%
30%
28%
7
7.2%
16%
13%
12%
11%
10%
11
3.1%
7.3%
6.3%
5.8%
5.2%
5%
13
2%
4.9%
4.2%
3.9%
3.6%
3.3%
17
1.2%
3%
2.4%
2.2%
2.1%
0.9%
19
1.1%
2.2%
2%
0.8%
0.7%
0.4%
In recent years, with the wide application of frequency converters, the harmonic filters of inverters have also achieved great development. Many inverter manufacturers and filter manufacturers have developed good performance harmonic filters. The aerospace green card HTHF harmonic filter developed by Aerospace Science and Industry Group has reached the level of international similar products in terms of filtering performance. Table 2 shows the harmonic test results of the HTHF harmonic filter installed in the control cabinet of a centrifugal fan (including the inverter). It can be seen from Table 2 that after installing the HTHF filter, it can fully meet the requirements of the GB17625 standard.
Table 2: Filtering effect of 22kW centrifugal fan control cabinet
Harmonic number
GB17625 requirements
Current distortion rate before filter
Current distortion rate after filter
3
21.6
5
10.7
66.8
7.6
7
7.2
43.9
2.8
9
3.8
1.3
0.5
11
3.1
9.0
1.8
13
2
7.8
1.6
15
0.7
0
0
17
1.2
5.6
0.9
19
1.1
4.7
0.8
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