Analysis of Harmonic Amplification Phenomenon

1. Summary

Due to the advantages of simple control and low cost of uncontrolled rectifying converter, it is widely used in switching power supply, frequency converter and other equipment. However, this kind of converter will inject a large amount of harmonic current into the power grid, and increasingly become the most important harmonic source in the power grid. Most of the frequency converters on the market are typical voltage source type nonlinear load, three-phase uncontrolled rectifier circuit with DC side capacitor. Compared with the inductance filtered diode rectifier circuit, the total current harmonic distortion rate is susceptible to the AC side impedance. In contrast, the AC side voltage characteristics are less affected by the AC side impedance, and the whole load appears more like a voltage source, so it is called a voltage source type nonlinear load.

For harmonic sources, the parallel active filter is generally used for governance. The active filter is essentially a voltage pulse width modulation converter. By detecting the load harmonic current, then the compensation current with equal amplitude but opposite phase is injected into the common coupling point for harmonic filtering, which is equivalent to a current source.

The voltage source inverter usually adopts current source. This paper analyzes this phenomenon and puts forward measures to suppress the harmonic amplification effect, so that the parallel AHF can achieve good compensation effect on the voltage source nonlinear load.

2. Analysis of load harmonic amplification effects

Figure 1 Single equivalent circuit of parallel compensation system under single harmonic

Figure 1 is the single-phase equivalent circuit of the three-phase parallel compensation system, and the IC is the parallel type AHF, which will be derived based on the equivalent circuit in this paper.

ILh-----Harmonic content after compensation
ILh-----Harmonic content before compensation
ZSh------System impedance
ZLh -----Load impedance
λ----------AHF compensation rate
m----------Voltage variation on the AC side of the load

As can be seen from the formula, after compensation with parallel AHF, the voltage source nonlinear load harmonic current will change, and the magnification is mainly related to the load voltage change rate μ, the harmonic compensation rate of AHF λ, the system impedance ZSh and the load AC side impedance ZLh. In general, the voltage source of nonlinear load changes very little, so m is about equal to 1; and the system impedance is also certain. If the load is connected to the PCC point, only through a section of ordinary wire, the AC side impedance is only the equivalent impedance of the wire and the ZLh is small. In order to achieve a better effect after AHF compensation, the harmonic compensation rate should generally reach more than 90%. It can be seen from the formula that the ratio of the harmonic current after compensation and before compensation will be much greater than 1, that is to say, more serious harmonic amplification effect will occur.

The load harmonic amplification effect is the result of the joint action of parallel AHF, nonlinear load and power grid. In the process of parallel AHF compensation current output, the load harmonic current increases constantly, and the AHF adjusts the current command to output more compensation current according to the detected load AC side harmonic current, forming a vicious cycle.

3. Measures to suppress the harmonic amplification effect

1. Analysis of the factors affecting the harmonic amplification effect

The load harmonic current amplification effect is mainly related to the four parameters: the load voltage change rate, the harmonic compensation rate of parallel AHF, the system impedance and the load AC side resistance. In practical application, the load AC side impedance and the harmonic compensation rate of parallel AHF can be easily changed. Therefore, in order to suppress the load harmonic current amplification effect, measures can be taken from the system structure and the AHF harmonic compensation rate.

2. Solution

Load AC-side series inductor:

It can be seen from the formula that increasing the impedance on the AC side of the load can reduce the amplification ratio of the load harmonic current, and the simplest and most effective method in practical application is to connect the series inductors on the load AC side. The larger the series inductance, the better the effect of limiting the harmonic current level of the load. If the inductance is too small, it is difficult to play a better inhibitory effect. The size of the series inductance shall be determined according to the initial harmonic current level of the load and the system impedance size.

Reduces the AHF compensation rate:

 According to the formula, assuming that μ = 1 and ZLh/ZSh = 0.01, the simplified formula is

With the improvement of the compensation rate, the greater the load harmonic amplification multiple, so can reduce the compensation rate to reduce the amplification multiple, selective harmonic compensation is also an effective method, through the current content in the characteristics of harmonic compensation, on the one hand, can make the total THDi control within the allowable range, on the other hand, can also avoid other times of harmonic current amplification.