Research on harmonic internal compensation of converter circuit

Abstract: High-frequency power active filter is a new harmonic and reactive dynamic compensation device. As a harmonic generator, the power active filter is added to the DC side of the converter to offset the harmonics generated by the converter and play a role in harmonic internal compensation. At the same time, the active filter replaces the electrolytic capacitor to achieve the "silicon solution" of the electrolytic capacitor. This paper introduces its working principle and performs the corresponding PSPICE simulation.

Keywords: harmonic compensation, active power filter, silicon solution

1 Introduction

With the rapid development of power electronic devices, two problems have arisen: First, the harmonic interference generated by power electronic converter circuits has become the main source of harmonics in power systems, and the harm caused by harmonics is becoming increasingly serious. Second, the passive components in power electronic systems, especially electrolytic capacitors, have not made much improvement in terms of cost, volume, and reliability. Since the heat caused by harmonics endangers the service life of electrolytic capacitors, electrolytic capacitors have become the weakest link in power electronic systems.

In order to eliminate the influence of harmonics, research on voltage-type and current-type active filters has been conducted at home and abroad in recent years to compensate on the AC side of the power grid. In the power electronic system, the loaded converter is a harmonic source, which causes fluctuations in current and voltage on the DC side. If an anti-phase harmonic source is artificially added to the DC side, the controllable active filter can make the two harmonics cancel each other out, achieving the effect of internal harmonic compensation. Replacing electrolytic capacitors with high-frequency active filters is a way to "siliconize" passive components. This new structure can prevent harmonics from being transmitted and eliminate electrolytic capacitors.

This article introduces the principle and solution of using voltage source inverter as active power filter on DC side to filter out harmonics and replace electrolytic capacitors.

2 Harmonic compensation principle

The basic element of the voltage source converter in the DC link is the electrolytic capacitor, which can absorb the harmonics generated by the load and the power supply. Replacing the electrolytic capacitor with an active power filter can, on the one hand, "siliconize" the electrolytic capacitor, and on the other hand, better achieve harmonic compensation.

Figure 1 is the main circuit diagram of the voltage source inverter. To simplify the analysis, the circuit is powered by a DC voltage source. In the figure, UB is the DC voltage source, RB and LB are the internal resistance and inductance of the voltage source respectively. The dashed box is the power active filter that replaces the electrolytic capacitor.

When the load is inductive, the inverter input current consists of two parts: the DC current id.dc and the ripple id.ac, namely:

id=id.dc＋id.ac(1)

In order to compensate for the harmonic current id.ac, the active power filter input current ig is required to be equal to id.ac in magnitude and opposite in direction, that is:

ig=－id.ac(2)

At this time, the output current of the DC voltage source is:

i=id＋ig=id.dc(3)

The DC voltage source output current is equal to the DC component of the inverter input current.

Figure 1 Single-phase inverter with active filter replacing electrolytic capacitors

The power active filter is a single-phase current type, and the switch devices S1-S4 use power MOSFETs, which have high operating frequency and fast response speed. LF is the DC side energy storage element of the active filter, and C is the demodulation AC capacitor.

The current if in the reactor LF is switched by the switching devices S1-S4, and a modulated wave current if.ac including the inverter input ripple current id.ac is generated at the input end of the active filter. After the current if.ac is demodulated by the AC capacitor, a current ig is generated which is approximately equal to the inverter input ripple current id.ac but in the opposite direction, thereby compensating the inverter input ripple current id.ac.

3. Control of Active Power Filter

The active power filter adopts the DC loop voltage hysteresis control method, and the control circuit block diagram is shown in Figure 2. It includes basic control, average current control of the reactor LF and resonance suppression.

Figure 2 Active filter control block diagram

The basic control part is composed of low-pass filters (1) and (2), hysteresis comparator, zero-crossing comparator, rectifier circuit and comparator. The DC loop voltage ud is subtracted from the DC component ud.dc detected by the low-pass filter (1) to obtain the ripple component ua.ac, and ua.ac generates a logic signal I1 through the hysteresis comparator. The inverter input current id is subtracted from the DC component id.dc detected by the low-pass filter (2) to obtain the ripple current signal id.ac. On the one hand, id.ac passes through the zero comparator to generate a logic signal I2 representing the current polarity, and on the other hand, it passes through the rectifier circuit and comparator to generate a logic signal I3 to judge whether |id.ac| exceeds the threshold A. I1-I3 generate control signals Ugs1-Ugs4 of the switching devices (S1-S4) through the decoder.

The average current control part of the reactor LF consists of a PI regulator, an advance compensator and a low-pass filter (3). This part can keep the average current of the reactor constant and make the reactor have sufficient energy. if* is the reference current, and if.dc is the average component of the reactor current if detected by the low-pass filter (3). The deviation voltage signal ue reflects the deviation between if.dc and if*. ue and ud.ac are superimposed, and the duty cycle of the active filter current if.ac is changed through the hysteresis comparator, so that the reactor is subjected to a positive average voltage.

The harmonic suppression part includes a bandpass filter and an amplifier As. This control part is used to suppress the resonance that may occur between LB and C when the system is put into operation or the inverter input ripple current suddenly changes.

4PSPICE simulation results

The single-phase bridge PWM inverter circuit shown in Figure 1 takes UB as 300V, RB as 0.1Ω, LB as 100μH, and the output frequency as 30Hz. The load is a resistive-inductive load, where the resistance is 10Ω and the inductance is 700mH. The inductance LF of the active filter is 85mH and the capacitance C is 10μF. S1-S4 uses power MOSFET. The simulation control circuit block diagram is shown in Figure 2.

Figure 3 (a) (b) are the PSPICE simulation waveforms of the inverter DC loop voltage ud and DC loop current i. The results show that the DC loop voltage and current are basically flat voltages and currents.

5 Conclusion

(a) DC loop voltage ud

(b) DC loop current i

Figure 3 PSPICE simulation results

As a harmonic source, the active power filter can replace the electrolytic capacitor, compensate the inverter input ripple current, realize the harmonic compensation of the AC circuit and the silicon solution of the electrolytic capacitor. With the high frequency of power electronic devices and the development of power electronic technology, this resonant adaptive system will have a high application value.