High-power active power filter realized by multiple main circuits

Abstract: In order to solve the contradiction between large capacity and switching frequency faced by large-capacity active power filters, this paper introduces a large-capacity active power filter using multiple main circuits, and gives the main circuit structure and control method of the system. Experiments show that the system effectively solves the contradiction between the capacity and switching frequency of the active power filter and achieves satisfactory results.

Keywords: Active power filter, multiple harmonics

Large Volume Active Power Filter Consisted of Multiple Main Circuit Abstract: It's a trouble that how to solve the contradiction between the volume and switch frequency in active power filter.In this paper, a large volume active power filter which main circuit consisted of multiple is introduced .The main structure of circuit and control method were presented.The experiment results show that good effects have obtained.

Keywords:Active power filter Multiple Harmonics

1 Introduction

Active power filters (APFs) can compensate for harmonics of varying magnitude and frequency as well as varying reactive power, and can effectively overcome the shortcomings of passive filters. Therefore, they have aroused great interest and attention, leading to rapid development of the research and application of active power filters [1,2,3].

How to use active power filters to compensate for harmonics and reactive power in large-capacity industrial devices is one of the issues that many researchers are concerned about. The harmonics generated by large-capacity industrial devices are very harmful to the power grid and must be given sufficient attention [2]. When using active power filters for harmonic suppression, the capacity of the active power filter is required to be large enough. Therefore, the power electronic devices used in active power filters must meet very high requirements in terms of capacity. As the capacity of power electronic devices increases, the allowable switching frequency of the devices becomes lower and lower, and the lower switching frequency will affect the compensation effect of the active power filter. Therefore, when using active power filters for large-capacity harmonic compensation, there is a contradiction between the switching frequency and capacity of the devices.

To solve this contradiction, there are three options: one is to connect the devices in series and parallel to meet the capacity requirements; the second is to use multiple independent active power filters in parallel; the third is to use multiple main circuits. Among the three options, the use of multiple main circuits is the most reasonable and effective option. Compared with the solution of connecting devices in series and parallel, the multiple main circuit can meet the capacity requirements while also increasing the equivalent switching frequency. Compared with the method of using multiple devices in parallel, in addition to increasing the equivalent switching frequency, only one set of control circuits is required, which is more economically reasonable.

Based on the above assumptions, we designed and manufactured an active power filter experimental device with a quadruple main circuit. The main circuit consists of four groups of PWM converters connected in parallel. The capacity of each PWM converter is 30kVA, and the harmonic compensation capacity of the entire active power filter device reaches 120kVA. In the control circuit, the calculation of the harmonic compensation current command adopts the harmonic current detection method based on instantaneous reactive power theory [4]. The compensation current command is distributed to the four groups of PWM converters. After proper control, the equivalent switching frequency of the entire device can be made four times the switching frequency of the device, achieving a good compensation effect.

2 System composition and working principle

The main circuit and control circuit schematic of the active power filter are shown in Figure 1. The main circuit consists of four groups of PWM converters connected in parallel. The DC sides of the four groups of PWM converters are connected in parallel and share a group of DC capacitors. Their AC sides are connected in parallel with the load harmonic source. When working, each group of PWM converters generates its own harmonic compensation current through its own current tracking link according to the harmonic command current calculated by the control circuit. After adding, it offsets the harmonic components in the harmonic source load current, so that the power supply current flowing into the power supply side is a sinusoidal fundamental current without harmonics [4].

The schematic diagram of the control circuit is shown in Figure 2. In the figure, the calculation of the compensation current command adopts the harmonic detection method based on the instantaneous reactive power theory of the three-phase circuit, that is, by detecting the voltage and current of the compensation object (i.e., the load in Figure 1), after coordinate transformation, low-pass filtering and coordinate inverse transformation, the fundamental current component in the load current is obtained, and then the load current is subtracted from it to obtain the command current signal of the harmonic compensation current. In order to balance the current outputs of each module, the calculated command current needs to be distributed, that is, the compensation current output by each PWM converter is 1/4 of the calculated command current. The command current after passing through the distribution circuit is output to each module, and the current tracking control circuit in each module is used to control the main circuit of each module to generate the required compensation current.

Figure 1 Structure of active power filter

The current tracking control circuit adopts the tracking PWM control mode. The timing comparison mode is used. The increase of the equivalent switching frequency of the multiple main circuit is achieved by staggering the phase of the reference clock of each PWM converter, that is, the reference clock input to each PWM converter is delayed by 90° in phase, the reference clock of the second group of PWM converters lags behind the first group of reference clocks by 90° in electrical angle, the reference clock of the third group of PWM converters lags behind the second group of reference clocks by 90° in electrical angle, and so on. The reference clock distribution principle diagram of each PWM converter is shown in Figure 2. In this way, the switching frequency of the entire system is increased to 4 times the switching frequency of the power electronic devices of each PWM converter.

3 Experimental results

The developed active power filter device was used to carry out compensation experiments on the harmonic source load shown in Figure 1. The harmonic source consists of a three-phase rectifier bridge with an inductive load. Figures 3 to 5 show the experimental results. Figure 3 shows the power supply current waveform before the active power filter is put into use. It can be seen that the waveform of the three-phase current is seriously distorted. Figure 4 shows the power supply current waveform after only two groups of PWM converters in the active power filter are put into compensation. It can be seen that the harmonics in the three-phase current waveform are compensated for 1/2 of the total harmonics. Figure 5 is the power supply current waveform after all four groups of PWM converters of the active power filter are put into compensation. It can be seen that the harmonics in the three-phase current waveform are basically eliminated. This shows that the active power filter has a good compensation effect.

4 Conclusion

This paper introduces a large-capacity active power filter realized by multiple main circuits, and gives the system structure and control principle. Experiments show that the system can effectively solve the contradiction between capacity and switching frequency encountered by active power filters at large capacity. The following conclusions can be drawn:

(1) The use of multiple main circuits can solve the problem of too low switching frequency of large-capacity single power electronic devices. Multiplexing can increase the equivalent switching frequency of the system exponentially, so that the active power filter has a good compensation effect.

Figure 2 Schematic diagram of control circuit principle and clock pulse distribution diagram

Figure 3 Power supply current waveform and spectrum analysis diagram when compensating the resistive and inductive load of the three-phase rectifier bridge before

Figure 4: When only two groups of multiplexed main circuits are working
Waveform and spectrum analysis after power supply current compensation

Figure 5: When the four groups of multiplexed main circuits are working
Waveform and spectrum analysis diagram after power supply current compensation

(2) The use of multiple main circuits can multiply the compensation capacity of the device.