Research on the realization of large-capacity active power filter * Zhuo Fang, Xie Jiaotong University, He Yihong, Li Hongyu, Wang Zhao'an (Xi'an 710049). A method of implementing a large-capacity active power filter with a quadruple PWM converter without a transformer as the main circuit is presented. The topology of the system structure is given and the control principle of the system is analyzed. It can be directly incorporated into or cascaded into medium and low voltage distribution networks without transformers, for voltage regulation, harmonic suppression, and reactive power compensation. The feature of this active power filter is that the switching frequency of the main circuit components is the same as the frequency of the power grid. Each device only switches once during one power network cycle; secondly, it can directly connect to the power grid without a transformer; The system's switching frequency is low, so electromagnetic interference, switching losses are relatively low. Therefore, it is more suitable for reactive power compensation and harmonic suppression in medium and low voltage power grids. Among the above three schemes, multiplexing the main circuit is the most reasonable and effective solution. Compared with the parallel-connected scheme of the small-capacity switching device, the multiplexing of the main circuit meets the capacity requirement, and the control circuit can also use only one set, which is economically more reasonable. In addition, another advantage of using multiplexed main circuits is that they can take control measures. Increase the equivalent switching frequency of the entire device. Make the system | system last. The resulting switching frequency is several times greater than the switching frequency allowed by the power electronics itself to achieve good compensation characteristics. Based on the above assumptions, an active power filter experiment device using a quadruple main circuit was designed and manufactured. The main circuit of the PWM power converter was connected in parallel by four line PWM reactors. The capacity of each PWM inverter was 30 kVA. The harmonic compensation capacity of the entire active power filter device is 120 kVA, and four sets of converters are controlled by a set of control circuits. Harmonic compensation current command calculation uses a harmonic current detection method based on instantaneous reactive power theory to achieve good dynamic response performance. To balance the output power of each converter, the control circuit distributes the calculated compensation signal evenly to the current tracking control circuit of the four groups of converters, so that the output capacity of each group of converters is 1 of the total capacity. /4. In order to increase the equivalent switching frequency of the system output, when each group of converters performs current tracking control, the control of the switching time is no longer independent, but is coordinated in a unified manner. Each group of converters is on the time of opening. Misaligned with each other at a certain angle, the equivalent switching frequency of the entire device is 4 times the switching frequency of the device, so as to obtain a good compensation effect. 3 The development of high-power active power filters realized by the quadruple main circuit combined with the development of high-power active power filters at home and abroad, and a quadruple PWM converter without transformer designed for normal industrial loads The main circuit of the active power filter, the circuit can directly enter the industrial grid, reducing equipment investment. Main circuit diagram of active power filter. The main circuit is composed of 4 PWM inverters connected in parallel by line reactors. The DC sides of the four groups of PWM converters share a set of DC capacitors in parallel. During operation, the harmonic command currents calculated by the control circuits in each group are controlled by the respective current tracking links to generate harmonic compensation currents. These harmonic currents are added to the total harmonic current component of the load harmonics source. Cancel harmonic components in the harmonic source load current so that the current flowing into the power supply side is a sinusoidal fundamental current with no harmonics. 4. According to the topology of the main circuit, combined with the working principle of shunt active power filter, An active power filter device developed using a quadruple mains circuit was used to perform harmonic compensation experiments on the circuit. Harmonic source consists of an inductive load: male, born in May 1962, associate professor. The research direction is harmonic suppression and power electronics technology. When harmonic compensation is performed, the harmonic compensation current generated by the four groups of converters can be regarded as a superposition operation between the current output from the four current sources and the load harmonic current iL. The current flowing into the power supply side is the superposition between the load current containing the harmonic current and the current output from each active power filter. During the control, the output capacity of each group is controlled in an equilibrium manner so that the harmonic current generated by each group is 1/4 of the harmonic current in the load. In this way, when each group of currents id, ic2, ic3 and the load current iL After the addition, the current is flowing into the power supply becomes a fundamental wave current that does not contain harmonic components. Quadratic PWM inverter single-phase equivalent circuit In order to increase the equivalent output switching frequency of the active power filter, the calculation circuit schematic block diagram of the entire control system and the time base pulse distribution diagram are given. In order to coordinate the compensation control of the 4 groups of PWM converters, each group is provided with a current tracking control circuit and a drive protection circuit. The time-base pulse of the current tracking control is uniformly processed by the control circuit and is distributed to each tracking control module according to a certain timing. In specific implementation, let the reference clock of the second group lag behind the first group of reference clocks by an electrical angle of 90°, the reference clock of the third group lags the second group of reference clocks by an electrical angle of 90°, and so on. In this way, the final switching frequency of the system is 4 times the frequency of each group. The timing-based pulse distribution diagram for each PWM converter can be seen. Three-phase rectifier bridge composition. The experimental waveforms and corresponding spectrograms before and after the active power filter is input are given. It can be seen from the given waveform diagram and spectrum diagram that before compensation, the three-phase current waveforms are positive and negative alternating trapezoidal waves, the waveform distortion is serious, and the higher harmonic components in the spectrum diagram have a certain amplitude; after compensation The waveform of the power supply current is significantly improved and basically becomes sinusoidal. In the frequency spectrum diagram, the higher harmonic current components are basically eliminated, indicating that the active power filter has a good compensation effect. 5 Conclusions The realization method of a large-capacity active power filter with a quadruple PWM converter without a transformer as the main circuit is proposed. This method effectively solves the capacity of the active power filter encountered in large capacity. Contradictions with switching frequency. The conclusions are as follows: using a multiplexed main circuit can solve the problem of too low switching frequency for large-capacity single power electronic devices. The multiplexed main circuit can double the equivalent switching frequency of the system, enabling the active power filter to have a good compensation effect. Using multiple main circuits can multiply the compensation capacity of the device.
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