Developing a voltage-source shunt active power filter for improving power quality

Abstract

Active filters are controlled current or voltage sources that can be used, for example, to compensate current harmonics, interharmonics and reactive power. They offer a wide and/or selectable filtering bandwidth and they are small in size. In addition, active filters can solve almost all the problems that exist with conventional passive filters. This thesis is concerned with developing a digitally controlled three-phase voltage-source shunt active power filter. First, the current compensation characteristic of the active power filter is studied and methods to improve this by compensating and minimizing effects caused by control system delays are investigated and proposed. Computational and prediction-based delay compensation methods are presented. Also, two methods in which the effect of the processing delay is eliminated by applying current-sensorless control and modified main circuit structure are proposed. Both the theoretical study and the experimental results presented show that all the studied methods provide effective compensation characteristics. The use of the LCL-type supply filter in an active power filter is studied by comparing an active and a passive resonance damping method and by assessing the suitability of each for the active power filter application. The results presented show that both of the damping methods provide the fast dynamic responses required in using the active power filter as well as efficient current ripple attenuation. In addition, the results obtained show that the passive damping method increases the power losses only slightly. In contrast, the active damping requires several current sensors and more complicated control than the passively damped system. The power loss profile of the active filter is determined and the effect of replacing the antiparallel silicon diodes in the IGBT bridge with their silicon carbide (SiC) counterparts is studied. The calculation and measurement results show that SiC diodes provide a reduction in the semiconductor power losses of the active filter. The reduction is important, since this would make it possible to reduce the cooling or to increase the switching frequency. The higher switching frequency would enable the use of smaller filter chokes. A comparison of the digitally controlled and vector-modulated voltage-source and currentsource active power filters is presented. The main circuit configurations and space-vector modulation techniques used are discussed as well as the load current detection -based control systems. In addition, the filtering characteristics, power loss distributions, and efficiencies of both systems are studied and compared in various operating points. Finally, a case study in which a combined active and passive compensator is applied to mitigate the voltage flicker problem caused by a resistance spot welding process is presented. The compensation characteristic of the solution is considered comprehensively, using simulations and practical measurements. Furthermore, the resulting flicker severity indices are assessed. The results show that the compensator offers a great reduction of the voltage drops causing the flicker.