Comparative study of microcontroller controlled four-wire voltage and current source shunt active power filters

Abstract

During the past two decades, active power filters have increasingly grown their popularity as a viable method for improving electric power quality. The main reasons for this have been the advent of fast self-commutating solid-state devices, the progression of digital technology and the improved sensor technology. Four-wire active power filters provide an efficient solution for improving the quality of supply in grounded three-phase systems or three-phase systems with neutral conductors, which are commonly used for powering residential, office and public buildings. Four-wire active power filters are applicable in compensating current harmonics, reactive power, neutral current and load phase imbalance. This thesis presents a comparative study of microcontroller controlled four-wire voltage and current source shunt active power filters. The study includes two voltage source topologies and a current source topology with two different dc-link energy storage structures, which are compared on the basis of their filtering properties, filtering performance and efficiency. The obtained results are used for determining the suitability of current source technology for four-wire active power filtering and finding the most viable four-wire shunt active power filter topology. One commonly recognized disadvantage of the current source active power filter has always been the bulky dc-link inductor. To reduce the size of the dc-link inductor, an alternative dc-link structure for current source active power filters was introduced in the late 80’s. The hybrid energy storage consists of both inductive and capacitive energy storage elements, two diodes and two controllable semiconductor switching devices. Since the capacitive element is used as a main storage unit, the inductance of the dc-link inductor can be considerably reduced. However, the original dc current control method proposed is not able to utilize the full potential of the hybrid energy storage and the inductance required in the dc-link is still quite large. To solve the problem, this thesis presents an enhanced dc current control method for the hybrid energy storage that makes it possible to minimize the inductance of the dc-link inductor. The performance of the proposed method is experimentally verified by applying it to the four-wire current source active power filter topology. The results show that all topologies included in the study have good filtering performance, but the efficiency of the voltage source topologies is much better than that of the current source topology with conventional dc-link structure. However, by using the hybrid energy storage and the enhanced dc current control method, the efficiency of the four-wire current source active power filter can be improved to be close to the efficiency of the voltage source topologies. Nevertheless, all things considered, the voltage source topologies are found to be the most suitable for four-wire active power filtering.