Power Quality Compensator For Low-voltage Users

Along with the increment of different kinds of nonlinear loads, especially the widely applied power electronic equipment in distribution systems, power quality becomes more and more deteriorated. On the other hand, due to continuous improvement of automation, need to effectively solve power quality problems of important customers with many computer applications is also more urgent. Unified Power Quality Conditioner (UPQC) has been studied and developed as a new device to compensate harmonics and reactive power actively. This paper introduces the basic conception of harmonics and reactive power, their harm to power system and the methods to compensating harmonics and reactive power. Then the foundation of UPQC is proposed and its operation principle is analyzed.The conventional control strategy for UPQC is that the control voltage and current reference by detecting and calculating the harmonic and reactive currents of nonlinear loads or ac main must be obtained firstly, and then the UPQC voltage and current are controlled to track the reference. Several methods of measuring harmonic and reactive power current are introduced and compared in this paper. Now another UPQC control strategy which the ac main voltage and current are controlled directly has been attracting researcher's interests gradually because there is no need to measure and compute the harmonic and reactive currents of nonlinear loads. This paper analyzes and demonstrates the inherent relations between the two strategies in details with several approaches, and draws an important conclusion that the latter strategy is equivalent to the conventional strategy mentioned above on the voltage and current control for UPQC. Simulation results are given. The equivalence principle of the two strategies assures us it is no longer necessary actually to detect the voltage harmonic and reactive currents.This paper proposes a new measurement of voltage and current, that there is no need of the measurements of voltage fluctuation, harmonic voltage, harmonic and reactive current, discusses the voltage equivalent measurements for series active power filters and the current equivalent measurements for shunt active power filters.Simulation results show that the UPQC controlled by the new equivalent measurement has simple control arithmetic, good performance. The UPQC can not only compensate for harmonic and reactive currents produced by nonlinear loads, improve power factor, but also eliminate voltage fluctuation and harmonic voltage.Simulation and experiment results indicate that UPQC can improve the power quality efficiently through new control strategy. With the development of power electronics, UPQC can be applied to power system and taken effect perfectly.