In a three-phase power system, the voltages at the generation side are sinusoidal waves equal in magnitude with 120° phase difference. At the load side, however, voltages may become unbalanced due to phase angle deviations or unequal distribution of single phase loads caused by unequal voltage magnitudes of each phase. Voltage unbalance is a major power quality issue because a small unbalance in the phase voltages can even cause a larger unbalance in the phase currents.
A completely balanced three-phase system contains only positive sequence components of voltage, current and impedance, whereas an unbalanced system contains both positive and negative sequence components of voltages and currents. The negative sequence component of current in the unbalanced system increases temperature loss in the equipment. Hence, it is necessary to mitigate this problem by removing the negative sequence current at the load side and keep the source side balanced.
Reactive power compensation can be ensured by using Static Var Compensators (SVCs), DSTATCOMs, Static Synchronous Series Compensators (SSSCs) and Unified Power Quality Conditioners (UPQCs), thus improving power quality.
Jong Il Bok, a researcher at the Faculty of Electrical Engineering, proposed a controller to determine the difference value between load current and positive sequence current as the standard value of compensating current and a proportional complex integral controller to control the current of each phase for compensating asymmetry and harmonics of load using a distribution static compensator (DSTATCOM). Then, he studied the performance of the controller by simulating the entire system in the MATLAB/Simulink environment.
This technique makes it possible to compensate asymmetry or harmonics of reactive power of load and load current. In addition, it makes switching frequency constant and improves the current tracking ability.
© 2021 KumChaek University of Technology