The Research On The Method Of Harmonic Suppression Of D-STATCOM In Three Phase Four Wire System

With the improvement of requirement about power quality, especially for the greatly increased application of power electronic devices and nonlinear loads, the significance of using the dynamic compensation devices to improve the power quality is becoming more and more prominent. The research of applying static Var generator(STATCOM) to realize dynamic compensation is one of the active fields in power system. The core of D-STATCOM is a self-commutation bridge circuit. Connect the device in parallel with grid and adjust phase and amplitude of the voltage or the current injected into power system, can regulate power quality indicators and realize the purpose of dynamic compensation.At present, the STATCOM is mainly used in high voltage transmission system for reactive power compensation and system static stability improvement. With the development of power electronic technology, the light device suitable for low voltage distribution network emerges as the times require, namely D-STATCOM. Because of its advantages of small volume,speedy adjustment,easy maintenance,etc, D-STATCOM has gradually become the focus of research in recently years.Especially for inverter control strategy research, domestic and foreign scholars have put forward many methods.In this paper, the power quality measure standards of city low voltage distribution network are considered, especially the three phase current imbalance caused by three-phase load unbalance. An effective method that produce corresponding compensation current to eliminate system three phase current unbalance by changing inverter control strategy of the traditional reactive power compensation device D- STATCOM is studied. At present D-STATCOM control strategies are mostly applied to three wire transmission systems, but in four wire system distribution system are used less.The main difference is that the introduction of zero line will provide access for the zero sequence current. In order to accurately detect the positive, negative and zero sequence current, the instantaneous symmetrical component method combined with the traditional instantaneous reactive power theory is applied to the control strategy instruction current algorithm. Thus, the D-STATCOM compensation to unbalance current is realized through the proposed new sequence component direct current control method.Finally, simulation is carried through the PSCAD/EMTDC software environment for three-phase unbalanced contain harmonic source load and R-L series three-phase unbalanced load two kinds of conditions, and the results are analyzed. The experimental data verifies the effectiveness of sequence component direct current control method applied in this paper.