Research On STATCOM Based On AC-AC Conversion Technology

As important device of flexible AC transmission system,static synchronous compensator(STATCOM),is a new technology in the field of reactive power compensation.The traditional STATCOM is based on inverter topology,and it is not reliable,damageable and expensive because of the DC side large-capacity electrolytic capacitors.In this paper,inverter-less static synchronous compensator(IL-STATCOM)based on AC-AC conversion technology is studied,which avoids using DC energy storage components.First of all,the theory of virtual quadrature source(VQS)is introduced,and limitations of the AC chopper are pointed out and the even harmonic modulation(EHM)is deduced.The VQS is verified by saber.Buck-type AC-AC converter topology family for EHM is studied and the push-pull forward topology is analyzed in detail.Taking everything into account,the high-frequency isolated push-pull forward AC-AC converter is chosen as the research object and is applied to IL-STATCOM.Secondly,the working principle of single-phase IL-STATCOM for fundamental reactive compensation is studied in detail.Single-phase reactive current is detected according to the mathematical relationship of trigonometric functions.In addition,a direct current control scheme is put forward.Then,a three-phase IL-STATCOM system is proposed.Working principle of three-phase IL-STATCOM based on EHM technique is analyzed.Fourier series decomposition is applied to study compensation characteristics.EHM coupling problem is discovered and solution is proposed.The method of harmonic detection based on instantaneous reactive power theory is studied.The detection of positive and negative sequence components of high harmonic current is realized,and a closed-loop control scheme of successive harmonic compensation is designed.The feasibility of the three-phase IL-STATCOM system and the control scheme is verified by simulation of MATLAB.Three-phase IL-STATCOM can achieve comprehensive compensation of reactive power and harmonics,and is suitable for three-phase three-wire balance system.Finally,a prototype of single-phase IL-STATCOM is developed on the basis of theoretical analysis and simulation.Experimental system includes control circuit,main power circuit,current detection and voltage synchronization circuit,and driver circuit.The experimental results show that single-phase IL-STATCOM has good reactive power compensation effect.