Research On Key Issues Of Multi-STATCOM System

Static Synchronous Compensator(STATCOM),as the most advanced reactive power device,can compensate reactive power and lower power loss in power system,which has gotten major league attention.As the demand for reactive power compensation increases,there are two broad scenarios for STATCOM to develop,one is to adopt high voltage and large capability for single device,the other is to use many STATCOM of small capability in parallel instead.The latter gains better performance in flexibility and reliability,besides,it is easier for mass production.This paper takes three-phase three-wire STATCOM as the research topic,focus on two key issues in multi-STATCOM system: one is resonance caused by the existence of grid impedance,the other is harmonic interference caused by harmonic distortion at grid connection point.On the harmonic resonant issue,using grid-connected-inverter model for reference,a Norton equivalent model of STATCOM is established based on closed loop transfer function.After that,the model for multi-STATCOM system is established based on the Norton model.The influence on frequency response caused by the number of parallel STATCOM,parameters of LCL filter,active damping and load changing is mainly discussed.At last simulation model of two STATCOM in parallel is carried out,which validates the correctness of the proposed mathematical model and theoretical analysis.As for harmonic interference issue,the method of grid voltage proportional feedforward is mainly studied through modeling analysis.The performance of grid voltage proportional feedforward strategy is discussed when grid impedance exists.It can be found out that the existence of grid impedance may bring down the phase margin of proportional feedforward strategy a lot,and if the impedance is large enough,the STATCOM system may lost its stability.As in multi-STATCOM system,the equivalent grid impedance for every single STATCOM is larger than the actual grid impedance,which makes the feedforward issue much more obvious.To this end,an improved grid voltage feedforward strategy is proposed to address the grid-impedance issue,in which the voltage drop caused by grid impedance is removed from the feedforward passage.After that,the approach is generalized to multi-STATCOM system.At last,the prototype of single STATCOM and multi-STATCOM system is constructed,which validates the influence on feedforward strategy caused by grid impedance.The effectiveness of the proposed approach and correctness of theoretical analysis is proved as well.