Research On Compensation Current Detection Method Without Phase Lock Loop For D-STATCOM

The nonlinear,reactive and asymmetric load in distribution network will bring some power quality problems such us harmonic pollution, power factor reduction and three phase unbalance, which will directly affect safe and economical operation of distribution network. Distribution static synchronous compensator can resolve the issues of appeal and has the advantages of fast response, wide operating range and dynamic continuous compensation, which has become a research hotspot for improving the power quality in today's society. However,the ability of detecting compensation command current in real time and accurately decide the compensation performance of D-STATCOM. Traditional compensation current detection method based on instantaneous reactive power can't accurately detect the compensation command current under distorting or unbalanced grid voltage due to inability to extract phase information of grid fundamental positive sequence voltage,and the phase locked loop circuit will bring detection delay, which ultimately affect the compensation effect of D-STATCOM.This paper introduced the topology of D-STATCOM firstly, analysed the error sources of ip-iq method based on instantaneous reactive power theory, and proposed an improved compensation current detection method without phase locked loop. The proposed method eliminated the detection delay of phase locked loop and can accurately extract the phase information of fundamental positive sequence voltage in non-ideal grid voltage condition, and the average filter and sampling optimization were used to reduce the filter delay and the amount of calculation in the detection, which improved the detection effect of D-STATCOM from detection accuracy and response speed. By analyzing that DC voltage fluctuation of D-STATCOM caused by the unbalance condition bring the impact to compensation current detection, filtering process is taken on the control output of DC voltage in order to improve the detection effect of compensation current under unbalance condition. In addition, in order to improve the tracking performance to compensation command current of D-STATCOM, taking the reduction of switching frequency and current tracking error as control target, a novel double hysteresis current control method based on voltage space vector is proposed. Finally, results obtained by simulations system built in Simulink indicated the effectiveness and feasibility of the proposed improved compensation current detection method and control methods.In order to verify the usefulness of the improved compensation current detection method, a 10 k VA experimental prototype of D-STATCOM used for 380 V distribution network was developed. This paper described the hardware parameters and software design of D-STATCOM device, wherein the hardware part includes: parameters selection of DC capacitance, AC contactor and filter reactor from main circuit,design of the switching element and the snubber circuit. The selection of Hall element, A/D converter module, digital signal processor and the design of signal filtering circuit and hysteresis circuit from detection and control system. Software section includes: the design process of main program, compensation command current detection program and other function subprogram. By making experimental verification of the proposed improved compensation current detection method in the prototype, experimental results showed that the experimental prototype using improved compensation current detection method can accurately achieve effective compensation to the reactive power, harmonic and asymmetrical current component under non-ideal grid voltage and nonlinear unbalanced load conditions.