Analysis And Research Of D-STATCOM Control Strategy

In recent years,a large number of non-linear loads have been used in the power grid,resulting in power quality problems that cannot be ignored.These power quality problems not only increase the power transmission loss,but also reduce the reliability of power supply.So it is of great significance to improve the power quality problem.D-STATCOM as an important equipment to improve the quality of power,can quickly compensate for the system's reactive power,inhibit the voltage fluctuations and flicker,improve system stability.In this paper,firstly the basic principle of reactive power compensation is introduced,and the mathematical model of D-STATCOM system is established.It can be seen from the model that D-STATCOM system is a nonlinear and mutual coupling system.Secondly,the method of detecting the compensation current is analyzed.Based on the study of the p-q and p qi-i method,it is found that the method cannot accurately detect harmonic,positive sequence and negative sequence components when the grid voltage is unbalanced or distorted.This paper presents an improved detection method,and through simulation to verify that it can accurately detect the various components when the grid voltage is unbalanced or distorted.On this basis,this paper adopts model predictive control.Firstly,the model predictive control of the finite switch sequence is studied deeply,and it is found that there is a drawback that the switching frequency is not fixed and the mathematic model used for prediction is not accurate.A model predictive control strategy with fixed switching frequency based on disturbance online observation is proposed.The simulation results show that the proposed method is effective in fixing the switching frequency and improving the accuracy of the model.On the basis of this,this paper further analyzes the running characteristics of D-STATCOM under unbalanced grid voltage and finds that it can cause double frequency fluctuation of the DC side voltage and overcurrent.The double frequency fluctuation of the DC side voltage passes through the switching function modulation,will lead to the output side AC current of the third harmonic.In this paper,the improved switching function is used to suppress the third harmonic.The ability to suppress overcurrent is analyzed by using model predictive control strategy with disturbance online observation.Finally,the effectiveness of the third harmonic and overcurrent suppression is verified by simulation.Finally,the experimental prototype and experimental results are introduced.The experimental prototype includes the main circuit,the detection circuit,and the control circuit,etc.The control system software and hardware were designed.Hardware design includes DSP and FPGA bus circuit design,A/D and D/A circuit design,relay circuit design,communication circuit design and so on.The software design mainly includes the design of the main program,the design of the interrupt program,the design of the fault protection program,the design of the communication program and so on.Finally,the experimental results are analyzed to verify the effectiveness of the model predictive control strategy.