Research On Control Strategy Of Chain STATCOM

With the rapid development of China’s power industry,industrial load and residents’ living load are increasing day by day,which may produce more reactive power and affect the normal operation of the power system.China has high requirements for reliability and practicability of power industry.Energy saving,environmental protection and high efficiency have become the main trend.Therefore,it is very important to carry out reactive power compensation and improve power quality.Static synchronous compensator(STATCOM)is a kind of dynamic reactive power compensation device based on voltage source converter.It is a typical representative of the second generation flexible AC transmission system.Cascaded H-bridge(chain)STATCOM has the advantages of small volume,small loss,fast dynamic response,flexible output,small output harmonic content and low cost.It is widely used in power system and plays an increasingly important role in the field of reactive power compensation devices.Firstly,this dissertation introduces the multilevel inverters with different topologies,and analyzes the reason why the chain structure is selected as the main circuit topology of STATCOM;According to the main circuit topology of three-phase star chain STATCOM,its working principle is analyzed in detail;After coordinate transformation,the mathematical model of chain STATCOM is established in the synchronous rotating coordinate system,and the parameters of the main circuit are designed according to the model.Secondly,a double closed-loop control system is constructed in the synchronous rotating coordinate system to control the output voltage,current and DC capacitor voltage at the AC side.The inner control loop of the double closed loop control system is the current control loop,and the outer control loop is the voltage control loop.This dissertation mainly analyzes the current inner loop decoupling control,including current feedforward decoupling control and internal model decoupling control;The design of the voltage outer loop control part is based on the principle of energy balance.According to this principle,a three-level control strategy of DC side capacitor voltage is proposed,including overall,phase to phase and in phase,to ensure the stability of DC side capacitor voltage,so that it will not interfere with the stable operation of STATCOM.Then,the control strategy of STATCOM when the system is unbalanced is introduced.The current loop control adopts the positive and negative sequence double current inner loop control strategy,and the voltage loop control still adopts the three-level voltage control strategy.At the same time,it also introduces the method of real-time reactive current detection-IP IQ method,and the modulation strategy of chain STATCOM-carrier phase-shifting modulation strategy(CPS-SPWM)used in this dissertation.In order to verify the correctness of the theoretical analysis,the STATCOM model is built in MATLAB/Simulink and simulated.From the simulation results,it can be seen that the decoupling control effect is good,and the chain STATCOM can maintain continuous operation,which verifies the effectiveness of the control strategy used in this dissertation.Finally,the reactive power compensation strategy used in this dissertation is applied to the wind power grid connected system,and the effectiveness of the reactive power compensation strategy used in this dissertation is verified by building a simulation model in MATLAB/Simulink.