Research On The Control Strategy For MMC Based STATCOM

In recent years,with the continuous improvement of the voltage level of the power system and the expansion of the power system scale,the STATCOM(static synchronous compensator)is also developing towards high voltage and large capacity.The MMC(modular multilevel converter)has many cascaded sub modules,so the output voltage is closer to the sine wave and the harmonic content is very low.It will not bring great harmonic pollution to the power grid when it is used in parallel reactive compensation.Therefore,the adoption of more advanced control strategy to control modular multilevel converter is of great significance to improve the performance of STATCOM for reactive compensation of power grid.Aiming at the problem of capacitor voltage balance control of MMC sub module,the mechanism of MMC internal energy change and transfer is analyzed,and a three-level balance control strategy of MMC-STATCOM capacitor voltage based on energy analysis is proposed.Aiming at the problem of poor dynamic performance of MMC controller,feedback linearization theory is used to decouple MMC-STATCOM system first,and then an inner and outer double sliding mode controller is designed.The main contents are as follows:(1)The working principle and mathematical model of MMC-STATCOM and the method of detecting reactive current are all introduced systematically.(2)An energy transform and transfer for MMC is deep analyzed,which explains how the energy can be transmitted from one phase to another phase,between the upper and lower arms,and among the SMs.The determinant of influencing energy balance of MMC is discussed.The circulating current suppression strategy based on quasi-PR controller is adopted.Thereby,the three-level energy balance control strategy is used for reference to realize the balance control of capacitor voltage.(3)The feedback linearization theory and the sliding mode control principle are introduced.Aiming at the characteristics of nonlinear and strong coupling of MMC-STATCOM system,the complex nonlinear system is decoupled and reduced to a first order linear system by using the principle of feedback linearization.Then,the voltage outer loop sliding mode controller and the current inner loop sliding mode controller are designed to control the reactive power compensation and reactive power generation.(4)The three-level capacitor voltage balance control strategy and the double sliding mode control strategy are simulated respectively by building a STATCOM simulation model based on MMC.The simulation results are analyzed and researched.The simulation results show that the capacitor voltage large fluctuations caused by load unbalance between phases can be smoothed by utilizing the three-level energy balance control of MMC.The occurrence of the STACOM's unstable operating which is caused by the load unbalance between phases can be suppressed as well.Therefore,the proposed capacitor voltage balance control strategy can effectively enhance the security and stability of the MMC-STATCOM.The double sliding mode control for feedback linearization decoupling strategy has good dynamic performance when the system operates in reactive power compensation and reactive power generation.