Research On Control Strategy Of Cascaded H-bridge Multilevel STATCOM

With the increase of distributed energy and nonlinear load in distribution network,the power grid presents the trend of large power grid interconnection,which will produce power quality problems such as reactive power shortage,harmonic pollution and three-phase system imbalance.Static synchronous compensator(STATCOM)has been widely used because it not only compensates reactive power but also suppresses harmonics.In this paper,the current tracking control strategy of cascaded H-bridge multilevel STATCOM and the DC voltage sharing under unbalanced conditions are studied to ensure its stable operation in power system.Firstly,by analyzing the principle of cascaded STATCOM,the mathematical model of STATCOM in three coordinate systems and the design of main circuit parameters are given.The mathematical model of STATCOM in abc three-phase coordinate system andαβ,dq two-phase coordinate system is established by using input-output method,and the parameters of the main circuit are designed according to the voltage level and system capacity,which provides a mathematical basis for the analysis of the current and voltage control strategy.Secondly,the PI-VPI current tracking control strategy in dq coordinate system and the FCS-MPCC command current tracking control strategy in αβ coordinate system are compared and analyzed.The design method of PI-VPI controller is given,and the tracking compensation effect is analyzed according to the Bode diagram.The finite control set model predictive current control(FCS-MPCC)strategy is studied,and the basic principle,discrete predictive model and evaluation function of model predictive control are given,respectively.The model with current tracking control as the main goal and DC side voltage balance and switching frequency constraint as the auxiliary goal is established The multi-objective FCS-MPCC strategy seeks the optimal output through rolling optimization to minimize the error of compensation current tracking instruction current.The theory and simulation show that the FCS-MPCC strategy is superior in the control system structure and performance.Thirdly,in order to reduce the number of rolling optimization and the amount of calculation of FCS-MPCC,on the basis of model predictive current control,the adjacent vector group optimization model predictive voltage control(AVGO-MPVC)strategy is proposed.By Sector judgment,the reference voltage vector is located in a small sector,and the adjacent group optimization of a limited number of voltage vectors is carried out in a small sector,which greatly reduces the number of optimization.Aiming at the mismatch phenomenon of inductance parameters,an inductance observer is designed by using the method of parameter identification,and the system stability under AVGO-MPVC strategy is analyzed;the causes and Countermeasures of each delay phenomenon in STATCOM device are analyzed.Finally,aiming at the unbalanced DC side voltage under unbalanced conditions.On the basis of hierarchical control,the mathematical model of active power exchange of cascaded H-bridge STATCOM under unbalanced conditions is constructed.For the control of intra phase DC voltage balance,a control strategy based on carrier phase shifted PWM(CPSPWM)modulation correction method is proposed.For the control of inter phase voltage balance,an improved zero sequence voltage injection method based on superposition feedforward control is proposed,which is compared with the existing zero sequence voltage injection method In contrast,the voltage injection method.Simulation and experimental results show the effectiveness of the proposed strategy.This paper has59 pictures,5 tables and 94 references.