Research On Cascaded H-bridge Multilevel STATCOM Control Strategy

In recent years,power quality problem has become more and more serious with the continuous increase of non-linear,impact and unbalanced loads,the problems of reactive power,harmonics and three-phase imbalance are the most prominent.Cascaded H-bridge multilevel STATCOM is widely used as a reactive power compensation device to improve above power quality problems,in this thesis,the control strategy of the cascaded H-bridge STATCOM in reactive power compensation mode and voltage compensation mode is studied.Firstly,introduce the main circuit topology and working principle of the cascaded H-bridge multilevel STATCOM,using its equivalent circuit structure,the mathematical models of STATCOM in abc and ?? coordinate system are established respectively.Through the coordinate transformation relationship,the mathematical models based on the dq coordinate system in the balanced and unbalanced working conditions are deduced,so as to provide a basis for the subsequent chapter research.According to the system performance requirement,the main circuit parameters of STATCOM are designed.Secondly,study the feedforward decoupling control strategy of STATCOM in reactive power compensation mode,for improving the current tracking accuracy of STATCOM,repetitive control is introduced.In order to solve the problem of slow dynamic response of repetitive control,this thesis proposes a compound repetitive control by reducing the repetitive control cycle and using repetitive control in series with PI control,at the same time,gives the system parameter design process.So as to solve the problem that the order of internal model in compound repetitive control is non integer,which affects the current tracking performance of STATCOM,the fractional compound repetitive control is applied to STATCOM,simulation results show that the fractional compound repetitive control has better harmonic current suppression performance.The DC-side voltage control adopts three-level balance control strategy,the active power exchange between STATCOM and power grid is used to achieve overall voltage control,phase-to-phase voltage control is realized by the zero-sequence voltage injection method,by using improved same carrier stacking modulation method,the modulation wave of each H-bridge unit is adjusted in real time according to the level of the capacitor voltage and the charge or discharge state,thereby achieving the phase-in-phase voltage control,the simulation shows that the control strategy is correct and effective.Thirdly,in order to improve the dynamic response performance of STATCOM,a multi-objective model current predictive control(MCPC)strategy based on ?? coordinate system in reactive power compensation mode is proposed,and the realization method of multi-objective MCPC strategy is given.Taking five-level STATCOM as an example,sector partitioning is used to reduce the number of rolling optimization for the optimal switching state.For the model mismatch phenomenon that may appear in the STATCOM system is corrected by adopting the inductance parameter observation method.Simulation has verified that the multi-objective MCPC strategy can effectively compensate reactive current,harmonic current and unbalanced current,compared with the feedforward decoupling control strategy,its harmonic current suppression performance is slightly worse,but it has the advantage of fast dynamic response.Finally,study the voltage compensation control strategy of STATCOM under unbalanced load condition.In order to achieve point of common coupling(PCC)voltage phase locking,a phase-locked loop based on cross decoupling resonator is studied,for the DC voltage component in the power grid,a phase-locked loop based on improved cross decoupling resonator of DC component elimination is proposed.By analyzing the active power absorbed by STATCOM under unbalanced condition,the zero-sequence voltage for phase-to-phase voltage equalization under unbalanced working condition is derived.In order to eliminate the voltage drop and voltage imbalance of PCC caused by unbalanced load,a voltage compensation control strategy based on the positive and negative double-sequence current loop superposition is studied,simulation verifies the effectiveness of the studied control strategy.This thesis includes 76 figures,9 tables and 92 references.