Research On The Control Strategy Of Medium Voltage Cascaded STATCOM With LCL Filter

At present,with wide application of non-linear,impact,and unbalanced load which is due to industrial development,the problems of electrical power quality is more and more serious including low power factor,harmonic pollution,three phase unbalance.The static synchronous compensator(STATCOM)which can solve all the aforementioned problems receives much concern,and has a wide application prospects in distribution network.This thesis studies several key technologies of Cascaded STATCOM.Firstly,mathematical model of STATCOM with LCL filter is derived based on abc coordinate frame and dq coordinate frame,which provide the theoretical basis for designing control system.In order to design LCL filter parameters,the filter principle is analysed and the relationship between filter performance and its parameters is studied as well.Then the design constraint conditions and procedures of LCL filter are given.At last,the simulation results demonstrate the effectiveness of filter parameters design method.Secondly,LCL filter damping method and current tracking are studied.In digital control system,the control delay has influences on the STATCOM stability and active damping of instant capacitance current feedback is researched,which can reduce the control lag to 0.5-stepdelay to enhance system stability.In addition,an integrated controlling method combining PI and repetitive control is introduced to improve the harmonic wave depression ability.Considering the compensation current distortion when load fluctuates,an optimal control method,which compares current tracking error present and 1/6 cycle before to adjust repetitive controller input,is given.When the load fluctuation is severe,the PI controller works alone,and the repetitive controller is put into operation when the load is stable.Simulation results verify the strategy and the control delay of repetitive controller drops to 1/6 of fundamental,dynamic response speed is improved correspondingly.Thirdly,a hierarchical control strategy for capacitor voltage,including overall control,clustered control and individual control,is adopted to guarantee the voltage balance of DC bus.A zero sequence injection method is used to realize clustered balancing.To achieve individual balancing control,PD-PWM is used and modulation signals are distributed according to voltages and charge-discharge state of each H-Bridge to realize energy balance.Simulation results confirm that this method is effective.Finally,the control strategy for compensating unbalanced load is researched.In order to compensate fundamental reactive power and negative sequence current at the same time,the positive-sequence and the negative-sequence mathematic model of STATCOM are set up,and then positive and negative sequence closed-loop current control strategy is adopted to realize current tracking.Aiming at DC voltage balancing among phases of three-phase wye configuration,zero-sequence voltage injection method is applied and it is influenced by the ratio between reactive component and negative component of compensation current.Considering limited output voltage of device,the output voltage will cause over modulation when zero-sequence voltage is excessive,so an improved control strategy is proposed to suppress the negative-sequence component of instruction current which depends on the operation limits of STATCOM.The simulation results show that the device can still work stably at high unbalanced degree condition,which reduces the unbalanced degree of grid current effectively.