Study On The Topology Of A Novel Generalized D-STATCOM And Its Robust Current Control Strategy

With the development of the national economy and advancement of industrial technology,people have higher requirements for the power quality of distribution networks.Distribution static synchronous compensator(D-STATCOM)is an important power technology device.Compared with traditional reactive power compensation devices,it has the advantages of fast response,small size,large capacity and ideal output characteristic,it has become an important means to improve the power quality of distribution network.When the D-STATCOM compensate for reactive power,it can also generate negative-sequence current to compensate for three-phase unbalance.For the cascade H-bridge D-STATCOM,dc side isolated from each other,and three phases are independent each other,so the active power can't flow freely between the three phases,and when there is a negative-sequence component,interphase balanced control of dc-side voltage is difficult to achieve.In order to stable operation under unbalanced conditions,this paper propose a new hybrid multi-level topology D-STATCOM with cascaded H-bridge topology as the main inverter and dc-side common capacitor topology as the slave inverter.When the new D-STATCOM working in the unbalanced conditions,the new D-STATCOM reasonably distribute the output voltage of the main inverter and slaver inverter of a certain phase under master-slave coordinated control,so that the output voltage is perpendicular to the output current of the phase,it can effectively solve the problem of interphase balanced control of the dc-side voltage.Due to the existence of negative-sequence components during the new D-STATCOM compensating for three-phase unbalance,it is difficult to design the current controller.By establishing the positive-sequence and negative-sequence equivalent model,individual sequence control is performed under dq coordinate system,and PI controllers' parameters are designed at the same time.Because the new D-STATCOM is a nonlinear,strong coupling and multi-variable complex control system,the conventional linear decoupling control method can't achieve the effect of control,in order to improve the dynamic performance and robustness of the new D-STATCOM,the affine nonlinear mathematical model of the new D-STATCOM is established,and state feedback exact linearization control strategy is proposed based on differential geometric principle,active and reactive power decoupling control can be realized.For the perspective of the stability of the new D-STATCOM,this paper proposes a nonlinear control strategy based on passivity theory and established Eular-lagrange system model in the dq coordinate system.The system accelerate the dissipation of energy and improve the speed of convergence of the control system by injecting damping r1 and r2.The simulation proves that the new D-STATCOM nonlinear control strategy based on passivity theory have more robustness.Dc-side voltage stability is the key to safe and reliable operation.But the series loss,shunt loss and hybrid loss always occur during the operation.It is analyzed that the dc-side voltage fluctuation is related to the shunt loss,hybrid loss and pulse delay by establishing the dc-side state equation for the i-th H-bridge unit.From the deviation of the active power absorbed by each phase respect to the average active power of three-phase,it can see that the negative-sequence component is root cause of dc-side voltage interphase unbalance.The dc-side voltage is modulated by global control,phase balance control and phase internal equilibrium control in this paper.The global control considers all the dc-side voltage as a whole,and obtains the corresponding active compensation current through PI control,the output voltage of the chain main inverter is perpendicular to the compensation current by reasonably distributing the output voltage of the master and slave inverter in phase balance control,the phase internal equilibrium control is realized by energy exchange between modules.The effectiveness of the dc-side voltage control strategy is verified by simulationThe above control strategies are verified by simulation,and the simulation platform is built by using RT-LAB.A low-voltage new D-STATCOM prototype development plan is presented in the ending.The main controller based on DSP+FPGA architecture is used to design a software-to-hardware system-level development plan.