Study On DC Side Voltage Balance Control Of Cascaded H-Bridge STATCOM

Cascaded H-bridge static synchronous compensator(STATCOM)is one of the important means to stabilize the voltage level of power system by dynamically compensating the reactive power in the network.The device has the advantages of high degree of modularization,easy expansion of the level and adaptability to different voltage levels,which represents the development direction of the new reactive power compensation device,and has become the research hotspot in the field of electricity.However,because the DC side capacitance of H-bridge module is independent,the DC side voltage is easily unbalanced,which directly affects the output waveform quality of the device and endangers the safe operation of the device,so the balance control of DC side voltage is one of the key technologies of cascaded H-bridge STATCOM.This paper takes the cascade H-bridge STATCOM device as the research object,mainly carries on the following four aspects work.This paper introduces the topological structure and working principle of cascaded H-Bridge statcom,establishes the equivalent model of the main circuit,and analyzes the control strategy of Feedforward decoupling according to the characteristic of equivalent model.Based on the working state of H-bridge module,by using the bipolar carrier phase shift modulation strategy and the frequency doubling unipolar phase shift modulation strategy,the statcom output waveform is analyzed mathematically,the characteristics of the two kinds of modulation are obtained,the validity of the theoretical analysis is verified.In order to solve the problem of DC side voltage imbalance,this paper establishes the equivalent model of H-bridge module's active loss,obtains the mathematical expression of the DC-side capacitor voltage in steady state,and according to the expression,the factors that affect the DC side voltage are the difference of parallel loss,switching loss and pulse delay.Based on this,the simulation model is built and the above three factors are simulated and validated.Meanwhile,the series losses that may occur in the power grid,the differences of voltage harmonics and capacitance capacity of DC side is also simulated,and the results show that the DC side voltage imbalance is related to the difference of the parallel loss,the switching loss and the pulse delay,but it has nothing to do with the series loss,the harmonic of the power grid and the capacitance value,The correctness of theoretical analysis is verified.In this paper,a hierarchical control strategy is used to control the DC side voltage of cascaded H-bridge STATCOM.The power model of the device is established,the flow of active power is analyzed,and the phase equilibrium strategy of 0-sequence voltage injection and the phase balance strategy of negative sequence current injection are studied according to the factors influencing the active power flow of interphase.Then the required 0-sequence voltage and negative sequence current are deduced.Two phase balance strategies are validated by building simulation,and the results show the effectiveness of the two strategies.Aiming at the voltage balance control in the DC side,because of the characteristic of pulse delay in the unipolar frequency carrier phase shift modulation,this paper analyzes the modulation strategy of carrier rotation,which can alleviate the voltage imbalance caused by the modulation method,but cannot cope with the voltage imbalance caused by the loss difference.Therefore,this paper studies the balance strategy based on modulation wave adjustment,and builds the simulation to verify that the method can effectively balance the DC side voltage.In this paper,based on the experimental platform of Cascaded H-bridge STATCOM,the control circuit of DSP+FPGA is designed,and the modulation strategy and the voltage balance strategy are validated experimentally.The results show that the control strategies adopted in this paper can achieve the control target effectively and have good dynamic and static characteristics.