Research On DC-Side Voltage Fluctuation And Control Strategy Of Low-Capacitance Cascade H-Bridge STATCOM

As the current advanced reactive power compensation device,Static Synchronous Compensator can quickly compensate reactive power in the power grid,effectively reduce power transmission losses,reduce power equipment capacity,improve power grid quality,and maintain grid voltage stability.The cascade H-bridge STATCOM has the characteristics of large capacity,easy expansion,and low output voltage harmonics,and is widely used in high-voltage and high-power applications.However,when the power of the device is large and the number of output levels required is large,the demand for the number of sub-modules will be greater,which means that there are a large number of DC capacitors in the system.In order to support the DC-side voltage buffering double-frequency fluctuation power,electrolytic capacitor with high power density is usually selected.Compared with electrolytic capacitors,film capacitors have a long life,good heat resistance,and are not affected by environmental changes.In order to improve the reliability of the cascaded H-bridge conversion device,extend the life of the device,and reduce the cost of capacitor maintenance in the later period,it can be considered to replace the electrolytic capacitor with film capacitor.However,the thin film capacitor has a low power density and a relatively small capacitance value,which brings great challenges to the control of the cascaded H-bridge converter.Therefore,it is of great significance to study the DC-side voltage control strategy of low-capacitance cascaded H-bridge converters to improve system reliability.Decreasing the capacitance will cause the STATCOM DC-side voltage ripple to increase,which will affect the system's compensation capacity when compensating inductive reactive power;it will cause the system's DC-side voltage ripple peak to be too large,which will increase the system switching device loss;and it will affect the accuracy of the low-capacitance cascade H-bridge STATCOM modulation strategy.This thesis analyzes the problems existing in the LC-CHB STATCOM and discusses its solutions.This thesis analyzes the topology of the cascade H-bridge STATCOM,establishes the cascade H-bridge STATCOM equivalent model,and analyzes its working principle in detail;The cascade H-bridge STATCOM mathematical model is established,and its double-loop decoupling control strategy is studied on the DQ axis.Simulation experiments verify that the STATCOM has good reactive power compensation performance using feedforward decoupling control.The factors affecting the voltage balance of the DC-side modules of the cascade H-bridge are analyzed,and the three-layer voltage balance control strategy of overall voltage balancing,clustered voltage balancing,and individual voltages balancing is adopted to control the DC-capacitance fundamental voltage of the H-bridge sub-modules to be consistent;The reasons for the double-frequency fluctuation of the DC side voltage of the cascade H-bridge STATCOM device are explained in detail;the effect of capacitor value on the DC voltage peak of the cascaded H-bridge STATCOM is investigated;At the same time,the impact of large fluctuations in capacitor voltage on carrier phase shift modulation strategy under the condition of low capacitance is analyzed.The combination of adjustable carrier amplitude PWM technology and single-pole frequency doubling carrier phase-shift modulation strategy can make it suitable for occasions where the capacitor voltage fluctuates greatly on the premise of meeting a higher equivalent switching frequency.The effectiveness of the voltage balance control strategy and the unipolar carrier phase shift modulation with adjustable amplitude of carrier is verified by simulation.To reduce the DC-side capacitor voltage fluctuation of low-capacitance cascaded H-bridge STATCOM devices,the model predictive control is combined with the control scheme of suppressing capacitor voltage fluctuations by injecting zero-sequence voltage,and a capacitor voltage fluctuation control strategy based on model predictive control is proposed;Control the amplitude of the zero sequence voltage injected into the cascade H-bridge within the range of the remaining modulation voltage,which avoids the possibility of over-modulation in the system.The model predicts control to predict the fluctuation of the capacitor voltage after the zero-sequence voltage is injected.The cascaded H-bridge capacitor voltage achieves the goals of minimal fluctuations and phase-to-phase voltage balance.Simulation and experimental results verify the effectiveness and feasibility of the proposed voltage fluctuation control strategy based on model predictive control.Analyze the impact of low-capacitance cascaded H-bridge STATCOM DC-side voltage fluctuations on the compensation capacity under different operating conditions,The corresponding relationship between the peak value of the capacitor fluctuation voltage and the peak value of the cascade H-bridge output voltage under the capacitive working condition is obtained.And it is concluded that the DC side capacitor voltage fluctuation has no effect on the compensation capacity of STATCOM to compensate the capacitive reactive power.According to the analysis of its voltage fluctuation rule,when the low-capacitance cascade H-bridge system compensates the capacitive reactive power,a control strategy to suppress the peak voltage of the capacitor fluctuation is used to inject a specific three-frequency zero-sequence voltage to control the STATCOM,make it when compensating capacitive reactive power,the peak value of capacitor voltage reaches the minimum under the premise of meeting the system modulation requirements.Simulation analysis verifies the correctness of the relationshipbetween LC-CHB STATCOM capacitor voltage fluctuation and system compensation capacity,And verifies the feasibility of the peak voltage suppression strategy of the cascaded H-bridge capacitor voltage under capacitive conditions.Combining the Matlab / Simulink simulation platform and the established experimental platform,the capacitor voltage balance control strategy,the capacitor voltage control strategy based on model predictive control,the capacitor voltage peak control based on triple-frequency zero-sequence voltage injection,and the technology of unipolar carrier phase shift modulation with adjustable amplitude of carrier proposed in this paper is used for verification.Simulation and experimental results show the correctness of the theoretical derivation results and the effectiveness of the proposed control strategy.