Research On Main Circuit And Control Strategy Of H-Bridge Cascaded STATCOM

In recent years, static synchronous compensator (STATCOM) has become one of the major equipments used in improving power quality, with the advantage of reactive power compensation, voltage stability, better performance of harmonic suppression and three-phase balance. With the rapid development of high voltage and ultra-high voltage power system, the demand for high voltage and large capacity static synchronous compensator is increasingly vigorous. Since H-bridge cascade STATCOM (H-C STATCOM) has the characteristics of low harmonic output, high utilization of the device, and it is easy to realize the application of high voltage and large capacity expansion, it has attracted much attention. However, H-C STATCOM is a complex of flexible AC transmission system controller, there are still many problems to be solved on the main circuit design, the method of decoupling control, the current inner loop and voltage outer loop, node voltage control and capacitor voltage balance control.In this paper, the main circuit and control strategy of H bridge cascade static synchronous compensator is took for the study. Firstly, the working principle and mathematical model of two-level STATCOM is analyzed. The mathematical model for cascaded STATCOM of H-bridge is derived from the principle of two level STATCOM, and the H bridge module level superposition principle and the cascade is analyzed by switching function method. Based on these studies, makes an intensive study of the H-C STATCOM output control, and according to the H type of cascade STATCOM system characteristics of strong coupling and nonlinear, exploring the decoupling control strategy based on single neuron, the inverse system model with PI power outer loop control method, the parameter self-tuning fuzzy PI set PCC node voltage control technology. Secondly, as to the problem of DC capacitor voltage unbalance in H-C STATCOM, through the analysis of the DC capacitor voltage unbalance mechanism and influence factors of H-C STATCOM, the control method of fractional inhibitory imbalance is discussed, the basic principle of which is applying the positive sequence current controls H-C STATCOM overall voltage, and applying negative sequence current controls DC voltage balance in each converter, then by adjusting the voltage output amplitude and phase difference, the capacitor and voltage control of single HBI module can be achieved. Finally, in consideration of cascade number, connection inductance, determination of DC capacitor, power electronic device selection, the influencing factors of connection inductance, DC capacitor and switching frequency, the design method, principle and process of H-C STATCOM is analyzed, and a simulation experiment is carried on and the results of which are also given.The research of H-C STATCOM in this paper has practical value. The main circuit structure ofH-C STATCOM can provide a solid material foundation for devices with low harmonic output; the output control of H-C STATCOM can be completely decoupled under the condition of parameter perturbation, solving the problems of slow response speed and low precision due to incompletely control decoupling in ordinary feedforward decoupling method. The control method of H-C STATCOM DC side capacitor voltage unbalance can improve the robustness and disturbance resistance capacity of H-C STATCOM, and the design method of H-C STATCOM provides a reasonable technical route for H-C STATCOM engineering design. The simulation results further verify the correctness of H-C STATCOM main circuit and its control strategy.