Research On Power Quality Active Control Strategy Of Microgrid

Microgrid is proposed to solve the problems caused by distributed generation on grid effectively. In order to ensure the efficient and reliable operation of microgrid and meet the higher power quality of the customer demand, power quality control becomes one of the key techniques in microgrid research. As a result of the widely use of power electronic-interfaced microsources, microgrid is quite different from distribution grid. By adopting proper control strategy, power quality active control becomes possible, so microgrid can realize the active control on power quality mainly by composite power quality control and improving the operation performance of microgrid converter. This thesis takes microgrid as the research object to conduct research on power quality active control strategies, including multi-objective control strategy of energy storage interface converter for power quality compensation, microgrid power quality control strategy based on energy storage interface double four-leg converter, and control strategy of microgrid converter for better performance under non-ideal grid voltage conditions.Firstly, for the topology, operating characteristics and load characteristics, the structure and control strategy of microgrid are analyzed. The analysis indicates that energy storage microgrid converter has the same topological structure as power quality management equipment, so it can also be used in power quality active control. Secondly, by analyzing and comparing the three microgrid PQ control strategies, the existing problems are pointed out. The analysis indicates that inductance parameter error and grid impedance cause reduction of system performance. In order to improve performance, the thesis proposes a novel microgrid PQ control strategy to realize decoupled control of the power and current separately. Finally, the norton equivalent circuit of the microgrid converter using the PQ control strategy is established. Based on the norton equivalent circuit, a equivalent model of microgrid is established to analyse the reasons of voltage fluctuation and the harmonic resonance mechanism in detail.In order to realize compensative current detection and reference current calculation, the decomposition method of positive and negative sequence components based on adaptive notch filter(ANF) is used to extract the fundamental positive and negative components of grid voltage, which can effectively restrain the voltage unbalance and harmonic interference. To improve energy storage utilization efficiency and achieve active compensative function of power quality, the multi-objective control strategy of microgrid energy storage converter with active power quality management functions is researched. With the proposed control strategy, the energy storage system can not only smooth the power fluctuations, but also compensate current harmonics, reactive power and current unbalance under the control mode of all compensation, and selectively compensate current harmonics, reactive power and current unbalance under the control mode of selective frequency-division compensation based on proportional vector PI (PVPI) control. Adopting the proposed two control strategies, the utilized rate of energy storage system can be enhanced effectively, resulting in the improvement of microgrid power quality and the suppression of its impact on distribution grid.Furthermore, based on research results of the multi-objective control for microgrid energy storage converter, the control strategies of super capacitor(SC) energy storage double four-leg converter(Microgrid Power Quality Controller, MGPQC) are studied to solve interactive influence of the power quality between distribution grid and microgrid. In order to compensate grid voltage sags at the point of common coupling(PCC), a coordinated control strategy based on SC-SOC is proposed to when MGPQC working in series-parallel mode. The proposed control strategy keeps the normal level of grid voltage at the PCC through coordinated microsources, SC and load shedding. In series mode, the influences of voltage unbalance and distortion on the power transmission characteristic of MGPQC is analyzed. In order to improve microgrid power quality and avoid the influence of distribution grid on microgrid, a control scheme using SC energy storage to supress the DC-link power fluctuation is proposed. In the current inner loop, a multi-resonant proportional resonant PVPI controller is adopted to realize the unified positive-and negative-sequence current control. Based on the above methods, power quality of microgrid is improved, and the operation ability connected with grid is elevated effectively.In order to enhance the operation performance of microgrid converter using LCL filter under the unbalanced and harmonic distorted grid conditions, combined with the capacitor-current-feedback active damping and grid-voltage-feedforward control, a microgrid converter PQ control strategy is proposed based on PVPI controller. With the proposed control strategies, neither phase-locked loop nor extraction of positive/negative sequence current feedback signal is needed, simplifying the control structure, improving the power quality of output current and enhancing the operation performance of microgrid converter under non-ideal voltage situations. The system is of high order, many parameters and complicated. Besides, the proper parameter design method should be used to meet the design requirements that enhance the robustness of system. Therefore considering these factors, a simplified parameters design method which can realize better robustness is proposed. The effects of the capacitor-current-feedback coefficient and the parameters of PVPI controller on the current-loop performance are investigated with frequency characteristics analysis, i.e. the stability, phase and amplitude margins. According to the requests of the stability, phase and amplitude margins, the capacitor-current-feedback coefficient, the relative resonant gain factor and the proportional factors could be designed separately. The proposed method simplifies the parameters design procedure using analytic calculation and further improves stability, highly robustness, fast transient response performance of the microgrid converter.