Abstract
The microgrid is an effective distributed energy utilization form, and energy storage system plays an important role in it, which realizes the energy flow from the grid side to the energy storage system through a bidirectional DC/DC converter. In a distributed power generation system, the DC/DC converter is generally considered as the first stage converter, and the microgrid inverter is seen as the second, forming a two-stage cascade system. Now the most commonly used DC/DC converter is the buck-boost converter, but a low-frequency right half plane (RHP) zero exists in the small signal model of the converter when working in the boost mode, which limits its control bandwidth and results in a mismatch with the microgrid inverter. This situation will reduce the stability of the cascade system. This paper uses the impedance ratio criterion in the cascade system stability study to analyze the causes of this phenomenon. Through simulation in MATLAB/Simulink, it can be seen that the control bandwidth mismatch leads to low-frequency oscillation and slow response speed of the DC bus voltage, and the stability of the cascade system is poor. To solve the above problems, the sliding mode control method is used to design the control system of the first stage converter. The simulation shows that adopting sliding mode control, the DC bus voltage has no low-frequency oscillation and has quick response speed under different operating conditions. The system stability is greatly improved.
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