| As the key equipment of battery energy storage, a battery energy storage bidirectional converter’s performance has important effects on battery charging performance, battery life, rate of power loss, quality of electric flows into grid, and grid stability. Based on the characteristics of battery charging and discharging, this paper designs smart battery charging and discharging curves of which charging combines constant current charging, constant voltage charging, and constant float charging, and discharging is constant current discharging. Based on the smart battery charging and discharging curves, system control structure of the cascade system of three-phase PWM rectifier and bidirectional DC/DC converter is designed. The three-phase PWM converter uses double closed loop control strategy to rectify and ensure the grid running under unity power factor. The bidirectional DC/DC converter uses a closed-loop negative feedback control to guarantee the constancy of output voltage or output current. This paper mainly focuses on bidirectional DC/DC converter control strategy, design of output filter, and interactions of cascade system.Firstly, main circuit of bidirectional DC/DC converter is simplified and mathematical model is established. In order to improve the input voltage disturbance rejection capability of bidirectional DC/DC converter, indirect sliding mode control based on double integrals is studied. Results show that indirect sliding mode control based on double integrals changes the system type, improves the steady-state performance, eliminates resonant peak, increase the closed-loop bandwidth, improves dynamic performance, and enhances the ability to inhibit the input voltage disturbances.Secondly, an improvement is made in output filter of the bidirectional DC/DC converter. The original LC filter is altered to LCL filter, and designing steps are given in this paper. Results indicate that when LCL filter is used at the output terminal of battery energy storage bidirectional converter, it can provide a low impedance path for switching ripple component of the output current, reduce the switching ripple of output current.Finally, mathematical models of three-phase PWM rectifier and bi-directional DC/DC converter are established using two-port parameters method. Interaction between the two is analyzed by establishing equivalent circuit of the impedance model and system stability determination method is given in this paper. Studies results show that when parallel resonance occurs in cascade system, the existence of bidirectional DC/DC converter increases the switching ripple of the DC bus voltage. |
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