Research On An Improved Bi-directional Z-source Converter For Energy Storage

Energy storage system which can increase energy inertia of system, improve system stability and operation effectiveness is widely used in power systems, electric vehicles, rail transport, and renewable energy generation. It is becoming a research hotspot now. Usually, energy storage system connects to the grid through a converter which controls the input and output power of the system and determines the system performance and efficiency. The converter is generally divided into single-stage and double-stage structure. Single-stage is simple in structure, but it does not have the boost ability for voltage and imposes exact restrictions on input voltage. Double-stage structure can bear wide range variation of the input voltage, but it has a low efficiency and high cost. So it’s significant to find a structure which has advantages of both single-stage and double-stage structutres.Z-source inverter which can achieve boost ability through sing-stage structure has no dead zone for output waveform, low harmonic content and high efficiency features, then combined merits of two structures. However, it can’t be used in energy storage system because of short of dual power flow ability. In this paper an improved Z-source converter is proposed. It retains the advantages of traditional Z-source inverter, then eliminates the abnormal state, reduces capacitor voltage stress and implements dual power flow. The operation principle based on improved topology is discussed and detailed operation modes from the view of voltage and current both in inverter and rectifier operation have been analyzed. Then the theoretical foundation of elimating abnormal state and achieving dual power flow is also presented. The mathematical model of the improved Z-source converter is gived and the relationship between the minium phase characteristics and system parameters has been analyzed. Also a dc-link control method with dc-input feed-forward compensation is proposed. Based on these theoretical analysises, an improved SVPWM modulation strategy with shoot-through vector and its simple implementation algorithm are put forward too.Then the simulation and experiments are applied to verify the theories presented before. The hardware circuit based on simulation model is designed containing key components parameters, schematics and software method. Simulation and experiment results indicate that the proposed topology not only reserves advantages of low capacitor voltage stress and small start-up inrush current, but also realizes operating at reactive load and dual power flow. The proposed dc-link control strategy can keep dc-link voltage stable under the disturbance of ac output and dc input. In conclusion, this new topology can meet the basic needs of energy storage applications and provide a new development idea for converter topology.