Research On Control Strategy Of Power Electronic Transformer With Access Of Photovoltaic-Storage System And Charging Station

Power electronic transformer(PET)is one of the key equipments of Ubiquitous Power Internet of Things and important interface for the interconnection of AC/DC hybrid microgrid and distribution networks.It can realize the controllable electric energy conversion and isolation of power quality disturbance to some extent.Studying coordinated control strategy of PET,distributed generation,energy storage and AC/DC load helps improve the power grid’s new energy consumption capacity,optimize the performance of microgrids and the quality of load power supply.This thesis focuses on PET with access of photovoltaic-storage system and electric vehicles charging station.The second,third and fourth chapters introduce the control strategies of each part.The specific research content is as follows.Firstly,the research status of PET at home and abroad in terms of typical topology,voltage sag isolation capability,and coordinated control with DC microgrid is summarized.Then,a new PET topology with access of electric vehicles charging station used in this paper is selected.On this basis,a PET working mode switching control strategy is presented.The strategy selects the switchable power module working mode based on the predicted load power and the current AC capacity,and then uses ramp control to achieve a smooth switching of the switchable module working mode.Secondly,the photovoltaic power generation system and a hybrid energy storage system composed of multiple parallel storage batteries and super capacitor are connected to the PET low-voltage DC bus.The photovoltaic power generation system uses a Boost+unidirectional full-bridge two-stage DC/DC converter.The battery and super capacitor use non-isolated half-bridge bidirectional DC/DC converter.A supercapacitor control method based on the instantaneous power of PET low-voltage DC capacitors is proposed,which can quickly stabilize the voltage of PET low-voltage DC buses under load power fluctuation conditions.And the coordinated control strategy of the converters under various working conditions is designed to makes full use of the complementary operating characteristics of the battery and supercapacitor and the photo voltaic power generation and improve the disturbance isolation capability of PET.Finally,a battery control strategy that considers the voltage sag recovery process and SOC is proposed.By improving the droop control characteristic of the battery,this strategy can not only achieve the balanced control of the SOC during the battery charging and discharging process,but also realize the smooth transition of the battery charging and discharging power,preventing the battery from impacting the DC bus when the voltage sag is restored.The above main circuit model of PET with access of photovoltaic-storage system and charging station and corresponding controller model were built in the PSCAD simulation environment,which verified the correctness of the theoretical analysis and the proposed coordinated control strategy.