Hybrid Cascade Power Conversion System Based On Battery Energy Storage

The global electricity demand is increasing in recent years, and meanwhile, the traditional petrochemical resources are becoming scarce. New energy technologies have been widely applied and rapidly developed. However, the distributed power resources connecting to power grid has brought great challenges to the stability and controllability of power system. Battery energy storage system will be able to solve this problem by providing new technologies and methods. As an important part of the battery energy storage system, power conversion system will become the key point of research.This paper is supported by the Beijing Science and Technology Commission "Research and demonstration application of large-scale photovoltaic power plants based on the unified power control equipment and DC microgrid technology (D131104002013003)". It has done some theoretical research and simulation on the hybrid cascade power conversion system based on battery energy storage, and the main content is as follows:First, this paper presents a topology of the hybrid cascade power conversion system. The device mainly consists of battery packs, isolated half-bridge DC/DC converter and cascade DC/AC converter. The basic principles, control mode and power transmission characteristics of the two kinds of converters have been analyzed. Combined with development of the low voltage physical model in the laboratory, the power conversion system main circuit topology structure and the main parameters have been selected.Second, this paper mainly designs the coordinating control strategy of power conversion system:DC/DC utilizes phase shift control and duty cycle control to ensure the dynamic balance of power, maintaining DC capacitor voltages constant. In order to meet the power transmission target of the system, DC/AC side uses direct current decoupling control. In order to increase the device response speed and improve the quality of DC capacitor voltage, this paper presents the coordinating control strategy of DC/DC and DC/AC, real-time power instructions of the grid side are fed back to the DC/DC side. Based on Lithium battery energy storage model, a detailed hybrid cascade power conversion system model is established on a PSCAD/EMTDC platform. Simulations on normal regulation conditions and the conditions with different battery nominal parameters and state of charge have been conducted by applying the proposed coordinating control strategy. Simulation results demonstrate that under the. proposed control strategy, this device has the advantages of wide voltage matching capabilities, strong battery status adaptability, fast control response, large capacity energy storage and bi-directional power regulation.Finally, in order to deal with SOC imbalance problem of the cascade battery energy storage system, SOC self-balance control strategy is studied based on the amplitude of modulation wave fine-tuning. Then, the battery SOC imbalance condition has been simulated, which verified the effectiveness of the proposed control strategy. In addition, harmonic analyses are conduced respectively on carrier phase shifting SPWM control and SOC self-balance control. The study results show that SOC self-balance control strategy based on the amplitude of modulation wave fine-tuning can reasonably distribute power regulation amount of each battery, which will make all the SOC converge. Thus, it will improve the capacity and availability of the entire power conversion system, but due to the change of modulation amplitude, it will cause certain influence to the output voltage harmonic performance simultaneously.