Control And Applications Of Electronic Power Transformer

With increasing pressure of global resource shortage and environmental pollution, human society has improved their requirements of environmental protection, energy conservation and emission reduction, and sustainable development. Smart grid is the ideal scheme to solve the energy transmission problem in the future and is the core of China’s energy structure transformation, and energy conservation and emission reduction. However, the realization of smart grid’s various objectives and functions mainly depends on the performance and smart level of electrical elements in the power grid. Obviously, the traditional transformer can hardly meet all the demand of future power system, while the Electronic Power Transformer (EPT) could provide an effective way to manage the smart grid owning to its excellent controllability, rich AC and DC interfaces and other functions. This paper mainly studies the modeling of EPT and its application in power system.This paper firstly establishes power flow calculation model and transient stability analysis model of EPT. The steady state mathematical model of EPT is built for Newton’s power flow calculation, and a power flow calculation algorithm, which calculates AC system and DC system alternately, is studied to calculate the power flow of a power system with EPT. Based on the method of injecting current at nodes, a detailed model of EPT is established for transient stability analysis. The steady-state simulation and electromechanical transient simulation of power system with EPT is realized by making use of user program interface (UPI) function in power system analysis software package (PSASP).Small disturbance stability analysis model is built for EPT. A new method is proposed using EPT supplementary damping controller to improve power system damping, and the design procedures of the supplementary damping controller parameters bases on frequency response method are presented in detail. The simulation results indicate that EPT equipped with supplementary damping controller can significantly increase system damping and effectively restrain low frequency oscillation.The application of EPT in microgrid is investigated. The mathematical model of directly driven wind turbine with permanent magnet synchronous generator (D-PMSG) integrated to the grid via an EPT with energy storage system is established, and a control strategy is proposed to smooth the wind power fluctuations and enhance the low voltage ride through (LVRT) capability of the system via the appropriate control of the charge and discharge current of the energy storage system. The mathematical model of EPT used in distribution network is established, and a control strategy is proposed to improve the capacities of low voltage ride through and unbalance load current management. The positive and negative sequence decoupled control strategy used for the high voltage side cascaded H-bridge converter, the phase shift control strategy for the isolation stage, and the positive-, negative-and zero-sequence decoupled control strategy for the low voltage side three-phase four-wire converter are presented in detail. The design of zero sequence voltage resonant controller based on root locus method is investigated, which could follow-up zero sequence voltage without steady error.At last, the design and implementation of the10kV/400V1MVA EPT and the experimental results are presented in detail. Control strategies are proposed for EPT to connect two grids and to limit inverter current under fault conditions, the effectiveness of the control strategies are verified by the experimental results.