Research On Cascaded Power Electronic Transformer Based On Re-purposed Batteries Energy Storage

In recent years,the proportion of renewable energy in the power system has gradually increased,but its output power has the characteristics of volatility and intermittentness,which limits the ability of renewable energy power generation to access the power grid on a large scale.The renewable energy microgrid can effectively improve the stability of the grid and improve the ability of the grid to accept renewable energy through the supporting battery energy storage system.The cost factor of large-capacity converter and energy storage battery limits the promotion and application of battery energy storage system,and decommissioned lithium battery and chain topology provide a new solution to solve the cost problem.This paper first analyzes the research status of the topological structure and control strategy of energy storage system,and designs a topology structure of cascaded Power Electronic Transformer(PET)with independent DC output.The working principle of cascaded H-bridge topology in PET is analyzed.Based on the mathematical model,single-phase dq vector analysis is used to realize power decoupling,and a current closed-loop controller is designed.The working principle,transmission characteristics and optimal modulation strategy of current-type bidirectional isolated DC-DC converter in PET are analyzed.The modal small-signal model is established and the voltage regulator controller is designed based on mode I.The energy storage system combines the control target and topology to design a three-layer control architecture of the energy layer,power layer and SOC layer.The energy layer responds to the scheduling command to stabilize the power fluctuation,the peak clipping and the reactive power compensation;the power layer distributes the active power based on the State of Charge(SOC)and equally divides the reactive power,making full use of the system capacity;SOC layer equalization The state of charge of the decommissioned lithium battery avoids the short board effect and stabilizes the high voltage bus voltage.Aiming at the problem of secondary ripple caused by AC/DC power mismatch,the power feedforward suppression strategy is designed.At the same time,the fault-tolerant design of each module is analyzed in view of the complexity of the system.The main circuit parameter design method and device selection of the system are given.The 3MVA/12 kV medium voltage system is designed.The simulation verifies the correctness of the control strategy.At the same time,a 1kW energy storage system verification platform and corresponding simulation system are designed.The feasibility and scalability of the system design scheme designed in this paper are verified by experiments and simulations.