| Wind and photovoltaic power generation are influenced by wind speed and illumination intensity,leading to intermittent and randomness of Renewable Energy Generation System(REG).Traditional power balance device used in DC microgrid,absorb Renewable power fluctuations directly by the energy storage device,with the large capacity storage equipment and high cost.Aiming at the above problem,the DC Electric Spring(DCES)has been presented as a new device for power balance.The DCES divides the load of DC microgrid into the critical load(CL)and the non-critical load(NCL),according to the range of voltage fluctuations that the load can withstand,while the requirements for the CL voltage are more precise but the NCL can withstand larger fluctuations.DCES transfers some of the power fluctuations from DC microgrid to the NCL,while the remaining energy fluctuations are borne by the battery,to maintain the power balance and voltage stability of the CL on the DC bus.Therefore,the proposed device can realize the concept of matching the power generation with consumption,while reducing the capacity of energy storage system.In view of the defects of the existing DCES topology,such as large volume,complex control,this paper proposes the topology,control method,and soft-switching analysis of the new DCES.Besides,this paper also proposes the energy management system for signal DCES applied in household photovoltaic system,and the distributed cooperative control strategy for multiple DCES applied in DC microgrid.The novel DCES proposed in this paper consists of a three-port DC converter(TPC),a bidirectional buck-boost converter(BBC),and a battery pack.Based on the topology,this paper firstly introduces the equivalent circuit,establishes the power and voltage equations of the system,and establishes a small signal mathematical model at the static operating point.For the TPC part,this paper adopts control methods such as phase shift and decoupling control,designs the coupling network.This paper also analyzes the ZVS soft-switch of the new DCES,expresses the ZVS boundary conditions with equations,and analyzes the influence of circuit parameters on the ZVS region,such as three winding leakage inductances,resistance of CL and NCL,so that expand the ZVS zone by selecting appropriate circuit parameters.Based on the new DCES,a simulation model is built in this study to analyze the feasibility of the control strategy and the stability of the system under different load conditions.Besides,this study builds an experimental hardware platform,introduces the system software and hardware design,and verifies the effectiveness and practicability of this new type of DCES.Based on the structure of the DCES internal energy storage system,this paper proposes an energy management system(EMS),applied for household photovoltaic system.The proposed EMS detects the Critical DC bus and battery voltage,and controls the operating mode transition of the BBC under normal,specific and emergency conditions,based on the influence of weather information.By detecting and calculating the Stage of Charge(SOC)of the battery system,the EMS can change the operating status of the battery system and implement a variety of charging and discharging control,to maintain continuous and reliable operation of the battery and the DCES.Finally,simulation models and laboratory prototypes verify the effectiveness of the proposed EMS.Finally,this paper studies the layout structure and cooperative control strategyof multiple new DCES paralleled in the DC microgrid.The proposed distributed cooperative control method,consisting of primary control and secondary control,can be used to achieve voltage stability,power distribution of multiple DCES,and consistency of critical DC bus voltage and battery SOC between multiple DCES.The proposed distributed cooperative control solves the problems of collaborative control,configuration of energy storage system,and system stability of multiple DCES in practical applications. |
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