Research On Electric Vehicle Charging Pile Converter And Its Control Strategy Responding To Grid Frequency Modulation

In order to alleviate the negative impact of the large number of electric vehicles connected to the power grid,electric vehicle charging piles,as the connection equipment between electric vehicles and the power grid,should have better vehicle-network coordination capabilities.The converter of the electric vehicle charging pile is responsible for the conversion and control of electric energy,and is the "heart" of the electric vehicle charging pile.In order to increase the voltage output range of the electric vehicle charging pile converter,reduce the grid-connected harmonic content,and optimize the ability to participate in the grid frequency modulation in the charging mode,this paper cascades the VIENNA rectifier with high power factor and low harmonic content and the half-bridge three-level LLC resonant converter with wide voltage output range to form the circuit structure of the electric vehicle charging pile converter,and conducts research on it.(1)Firstly,it analyzes the current status of the research on the topology,control strategy and grid regulation technology of the electric vehicle charging pile converter.The topology structure of the electric vehicle charging pile converter studied in this paper is given.The VIENNA rectifier is used in the front stage,which improves The converter power factor reduces harmonic pollution.The latter stage adopts a half-bridge three-level LLC resonant converter,which improves the power density of the electric vehicle charging pile converter and widens its voltage output range;at the same time,the three-level structure of the front and rear circuits effectively reduces the switching device Voltage stress improves the voltage application level of the device.In addition,the working principle of the electric vehicle charging pile converter was further studied,and the related mathematical model was established.(2)Secondly,the basic control strategy of electric vehicle charging pile converter is studied.Both the front and back stages adopt dual closed-loop control strategies,which are independently controlled.The front stage provides a stable and reliable DC power for the back stage circuit while ensuring the quality of the grid-connected current.A voltage feedforward current loop controller is designed for it,which effectively solves the problem of zero-point distortion of the grid-side current;by controlling the SVPWM the redundant small vector in the DC port realizes the midpoint potential balance of the upper and lower capacitors at the front-end DC port.The back-end circuit adjusts the higher-level voltage of the front-end DC port to a DC voltage output suitable for the needs of the power battery.A frequency modulation and phase-shift hybrid control method(PFM-PWM)is designed for it,which expands the voltage output range of the converter.At the same time,the use of soft switching technology improves the power density of the converter.A simulation model was built based on MATLAB/SIMULINK to verify the correctness of the circuit topology and basic control strategy.(3)Finally,in order to further optimize the ability of electric vehicle charging pile converters to respond to grid frequency adjustment,the virtual synchronous motor(VSM)technology is introduced on top of the basic control strategy of the front-stage VIENNA rectifier of the electric vehicle charging pile converter.Using the energy storage effect of the front-stage DC port capacitor of the electric vehicle charging pile converter,the front-stage DC port voltage control method is designed,and an optimized control strategy for the charging power of the electric vehicle charging pile converter is proposed by combining the above two methods.Based on MATLAB/SIMULINK,a simulation model under the control of the charging power optimization control strategy of the electric vehicle charging pile converter is built.The simulation results show that when the grid frequency is lower than the allowable value,it can be achieved for a short time without reducing the charging power to the load.The electric vehicle charging pile converter absorbs the active power from the grid to achieve the purpose of responding to the grid frequency adjustment.The response process has a certain inertia and damping effect,which optimizes the ability of the electric vehicle charging pile converter to respond to the grid frequency adjustment.The simulation results verify the effectiveness and superiority of the research method in this paper.The large-scale application of this strategy to electric vehicle charging pile converters will significantly improve the influence of electric vehicle charging on the frequency stability of the grid.