Operation And Control Strategy For Bipolar-type DC Microgrid With Wind And Energy Storage System Based On Three-level DC-DC Converter

DC microgrid as a new energy framework has been a research focus.The DC microgrid integrated new energy source units such as wind power and photovoltaic,energ storage devices and loads,can achieve the high efficient use of renewable energy.At present,the research on DC microgrid is mainly focused on the unipolar-type DC microgrid structure,which reduces the system operating efficiency.In this paper,a bipolar-type DC microgrid structure based on three-level DC-DC converters is proposed,which can reduce the bus voltage to ground and improve the system efficiency,meanwhile meet the requirements of different voltage levels for various converters.At the same time,a corresponding operation and control strategy is also proposed.The main research contents of this paper include:First,the topology and principles of the three-level DC-DC converter in the bipolar-type DC microgrid is analyzed in detail.According to the energy bi-directional flow characteristics,the work mode of this converter contains Boost and Buck,and a detailed description of the working state and the input-output relationship is analyzed.In addition,the state-space model is built to analyze the three-level boost converter deeply which used in wind power unit.Second,the wind power unit based on three-level Boost converter is studied emphatically,including the composition and the operating characteristics of the wind turbine at different wind speeds.A maximum power point tracking(MPPT)control method in wind energy utilization coefficient constant region and the constant rotor speed and constant power control strategy in high speed region is proposed.According to the output characteristics of the three-level Boost converter,the neutral-point potential balancing control is supplemented.The effectiveness of strategy in different wind speed region is verified by Matlab/Simulink.This strategy eusures the stable operation in different wind speed regions,and realizes the smooth transition,and improves the utilization efficiency.Third,the energy storage unit based on the three-level bi-directional DC-DC converter is researched.The working principle and equivalent circuit model of supercapacitor are analyzed and the voltage droop characteristic is built.According to the voltage fluctuation range in DC-bus,there are three operation stages--no action,droop control and constant-current charge and discharge.Meanwhile,a charging and discharging mode separation control strategy is proposed.Finally,this control strategy is verified by Matlab/Simulink.There are only two switches turned on when in Boost or Buck mode in this strategy,which can reduce the complexity of control.Fourth,DC-bus voltage interval separation operation and control strategy is proposed.The power flow relationship and the structure of bipolar-type DC microgrid with wind and energy storage are analyzed,and the bus voltage will operate in four regions according to the operating status.According to wind power control and power-limitation control based on droop characteristic in wind power unit,charging and discharging mode separation control in energy storage unit and load shedding control,it can implement stable running in different states and smooth transition in different intervals.Fifth,a bipolar-type DC microgrid experimental platform containing the wind power,supercapacitor energy storage device and DC load is built,in order to verify the operation in different bus voltages and the transition in different states of the control strategy.The experimental results show that the overall control strategy of the system can realize the bus voltage stabilization and power balance,which means realize the stable operation of bipolar-type DC microgrid with wind and energy storage.