Research On Smooth Switching Control Strategy Of Microgrid Operating State

With the rapid development of distributed renewable energy(especially distributed photovoltaics),distributed photovoltaic + energy storage will become an important part of the future energy supply.Microgrid is a form of distributed renewable energy.Its operation control is a research hotspot in recent years.Among them,the smooth switching between two modes of island operation and grid connection is the key to research.This paper studies the smooth switching control technology of microgrid operating status.The main contents include:1.The principle of photovoltaic power generation,mathematical model and grid-connected photovoltaic inverter control method are analyzed.Above this,the principle of the energy storage battery,the mathematical model,the charge and discharge control method,and the control strategy of the energy storage inverter in grid/island mode are studied.Besides,the main coordination and control methods for the independent operation of the microgrid are sorted out.2.For the phenomenon of instantaneous overvoltage impact of the microgrid when the large-scale power grid is disconnected,this paper proposes a control method based on voltage amplitude following of the energy storage inverter.The final value of the inverter output voltage amplitude under the PQ control mode used in the grid connection of the microgrid is used as the initial value of the voltage given in the islanding operation V/f control mode,and the PI controller is used to transition to the target voltage.It eliminates sudden changes in voltage before and after the operating mode.3.This paper proposes a method for controlling the energy storage inverter followed by the phase of the large power grid,which enables the microgrid to"remember" the instantaneous voltage phase of the large power grid and use it to establish a new microgrid voltage.This allows the microgrid voltage to continue the phase of the large grid voltage at the instant of the fault,so that the new AC voltage re-established in the island mode of operation maintains a smooth transition.4.In order to solve the problem of slow synchronization in the PI control method in traditional pre-synchronization control,this paper proposes a phase synchronization control method based on fixed frequency difference,and analyzes the feasibility of this method and the phase synchronization rate.In this way,under the premise of guaranteeing the microgrid power quality,the smooth grid connection of the microgrid is realized most quickly.5.Based on the above method,a complete master-slave structure of photovoltaic energy storage microgrid was built in MATLAB/Simulink.Through simulation,the changes of the amplitude,frequency and phase of the voltage of the microgrid during the transition from grid-connected to island-to-grid are analyzed.The harmonic analysis of the output voltage and current of the inverter proves the effectiveness of the smooth switching control strategy proposed in this paper.