Research On Hierarchical Control Strategy Of Microgrid Based On Improved Droop Control

As one of the effective ways to solve the problem of energy shortage,microgrid has attracted much attention in recent years,but when it’s paralleled to the grid directly,the voltage and current of the grid will be impacted.Since the inverter is the most important part of the microgrid,the power of the micro source and the energy store device is provided by it.Therefore,to control the microgrid is to control the inverter,but both have disadvantages.PQ control can only be used for grid-connected operation,V/f control can only be used for island operation.While droop control can be used in both operating states of microgrid,droop control will cause frequency deviation and voltage deviation.In addition,droop control cannot suppress harmonics.Therefore,this paper improves the traditional droop control and uses the hierarchical control idea to solve the deviation problem caused by droop control.The improved control strategy can not only suppress harmonics,but also compensate the frequency deviation.And the improved control strategy also can synchronize the output voltage phase of microgrid with the output voltage phase of the grid.First,this paper analyzes the mathematical model of VSI and the principle of each controller,and designs the one-layer control,optimizes the parameters of each controller,and the one-layer control simulation model based on traditional droop control is built with the help of MATLAB/SIMULINK platform.From the simulation results obtained,it can be seen that: 1)the control strategy realizes power sharing;2)the deviation of the output frequency is large,and the fluctuation range is 49.06-50.08Hz;3)the phase of the output voltage cannot be synchronized;4)the harmonic content is also relatively high,when the microgrid is connected to the grid,the voltage THD is 2.90%.Secondly,in view of the large harmonic content in the one-layer control strategy of microgrid based on traditional droop control,an improved voltage and current dual-loop control strategy is proposed in this paper,and the corresponding simulation model is built with the help of MATLAB/SIMULINK platform.From the simulation results obtained,it can be seen that: 1)the moment when the microgrid is connected to the grid,the voltage THD will drop from 3.29% to 2.19%,and the current THD will drop from 1.11% to 0.47%;2)the moment when the microgrid is off-grid,the voltage THD will drop from 3.85% to 2.32%,the current THD is reduced from 1.19% to 0.39%.It can be seen that the improved voltage and current double-loop control can effectively suppress harmonics.Finally,in view of the problems of frequency deviation and phase out-of-sync in traditional droop control,this paper proposes a pre-synchronization control strategy that can be used for two-layer control,and builds a corresponding simulation model with the help of MATLAB/SIMULINK platform.From the simulation results obtained,it can be seen that: 1)the frequency fluctuations range is reduced from 49.96-50.08 Hz to 49.992-50.02Hz;2)the phase of the microgrid is synchronized with the grid.It can be seen that the pre-synchronization control strategy can not only realize the compensation of the frequency deviation,but also realize the phase synchronization of the microgrid and grid.The research results of this paper solve the problem that traditional droop control can not achieve harmonic suppression,improve the frequency accuracy of the microgrid when operating modes are switched,and the phase of the microgrid is synchronized with the phase of the grid.It is suitable for the microgrid and the grid parallel/off-grid operation.