Research On Improved Droop Control For Multiple Energy Storage Systems In Microgrid

With the increasing demand for energy and the deterioration of ecological environment,developing the green and more efficient sources of energy has been widely concerned these years.Distributed power generation is intermittent and volatile,which may cause shocks to the grid when it is connected to the grid on a large scale.Energy storage systems have become an important part of microgrid with the advantages of smoothing grid fluctuations and improving power quality.Using energy storage control technology to obtain stable voltage and frequency is an effective way to improve power quality.AC microgrid is widely used,which is conducive to meeting the diversified electricity needs of customers.Multiple energy storage units can be connected to the grid through converters to achieve grid expansion.Therefore,it is significant to investigate the control strategy of multi-storage microgrid.This paper takes an AC microgrid under islanded operation as the research object,focusing on both active power control and reactive power control of the energy storage in the microgrid.Firstly,this paper introduces the development of microgrid and its control technologies,outlines the main features of energy storage technologies and distributed control.The basic principles and concrete implementation of droop control are described in paper.Secondly,a multiple storage microgrid improved droop control strategy based on the state of charge(So C)is proposed to address the problem that the active power output of energy storage cannot be reasonably distributed in the conventional droop control.The strategy introduces a power exponential function term relating the droop coefficient to the charge state of the energy storage,which allows the storage unit to dynamically adjust the output active power according to the level of its own charge state.Energy storage units with a high state of charge output high power and conversely output low power,which leads to a dynamic balance of the state of charge.The proposed power exponential function droop control(PEFCD)eliminates the need of communication between energy storage units and has better control efficiency than conventional droop control(CDC)and power function droop control(PFDC).The stability of the proposed control strategy is analysed,and guidance is provided on the choice of parameters for the function term,which enhances the feasibility of the strategy implementation.Then,a distributed reactive power control strategy based on adaptive virtual inductive resistance is proposed for the problem that the reactive power output from the converter cannot be equally distributed due to the presence of line impedance and that the converter cannot output reactive power according to capacity in conventional droop control.The reactive power output from the energy storage is analyzed and the process of solving for the given values in the proposed distributed control strategy is described.For voltage deviation occurring at common points,an adaptive voltage compensation controller is proposed.The control strategy is based on a distributed control algorithm,only requiring the exchange of information from adjacent energy storage,and no central controller is required to achieve precise allocation by capacity,which improves the reliability of the system.Importantly,the reactive power control strategy considers the inconsistent impedance of the transmission lines and the inconsistent capacity of the converters,which makes the strategy more general and better suited to practical engineering applications.Finally,models were built in MATLAB/Simulink for simulation,and a semi-physical experimental platform was built for hardware-in-the-loop simulation to verify the proposed control strategy and compare the results.Simulations and semi-physical experiments have verified that the proposed improved droop control and distributed reactive power control strategies are superior and reliable under steady-state,storage fault withdrawal or sudden load changes,and can effectively stabilize the system voltage and frequency and achieve reasonable distribution of active power and reactive power.