Research On Control Strategy Of Battery Energy Storage System Under Primary Frequency Regulation Of Power Grid And Unbalanced Grid Voltage

The battery energy storage system(BESS)has been widely used in power grid with the advantages of high control accuracy and fast response speed.In order to play advantages of BESS in primary frequency regulation(PFR)of power grid,this dissertation proposes a PFR strategy based on state of charge(SOC)and frequency deviation.Firstly,the droop coefficient and the virtual inertia coefficient based on SOC are designed to improve battery cycle life.After that,the weighting coefficients based on frequency deviation are introduced to combine the droop output with the virtual inertial output.When the frequency deviation is small,the weighting coefficient of the virtual inertia output is increased to stabilize the frequency.When the frequency deviation is large,the weighting coefficient of the droop control is increased to quickly adjust the frequency deviation.And when the frequency deviation exceeds a certain limit,BESS performs frequency support during fault ride through.In addition,when the system is in good condition and the SOC is low or high,the SOC recovery is performed.Secondly,the evaluation index of BESS participating in PFR of power grid is proposed to quantitatively evaluate the frequency regulation performance and the SOC maintenance performance of this strategy.Finally,a simulation model of BESS is built in PSCAD/EMTDC,and the frequency regulation performance and the SOC maintenance performan ce of this PFR strategy are verified by simulation under step load disturbance,random load disturbance,transient short-circuit fault and intermittent output of PV conditions.The simulation results indicate that this PFR strategy can improve the frequency regulation performance and maintain the SOC within a reasonable range.In addition,a multi-objective coordinated and optimized control strategy based on weighting coefficients is proposed to solve the control problem of BESS under unbalanced grid voltage.Firstly,the mathematical model of BESS under unbalanced grid voltage is established.After that,the control principle of the positive and negative sequence separation circuit based on the T/4 delay method is introduced,and the negative sequence current suppression strategy of BESS under unbalanced grid voltage is discussed.Secondly,based on the instantaneous power theory,the positive and negative sequence current expressions of BESS under the power fluctuation suppression are deduced.Then a multi-objective coordinated and optimized function is constructed to suppress negative sequence current,active power double-frequency fluctuation and reactive power double-frequency fluctuation.And coordinated and optimized control of BESS under unbalanced grid voltage is achieved by using the fruit fly algorithm for global optimization.Finally,a simulation model of BESS under unbalanced grid voltage is built in PSCAD/EMTDC,and the effectiveness and superiority of the multi-objective coordinated and optimized control strategy are verified by comparing this strategy with the negative sequence current suppression strategy,the active power double-frequency fluctuation suppression strategy and the reactive power double-frequency fluctuation suppression strategy.The simulation results indicate that the suppression performance of negative sequence current,active power double-frequency fluctuation and reactive power double-frequency fluctuation is good after the multi-objective coordinated and optimized control strategy is adopted,and the multi-objective coordinated and optimized function value is the smallest,so the effectiveness and superiority of the multi-objective coordinated and optimized control strategy are verified.