Stability Research Of Energy Storage Inverter Based On Sequence Impedance

With the rapid development of renewable energy power generation and the electric vehicle technology,more and more Battery Energy Storage Systems(BESS)are introduced into the power grid.Energy storage inverters are used as the interface device between battery and the power grid,the stability of energy storage inverter is related to the grid-connected power quality.The influence of grid impedance is ignored in the conventional design of energy storage grid-connected inverter.As a result,the energy storage inverter may exhibit harmonic resonance and instability problems.The impedance-based analysis method is an efficient tool to assess energy storage inverter-grid stability.In this paper,a 50kW energy storage inverter was designed at first.The simulation result showed that it can work stably under the ideal grid condition.The sequence impedance-based stability criterion for three-phase energy storage grid-connected inverters was derived with symmetrical component analysis,the restricted area of impedance ratio and the impedance standard specification are introduced as well.In addition,the improved restricted area of impedance ratio was proposed,it can provide reference for the stability analysis of energy storage inverters.Small-signal methods and the harmonic linearization method were used to build the small-signal model of PLL and current loop.Considering the influence of digital control delay,the positive-sequence and negative-sequence impedance of energy storage grid-connected inverter were built at last.In addition,the impedance models were verified by simulations.Then the output impedance characteristics were analyzed.The influence of PLL,current control loop and filter on the output impedance was analyzed.The works should be helpful for better understanding of the output impedance model.The stability of energy storage grid-connected inverter was analyzed by applying the impedance ratio criterion.The effect of different grid impedance for inverter-grid stability was researched,and the correctness of theoretical analysis was verified by simulation experiments.At the same time,the factors which affect the system stability were discussed,the influence of battery SOC and power level for inverter-grid stability was analyzed.In addition,the mechanism of harmonic resonant was analyzed,and the methods to improve the stability were summarized.A passive damping strategy was introduced.Moreover,the method of PLL parameters optimization was proposed.Theoretical analysises and simulations showed that this method could effectively solve the stability problem which caused by grid impedance.Finally,according to the grid connected system of multi-paralleled energy storage inverters,the sequence network model was established and the influence of the number of parallel inverters on the system stability was analyzed.The research of this paper can provide reference for the stability problem of high-power PCS grid connected systems.