Research On Controlling Characteristic Of Energy Storage Device In Microgrid Supplies Nonlinear Loads

With the development of renewable power generation, the microgrid has become a focus recently. Energy storage device act as an important role in microgrid, and it’s the key to the safe operation of the microgrid in island mode. With the increasing of nonlinear loads in power grid, this result in a more serious requirement on operation of energy storage systems. Due to the limit of switch frequency, voltage loop bandwidth is not so high, that result in conventional dual loop control strategy used in energy storage inverter has harmonic problems when supllying local nonlinear loads. Traditional droop control scheme used in energy storage inverters for parallel operation also cause the problem of too much harmonic pressure drop with nonlinear loads. This article mainly launches the research to solve these problems. Firstly, the equivalent circuit model of energy storage battery is briefly introduced, the charging and discharging mode and control strategy is designed based on the bidirectional Buck/Boost circuit, and then verified by simulation. According to the coordinate transformation theory, the mathematical model of energy storage inverter in d-q coordinates system is established. On this basis, the operation mode of energy storage device is analyzed, then the voltage and current double loop control strategy is designed for the energy storage system in grid-off mode, and the controller parameters are achieved by using the method of pole assignment. The limitation and its essential reason of conventional double loop control with nonlinear load is analyzed in detail. By designing of single voltage loop control strategy and the comparative analyzing with double loop control, the combination of single and double loop control composite control strategy is proposed, the inadequacy of the double closed loop control is effectively compensated, the effectiveness of the proposed control strategy is proved by simulation in PSCAD. Moreover, the parallel operation mode of energy storage inverter based on droop control is also studied. With the introduction of virtual impedance method, the connection inductance is instead. Then the traditional virtual impedance is improved to make the output impedance at the fundamental frequency is inductive, and the output impedance at harmonic frequencies is suppressed, so the adaptability of nonlinear load is enhanced, the effectiveness of this proposed control strategy is also verified in PSCAD simulation.