Research On Control Strategy Of Bidirection Inverter Applied In Energy Storage System Of Micro-grid

In this paper, control strategies of a bidirectional inverter, which is used to interface an energy storage system of battery with an AC micro-grid, are researched. The main function of an energy storage system is to compensate the intermittent output of renewable energy, match the unequal power between generators and loads and increase the stability of the AC micro-grid, especially when the micro-grid operates as an intentionally islanded system. In order to achieve these functions, bidirectional inverters, which are with high reliability, redundancy, ability to operate in grid connected mode and regulate voltage amplitude and frequency of the AC bus in islanded mode, is required. Based on these requirements and the comparison of variety kinds of inverter parallel strategies, droop control algorithm is used to control the bidirectional inverter.An experiment platform is build in order to verify the inverter control strategy. The LCL filter is designed by the inductors current ripple and resonant frequency of LC. DSP is used to realize the control algorithm and program flow chart is shown.A droop control strategy with single power loop is proposed, in which the active and reactive output of the inverter is decoupling as the impact of LCL’s inductance. The proposed strategy enable the inverter to operate in both grid-connected and island mode by regulating the inverter’s frequency and voltage amplitude through active and reactive current respectively. In dq reference frame, linear small signal model is built to analysis the stability of the inverter in grid-connected mode, and then the compensation network design and optimization of the control parameters are following. The inverter stability with the same control method and parameters is further analysis in island mode to prove that the proposed control can be used in both modes, so theoretically the inverter can switch between these two modes seamlessly. The simulations verify the feasibility of the control strategy and accuracy of the modeling. Validity of the theory analysis and simulation is further confirmed by experiments. In order to solve the problem cause by dead time effect and grid voltage harmonics, a virtual high frequency impedance loop is added to the proposed strategy, suppressing the harmonic current in the inductors to decrease THD of its output current. Meanwhile, this virtual impedance loop doesn’t impact the steady-state droop characteristics of the inverter’s output voltage amplitude and frequency, so that it can operate in both two modes. Linear small signal model is also built, and then verified by simulation and experiment results.