Control method of grid-connected power for battery storage inverters based on digital phase-locked loop

Micro-grid has been becoming a focus all over the world because of its advantages. Usually, energy storage unit plays an important role and battery storage inverter is an integral part as a result. While a micro-grid running in grid-connected mode, it’s necessary to control the power at the point where the inverter joined to the grid. So far, there are two main kinds of strategies for the grid-connected power control: (1) closed-loop control for active and reactive power respectively; (2) closed-loop control for the active and reactive component of the grid-connected current. These methods require calculation of the active and reactive power or active and reactive component before closed-loop control. And the calculation usually is very complex, which lead to slow response.In grid-connected mode, the voltage of the storage inverter will be clamped by the grid. It means the power will be determined by the grid-connected current. As a result, the power can be controlled by indirect control of the grid-connected current. With the background mentioned above, a new strategy of power control for single phase battery storage inverter, based on advanced phase-locked loop, was proposed in this paper. The phase angle and RMS value of the current was used to control the active and reactive power instead of common methods, which use direct power feedback control strategy in this method. Comparatively, this strategy could accelerate the response without complex calculation. The detailed theoretical analysis for the strategy was presented in the paper. Firstly, an introduction for an advanced phase-locked loop method was given this paper. This method could lock the voltage on the grid, which will be the premise for the power control. Then, the detailed step getting the reference phase angle and RMS value of the current was introduced. With this strategy presented, it can control the active and reactive power at the interface of the inverter accurately. As a result, the requirement for power’s bi-directional interaction between energy storage unit and micro-grid can be realized. And it also supports the energy optimization and dispatch for the micro-grid system. A simulation model was established with MATLAB/Simulink module according to the method mentioned in this paper. The results of the simulation corresponding to four quadrant typical reference active power Pref and reactive power Qref was given .And the results have proved that this method could track the reference power quickly and exactly. Both the theoretical analysis and the results of the simulation have shown the correctness of the method.