This paper addresses operation methods of energy storage systems in a low voltage micro-grid. In order to supply active and reactive power stably, energy storage systems(ESSs) connected to low voltage distribution system should be operated in grid-connected mode under normal conditions or in islanded mode under emergency conditions. It is important for stable operation in micro-grid to select appropriate power control methods of Power Conditioning Systems(PCSs) of ESSs.
The frequency-voltage droop control method is usually used for parallel operated PCSs to achieve autonomous load sharing under islanded conditions. The droop control method have frequency and voltage deviations after autonomous load sharing. To compensate for deviations, a secondary control is proposed. In grid-connected mode, the active and reactive power control method operated is applied to PCSs. Recently, several papers have been proposed the droop-based control method to operate micro-grid without changing control method even though operation mode is changed from islanded mode to grid-connected mode. This method is named the tertiary control method.
In many papers, droop control methods are proposed to control active and reactive power considering transmission line impedances of ESSs in low voltage micro-grid under islanded condition. However, if line impedance of micro-grid is considered not only transmission line impedances of distributed generations but also the line impedance between local loads, the conventional droop-control method may not be suitable. Therefore, to ensure proper active and reactive power sharing, the proposed controller uses additional control stage. The proposed controller contains the bus voltage control stage to share reactive power of PCSs.
In case of grid-connected mode, each PCS should be controlled through the tertiary control method. The purpose of this method is to control power flow between main grid and micro-grid. But ESSs should be controlled individually, because they are operated in the charge mode or the discharge mode depending on a state of the ESS.
For operation of low voltage micro-grid, the proposed method consist of droop control including virtual inductor method, secondary control, and improved tertiary control. The proposed power control method was verified through PSCAD/EMTDC software package.