Studies on fault analysis and protection configuration schemes in an isolated micro-grid

Different fault characteristics are shown in the micro grid due to distributed generators (DGs) interfaced with inverters. Overcurrent protection cannot operate correctly in this type of micro-grid. In this paper, we analyze the characteristics of the fault currents which are supplied by photovoltaic (PV) units, permanent magnet synchronous wind generators (PMSG) and power storages. These generators are always seen as typical inverter-interfaced DGs. Besides, the performance of overcurrent protection schemes is studied by taking a typical isolated micro-grid as the example. Due to the limited fault current supplied by DGs and frequency deviation of fault current waveform, the overcurrent relays can not operate correctly. Voltage controlled overcurrent relays work well if the fault is not very serious. Due to its reliability in fault detection, the differential protection is suggested to be used in the micro-grid with inverter-interfaced DGs. Finally, a practical protection configuration scheme is proposed for the micro grid in this paper. Differential relays are suggested to act as primary protection and voltage controlled overcurrent relays act as backup protection in the proposed scheme. Digital simulations are performed to verify the fault analysis results and proposed protection configuration schemes. The proposed protection configuration schemes have been used in an isolated micro grid and work well.