Islanded operation of distribution grid with high penetration of renewable energy sources

Renewable energy sources are being connected not only to transmission but also to distribution grids. Medium voltage distribution grids are meshed grids that are operated radially. Their reliability is determined by their reconfiguration capability. However, portions of medium distribution grids can lack of supply in case of specific disturbances or abnormal operating conditions.Al alternative to the lack of supply in medium voltage distribution grids is the use of the available renewable energy resources. The feasibility of the use of renewable energy sources depends on their ability to control both voltage and frequency to guarantee the quality of the electrical supply.This contribution will show from real life tests [1] that stable operation islanded operation of medium voltage distribution grids is not feasible due to the limited control capability of renewable energy sources connected through power electronic converters under grid feeding control scheme. It will show that medium voltage distribution grids can run properly while isolated from the main grid if they incorporate battery energy storage systems. Records of real life tests will prove it.In addition, this contribution discusses the conditions por parallel operation of several battery energy systems under grid forming control scheme with inverter based generation under grid feeding schemes [2]. Stability conditions will be drawn.