Safely integrating microgrids into the power system
The penetration of distributed generation resources, such as photovoltaics, combined heat and power (CHP) microturbines, small wind turbines and possibly fuel cells alters the traditional operating principle of the low voltage (LV) grids. A particularly promising aspect of the proliferation of small-scale, decentralised generation is the possibility for parts of the main power network to operate separately from the main grid. Reduction of congestion in the transmission system and local voltage support are only some of the benefits that microgrids may offer to the global system. From the customers' point of view the potential to improve power supply quality and enhance local reliability, as well as to reduce costumer minutes lost (CML) renders microgrids an attractive option. Although distributed energy resources (DER) are already being used extensively for selected applications in response to specialised customer needs, their electrical safety has received little attention to date. Electrical safety is an overriding operational requirement, and both the earthing system of the microgrid and the transformer connecting it to the utility network need to be carefully analysed. A fault current may generate substantial ground potential rise, even if the energy sources operate at low voltage, influencing the choice of protective devices. Within the MICROGRIDS project, the fault current distribution in a generic microgrid interconnected to the main grid and during islanded operation, it was extensively investigated. Simulation results indicated that in order to provide a path for electric currents into the earth and ensure people's safety, the micro-sources could be operated without earthing their neutral points locally. Protective earthing should be achieved by connecting the generator frame and all conductive parts to a main earthing terminal. The performance of different earthing techniques of the medium voltage / low voltage (MV/LV) transformer and the load equipment are being specified by project partners in application studies.