Digital Protection Scheme for the Microgrid

Abstract

As today's world power demand is increasing day by day, the penetration of Distributed Energy Resources DERs like Solar PV, Wind Energy, Flywheels, Bio generation, Fuel Cells are gaining their own portion at the Load End of the conventional power systems. The low voltage grid at the consumer end which is compromising of DERs and variety of loads installed and is connected as a Single Electrical Entity with the conventional grid through Point of Common Coupling (PCC) is called Microgrid. There is an obvious discrimination between the old lazy relaying protections of main grid and the protection schemes of the Microgrid, as it is having lower Fault levels compared to the conventional grid. Microgrid faults are suppressed by 2 times [4] of the rated capacity due to the limitations of the power electronic switches and their power handling capacity. While in case of conventional grid the fault level may rise up to 14 to 16 times of the ratings. So the protection systems of main grid are not ideal for the microgrids as the fault parameters and fault levels are very different, which may lead the Sympathetic tripping and mal-operation of the relaying schemes. It requires the development of new fast acting Protective Relaying schemes for the microgrids. The scheme must be smart enough to operate for genuine faults only when it is in Grid connected and the Off Grid (Islanded) mode of operation. Digital protection is one of the best alternatives because of its vast programming capabilities, real time data collection and communication with the other components of the protection. DSP based numerical relays provide high computational speed and vast flexibility of the Generic Object Oriented System Event (GOOSE), Future Extraction and Selection (FES), Wavelet Packet Transform (WPT) techniques for the calculation of THD, Travelling waves etc. New schemes have to consider the two way power flow when it is operated with the conventional Utility Grid through PCC. Scheme requirements vary as it has changing fault levels and hence it is a critical practice to discriminate the faults of microgrid and abnormalities of the main grid in grid connected mode. So requirements arise to develop changing "relay coordination schemes" and "trusted data groups" as per the mode of operation the microgrid.