Microgrid protection with conventional and adaptive protection schemes

Abstract

One of the driving force of the Microgrid research in the past decade has been to accelerate the integration of renewable energy sources (RES) including wind and photovoltaic (PV) based generation into distribution networks. The islanded mode of Microgrids on the other hand would not only increase the reliability of existing distribution networks but also increase the security of supply for geographically islanded systems. But as a matter of fact, wind and PV systems are inherently intermittent resources so there is always need of some kind of energy storage to smooth out the power fluctuations caused by weather related intermittency. It is also a fact that most of the well-established renewable energy generators and energy storage technologies are coupled to the distribution networks using power electronic converters. The power electronics converters are, on the one hand very flexible and quick to operate, but on the other hand lacking inertial response and very limited fault current providers. Present day protection schemes rely on single setting group which becomes ineffective with increasing penetration level of distributed energy resources (DERs) including renewable energy sources and energy storages. During faults in grid-connected mode of Microgrids a high magnitude fault current of 10–50 times the nominal current will be expected from the main grid but in islanded mode only limited fault current magnitude of 1–2 times the nominal current will be available due to converter-based generators. The conventional single setting overcurrent protection causes the immediate disconnection of DERs during faults in grid-connected mode and does not allow any islanded mode operation of DERs due to safety reasons. An adaptive protection will be necessarily required for grid-connected Microgrids to continue operation even in islanded mode after disconnection from the main grid due to faults. An adaptive overcurrent protection will have at least two reliable settings groups, one for grid-connected and other for islanded mode of Microgrid. The adaptive setting group of islanded mode overcurrent protection will have to be more sensitive as compared to grid-connected mode considering the reduced fault current contribution of DERs in islanded Microgrid. The adaptive setting groups could be changed from one setting group to other based on the status of Microgrid connecting switches. The status of switches and thus Microgrid mode (grid-connected or islanded mode) information can be exchanged through for example IEC 61850 GOOSE communication protocol with centralized or decentralized communication architecture or through offline communication-less local measurements for example local voltage magnitude.