PROTECTION AND IMPACT OF CONVERTER DOMINATED NETWORKS AND LOW INERTIA SYSTEMS

Abstract

Microgrids are becoming very crucial in modern power systems due to the increasing use of distributed generation by renewable energy over the fossil fuel-based generation. The distribution system conventionally fed by synchronous generators majorly uses directional and inverse definite minimum time overcurrent relays, which is based on the principle of overcurrent detection. The short circuit faults create low impedance paths resulting in high currents of about 10 times of the rated current to flow through the system. This abnormal rise in currents is detected by overcurrent relays to detect and locate the fault in the system. But in a distribution system or microgrid which is fed completely or having high dominance of renewable energy sources using the inverter interface, this concept of overcurrent detection cannot be used efficiently because the power electronic devices such as IGBTs and Power MOSFETs used in the inverters do not allow the currents of such high magnitudes to flow through them due to their thermal and current carrying limits, also devices with such high limits can be very expensive if available. Hence the need of using new strategies for protection of such systems needs to be developed. Differential protection principle could be used for this purpose efficiently but it is very costly due to the requirement of high bandwidth communications. Hence in this project, fault detection by under voltage principle is proposed, as the voltage decreases at the time of fault and the zero sequence currents increases during ground faults. For undervoltage principle to be used effectively the coordination between relay is a main problem. Communication based methods solve this problem but they are expensive. Hence this report uses voltage based time characteristics that uses local voltage measurements and do not need communication. The strategy works on all kind of DGs, fault types, islanded and grid connected mode of operations. To verify the proposed strategy, a seven bus microgrid is tested in PSCAD. First the characteristics of inverter with LVRT feature and current limiting action in PQ control mode is studied and then, it is modelled as a voltage controlled current source. This model is then used for testing the system in PSCAD. The system is subjected to faults at various location in grid connected and islanded modes of operation. The protection strategy successfully works in all cases.