Harmonic-Based Overcurrent Scheme for Optimal Protection Coordination of Islanded Microgrids

Abstract

Protection of microgrids dominated by inverter-based distributed generators (IBDGs) is challenging as these IBDGs are characterized by their low contribution to fault currents. Therefore, their fault currents can be confused with the normal operating current, which imposes a major challenge for overcurrent protection relays to distinguish faulty conditions. Furthermore, the change in the microgrid's topology due to the intensive use of IBDGs imposes a major concern. The IBDG has an interfacing LC filter with relatively high impedance compared to the microgrid’s lines. This affects the sequence networks; hence, the three-phase fault is not the most severe. In this thesis, harmonic voltage generation methodology is deployed. Performing the protection coordination in a new harmonic layer formed during faults helps to avoid the problem of distinguishing faulty conditions. It is proposed to include the symmetrical as well as asymmetrical faults in the optimal protection coordination (OPC) problem formulation. The inclusion of asymmetrical faults ensures a more reliable operation of the proposed overcurrent protection scheme, as relying only on symmetrical faults can be deceptive. A comparison between current levels for different fault types and different transformer connections is presented to highlight the importance of asymmetrical faults inclusion in the OPC as well as provide evidence of symmetrical-based approaches failure. After modifying the OPC by including asymmetrical faults, the results are compared to a newly proposed fault type multi settings (MS)-based OPC. The OPC problem is formulated as a non-linear programming (NLP) that is developed in GAMS software. The results are validated using a test system which is a section of a Canadian distribution system, as well as the IEEE-33 bus system. Results confirm the proposed MS-based method’s ability to enhance the relays’ total operation time. 14% and 38% reduction in the total operating time are achieved for the 9 bus and 33 bus respectively. Several case studies are conducted to demonstrate the effectiveness of the proposed methodology in providing OPC considering symmetrical and asymmetrical faults with high fault resistances besides evaluating the proposed protection scheme capability to maintain precise coordination in case of expanding the IBDGs capacities.