MITIGATION OF MICROGRID INTERACTIONS ON PROTECTION SYSTEMS IN UTILITY NETWORKS

This thesis presents novel schemes and techniques to overcome the difficulties associated with the integration of distributed generation (DG) and microgrids in the context of existing short circuit characteristics and protection infrastructure adequacy. One such inadequacy is associated with the loss of coordination (LOC) in existing protection infrastructure, with disruption to an expected sequence between utility reclosers and fuses. This thesis aims to offer solutions to these issues, allowing for DG sources and microgrids to be integrated into utility distribution networks without significant effect on existing protection infrastructure. The integration of DG units into radial distribution networks can result in LOC between upstream reclosers and downstream fuses. To overcome this issue a novel reclosing scheme is proposed whereby a control unit, variable load bank and dedicated recloser are integrated at the point of common coupling (PCC) between the DG unit and the network. This scheme works by receiving a control signal from the distribution network head-end recloser via a communication channel to signal the detection of a fault. Post fault detection, in conjunction with the DG current exceeding pre-specified pick up levels, the control unit disconnects the DG unit from the network to a transfer impedance. This transfer allows the DG unit to continue to supply the transfer impedance at the pre-fault load sharing condition, without the requirement for a shut down. This causes the DG unit to maintain its pre-fault speed and frequency, resulting in a fast reconnection time once the system fault is cleared by the existing protection infrastructure. The scheme is also compared to another potential method, namely fault current limiters (FCLs). To address the possibility of communication failure in the novel reclosing scheme, a fault detection technique is proposed based on measurements of the rate of change of current output by DG sources. The rate of change of current (ROCOC) is measured over a specified time window to generate a fault detection signal when the ROCOC exceeds specified pickup values. A hybrid adaptive overcurrent and differential protection scheme is proposed to protect microgrids that operate in both grid and islanded modes. Differential relays are utilized for feeder backbones and buses while adaptive overcurrent relays are concurrently used for load points. The hybrid approach is to reduce both infrastructure upgrade requirements and setting computation complexity, whilst also addressing the potential lack of coordination when differing protection mechanisms are merged. The proposed scheme is validated through multiple time-domain simulations while the microgrid is in both grid and islanded modes of operation. A smart protection scheme is then proposed to predict and mitigate the short circuit contribution of a microgrid to a utility fault at a magnitude below the LOC limit. The scheme utilizes polynomial regression analysis (PRA) and particle swarm optimization (PSO) in conjunction with a directional element of a relay to allow for partial continual microgrid connection during utility faults. The directional element specifies the direction of short circuit current flow, only allowing the scheme’s operation when the microgrid current is flowing to the utility. The PRA and PSO utilize wind speed, irradiance and operating conditions of synchronous machine based (SM-based) generators to determine the short circuit contributions to utility faults from plants and units within the microgrid. The predictions are used to minimize generation source disconnection to reduce the microgrid short circuit contribution to below the LOC limit dictated by the utility network allowing for the original utility coordination to be maintained. Finally, a case study is offered to demonstrate the capacity of every approach to mitigate microgrid short circuit contributions while restoring pre-fault operating conditions shortly after fault clearance by utility protection infrastructure. In this thesis, all case studies have been conducted using realistic distribution network and microgrid designs and settings, ensuring the efficacy of the proposed approaches. Time-domain simulations are carried out on these test benchmark models within the EMTP-RV software environment for validation purposes