Protection challenges facing Distributed Generation on rural feeders

Experience with connecting farm biogas generation to rural distribution feeders in Ontario has revealed several challenges with electrical protection. The biogas projects are small (<;500kW), synchronous, farm-based, and looking for an affordable protection solution involving the use of only low-voltage equipments. The utility feeders are generally at the 8.3kV and 12.5kV voltage levels, can involve long lengths and contain significant imbalance due to the presence of single-phase laterals. The main challenges arise from two requirements. First, a utility requirement to detect feeder faults to the end of every feeder spur. In view of the potentially long electrical distances, conventional over-current protections may not be able to discriminate from normal running conditions. In order to detect the most common fault type (phase-ground) at these remote locations, recourse is being made to sensitive imbalance protections. Second, as the load on the feeder downstream from the last recloser set is small, special anti-islanding protection is often required. In view of the fast reclose time of utility reclosers, the utility solution in the past was to require expensive fast transfer trip protection. Although regulatory developments are now in place to not pass on such costs for future projects, the utility allowed small projects, as an interim measure, to offer various alternatives. Based on experience over three projects, a potential alternative to transfer-trip has evolved based on the rapid detection of feeder faults combined with multiple passive anti-islanding protections. As the strategy involves the use of sensitive imbalance protections, information on the background levels of feeder imbalance is particularly important. This paper reviews recent developments covering the design and piloting of these protection strategies on farm biogas facilities in Ontario.