Load blinding stabilisation of rough balance busbar protection relays

Distribution Network Operators (DNO) face the challenge of increasing load demand on networks with low fault levels. This challenge is present at substations if the busbar protection scheme uses the overcurrent principle. The challenge is even more pronounced if that protection is to provide circuit breaker fail protection for feeder faults because fault levels at the end of the feeder can be lower than the busbar load. Both installation of additional circuit breakers for redundancy and additional busbar sections for load sharing are expensive solutions for this challenge. Circuit breaker fail back-tripping schemes on feeders pose an operational risk of customer interruption. UK Power Networks (UKPN) already use load blinding to stabilise directional overcurrent protection (DOC) on transformer incomers to enable increased distributed generation on the network. This allows the distribution transformers to export more current than DOC would see for a fault on the network. Following system studies, bench testing and site testing, UKPN is now using load blinding at applicable sites to stabilise overcurrent busbar protection so that the protection settings can be set below load levels. The function can be enabled in approved protection relays therefore the overall savings are very high.