Find a better way: National Grid experience on circuit breaker fail replacement at mesh corner substation

National Grid Electricity Transmission (NGET) policy requires Circuit Breaker Fail (CBF) protection for 400kV and 275kV circuit breakers. CBF is provided to detect and interrupt any uncleared system faults due to a failure of the circuit breaker mechanism or a failure of the circuit breaker current interrupting device. When a fault occurs and a circuit breaker fails to clear the fault, it is necessary to trip all the circuit breakers beyond the faulty breaker to isolate the uncleared fault. NGET is facing a large volume of P&C asset replacement target in the RIIO T2 price control period, system access and resources are difficult to obtain, especially for mesh corner substations as one circuit breaker controls multiple circuits. Traditionally, the outage for secondary equipment intervention is tagged along with other work against primary equipment which requires longer outage. However, this has proven to be a challenge from a co-ordination of planning point of view. Also, the design needs to be staged depending on the outage availability. If the outage sequences are altered at any point during delivery, the design stages has to change according which is not cost effective and forces tremendous pressure on resources. Therefore, a new way of working has been considered in order to achieve accelerated delivery for this type of asset intervention. This paper introduces new design and commissioning strategy to deliver the Circuit Breaker Fail protection replacement for mesh corner substation. The focus will be on the protection design and installation, comparing two sites with different methodologies, and will try to overcome some of the challenges from the experience.