Direct Transfer Trips (DTT) are initiated from station relays when a serious event occurs in the substation. Some of these events are breaker failure, bus faults, transformer failure, etc. A lockout relay (86 device) is assigned to each event.
Substation Protection and Control
Power substations contain expensive pieces of equipment. If protection is not installed then you can watch them explode. A piece of protection also has to do with safety of personnel for instance fast trip settings, disabling breaker reclose, arc-flash settings etc.
Pilot relaying refers to the communication network implemented on the high voltage transmission line (T-line) to transmit “trip or don’t trip” signal to and fro between two or more substations. The intent here is to trip the circuit breakers as fast as possible when a fault strikes the T-line, therefore, protecting it.
The Directional Comparison Blocking (DCB) scheme is the most popular pilot relaying scheme, implemented to protect extra high voltage power lines. This scheme is more dependable than permissive transfer trip schemes because it trips the breaker even if there is no carrier signal from the remote end pilot relay. Ofcourse, the protective relays need to see the fault first.