The procedure for disconnecting transformers - the answer to the question
A question
What should be the load tripping order of transformers working singly and in parallel with each other? Is it possible to disconnect them with a disconnector, specifically a bus?
Answer
When two or more transformers are operating in parallel, as well as when they are operating separately, the transformer must be disconnected from service by a switch, not by a disconnector, especially a busbar.
When the disconnector disconnects an operating transformer, the disconnector interrupts power equal to the transformer's load.
If one transformer is working in parallel with another, then the supply switched off by the disconnector is 5 — 10% of the load of the transformer to be switched off, i.e. in this case switching off the transformer is much easier compared to switching off of a working transformer.
However, in this case, it is possible not only the formation of sparks between the knife and the jaws of the disconnector, but also their passage in an arc, which can cause not only the burning of the knife and jaws, but also go into a phase-phase short circuit.
It is necessary to accept it as a firm rule — to disconnect the transformer under load in all cases only with a switch and not with a disconnector.
Deviation from this rule for transformers operating in parallel is also unacceptable, since the adoption of a different order of operation (in some cases the start of the opening operation from the circuit breaker, and in others from the disconnector) can cause accidents with tripping of disconnectors under load.
On-load tripping of a transformer by a bus-disconnector has more serious consequences than tripping by a line-disconnector, because when a short circuit occurs in the first instance of the buses, the entire substation is taken out of service while the bus is repaired. When disconnecting from a short circuit with a line disconnector, disconnect only one damaged power supply unit and the substation can continue to operate.
See also on this topic:
Parallel operation of transformers
Performance characteristics of power transformers
Actions of service personnel when tripping gas protection of transformers