I’ve paralleled 11/.44KV 1600KVA transformers several times and each time we knew we would be exceeding the boards maximum fault level but for only a mater of seconds. The chances of a fault on the board in that space of time would be minimal.
This was a modern system but due to the failure of a VCB it was decided all VCB’s were to be taken out of service in turn and tested. We did all we could to make the operation as safe and quick as possible, it wasn’t possible to shut the plant down. One beauty of this particular system was every boards loading was such that if one transformer failed the other could take the full load. When I first saw the system I thought it laughable that a transformer capable of supplying 2133A was bumbling along at say 900A only when we had to go through this exercise did I realise how good it was. OK some bits of kit had to be shut down where there was no parallel feed, (11KV motors) for a few minutes while we swapped the VCB for a stand by one.
The four incoming 33/11KV 20MVA transformer OLTC’s were always in leader and follower mode.
At another works two of us were separately asked to write a switching procedure so part of the intake board could be shut down, we compared them and made the final out of the two. There were 77 steps to the procedure, a lot of load shedding, a lot of driving and radio contact, but most things kept going!
Never worked on such large networks, and TX's the biggest network i had to deal with was on the Libyan Great Man Made River Project. Which was a sub-distribution from 66 Kv feeds down to the well fields some 600+ km into the Sahara desert, feeding 2 sub-station/distribution centres (66KV/33KV/11KV) that went on to feed numerous TXs around the well fields for pumping purposes and supplying purpose built self contained maintenance villages. Everything in the sub-stations on the 66KV side was SF6 Gas filled (by ABB), looked more like a pumping station, than a anything to do with electrical...lol The 33KV and 11KV side of things was via conventional M/Switchboards (by MG), But all breakers were GCBs.
Can't remember the size of the main 66KV TXs at the 2 well field substations, but they were big buggers, and never really had any dealings with them to be honest, except the repair of one tail leg that failed on the secondary side that went to earth via a nice clean hole thru the tail insulation about 12mm in dia and a dammed black, burned, and deformed cable protection cover, about a metre from the failed tail!! ...lol!!
Most of the TXs i was involved in were 33KV and 11KV, mostly the later ranging from 1 to 3MVA, but with pretty sophisticated relay/auto control and protection all the same.
By the way, all the main switchboards WERE rated for the full fault current of the respective connected TXs in paralleled configuration, and tested as such, but normal operation was with open bus couplers, 3 to a switchboard.
I won't tell you what the Libyan supplying main distribution centre in Benghazi was like, except it scared the crap out of me, walking round the place!!! lol!!! :vanish:
