Transformer Wye-Wye both windings grounded
Transformer Wye-Wye both windings grounded
(OP)
Is a Wye-Wye transformer with both windings grounded an acceptable practice? Are faults on the primary detrimental to the secondary or load on the secondary?






RE: Transformer Wye-Wye both windings grounded
The concerns about wye-wye transformers are generally:
Triplen harmonics can pass through the transformer - in the old days, this could cause problems with telephone interference on overhead power lines, but this is not common problem these days.
Ground faults on the secondary are seen as ground faults on the primary since the zero sequence current goes right through the transformer, essentially. This makes relaying more difficult. With a delta-wye transformer, the secondary ground faults do cause zero sequence current to flow in the primary circuit (except in the transformer delta).
Utilities like wye-wye transformers because they are generally cheaper than delta-wye and they feel they greatly reduce the risk of ferro-resonance if the transformer loses an incoming primary phase.
If I had a choice, I'd still go with a delta-wye in most applications.
RE: Transformer Wye-Wye both windings grounded
RE: Transformer Wye-Wye both windings grounded
See Thread238-88701
RE: Transformer Wye-Wye both windings grounded
One place to use a wye/wye trsf is on a power station auxiliary system with two supplies. On a generator with a step-up trsf and with a unit trsf providing a supply to a unit switchboard - while the generator is being started up, the unit board will normally be supplied via a station trsf which will take its supply from the HV. At some stage during the loading of the generator, supplies will need to be changed from station to unit which will involve briefly paralleling these, and so both supplies must be in phase, hence the wye/wye unit. (dpc is this what you suggest with service trsf...?)
It is possible, and may be necessary to provide a wye/wye unit with a delta-connected tertiary winding in order to eliminate third-harmonic voltages.
Regards
Ralph
RE: Transformer Wye-Wye both windings grounded
I agree. Further, Tertiary delta is not considered necessary for transformers below 100MVA rating. Many power stations in India have these Y/Y connected station / start-up transformers.
RE: Transformer Wye-Wye both windings grounded
Watch out for add zero sequence sources on distribution feeders - you can get a fault on a neighbouring feeder tripping the feeder with the zero sequence source unless you are using directional ground fault relays.
Bung
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