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?
Wye-wye transformers are very common. Utilities in the US use them all over the place for service transformers.
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.
I concur with dpc and would add that faults on the primary do not affect passive secondary loads except for the loss of voltage. Motors and generators will contribute fault current to primary faults.
for some "pros and cons" on the different transformer connections.
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.
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.
Just a small correction to dpc's post - an earth fault on a YNyn transformer secondary IS seen as an earth fault on the primary side - the zero sequence impedance is straight through from primary to secondary. A delta tertiary winding (and sometimes the tank acting as a 'delta') adds in a zero sequence source which messes up your earth fault relaying. The Dyn transformer reflects a ground fault on the y side as a phase fault on the D primary.
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.
