We have a yard transformer as follows:
10 MVA
34.5 KV Y primary (grounded Y)
7.2 KV Y secondary
we are looking to replace this transformer and have found a transformer that has:
34.5 KV Delta primary
7.2 KV Y secondary
can we replace the existing transformer with the Delta primary transformer?
Please explain why or why not.
There is a de facto phase-shift difference in the old/new units. Does the transformer ever operate with a closed tie to other local/remote equipment?
Determine if anything on the 34.5 side is affected by loss of the H0 connection. The grounded-wye winding may be used to establish a grounded-neutral point or “zero-sequence source.” Will insulation levels or surge-arrester ratings be affected by changing to an ungrounded-delta configuration?
A big issue is changed or unanticipated operation/miscoordination of protective relaying on both sides of the transformer. It’s probable that part of protective relaying expects to see current flowing in the grounded-wye bushing during certain faults. Will removal of the connection affect operation of any ground or other protective relaying out on the system? Does a different concern have protective equipment connected to either winding? I.E., is this a part-utility/part-industrial system? If not allowed for, serious misoperation of protective equipment may occur in another concern’s system with your reconfiguration.
Depending on the region and supporting equipment, delta-primary power transformers are common, and desirable for protective-device coordination. For instance, with a Y-Y system, ground faults on one winding can be reflected through the transformer and affect current flow on the other side of the transformer.
In a delta transformer winding, triplen harmonic currents will be manifested/dissipated as heat in the delta winding, whereas triplen harmonics will be passed through the Y-Y transformer.
Without an adequate study of the installation, problems may not surface for awhile, and be very difficult to comprehend and resolve when they show up down the road. Also, it may be desirable to review station ground-mat adequacy and touch-step potentials.
[This message has been edited by Bjarney (edited 04-23-2002).]