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Delta Winding with grounding transformer on utility sub-transmission & distribution

rockman7892

rockman7892

Electrical
Apr 7, 2008
1,178
I've come across a few small municipal distribution systems recently that had either their sub-transmission or distribution systems served from and interconnection substation to local transmission system owned by others (larger IOU) and noticed that the low voltage side of these transformers for municipal sub-trans or distribution (IE 26kV or 12.47kV) was a delta winding with a grounding transformer.

I found this a bit odd as most of these transmission to distribution level transformers I've come across in the past have always had a wye winding. Would there be any particular reason for having a delta winding on the low side of a sub-transmision or distribution transformer? Only thing I could think of would be the presence of generation on sub-transmission or distribution?

Appreciate any insight from others who may be familiar with approach.
 
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That would stabilize the incoming voltages.
That MAY allow a ride through of a primary loss of phase if the incoming was a wye configuration.
The downside of that is that it would back-feed full voltage into a lost primary phase.
 
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Having a DELTA on the 26kV or 12.47kV distribution system is little odd because,
1) when that system is connected to a customer he will definitely have a DELTA winding in his 26kV or 12.47kV side. That means the distribution system will become a DELTA/ DELTA ungrounded system.
2) The distribution system has always to be effectively grounded to avoid over voltages during a ground fault and also from the safety point of view.
Looks like a ZZ transformer is already installed so that during phase to ground faults the COG is kept within at least 0.8 so that all phase to ground connected equipment such as surge arresters, VTs etc are operating within their limits.
 
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In the areas that I am familiar with, there are seldom any 26kV or 12.47 connected customers.
Almost all customers are served by utility owned transformer connected line to neutral/ground and delivering 120/240V, 120/208V, 277/480V or 347/600.V
Hence the need for a neutral point by a grounding transformer or a wye connection.
 
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How old are these installations?
I only ask because in my company a very similar situation is present, but it’s due to a change of system grounding philosophy over the last 100 years. The 55kV transmission system, which was the original system transmission voltage, was originally ungrounded. I’ve been able to confirm by pulling old drawings as far back as the 1920s that show the 55kV system delta connected with no ground reference. In the 1960s timeframe the company began to effectively ground the 55kV system by installing grounding banks.
Perhaps with these smaller utilities a similar change occurred.
 
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rockman7892 said:
I've come across a few small municipal distribution systems recently that had either their sub-transmission or distribution systems served from and interconnection substation to local transmission system owned by others (larger IOU) and noticed that the low voltage side of these transformers for municipal sub-trans or distribution (IE 26kV or 12.47kV) was a delta winding with a grounding transformer.

I found this a bit odd as most of these transmission to distribution level transformers I've come across in the past have always had a wye winding. Would there be any particular reason for having a delta winding on the low side of a sub-transmision or distribution transformer? Only thing I could think of would be the presence of generation on sub-transmission or distribution?

Appreciate any insight from others who may be familiar with approach.
Click to expand...
Having more number of transformers with Star connected windings and neutral solidly earthed, helps reduce overall zero sequence impedance of the utility transmission system.
Having a Delta on lower side insulates the utility system from L-G faults in the downstream / consumer power system.
 
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Yeah I’ve seen this setup before in some smaller utility co ops. My take: they’re probably using delta on the low side for ground fault isolation. Keeps L-G faults downstream from reflecting back up into the transmission system. Less mess for the upstream utility.
Also seen it where they don’t want to bother grounding at every step. They just ground once with a grounding bank and let the rest float.
Sometimes it’s legacy too, like wcaseyherman mentioned. If the gear is older or from a time before grounding was standardized, delta might just be what was available or preferred at the time.
You seeing any generation on the 26kV or 12.47kV side though? That’d change how you’d want to ground things too
 
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Amp Dylan said:
Yeah I’ve seen this setup before in some smaller utility co ops. My take: they’re probably using delta on the low side for ground fault isolation. Keeps L-G faults downstream from reflecting back up into the transmission system. Less mess for the upstream utility.
Click to expand...
Maybe not. with a delta secondary, line to ground faults on the primary will cause fault current to flow in all three primary phases of a wye/delta system.
 
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waross said:
Maybe not. with a delta secondary, line to ground faults on the primary will cause fault current to flow in all three primary phases of a wye/delta system.
Click to expand...
Yup, fair point. If the delta is on the secondary, L-G fault on the primary can still drive current through all 3 phases depending on the core and grounding of the primary side. Especially true if it’s a solidly grounded wye primary.

I was mostly thinking of the L-G faults on the delta side not coupling back upstream too hard. But yeah, fault current behavior across wye-delta isn’t always intuitive.
 
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