J.P.
Senior Member
- Location
- United States
- Status
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I believe the problem doesn't exist on multiple core transformer banks - like when POCO builds a bank out of three individual single phase transformers - they typically are a wye primary with a connection to neutral around here anyway, otherwise you would see two bushing transformers - very rare in this area, maybe even non existent on distribution lines.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
A good observation, and worthy of comment:
It may be worse with a single core three transformer, bit it happens with three independent pots too.
Your observation tells us two things:
1. POCO does a very good job of maintaining balance, at least on their primaries. Less so on a random load customer service.
2. The caution on this config in the PDF stresses that the taps must be set identically and the transformer impedances must match.
And I will add:
3. In the POCO situation the wye-delta will actually try, via circulating currents, to force voltage balance on the primary side. POCO may actually appreciate this at the end of a long run. A customer transformer, OTOH, on the other hand will destroy itself trying to wag the POCO dog.
Iron_Ben
Senior Member
- Location
- Lancaster, PA
We don't build three transformer wye-delta banks very often any more. Two pot open delta is the standard. However, we have thousands of three pot wye-delta in service from 30 and 40 years ago and they give no more trouble than any other configuration. I know of a number of large banks, say two 50's for the wing pots and a 100 for the lighting pot, with maybe 15 or even 20 customers connected. This would typically be in an urban, heavily populated area, a mix of residential and commercial. There might be 2 three phase customers on the bank and the other dozen plus single phase. You do the calculations and you say, man, that bank is being pushed to the limit. The circulating currents on the delta side are high. It's overdue to fail. Then you go in the field to take a look and see that all three pots and all connections are original. Thirty or forty years old and still rocking along. You shake your head and mumble, "They really knew how build transformers back then," and move on to the next job.
As far as POCO's maintaining good primary voltage balance, that's mostly dumb luck. You have to have the individual primary phase loadings very far out of whack to introduce significant primary voltage imbalance. The most I can recall seeing on the 13.2 kv side was around 2%. This was enough, however, to wreak havoc with a rural water plant's solid state controllers, which tripped on current imbalance for some 75 HP pump motors.
As far as POCO's maintaining good primary voltage balance, that's mostly dumb luck. You have to have the individual primary phase loadings very far out of whack to introduce significant primary voltage imbalance. The most I can recall seeing on the 13.2 kv side was around 2%. This was enough, however, to wreak havoc with a rural water plant's solid state controllers, which tripped on current imbalance for some 75 HP pump motors.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
D'oh! I meant 120 degrees, of course. Please forgive the brainfart. As you were.
But my question still is: if you compare A to N with A to B, what is the phase relationship of the waveforms?
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