Kitchen circuit suddenly reading 40V

Salmon Egg wrote:


Yes.
No. One of the primary phases feeding the neighborhood, including the wye-delta opens.

Then picture it with a neutral.

With the neutral connected, the unbalanced load on the disconnected primary phase is backfed through the bank's delta and the two remaining primary phases.
We used to use these wye-delta banks quite a bit in rural areas to feed irrigation pumps. When the pumps were off, the banks were of sufficient capacity to support the loads on the open phase for quite some time. Sometimes we'd get a call about low voltage. Or sometimes it wouldn't get picked up until a crew would happen to spot an open fuse. Sometimes weeks later.
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I still seem to missing something. I one of the feeders to a primary wye opens, then the secondary delta, becomes an open delta. Under these circumstances, I would expect a consume to hardly notice until the remaining phases are no longer able to provide the current demanded.
Bill
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Salmon Egg wrote:

No. Its still closed. Which means that the unbalance current can circulate around the delta. Draw the phasors and see what happens.

That's correct. Even for the consumers connected to the 'open' phase. Of course, back feeding a substantial load through the high impedance of a single bank results in low voltages.
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I still am trying to understand what you are saying. As I see it, the secondary delta is working like an open delta. If you were to disconnect the unexcited winding of the secondary, you would have an open delta. When you then reconnect that winding, the secondary works like an open delta. The only difference between having the three windings connected in a delta arrangement is hat magnetizing current is flowing in the unexcited leg of the delta. What am I missing?
My argument does not address whether the transformer will be overloaded or dealing with harmonics. I do not understand how the voltage across the unexcited (from magnetic flux in the core) leg is 120VAC. How can this voltage be reduced to 40VAC by having one end of the wye on the primary side get disconnected?
Bill
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Consider a Y-delta transformer and a 3 phase load downstream of a set of fuses. Fuse on Phase "C" blows. On the Y-Delta transformer, two of the transformers A and B are powered normally, and operate as an open delta configuration. Assume it's 480V phase-phase on the secondary. The "C" transformer has its _secondary_ connected to the A and B transformers in series. The voltage here is still 480V (assuming the mythical perfect lossless transformers). It operates in reverse, stepping up the 480V to the primary voltage. The three phase load downstream sees all three phases still present and may work normally -- if (big if) the Y-delta transformer bank doesn't get overloaded.
I think in practice they frequently do _not_ connect the Y's neutral point to the system neutral to prevent this.
re my comment "I wonder what utility problem would cause me to see 40-48V": I was thinking what may happen if one phase was lost upstream of a delta-wye transformer that fed my local distribution net. I'd think that should feed me ~60V, but loading may have pulled it down.
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Michael Moroney wrote:

Its a trade-off. Often, Wye-delta banks serve combinations of 240V 3phase loads and 240/120V single phase off one center-tapped winding. The result is an unbalanced load on the bank. Many of these installations oversize the 'lighter' transformer (the 240/120 unit). The resulting loading has significant zero sequence components which must be suppored by a primary neutral so as not to produce voltage regulation problems.
For large single phase loads combined with small 3 phase, the solution is the open delta secondary. The backfeed problem is eliminated. Regulation is OK so long as the 3 phase load is small.
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I think that it can be mathematically proven that in a Y-delta configuration, if all transformers are equivalent, there will never be any net neutral current for any delta load you can come up with. In practice it may not be so. Note that even if there is no three phase load (suppossedly the entire load is supplied by the "lighter" transformer), all three transformers contribute to the load, and the neutral of the two transformers counters that from the main one, so the net is zero.
I do know this: I used to drive by a setup where there were three large transformers on three adjacent poles. Downstream of this everything was older and it appeared everything had delta-connected primaries. This included branches into neighborhoods which had two "hots", not one, not three. I looked closer at the transformer configuration, it was Y-delta but the "neutral" was insulated for full voltage and was connected _only_ to the three transformers.
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On Tue, 16 Mar 2010 23:33:28 -0700, Salmon Egg

In my experience distibution transformers are almost always DELTA on the hi voltage side and WYE on the consumer side with the star point,the neautral and grounded.
In USA usually there is only a single phase transformer (120-0-120 volts) at each distribution point with these balenced on the three supply phases along the street.
John G.
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wrote:

------------------------------------ I know of many cases where the distribution system in a given town was near its load limit and a cheap and effective fix was to convert the existing sub-transmission/distribution transformer from Y-delta to Y-Y (grounded both sides) and the distribution/ consumer transformers treated the same way. This could be done quite quickly as grounds are available at all stages and for single phase units, it is simply a case of tying one end to ground. While this doesn't increase the capacity of the sub-transmission/distribution transformer, (which then can be replaced if needed), it does increase the capacity of the distribution system itself. the gains, in the cases where this has been done, outweigh the disadvantages.
This has been in Canada but, I am sure, the utilities in the US also do this where it is advantageous and I don't know who tried it first.
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