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Power Transformers Offline

Power Transformers Offline

Power Transformers Offline

(OP)
I would like to ask if anyone had personal experience in taking a power transformer (PXF) offline for a time.  If there are any consequences which could be harmful to the life and efficiency of the PXF.  And what could be done to maintain its integrity while offline.

I have seen a thread on "Long Time Storage of Transformers" thread238-98650 dated July 2004 but the length of time concern is about 2-3 years and even more than 10 years, which to my particular application would be too long.

We are operating a distribution utility with a primary voltage of 115 kV and secondary voltage of 34.5 kV, both L-L.  We have 3 wdg PXFs with sizes ranging from 50MVA, 83 MVA to about 100MVA and 133MVA.  Most of our substations have at least two to three PXFs.  This was done based on previous medium/long range planning taking into consideration forecasted load growth.  However, with the recent development in the electric industry in our country, much of concerns were directed towards loss cap limit.  This is such a concern especially for distribution utility like us since we are forced to shoulder the excess loss above the loss cap.

Initial studies we made recently proved that in many cases, substations may well operate using one or two PXFs only, especially in off peak seasons, which is could be about 3-6 months a year.  In short, there is a possibility of taking one PXF offline for a certain time of the year, say 6 months.  This may be done by transferring the loads of the other PXF to the remaining PXF by closing the bus tie breaker.  Now, assuming that I will have an overall loss reduction in operating one less PXF, I am concern if the PXF that would be idle (in place) for 6 months would degrade rather more than if we maintain it on-line?  Should there be a concern in doing such?  Are there any manufacturer's guide or warnings to go with this activity?

RE: Power Transformers Offline

First off, look at the relative magnitudes of core (iron) and load (copper) losses.  Two transformers splitting a load will have half the load losses and twice the core losses compared to one transformer carrying the entire load.  If load loss is large enough compared to core loss, you will have more losses with one transformer than with two.

RE: Power Transformers Offline

(OP)
Hi David!  I agree with you! But as I have posted, "assuming that I will have lower overall loss with 1 PXF than with the 2 PXF", my concern is if there is some danger in the life and efficiency of the PXF...or what would be the drawbacks in taking 1 PXF offline for about 6 months and then having it on-line again...because if there is a great danger, there is no point of considering it even if it could mean savings in terms of lower loss.  The cost of PXF at the voltage level I am referring in this case is quite huge.

RE: Power Transformers Offline

We have several deenergized substation transformers sitting as spares in our equipment yard.

We make certain the nitrogen blanket shows normal pressure, and also perform annual oil tests (which includes moisture), as well as power factor (insulation condition) tests.

Lastly, we make certain all the high (H) and low (X) bushings are tied together and to a ground rod with copper wire, to eliminate the possibility of static getting into the core via the bushings.

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