Close transfer of load between two power transformers

[farooquekhann]

We have two power transformers of 132 kV/6.6 kV each. Both are of different make. One is 25 years old, the other is brand new. The only common points between the two are: same volt-amp ratings i.e 10/13 MVA, same turn ratio corresponding to 132 kV/ 6.6 kV, and they both have OLTC (of different make and technology).
Load is to be shifted from running transformer to the standby transformer without causing any interruption. The two transformers will come in parallel for at most a few seconds. What are the necessary condition to fulfill? We have the Transfer Switch but I do not find the conditions for this close transfer between transformers.

 

[GroovyGuy]

I have done this before with three (3) x 45MVA units rated 230kV-13.8kV. In this case the only condition was to ensure that both tap-changers were set to manual and that the same taps were selected (ie the secondary voltage was identical). There was a timer that only permitted the three CBs to be closed for a few seconds, after that the tie would automatically open. OF course this was a new Site so the three xfmrs were identical, but I don't think that the fact that your xfmrs are not identical should be an issue for you.
BTW you did nit state if the xfmrs are connected to the same primary source ( I am sure that they are, but I had to ask).

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[crshears]

Building on what GroovyGuy wrote, having both xfmrs on the same primary supply is a big help, as system-induced circulating flows during the transition will be minimized. [Edit/addition: if the two transformers are not in fact close-coupled to the same primary supply, your local utility scrutineers can be expected to ask some very pointed questions, since they not only must answer to the local AHJ [authority having jurisdiction] but won't want any "back-splash" from your site into their grid.]

Beyond that, doing the usual standard stuff should get you through OK; ensure the low side phase rotations match, the individual phases are correctly matched up, and the secondary winding voltages are balanced.

During the brief period that the two xfmrs are in parallel, the loads on the two units can be expected to vary inversely as their impedance. Also, since for this period the supply impedance will be much lower than normal, the fault current infeed capability will likely greatly exceed the switchgear's design standard, so it would be wise to establish a personnel exclusion zone from the vicinity of the xfmrs and switchgear, or at the very least minimize the number of personnel involved. For the same reason, do not unduly delay the unloading of the "old" xfmr.

Don't let this scare you off, however; utilities execute this procedure quite routinely and as a matter of course, so as long as you exercise your due diligence there shouldn't be an issue. One thing that would make me leery of doing this would be if the existing switchgear or its ancillaries has not been well maintained, leading me to doubt whether a spectacular and/or explosive equipment failure might occur at an indelicate moment...in my utility we have on rare occasions flat-out refused to do live transfers for just this reason, gripe though the customers might at having to take a complete site interruption.

Hope this helps.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[farooquekhann]

Yes, both transformers are connected to same supply on primary side. Also, the two are of same vector group.

 

[RonShap]

Do a voltage check from Phase A of one transformer secondary to the other and you should measure ~0. Same for B and C, and if it is true, then parallel at will.

 

[davidbeach]

It's more important to match transformer ratios than to match voltage.

If you match voltages, the ratios will be different and there will be circulating currents between the two transformers. As soon as you close to make parallel the voltage will rise because you now have twice as much transformer carrying the same load. At that new voltage one of the transformers will be supplying Vars that are being absorbed by the other transformer.

If you match ratios, the voltages will be different when you close, but will instantaneously match. Both will share the real and reactive power based on their impedances but there will not be any circulating currents.

If the impedances are radically different, it may be desirable to split the difference some, but in most cases matched ratios will give the best results.

 

[waross]

To add to David's information:
When you close the connection between two identical transformers with the same ratios the load will be split between them. The lessened load on the original transformer will cause the voltage to rise as predicted by the transformer regulation characteristics. When the transfer is complete and the first transformer is disconnected the voltage will drop back to the original value. The load on the incoming feeder will be the same and so there should be no change in the voltage drop in the feeder.
For transformers with different impedances and/or regulation the calculations are a little more difficult, and the voltage drop can be expected to be a little worse.
For transformers with different X:R ratios, both the calculations and the voltage drop may be worse again. The transformers may be operating at different phase angles.
For different impedances and different X:R ratios, it gets worse again.
Now if the taps are set differently it all gets worse again and the calculations ......... you get the idea.
I agree with David.

If the impedances are radically different, it may be desirable to split the difference some, but in most cases matched ratios will give the best results.

In extreme cases a little circulating current may be beneficial.

That said, loads are transferred and transformers are connected in parallel every day. If you match ratios you should be able to live with the results.
Note: You cannot "Soft Load" a transformer in parallel by adjusting the voltage. You must use a phase shifting transformer.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

There is another issue with paralleling transformers that has not been mentioned yet. Don't forget to review your arc flash levels.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stevenal]

I agree it is important to match the steps. RonShap's suggestion is possibly more important if this transfer has never been done before. It is a phasing check, generally performed across the open switch prior to closing it to make parallel. If the voltage is closer to line to line than to zero, don't close.

 

[bacon4life]

In addition to arc flash, review withstand ratings of the switchgear for the additional fault current.

I assume you are using a purpose built transfer switch or circuit breakers in a switchgear. If you use some other kind of disconnect switches, check that the switches are rated for loop splitting.