Phase rotation query

[MaxDmand]

Can somenone verify this please - A 20kV/11kV 15MVA, YNd11 trasnformer is connected between a 20kV network and an 11kV switchboard. On checking the phase rotation at the 11kV switchboard it was found to be counter-clockwise. This was also the case on the 20kV side, which I suspected, due to some earlier situation on the same system. I suggested changing the phasing at the 11kV switchboard, but the contractor said he would rather do it on the 20kV line for practical convenience. After the change over (R/Y phases)the phase rotation on the 20kV side had changed to clockwise but the 11kV side remained counter-clockwise. Without making any more changes on the 20kV side, we changed the phasing on the 11kV side,; the result being that the phase rotation became clockwise. I am a bit non-plussed as I would have thought that the change on the upstream side of the transformer would have resulted in a similar change on the other side. I have done some vector analysis, drawn it out in autocad, but still don't seem to get an explanation. Any input would be much appreciated.

 

[Skogsgurra]

I am as puzzled as you are. The only solution to this puzzle is that the 11 kV measurement was wrong in the first place. (I rule out that the 20 kV side wasn't changed or that your 11 kV has another origin than the changed 20 kV).

How was the rotation checked?

 

[MaxDmand]

Further to my puzzle!!

1. The measurement (phase rotation) at the 11kV side was taken on the secondary of the line VT and again at the busbar VT. This was to confirm that the wiring from the VTs (although verified earlier) was correct.
2. The measurement (phase rotation) was rechecked using another phase rotation meter to prove that the meter wasn't at fault.
3. The change was made on the 20kV line - 20 meters above ground - seeing is believing.
4. I don't understand where you are coming from when you say the 11kV has a changed origin. There is no other origin than the primary input (20kV) to the transformer. If you change 2 of the primary input lines-ie r/y, y/b, or b/r, then the only change is that first the phase rotation into the transformer primary is R/Y/B then it becomes Y/R/B.

In my humble opinion, surely the phase rotation on the secondary side should have changed in consequence???

As for the measurement being wrong in the first place, well I checked it on the 20kV incomer VT aswell and this proved to be CCW. Then, when the lines were changed the 20kV 'measurement' <-- or --> was CW, but alas the 11kV 'measurement' was still CCW!

If only Tesla was still alive.

 

[busbar]

 
Is it possible someone flopped 11kV phases without telling others? Or attempted to roll 11- or 20kV phases incorrectly—actually flopping two phases? An old contractor trick is to flop phases at one cable termination to “correct”/reverse phase sequence, but modify and leave behind phase markers that look correct but are in fact wrong.

Have reversed readings shown in var, varhour or power-factor meters on the 11kV bus? One way to check may be at the in the 11- and 20kV VT-terminal boxes, doublechecking that polarities and wiring connections. A further test may be to measure and record {and sketch} all VT-secondary phase-to-phase and phase-to-neutral voltages, including secondary-side 20kV-phase to respective 11kV-phase checks—accounting for vector group 11 [30° shift.] A magnitude-indicating phase-angle meter may help.

However petty and agonizing, sometimes one has to reduce tests and analysis to the simplest level to find the hitch, having one other experienced person ‘birddog’ you on each step. It’s likely a minor error that may look silly when found. Another help is to run off all the ‘Looky-Lou’ rubber-neckers and “official observers”/”tourists.”

Lastly, it may take a correctly-rated. stick-mounted phasing voltmeter to sort out the problems, taking VTs and wiring out of the picture.

 

[dpc]

Well, unless you're governed by a separate set of the laws of physics, the problem is in your measurements, not the system.

PT wiring is always highly suspect in these situation, as is the infamous "phase rotation meter".

I agree with busbar that a couple of sets of phasing sticks may be the fastest means of verifying the actual phasing. But wear your PPE.

 

[Skogsgurra]

eddielaird,

Your comment:
"4. I don't understand where you are coming from when you say the 11kV has a changed origin"

I come from Sweden. Does that answer your question?

And, I did NOT say that it came from somewhere else. I said that I ruled that possibility out. Mainly because I think that you should know about that possibility, if it existed. (It does in many switchyards where two redundant sets of busbars are used to increase availability).

As I said. Either one of the measurements (before or after) was/is wrong or I have to go back on my assuption that the change-over didn't take place.

 

[MaxDmand]

Just to put you all in the picture, I did what I suggested to the contractor, in the first place, and changed the phasing at the 11kV line into the switchgear. This remedied the situation. If my suggestion had been heeded we would not be talking now.