Sudden Phase Shifts in Supply 3

[ScumPunk]

Hi,All
We've recently had a potential customer at our lab performing some tests on various drives. They are mainly interested in performance/recovery from mains disturbances. This started out as the usual mains dips/brownouts, etc. but as the day progressed, one of the engineers began asking us to shift ALL 3 PHASES by up to 90 degrees instantaneously. I asked how this could happen in the real world, and got a long-winded reply about lightning strikes. I get the impression he was "playing" with our programmable mains source, and having asked a few of our engineers, who are much more knowledgable than myself, have come to the conclusion that this situation is not possible. Can any of you correct this impression?
Thanks,
Mort

 

[slavag]

Oph. Found it.
Please see reports.
SR is regular supply.
SE is emergency supply.
VTBB voltage on BB 6.6kV with motor load only, sveral funs and one 2MW compressor.
Start auto transfer is command from protective relay (operated by undervolatge, underfrequncy, df/dt).
Tested 08.2005.
Regards.
Slava

 

[Marke]

Gunnar

If you look at the phase shift on trace 1 during the brown out, and the phase shift on trace three, they are almost identical.
In the first case, you have a brown out and the supply could be pulled low by other load on the supply. Some of the voltage will be supplied by the synchronous compensator.

If you had a temporary disconnect with a very light load, then the synchronous compensator could supply full voltage albeit at a phase shift for a period of time. In the extension of trace three, is there a second disturbance that brings things back into synch again?

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[Skogsgurra]

Yes Mark,

I guess that could be it. I haven't found any second trigger showing things go back to normal, though. But that may have happened when the recorder was out of paper. Which it was once during the period. It took some time before the guys noticed and replaced the roll.

I think that is exactly what happened. The compensator took over for some time. It is the only explanation that I find plausible so far. The compensator was probably a bit overexcited and that explains why the voltage stays approximately constant.

Would it also explain the phase shift? Can't picture the vectors. Have to draw a few arrows to see what happened.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[davidbeach]

I highly doubt that it is the compensator. If all phases shifted together then fine, the compensator could well be the source. But when the apparent phase sequence changes it probably isn't the result of a three phase machine.

 

[slavag]

Gunnar.
See attached document.

http://www.multi.fi/~saunaaho/Files/VectorSurgeRelayEffectiveness.pdf.

in both of recorders time between triggers about 70ms.
It seems more as time between start faults on the 70kV line and clean the faults time. I'm not so believe that is some ATS. Autorecloser: min time are 250-300ms.
Regards.
Slava

 

[slavag]

Hello.
And see attached, I think it's explained what was happened.
Regards.
Slava

 

[ScumPunk]

Hi, all,
I seem to have tweaked a few peoples' interest! Thanks a lot for all the replies, it's interesting to hear the different ideas.
I've showed Gunnar's plot to a few people here, and the consensus is that two phases have swapped (for whatever reason). They are surprised that there isn't a return event detected, as the phase swap would have a "detrimental" effect on any machinery connected to it.
Marke's suggestion re. the "Compensator" (sorry I'm not familiar with this equipment) sounds plausible, but I'm afraid my lack of experience in this field will preclude from either i) Declaring a winner; or ii) Coming up with an explanation myself. I may get some ideas from my colleagues which i will pass on, but there is not much experience of distribution grids here. We do VFD's and not much else..
Gunnar: No, I'm in Cheshire, not far away. "Very Big German Company" should allow you to pin me down! What effect did the phase shift have on the connected motors? If there was no return event, they should still be running backwards?
Again, many thanks for your enthusiasm and replies. I'm afraid I didn't work Sunday either...
Regards,
Mort

 

[Nealsn]

I work for a medium sized utility. I know that here we have caused a significant shift in all 3 phases at one on multiple occasions. This has been caused by some of our large units (550 MW) being closed up out of phase (due to operator error and failed relays).

When the units were closed out of phase it was felt all across the system including our other plants 100-150 miles away.

 

[waross]

Gunnar, If the event in trace three involved a synchronous condenser, would we not expect the phase rotation to be the same?
ScumPunk; I believe the Synchronous Compensator is also known as a Synchronous Condenser. Basically a large synchromous motor floated on the line and over excited to supply VARs. In practice it may not have a shaft extension and it may be designed to allow the higher excitation without overheating the rotating field winding.
Yours

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

 

[Skogsgurra]

Cheshire was what I meant. Stayed at the Ploughman's Inn for a couple of nights in May last summer. We probably met. I gave a lecture on EDM in bearings with five or six guys present and then had a tour on the premises. Visited the lab, climate testing, sand storms, and a huge mains simulator. I think that we shook hands.

I have got in touch with the utility guys. They have no recordings from that time. But one of them said "This can't happen". Like we all thought.

We then traced the curves and found that it is possible that what looks like curves crossing actually isn't. The crossing just before the disturbance is a true crossing. But the second probably isn't. It could also be that the two curves just touch and then return to their normal sine shape. That would not only conserve phase relation but also phase rotation - isn't that what we all thought?

I have colored the curves and the "notch" interpretation is a lot more plausible than the phase change and rotation reversal interpretation.

So, Scum, I think that you can tell that potential customer that such a waveform is like Nessie - there are pictures of it, but does it really exist? I would say NO.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

Thanks for sharing this with us Gunnar. That is an overwhelmingly acceptable explanation.
Here is the first of the several stars you deserve for this one.

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

 

[davidbeach]

A notch explanation would be very convenient, but I just can't see it; it seems to be asking too much to have the curves touch and turn around rather than crossing. The traces are too smooth at both crossings.

Any chance that we are looking at currents rather than voltages in the third plot? That was my first impression and had to delete a bunch of my first response when I remembered that you said voltages. Power flow reversal would be a good explanation if these were currents rather than voltages.

 

[buckeye3]

I would listen to Marke, he is the smartest one.

 

[stevenal]

Here's a regulating winding that can shift your source voltage nearly 60 degrees.

http://www.mathworks.com/access/hel...47,GGIC:en&q=phase+shifting+voltage+regulator

Change would not be instant, though, but come in steps.

 

[ScumPunk]

Hi, All,
Well, I must say, for my first thread I'm very impressed with the response from you all. Actually, now that Gunnar mentions it, if you look at it long from that angle, it certainly does look like a notch. The now famous Mains source (Not that massive, but around £60k all the same) is used to produce this sort of thing all the time. A 100% notch is unusual as far as I know, but I guess on a low impedance circuit it's possible. Shame the recorder wasn't colour, or the argument could be settled instantly. I still can't get past the fact that if the phase order changed, any motors connected would reverse..
Gunnar, I'm afraid I can't remember meeting you, we get so many people visiting, but if you visit again, we must go for a beer..it really is a small world.
Once again, thanks for everyone's input. I think I can tell our (potential) customer the chances of this happening are very remote. I'm awarding a star to Gunnar for the education. I now know a lot more about "Big Lad's Electricity".
Mort