Problem In Parallel Operation Of Transformers

1. Colorblind electrician? (No kidding, I've heard of that happening before.)

2. One or more ground faults, resulting in one or more corner-grounded transformers?

3. What are the various phase-to-earth voltages?

Dear Tornado,

First point is definitely not the case

How to check second point, corner grounded transformer?

Phase to ground voltages are:

R1-E: 247V

Y1-E: 248V

B1-E: 245V

N1-E: 0.01V

R2-E: 250V

Y2-E: 253V

B2-E: 253V

N2-E: 0.22V

Please confirm that either only one transformer's neutral is in use or that the appropriate switching for the neutrals to maintain 'one only in use' is correct?

I agree with you.One transformer has to be Dyn-11 indeed but the second has to be

Dyn-1.

Check up the values of the Neutral earthing of both the transformers and also check the terminations of both transformers.

It could be that some of the transformer leads are incorrectly marked.

I suggest you de-energize both transformers then disconnect all wiring and perform a wire-by-wire checkout and labeling of all transformers windings.

Be sure to record all impedance readings and compare the readings from one transformer to the other.

If you are unsure of how to perform the required testing procedure I suggest you contact the transformer manufacturer field service group for guidance and support.

Here is a link to an excellent site with good transformers testing procedures available for free.

http://engineering.richmondcc.edu/Courses/EUS%20215/Notes/Transformer%20Winding%20Resistance%20Test.pdf

Good luck and stay safe.

That should be fun given the size of the transformers.

I would expect 5 x 500mm² Cu (600mm² Al) singles per phase 5 or 3 for the neutral depending if they are type "K" or not.

Yep. Been there and done that more than once.

For some reason it doesn't get any easier the older I get.

Executing a 345KV or 500KV transformer conductor disconnect in my opinion should be restricted to those less than 40 years of age.

I suspect the connection issue(s) if there are any is/are on the paralleled load side conductors.

I'm glad I retired, it is a young mans game.

I'm 61 now but haven't done much hands on with substations since my early forties.

Here's a good read

Since the high-side is "only" 22kV you could check the phasing, IFF (If and only if) you have the proper hot-stick equipment, training, and PPE (Personnel Protective Equipment).

I suspect that you will find that, as others have suggested, somehow your source or internal transformer/switchgear connections were crossed during your last maintenance outage. The leads are only a piece of #4 cable and easily bent.

I'm fascinated to see that the meter on the pictured hot-stick equipment is analogue, of a design which would have been entirely at home on WW2 army surplus equipment.

Have you ever used one?

Army surplus equipment, yes. Hot-stick equipment, most emphatically no.

If you had you would have realized that the electric fields are pretty intense and can have an adverse effect on modern readouts and their electronics. A properly shielded moving coil meter is fairly immune to those fields, and if the meter deflects you know that there is voltage at the test point. Archaic perhaps, but it gets the job done.

Thank you. I understand now the reason for the analogue meter. What still intrigues me is the design, which remains at this stage

when it could have moved on to

or

Start a new thread about "hot sticks" if you want more information.

Is it possible that your two transformers have differently connected delta primaries - ie one of them configured as dy1 and the other as dy2? This is easily done and would not be evident with normal checks.

In other words, on the transformer primary configured as dy1 - A1 is connected to C2, A2 to B1, B2 to C1 with A2, B2 and C2 connected to respective phases. While the transformer configured as dy2 would have A1 to B2, B1 to C2 and C1 to A2 with A2, B2 and C2 to respective phases.

A Delta-star transformer connected as d1 would have a 30° lagging secondary voltage, and one connected as d2 would have a 30° leading secondary voltage.

This would result in a 60° phase shift between the two transformer secondaries, which may explain your numbers.

Dear Spades,

It may be the case..

How can I check it?

...........

You can generally parallel Dy1 and Dy11 transformers by crossing 2 of the incoming and outgoing phases on one of the transformers - ie swap A with B or A with C or B with C on both primary and secondary sides. That will move the phase shift back from -30° to +30° or vice versa and bring it into phase with the other unit

So to test if this is your problem, swap 2 of the primaries on one transformer and then test for zero voltages between the two transformers, the secondary terminals of each transformer that read zero can then be paired.

If they only read close to zero then your 2 transformers are not evenly matched and you will get some circulating currents the magnitude of which may or may not be hazardous dependent on the degree of difference.

That will reverse the phase rotation so it will still be 60° out but also going the wrong way.

As for circulating currents, even if the voltage difference is minimum the circulating currents can be considerable.

You got me thinking whether my memory was failing me so I went looking for answers.

Below is a cut and paste excerpt from halfway down the article on this link on that very subject which would appear to support my proposal. Maybe the article is also incorrect and perhaps you are right, I'll leave that alone as I never had to do it but thought I knew the theory. The last paragraph on the excerpt is the most relevant.

"5. Same phase angle shift (zero relative phase displacement between the secondary line voltages)

The transformer windings can be connected in a variety of ways which produce different magnitudes and phase displacements of the secondary voltage. All the transformer connections can be classified into distinct vector groups.

Group 1: Zero phase displacement (Yy0, Dd0, Dz0)
Group 2: 180° phase displacement (Yy6, Dd6, Dz6)
Group 3: -30° phase displacement (Yd1, Dy1, Yz1)
Group 4: +30° phase displacement (Yd11, Dy11, Yz11)

In order to have zero relative phase displacement of secondary side line voltages, the transformers belonging to the same group can be paralleled. For example, two transformers with Yd1 and Dy1 connections can be paralleled.

Thetransformers of groups 1 and 2 can only be paralleled with transformers of their own group. However, the transformers of groups 3 and 4 can be paralleled by reversing the phase sequence of one of them. For example, a transformer with Yd1 1 connection (group 4) can be paralleled with that having Dy1 connection (group 3) by reversing the phase sequence of both primary and secondary terminals of the Dy1 transformer.

We can only parallel Dy1 and Dy11 by crossing two incoming phases and the same two outgoing phases on one of the transformers, so if we have a DY11 transformer we can cross B&C phases on the primary and secondary to change the +30 degree phase shift into a -30 degree shift which will parallel with the Dy1, assuming all the other points above are satisfied."

Dyn1 can be also be changed to Dyn11 by reversing the secondary (if you have HeMan on your team).

I'm lazy so would use the easy way. (The OP will still get covered in oil).

I've saved that article to read later.

There are a couple of anomalies regarding parallel phasing. The major one would result in 30° 30° 30° on one half of the vector rotation/cycle (a tricycle/unicycle? All the wheels are on one side.)

Seriously, the only practical way is to re-jig the primary tappings. The secondary tappings are going to be massive on a 2.5MVA transformer.

I've finally worked out what the article is on about. Reverse two primary windings and the third secondary. The clue was in the photograph, three small single phase units.

Easy with small units, it's virtually impossible to reverse a secondary winding in a 2.5MVA unit.

What effect it would have on the transformer impedance I don't even want to think about.

I don't see that they are suggesting reversing a secondary winding but just the external phase connections to 2 of the primary and 2 of the secondary windings, a relatively easy task. No internal changes involved.

A quick web search found that there are quite a number of sites that suggest this same solution. Here's one and another, and another and plenty more if you care to look. I doubt they are all wrong.

I also don't believe that they are talking about separate single phase transformers on that earlier link as they mention changing the connections on one only transformer, whereas if they were single phase units they would have to change the connections on 2 transformers, and they mention swapping connections B and C which would not exist together on a single phase unit

I would also be very surprised if that photo (the only one that I see in the article) was of a single phase unit.

On a power transformer normally only four connections are brought out on the LV side c, b, a, n. only three on the MV side A, B, C

How do you swap the connections?

I'm not saying you can't alter the connections as suggested but it is totally impractical.

This is the LV side of a small 500KVA unit I installed 20+ years ago 600mm² 4 sector SolidAl. You try swapping them around.

"How do you swap the connections?"

Endeavor to persevere.

It is relatively easy to change a Dyn1 to Dyn11 so long as you know what you're doing.

In a touch of pure cost cutting genius we acquired a 2^nd hand 1000KVA 11/.433KV. Pity it was Dyn1, I'd transferred to a different department and so it wasn't my problem. I enjoyed watching the head scratching, eventually I dropped a hint and ran away.

I didn't escape sorting someone else's mess out.

I do not had any personal experience of running 3-ph transformers in parallel. Hence some questions that reveal my ignorance.

1) Are these transformers already running and you've just found out about the different voltages and ask CR4 out of curiosity?

2) If not yet running, and you went ahead and connected them as they are, what do you think would happen when you switched them on.

Just curious.

The phase rotation would be the same so you wouldn't really notice anything. That is until you tried to parallel them, then you would definitely know about it. Then the race would be on as to what trips or blows up first.

I recently took a transformer out of service, to do so meant paralleling two first. As it was a system I was unfamiliar with, before I closed the bus-section I did the same checks the OP has done.
Self preservation.

Thanks for the tip.