250 KVA Hydro Generator - Pick apart the design 4

[mhulbert]

I have a client that has a small hydro generating setup. It's about 30 years old and experiencing some problems. I think there are a few areas of the design that may need fixing. Some of this is outside what I deal with typically, and I would like you guys and gals to give me your opinion.

The system runs parallel to the grid. All of the water side items seem to work correctly, so I will focus on the power side. The generator is a 250kw 3 phase induction motor, labeled for generator use (by GE). It is connected for 480V. This goes to a contactor to connect it to the line, fuses, current monitoring, in-phase/in-voltage monitor, revenue metering, and then out to a transformer. There is also a capacitor that is brought on or off line for PFC (I think). Electrically speaking, the generator is hooked up just as you would a DOL motor starter, except with fuses instead of OL's.

The 480V heads to a pad-mount transformer(300 KVA, 12470Y/7200-480 Delta, 1.9% impedance), and here is where things get a little uncertain for me. 480V goes in on the delta side of the transformer (X1, X2, X3). 12kv comes out of the Wye side (H1, H2, H3). H0 is tied to the building/site ground through a resistor and 12KV/120V single phase transformer arrangement. The 120V winding feeds a voltage relay which I believe opens the contactor when voltage goes above a threshold. The 12kv lines go underground 300', and are connected through fused cutouts (with lightning Arrestors) to the 12kV grid.

So, what problems have we had-
-Hydro plant is running, owner is present fine tuning controls
- A 480V lightning arrestor shorted out. It was unfused so a lot of current and sparks flowing. The owner was present and disconnected the system at a 480V disconnect just before the transformer. The lightning arrestor was connected on the line side, so it kept burning up! The rest of the 480V equipment is disconnected at this point
-Transformer starts thumping, lightning arrestor still burning up
-After about 5 minutes, one of the fused cutouts blows out, everything stops.
-Upon removing the lightning arrestor, and refusing the cutout...fuse blows immediately
-Not sure on a plan of action at this point. I have meggered all 480V wiring and equipment, tested fine

After reading all this, and thanks to anybody who got this far- what problems do you see with this setup? I am concerned that there is no grounding of the 480V side of this system so L-G voltages could become elevated. That may be why the 480V lightning arrestor shorted out. Could this have triggered a fault in the transformer? Would a transformer fault only take out one fuse on the 12kV side? Is this how these are usually grounded?

We are thinking of having an oil analysis done on the transformer, but are hesitant to bring out a firm to test the underground cabling as it is $3500+ My gut says the transformer is faulting to ground, but I find it odd that it did it exactly when the 480V lightning arrestor faulted out too.

Any thoughts on this are incredibly appreciated. I have pictures and can answer questions if you have any.
Thank you Mike

 

[rasevskii]

In addition to what Smallgreek anf Edison123 have suggested there are other easy tests that can be done.

Before doing anything on the 12KV side, insure that the fuse cutouts are really open (visible break) and tagged out. Put a set of grounds on the 12kV cable at the working place (your transformer).

Preferably a Variac should be available, but if not do a simple test with a 100 or 150 Watt light bulb (not a CFL)connected in series on your local 110V supply on the Phase (hot) side. Connect the Neutral of this supply to the H0 terminal (disconnect the transformer from all other connections) Now touch the light bulb wire to each phase H1, H2, H3 one at a time. The bulb should not light, or only dimly. On each test you should be able to measure a few volts on the corresponding phase of the 480 side.

Repeat the test using instead of the light bulb, a 1KW 110V heater element instead (this will limit the current to about 10A), again measure the secondary voltage on the corr. phase of the 480. You should read about 7.3 Volts on the 480 (delta) side on the corr. phase. There should be actually only a small current on your 110V input, if there is a heavy current (say 10A) that is bad news, there is a fault in the transformer.

This is a Poor Mans ratio test. There is no way that you can damage this relatively large transformer with this weak 110V supply.

If that was all successful you still have megger the transformer H1-H2-H3 to ground (H0 disconnected) and X1 or X2 or X3 to ground (this a delta so it does not matter which phase), in each case some 10s of M ohm should be read.

Use all reasonable safety precautions. 110V can also be dangerous if you are in wet weather outside.

None of this will prove that the transformer is really fit for service. It is only a damage assessment. In reality a commercial company shoud test it, or the utility company on a payment basis.

rasevskii

 

[electricpete]

Muthu – thanks for the info. Apparently I was wrong about the fact that the center phase would act differently during surge test.

I will say surge test does not appear to be a “standard” test for transformers as it is for motors based on the following:
[ul]
[li]EASA AR100 for motor repair discusses surge testing, but EASA AR200 for transformer repair does not mention surge testing.[/li]
[li]IEEE has a standard for surge testing motors, but none for surge testing trasnformers. (Impulse testing is used as a factory test, but is a slightly different animal... as I understand more of a pass-fail like a hi-pot and not a comparison).[/li]
[li]IEEE 62-1995 “Ieee Guide For Diagnostic Field Testing Of Electric Power Apparatus, Part 1: Oil Filled Power Transformers, Regulators, And Reactors” – does not mention impulse testing or surge testing. Provides quite an extensive list of other tests.[/li]
[/ul]
So, I tried to fit a reason with the above “standard” practice and I came up with the magnetic asymmetry of the center core leg (which does show up in other tests like single phase excitation test). Thanks for correcting me.

Now I’ll try to fit another reason to the standard practice: for motors we don’t have the option of TTR. For transformers we have TTR which is easier and tests the whole winding rather than just the 1st few turns. That is probably the reason that most people don’t do surge test on transformers. Would you agree?


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(2B)+(2B)' ?

 

[rhatcher]

The testing routine that edison123 recommends is a good one. You could substitute a more conventional turns ratio test for the surge comparison test if desired.

 

[edison123]

pete - I would say induced over voltage test in trafos (which basically tests the inter-turn insulation) is akin to surge comparison test. To avoid saturation, the induced over voltage test is typically done at twice the rated frequency, which of course means only a factory test and not a field test. Surge comparison test similarly tests the turn insulation though at much higher frequency thus reducing the current and power requirements.

And in both rotating machines & trafos, I have found this test can detect the turn insulation failure located anywhere in the windings and not just at the first few coils.

Of course, no standards I know of suggests the surge comparison test as a standard test for trafos but I am using it extensively in my shop given its utility and portability.



Muthu

http://www.edison.co.in

 

[electricpete]

Thanks Muthu.
I do know the surge test is widely believed to be not as effective for finding turn insulation weakness deep in the winding, if for no other reason than the surge magnitude decreases as it travels down the winding. (This is one reason many customer specs require reduced-level surge testing of individual coils after inserted into the slot but before connected into a complete winding). I’ll take your word that it is effective for finding turn insulation shorts even deep in the winding, which is probably the more meaningful context when comparing to TTR..., because TTR also would likely find only shorts, not weaknesses.

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(2B)+(2B)' ?

 

[mhulbert]

Muthu,
My apologies on the readings. These were of the cables, not the transformer.
the readings were taken with the cutouts open, cable disconnected from the transformer. Only thing that was still connected were the 12KV lightning arrestors on the pole. One of these could be bad, we are working with utility to temporarily disconnect and close cutouts with cable removed from transformer.

Smallgreek and rasevskii thank you for the poor man's test ideas. They seem so obvious now! Going to try these if we can secure a generator at the site.

When we tested for continuity, H1 to ground/transformer case, with H0 floating, read at 155 Ohms. Same for H2 (152 Ohms), H3 (149 Ohms). 12 kv cables were not connected, 480V side all terminals clamped to ground. This seems wrong, but the resistance was very consistent...

Also, with 12KV and 480V cables detached-
H1-H0: 2.7 Ohm
H2-H0: 4.26 Ohm
H3-H0: 2.5 Ohm
Ho-Ground/Case: 149 Ohm (Probably the NGR and GFD Transformer Primary)
(These values were taken with a DMM)

We will try additional poor-man tests if possible.