250 KVA Hydro Generator - Pick apart the design
250 KVA Hydro Generator - Pick apart the design
(OP)
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
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





RE: 250 KVA Hydro Generator - Pick apart the design
RE: 250 KVA Hydro Generator - Pick apart the design
RE: 250 KVA Hydro Generator - Pick apart the design
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 250 KVA Hydro Generator - Pick apart the design
Why would you build an ungrounded system like this, and what would be the preferred way to establish a ground on the 480V side?
Waross, I'm not sure I fully understand your description. Are you saying that the 12KV winding faulted, and this raised the 480V side in relation to ground? Can you elaborate on common mode voltage?
It sounds like we will have to replace or rebuild the transformer. The unit is 30+ years old and I don't believe it has ever had any oil analysis or testing done to it.
Keep the comments coming, I am curious to hear more.
Thanks,
Mike
RE: 250 KVA Hydro Generator - Pick apart the design
Mike:
Sometimes problems are being placed in this forum with little or no relevant information, making it difficult if not impossible to deal with. Your thread is a shining example how a particular problem should be described. This star is well deserved.
Wolf
www.hydropower-consult.com
RE: 250 KVA Hydro Generator - Pick apart the design
Was the lightning arrestor connected phase to ground in the conventional way or phase to phase? If it was phase to ground then there had to be another ground somewhere on the 480V side to have had it burn up. It can be that the transformer had a ground fault on the 480 side, inasmuch as you said that upon opening the contactor and disconnecting the generator, it continued to burn.
As davidbeach said, the transformer is likely faulted and has to be replaced. The primary fuses on the pole were likely oversized, catering only for a dead short in the cable or transformer and not as any sort of conventional overcurrent protection.
Now you should megger the transformer primary and secondary after disconnection to verify that it is faulkted or ground faulted.
On the generator: is there a neutral nad if so is it connected to anything?
rasevskii
RE: 250 KVA Hydro Generator - Pick apart the design
I'm not sure how the 480V lightning arrestor was connected, as all that remains of it is a pile of debris and a scorch mark on the concrete wall. I will ask the owner to see if he remembers. I'm not quite sure why it was even installed, and putting it on the line side of the main disconnect, without fuses, was certainly the wrong way to do it.
I'm sure the fuses are for cable protection only, as they are provided by and coordinated with the utility. Ideally this setup should have fuses at the high side of the transformer as an underground cable fault would just burn up if the generator was online. There is some sort of ground fault protection on the 12KV side of the transformer but I'm not sure if it is appropriate for this setup.
The generator has no ground, as it's essentially a 3 phase induction motor being run by the hydro turbine via gearbox. So we have a delta connected motor feeding into the delta side of a transformer. This seems wrong to me but I don't have a good reason!
Wolf- I enjoyed looking at your website! You are dealing with some huge facilities, makes my project look like a toy.
Thank you for all of the help, I will update this thread if we find anything else
RE: 250 KVA Hydro Generator - Pick apart the design
In this case an undervoltage and/or overspeed trip has to open the 480V contactor, and shut down the turbine (close the wicket gates)
allowing the unit to come to a safe stop.
There should also be a loss of phase protection included in case only one pole-top fuse were to fail.
Let us hope this is implemented in the control scheme.
rasevskii
RE: 250 KVA Hydro Generator - Pick apart the design
Mike:
I don't know whether this important for your problem, but large synchronous hydro generators usually are star connected, feeding into delta winding transformer windings.
Wolf
www.hydropower-consult.com
RE: 250 KVA Hydro Generator - Pick apart the design
that is what I thought. I am very much comfortable with a star arrangement. This generator has only 3 leads so we are stuck with what we have. It seems like there should be some sort of grounding scheme on the 480V side, perhaps a zig-zag transformer.
Rasevski, I believe there is phase loss and undervoltage protection. The control cabinet has seen a lot of modifications over the years, but it looks like the required relays are all still working. I had not thought about losing excitation voltage from the grid. Luckily the whole thing fails closed. There are a pair of NO solenoid valves that open on power loss, moving a hydraulic cylinder which diverts the water to the outlet.
Thank you for all the insight into this setup, this site is a great resource.
Mike
RE: 250 KVA Hydro Generator - Pick apart the design
If you want to ground the 480 volt system consider a wye:delta transformer bank rather than a zig-zag.
Decide on the value of your NGR and the resulting ground current.
For a ground current of 5 amps, select a transformer that will carry 5 amps in the primary winding. That is 5 Amps times 480 volts or 2400 VA. Use three 480 Volt dry type transformers rated at over 2.4 KVA.
The secondary voltage is not important.
Connect the 480 Volt windings in wye and connect the wye point to ground through your neutral grounding resistor. Connect the secondaries in delta. The delta winding will stabilize the wye point of the primary. This is a conservative design as the transformers see only 277 Volts on the 480 Volt windings. The current in the delta is proportional to the current in the NGR.
This is an off the shelf solution rather than a special design and build zig-zag transformer.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 250 KVA Hydro Generator - Pick apart the design
I will check for connection between primary/secondary with a megger. We meggered everything at 1000V (obviously not that helpful for the 12 KV side, but a good sanity check on the 480 side)., but I did not check from X1--H1, etc.
That is an interesting solution to grounding the system, I like it. So the secondary winding are connected in Delta only, with no other connections to them? Interesting
Will have to float this by the owner once we get things up again. Have to find an affordable 250kva transformer first...
RE: 250 KVA Hydro Generator - Pick apart the design
I sugest you stay with the delta on the generator side of the GSU transformer. The delta winding often does a better job of distributing single phase loads and unbalanced loads on the generator but this may be moot if you have a delta generator winding.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 250 KVA Hydro Generator - Pick apart the design
1. HV to Ground IR
2. LV to Ground IR
3. HV to LV IR
4. Surge comparison test of all 3 HV phases
5. Surge comparison test of all 3 LV phases
Muthu
www.edison.co.in
RE: 250 KVA Hydro Generator - Pick apart the design
I think more standard tests for transformer would include Transformer turns ratio test which can be fairly sensitive indicator of shorted turn.
=====================================
(2B)+(2B)' ?
RE: 250 KVA Hydro Generator - Pick apart the design
We are trying to get the local utility to close the cutouts with the transformer disconnected from the 12KV line to make sure we don't have a bad cable on top of this. With a 1000V DC megger, one phase measured 3.3 G-Ohm to ground, while the other 2 were over 11 G-Ohm (the range of my meter).
Wish I had access to more test instruments on this one.
RE: 250 KVA Hydro Generator - Pick apart the design
#1 I would recommend renting a small single phase unit. It can be performed without a dedicated test set, however if you are considering replace/repair of the xfmr, better to have dedicated equipment.
#2 - Completely isolate transformer from system. Apply voltage (10,40 or 100V) from H1-H0 to corresponding phase on LV side. (look for the parallel lines (H1-H0 / X1-X2) for secondary connection. The connections can be easily determined from the vector diagram on the transformer nameplate. The calculated ratio will be (12470/1.732)/480, assuming you are in the middle tap position. Repeat for the other 2 phases. Your measured ratio vs. your calculated ratio should match within 0.5%. A decent TTR should also measure exciting current. Compare phases. You should see a pattern of 2 highs and one low.
Another test to consider is a low voltage backfeed of the unit, using a variac (0-150V). Again follow all safety precautions. Compare currents (connect ammeter on the neutral (grounded) side of the variac output). You will be backfeeding the transformer, at 1/4 normal voltage (120v output). I have used this technique when there is a suspected transformer is involved and the budget is tight. If the variac trips, or shows excessively high current this is a very good indication of a failed transformer.
Good luck.
RE: 250 KVA Hydro Generator - Pick apart the design
I routinely do surge comparison tests on test trafos and have not seen magnetic flux playing any role in the results. I have attached two reports One for HV tested at 12 KV & one for LV tested at 1 KV) on a 11 KV/415 V, 1.6 MVA recently rewound by us. You can see no difference in the waveforms in all 3 phases in both HV & LV. The magnetic flux plays no role in surge testing since the current taken is so low that the flux is nowhere (since amp-turns is so low) near the rated levels.
mhulbert
Your IR readings are confusing
"With a 1000V DC megger, one phase measured 3.3 G-Ohm to ground, while the other 2 were over 11 G-Ohm (the range of my meter"
If these readings are about the same winding (HV), then you can't have different readings for each phase of the same windings (unless you separated the starts and ends of each phase or the each of these windings got cut inside, in which case you will read an insulation resistance value between the phases)
Muthu
www.edison.co.in
RE: 250 KVA Hydro Generator - Pick apart the design
Here is HV winding surge test photos.
Muthu
www.edison.co.in
RE: 250 KVA Hydro Generator - Pick apart the design
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
RE: 250 KVA Hydro Generator - Pick apart the design
I will say surge test does not appear to be a "standard" test for transformers as it is for motors based on the following:
- EASA AR100 for motor repair discusses surge testing, but EASA AR200 for transformer repair does not mention surge testing.
- 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).
- 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.
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?
=====================================
(2B)+(2B)' ?
RE: 250 KVA Hydro Generator - Pick apart the design
RE: 250 KVA Hydro Generator - Pick apart the design
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
www.edison.co.in
RE: 250 KVA Hydro Generator - Pick apart the design
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.
=====================================
(2B)+(2B)' ?
RE: 250 KVA Hydro Generator - Pick apart the design
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.