Transformer secondary bushing overheating 2

[H1H2N3F4]

Good morning all.
We have a double ended switchgear that is rated for 3000A. This switchgear is supplying power to four ( 4) Gas compressors ( 20,500 hp each) and are continuously running. In addition, this switchgear is being supplied by two 60/80MVA, 69kv/13.8kv Bruch type transformers. My problem here is that I usually notice a sign of burn on the bus insulation (boots) covering the flexible joint between the secondary bushings and the bus bars. Moreover, those bushing are always leaking and I think that this is because of this overheating. We did check the torque on the bolts connecting those jumpers but found them ok. I need you valuable help in this matter in the whole set up and what cause this overheating.


MMQ

 

[Marmite]

Without seeing the setup it's difficult to be specific. I believe you are talking about the transformer secondary bushing. If the connection you checked the torque on looked OK then it might be that heat is being conducted along the stud from a problem on the other side of the bushing. If there's enough heat to burn the bushing shroud you should be treating this as a matter of urgency. It has the potential to lead to catastrphic failure.
Regards
Marmite

 

[Gianoli]

I would try to have a infrared camera scan done ASAP. The results of the scan should give you some idea of the condition of the connections.

Bushing leaks could be the result of a number of problems not just overheating. Start with the IR scan and proceed based on the results you receive.

 

[magoo2]

With all four compressors running, you have 82 MW of load at 13.8 kV. This gives you 3431 amps at 1.0 power factor. You are already overloading the switchgear since it's only rated for 3000 amps.

Chances are, the power factor is lower. like 0.8 or so. This just makes things worse.

 

[ScottyUK]

magoo,

4 x 20,500HP [≠] 82MW

The four motors together are more like 62MW or so. The gear is likely heavily loaded but probably within rating even assuming a pf of 0.8 at full load.


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If we learn from our mistakes I'm getting a great education!

 

[magoo2]

You are correct, Scotty. I usually equate HP and kVA as a first cut.

In truth, it (kVA) should be equal to

HP * 0.746 / (eff * pf)

If eff is 90% or so and power factor is 80% or so, the product takes care of the 0.746 term.

So, I should have said that the MVA is 82. With that said, the current at 13.8 kV still indicates an overloaded condition for 3000 amp switchgear.

 

[H1H2N3F4]

Thank you all for you valuable replies.
To answer Gianoli, the use of the infrared camera will not give us any clear indication since the secondary side of the transformers encased in a steel box. But I think as was said with the rest of the group ( magoo2, ScottyUK & Marmite), It is an overload problem that should be discussed with our engineering and operation. I will comp back to you when I get a clear answer. For information, we did not notice any upnormality on the switchgear or the bus bars, but the problem is very clear on the transformer. However, from the manufacturer specification, the rated current of the transformer is as follows: OA: 2510.2 Amp. & FA: 3347 Amp.



MMQ

 

[Gianoli]

“To answer Gianoli, the use of the infrared camera will not give us any clear indication since the secondary side of the transformers encased in a steel box. “


Don’t give up on the IR camera idea yet. It sounds like the transition from the transformer to the switchgear in your case may utilize buss duct. If this is the case it may be possible to remove the buss duct covers temporarily to do the IR scan. The IR scan really helps pin point the problem areas when it comes to hot connections, especially in the hands of an experienced operator.

However, to safely remove the covers on medium voltage buss duct to do an IR scan requires the transformer and buss duct to be de-energized and safety grounds to be installed prior to removing the covers. Energizing the transformer and buss duct with the covers removed will require that special safety precautions be put into place. Do not try this unless the all the personnel involved are intimately familiar with potential hazards and safe work practices required.

If the IR scan is not possible another possible diagnostic tool for this condition is the Ductor. The Ductor is a micro-ohmmeter, which in the hands of a qualified person can be a valuable tool in finding high resistance connections (again with the transformer and buss duct in a de-energized and grounded condition).

I am not sure why you would be in an overload condition. My calculation looks like this:
20,500HP X 4 = 82,000 HP.
82,000 HP X 746 (Watts per HP) = 61,172,000 Watts
61,172,000/13,200/1.723= 2676 Amp
2676/.93 (motor efficiency) = 2877 Amp

I am assuming these are likely synchronous motors operating at 1.0 PF. 93% efficiency or higher is not unusual for this type of motor. Unless the motors are loaded into the service factor or not running at PF of 1.0 you should not be overloaded. Even if you are slightly overloaded for some reason the transformer secondary bushings o buss connections should not be getting hot enough to burn the vinyl boots. You likely have a poor connection(s).

 

[ScottyUK]

It's a fair approximation magoo, I had half guessed what you meant but thought it was worth clarifying.

It's bad luck (or optimistic design) if those motors are fully loaded to their nameplate rating.

Have you any DGA results from this transformer? If there is a hot joint on the oil side of the bushing it would show up on the air side because of the conductivity of copper. There should be some signature gases in the DGA results if this is the case.


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If we learn from our mistakes I'm getting a great education!

 

[ELEP]

If possible check the capacitance & tan(delta) of bushing, and compared with origional value. As this bushing is 13.8kV bushing, there might not be measuring point but you can measure capacitance at least.

 

[H1H2N3F4]

Utilizing the Infrared camera with transformer energized and coveres off is a risky task at least for us with limitation on our maintenance staff.

To clarify, I meant by overload, not on the transformer itself but on this type of bushings. As I said previously, we have two transformers; each is supplying power to two compressors. In some occasion like during maintenance of the transformer or its bus bars, we need to have all four compressors fed from one transformer.

For information we have four transformer of this type and all have the same problem. I don't think that we have loose connection on all of them.


MMQ

 

[Gianoli]

I would have to assume the transformer manufacturer installed secondary bushings that have a continuous current rating equal to or higher than the highest secondary current the transformer is rated to deliver. If this assumption is not valid I guess you need to replace the bushings.

If the bushings are correctly sized then you may want to confirm that the flexible jumpers between the bushings and the buss duct buss are also properly rated. Just because the connections are tight does not guarantee a good electric connection. The ductor I mentioned in an earlier post is a good instrument to use to check the resistance of the connections. Compare the resistance of the connections that are heating to those that are not. I suspect you will find a significant difference in resistance between the hot connections and the cool connections.

 

[H1H2N3F4]

Gianoli; as you said the bushings or the flexible jumper between the bushing and the bus bars could be under sized. But, what is interesting is that the heat is observed on the side of the bushings and not on the bus bar side.

I will check the specification of the bushing; hopefully I have that, and will do some oil analysis and some resistance test. In addition I want to check the ratings of the jumpers, but I wonder how can I do this? Any suggestion?


MMQ

 

[apowerengr]

Are the "boots" rated for this application given the thermal rise of the flexible conductors and/or bushings at near full load over ambient conditions? With the "steel box" you probably have no air cooling which adds to the issue.

 

[Ceast]

As Scotty eluded to a poor internal connection between the bushing and winding termination will cause heating (acetylene?) which should show up in the DGA report. Also for ease of thermography you need to get some IR windows installed in the transition section.

EastB

 

[Gianoli]

“In addition I want to check the ratings of the jumpers, but I wonder how can I do this? Any suggestion?”


Not really. They probably don’t have a name plate or even a tag with a part number. If this problem is common to all your transformers I would suggest need to review the design with the original equipment manufacturer. If that is not possible you are going to have to solve the problem yourself. That will involve determining with reasonable certainty were the heat is being generated. This probably means the IR scan as I mentioned in a previous post. You may want to bring in a company with some expertise in the field to assist you with the scan. Good Luck!

 

[H1H2N3F4]

Thank you all for your valuable comments and analysis.

We have formulated a team to investigate this problem. We are intending to involve the manufacturer and designer in this task and will get back to you when we come to a conclusion.

I am still looking for your valuable input in this if you have some thing new.

Thanks again



MMQ