Reason for transformer failure?

[steelerfan28655]

This week we had an interesting situation here that still has some of us scratching our head. We have an industrial customer that is supplied by two parallelled pad mounted transformers. The primary voltage is 12,470/7200 GND-Y. the secondary is 480/277 GND-Y. Both transformers were installed four years ago as brand new 1500 kVA units, with identical impedances of 5.14. The secondaries were connected to a common bus by five, 500 MCM copper conductors per phase. The primary was fed thru transformer one to transformer two.

The secondary of transformer two reached a temperature great enough to ignite the oil that had leaked into the secondary compartment. we do not know if the temperature caused the oil leak, or if the lower level of oil caused the extreme temperature. The transformer has been sent for testing and evaluation.

The only difference between the two setups that we can find is the length of the secondary conductors. The conductors of transformer two are about 2' shorter than the conductors of transformer one. The total length of the transformer two secondaries is about 15'.

Can this be the cause of the temperature increase in transformer two?

We have had to install an emergency replacement transformer with an impedance of 5.77. Amp readings on the secondaries of both transformers differ by about 2%. Full load measured on either secondary max out at 1450 amps, so neither transformer is overloaded. The customer only operates one shift each day.

Any ideas? Are we just waiting for the replacement to fail?


Happiness is a way of travel, not a destination.

 

[rbulsara]

Cable lengths could have nothing to do with this failure. There could be many reasons for oil leak or temp rise, loose connections, overload, lack of cooling, blocked cooling fins/pipes, restricted air flow, high ambient, internal fault, etc.

Oil test should also provide a clue. Wait until results of the test.

By the way, wasn't the oil temp monitored? If not make sure you do.

Rafiq Bulsara

http://www.srengineersct.com

 

[Zogzog]

I assume you pulled a DGA sample, those results should lead you to the cause or at least type of failure.

Now having 2 transformers with different impedences feeding a common bus? That could be an issue.

 

[electricpete]

Load of parallel transformers shares in proportion to impedance, but the difference in impedance caused by 2' of large cable would be neglgible compared to the transformer impedance. So I agree with Rafiq is seems very unlikely that the length had any effect.

As Rafiq also mentioned, if this machine monitored by temperature alarms, we would expect alarm long before any kind of failure if the failure was caused by impedance-induced overload (or other types of overload for that matter).

=====================================
(2B)+(2B)' ?

 

[ScottyUK]

Are the transformers commoned at the HV side, or are they fed from two distinct sources? Bad things can happen if you unwittingly link two sections of the HV system via transformers paralleled at an LV board.

If fitted, were the tap selectors in matching positions?


----------------------------------

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

 

[steelerfan28655]

Both transformers were fed from a common source. Actually, transformer two is fed from transformer one.

Tap selectors were the same. These transformers had operated with no symptoms for four years.

When i arrived at the scene of the smoldering transformer, it was still energized, with the plant equipment in operation. Indications were that there were no major electrical faults inside the transformer.

The replacement transformer has an impedance of 5.77% compared to the existing transformer one that has an impedance of 5.44%. Transformer two is now running almost 20 degrees F hotter than transformer one. Fans have been added to transformer two. But i am wondering if we are just treating the symptom and not removing the cause.

DGA will be performed on the old transformer at the test lab next week.

Oh and the company electricians have replaced the secondary conductors and it looks like they went against my recomendations and actually installed shorter wire. They are primary metered so the secondary conductors are their responsibility. But as the utility we are getting ready to force a reconfiguration of their electric service. They have added load to different services without giving notice.

Happiness is a way of travel, not a destination.

 

[Ceast]

Did anyone record the max oil temp pointer? It should have been on the sample form. What is the replace xfmr temp running?

What does the load PQ signature look like? Harm heating? Just throwing it out there.

 

[waross]

I would be looking at oil leaks and poor connections.
I had a 100 KVA transformer destroyed by a poor connection.
An unskilled crew added an aluminum conductor to the split bolt type connector on the transformer terminal. The poor connection developed enough heat to heat the oil to the point that it started to coke out just before the transformer failed completely. The heat also cooked a bushing seal and started a small oil leak.

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

 

[PHovnanian]

Sometimes transformers fail just because it's their time. Nothing to do with the parallel arrangement.

If the load sharing between the two was OK up to the failure, it was due to a problem within the failed unit. If the load sharing with the replacement unit is within 2%, I wouldn't concern myself the parallel arrangement. Just keep an eye on the load split (particularly after maintenance or primary switching operations).

 

[Bronzeado]

Hi folks!

Back to the forum after a long "silence". A happy 2011 for all!

My 2 cents in this discussion....

Transformers Y-Y connected may heat it up if the load fed by them is unbalanced, mainly if they are three-legged core (is it the case?)

The load imbalance produces a zero sequence flux (induction) that leaks the core and goes throught the tank. This magnetic flux (induction) induces eddy currents in the tank, heating it and therefore heats the oil up.

One recommendation is to use a Y-delta connected transformer in this configuration.

Mind that load imbalance may be caused by poor connections.

Best regards,

Herivelto S. Bronzeado
Chesf, Recife, Brazil

 

[steelerfan28655]

Thank you everyone for your replies. We have had some major snow here in the South East United States so I have been away from the desk for a few days. Before I left though, we actually found that the new connections in the transformer were not tightened properly. We made the customer shut down to verify the connections in the outdoor switchgear. They were also loose.

The conductors that were removed after the transformer failure showed extreme heating on the transformer end. There was about two feet of damaged insulation on that end. The remainder of the wire looked fine. I am leaning towards either an oil leak in the transformer caused the whole thing, or that the secondary connections were loose.

We have installed individual CT's on the pair of transformers. We have found that the power factor dips into the 60's for a few seconds during the startup of their compressors.

We have recommended the installation of capacitors downstream from the transformers to solve the power quality problem.

Thank Y'all again for the advice and suggestions I still have a lot of work to do before I can pin this on just one thing, or a combination of problems.

Happiness is a way of travel, not a destination.

 

[waross]

A power factor dip is normal when starting large motors. Trying to correct it may be quite onerous.
A meaningful correction may take many times more capacity than normal motor running correction.
As an example of the amount of capacity required to correct the motor starting power factor, this has been proposed as a method of reducing starting current. The amount of capacity needed to correct the power factor of a large motor starting is so great that it may not be safely left on the line after the motor starts. If capacitors are used as a starting aid for large motors, they must be switched out in steps as the motor accelerates to avoid system over voltage issues. I have seen a paper on this starting method complete with calculations but I have never seen it applied in practice nor have I ever heard of it being used.
My suggestions:
1> If this is not causing issues for any other customers, let them live with it, under the condition that this is a temporary permission, subject to change should the motor starting affect other customers in the future.
2> If the motor staring is causing issues with other customers, require your customer to limit the starting current of the compressors with a soft start, a VFD or other suitable means.
3> I have heard of some extremely large booster compressors on natural gas transmission lines that are only allowed to be started between the hours of 2 AM to 4 AM. These compressors normally run 24/7 but if they do go down, they may not be restarted until the next early morning.


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

 

[PHovnanian]

What Bill (waross) said.

The transformer failure appears to be due to overheating/overload for some unknown reason. This tends to be due to some steady state rather than transient condition. The CTs on each transformer will be useful to identify potential imbalance or unit overload conditions.

There are several possible causes of the onset of an unbalance condition. One transformer could blow a primary fuse on one phase and, due to the parallel feed, this could go unnoticed for some time. Its also possible that one primary phase feeding one transformer was not properly closed following some switching operation. I've heard of load break elbows not being properly plugged in. Monitoring the loads, or better yet adding some alarms to the units ammeters will go a long way toward identifying imbalance conditions.

 

[waross]

Given your experience with multiple loose connections, I would go with loose connections as the root cause of the damages. The presence of a parallel transformer would mask the condition for quite awhile.
A loose connection may work for a long time but the eventual failure mode is heat corrosion inside the connection.
The load cycling due to the one shift per day operation is a further factor. Eventually the heat corrosion becomes severe enough to develop significant heat. From that point on, the deterioration will be quite rapid.


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

 

[Ceast]

Did you have a quality NETA company commission your transformers? I have seen a few poor internal terminations cause heating of the bushing and subsequent oil seepage. Also some Noload LTC's with poor terminations cause heating as well. That typically shows up during the on-site commission testing.

 

[electricuwe]

Although it might not be important to understand the reason of this fault, I'd like to ask for some background information.

Why this arrangement has been chosen instead of installing one 3000 kVA unit ?

 

[steelerfan28655]

We are a small utility with limited resources. The largest standard transformer size that we hav on our system is a 1500 kVA. We limit the size because we do not have the equipment needed to lift any larger transformers in an emergency situation.

We are monitoring and collecting load data from the installation. I will post a reply if we find anything out of the ordinary.

Thanks to all for your input !

Happiness is a way of travel, not a destination.