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Participant

Join Date: Dec 2009
Posts: 4

Transformer Temperature Problem

12/08/2009 9:54 AM

Dear All,

lately I have been having some problems regarding one of the three transformers of the factory in which I am working. Two of these transformers are twins:

HV: 20+2x2,5% kV, 46.19A, LV: 400V, 2309A, Y

ins.Cl 24kV, Δ, V Group Dyn 11, impedance %6, cast resin, 1600kVA

The fist one (TR1) is loaded almost all year with 1500-1600A and has a temperature of 45~50 oC during the winter and 55~65 oC during the summer. The second (TR2) is loaded during the summer with 1500~1600A and during the winter 700~800A, its temperature is 75~95 oC during the summer and 45~50 oC during the winter.

I have been monitoring the voltage on the secondary and have found it to reach 415~420V for small periods of time during the night. Some times, also during the night, the transformer gets very noisy. With a power analyser (siemens sentron PAC3200) I have been measuring the PF=0.99 and the THD I=8~10%. A couple of weeks ago a capacitor bank (150kVar) connected on the TR2's secondary exploded.

In my opinion since the transformers are identical they should have the same temperature under the same load. Taking into account the rest of the problems what do you think I should do?

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Power-User

Join Date: Jan 2009
Posts: 187
Good Answers: 9
#1

Re: Transformer Temperature Problem

12/08/2009 11:19 AM

Check for harmonics from the load. The most probable culprit would be a variable frequency motor drive, although a large switch-mode power supply or lighting load with electronic ballasts is a possibility. Harmonics can cause all sorts of mischief, including over-voltages by constructive interference and overheating of the transformer, wiring and other loads on the transformer. (Any effects on the capacitor bank would depend on its high frequency characteristics.) Maybe the harmonic currents also caused vibration in the transformer.

Is there some means to test the mechanical integrity of the transformer windings? If the transformer has been making noise, this may be an issue. Perhaps the voltage spikes are the result of insulation breakdown between windings on the primary (decreases the turns ratio), possibly caused by the coating being rubbed off of the vibrating wire. This should be detectable by electrical testing. The capacitor explosion could have been caused by an extended short or arc inside the primary, resulting in a spike much larger than usual. It would be a good idea to make sure the primary winding still meets specifications.

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Participant

Join Date: Dec 2009
Posts: 4
#2
In reply to #1

Re: Transformer Temperature Problem

12/08/2009 12:02 PM

Last time I checked the primary was last August and it was within specs. Keep in mind that the temperature problem first bacame evident during June, while the capacitor exploded recently. What troubles me the most is that the other transformer has a much larger load with many more AC motor drives, without presenting any alarming temperature issues, despite the fact that the same maximum voltage (415~420V) is measured during the night.

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Power-User

Join Date: Jan 2009
Posts: 187
Good Answers: 9
#3
In reply to #2

Re: Transformer Temperature Problem

12/08/2009 4:16 PM

If both transformers are producing an increased voltage at the same time, then that is likely caused by an increase in primary voltage. This would be consistent with a reduced night-time load on the power company grid. The capacitor explosion could be the result of gradual degradation culminating in catastrophic failure. What is the temperature behavior of the capacitor banks?

I'd advise checking the power quality. The THD reading is quite high, so that is a warning that either the transformer core is saturating (not likely with the load given) or you have a load generating harmonics. All it takes is one bad harmonic filter on a big motor to cause major headaches. I once talked to the maintenance tech at a bowling alley, who told me they had problems with erratic pin setter operation that were traced to harmonics on the power line generated at a foundry a mile away across the river!

If you find harmonics, you may be able to identify the source by correlating changes in harmonic content with varying motor speeds. Do you have the same distortion without the summer loads?.

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Participant

Join Date: Dec 2009
Posts: 4
#4
In reply to #3

Re: Transformer Temperature Problem

12/09/2009 1:55 AM

The summer loads are basically two large cooling units 700A total, without AC drives, and yes the distortion is the same.

I cannot check the temperature behaviour of the capacitor banks because they are in an enclosure right next to the transformer.

I am thinking of removing all capacitors connected directly to the transformers secondary and only use the ones connected on the low voltage field which I can disconnect without the need to disconnect the transformer, given that my PF remains high. As for the harmonics I have contacted a local company specializing in power quality measurements to assist on the matter.

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Anonymous Poster
#5

Re: Transformer Temperature Problem

12/09/2009 4:17 AM

Temperature rise may be caused by several reasons...

But before you do anything check that the temperature indicator is correct.... (nothing like trying to find a non existent fault)..

Check also that the cooling system is functioning... or that the airways are not blocked.

2) If the TX is not overloaded check for harmonics in your circuit.

or

3) you could have an internal fault in the TX...

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Participant

Join Date: Dec 2009
Posts: 4
#6
In reply to #5

Re: Transformer Temperature Problem

12/09/2009 10:00 AM

The cooling system is 6 fans 1800m3/h total supply located at the bottom of the transformer. One of them was broken and replaced without any noticeable effect on the temperature. A possible fault of the temperature sensor has also crossed my mind. But the transformer has three (one in each winding) and at high temperatures all three have the same indication

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