Regulating Transformer Tap Changer Compartment Oil Temps Getting High 1

[mudbone]

I have a McGraw Edison Regulating Transformer. Main tank, Selector Tank, Interrupter Tank. Main tank oil temp 45C, Selector Tank up to 75C, Interrupter Tank 55C. During summer months, Selector tank will start trending upward past 80C alarm point, cut load on unit temp. begins to fall. Have Done internal inspection, cleaned all contacts on tap changer to no avail. Does anyone have any suggestions as to what could be the cause. Am considering installilng pump and cooling exchanger on Selector compartment.

 

[hacksaw]

check your sensor, transducer, and wiring

 

[mudbone]

Hello Hacksaw,
Thanks for the reply. I just wanted to let you know that we haved checked and rechecked the monitoring devices. We have digital and analog type equipment and the temps. agree. We have even done an infrared on the transformer tank and this proves as well the heat being generated.

 

[RalphChristie]

Mudbone,

If you have verified the temp-devices are correct:
What is the minimum and maximum temperatures? Is the selector-temperature just high during the daytime or also during the night? The reason I ask this is because I have seen some of our transformers doing the same. It is always the transformers standing directly in the sun for the greatest deal of the day. Many times between 70°C and 80°C. Day temperatures are many times in the 40s.(summer-months in the Kalahari, South Africa)
I assume the selector tank is the tap-changer compartment. Is it working hard - tapping a lot? That could also be a possible reason for high temperatures. What is an interupter tank?

Also, I do not know how do your probes look like, but if it is the type that you have to insert into a pocket, make sure the pocket is full of oil.

Regards
Ralph

 

[hacksaw]

the bottom line is that if these are justifiably real temperatures and you are not overloading the equipment, then you need to remove the heat

 

[mudbone]

Hi Ralph, In all honesty, I am not too concerned about minimum temperatures. I get concerned when the transformer is at full load and the temp. hits 80deg C and appears to be headed higher according to the trend. One thing to note, is if we tap the unit back, even during the dead of summer we can turn the temp. downwards. I agree with your comments during the summer the temps. can get to 70/80 fairly easy. But as we trend the temp. it appears as though it is not going to line out. As for the probes, they are all in thermowells, they are not actually inserted in the oil. Fortunately, the unit does not tap a real lot, for the most part, the load is steady.

Thanks Mudbone

 

[RalphChristie]

Mudbone:

Actually what I wanted to see was the temperature trend (min and max temperatures) to determine if the problem is external (sun) or internal.

Regarding the thermowells - I think the probe still have to be inserted into oil (thus there have to be oil inside the thermowell too) This is to ensure the oil inside the transformer and inside the well is at the same temperature level.

But, if I read through your post, it seems as if the fault is inside the tap-changer compartment: "we can turn the temp. downwards" This indicates there are high currents flowing on certain taps. The only things I can think of are loosened connections or arcing during certain taps. Or, if you have more than one transformer in parallel, circulating currents. (Due to uneven tap-positions between the two transformers)

Just my thoughts.

Regards
Ralph

 

[RalphChristie]

Another thought regarding circulating currents:

What is your transformer's vector-group and where is the tapchanger situated? You can have a small turn-to-turn fault (in the tapchanger, if situated on a delta or star connection) or a winding-to-earth fault,(if tapchanger is situated near the star-point, just for star connections) If there is one of these faults, it will be in the tapchanging-windings. (if you tap it back, the temp turn backwards)

Turn-to-turn faults:
Differential protection will not always see turn-to-turn faults. (depending on the size of the fault)
High circulating current in the faulty windings

Winding-to-earth fault near the star-point:
Currents from a phase viewpoint small, but big from a neutral viewpoint.
Can eliminate it with REF-protection (if applied on the trsf) but there will be a certain percentage of the winding (towards the neutral point) not protected.

Regards
Ralph

 

[stevenal]

Isn't the regulating winding in the main tank? How about the preventive autotransformer? Main or LTC tank?