Need HELP understanding calibration procedure of Power Transformer Winding Temperature Indicator 1

[billcasper5968]

Our procedure to calibrate our WTI (AKM Qualitrol) is to subject it to a constant temperature say 30 deg C for 10 minutes. After which, we inject 1A for 1 hour and our indication should read 50 deg C. This means that for 1A injection the increase should be 20deg C (based on a given graph). Since we are new personnel assigned to the plant, we don’t know the logic of this procedure. This was just an instruction handed down to us.

My concern at this point is the actual reading of our WTI during operation is above 100 deg C. If the WTI is reading correctly since it was calibrated per the instruction we might have a problem with our 47MVA power transformers. However, I asked the old personnel at the plant and they informed me that it had been above 100 deg C for a long time now (years). From reading other posts, it would mean that our transformers should have degraded considerably at this condition (100 deg C winding temp). However, this is not manifested on the electrical test and DGA so I am concerned that it is our calibration procedure that has an error. I want to know your views on this.

 

[DTR2011]

is there a liquid temperature gauge to correlate to?

 

[billcasper5968]

Hi DTR2011... Yes, the 30 deg C sensor connection that I mentioned is connected to the top oil temperature sensor during the actual installation. This means that after compensating for the ambient, the total reading would be the top oil temp plus the contribution from the current (via CT) of the transformer.

 

[collies99]

You can only test the calibration for the bulb type temperature indicator itself. This could be done through a lab type dry-well calibrator or using reference temperatures as in an ice-water bath at 0 deg C at/near sea and boiling water at 100 deg C both at sea level elevation.

The calibration check of the heating well is not done in the field since thermodynamic interactions between the oil, static oil vs circulating oil medium, ambient temperature, oil temperature and others cannot be controlled or unknown; at least I have never seen it done. Doing current injection on the heating well is simply a functional test to ensure that the indicator read more than the oil temperature by the added heating effect of current flowing in the heater, that is common practice.

Therefore i question the validity of 1 amp in 1 hour gives accurate added temp of say +20 deg C.

if your well CT have taps and your cooling system is in good working order, valves/rads/fans/ambient temp are ok but at what level and loading within nameplate rating both oil temp and winding temp indicator should be within range and not in alarm stage. If oil ok and winding temp indicator shows very hot as per your comments and loading ok, perhaps CT tap needs to be changed.

is it possible that the well heater is the wrong ones installed ? And is hotter than it should be ? Are there various rating fir the well heater and is the correct ones installed ?

sorry no help but may have you questikns other things.

 

[collies99]

Question: What us the make and model ? Or is there a CT currene rating or a cakibration resistor orovided ?

 

[collies99]

What is the nameplate temperatute rise and max ambient temp rating ?

 

[prc]

Please understand that WTI of a transformer is a slave that will show what is programmed it for and not a direct reading one. So whether the reading is actually showing the correct value or not- you have to check with transformer factory test report and instrument(WTI) manual. In case WTI is showing the correct hot spot, a reading of 100C is nothing to worry about.

AKM WTI consists of a sensing bulb at the top cover which will take the actual top oil temperature. The winding gradient( the amount by which winding hot spot is above top oil) is added to above top oil temperature by a heating coil inside the instrument casing. Heating coil is fed by a bushing CT ( usually 1.5 A at full load) in any of the winding. Transformer manufacturer will adjust the heating current in coil by an adjustable parallel resistance, based on the winding gradient obtained at the time of temperature rise test in factory to get gradient as per the curve mentioned by you.

 

[crshears]

Ah! In my utility we call that a "winding temperature simulator."

How to properly evaluate and calibrate one is way beyond me, though...

CR

 

[billcasper5968]

To Collies99

Our WTI is AKM Qualitrol, WTI 35 with matching resistance TD50/76

Yes there is a CT and the ratio is 1135/2A. Yes there is a calibration resistor which we adjust for the 1A to 20C during our calibration.

These are the data that could be found on our manual;
1. Winding temp rise by resistance 55C
2. Rated Output 47MVA @ 55C
3. Cooling ONAN
4. Ambient Temp 0-40C

I have a couple of questions (correct me if my understanding is wrong);
To Collies99: Are you saying that we should have not calibrated it on site? And we only have to do a functional check?
To Prc: We are using a gradient table of the WTI in which we got the 1A=to temp rise of 20C. Is this wrong?

Thanks to all

 

[prc]

Normally only functional check is necessary at site.

You can see from the graph that at 2A current, the gradient may be 35C or so. So at 1A current,20C seems OK. By calibrating it you are checking that the gradient addition is OK.

 

[mgtrp]

To set the calibration resistor, you need to know what the winding temperature gradient is for the transformer. This is usually calculated by measuring the winding resistance of the transformer at ambient temperature, and then again immediately after a heat run, which is compared to the average oil temperature rise - usually by the manufacturer during factory testing, and then never again.

If you have this information, then the AKM manual should provide some guidance with regards to setting the resistor to achieve the required gradient.

Typically, the only checks done in the field are to check that the bulb is measuring correctly and that the alarm/trip contacts close and open at the right spots.

 

[DiscoP]

If I understand the question, you are concerned that the value the WTI is giving you is too high.

I think the two most likely reasons are

1. The temperature measurement is out
2. The calibrating resistor in parallel with the heating element is not set correctly (or maybe even open circuited)

You can do a rough check of the temperature measurement, by placing the probe in a temperature calibrator, or if you don't have one use boiling water (but take care !)

It seems like you know that you want a 20 deg rise, and this will happen when 1 A flows through the element.

So inject full load current as a secondary value to replicate CT current and use a clamp on ammeter to confirm that the correct amount of current is being shunted away by the calibrating resistor so that 1 A is flowing in the heater element.

 

[collies99]

Billcasper:
Normally we do not adjust the calibration resistor.

Seeing that you have already done so, you need to test as stated above at the max rating based on the cooling class. Check CT tedt dats and confirn ratio is matched to indicated nameplate rating giving 1A out rated cooling class.

Since you have already calibrsted ghe resistor the thermometer in the only item left.

good luck

 

[collies99]

To billcasper:

The manual shows for TD50 is good for 2.2A max CT output and TD75 is good for 2.65A max.

Depending on your CT ratio, you need to calculate the CT sec output at 100% load.

Depending on the sec current output, the calibration resistor between term 5-5 needs to be set. Ie: if your CT at 47MVA puts out 2A, then resistance for TD50 between 5-5 (CT wire removed) needs to be set to about give 1.08 ohms (calibration resistor snd heater in parsllel)for a 20 deg C gradient curve.

my big question is do you have the transformer gradient curve number ?

 

[billcasper5968]

Thanks everybody. It is now very clear to me that we need to re-evaluate our procedure and request the gradient curve from the manufacturer. We could have not understood this fully without all your help. Thanks to all of you.

 

[stevenal]

It seems like you know that you want a 20 deg rise, and this will happen when 1 A flows through the element.

So inject full load current as a secondary value to replicate CT current and use a clamp on ammeter to confirm that the correct amount of current is being shunted away by the calibrating resistor so that 1 A is flowing in the heater element.

20 deg rise at 1 A does not represent full load, since this is a 55 deg rise transformer per billcasper5968. This can only be a calibration mid-point at best.

 

[collies99]

Temperature gradient in the difference between mean winding temp and mean oil temp. The numbers given typically range around the 20C area for a transformer at full load rating.

I think billcasper's number is not that far out. With a bit more leg work he should be able to calibrate the AKM Quslitrol winding temp indicator do that in can be more meaningful, just my thoughts.

 

[DiscoP]

I would be very surprised if the gradient was 55 degrees.

This is an important point - you need the expected gradient at full load current.

I had assumed was already known because that was what was written in the initial question. (It is also important that you never assume ! )

The transformer manual may give you this figure, there are specific tests done to determine the gradient. It is not normally a figure on the transformer nameplate.

Once you have this figure, use the akm manual to work out how much current needs to flow in the element to achieve this rise. Then work out the shunt resistor value required so that this current flows in the element when the CT is producing full load current using current divider theorem (or ohms law).

The gradient varies for each transformer, on ours it normally varies between about 5 to 25 deg.

If it has been calibrated for 55 degrees, maybe that is why the measurements you are getting are unrealistic