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nani12
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WTI & OTI Indicator

01/08/2019 1:54 AM

Why the WTI indicator is showing lesser reading compared to OTI when transformer is working on no load?? and on load vice versa?? please help
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PWSlack
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#1

Re: WTI & OTI indicater

01/08/2019 3:54 AM

For the benefit of readers that are not familiar with the terms and wanting to learn from the post, please define TLAs before using them.

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#2

Re: WTI & OTI indicater

01/08/2019 7:05 AM

The excessive use of acronyms has got to stop ASAP!

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PWSlack
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#3
In reply to #2

Re: WTI & OTI indicater

01/08/2019 8:57 AM

NIMBY! ��

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jesw55
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#5
In reply to #3

Re: WTI & OTI indicater

01/08/2019 11:30 AM

In the past might one get run over by a train IYBY (In Your Back Yard)

John

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PWSlack
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#13
In reply to #5

Re: WTI & OTI indicater

01/11/2019 10:38 AM

WTF WTH.

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#4

Re: WTI & OTI Indicater

01/08/2019 10:00 AM

WTI = Winding Temperature Indicator

OTI = Oil Temperature Indicator

My guess is that heat is being generated in the windings when the transformer is under load and when the load is removed the oil cools slower than the windings.

http://www.electricalunits.com/wti-and-oti-of-transformer/
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Brave Sir Robin
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#6
In reply to #4

Re: WTI & OTI Indicater

01/08/2019 11:44 AM

Doesn't that seem strange since the windings are bathed in the oil?

My only guess is that under no load, the terminations to the windings are removing heat faster from the wire than the oil cools.
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#9
In reply to #6

Re: WTI & OTI Indicater

01/08/2019 3:40 PM

The WTI has two inputs, hottest oil temperature (from sensing bulb) and a heater driven by winding current (current transformer). The OTI measures the oil temperature via a different method (gas pressure). The WTI reads higher when the transformer is under load due to the winding current input from the current transformer.

"Details of Winding Temperature Indicator(WTI):

Winding temperature indicator(WTI) consists of a sensor bulb placed in the oil filled pocket in the transformer tank top cover. The bulb is connected to the instrument housing by means of two flexible capillary tubes. One capillary is connected to the measuring bellow of the instrument and the other to a compensation bellow.

The measuring system is filled with a liquid, which changes its volume with rising temperature. Inside the instrument is fitted with a heating resistance which is fed by a current proportionate to the current flowing through the transformer winding.

The instrument is provided with a maximum temperature indicator. The heating resistance is fed by a current transformer associated with the loaded winding of the transformer. ( The heating resistance is made out of the same materials as that of the winding ) The increase in the temperature of the resistance is proportionate to that of the winding. The sensor bulb of the instrument is located in the hottest oil of the transformer, therefore, the winding temperature indicates(WTI) a temperature of hottest oil plus the winding temperature above hot oil i.e the hot spot temperature."

"The (OTI)oil temperature indicator consists of a sensor bulb, capacity tube, and a dial thermometer, the sensor bulb is fitted at the location of hottest oil. The sensor bulb and capacity tube are fitted with an evaporation liquid. The vapour pressure varies with temperature and is transmitted to a bourdon tube inside the dial thermometer, which moves in accordance with the changes in pressure, which is proportional to the temperature."

http://www.electricalunits.com/wti-and-oti-of-transformer/
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Brave Sir Robin
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#10
In reply to #9

Re: WTI & OTI Indicater

01/08/2019 7:41 PM

Thanks for the clarification and edification!!

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67model
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#7

Re: WTI & OTI Indicater

01/08/2019 1:34 PM

My thought is that....

1/ OTI sensor is at top of tank.

2/ WTI is lower in tank.

3/ No-load, oil circulation is minimum & top of tank is hotter than lower parts. Most of loss is in iron & distributed uniformly in iron - probably more heat at top & bottom of assembly, where there is more iron in three phase transfo with height < width.

4/ Transfo may have pump cooling at high load, reducing temperature variations within oil.

5/ It is normal for actual winding temperature to exceed oil temperature on load due to resistive heating of windings by load current.

6/ You should be able to check this idea by looking at drawings of transfo showing location of WTI & OTI sensors & checking for temperature of tank exterior at several locations bottom to top at no-load & high load. Many professional multimeters have a thermocouple set included & range for temperature.

67model
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#8

Re: WTI & OTI Indicater

01/08/2019 3:39 PM
  • One of the possibilities is a calibration error.
  • Another is a fault.
  • Another is that the sensors are in two different places on the thing and the indicated readings are actually valid.

Quantification detail of the variation between them has been withheld from the forum. A judgement could then be made locally as to whether what follows is worth carrying out:

  • It would be good to check the content of the calibration and commissioning records to see whether it has actually been carried out. It would be good to check the next due date for a calibration check. It would be good to check the calibration protocol for these two different sensors to see whether there is true validity in it and how close the readings are to locally-applicable standards. However, none of these things can be done from here.
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nani12
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#11
In reply to #8

Re: WTI & OTI Indicater

01/10/2019 7:38 AM

The sensors calibrated before commissioning..And there is no fault for sure..I need to check the locations of the sensors. But i dont think its due to location. FYI the readings of WTI and OTI are differing by only 1 degree(OTI>WTI) in all transformers...
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67model
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#14
In reply to #11

Re: WTI & OTI Indicater

01/11/2019 12:21 PM

If reading difference is only 1 degree, then this is within the tolerance of the two instruments - the real difference could be +/- 1 degree - explaining the unexpected hotter/colder readings.

There is nothing to be gained by worrying further - but of course you have learnt more about operation/interpretation of WTI/OTI, which is worthwhile.
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nani12
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#12
In reply to #8

Re: WTI & OTI Indicater

01/10/2019 7:41 AM

I also have a doubt on calibration. If the sensors are in different locations such that the readings they give are not true values then should the meters be calibrated accordingly???
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nani12
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#15

Re: WTI & OTI Indicator

01/14/2019 12:16 AM

And Finally the meter is gone. some smokes started coming from the meter and we cut off the supply to that meter.
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Users who posted comments:

67model (2); Brave Sir Robin (2); HiTekRedNek (1); jesw55 (1); nani12 (3); PWSlack (4); Rixter (2)

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