8
APPLICATION NOTES
Testing Delta Configured Windings
Testing Delta winding resistance may be a very time consuming procedure, in particular LV winding deltas the correct
balance time can take up to 30-60 minutes for a large transformer. The method for improving test time for delta
configurations requires that both the high side and low side be connected in series with the MTO’s current source (see
connection Table 1). By using both HV and LV windings to magnetize the core, the effective test magnetization current
increases with the turn ratio (simultaneous winding magnetization, SWM).
As an example, testing the Low Voltage side of a 100/10 kV Yd transformer with dual side injection at 10A is the same
as testing only the Low Voltage side of that same transformer 68 A. Even if only one side of the transformer needs to
be tested, connecting both high and low windings in series will speed the test up by a factor of 10 or more.
Delta Winding Resistance
Manufacturer’s winding resistance data are usually presented as per winding for Y configurations and per terminal pairs
for Delta windings. In the rare case that manufacturer’s data is presented per winding also for a delta connection, the
recommendation is to recalculate the numbers to terminal pairs and compare with the field measurement results.
Ravg = Average individual winding resistance
Rtp = Winding resistance between terminal pairs
Rtp=Ravg*0.6667
Temperature Correction
It may be necessasry to convert the resistance measurements to values corresponding to the reference temperature in
the transformer test report.
To estimate the winding temperature at the time of measurement is important. There are three things needed to get
the resistance readings corrected, Winding Material, Winding Temperature and Corrected to Temperature.
Temperature Entry
If the transformer has winding temperature meters, use theses readings. Most transformers will have a meter for each
winding. Most meters are reading about the same value for a static transformer. Use the average of the three readings
and place this number in the Winding Temp field of the Transformer nameplate section of the form. A secondary
method is to use the oil temperature. Use this value in the winding temp field and oil temp field of the form.
Other information
The transformer has been out of service for at least 3 hours
The temperature of the insulating liquid has stabilized, and the difference between the top and bottom
temperature does not exceed 5ºC.
If the transformer is measured without oil, the winding temperature is normally assumed to be the same temperature
as the surrounding air.
Once the information and results are in the form, the results can be switched from measured resistance to corrected
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MTO3XXX-UG-EN Rev 1 Feb 2018
33
Application notes
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