Transformer Testing
AVTMTO3XX Rev 3 Sept 2013
35
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
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