[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)
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FIGURE 3.6.17 Circuit for measuring winding resistance.<br />
FIGURE 3.6.15 Test circuit for load-loss measurement.<br />
FIGURE 3.6.16 Equivalent circuit and phasor diagram for load-loss test.<br />
recently been developed by a working group of the IEEE PES <strong>Transformer</strong>s Committee. When generally<br />
available, it will provide a complete background and the basis for carrying out measurements and<br />
calibrations to ensure accurate values of reported losses as required by ANSI and IEEE standards [1].<br />
Because the magnitudes of load losses and impedance depend upon the tap positions of the deenergized<br />
tap changer (DETC) and the load tap changer (LTC), if present, load-loss and impedancevoltage<br />
measurements are usually carried out in the rated voltage connection and at the tap extremes. If<br />
the transformer under test has multiple MVA ratings that depend on the type of cooling, the tests are<br />
normally carried out at all ratings.<br />
3.6.6.3 Winding Resistance Measurements<br />
3.6.6.3.1 Purpose of Winding Resistance Measurements<br />
Measurements of dc winding resistance are of fundamental importance because they form the basis for<br />
determining the following:<br />
Resistance measurements, taken at known temperatures, are used in the calculation of winding conductor<br />
I 2 R losses. The I 2 R losses at known temperatures are used to correct the measured load<br />
losses to a standard reference temperature. Correction of load losses is discussed in section 3.6.6.2.<br />
Resistance measurements, taken at known temperatures, provide the basis to determine the temperature<br />
of the same winding at a later time by measuring the resistance again. From the change in<br />
resistance, the change in temperature can be deduced. This measurement is employed to determine<br />
average winding temperatures at the end of heat run tests. Taking resistance measurements after<br />
a heat run test is discussed in section 3.6.6.4.<br />
Resistance measurements across the transformer terminals provide an assessment of the quality of<br />
internal connections made to the transformer windings. Loose or defective connections are indicated<br />
by unusually high or unstable resistance readings.<br />
3.6.6.3.2 Nature of the Quantity Being Measured<br />
The dc winding resistance differs from the value of resistance indicated for the resistor shown in Figure<br />
3.6.16 or the resistors that appear in textbook illustrations of the PI or T equivalent circuits of transformers<br />
to represent the resistance of the windings. The resistors in the equivalent circuits include the effects of<br />
winding I 2 R loss, eddy loss in the windings, stray losses in structural parts, and circulating currents in<br />
parallel conductors — namely, they represent the resistive components of the load loss. The resistors<br />
shown in the equivalent circuits can be thought of as representing an equivalent ac resistance of the<br />
windings. The dc resistance of the windings is a different quantity, one that is relevant for calculating<br />
I 2 R, for determining average winding temperature, and for evaluating electrical connections.<br />
3.6.6.3.3 How Winding-Resistance Measurements Are Made<br />
The measurement of power transformer winding resistance is normally done using the voltmeter-ammeter<br />
method or using a ratiometric method to display the voltage–current ratio directly. A circuit for the<br />
measurement of winding resistance is shown in Figure 3.6.17. A dc source is used to establish the flow<br />
of steady direct current in the transformer winding to be measured. After the R-L transient has subsided,<br />
simultaneous readings are taken of the voltage across the winding and the current through the winding.<br />
The resistance of the winding is determined from these readings based on Ohm’s law.<br />
3.6.6.3.4 Discussion of the Measurement Process<br />
If a dc voltage is applied as a step to a series R-L circuit, the current will rise exponentially with a time<br />
constant of L/R. This is familiar for the case where both resistance and inductance remain constant<br />
during the transient period. For a transformer winding, however, it is possible for the true resistance,<br />
the apparent resistance, and the inductance of the winding to change with time. The true resistance may<br />
change if the direct current is of high-enough magnitude and is applied long enough to heat the winding<br />
substantially, thereby changing its resistance during the measurement. The inductance changes with time<br />
because of the nonlinear B-H curve of the core steel and varies in accordance with the slope of the coresteel<br />
saturation curve. In addition, there is an apparent resistance, Ra, during the transient period.<br />
V<br />
Ra R<br />
L dI<br />
<br />
(3.6.3)<br />
I I dt<br />
Note that the apparent resistance, Ra, is higher than the true resistance, R, during the transient period<br />
and that the apparent resistance derived from the voltmeter and ammeter readings equals the true<br />
resistance only after the transient has subsided.<br />
Resistance measurement error due to heating of the winding conductor is usually not a problem in<br />
testing transformers, but the possibility of this effect should be taken into consideration, especially for<br />
© 2004 by CRC Press LLC<br />
© 2004 by CRC Press LLC