[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

FIGURE 3.6.18 Current, voltage, and apparent resistance with time.
some low-current distribution transformer windings where the dc current can be significant compared
with the rated current. It is more likely that errors will occur because of meter readings taken before core
saturation is achieved. The process involved in core saturation is described below.
Compared with the exponential current-vs.-time relationship for the R-L circuit with constant R and
constant L, the current in a transformer winding, when a dc voltage is first applied, rises slowly. The slow
rate of rise comes about because of the high initial impedance of the winding. The initially high impedance
results from the large effective inductance of the winding with its iron core. As the current slowly increases,
the flux density in the core slowly rises until the core begins to saturate. At this point, the winding no
longer behaves like an iron-core coil and instead behaves like an air-core coil, with relatively low inductance.
The rate of rise of the current increases for a period as the core saturates; then the current levels
off at a steady-state value. Typical shapes for the voltage, current, and apparent resistance are shown in
Figure 3.6.18. The magnitude of the dc voltage affects the rate at which flux builds up in the core, since
V = N(d/dt). The higher the magnitude of the dc voltage, the shorter is the time to saturation because
of a higher value for d/dt. At the same time, though, the coil must be able to provide the required
magnetomotive force in ampere turns, N I, needed to force the core into saturation, which leads to a
minimum value for the dc current. Of course, there is an upper limit to the value for dc current, namely
the point at which conductor heating would disturb the resistance measurement.
Note the time scale of the graph in Figure 3.6.18. It is very important that the steady-state dc current
be attained before meter readings are taken. If this is not done, errors in excess of 20% are easily realized.
3.6.6.3.5 Winding Resistance and Average Winding Temperature
Two of the three purposes listed above for measuring the dc resistance of a transformer winding inherently
involve a concomitant measurement of temperature. When measuring resistance for the purpose of
calculating I 2 R at a given temperature, the I 2 R value obtained will be used to determine the load-loss
value at a different temperature. When the winding resistance is measured before and during a heat run,
the determination of average winding temperature at the end of a heat run test requires knowledge of
winding resistance at two temperatures.
The winding dc resistance at two temperatures, T1 and T2, will have values of R1 and R2, respectively,
at the two temperatures. The functional relationship between winding resistance and average temperature
is shown in Equation 3.6.4:
R1
T1
Tk
R2
T2
T
k
(3.6.4)
where
R1 is the value of winding resistance, corresponding to average winding temperature of T1
R2 is the value of winding resistance, corresponding to average winding temperature of T2
T k is 234.5C for copper, 225C for aluminum
Correction of load loss for temperature is covered in section 3.6.6.2. Determination of average winding
temperature in a heat run test is covered in section 3.6.6.4.
3.6.6.4 Heat Run Tests
3.6.6.4.1 Purpose of Heat Run Tests
The maximum allowable average and hottest-spot temperature rises of the windings over ambient
temperature and the maximum allowable temperature rise of the top oil of the transformer are specified
by ANSI and IEEE standards and are guaranteed by the manufacturer. The purpose of temperature-rise
tests is to demonstrate that the transformer will deliver rated load without exceeding the guaranteed
values of the temperature rises of the windings and oil. According to the ANSI and IEEE standards, these
tests are performed at the minimum and maximum load ratings of a transformer.
3.6.6.4.2 Test Methods
For factory testing, it is not practical to connect the transformer to a load impedance with full rated
secondary voltage applied to the simulated load. Although this would most directly simulate service
conditions, most of the total test input power would dissipate in the load impedance. The load power,
which equals the load rating of the transformer, is much larger than the sum of no-load and load losses
that dissipate in the transformer. The electrical heating of the load would not contribute to transformer
heating. Electric power consumption for the test would be excessive, and the test would not be practical
for routine testing.
In factory tests according to methods specified in the ANSI and IEEE standards, several artificial loading
schemes can be used to simulate heat dissipation caused by the load and no-load losses of the transformer.
The back-to-back loading method, described in the IEEE test code [2], requires two identical transformers
and is often used for heat runs of distribution transformers. For power transformers, it is most common
to use the short-circuit loading method, as specified in the IEEE test code [2]. The test setup is similar
to that used for measurement of load loss and impedance voltage. One winding is connected to a short
circuit, and sufficient voltage is applied to the other winding to result in currents in both windings that
generate the required power loss to heat the oil and windings.
3.6.6.4.3 Determination of the Top, Bottom, and Average Oil Rises
This discussion applies to the short-circuit loading method. Initially, the test current is adjusted to provide
an input power loss equal to the no-load loss plus the load loss. This can be called the total loss. The
total loss is corrected to the guaranteed temperature rise plus ambient temperature. During this portion
of the heat run, the windings provide a heat source for the oil and the oil cooling system. The winding
temperatures will be higher than expected because higher-than-rated currents are applied to the windings,
but here only the oil temperature rises are being determined. For an oil-filled power transformer, the
total power loss for a given rating is maintained until the top-oil temperature rise is stabilized. Stabilization
is defined as no more than 1C change in three consecutive 1-h periods.
After stabilization is achieved, the top- and bottom-oil readings are used to determine the top, bottom,
and average oil temperature rises over ambient at the specified load.
3.6.6.4.4 Determination of the Average Winding-Temperature Rise
After the top, bottom, and average oil temperature rises are determined, the currents in the windings are
reduced to rated value for a period of 1 h. Immediately following this 1-h period, the ac power leads are
disconnected, and resistance measurements are carried out on both windings. The total time from
disconnection of power and the first resistance readings should be as short as possible, typically less than
2 min — certainly not more than 4 min. Repeated measurements of winding resistance are carried out
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC