[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

FIGURE 3.6.8 Impulse-test setup.
seen at the tested terminals of the transformer, causing a deviation in the test waveforms compared with
the reference waveforms. The act of comparing the reduced full-wave records and the full-wave records
is sometimes called “matching” the waves. If they are identical, the waves are said to be matched. Any
differences in the waves, judged to be significant, are said to be mismatches. If there are mismatches,
something is not correct, either in the test setup or in the dielectric system of the transformer. Various
waveforms of voltages and currents associated with different types of defects are presented in great detail
in the IEEE impulse guide [4]. When digital recorders are employed, methods of waveform analysis using
the frequency dependence of the transformer impedance, transformer transfer function, and other digital
waveform-analysis tools are now being developed and used to aid failure detection. Measurements of the
voltages and currents in various parts of the transformer under test can aid in location of dielectric
defects. These schemes are summarized in Figure 3.6.9.
3.6.5.1.3 Switching-Impulse Test
Man-made transients, as opposed to nature-made transients, are often the result of switching operations
in power systems. Switching surges are relatively slow impulses. They are characterized by a wave that:
1. Rises to peak value in no less than 100 s
2. Falls to zero voltage in no less than 1000 s
3. Remains above 90% of peak value, before and after time of crest, for no less than 200 s
This is shown in Figure 3.6.10. Generally, the crest value of the switching-impulse voltage is approximately
83% of the BIL.
Voltages of significant magnitude are induced in all windings due to core-flux buildup that results
from the relatively long duration of the impressed voltage during the switching-impulse test. The induced
voltages are approximately proportional to the turns ratios between windings. Depending upon the
transformer construction, shell-form versus core-form, three-leg versus five-leg construction, etc., many
connections for tests are possible. Test voltages at the required levels can be applied directly to the winding
under test, or they can be induced in the winding under test by application of switching impulse voltage
of suitable magnitude across another winding, taking into consideration the turns ratio between the two
FIGURE 3.6.9 Impulse-current measurement locations.
windings. The magnitudes of voltages between windings and between different phases depend on the
connections. This is discussed in great detail in the IEEE impulse guide [4].
Because of its long duration and high peak-voltage magnitude, application of switching impulses on
windings can result in saturation of the transformer core. When saturation of the core occurs, the resulting
waves exhibit faster-falling, shorter-duration tails. By reversing polarity of the applied voltages between
successive shots, the effects of core saturation can be reduced. Failures during switching-impulse tests
are readily visible on voltage wave oscillograms and are often accompanied by loud noises and external
flashover.
Switching-impulse tests are generally carried out with impulse generators having adequate energy
capacity and appropriate wave-shaping resistors and loading capacitors.
3.6.5.2 Low-Frequency Dielectric Tests
3.6.5.2.1 Purpose of Low-Frequency Dielectric Tests
When high-frequency impulse voltages are applied to transformer terminals, the stress distributions
within the windings are not linear but depend on the inductances and capacitances of the windings. Also,
the effects of oscillations penetrating the windings produce complex and changing voltage distributions.
Low-frequency stresses that result from power-frequency overvoltage, on the other hand, result in stresses
with a linear distribution along the winding. Because the insulation system is stressed differently at low
frequency, a second set of tests is required to demonstrate dielectric withstand under power-frequency
conditions. The low-frequency dielectric tests demonstrate that the power transformer insulation system
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC