[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

FIGURE 3.6.7 Discharging the capacitors of an impulse generator.
FIGURE 3.6.5 Marx generator with four stages.
FIGURE 3.6.6 Charging the capacitors of an impulse generator.
The discharge path shown in the figure is somewhat simplified for clarity: Significant currents do flow
in the shunt wave-shaping resistors, R p , and significant current also flows in the loading capacitor, C Loading .
These currents, which are significant in controlling the waveshape, are ignored in Figure 3.6.7.
The measurement of impulse voltage in the range of a million volts in magnitude requires the use of
voltage dividers. Depending upon requirements, either resistive, capacitive, or optimally damped (RC)
types of dividers, having stable ratios and fast response times, are utilized to scale the high-voltage
impulses to provide a suitable input for instruments. Most impulse-test facilities utilize specially designed
impulse oscilloscopes or, more recently, specially designed transient digitizers, for accurate measurement
of impulse voltages.
Measurement of the transient currents associated with impulse voltages is carried out with the aid of
special noninductive shunts or wideband current transformers included in the path of current flow.
Usually, voltages proportional to impulse currents are measured with the impulse oscilloscopes or transient
digitizers described earlier.
3.6.5.1.2.2 Impulse Test Setup — For consistent results it is important that the test setup be carefully
made, especially with respect to grounding, external clearances, and induced voltages produced by
impulse currents. Otherwise, impulse-failure detection analysis could be flawed. One example of proper
impulse-test setup is shown in Figure 3.6.8. This figure illustrates proper physical arrangement of the
impulse generator, main circuit, chopping circuit, chopping gap, test object, current shunt, voltage
measuring circuit, and voltage divider. High voltages and currents at high frequencies in the main circuit
and the chopping circuit can produce rapidly changing electromagnetic fields, capable of inducing
unwanted noise and error voltages in the low-voltage signal circuits connected to the impulse-recorder
inputs. The purpose of this physical arrangement is to minimize these effects.
3.6.5.1.2.3 Impulse-Test Failure Detection — To accomplish failure detection or to verify the absence of
a dielectric failure in the transformer insulation system, the impressed impulse-voltage waveforms and
the resulting current waveforms of the full-wave test are compared with the reduced full-wave test
reference waveforms. The main idea behind failure detection in transformers is that if no dielectric
breakdowns or partial discharges occur, then the final full-wave test voltage and current waves will exhibit
waveforms identical to the initial reduced full-wave reference tests when appropriately scaled. The occurrence
of a dielectric breakdown would produce a sudden change in the inductance-capacitance network
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC