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