[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 3.6.3 Standard full-wave lightning impulse.<br />
1.2 s, then decays to 50% of crest magnitude in 50 s. This is shown in Figure 3.6.3. Such a wave is<br />
said to have a waveshape of 1.2 50.0 s. The term waveshape will be used in this article to refer to the<br />
test wave in a general way. The term waveform will be used when referring to detailed features of the test<br />
voltage or current records, such as oscillations, “mismatches,” and chops. The difference in meaning of<br />
these two terms can be found in the IEEE dictionary [3].<br />
In addition to the standard-impulse full wave, a second type of lightning-impulse wave, known as the<br />
chopped wave, or sometimes called the tail-chopped wave, is used in transformer work. The chopped<br />
wave employs the same waveshape as a full-wave lightning impulse, except that its crest value is 10%<br />
greater than that of the full wave, and the wave is chopped at about 3 s. The chop in the voltage wave<br />
is accomplished by the flashover of a rod gap, or by using some other chopping device, connected in<br />
parallel with the transformer terminal being tested. This wave is shown in Figure 3.6.4. The choppedwave<br />
test simulates the sudden external flashover (in air) of the line insulation to ground. When the<br />
voltage applied to a transformer terminal suddenly collapses, the step change in voltage causes internal<br />
oscillations that can produce high dielectric stresses in specific regions of the transformer winding. The<br />
chopped-wave test demonstrates ability to withstand the sudden collapse of instantaneous voltage.<br />
In addition to the full-wave test and the chopped-wave test, a third type of test known as front-ofwave<br />
test is sometimes made. (The test is sometimes called the steepwave test or front-chopped test.)<br />
The front-of-wave test simulates a direct lightning strike on the transformer terminals. Although direct<br />
strokes to transformer terminals in substations of modern design have very low probabilities of occurrence,<br />
front-of-wave tests are often specified. The voltage wave for this test is chopped on the front of<br />
the wave before the prospective crest value is reached. The rate of rise of voltage of the wave is set to<br />
about 1000 kV/s. Chopping is set to occur at a chop time corresponding to an assigned instantaneous<br />
crest value. Front-of-wave tests, when required, must be specified.<br />
Lightning-impulse tests, including full-wave impulse and chopped-wave impulse test waves, are made<br />
on each line terminal of power transformers. The recommended sequence is:<br />
1. One reduced-voltage, full-wave impulse, with crest value of 50 to 70% of the required full-wave<br />
crest magnitude (BIL) to establish reference pattern waveforms (impulse voltage and current) for<br />
failure detection.<br />
2. Two chopped-wave impulses, meeting the requirements of crest voltage value and time to chop,<br />
followed by:<br />
3. One full-wave impulse with crest value corresponding to the BIL of the winding line terminal<br />
When front-of-wave tests are specified, impulse tests are carried out in the following sequence: one<br />
reduced full-wave impulse, followed by two front-of-wave impulses, two chopped-wave impulses, and<br />
one full-wave impulse.<br />
FIGURE 3.6.4 Standard chopped-wave lightning impulse.<br />
Generally, impulse tests are made on line terminals of windings, one terminal at a time. Terminals not<br />
being tested are usually solidly grounded or grounded through resistors with values of resistance in the<br />
range of 300 to 450 . The voltage on terminals not being tested should be limited to 80% of the terminal<br />
BIL. Details about connections, tolerances on waveshapes, voltage levels, and correction factors are given<br />
in the IEEE test code [2] and the IEEE impulse guide [4].<br />
3.6.5.1.2.1 Lightning-Impulse Test Equipment — The generation, measurement, and control of impulse<br />
voltage waves is a very specialized subject. In this section, only a very brief general introduction to the<br />
subject is provided. Most impulse-generator designs are based on the Marx circuit. Figure 3.6.5 shows a<br />
schematic diagram of a typical Marx-circuit impulse generator with four stages. In principle, voltage<br />
multiplication is obtained by charging a set of parallel-connected capacitors in many stages of the impulse<br />
generator to a predetermined dc voltage, then momentarily reconnecting the capacitor stages in series<br />
to make the individual capacitor voltages add. The reconnection from parallel to series is accomplished<br />
through the controlled firing of a series of adjustable sphere gaps, adjusted to be near breakdown at the<br />
dc charging voltage. After the capacitors are charged to the proper dc voltage level, a sphere gap in the<br />
first stage is made to flash over by some means. This initiates a cascade flashover of all the sphere gaps<br />
in the impulse generator. The gaps function as switches, reconnecting the capacitor stages from parallel<br />
to series, producing a generator output voltage that is approximately equal to the voltage per stage times<br />
the number of stages.<br />
The desired time to crest value on the front of the wave and the time to half-crest value on the tail of<br />
the wave are controlled by wave-shaping circuit elements. These elements are indicated as R c , R p , and<br />
C Loading in Figure 3.6.5. Generally, control of the time to crest on the front of the wave is realized by<br />
changing the values of series resistance, the impulse-generator capacitance, and the load capacitance.<br />
Control of the time to 50% magnitude on the tail of the wave is realized by changing the values of parallel<br />
resistors and the load capacitance. Control of the voltage crest magnitude is provided by adjustment of<br />
the dc charging voltage and by changing the load on the impulse generator. The time of flashover for<br />
chopped waves is controlled by adjustment of gap spacings of the chopping gaps or the rod gaps.<br />
The capacitor-charging current path for the impulse generator is shown in Figure 3.6.6. At steady state,<br />
each of the capacitors is charged to a voltage equal to the dc supply voltage. After the cascade firing of<br />
the sphere gaps, the main discharging current path becomes, in simplified form, that of Figure 3.6.7. The<br />
RC time constants of the dc charging resistors, R c as defined in Figure 3.6.5, have values typically expressed<br />
in seconds, while the waveshape control elements, R p and R s as defined in Figure 3.6.5, have RC time<br />
constants typically expressed in microseconds. Hence, for the time period of the impulse-generator<br />
discharge, the relatively high resistance values of the charging resistors represent open circuits for the<br />
relatively short time period of the generator discharge. This is indicated by dotted lines in Figure 3.6.7.<br />
© 2004 by CRC Press LLC<br />
© 2004 by CRC Press LLC