[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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