Full resolution (JPEG) - On this page / på denna sida - 1958, H. 6 - Up-to-date Criteria in the Construction of Equipment for High Voltage Power Systems According to the Experience of the 400 kV System in Sweden, by Gunnar Jancke
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Fig. 4. Diagram of overvoltage factors measured on
interrupting a 500 km unloaded 400 kV line. For each
circuit-breaker type the first stem shoivs
over-voltage on the station side and the second stem
overvoltage on the line side. .4 — air-blast breaker,
() -— oil-poor breaker. Unfilled stem — maximum
value, filled stem — mean value from the test series.
faults and protect the transformers and other
expensive equipment by means of lightning arresters. The
insulation level is determined solely on the basis of
the internal overvoltages and the characteristics of
the arresters.
The voltage rise on disconnecting a loaded line is
a function of the line’s length, the available
short-circuit power and the load on the line. A further
rise is experienced at the open line terminal owing
to the Ferranti effect, and furthermore, a
considerable voltage rise occurs at this point in the sound
phases on earth faults.
Extensive tests have been carried out in the
Swedish network with seven different types of 400 kV
circuit-breakers in order to determine the
magnitude of switching surges in the most unfavourable
cases. When disconnecting a 500 km unloaded line
the results shown in fig. 4 were obtained. The tests
were carried out with an operating voltage varying
between 400 and 540 kV at the feeding station. The
air-blast circuit-breakers were entirely free from
re-strikes and therefore did not set up any overvoltages.
On the other hand, the oil circuit breakers restruck,
which resulted in overvoltage factors up to about 2.5.
In addition, a series of voltage jumps with a very steep
wave front and amplitude were obtained with
restri-kes, which occured at extremely short time-intervals.
These voltage jumps took place symmetrically around
zero potential so that the arresters were unable to
protect the transformers. In connection with the
testing of one oil circuit-breaker, which on account
of its total failure has not been included in the
figure, on one occasion 20 such jumps of about 1 500—
2 000 kV were obtained with a wave-front time of
approximately 3 [.is and time intervals of about
100 us. In the course of this test the transformer in
Fig. 5. Diagram of overvoltage factors measured on
interrupting a reactorloaded or unloaded transformer.
For each circuit-breaker type the first stem shows
overvoltage on the station side and the second stem
overvoltage on the transformer side.
one phase flashed over at the entrance coils. We
now only employ circuit breakers that do not
re-strike for our lines, which not only eliminates the
risk of such transformer stresses but also lowers the
figure for the necessary insulation level. In
consequence of our experience attempts are now being
made in various quarters to construct oil
circuit-breakers which do not restrike.
The results obtained when disconnecting unloaded
and reactor-loaded transformers are shown in fig.
5. Here the different types of breakers are equal to
one another, so that one’s hands are freer with
respect to their selection. In this case arresters placed
on the transformer side of the breaker can give
protection against overvoltages without a risk of them
being destroyed themselves.
Certain transient oscillation phenomena set up on
connecting long lines in circuit were dealt with at an
early stage by Professor Harold Peterson in his book
"Transients in Power Systems". On the basis of the
latter the matter was studied when planning the first
sections of the Swedish 400 kV network which
included two line sections each about 500 km in
length. Only insignificant oscillation phenomena
were observed both in the model experiments and
later during the field tests. On the other hand, a
maximum voltage crest of 1.7-times the stationary
overvoltage when connecting up a 650 km line
section was observed.
The phenomenon has therefore now been studied
more closely in a transient analyzer. It was then
found that the overvoltage factor rises with an
increasing degree of compensation and length of line
and with a decreasing short-circuit power. Series
capacitors should not be placed at the line terminals
but should be distributed symmetrically around the
1 gg ELTEKN I K 1958
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