Urs Krüsi, Bestimmung und Beurteilung der Charakteristika von ...

viii Abstract
nical scatter could be reduced to -3.1 ms to 2.7 ms. This scatter can now
be used to calculate the breaker’s suitability for controlled switching. A
method is developed that calculates the frequency of occurrence of the expected
switching transients. The requirements concerning these transients
vary greatly among power utilities. It is therefore not practical to specify a
hard limit. Thus, the suitability of a circuit-breaker can hardly be judged
absolutely, but rather only whether it is more or less suitable. One of the
main features of the presented method is that no time-consuming statistical
simulations of a circuit-breaker are needed. The application of the
method is demonstrated on an example of a fictitious case study.
The de-energization of capacitive loads is another switching duty that is
closely related to the statistics of the dielectric strength of moving contacts.
A similar mathematical description of the dielectric strength of the opening
contact gap was able to be used. The goal is to determine the influence of
the prolonged arcing time on the restrike probability in a quantitative manner.
In combination with results from development tests, this model allows
the estimation of the circuit-breaker’s capability to de-energize capacitive
loads at higher voltages and/or frequencies, by taking into account the
statistical behavior of the opening gap. The resulting restrike probabilities
are given as a function of arcing time and RDDS. These results are applied
to the evaluation of laboratory tests 1 on a HVAC SF6 circuit-breaker. In
this case, an arcing time of 2.8 ms allows an increase of the frequency to
66 Hz and an increase of the voltage by 15%. The calculated restrike probabilities
allow determination of the uprating potential of a circuit-breaker.
The higher restrike probability at higher frequencies and/or voltages can
be compensated with a prolonged arcing time — in some cases even overcompensated.
Two new possible applications of controlled switching have been demonstrated
in this thesis. The benefits of controlled switching for installed
circuit-breakers is now quantifiable, and the influence of the arcing time on
the restrike probability of a circuit-breaker can be estimated reasonably.
1 Per Jonsson, ABB Sweden