The mechanical effects of short-circuit currents in - Montefiore

Figure 5.11 Histogram of distribution of the clearance time (TE)
for phase to earth faults
This parameter has a crucial influence on the
amplitude of the mechanical loads. It is also worth
pointing out that the longest clearance times occur
with resistive faults, with the result that their
amplitudes are lower. These clearance times will
depend on the coordination of network protections,
the location of the faults and their nature (single or
polyphase). For each fault pinpointed, the following
sequences may be considered:
normal clearance time: ex t k =
120 ms
normal operating
rate: 0.988
In the event of a failure, the clearance sequences are
often complex and the current inputs are not all cut
off simultaneously (see diagram below).
I P − I L
current input
O 220ms 700ms 1s
Figure 5.12 Variation of SCC during a fault
in ms
This raises the question of how the dynamic response
of mechanical systems should be calculated for such
clearance sequences. Several remarks are necessary:
• First, regarding the Laplace forces, the effects of
which diminish very rapidly due to their quadratic
dependence on the amplitude of intensity.
• Beyond a certain duration (between 80 ms and
200 ms), the loads on mechanical structures often
83
reach their maximum, and this is particularly true for
rigid structures like tube busbars. For flexible
structures, the response time may be longer.
In light of the above remarks, it is often possible to
chose an equivalent mechanical time. Simpler
scenarios can then be considered, such as those
proposed below:
clearance time in the event of
failure of the protection
systems:
ex t k = 220 ms
rate of failure per
year: λ
λ is the overall failure rate established in accordance
with the low voltage devices of a bay. It is established
on the basis of the failure rate of the protections
composing the unit (e.g., for one protection per year:
~10 -2 ).
or:
clearance time in the event of
failure of the circuit breaker:
ex t k = 300 ms
5.2.3.1.5. Reclosure
rate of failure per
year and per
device:
~2 10 -3
Three parameters characterize a reclosing cycle, t k1 / t i
/ t k2. There are two types of reclosure operation in the
networks, the first described as rapid, after a few
hundred ms (t i>100 ms) and the second as slow, after
several seconds (t i>2s). These reclosures may if the
fault recur (~1 time out of 10), lead to final trips. A
good knowledge of the ranges of variation of these
parameters is important in making the choice. If the
basis of consideration is a type of substation
frequently encountered in the network, a uniform
distribution of the isolating time may be taken. The
designer may have to choose carefully in order to
make an accurate estimation of the distributions of
these parameters.
5.2.3.1.6. Other Parameters
If the risk situations are classified by probability of
occurrence, an ultimate load frequency approach then
becomes possible. In that case, a rule can be set:
• either regarding the assumptions to be taken into
account.
In this field, combining the loads (wind + short
circuit) may avoid arbitrarily choosing an overly high
or ill-suited safety factor based solely on the
electrodynamic assumption.
• or as to the maximum capability of the existing
substations, acceptable in terms of risk, and thus as to
the conditions for rebuilding or reinforcing an
existing substation.