The mechanical effects of short-circuit currents in - Montefiore

The wind may act in the same direction as the
electrodynamic loads. This is the case for phases 2, 4
and 6 in the above histograms. But for the other phases
it acts in the opposing direction. With respect to the
number of loaded elements, the cumulated risks on the
various phases of the two busbars gives a mean number
n per suite of loaded post insulators of less than 1.1 in
the case of combined loads and of approximately 2 in
the case of a simple electrodynamic hypothesis.
5.2.3.7.4. Structures with a single busbar
For certain substations with only one busbar or for cross
connections comprised of rigid bars, the analyses made
in 5.2.3.7.2 for the fault on a single busbar can be
reused.
5.2.3.7.5. Influence of connectors
Another factor reduces the number of post insulators
concerned, i.e., the distribution of loads between post
insulators according to the type of connector. The types
of connector must frequently installed are:
a) successions of clamped - clamped and pinned -
pinned connectors. In this case, half the post
insulators (n=2) are loaded (ρ=0.5).
b) successions of clamped - clamped then sliding, -
sliding then pinned - pinned connectors. In this case,
two-thirds of the post insulators (n=3) are loaded
(ρ=0.67).
c) clamped - pinned fittings which distribute loads
uniformly, which means that all post insulators
located relatively far from the extremities along the
path of maximum currents are loaded (ρ=1).
We define ρ as:
(5.24) ρ =
1
n = 2ou3 n= 2 ou 3
∑
n=
1
Risk( F / F )
Risk( F / F )
0
n R
The set (Fn) corresponds to the loads on the post
insulators of the most loaded busbar (longitudinal
variation of load), but limited in size to two or three
elements defined in paragraphs a) and b).
5.2.3.7.6. Common mode faults
A distinction must be made between a substation and a
family of substations. In the case of one substation,
there may be common mode faults such as those
resulting from component manufacture or assembly.
Example: On a given substation, it is very likely that
many of the ceramic post insulators come from the
same production batch. This may give rise to
statistically above-average or below-average strength.
In short, for the analysis of a family of substations, the
advantages of the probabilistic approach are clearly
apparent.
R
95
5.2.3.7.7. Conclusions
For N sequences on a section of busbars, we thus take
. ρ . post insulators subject to maximum loads.
n N
5.2.3.8 FAILURE RECURRENCE TIME
5.2.3.8.1. Risk at a substation
For line fault
The overall maximum risk at a substation is given by
the expression:
(5.25) R l n N Risk F
L
o
P = νλη . . . P.
. ρ.
. ( )
F
with ν : normal operating rate of fault elimination (close
to 1).
λ : number of faults on overhead lines connected per km
and per year,
η : rate of high-amplitude polyphase faults (hence nonresistive),
lP : cumulative length of the risk zone in km,
n : number of loaded post insulators per busbar suite,
ρ : coefficient depending on the type of connector
N : suite number,
Example:
For λ=20x10 -2 per km and per year, η=30%, l=6km,
n =1,5 ρ=1, N=14 suites, Risk Fo
R
( )= 10
FR
-5 , there is
thus a probability of substation failure of around 7.6 10 -
5
, corresponding to a recurrence time of more than
13000 years for a safety factor of 0.7.
For line fault in the event of failure
The previous analysis is also applicable in the event of
failure (circuit breakers or protection devices), though
the elimination times may be different and the resulting
loads increased. It is important to determine the
amplitude of loads amplification, in view of the remarks
made in section 5.2.3.1.4 on the effect of saturation
when the clearance time exceeds the mechanical
reaction time. In (5.25), Fo is replaced by F1 and the
failure rate: ν : (protection system and/or circuit breaker
failure rate) is taken into account.
The increase in risk is often low and the reduction in the
failure rate means that this term can be ignored.
For substation fault
The maximum overall risk at a substation is given by
the expression: R n N Risk F
P o
P = λη . . . ρ.
. ( ) with
F
λ : annual frequency of faults in the substation,
η : rate of high-amplitude non-resistive polyphase faults
(for example, accidental earthing with all available
network power).
R