The mechanical effects of short-circuit currents in - Montefiore

The first case (Figure 5.14) corresponded to the
dynamic response of a system to a degree of freedom
at a constant Laplace force. The second case (Figure
5.15) may be resolved through a purely functional
approach or by modal superposition.
7.2.3.2.2.5. Reclosure
When a structure has been set in motion by an initial
fault, the possibility of a recurring fault after
reclosure may lead to increased mechanical stresses.
Between the maximum loads due to the first fault and
the maximum loads resulting from the two faults,
there may be a build-up factor of as much as 1.8 [Ref
95]. Nevertheless, a fault recurs at reclosure roughly
one out of ten times.
One property, superposition, will be particularly
useful in studying reclosure.
PROPERTY OF SUPERPOSITION
The displacement of the system is linked to the
excitation force by a linear differential operator L and
denoted in the following manner:
r
r FL
( t)
during the short circuit
(5.16) L ⋅ U = r
0 outside the short circuit
Following a first fault, for a definite force F () t , a
L
fixed short-circuit intensity, voltage phase and circuit
time constant, and for a given fault duration, only one
physical solution describes the system's displacement
over time on the basis of initial break conditions.
r r r r
(5.17) U( M, 0 ) = o ∀ M
r
where: M : spatial variables.
Following a second fault, the system's response is the
superposition of the dynamic responses of the faults
over an initial break condition with, however, a time
lag tR corresponding to the reclosure time:
r r r r r r
(5.18) U( M, t) = U1( M, t) + U2 ( M, t −tR)
r r r r
where U M , t)
and U ( M , t)
satisfy (7.16).
1(
2
Because of this lag property, the search for the
maximum following the two short-circuit faults is
simplified. In order to superimpose two staggered
faults and calculate their optimum, all that is needed
is to characterize the dynamic response of a rigid
structure to each of them separately. Where the two
faults are of the same type, amplitude and duration,
this superposition principle is especially useful.
86
7.2.3.2.2.6. Combined Loads
Each company has to define its own design policy in
accordance with the occurrence of extreme loads. We
propose here to examine the case of combined shortcircuit
and wind loads. The action of the wind on the
busbars depends on the angle of the wind's direction
relative to the tube axis, whereas that relative to the
insulating bushings is constant for a given wind
velocity, despite its changing orientation. The
maximum dynamic response thus varies according to
the following formula (5.19) :