The mechanical effects of short-circuit currents in - Montefiore

3.7. SPECIAL PROBLEMS
3.7.1. Auto-reclosing
a) Introduction
In order to reduce outage duration, reclosure is very
often used to re-energise overhead lines after a tripping
due to a fault on the power system. A reclosure cycle
will be indentified by : Tk / Ti / Tk2 where Tk is the
first short circuit duration, Ti the reclosing interval and
Tk2 the second short circuit duration. For example, on
RTE transmission network, the reclosure cycle is 120
ms / 2 to 5 s / 90 ms for 400 kV and 210 ms / 2 to 5 s /
150 ms for 225 kV depending on the protection devices.
But fast reclosure is sometimes used to minimize the
reclosing interval (Ti = 300 ms) on 90, 63 kV customers
lines.
When the fault is maintained, the stresses can be larger
than before. The calculation of the effects requires the
use of finite element techniques and powerful
computers. IEC 60865 modelling proposes a simplified
method to calculate the tensile force during or after the
short-circuit for the simplest arrangements with or
without pinch effects. But reclosure is not taken into
account.
Span [m] 68 68 34 34
Phase n° 1 2 1 2
Static conductor
tension [N]
5740 5370 3579 3390
First short-circuit force
[N]
10040 8559 9919 7952
First drop force [N] 12824 13694 9203 9045
Second short-circuit
force [N]
11432 9968 10532 6660
Second drop force [N] 21870 22051 8181 9146
Table 3.2 Test results
b) Tests
The case 9 tests in the Cigre brochure concerns two
spans of 34 (Figure 3.56) and 68 m (Figure 3.55) using
1 x ASTER (570 mm 2 ) cable per phase made by EDF.
The reclosure cycle is 85 ms / 2 s / 95 ms. The
difference of the tensile force between two phases
(Table 3.2) is due to several causes : static tensile force,
portal stiffness (between middle and edge), shift in time.
After the second short-circuit, maximum short-circuit
force on phase 1 of 68 m span is increased by 13 % (16
% on phase 2) and drop force by 70 % (61 % on phase
2).
2 5 0 0 0
2 0 0 0 0
1 5 0 0 0
1 0 0 0 0
5 0 0 0
0
0
0,17
0,34
0,51
0,68
0,85
1,02
1,19
1,36
1,53
1,7
1,87
Figure 3.55 Tensile force measurement (N) versus time (s) – phase
68 m – phase 1.
2,04
2,21
2,38
2,55
2,72
2,89
3,06
3,23
3,4
3,57
3,74
3,91
4,08
4,25
61
10000
5000
0
0
0.17
0.34
0.51
0.68
0.85
1.02
1.19
1.36
1.53
1.7
1.87
2.04
2.21
2.38
2.55
2.72
2.89
3.06
3.23
3.4
3.57
3.74
3.91
4.08
4.25
Figure 3.56 Tensile force measurement (N) versus time (s) – span 34
m – phase 1.
c) Extension of the IEC method
Annex 8.4 proposes an extension of the IEC pendulum
model for autoreclosing in the simplest cases
0
( δ m < 70 ) without dropping. Several points have
been studied relating the second short-circuit to the
influence of the first-fault heating. Annex 8.5 gives a
synthesis of this proposal and a sample of calculation.
d) Comparisons
These calculations are made in the conditions of EDF
tests and give a comparison (Table 3.3) between this
extension and tests within several hypotheses.
Span [m] Maximum tensile force [N]
case 9 link 1 link 2
68 22051 22469 22276
(+1,9%) (+1%)
34 10532 11698 12758
(+11%) (+21%)
Table 3.3 Influence of linking conditions
Link 1 : δM = arccos(χ)
Link 2 : δM IEC 60865 (*31)
e) Influence of reclosing interval
Reclosure operates on a structure that is already in
movement because of the initial fault and depending on
whether the second fault occurs within a cable
acceleration or deceleration stage (rising to the first gap,
drop), the influence of mechanical and geometrical
stresses thus differs : either the effect of reclosure is
combined with that of the initial fault, amplifying the
stress, or its effect opposes the movement of the cables
caused by the initial fault and thus reduces forces,
contributing to damping their rocking motion.
It will be observed that :
a) reclosure can cause larger forces than a simple fault.
This is the hypothesis retained in IEC 60865 on rigid
conductors.
b) a small proportion of faults occurring on reclosure
leads to forces exceeding those of the initial fault.
c) a reclosing interval will enable the structure to damp
out its oscillations before reclosure occurs and thus
contributes to reducing the maximum force obtained
on reclosure.
Knowledge of the most stressing reclosure instant is of
interest for the engineer. The optimum force of drop
after reclosure is indeed obtained slightly after the