The mechanical effects of short-circuit currents in - Montefiore

5.2.3.1 PRIMARY VARIABLES
The mechanical loads in a substation due to short
circuits depend on:
- the high-voltage and low-voltage structural design,
- the amplitude of the fault currents and their
distribution over the busbars,
- the instant of occurrence of faults,
- the time constant of the networks,
- the clearance times,
- but also, in the event of a reclosure, the isolating
time and the parameters of the possible second short
circuit,
- the variation in concomitant parameters such as
temperature, wind, ice, etc.
Many of the above are random parameters.
Given the random nature of these parameters, the
conservative approach is to consider the maximum
number of stresses under the worst conditions and
assume no limits or choices. Hence, this approach
may result in expensive over-sizing associated with
low probabilities of occurrence.
We propose to define the broad outline of a
probabilistic approach drawing on existing
approaches (Ontario Hydro, IEC 60826 on overhead
lines [Ref 93]).
By classifying the penalizing situations according to
their probability of occurrence, we can approach the
ultimate stress loading frequencies and set a rule of
choice in order to determine:
• either the assumptions to be made, depending on
the climatic, electrical and structural (HV, LV)
stresses,
In that respect, the combined load (Wind + Short
Circuit) offers an alternative to the arbitrary choice of
a higher safety factor for the isolated electrodynamic
assumption.
• or the conditions for rebuilding or reinforcing an
existing substation. The analyses in progress focus on
this aspect, and more specifically the largest rigid
structures, when the primary busbars are concerned.
Here, we propose to define the main features of a
probabilistic approach on the basis of an existing
approach.
5.2.3.1.1. Short-Circuit Currents
The amplitude of the fault current depends on the
type of fault, the number of generating sets in service,
the network layout (number of transformers or lines
connected at a given instant), and the location of the
faults (substation, lines).
In seeking the maximum electrodynamic loads on
rigid busbars, the most significant faults are
polyphase.
81
a) An initial approach to the variability of fault
current amplitudes is shown in the Figure 5.8 below.
The various distribution functions concerning the
intensity of three-phase short-circuit currents are
plotted on the following graph:
%
100
90
80
70
60
50
40
30
20
10
0
Cumulative distribution function
0 10 20 30 40 50 60 70 80 90 100
%I/Imaximum
Intensity A
Intensity B
Intensity C
Figure 5.8 the different distribution functions for the intensity of
short-circuit currents.
Intensity A: C.D.F. of the short-circuit current in a
substation due to variation over the year. The
variation over the year in the amplitude of the shortcircuit
current (Intensity A) is noticeable in the
networks where the main generating units are located,
but may be reduced in certain networks, when
transformers supply the power. In this case, it may be
more practical to take a fixed value of the current.
Intensity B: C.D.F. of the short-circuit current
passing through a substation for a short circuit located
on the connected lines, calculated at the instant of
maximum short-circuit power for that substation,
At a constant short-circuit power, the impedance of
the lines reduces the amplitude of the fault current as
a function of the latter's distance.
Intensity C: the combination of curves A and B thus
concerns the variation in the fault current passing
through a substation as a function of the location of
the fault and the time of year.
b) A second approach, which is very useful and
practical, concerns constant short-circuit intensity
analysis.
The system manager often considers the peak
withstand current of a substation. In the event of a
predictable overload, the manager will need to take
steps to operate the substation as several electric
nodes (7.2.3.2.2.7.) or to upgrade the station to
increase short circuit capability. In such situations,
the language of probabilities is highly useful in
evaluating a mechanical risk of failure for the
different substation layouts,.In particular when nodes
are interconnected in order to transfer a load from one
node to another, the risks of network weakening due