The mechanical effects of short-circuit currents in - Montefiore

to multinode operation can compared with risk of
another drawback, mechanical failure.
5.2.3.1.2. Instant of Occurrence of Faults
The instants of occurrence of faults (Figure 5.9 and
Figure 5.10) were analyzed on the basis of 129
recordings made on RTE's 400-kV network using five
digital fault recorders over a period lasting from 1981
to 1984.
Phase to earth fault
The angle ϕ is defined by the phase angle difference
between the rising voltage zero and the instant of
occurrence of the fault.
U = U t +
(5.7) ( )
0 2sinω ϕ
Figure 5.9 Histogram of angle ϕ distribution for a phase to
earth fault. Frequency in %
We note that the events are not equally likely. There
is a preference for positive alternation over negative
alternation, which is due to the short circuits
produced by lightning and wind and explained by the
preferentially negative ionization of air. It also
appears that the maximum asymmetry ϕ =0 is not the
most likely event. In other words, the phase
distribution may differ significantly from uniform
distribution.
Phase To phase faults
The phase to phase fault incidence angle is defined by
the phase angle difference relative to the zero point of
the phase-to-phase voltage.
82
Figure 5.10 Histogram of angle ϕ occurrence of phase to phase
faults. Frequency in %
We note that the frequencies of fault occurrence reach
their maximum when ϕ approaches 90° or 270°, i.e.
when the phase-to-phase voltage is maximum, with a
preference for positive alternation.
Three-Phase Faults
For these more infrequent faults, establishing
statistics with the available data proved impossible.
Proposed choice of an instant of occurrence
distribution
Uniform distribution tends to raise the likelihood of
maximum asymmetry, thereby increasing the
maximum stresses, which is still conservative. Hence,
to simplify calculations, we propose a uniform
distribution for all faults (Figure 5.18).
5.2.3.1.3. Time Constant
The time constantτ of the network varies depending
on the type of fault (single, two or three-phase) and
its location. Frequently, the highest time constants
arise from the transformer inputs, whereas the lines
have lower time constants. For a given substation, the
result is a drop in the time constant as the distance
between the fault and the substation increases. The
usual choice of a standard value of 45 ms does not
always cover the values encountered in networks. In
other words, this parameter must be chosen with care.
5.2.3.1.4. Clearance Time
The clearance time may vary from one line to another
for the same protection threshold, depending on the
element on which the load is applied, the possibility
of failure of a component such as the circuit breaker
or the protection system. The Figure 5.11 shows the
variation in clearance time for an extra high voltage
line busbar connection in the RTE network.