The mechanical effects of short-circuit currents in - Montefiore

5.2. SECOND LEVEL PROBABILISTIC
METHOD
5.2.1. Stress and strength relationship for
supporting insulators
5.2.1.1 Static and dynamic break-load
In many cases post insulators are mounted in
disconnectors. The permissible (tensile-) load for the
disconnector depends on the minimum breaking load
of the post insulator. Post insulators can be stressed
by static and/or dynamic loads. In the normal case it
is a bending stress. Usually, ceramic materials are
inflexible and differences between static and dynamic
strength are not known [Ref 94]. Experimental [Ref
95] and theoretical [Ref 96] investigations of 110 kV
post insulators under short-circuit loading have not
shown any significant differences between static and
dynamic porcelain strength. For that reason it is
possible to convert the maximum stress at a specific
point of the porcelain in the case of a short-circuit
(i.e. measured as strain εdyn on the surface of the
insulator core over the lowest shed) to an equivalent
static load F max which generates the same stress at the
same point. This load can be compared with the
minimum breaking load.
Figure 5.3 Current-conducting-equipment with conductor, post
insulator and foundation (scheme)
5.2.1.2 One point distribution of strength
From a statistical point of view, the minimum
breaking load fixed by the manufacturer could be
considered as a one point distribution of the
mechanical strength of post-insulators.
This would mean that each post insulator withstands a
load lower than the minimum breaking load. If the
load is higher than the minimum breaking load all
post insulators would be damaged. This approach
does not, correspond with reality. The strength of post
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insulators is distributed statistically as a result of the
fact that the strength of a ceramic body is statistically
distributed, due to differences in the structure inside
the ceramic body [Ref 97]. In most cases the real
bending strengths of post-insulators are higher than
the minimum breaking load. But it is possible that
some post insulators are damaged by loads lower than
the minimum breaking load. Exact knowledge of the
strength distribution function is necessary (especially
for little quantils of strength) for determination of
mechanical reliability of a post-insulator.
5.2.1.3 Strength distribution function of 110 kV post
insulators
111 ruptures of post insulators with a minimum
breaking load of 8kN were evaluated for
determination of the strength distribution function of
brand-new post insulators. The statistical evaluation
of these test results showed three break value ranges
(Figure 5.4) :
1. ruptures due to loads up to 10 kN
2. ruptures due to loads between 10 kN and 20 kN
3. ruptures due to loads greater than 20 kN which
can be approximated by a double exponential
distribution.
Figure 5.4 Distribution-function of mechanical strength of 110kV-post
insulators with different minimum breaking
load Fumin and age (courtesy ABB, Pr. Böhme)
The same procedure is valid for 25-year-old post
insulators with a minimum breaking load of 6 kN. To
determine the distribution function of this type, 21
post insulators were tested to failure. The theoretical
distribution function is also a double exponential
distribution with the following break value ranges :
1. ruptures due to loads up to 7,5 kN
2. ruptures due to loads between 7,5kN and 11 kN
3. ruptures due to loads greater than 11 kN.