The mechanical effects of short-circuit currents in - Montefiore

the electromagnetic force at 2f is two times higher and
the constant term is zero. For fc/f < 1 outside the
resonances, the stress for the outer phases L1, L3 is
higher caused by the constant term.
The amplifications due to resonance require that the
stress in the conductor is always in the elastic range. In
fact, during or near resonance the stress will increase as
far as plasticity is reached. By this the mechanical
frequency of the conductor is shifted to lower values
and the condition for resonance which is only valid in
the elastic range is no longer met. Because the increase
of the stress due to resonance needs a constant
eigenfrequency resonance, resonance phenomena
cannot occur in plastic limit state [Ref 23]. The factor
Vσ cannot become greater than 1. The dynamic
calculation is equivalent to a static calculation with the
maximum values of the electromagnetic force as a static
equivalent force which cause the ultimate load MTr
according to equation (2.27). Lower outer moments give
elastic or elastic-plastic deformations lower than the
ultimate load, but equations (2.28) and (2.29) are always
fulfilled.
Figure 2.2 is calculated for beams fixed at both ends. In
substations, the conductors are mounted on insulators
which are elastic and can oscillate. Thus there are no
fixed supports. For fc > f, the amplifications by
resonance are clearly reduced and also the range in
between [Ref 20, Ref 24]. For the simplified method it
is sufficient to state a curve which lies near the minima
in Figure 2.2. Attention should be payed to the fact, that
the calculated curves are related to the maximum force
at L1, L3 and the standardised one is related to the
maximum force at L2, which differ by about 7 %.
If the verification for the short-circuit strength is done
without consideration of the conductor frequency, a
calculation with the maximum Vσ = 1 can be done, this
value is stated in Table 2 of the standard. The use of
values Vσ < 1 especially with HV-arrangements needs
the knowledge of the conductor frequency, which is
shown in sub-section 2.2.4.
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