The mechanical effects of short-circuit currents in - Montefiore

Figure 6.13 Coefficients for determination of dynamic stress and
deflection. VF for insulator stress, Vσ for conductor
stress, and Vy for conductor deflection.
6.3.4. Thermal stress
Thermal short-circuit strength may be established
according to [Ref 2, Ref 3], where the thermal
equivalent short-circuit current I th is known. The
quantity I th for d.c. systems may be derived with the
help of coefficients mθ1 and mθ2 as given in section
6.3.2.
I
= i m I = i m
(6.24) th1 p θ 1 th2 p θ2
(6.25)
I
th
= i
p
m
t + m
t + t
θ1
1
1
θ2
2
Of course, it is as well possible to assess the thermal
stress without determining the parameters of the
substitute rectangular time function by assuming that
I th = i p. The result will be on the safe side, but may be
very inaccurate.
6.4. CONCLUSION
The time curves of short-circuit currents and of
electromagnetic short-circuit forces in d.c. auxiliary
systems are manifold and include a variety of
parameters. For the characteristic time curves of
current and of force, a standardized function is
2
t
105
introduced, which needs no more than 6 parameters
[Ref 64].
In order to avoid integrating the differential equation
of vibration of every system under investigation, a
rectangular time function is substituted for the time
curve of the standardised electromagnetic force. This
rectangular function approximately causes the same
maximum instantaneous value of mechanical stress as
does the original electromagnetic force, where the
short-circuit duration does not surpass half the
vibration period of the relevant natural oscillation.
After suitable modification, the substitute rectangular
time function may be used even where the shortcircuit
duration surpasses half the vibration period of
the natural oscillation.
The parameters of the rectangular function may easily
be determined with the help of the coefficients mθ, m s
and m Js. The thermal equivalent short-circuit current
I th, which is relevant for the thermal stress, may be
calculated from mθ as well.
The calculation of the mechanical stresses and
deflections starts from the substitute rectangular time
function and proceeds according to the method as
standardized in IEC/EN 60865-1 [Ref 2, Ref 3]; the
coefficients V F, V σ and V y which are required for d.c.
systems are given here.