The mechanical effects of short-circuit currents in - Montefiore

Figure 1.7 a) 245 kV circuit breaker
b) first modal shape (3,4 Hz)
c) second modal shape (20,3 Hz)
Figure 1.8 a) 245 kV Pantograph disconnector
b) first modal shape (3,8 Hz)
c) second modal shape (26,5 Hz)
a) b)
Figure 1.9 550 kV current transformer
a) Eigenfrequency 1,840 Hz
b)Eigenfrequency 7,603 Hz
8
1.3.3. Rough evaluation of the basic frequency of
insulators apparatus and supporting
structures.
1.3.3.1 Insulator support and apparatus
We can try to make a rough estimation of basic
frequencies for typical apparatus: In the absence of
data, Figure 1.12 can be used for a first rough
estimate of any structures in substations. The
middle dotted straight line can be used for
supporting insulators, the upper straight line is
upper border for insulator support and the lower
straight line is the bottom border for insulator
support (mainly composite). This can also be used
for any apparatus (current transformer, circuit
breaker, isolator, surge arrester). The height
includes supporting structure.
The figure gives access to the main first
eigenfrequency, which will be used in this brochure
to evaluate the Equivalent Static Load (see further).
The horizontal shaded areas highlight the resonance
risk areas for a 50 Hz network. There are three
zones corresponding to the cases of resonance on
the first eigenmode and the second eigenmode in
relation with either 50 or 100 Hz component of
excitation frequencies. There is evidence that 400
kV range is free of resonance problems and that
123 kV up to 245 kV is certainly more prone to
such problems.
It is not just a coincidence that all these different
kind of supports have similar eigenfrequencies with
only one main parameter: the height (means the
voltage). You have to remember that the
eigenfrequency of a clamped-free beam is inversely
proportional to the square of its length and
proportional only to the square root of its young’s
modulus (more or less the same for all apparatus,
except for fiber glass) and of its moment of inertia
(means mainly its outside diameter, also very
similar for all cases).
Table 1.1gives the range of basic data for typical
substation structures like post insulators and
gantries.
Rated Voltage/ nominal voltage (kV)
123/ 110 245/ 220 420/ 380
Post insulators
Height m 1..1,7 2,1..2,5 3,15..4,5
Mass kg 35..200 120..400 250..650
Cantilever
strength
kN 4..16 8..16 8..16
Stiffness N/mm 300..2500 40..200 40..140
First natural
frequency
Hz 20..60 7..14 4..12
Damping
log decr.
% 2 2 2
Supporting structures for flexible bus (gantries)
Height m 5..6 8..10 12..15
Stiffness N/mm 150..1300 500..800 600..3500
First natural
frequency
Hz 3..6 3..5 1..4
Table 1.1 Basic data for typical substation structures