The mechanical effects of short-circuit currents in - Montefiore

In Figure 3.5 to Figure 3.8, the maximum tensile
force due to pinch effect is the same magnitude
or lower than the tensile force during swing-out
of the bus. This indicates that the bundle
conductors clash effectively. The ratio λ is
greater than about 25 according to equations
(*43) and (*44) in [Ref 1, Ref 2, Ref 3] which
result from tests e. g. cases 1 and 3. Ratios λ less
than 25 should be avoided, Figure 3.9 to Figure
3.11.
In the case shown in Figure 3.11, common for
overhead lines, the pinch can depend
significantly on the ratios mentioned above. The
transient after the short-circuit ends is typical of
wave propagation along the subspan. This
phenomenon can be observed on the
accompanying video.
Depending on the geometry, similar short-circuit
have very different design loads. A structure will
not behave in a similar way if the load shape is
going from Figure 3.1 to Figure 3.11, even if the
maximum instantaneous peak would be similar
in all cases.
3.2.2. Application of advanced method on high
voltage busbar system
Figure 3.12 shows the complete test arrangement
consisting of two mechanically coupled spans
which is modeled using Finite Element Method
(FE).
The lattice type steel towers and cross arms are
welded constructions of T and L cross-sections.
The mid portal has more stiffness, as it is
movable.
Figure 3.12 FE modelling of the busbar system with force
measuring points K1, K2, K3, K4.
To stabilize this type of construction, the towers
need larger bottom part. Towers are 18m high,
portals 18m wide, cross arms are at ca.10m
height and the spans are 37.35m (South) and
27.40m (North). Phase distance of the conductors
1 x AC SR 537/53 is 1.84m.
The suspensions consist of 8-element chains of
glass or cap- and-pin insulators.
The components of the studied busbar system
represent an assembly. For the FE model, see
Figure 3.12 , this assembly is transformed into a
39
finite number of such finite element types that are
best suited for modelling the structural parts in
question. The discrete elements of the system are
connected by nodes.
For modelling the substation, element types from the
element library of the FE-programm ABAQUS [Ref
33] were used [Ref 40].
The aim of any modelling is to achieve a quality of
the FE model so the deviations between calculation
using the model and the behaviour of the real
arrangement become negligible. To test and verify
the adequacy of the FE model of the portal structures
the following comparative FE calculations were
performed [Ref 49].
Bending stiffness of the component towers and
crossarms, as well as of the complete portals is one
important parameter to characterise the mechanical
properties. The respective FE calculated
characteristic must be in conformity with those of
the real arrangement. The 7 step unidirectional test
cycles of two identical static loads applied at the
conductor suspension points had shown a practically
linear characteristic without hysteresis, so that
stiffness can be expressed in terms of a spring
constant [Ref 34]. Field tests performed by the
members of the WG 121.2 of the German
Electrotechnical Commission DKE on a variety of
portals have come to the same result.
The respective calculations therefore need only one
check for one convenient value of the loads and
under exact validity of Hook's law.
The calculated and the measured spring constants
are graphically compared in Figure 3.13 .The
achieved conformity for South/North portals of
lesser stiffness is excellent and it is very good for the
mid portal.
Figure 3.13 Comparison of calculated and measured spring
constants
For the dynamic studies on the model, the first, the
second, and the third eigenfrequencies should be the
same as those of the real arrangement. From
measurement only the first modes of the crossarms
and of the towers are known. While the calculation
of higher modes is easy, measurement is extremely
complicated for large mechanical structures and may
be, if at all, only available for components. The
conformity of the first modes is a very strong criteria
for good modelling.