Full resolution (JPEG) - On this page / på denna sida - 1958, H. 6 - Up-to-date Criteria in the Construction of Equipment for High Voltage Power Systems According to the Experience of the 400 kV System in Sweden, by Gunnar Jancke
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Fig. 2. Variation of reactance and permissible operating
voltage with number of subconductors at constant
total cross-section, about 1 200 mm’.
intermediate switching station equipped with 400 kV
shunt reactors in order to reduce the operating
frequency voltage rises.
At the time the first 400 kV lines were planned it
was unreasonable from an economic point of view
to demand complete operating reliability even in
the event of the worst faults that might occur on the
system. When determining the point of time at which
new lines should be installed, therefore, a special
cost item was introduced instead to cover the
probable number of total breakdowns in the main
power system to which such faults might be assumed
to give rise, on the basis of earlier operating
experience and of theoretical calculations. The costs for
disturbances in operation were estimated from a
national-economic viewpoint with respect to
reduced production and destruction of material (in
certain industrial processes) and other inconveniences
to the country which a breakdown in the supply
might entail. This estimate showed a cost which at
the present rate of exhange amounts to about 20 US
cents per kW disconnected and 40 cents per kWh
disconnected. The design of the system obtained in
this way has in fact given such an adequate margin
of stability that as yet only one major disturbance
has occurred, caused by personal error.
With an increase in the number of 400 kV lines
operating in parallel, the costs for achieving full
operating reliability have been reduced while at the
same time the extent of a serious breakdown has
increased. On this account the network is now built
in such a way that it is stable for every fault
location. Individual power stations are connected up
through single lines to the main network, however,
since the short-time disconnection of limited
generation does not entail a breakdown of a serious
nature. Hot-line maintenance work and single-pole
rec-losure will be introduced.
Choice of Insulation Level
It was obvious from the outset that only direct
grounding of all transformer neutrals could be
considered for the 400 kV network. With this
arrangement large transformer units could be built within
the scope of the available possibilities for transport.
Furthermore, according to the plans, the network
would rapidly increase to such an extent that
arc-suppression-coil grounding would be useless in
view of the active residual currents. According to the
experience from the Swedish 230 kV network and
several networks for 138 and 230 kV in other
countries the capacity of the arc-suppression coils to
eliminate single-phase faults without causing the
circuit-breakers to function disappears when
metallically connected line lengths exceeding about 2 500
km are in question, owing to the fact that too high
active residual currents are obtained from various
forms of losses.
In the choice of the insulation level due
consideration has to be given to internal overvoltages. These
are composed of the voltage rise on disconnecting a
loaded line, the voltage rise in sound phases on
ground faults, the switching surges from
circuit-breakers and certain transient phenomena on
switching very long lines. These factors give such
a high insulation level at 138 kV and higher
voltages that a further increase with a view to lightning
overvoltages has not been found justifiable for
Swedish conditions. A study of the distribution curve
for the latter shows that only a small number of
lightning strokes can give rise to higher voltages and
that the elimination of the risk of faults from such
causes would necessitate an appreciably higher
insulation, fig. 3. This is not economically feasible. In
place of it we allow for a certain number of line
//
Fig. 3. Approximate number of lightning overvoltages
leading to flash-overs at the insulation with a varying
impulse withstand voltage for the insulation. The
curves apply to a lightning frequency of 10
thunderstorm days per year.
ELTEKNIK 1958 1 1 9
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