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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A carrier current for telephony, recording measured
values and relay protection is transmitted over the
lines. In unfavourable weather the damping is too
great on the long line sections, and a reinforcement
is therefore arranged in the series capacitor stations.
The Stations
In an extra-high-voltage network every line has a
large transmitting capacity and it naturally follows
that transformer groups of the same order of
magnitude are selected. The operating reliability of a
high-voltage transformer is very great, so that no obstacles
are encountered in this respect. The cost per kVA
will be less, the larger the transformers, and costs
for switchgear will decrease with a reduction in
the number of units.
Sweden is an outstanding water-power country in
which the individual power stations are of moderate
dimensions. When the output of a station exceeds
300 MW direct transformation to 400 kV is found
most remunerative. For this purpose a common
transformer group is installed for all the generators.
The largest group of this kind will be put into
operation 1958 for a station with 4 x 150 MVA
generators. In the case of smaller stations the output
from a number of stations in one district, such as
a river valley, is collected in a 138 kV or 230 kV
local system and fed into the 400 kV network through
auto-connected transformers. These transformers are
installed in one of the district’s largest stations and
the generators in the latter are connected to the
delta-connected tertiary of the transformer group.
Auto-connected transformers for 400/138 kV are
used exclusively in the receiving stations or in
exceptional cases 400/230 kV. The sizes are determined
by available transport facilities. A transformer group
for 900 MVA is now on order. The synchronous
machinery and shunt reactors are connected to a
delta-connected tertiary winding.
The transformers consist of single-phase units. To
begin with, a set consisting of three units with a
fourth in reserve is being installed. In a receiving
station it is far more difficult to make arrangements
for a disconnection than is the case in a power
station, and consequently in these stations the
connections have been chosen in such a way that a unit
can be changed on-load by means of circuit-breakers
and isolating switches. As the need for
transformation grows, the equipment is increased to seven
units, whereupon it is usually possible to change
a unit with the help of isolating switches during
the disconnection of one set, lasting for
approximately 20 minutes. When a further set has finally
been installed it is normally possible to shift a unit
with an interruption of relatively long duration.
In the power stations, the turbines and generators
are usually installed underground for economic
reasons. Transformers in the power stations and
receiving stations are placed in chambers blasted out
of the röck for other reasons. In this case 400 kV
cables are employed which are led directly into
the transformers.
Lightning arresters are installed in every station and
are directly connected to the transformers or the
upper ends of the cables.
The 400 kV voltage transformers are designed in
the form of capacitive voltage dividers combined
with an inductive voltage transformer for about 16
kV. Their accuracy is 0.5 % at 150 VA burden. With
400 VA their accuracy is 3 %. They are connected
to the lines and are used simultaneously as coupling
elements for the carrier current on the line.
Sepa-rately mounted current transformers are used for
the lines, whereas the transformers are provided
with current transformers around the bushings or
the potheads of the cables.
The stations are now designed for an insulation
level of 1 425 kV. For those parts of the installation
which may be connected directly to a long" power
line, however, such as circuit breakers and
instrument transformers, a higher level of 1 550 kV is
selected with a view to the voltage rise along a line
connected at one end.
The switchgear is provided with a main busbar
and a transfer bus. The circuit-breaker of the
transfer bus constitutes a common standby for the other
400 kV breakers in the station. In some cases
breakers are not installed for short lines and the standby
breaker is then employed for these when it is not
needed for other connections. The symmetrical short
circuit power in the 400 kV network will be about
18 000 MVA in the ultimate stage, which does not
mean any appreciable difficulties.
Summary of 400 kV Experience
Sweden’s first sections of a superimposed 400 kV
system were placed in service in the spring of 1952.
This network now consists of 3 100 km of lines and
7 600 MVA of transformers.
Experience gained with the network is entirely
satisfactory. Transmission costs are about 40 %
lower than at 230 kV. The number of lightning faults
hitherto has been 0.14 per 100 km per year, which
agrees closely with the preliminary calculations.
Experience in connection with snow and ice loading
on the bundled conductors has been favourable.
Radio interference from the lines has been
inconsiderable. One serious breakdown has occurred in
the form of a falling-out-of-pliase due to faulty
manipulation on the part of the staff. A single-phase
transformer has been damaged in connection with
Fig. 9. Voltage distribution on a 20 unit suspension
insulator string. The unit numbers are counted from
cross-arm to conductors.
El TEKNIK 1958 () ]
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