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INTRODUCTION
Following the development of thermal and hydroelectric
power stations, at the beginning of the 30s, high current
transmission lines with screened busducts connecting big
generators with unit transformers began to be installed. The
contemporary solutions consist of transmission lines isolated
with air at atmospheric pressure, with duty-rated voltage values
reaching up to 36 kV and duty-rated current values reaching
up to: 10 kA for hydroelectric power plants, 20 kA for
thermal and nuclear plants whose duty-rated power values
reach up to 900 MW, 31,5 kA for nuclear plants with power
value of 1300 MW [1-3].
Power industry has been using high current transmission
lines with gas insulated phases since the 70s. The gas most
commonly used is SF6 (sulphur hexafluoride) whose pressure
values range from 0,29 to 0,51 MPa (at 20oC) . Recently SF6
has been replaced with a mixture consisting in 95% of N2 and
in 5% of SF6 of 1,3 MPa pressure, corresponding to 0,4 MPa
pressure in the case of pure SF6 [1-6].
Nowadays such transmission lines are built for voltage
values ranging from 72 to 1200 kV but most often for voltage
values ranging from 110 to 750 kV, duty-rated current values
ranging from 1 to 12 kA and duty-rated power values ranging
from 200 to 4000 MVA. The case when the application of gas
insulated high current transmission lines brings the most advantages is when they are used for voltages higher than 245
kV and transmission of power values ranging from 2000 to
4000 MVA. [1-6]