23-05-2012, 11:17 AM
GAS-INSULATED TRANSMISSION LINES - UNDERGROUND POWER
TRANSMISSION ACHIEVING A MAXIMUM OF OPERATIONAL SAFETY
AND RELIABILITY
[attachment=22810]
INTRODUCTION
Gas Insulated Transmission Lines (GIL) are a means of bulk
electric power transmission at high and extra high voltage.
GIL consists of tubular aluminium conductors encased in a
metallic tube that is filled with a mixture of Nitrogen and
Sulphur Hexafluoride gases for electrical insulation. Since
the first installation of GIL in 1975, second generation GIL
has been developed which is more economically viable and
its design was optimized both for installation and operation.
Where GIL is installed in combination with Gas Insulated
Switchgear (GIS), compact solutions can be delivered. As
such GIL can contribute in the mitigation of power flow
problems, reduce the risks of failure of electrical
transmission systems and enable the installation of optimum
solutions regarding technical, economical and environmental
aspects.
OPERATION-RELATED ADVANTAGES OF GIL
Energy transmission in pumped-storage power plants has
been facilitated in the past mainly using high voltage cables
in order to avoid complex OHL outdoor installations. When it
comes to reliability it is worth mentioning that for 35 years
several GIL installations in the voltage range of up to 500 kV
have been in service. So far, no single fault has occurred in
service due to failure of the insulating gas, nor of the epoxy
resin insulators. To date, experience with GIL in service with
Siemens amounts to approximately 35 km of phase length.
No ageing of gas insulation is evident in GIL.
Electromagnetic compatibility
Low electromagnetic field radiation was envisaged in order
to allow service personnel to enter the gallery under full load
conditions without time limitation. The value of inductance
with GIL amounts to <5 μT in
0.5 m of distance under full
load. The comparable value
with XLPE cables is
significantly higher.
Fully encapsulated design
The design of the GIL provides a 100% metallic, grounded
encapsulation. Due to this, the electrical field strength
outside the GIL will be zero at any point and at any time. As
such there will be no limitations imposed during service even
in case of staff being present in the gallery. This fact is
important for the use of the GIL gallery for access and
emergency exit purposes.
In the event of cable failure, the installation of repair joints
would not be possible due to the small dimensions of the
gallery. A 4th reserve phase would have to be laid. With GIL,
only 3 phases have to be installed in the gallery, keeping the
size of the gallery small.
Commissioning test
The HV commissioning test can be performed together with
the GIS test procedure. The relatively compact test
equipment can be transported through the gallery.
Environmental and operational aspects
“Global warming friendly design” is ensured through total
gastight welding of all tubes. No servicing is envisaged over
lifetime; no refilling of gas mixture is necessary. The leakage
rate lies one order of magnitude below the GIS standard of
0.5 % p.a. /1/. Moreover the GIL tubes are filled with a gas
mixture of 80% N2 and 20% SF6 in order to keep the
amount of SF6 low. This is feasible due to the electro
negativity of SF6 comprising high electric breakdown
strength even at low concentration of SF6.
TRANSMISSION ACHIEVING A MAXIMUM OF OPERATIONAL SAFETY
AND RELIABILITY
[attachment=22810]
INTRODUCTION
Gas Insulated Transmission Lines (GIL) are a means of bulk
electric power transmission at high and extra high voltage.
GIL consists of tubular aluminium conductors encased in a
metallic tube that is filled with a mixture of Nitrogen and
Sulphur Hexafluoride gases for electrical insulation. Since
the first installation of GIL in 1975, second generation GIL
has been developed which is more economically viable and
its design was optimized both for installation and operation.
Where GIL is installed in combination with Gas Insulated
Switchgear (GIS), compact solutions can be delivered. As
such GIL can contribute in the mitigation of power flow
problems, reduce the risks of failure of electrical
transmission systems and enable the installation of optimum
solutions regarding technical, economical and environmental
aspects.
OPERATION-RELATED ADVANTAGES OF GIL
Energy transmission in pumped-storage power plants has
been facilitated in the past mainly using high voltage cables
in order to avoid complex OHL outdoor installations. When it
comes to reliability it is worth mentioning that for 35 years
several GIL installations in the voltage range of up to 500 kV
have been in service. So far, no single fault has occurred in
service due to failure of the insulating gas, nor of the epoxy
resin insulators. To date, experience with GIL in service with
Siemens amounts to approximately 35 km of phase length.
No ageing of gas insulation is evident in GIL.
Electromagnetic compatibility
Low electromagnetic field radiation was envisaged in order
to allow service personnel to enter the gallery under full load
conditions without time limitation. The value of inductance
with GIL amounts to <5 μT in
0.5 m of distance under full
load. The comparable value
with XLPE cables is
significantly higher.
Fully encapsulated design
The design of the GIL provides a 100% metallic, grounded
encapsulation. Due to this, the electrical field strength
outside the GIL will be zero at any point and at any time. As
such there will be no limitations imposed during service even
in case of staff being present in the gallery. This fact is
important for the use of the GIL gallery for access and
emergency exit purposes.
In the event of cable failure, the installation of repair joints
would not be possible due to the small dimensions of the
gallery. A 4th reserve phase would have to be laid. With GIL,
only 3 phases have to be installed in the gallery, keeping the
size of the gallery small.
Commissioning test
The HV commissioning test can be performed together with
the GIS test procedure. The relatively compact test
equipment can be transported through the gallery.
Environmental and operational aspects
“Global warming friendly design” is ensured through total
gastight welding of all tubes. No servicing is envisaged over
lifetime; no refilling of gas mixture is necessary. The leakage
rate lies one order of magnitude below the GIS standard of
0.5 % p.a. /1/. Moreover the GIL tubes are filled with a gas
mixture of 80% N2 and 20% SF6 in order to keep the
amount of SF6 low. This is feasible due to the electro
negativity of SF6 comprising high electric breakdown
strength even at low concentration of SF6.