THE ADVANTAGES OF GAS INSULATED TRANSMISSION LINES BASIC INFORMATION AND TUTORIALS

GIL offers several advantages for high capacity power transmission, as listed below.

Low Transmission Losses 
Resistive losses are low because of the large cross-section of the conductor and enclosure pipes. Typical GIL resistances are 6–8 m /km depending on the outer diameter (500 mm or 600 mm) and the wall thickness of the enclosure and conductor pipe (6 mm to 15 mm). 

The transmission losses are related to the square of the transmitted current as Pv = I2 · R (I = current, R = resistance). When the current rating is high – as it is for GIL (e.g., 3000 A) – then the effect of low transmission losses is high. The losses through the insulating gas are negligibly small.

Low Capacitive Load 
Electric phase-angle compensation is only needed at very long lengths, because the capacity of the GIL is low, typically 55 μF/km. Therefore, no or low compensation coils are needed under most network conditions for transmission lengths of about 100 km. This also reduces the thermal operation losses.

Power Rating Like an Overhead Line 
The GIL is the ideal alternative or supplement to overhead lines. The high power transmission capability of the GIL (up to 3000 MVA per system at 550 kV rated voltage) allows it to go directly underground in series with an overhead line without power reduction. 

The GIL also allows the use of protection and control systems in the same way as with overhead lines. No differential protection is needed for failure location when a GIL is combined with overhead lines. The GIL has a low capacitance and, therefore, the inrush current is low.

High Level of Personnel Safety 
The outer enclosure pipe is solid grounded and no access to high-voltage parts is possible (gas-tight enclosure). Personnel safety is also guaranteed in case the GIL has to carry a short-circuit current (50, 63 or 80 kA up to 1 or 3 s). Even in case of internal failure and an arc between the enclosure and conductor pipes, tests have shown that no external impact occurs on the surroundings.

High Reliability 
The only purpose of the GIL is electric power transmission. No internal switching or breaking capability is needed. Based on this, the GIL can be seen as a passive high-voltage gas-insulated system with no active moving parts (e.g., switches). 

Today, more than 300 km of single-phase lengths has been in operation world-wide for more than 35 years. So far, no major failure (arc fault in the system) has been reported. This makes the GIL the most reliable power transmission system known.

No Electric Ageing 
Gas insulations do not age. The best example is an overhead line with ambient air as insulating gas. The electric field strength of the insulators and the maximum temperature of the GIL are too low to start the process of electrical or thermal ageing. 

This has been proven using long-term measurements in independent laboratories and also by extensive experience with the equipment in the network. The first GIL installations have been in operation since 1974, and the results are reported by the CIGRE [71, 224].

Operation Like an Overhead Line 
Overhead lines in the transmission network are operated with the so-called autoreclosure function. This means that in case of a ground fault detected on the line, the circuit breaker will automatically break the lines, wait some seconds (depending on the network condition) and then switch on again. 

In most cases the reason for the fault current detection will be gone and the transmission line will go back to normal operation (for example, if a tree branch gets too close to an overhead line, the branch will be burned away or if a lightning strike causes the fault current, that will also be gone after some seconds).

Electromagnetic Fields 
To protect the public and the operational personnel international regulations require electromagnetic field limitations. These values vary across regions and countries depending on laws and regional regulations. A trend can be seen worldwide that limiting values are getting lower and the restrictions harder. In densely populated areas and cities these electromagnetic field requirements are defining the allowed design of transmission lines.

The GIL is operated as a solid grounded installation and the inductive loop is closed through the ground connection. The coupling factor is about 95%. This means that the superposition of the two reverse currents reduces the outside magnetic field by 95%, and only 5% of the magnetic field of the conductor current is effective outside the GIL.

Because of the induction law, the current in the conductor will induce a current in the enclosure of the same size and with 180◦ phase shift. The superposition of both electromagnetic fields is close to zero. In case of limitation of the magnetic field in the surroundings, this solid grounded GIL can fulfil even very low magnetic field requirements. With a current rating of 3000 A, within a few metres’ distance a magnetic field strength of 1 μT can be reached (as required in some countries).

The advantage of a low magnetic field is important when residential areas are close to the transmission line for airports with their sensitive instruments, hospitals with their sensitive imagining systems, or all kinds of sensitive electronic equipment in private or business use. In Italy, electromagnetic field requirements for new installations go down to magnetic flux values of only 0.2 μT. When residential areas are involved, the GIL can reach such low values over a distance of a few metres.

No Thermal Ageing 
The GIL is designed for maximum operational temperatures given by the surrounding conditions – maximum 60 or 70◦C touching temperature in a tunnel, or 40 or 50◦C when directly buried. The different temperature values depend on individual countries and their applied standards and regulations.

In all cases the maximum allowed temperature of the conductor of 100 to 120◦C is not reached by far. Therefore, no practical ageing of the system can be expected under these operating conditions.

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