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Tracking the pros & cons of air and gas substations

Taking into consideration that the cost of a GIS indoor substation is much higher compared to an AIS substation, it’s handy to have a quick reference guide to the pros and cons of both indoor and outdoor substations, writes John Greyling, switchgear specialist and Director of Hevilex, South Africa.

This article first appeared in ESI Africa Edition 2, 2018. You can read the full digital magazine here or subscribe here to receive a print copy.

African utilities and municipalities are faced with increasing demand to manage a reliable electricity supply to a growing consumer market, which motivates the need to upgrade
aging infrastructure and to invest in new assets. The one area calling for attention when refurbishments and new projects are identified is at the substation level. To assist utilities in their choice of substation a few of the advantages and disadvantages of both outdoor and indoor switchyard is discussed below.

In earlier days, for any voltage ratings the outdoor Air Insulated Substations (AIS) were employed and are still being built. Indoor type substations like GIS were only employed in places where high pollution, aggressive saline environment, or heavy population existed.
It is notable that the AIS or outdoor substation holds all the switchgear, busbars, current transformers, voltage transformers, isolators, earth switches and other equipment installed outside, and this is fully exposed to the atmosphere. This obviously poses huge problems to the assets.

All the electrical components inside this equipment are protected by an insulation medium, which is usually oil, SF6 gas, a vacuum, or compressed air. With this gear being exposed to the elements, oxidation, rust, flange corrosion etc. increase the cost of maintenance and equipment degradation. As indicated, the cost of a GIS indoor substation is much higher compared to an AIS substation, but the benefits of a GIS are by far more desirable and include high reliability, less space and far less maintenance.

One of today’s main challenges in power transmission is to bring high voltage levels right into the centres of urban areas. This requires switchgear that features a small footprint,
the utmost reliability, and very low noise and electromagnetic emissions.

AIS advantages

This type of substation arrangement is best suited for those substations where there is ample amount of space available for erecting and commissioning the equipment of the substation.
• The construction work required is comparatively less than the indoor switchyard and the cost of switchgear installation is also much lower.
• In any future extension of the substation erecting and commissioning is much easier.

• The time required for the erection of outdoor AIS is less compared to an indoor  substation.
• All the equipment in AIS switchyard is within view and therefore the fault location is easier and related repairing work is also much easier.
• There is practically no danger of the fault which appears at one point being propagated to another point for the substation installation. Obviously this is due to the equipment being spaced far apart with no appreciable increase in the cost.

AIS disadvantages

• More space is required for an outdoor substation compared to an indoor gas-insulated substation (GIS) of the same specification.
• Outdoor switchyards are more vulnerable to faults, as an outside atmosphere has some influence due to the effects of pollution, saline air and other environmental factors.
• Deposition of saline particles on insulators can cause insulator failures. They are also vulnerable to direct lightning strikes and other external events such as heavy wind, rain and cyclones. Therefore reliability is relatively low compared to indoor substations.

More regular maintenance is required on outdoor substations than on indoor substations. This is obviously due to the fact that AIS equipment is exposed to the outside environment.

GIS advantages

• It occupies much less space (1/10th) compared to ordinary substations. This is probably the overwhelming reason for GIS popularity where the area for the substation is small: residential areas, industrial complexes where uninterrupted power is vital, mountainous regions and valleys, and where the location is underground.
• More reliable than AIS whereby the number of outages for GIS due to faults is far less.
• Almost maintenance free (no equipment is totally maintenance free).

• Can be assembled at the manufacturer’s facilities, and modules can be placed and commissioned at the plant quite easily.

GIS disadvantages

• Cost is much higher compared to ordinary conventional substations.

• No dust particles or moisture can enter into the live compartments, which could result in flashovers.
• When a fault occurs internally, diagnosis of the fault is difficult and then rectifying this would take a long time (outage time is high).
• SF6 gas pressure (SF6 density monitor) must be monitored in each compartment, as reduction in the pressure/density of the SF6 gas in any module results in flashovers and expensive downtime for repairs.

About the author

John Greyling has over 43 years’ experience in the HV/MV electrical industry, initially in
substation construction and later as a noted circuit breaker specialist. As the director
of Hevilex, John shares his expertise through master switchgear courses worldwide and
has contributed to numerous maintenance and monitoring modules.

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