The differences between air insulated substation engineering and gas insulated substation engineering are vast. There are five major considerations in the differences between the two high voltage substations: their construction, installation, operation, ongoing maintenance and total cost of ownership.
We’ll look comparatively at each of those five substation differences below.
GIS high voltage substation engineering uses the gas sulfur hexafluoride for insulation, whereas AIS uses air insulation in a metal-clad system. Sulfur hexafluoride is five times heavier than air and offers excellent extinction behavior. Another significant construction-based difference between the two switchyards is that a metal-clad AIS uses three-position draw-out circuit breakers (on, off and test). GIS systems use fixed, mounted circuit breakers. The sealed mounted breakers are a ‘sealed-for-life’ technology, whereas the AIS breaker can be removed for maintenance and troubleshooting.
Installing a GIS high voltage substation can be faster than installing its metal-clad AIS counterpart. This is largely due to the fact that GIS systems are significantly smaller and weigh less (despite the gas weighing more than air). Provided the technician doesn’t need to handle the gas itself, GIS substations are quicker to install. AIS systems need busbar connections and boots on the switchgear. The average installation time is reduced by approximately 30% with a GIS installation (when no gas-handling is required). GIS systems also take up less space than AIS. The physical footprint of a GIS is about 35% less than AIS.
GIS systems are significantly easier to care for on a regular basis as they offer front instead of rear access. They also contain their own integrated testing instruments.
Arc flashes are rare in GIS because all the interior elements are insulated, with only the cable compartment being accessible. As the parts are fully insulated, no cables or linkage can come in contact with the live parts.
As discussed in point one, GIS switchyards require significantly less maintenance. On average, GIS systems need only be visually inspected every four years or more, depending on the specific manufacturer recommendation for your device. GIS drives only need to be re-greased after about 20 years (again, this varies between manufacturers).
AIS systems should be visually inspected every year to two years (again, based on the specific manufacturer recommendation). When inspected, all compartments must be checked, unlike GIS where the individual compartments and elements are fully insulated and can be monitored. On average, an AIS breaker will need about four hours of maintenance for each two-year period.
In terms of maintenance, AIS systems require significantly more effort, especially due to the thoroughness of the inspections. Inspections include having a technician torque, draw-out, clean, lubricate and vacuum the unit. They also need to be checked for visual signs of copper corrosion, something that doesn’t occur in GIS due to the units being sealed. That sealing also protects the components from environmental damage.
Total Cost of Ownership
On a high-level comparison of AIS and GIS switchgear, you need to weigh the upfront costs against the required lifelong maintenance. These units generally have a 30-year minimum life span. GIS units tend to cost between 10% to 40% more than AIS.
In summary, AIS systems offer upfront cost savings, however they require more man-hours over the long run. GIS high voltage substations’ advanced electrical engineering requires a larger upfront investment, but their sealed technology means lower installation and maintenance costs.