Gas insulated substation (GIS) versus Air insulated substation (AIS)

Gas insulated substation (GIS) design

The metal-enclosed gas insulated switchgear> inherently follows the criteria for new substation design and offers a higher reliability and flexibility than other solutions. Due to the gas enclosed design, GIS is the most suitable solution for indoor and underground substations.

In outdoor and hybrid substations, the occupied area is tremendously reduced by using GIS technology!

GIS configurations can be applied to any type of bus bar arrangements:

• Single busbar,
• Double busbar,
• Single busbar with transfer bus,
• Double busbar with double circuit breaker,
• One and a half circuit breaker scheme and
• Ring busbar.

More about switching configurations you can read here. Figure 2 shows the layout of a GIS substation based on single line diagram in Figure 1.

In Figure 2 it may be observed how the compact design of GIS reduces substation area tremendously (at least 70%) compared to the same AIS configuration! This fact allows GIS to become the choice of preference for indoor and underground substation.

For a better appearance, an underground GIS substation can be design with an aesthetic view that hides its presence. The existence of a substation could be designed so that it cannot even be recognized, such as given in example in Figure 2.

GIS performs the same function as AIS. The compact and metal-enclosed design of GIS has prominent advantages and better performance than AIS. However, the high initial investment is a key obstacle in expanding the application of GIS.

In remote or rural area, industrial areas or in developing countries, AIS is still the best choice. In places where the cost of land or cost of earthworks is high or where the sceneries cannot be disturbed by AIS, the solution is to use underground or indoor GIS.

Different type of substations has different advantages which come from its components and design. The characteristics of GIS and AIS are given in the following table.

Design characteristic of GIS and AIS

From the table, the difference in design characteristics may be observed. As may be noted, the GIS designs are supposed to be applied where one or more of the following features are desirable:

• Limited space,
• Extreme environmental conditions,
• Required low environmental impact and
• Less maintenance.

Economics – Life cycle cost (LCC) comparison

Regarding economics, initial capital investmentis not enough to evaluate the overall substation project. Life Cycle Cost (LCC) should be considered, including primary hardware cost, maintenance cost, operation cost, outage cost and disposal costs.

The LCC comparison of AIS and GIS is as follows:

1. Primary hardware

Primary hardware for primary equipment, GIS is more expensive than AIS. However, the price of auxiliary equipment such as support, conductors, land, installation, control, protection and monitoring can lead to a cost difference between the two systems being small.

2. Maintenance

The failure rate of circuit breaker and disconnecting switch in GIS is one-fourth of that of AIS and one tenth in case of busbar, thus the maintenance cost of GIS is less than that of AIS over the lifetime.

3. Operation cost

The maintenance cost of GIS and AIS shall be equivalent. The cost for training in GIS is higher than in AIS.

4. Outage cost

Since the failure rate of GIS is lower, the outage cost of AIS shall be greater.

5. Disposal cost

The cost of decommissioning and disposal after use should be capitalized. The value of future expense must be taken into account.

Figure 4 shows the design of AIS and GIS solutions.

Table 2 – LCC evaluation of AIS and GIS (based on calculation conducted by a utility)

The latest GIS technology has less environmental impact than previous technology. The SF6 leakage rate is less than 1% (in experiment <0.5%). Due the design characteristics, GIS has a better impact on environment than AIS.

Table 3 – Environment impact comparison of AIS and GIS

As observed, GIS offers many prominent advantages over AIS. It meets all requirements for new substation design, except high initial investment and potential environmental risk.

This disadvantage and the ever increasing environmental awareness become the drivers for a new generation of GIS that complies with future green field substation criteria.

Substation GIS Overview (VIDEO)

Reference // The 21st Century Substation Design by Mladen Kezunovic, Ph.D., P.E. (Power Systems Engineering Research Center)