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The battle between insulated substation technologies is not new, says Ahmed Mousa, Manager: Distribution & 69kV Transmission Planning at PSEG, US but the differences between Air Insulated Switchgear (AIS) and Gas Insulated Switchgear (GIS) as well as the elements that make gas more attractive than air, remains a topic of interest.

You are probably familiar with AIS substations; at least you have become accustomed to seeing them near power plants and major highways, or near load centres.

This article originally appeared in Issue 2 2018 of our print magazine. The digital version of the full magazine can be read online or downloaded free of charge.

You are also aware of the importance of switchgear, which houses various types of equipment such as disconnect switches, fuses, circuit breakers, current and potential transformers.

The sad reality is that the public do not comprehend the critical role substations play or why utilities need to build more. They do however comprehend that they need to stand together during community meetings to protest/oppose the proposed substation or transmission lines, mainly due to the risk of their property value dropping and/ or increased utility vehicle traffic around the proposed infrastructure location, and rumors regarding these assets’ negative impact on health.

Utility’s last resort

There are many reasons why utilities have to build substations, whether to replace aged substations, meet load growth, accommodate a large customer, voltage transformation (converting 110kV to 230kV), etc. Also, the higher the available short circuit, the larger the needed breakers, switches, etc. So building a substation is the utility’s last resort due to the cost and complexity associated with building one.

Depending on your geographical location, politics and utility structure (government owned vs. investor owned), building a new substation may represent a major hurdle and require public approval, which is accomplished via a lengthy and complicated process.

In this regard, town boards and key stakeholders are more receptive to GIS proposals due to the aesthetic benefits and the fact that they can fit inside a building where the façade matches the neighbourhood.

Substations mustn’t look like a typical substation. Rendering is not just about looking pleasant; it’s about fitting in. If the substation is planned in a historical location then the substation must have a historical look to it: if the area is modern, then the substation must look modern; if the substation is planned next to Disneyland, then it should look like a fairy palace. This is accomplished with GIS, which can also provide added security against theft and vandalism, and there are no exposed live parts!

Why size matters

Acquiring land for substations is difficult or impossible to obtain in urban areas, thus the need for a modern design. The main reason for adopting GIS configuration is that GIS substations cover a smaller footprint compared to AIS; the reason AIS needs a large footprint is due to the required insulation and clearance factors.

Examining transmission towers, you will realise that the higher the voltage, the more robust the tower will be – that is, taller structures, more insulators, and larger distance between phases.

The American National Standards Institute dictates the minimum allowed distance between phases, between the tower and the individual phases, and between phases and nearby objects, such as buildings and billboards. The same applies to substations where appropriate distance between substation buses is required. In AIS substations, air is used as an insulation medium and thus the need for a larger footprint to accommodate all the incoming and outgoing circuit and equipment: breakers, switches, etc.

An 80kA breaker is much larger than a 40kA breaker. Due to all the aforementioned reasons, the GIS designs were adopted. GIS equipment is typically installed inside a building, whereby most of the equipment is not visible to the public. This makes it ideal for urban areas and will lengthen the life of the equipment since it is protected from the weather.

SF6’s makeup

Switches and breakers are used in all substations; traditionally both were oil filled. Due to environmental concerns, fire hazard and limited capabilities, gas filled equipment/buses, i.e. sulfur hexafluoride (SF6) switches and breakers, are now more common. SF6 is colourless, non-toxic and chemically stable. It has high dielectric strength, high arc interruption capability, and is odourless with a vapour density five times that of air. SF6 is thermally stable, has excellent thermal transfer characteristic, is non-flammable, has unmatched dielectric strength, and is self-healing.

Those were just some of the few SF6 characteristics and why it is widely used nowadays. However, SF6 is a greenhouse gas necessitating SF6 filled equipment being closely monitored for leaks. For this reason, GIS buses have built-in monitors and alarms to alert operators when leaks occur. It is also common to use thermal imaging, i.e. infra-red cameras, to detect leaks.

In addition to switchgear,transformers are now using SF6 as well. Most of the transformers are oil filled, where oil is used as an insulation medium and for cooling the transformers.

Although utilities are constantly adding intelligence and monitoring devices to detect signs of gases developing inside the oil tank, transformer failures do occur and in many cases result in massive fires.

The fact that they are oil filled means that the fire will last for a long time. Gas filled transformers (GIT) remove that hazard as they are not oil filled and will not result in fires during failures. GIT transformers use SF6 as the insulation/ cooling medium instead of oil; there are no conservators or pressure relief valves.

A noteworthy fact is that GIS requires less maintenance, which results in O&M reduction and longer life cycle.

It’s not all sunshine

We have been discussing the advantages so far; however, there are some drawbacks such as cost, the fact that SF6 is an inert gas, repair cost, time to restore after failure (dismantle several components), reliance on vendors for troubleshooting, special training required and variance in design between GIS vendors and difficulty expanding the design. Repair will take longer for GIS than AIS.

Although GIS has only existed for a few decades, the new compact GIS designs use bolted connections resulting in less stress on the insulators, are easier to dismantle, require less SF6 volume, and have lesser leak rates. They even occupy lesser footprint due to reduction in size and weight, and provide operational flexibility and better leak detection. In order to minimise potential leakage, the bus is sectionalised in several sections, known as gas zones, where physical barriers between different zones are positioned. Similar to AIS, several mechanical and dielectric tests (IEC 60517) are conducted.

The ‘battle` between the two technologies will continue, but GIS is here to stay. Look out for it in your neighbourhood, where recent GIS renderings are simply works of art that increase community values, and where some GIS stations have strategically been built underground as well. ESI

This article originally appeared in Issue 2 2018 of our print magazine. The digital version of the full magazine can be read online or downloaded free of charge.

About the author

Ahmed Mousa has over 15 years of field, planning, operations, design, project management, training, DERs, energy efficiency, DR, EVs, VVO, AMI, DMS/ADMS, load relief, load forecast, emergency response, DMS, best practice/benchmarking, risk management and climate change studies experience in generation, transmission, substations and distribution systems.