The last few years the switchgear market has shown a growing demand on gas insulated switchgear and this growth is prognosed to steadily continue over the next years. To a bigger extent, this is connected to the compactness of GIS and its very low maintenance. But at what costs do these features come? To understand the future of the market and what switchgear features drive better performance, we decided to take a closer look at the advantages and disadvantages of GIS as well as of its older sibling – air insulated switchgear (AIS).
Since the names are not self-explanatory, let's begin by understanding the main difference between the two designs: the core component in a gas-insulated system is usually a hermetically sealed vessel, which contains all the important switching components - like conductors, moving switching connectors, and mechanisms. This container is filled with an insulating gas which prevents any electric flash arcs between the conductors at high voltage levels. To keep the gas inside the container it has to be tightly sealed for lifetime of the switchgear. An air-insulated system solves this problem slightly differently: instead of using a special gas, the conductors are placed at larger distances and the container is only metal clad system which can be left with air holes for improved heat dissipation.
Both of them have their pros and cons, further in the article we present an overview of AIS & GIS based on the following criteria: size, simplicity, environmental impact, sensor integration, reliability.
While for the low voltage level up to 12 kV, the size difference between GIS and AIS is not significant, for higher voltage levels, the difference in size gets to be a defining factor. The dielectric strength of simple atmospheric air is up to 2.6 kV/mm, mostly less when it can be assumed that the air contains moisture or other particles. A specialized insulating gas such as SF6 on the other hand can show a dielectric strength of 8.9 kV/mm or more. This means the space between the conductors can be reduced by more than half when using a GIS approach or even more with pressuriesed insulating gas system. In reality, due to other constrictions and thoughtful design GIS usually show a footprint which is 30 - 40% smaller than an equivalent AIS switchgear.
Size is one of the main factors that leads to a broader adaptation of gas insulated switchgear in the cities, where the space is limited, and the potential of growing population and energy needs over the 30-40 years of switchgear lifetime should be taken into account.
The advantage of an AIS usually lies in its simplicity. The lack of a special insulation medium which needs to be contained, means that the switchgear can be constructed out of simple sheet metal parts as a bolted or riveted construction. Building a pressure vessel, as it is required for a GIS, leads to a number of technical constraints and weak points in the finished product. For safety reasons and to ensure the functionality of the insulation, the air tightness is absolutely essential. In an AIS on the other hand, parts can be highly simplified to optimize the system for costs.
In addition, handling of an AIS system is overall simpler, since no gas handling is involved, and no special personnel or training is required. There are no complicated chemical reactions to be considered when choosing the materials for components in an AIS.
An added advantage of the AIS solution is accessibility to live parts when there is requirement of a maintenance. However, such accessibility comes with disadvantages that live components are exposed to environmental factors such as salts, humidity, dust, insects and insulation degradation.
While an AIS is normally filled with air (and hence does not have a significant climate impact), majority of the GIS are based on SF6 – the most potent greenhouse gas that is 23,500 times stronger than CO2 in terms of its global warming potential. GIS use approximately 80% of all SF6 produced. Although the MV switchgears need to tested to have leakage rate less than only 0,1% per year, they normally do not take into account the leakage level during the lifecycle of switchgear production, decommissioning and transportation. One of the researches on the topic states that up to 80% of SF6 emissions are not reported at all (Levin et al. 2010: 2655). Studies also show that atmospheric concentrations of SF6 have increased by more than a factor of 10 since measurements began in 1973 (Rigby et al, 2010: 10305). This was measured and modelled extensively and independently by several research studies (Weiss and Prinn, 2011: 1934). They all came to the same general conclusion, namely that global SF6 emissions are greatly underestimated by emissions reported to the UNFCCC by developed countries (Weiss and Prinn, 2011: 1934).
Although the environmental impact of SF6-based GIS is large, not so many electricity providers know about it and hence this does not always affect their choice of switchgear.
In the age of digitalization and automation, being able to constantly monitor the conditions of a devise becomes more and more important for the owners. For most switchgear the currently implemented monitoring routine consists of regular check-ups by on-site personnel. Since switchgear overall require relatively rarely any maintenance, these scheduled service visits are unnecessary in many cases and it would be helpful and cost-effective to be able to switch to a predictive maintenance system instead, where technical personnel are only dispatched when actually required. For this, data analysis is necessary, based on real-life condition monitoring of the systems.
Such a monitoring system requires a sensor setup inside the switchgear itself to measure parameters like the system temperature, partial discharge levels or gas quality. The lifetime of a switchgear is usually calculated as 30 years and more, which is beyond the expectable lifetime of complex sensor electronics. For this reason, placing sensor inside a switchgear absolutely requires being able to access them during their lifetime, in case the sensors fail and have to be replaced. Unfortunately, most GIS units are hermetically sealed for lifetime, preventing any access. It is therefore in general easier to equip AIS with monitoring systems, but even in these cases the switchgear should be designed with sensor integration in mind to prevent dielectric interferences when introducing any measuring devices into the system.
By necessity GIS systems are conceived as place-and-forget solutions, which should only require a minimum of maintenance during their lifetime and run undisturbed. Thanks to their fully enclosed design the vital components are protected against environmental influences such as dirt, moisture and rodents. The flip side of this is that repair works inside a GIS are comparatively complicated to realize, since they would have to be evacuated and refilled afterwards. Many designs don't have a dedicated access point to the inside of the pressure container but are welded shut to ensure life-long gas tightness. Such a system cannot be opened but only replaced when it is damaged. In most cases such efforts are economically non-viable leading to exchange of products to new generation.
Looking at the advantages and disadvantages of both AIS and GIS we could predict already now that the future generation of switchgear should have all the advantages of both types – compactness, environmental friendliness, simplicity, reliability, predictive maintenance – without any of the challenges associated with current GIS and AIS. A number of big and small companies are trying to develop SF6-free alternatives to the existing GIS and have already developed solutions for low and medium voltage ranges, what proves that the industry is moving into the SF6-free direction.