Increased economic development and burgeoning population levels over the years have created power transmission challenges in urban centres. In such a scenario, transmission utilities need to take steps such as design modifications of substation equipment to improve their physical infrastructure and cope with higher voltages and larger capacity ratings. Moreover, with the increased cost of land being a major constraint, new designs that economise space requirements are becoming a key priority. In this context, the deployment of gas insulated switchgear (GIS)-based substations is being increasingly seen as a viable solution by transmission utilities.
A GIS substation is a compact, multi-component assembly enclosed in a ground metallic housing with compressed sulphur hexafluoride (SF6) as the primary insulating medium. The main components of a GIS substation are bus bars, circuit breakers, disconnecting switches, earthing switches, voltage transformers, current transformers, feeder disconnect or feeder earthing switches, and lightning or surge arresters.
SF6 is an inert, non-toxic, colourless, odourless, tasteless and non-flammable gas, about five times as dense as air. SF6 is used in GIS substations at moderate pressures of 400-600 kilopascals absolute for phase-to-phase and phase-to-ground insulation. The pressure is chosen so that the gas does not condense into a liquid at the lowest temperatures experienced by the equipment. SF6 has two to three times the insulating ability of air at the same pressure and is about hundred times better than air for interrupting arcs. It is universally used as an interrupting medium for high voltage circuit breakers, replacing the older mediums of oil and air. Moreover, SF6 decomposes at the high temperature of an electric arc, but the decomposed gas recombines back into SF6 so well that it is not necessary to replenish the SF6 in the substation; hence, the gas is recycled within the system.
Key demand drivers
Land acquisition is the primary challenge being faced by transmission utilities. Procuring right of way (RoW) is difficult and leads to time overruns, which, in turn, results in cost escalations. As such, utilities need to make the best use of the already available land resources. GIS substations could help resolve this “space problem”. The deployment of GIS substations is ideal in dense urban areas where land for new facilities is limited and available only at prohibitive costs. The use of SF6 gas in GIS substations reduces the distance needed between active and non-active switchgear parts, resulting in smaller overall space requirements. GIS substations take about one-tenth of the space required for conventional installations. The use of GIS substations proves to be more economical, including the cost of land coverage and construction, particularly for high and extra high voltage applications.
As compared to other types of substations, GIS substations provide better protection against environmental processes such as salt deposits in coastal regions, industrial vapours and precipitates, sandstorms, humidity, and high temperatures. Moreover, these can be installed as indoor as well as outdoor solutions and have a short erection time.
Further, GIS substations require minimum maintenance and have low operating costs. For instance, air-insulated switchgear (AIS) substations require regular hotline washing, more inventories of spares, and significant manpower. Such maintenance practices are not needed in GIS substations. In certain situations, these substations are completely unmanned and are operated remotely from a control room.
GIS equipment also incorporates safety and reliability features. Safety is enabled by optimising operating electrical stresses to safe levels through appropriate spacing between electrodes. Meanwhile, the increase in gas volumes and thermal inertia in the system ensure the safety of the substation as they provide better cooling and help maintain insulation strength. To address reliability issues, improved contact systems are used for circuit breakers and disconnectors. Multi-contact and friction-free surfaces are used for long operating cycles. The design of GIS is defined by electric field strength and thermal limits. For medium and high voltage GIS equipment, the minimum dimensions are usually set by the thermal limits to carry the rated current.
Some of the other advantages of GIS substations are better reliability, lower weight, ease of equipment handling, and better safety of personnel under operating and fault conditions.
Although GIS substations have been in operation for quite some time and continue to gain popularity, there are certain issues that are faced during their operation. Switching operations in GIS substations generate very fast transient overvoltages (VFTOs). VFTOs are generated primarily due to two reasons – disconnector switch operations and faults between busbars and enclosures. These VFTOs give rise to many problems such as flashover to ground at the disconnector switch contacts; failure of the electronic control circuits connected to the GIS due to the electromagnetic interference of VFTOs; reduction in dielectric strength, if a non-uniform electric field is formed by the particles, particularly metallic; impact on bushing, power transformers and instrument transformers; and transient enclosure voltage on the external surface of the sheath, which may cause flashovers to nearby grounded objects. As such, while designing the insulation for the substation, VFTOs need to be considered as an important factor.
In addition, there could be problems pertaining to potential insulation coordination. The compact nature of these substations when combined with short sections of the cable may complicate insulation coordination. Moreover, field non-uniformities reduce the withstanding levels of GIS-based substations and prolonged arcing may generate corrosive and toxic by-products.
Further, as sensitivity towards the environment has increased in recent times, GIS-based substations have been denounced, as SF6 has been identified as a gas with the highest global warming potential and a long atmospheric life.
Deployment in India
GIS deployment has gained traction in India owing to the twin challenges relating to land availability and increasing population levels. Power Grid Corporation of India Limited (Powergrid) has contributed significantly to the construction of GIS substations in the country. As of April 2016, Powergrid owned 24 GIS substations, of which 75 per cent (18) were at the 400 kV level, 17 per cent (4) were at the 765 kV level, and the remaining were at the 220 kV level.
At the state level too, various transmission utilities are increasingly deploying GIS substations. Delhi Transco Limited, for instance, is now prioritising GIS-based substations over AIS-based substations. The utility’s transmission infrastructure comprises four 400 kV substations and 33 220 kV substations, of which one 400 kV and nine 220 kV substations are GIS based. Meanwhile, Maharashtra State Transmission Company Limited has established GIS substations at Bhandup, Pune, and is in the process of commissioning a 400 kV GIS substation at Hinjenwadi, Pune, to cater to the needs of the city’s fast growing IT industry. The transmission utilities of Gujarat, Tamil Nadu and Andhra Pradesh have also undertaken the development of GIS substations.
The transmission utilities of the states of Uttarakhand, Himachal Pradesh and the Northeast have also commissioned GIS substations to overcome the space challenges posed by the hilly terrain. Some of the GIS substations deployed in these regions are the 220 kV Mori, Nogaon, Ghansali, Madkot and Dharchulla substations in Uttarakhand; and the Karian and Phojal substations in Himachal Pradesh. The first substation in the Northeast was established in February 2014. It is operated by Assam Electricity Grid Corporation Limited and serves the newly developed areas of Assam.
Going forward, Powergrid as well as state transmission utilities have several GIS projects in the pipeline. However, proper planning and layout engineering need to be undertaken to address the issues relating to insulation coordination and VFTOs that can arise during the operation of such substations.
Although GIS substations have been in commercial operation for about three decades, there is still immense scope for transmission utilities to achieve significant savings in area and volume by undertaking continuous design and technological improvements of substation components.