GIS Advantage

Efficiency gains drive deployment

As per the Ministry of Power, India’s electricity consumption is likely to increase by almost four times by 2030, from the existing 1,110 billion units (BUs) to around 4,000 BUs. Despite the slowdown in the economy and the roll-out of energy efficient schemes, electricity demand is expected to grow at about 10 per cent annually for the next 15 years. This demand is likely to be led by an increase in the levels of rural and household electrification, the elimination of diesel-based captive generation with improved access and quality of power and an increase in the overall economic activity in the country.

The power distribution segment needs to gear up to meet this future energy requirement. Substation equipment has to be made more efficient and reliable. In addition, urbanisation, increasing real estate prices and the need to accommodate more renewable energy for meeting climate change targets warrant the need to adopt compact, energy efficient and intelligent devices to ensure reliable power supply to consumers. In this regard, gas-insulated switchgear (GIS)-based substations are gaining popularity as the preferred technology in India. The Integrated Power Development Scheme (IPDS), one of the key initiatives being taken by the government for strengthening the sub-transmission and distribution network in urban areas, also envisages the setting up of GIS along with the associated transmission and distribution lines.

Switchgear is a combination of electrical fuses, switches and circuit breakers that are used to control and distribute electric power while protecting against current overload and short circuits. Switchgear is also used to de-energise equipment to allow work to be done and clear faults. At small substations, switches may be manually operated, but at important switching stations all devices operate using remote control. Switchgear can be classified on the basis of their load-bearing capacity (or voltage class), the medium used to interrupt the current, interrupting rating (which is the maximum short circuit current that the device can safely interrupt), construction type, operating method and type of current.

Based on the medium used to interrupt the current, the switchgear can either be a simple open-air isolator switch or it may be insulated by some other materials. GIS is an effective, although costly, form of switchgear, in which all the equipment is contained in a sealed metal enclosure with sulphur hexafluoride gas (SF6) as the insulating medium. Other common types of switchgear are air-insulated switchgear (AIS), oil-

insulated switchgear (OIS) and vacuum-insulated switchgear. OIS is not very popular as it presents an oil-spill hazard. Vacuum switching is more widely used in the medium voltage range and is emerging as an alternative in high voltage applications as well. These are more environment friendly than SF6 switchgear. GIS has several advantages over AIS and other technologies. Some of its advantages are as follows…

Low space requirement

GIS is particularly advantageous in areas where land availability is low and/or land prices are high; for example, in urban centres, mountainous regions with narrow valleys, or in underground power plants. GIS can help in extending substation capacity within the limited space considerably more than AIS. GIS installations take only about one-tenth of the space required for conventional installations. Although the cost of a GIS is higher than that of regular switchgear, in a project, when the total cost includes land coverage and construction, the use of GIS proves to be more economical, especially for high as well as extra high voltage applications.

Protection against environmental processes

The metal enclosure fully protects the GIS from environmental processes such as salt deposits in coastal regions, industrial vapours and precipitates, sandstorms, humidity and high temperatures.

Free choice of installation site

The compact switchgear can be installed as an indoor as well as outdoor solution. The small site area required for GIS saves expensive grading and foundation work. Another advantage is rapid installation and commissioning because of the short erection time, unlike other options like AIS, which involves significant planning and execution time.

Environmental compatibility

The necessity to protect the environment often makes it difficult to install outdoor switchgear of conventional design. GIS, however, can mostly be designed to blend well with the surroundings. It has a flexible design and meets all requirements for configuration related to operating conditions. The use of GIS also prevents electromagnetic pollution because of its closed metal cabinet. In comparison with AIS, the footprint of a typical GIS is ten times smaller.

Low maintenance and operating costs

As compared to other traditional counterparts, GIS requires minimum maintenance and has low operating costs. For instance, AIS requires regular hot line washing, more inventories of spares and more manpower, which GIS does not. GIS substations, in many cases, are completely unmanned and are operated from a distant control room. Other benefits include better reliability, lower weight and ease of equipment handling. It also ensures better safety of the personnel under operating and fault conditions.

These advantages make a case for the adoption of GIS technology. For substations in urban and highly populated load centres where utilities face land availability issues and high real estate prices, the use of GIS has an even stronger case. While the technology has gained popularity among transmission utilities in India, distribution companies are yet to fully adopt it. However, some distribution utilities, such as Tata Power Limited and BSES Yamuna Power Limited (BYPL), have made a beginning.

TPDDL

Tata Power Delhi Distribution Limited (TPDDL) commissioned its first 66 kV GIS substation at Prashant Vihar, Rohini, in July 2015. The new 50 MVA substation (with two transformers of 25 MVA each) has been set up in addition to the two existing substations of 75 MVA each in the area. It has been constructed on a 540 square metre plot, which is only 10 per cent of the area required for a conventional substation of the same capacity, and has been commissioned in a record time of nine months. It will benefit over 50,000 institutional, domestic and commercial consumers in the area. This is an unmanned indoor substation that will be operated from TPDDL’s remote control room.

The company, in September 2013, had installed a 33 kV, GIS-based, 200 MVA substation at Wazirpur as part of its distribution network augmentation programme. The substation, constructed at a cost of Rs 110 million, is benefiting approximately 200,000 customers.

BYPL

BYPL is in the process of implementing two 33/11 kV GIS-based substations in Delhi – at Krishna Nagar (32 MVA) and at Motia Khan (16 MVA). The projects have been approved by the Delhi Electricity Regulatory Commission. The company had invited tenders for the design, engineering/manufacture, supply, installation, testing and commissioning of the two substations. The projects are yet to be started. The estimated cost for the proposed substations is Rs 67.5 million and Rs 30 million respectively.

Conclusion

The higher cost of GIS-based substations as compared to conventional substations remains the biggest constraint in their widespread deployment, especially at the distribution level. GIS also involves risks of excessive damage in case of an internal fault, which may lead to longer outages.

That said, GIS is emerging as a preferred choice for many substation projects around the world owing to its technological and economic advantages over other technologies. Going forward, this technology is expected to gain traction among distribution utilities.

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