The deployment of digital substations is rapidly gaining traction in the power distribution and transmission segments. Digital substations integrate real-time data into the system, thereby reducing downtime and enhancing diagnostics. They help avoid the cumbersome process of troubleshooting with timely diagnosis of problems. In the current scenario of growing renewable energy penetration, digital substations play a key role in maintaining grid stability. Apart from this, there is an increasing deployment of gas-insulated switchgear (GIS) substations, primarily due to their compact size. The other key features of GIS include high modularisation, high safety index, lower maintenance requirements, and the ability to resist vibration and avoid electromagnetic pollution.
Digital substations
In modern switchgear, internet of things (IoT) is being leveraged to enable real-time information flow and improve predictive diagnostics, resulting in an energy efficient and reliable grid. Intelligent switchgear can connect with the internet to provide comprehensive monitoring and protection functions. It can also measure all electrical parameters in real time. Smart and automated switchgear has become the need of the hour, especially with the growing penetration of renewable energy sources. To this end, digital substations, which are more compact, reliable and safe than conventional substations, can serve as one of the core elements of a flexible power grid. Such substations enable the collection of real-time data on primary equipment and the conversion of this data into actionable intelligence in order to help utilities monitor, control and maintain assets, as well as achieve cost efficiencies.
Digital substations incorporate intelligent electronic devices (IEDs) with integrated information and communication technology, non-conventional instrument transformers (NCITs), merging units, and phasor measurement units that are interfaced with the process bus and station bus architecture.
For Power Grid Corporation of India Limited (Powergrid), the country’s largest power transmission company, modernising its substations has been a key focus area. Powergrid, in collaboration with Bharat Heavy Electricals Limited (BHEL), successfully commissioned India’s first indigenously developed 400 kV optical current transformer and digital substation components at Powergrid’s 400/220 kV Bhiwadi substation and energised it on September 12, 2020. It comprised a switchgear controller, an NCIT (optical CT), GPS, ethernet switches, a distance-protecting relay, and a capacitor voltage transformer merging unit. This collaborative research and development project was a major step towards the complete digitalisation of the substation as well as towards indigenisation.
There are several advantages of a digital substation. Digital substations completely eliminate the need for switchyard panel rooms, thus significantly reducing the project commissioning time. They also permit remote administration, help in independently regulating voltage through smart transformers, and provide real-time feedback on power supply parameters. Other intangible benefits of digital substations include improved productivity and functionality, greater asset reliability, substation operator safety, and lower cost and space requirements. It has also been observed that the installation of modern equipment to digitalise substations increases system availability while optimising manpower requirements.
Gas-insulated switchgear
GIS is essentially compact and metal encapsulated, consisting of high voltage equipment such as circuit breakers and disconnectors. In GIS, all the components including busbars, circuit breakers, current transformers, potential transformers and other substation equipment are placed inside modules filled with sulphur hexafluoride or SF6 gas.
Traditionally, GIS SF6 is an insulation medium. It maintains atomic and molecular properties even at high voltages, and has superior insulation properties. It also reduces the distance needed between active and non-active switchgear parts, thereby reducing the size of the equipment and making it ideal for urban areas and indoor spaces. The other advantages of GIS are its high safety index and low maintenance requirements. The high capital cost of GIS as compared to that of AIS is an issue, but if land as well as construction and maintenance costs are taken into account, it proves economical. Further, with the increase in voltage, the incremental investment required in GIS is less than that required for regular switchgear. Further, SF6 is a greenhouse gas with a high global warming potential. With the environmental concerns associated with SF6, efforts are under way to replace it with an alternative medium.
Hybrid switchgear
The use of hybrid switchgear is also fast gaining popularity. Hybrid switchgear leverages both AIS and GIS technologies, striking a balance between the cost of land and the facility construction cost. Hybrid switchgear is compact, with the AIS functionality integrated in a gas-insulated enclosure. With hybrid switchgear, the bay length is reduced as the circuit breaker and the disconnector earth switch functions are integrated into one module. As a result, there is an overall reduction in the area required for the substation.
The initial capital cost of hybrid switchgear is about 20 per cent higher than that of AIS and requires a moderate land size (50 per cent of AIS and 50 per cent higher than GIS). Hybrid technology helps optimise space and cost, and can be used for the future extension of existing substations. Besides, the use of SF6 gas for encapsulation has simplified the maintenance of hybrid switchgear and reduced its frequency. The use of SF6 gas also increases the operational reliability of switchgear and makes it safe for use even in very demanding environmental conditions like polluted environments and extreme climates.
Vacuum switchgear
Vacuum switchgear is suitable for medium voltage applications where the arc quenching takes place in a vacuum. It is now emerging as an alternative in HV applications as well due to its various advantages – compact size, higher reliability, lower maintenance and faster restoration. Vacuum switchgear has also seen renewed interest give the focus on reducing the use of SF6.
Vacuum switching, although widely used in the MV range, is emerging as an alternative in high voltage applications. This is primarily due to its high reliability, low maintenance and fast interruption advantages. Given its high dielectric strength, low open gap is a key characteristic of the vacuum switchgear. As such, it is more compact, requires less mechanism energy and is considered more reliable.
This kind of switchgear uses vacuum as the arc quenching medium, as vacuum has the highest insulating strength, and a vacuum switchgear has a much superior arc quenching property than any other medium. Hence, as soon as the arc is produced in a vacuum, it is extinguished owing to the fast recovery of dielectric strength in a vacuum. In recent times, as sensitivity towards environmental degradation has increased, the drive towards a reduction in the use of SF6 gas due to its global warming potential has gained momentum as far as the development of vacuum switchgear for transmission circuits (higher voltages) is concerned. Some of the key advantages that the vacuum switchgear offers are its ability to withstand a much higher rate-of-rise of recovery voltage than SF6 due to its higher dielectric strength and a smaller contact stroke as compared to the corresponding SF6-based switchgear. Moreover, this type of switchgear has a lower moving mass, due to which the mechanism energy is much lower as compared to SF6-based switchgear. The lower mechanism energy makes it more reliable and less prone to damage. As such, it tends to have a longer life and requires less maintenance.
Conclusion
The power grid is undergoing a significant transformation due to the growing penetration of renewable energy sources. Switchgear and substations, which are an integral part of the power transmission and distribution infrastructure, need to keep up with these changes and adopt advanced technologies to make the future grid more resilient and flexible. Given the growing focus on smart grids and smart cities, the adoption of digital substations is expected to increase in the country. The benefits of faster installation, and lower cost and space requirements are expected to find favour with transmission and distribution utilities, especially in light of right-of-way constraints. That said, the deployment of digital substations should be accompanied by solutions to manage data and turn it into actionable insights in order to derive maximum benefits.