The different kinds of switchgear can be classified on the basis of their load bearing capacity (or voltage class), the medium used to interrupt the current, the interrupting rating (which is the maximum short circuit current that the device can safely interrupt), construction type, operating method and type of current.
Switchgear is classified as low voltage — these have ratings below 1 kV and include air circuit breakers, moulded case circuit breakers, miniature circuit breakers, residual current devices, contractors and relays; medium voltage — these have ratings between 1 kV and 75 kV and include air-insulated switchgear (AIS), vacuum switchgear, gas-insulated switchgear (GIS); and high voltage – these have ratings above 75 kV and include AIS and GIS. In addition, based on the medium used to interrupt the current, a switchgear is classified as either a simple open-air isolator switch or it may be insulated by some other material like oil and vacuum.
Further, based on interrupting rating, a switchgear can be either circuit breakers, which can open and close on fault currents; or load-break/load-make switches that can switch normal system load currents; or isolators, which may only be operated while the circuit is dead or the load current is very small. As per the construction type, switchgear is classified as indoor, outdoor, industrial, live-front, dead-front, open, metal-enclosed, metal-clad, arc-resistant, etc. With regard to the method of operation, a switchgear is either manually operated, or motor/stored energy operated, or solenoid operated. Meanwhile, depending on the type of current, it operates either on alternating current (AC) or direct current (DC).
A look at some of the key technology trends that have emerged in the switchgear industry in recent years…
Ultra-high voltage switchgear
Worldwide in the transmission segment, utilities are moving towards ultra high voltages (UHV) and such developments necessitate the use of high voltage switchgear of corresponding ratings. For instance, China has started installing 1,100 kV AC and 800 kV DC systems. Meanwhile, in September 2015, Power Grid Corporation of India Limited commissioned the Biswanath Chariali-Agra line, which is India’s first +800 kV high voltage direct current (HVDC) link.
The switchgear industry’s growth so far has been due to the strong demand for low and medium voltage switchgear from distribution and low voltage transmission consumers. In fact, in these segments, the market has a large number of domestic suppliers that compete with global majors on prices. Going forward, though, the demand for high voltage (HV) and UHV switchgear is expected to drive the industry’s growth, given the government’s thrust on ramping up transmission infrastructure.
Over time, GIS has gained popularity over the regular air- or oil-insulated high voltage switchgear due to its several advantages, including small size, high modularisation, safety index, less maintenance, small land coverage, and ability to resist vibration and avoid electromagnetic pollution in the environment. The last four factors have significantly increased the deployment of switchgear for extra high voltage (EHV) projects.
In addition, GIS offers protection against environmental effects such as salt deposits in coastal regions, sand storms and humidity as all its parts are contained in a metal enclosure. Although the cost of a GIS is higher than a regular switchgear, in a project, when the total cost includes land coverage and construction, the use of GIS proves to be more economical for high voltage and EHV applications. Moreover, with an increase in voltage, the ratio of the total investment required for GIS to that required for regular switchgear decreases.
Continuous efforts are also being made to reduce the volume of SF6 gas used per GIS module. Meanwhile, gas mixtures like N2-He, N2-SF6, CO2- SF6 are being investigated as a substitute for SF6 gas.
Vacuum switchgear for high voltage projects
Vacuum switching, although widely used in the medium voltage range, is emerging as an alternative in high voltage applications as well. This is primarily due to its higher reliability, lower maintenance and faster 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 lower mechanism energy and is thus considered more reliable.
This kind of switchgear uses vacuum as the arc quenching medium. As vacuum has the highest insulating strength, a vacuum switchgear has a much superior arc quenching property than any other medium. Hence, as soon the arc is produced in vacuum, it is extinguished pertaining to the fast recovery of dielectric strength in 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 attracted renewed interest 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 at transmission voltages 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, owing to which the mechanism energy is much lower in a vacuum switchgear as compared to an 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.
Given the various advantages and the fact that the use of vacuum does not have any adverse impact on the environment, the deployment of vacuum switchgear at higher voltages will be inevitable in times to come and further research is under way for its development. In fact, in countries like Japan, vacuum switchgear rated at 72.5 kV and 145 kV is already in operation. However, a few challenges pertaining to capacitor switching, continuous current performance, voltage sharing during series connection, mechanical design, and testing-related issues still need to be addressed before vacuum switchgear can be successfully deployed at higher voltages.
The increased use of supervisory control and data acquisition has resulted in a growth in demand for intelligent switchgear. Switchgear manufacturers are now including built-in protection and control intelligent electronic devices (IEDs) in their switchgear solutions. These new IEDs combined with the latest information and communication technologies form a base for enhanced protection, control and monitoring. Intelligent switchgear will significantly enhance the efficiency and reliability of a grid and help utilities avoid blackouts and equipment failure. This switchgear overcomes the disadvantages of electric switchgear by utilising internal computer technology. It can also perform functions like system diagnosis, electric power fire predictions and electric power demand predictions.
In recent times, the switchgear industry has witnessed the evolution of hybrid switchgear. This is a combination of conventional AIS and high voltage GIS, and is primarily used in the the renovation and extension of substations along with AIS switchgear. The distinguishing feature of this type of switchgear is its compact and modular design, which allows for several functions in one module. The modular designs allow for a large variety of different layout configurations. Modularisation also helps in bringing about space, time and cost savings. Compact hybrid switchgear assemblies reduce space requirements by more than 50 per cent as compared to the conventional open-type switchgear. Moreover, as compared to AIS and GIS, hybrid switchgear can be erected and installed faster. The use of standard components also decreases the chance of design faults.
Further, due to the use of SF6 gas for encapsulation, the maintenance of hybrid switchgear is simple and is not required to be undertaken very frequently. The use of SF6 gas also increases the operational reliability of this kind of switchgear and makes it safe to use even in very demanding environmental conditions like polluted environments and extreme climates.
Industry players are constantly trying to develop different kinds of switchgear that are more compact, reliable, environment friendly and require minimum installation and commissioning time. Moreover, as the pace of renewable energy integration increases and there is widespread adoption of smart grid technologies, utilities would be required to increase the deployment of intelligent switchgear or to undertake modifications to transform the existing switchgear modules into “smart switchgear” as the availability of real-time data is critical in the context of both developments. In times to come, space challenges are also bound to get more acute. Hence, going ahead, switchgear equipment manufacturers need to undertake innovations and move towards smaller but smarter switchgear.