The power sector in India has been growing at a healthy pace supported by various policy initiatives by the government. Generation capacity has grown at a rate of over 10 per cent over the past five years, while the transmission and distribution network has witnessed a moderate growth of around 5 per cent. This has translated into a growing demand for robust power system equipment such as switchgear in the country. Switchgear is a combination of components such as switches, fuses, and circuit breakers that are used to control, protect and isolate an electrical equipment to enable continuous and reliable supply of electricity. The use of switchgear helps in protecting against situations of overload, short circuit, insulation failure etc. Power system protection is a bough of electrical power engineering and it is designed to continuously monitor the power system to ensure maximum continuity of electrical supply without damaging equipment. Since power system developments change its structure, the power system protection becomes very vital. The design of switchgear has to ensure that it will be able to detect abnormal or undesirable conditions and then trip the circuit to disconnect the affected area without affecting other undesired areas.
The different kinds of switchgears 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. 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, 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 or direct current.
Switchgear is available in low, medium and high voltage levels. Low voltage switchgear is generally rated up to 1 kV and caters to buildings, the power distribution system and industries. This segment includes circuit breakers, (moulded case circuit breakers and miniature circuit breakers), switches, high rupturing capacity fuses, residual current devices, contractors and relays. Medium voltage switchgear has ratings 1 kV to 75 kV and includes various types of circuit breakers such as air circuit breakers, minimum oil circuit breakers and vacuum circuit breakers. The high voltage switchgear has ratings above 75 kV and includes SF6 circuit breakers, gas-insulated switchgear, hybrid switchgear, lighting arresters and composite insulators. In addition, based on the medium used to interrupt the current, switchgear is classified as either a simple open-air isolator switch or it may be insulated by some other material like oil and vacuum.
Smart Grid Operation
In smart grid operation, automated switchgear operation is preferred over conventional operation. In conventional substations, all signals, controls and interlocks are hardwired and records are manually maintained in a logbook. Therefore, a lot of work and efforts is required to draw comparisons for analysis and trouble shooting. An automated substation, wherein all operations are automated, is more efficient and requires less manpower.
Traditionally, remote terminal units were used in substations as a link between the switchgear and the control centre. Some of these remote terminal units had intelligence features such as interlocking features, but no substation or region-wide automation was available. However, now more and more remote terminal units are being replaced by or complemented with specialized intelligent electrical devices that are capable of multiple protections and measurements in smart grid operation. In addition, intelligent gateways and concentrators have been introduced in the substation. In smart grid operations, monitoring and signaling are becoming an integral, in addition to protection & control functions. Manufacturers are including intelligent built-in protection and control electronic devices in the switchgear to enhance grid efficiency and reliability.
Historically, air-insulated switchgear has been the most commonly used switchgear in India due to the low price. However, gas-insulated switchgear is now gaining popularity and is emerging as the preferred technology in India, especially, in the transmission segment. Its compact and encapsulated structure makes it ideal for areas with space constraints. This kind of switchgear is also suitable for use in locations with severe weather conditions (high temperature and high altitudes) and in industrial environments.
New developments in the switchgear industry are vacuum switchgear, hybrid switchgear and intelligent switchgear. Vacuum switching through widely used in the medium voltage range, is also emerging as an alternative in HV applications. This trend is being driven by the fact that the vacuum switchgear is more environment friendly than SF6 switchgear. Hybrid switchgear is a combination of conventional air insulated switchgear and high voltage gas insulated switchgear. Some of the high voltage switchgears used in the power sector is as given below:
Gas Insulated Switchgear
Current interruption in a high-voltage circuit breaker is obtained by separating two contacts in a medium such as sulfur hexafluoride (SF6), having excellent dielectric and arc-quenching properties. After contact separation, current is carried through an arc and is interrupted when this arc is cooled by a gas blast of sufficient intensity.
The sulfur hexaflouride gas (SF6) is an electronegative gas and has a strong tendency to absorb free electrons. The contacts of the breaker are opened in a high pressure flow of sulphur hexa flouride gas and an arc is struck between them. The gas captures the conducting free electrons in the arc to form relatively immobile negative ions. This loss of conducting electrons in the arc quickly builds up enough insulation strength to extinguish the arc. Over time, gas insulated switchgear has gained popularity over regular air or oil insulated high voltage switchgear due its several advantages, including small size, high modularization, safety index, less maintenance, small land coverage, and ability to resist vibration and avoid electromagnetic pollution in the environment. These factors have increased the deployment of switchgear for extra high voltage projects.
Although the cost of a gas insulated switchgear is higher than a regular switchgear, in a project, when the total cost includes land coverage and construction, the use of gas insulated switchgear 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 gas insulated switchgear to that required for regular switchgear decreases. Continuous efforts are also being made to reduce the volume of SF6 gas used per module. Other gas mixtures are also being investigated as a substitute for SF6 gas.
A vacuum circuit breaker is the suitable for mainly medium voltage application circuit breaker where the arc quenching takes place in vacuum. Vacuum switchgear 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 higher 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, 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 time, as sensitivity towards environment 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 stoke. Moreover, this type of switchgear has a longer moving mass, owing to which the mechanism energy is much lower in vacuum switchgear 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.
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. However, a few challenges pertaining to capacitor switching, continuous current performance, voltage sharing, 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 in their switchgear solutions. These new intelligent electronic devices combined with the latest information and communication technologies from a base for enhance 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 utilizing internal computer technology. It can also perform functions like system diagnosis, electric power fire predictions and electric power demand predictions.
Hybrid switchgear is a combination of conventional air insulated switchgear and high voltage gas insulated switchgear, and is primarily used in 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 design allows for a large variety of different layout configurations. Modernization also helps in bringing about space, time and cost savings. Compact hybrid switchgear assemblies reduce space requirements by more than 50 percent 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.
Researchers are constantly trying to develop different kinds of switchgear suitable for smart grid operation and that is more compact, reliable, environment friendly and requires 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 times to come, space challenges are also bound to get more acute. Hence, going ahead, switchgear equipment manufacturers need to undertake innovations and more towards smaller but smarter switchgear.
With the arrival of smart grids, substation automation is expected to expand significantly, providing increased control of relays, capacitor banks, voltage regulators and feeders. Given their critical role in substation automation, the scope of switchgear is also expected to grow considerably in the near future. Given the government’s reform initiatives, the switchgear market is likely to continue on its growth trajectory. The expansion of key industries and infrastructure sectors like telecom, railways, airports and ports, and the increases emphasis on the development of smart grids and grid automation are likely to support the switchgear market.
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