A Circuit Breaker is anautomatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition by interrupting continuity to immediately discontinue power flow. Circuit Breaker (CB) is a piece of equipment which can make or break a circuit either manually or by remote control under normal conditions, break a circuit automatically under fault conditions and make a circuit either manually or by remote control under fault conditions.
A typical circuit breaker consistsof fixed and moving contacts called electrodes. Under normal conditions, these contacts remain closed and will not open until and unless the system becomes faulty. When faults occur in any part of the system, the trip coils of the CB get energised through relay circuit and the moving contacts are pulled apart by some mechanism, thus, opening the circuit. The circuit breaker contacts must carry the load current without excessive heating which must withstand the heat of the arc produced when interrupting (opening) the circuit. Contacts are made of highly conductive materials. Service life of the contacts is limited by the erosion of contact material due to arcing. The arc between the contacts produced due to ionisation of air or vapour of oil acts as a conductor. When a short circuit occurs, a heavy current flows through the contacts of CB before they are opened. At the instant when the contacts begin to separate, the contacts area decreases rapidly and large current increases the current density, thus, increasing the temperature result heat produced in the medium oil or air is sufficient to ionise.
Different techniques are used to extinguish the arc including lengthening or deflection of the arc, intensive cooling in jet chambers, division into partial arcs and connecting capacitors in parallel with contacts in DC circuits. Some time “zero point quenching” is also used as arc quenching method. In this method, the contacts of the circuit breaker open at the zero current time crossing of the AC waveform, effectively no load current at the time of opening. The zero crossing occurs at twice the line frequency i.e. 100 times per second for 50 Hz alternating current. Based on the medium used for arc quenching, the circuit breakers are classified being air circuit breaker, oil circuit breaker and gas circuit breaker. New developments in the CB industry are hybrid CBs and intelligent circuit breakers. Historically, air-insulated CB has been the most commonly used due to the low price. However, gas-insulated CB 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 CB is also suitable for use in locations with severe weather conditions (high temperature and high altitudes) and in industrial environments. 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 CBs are more environment friendly than SF6 circuit breakers. Hybrid CB is a combination of conventional air insulated circuit breakers and high voltage gas insulated CB. A brief of the commonly used CB’s in power sector are as follows:
Air Circuit Breaker
Air circuit breaker (ACB) is the most widely deployed switchgear in India. It uses air as the primary dielectric for phase to phase and phase-toground insulation. Air Circuit Breaker employs a high pressure air-blast as an arc quenching medium. The contacts are opened in a flow of airblast established by the opening of blast valve. The air-blast cools the arc and sweeps away the arcing products to the atmosphere. This rapidly increases the dielectric strength of the medium between contacts and prevents from re-establishing the arc.
ACB is popular where space is not an issue. It has low construction and maintenance costs. ACB is easy to maintain as all the equipment is within view and faults can be attended to without much delay. However, it is vulnerable to faults since the equipment is open to external elements such as human intrusion, pollution, deposition of saline particles, lighting strikes and extreme weather conditions.
Oil Circuit Breaker
In Oil Circuit Breaker (OCB) Transformer oil used as arc quenching medium. It insulates between phases and ground which provides the medium for the extinguishing of the arc. When electric arc is drawn under oil, the arc vaporizes the oil which creates a large bubble that surrounds the arc. The gas inside the bubble is around 80 per cent hydrogen, which impairs ionization. The oil surrounding the bubble conducts the heat away from the arc and thus also contributes to deionisation of the arc.
Gas Insulated Circuit Breakers
Gas-insulated circuit breaker (GCB) is essentially compact and metal encapsulated, consisting of HV equipment such as circuit breakers and disconnectors. In GCB, all the components are placed inside modules filled with Sulphur hexafluoride or SF6 gas. Current interruption in a highvoltage circuit breaker is obtained by separating two contacts in a medium of 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. 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 circuit breaker parts, thereby reducing the size of the equipment and making these ideal for urban areas as well as indoor spaces.
The sulfur hexa flouride 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 circuit breakers have gained popularity over regular air or oil insulated high voltage circuit breakers due to its several advantages, including small size, high modularisation, safety index, low maintenance requirement, small land coverage, and ability to resist vibration and avoid electromagnetic pollution in the environment. These factors have significantly increased the deployment of circuit breakers for extra high voltage projects.
The high cost of GCB as compared to others is an issue, but if land as well as construction and maintenance costs are taken into account, it becomes economical. Moreover, with an increase in voltage, the ratio of the total investment required for gas insulated circuit breaker to that required for regular circuit breaker decreases. Further, SF6 gas is a greenhouse gas with high global warming potential. With the environmental concerns associated with SF6 gas, efforts are under way to replace it with an alternative medium. Efforts are also being made to reduce the volume of SF6 gas used per module.
Vacuum Circuit Breakers
A vacuum circuit breaker (VCB) is mostly suitable for mainly medium voltage applications where the arc quenching takes place in a vacuum. In VCB, vacuum is used as an arc quenching medium which has the greatest insulating strength. When the contacts of the CBs are opened in vacuum, an arc is produced between the contacts. The arc is quickly extinguished because the metallic vapours, electrons and ions produced during arc rapidly condense on the surfaces of the CB contacts, resulting in quick recovery.
It is emerging as an alternative in high voltage applications as well due to various advantages primarily likecompact size, higher reliability, lower maintenance and faster restoration. Given its higher dielectric strength, low open gap is also a key characteristic of the vacuum circuit breaker. 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 areduction 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. Vacuum circuit breaker has also seen a renewed interest with focus on reducing the use of SF6. It is expected that the deployment of vacuum circuit breaker at highervoltage will increase in the time to come. However, a few challenges pertaining to capacitor switching,continuous current performance, voltage sharing, mechanical design, and testing related issues still need to be addressed before vacuum circuit breaker can be successfully deployed at higher voltages.
Circuit breaker manufacturers havestarted leveraging internet of things to enable real-time information flow, as well as improved predictive diagnostics that leads to higher energy efficiency and a more reliable grid. The increased use of supervisory control and data acquisition has also resulted in a growth in demand for intelligent circuit breaker. Intelligent circuit breaker can connect with internet and provide comprehensive monitoring and protection functions, as well as measures all electrical parameters in real time. 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 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.
Hybrid circuit breaker combines conventional air insulated and gas insulated technologies and can be installed both in indoor as well as outdoor areas. Some of the key advantages of hybrid circuit breaker are that it requires almost 30 per cent less switchyard area and lower foundations per bay. The distinguishing feature of this type of circuit breaker is its compact and modular design, which allows for several functions in one module. Moreover, as compared to AIS and GIS, hybrid switchgear can be erected and installed faster. 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.
There is need 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. 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. The expansion of key industries and infrastructure sector increases emphasis on the development of smart grids and grid automation and is likely to support the circuit breaker market.
Dy. Director (Generation)