High Voltage Circuit Breakers (HVCBs)

What is a High Voltage Circuit Breaker?

A High Voltage Circuit Breaker (HVCB) is a crucial component in power transmission and distribution networks. Its primary role is to protect electrical equipment by interrupting fault currents within the system. Beyond safeguarding equipment, HVCBs are vital for maintaining the integrity of the power grid. They isolate faults effectively, ensuring that adjacent systems remain unaffected and operational. By definition, circuit breakers operating at voltages typically above 1000 volts are classified as high voltage. In transmission systems, HVCBs are commonly used at voltage levels ranging from 33 kV up to 800 kV and even above.

Why Are HVCBs Important?

High voltage transmission networks carry large amounts of electrical energy over long distances. Any fault, such as a short circuit, lightning strike, or equipment failure, can result in severe consequences, including:
✅ Power outages
✅ Equipment damage
✅ Fire hazards
✅ Grid instability
Circuit breakers interrupt fault current and isolate faulty part of the system within milliseconds to prevent these issues and ensure continuous operation of the power grid.

How Do High Voltage Circuit Breakers Work?

Protective relays receive voltage, current, and system condition data from voltage transformers, current transformers, and other monitoring devices such as Buchholz relays or similar relay contacts. When an abnormal condition is detected, the relay sends a trip signal to the circuit breaker. This signal typically involves sending a DC voltage to the circuit breaker’s tripping coil. Once the tripping coil is energized, it displaces a plunger, which triggers the circuit breaker’s mechanism. This mechanism releases its stored potential energy, causing the moving contact lever to shift. As a result, the moving contacts separate, opening the breaker. When the breaker opens, an electrical arc forms between the contacts. This arc must be extinguished quickly to stop the current flow. In alternating current (AC) systems, the current naturally crosses zero at the end of every half-cycle. Various arc extinction methods are employed to prevent the arc from reigniting after this zero crossing. This principle, known as the Zero Current Crossing Method, is the primary approach used in high-voltage circuit breakers for arc quenching. Common arc extinction media include air, oil, SF6 gas, and vacuum.

Types of High Voltage Circuit Breakers

Different high voltage circuit breakers are used based on voltage levels, applications, and environmental conditions. These are mainly Air Circuit Breaker, SF6 Circuit Breaker, Vacuum Circuit Breaker, Oil Circuit Breaker etc.

Air Circuit Breaker (ACB)

An air circuit breaker uses compressed air to blow out the arc. This arc-quenching methodology does not rely on a precise zero crossing. Rather, in this method, the arc is lengthened by a blast of air, and the resulting increase in arc resistance leads to its extinguishment. This type of circuit breaker has become obsolete for high voltage levels. Although these are still popularly used in low voltage levels. But, mostly in older substations high voltage air circuit breakers are used and now these are being replaced by SF6 and vacuum breakers. Air CBs are mostly used up to 132kV level. This type of CB requires high maintenance, and generates significant noise during operation.

SF6 Circuit Breaker

This is a combination of SF6, vacuum technology. Here, mainly the arc quenching is performed in a vacuum chamber. This vacuum chamber is called vacuum interrupter (VI) or vacuum bottle. The conducting portion of the CB is insulated by SF6 gas. This technology is used in 33KV or 66KV modern Gas-Insulated systems. The main advantage of hybrid system is low usage of SF6 greenhouse gas, since no gas is required for arc quenching. This improves the compactness of GIS system. The main disadvantage of this technology is that due to presence of vacuum interrupting units hybrid circuit breakers are not suitable for 132KV and above system.

This type of CB uses sulfur hexafluoride (SF6) gas as an insulating and arc-quenching medium. These circuit breakers are most widely used for 132kV to 400kV even up to higher voltage systems. It has the excellent arc-quenching properties. The maintenance is significantly low compare to oil and air circuit breakers. Period of maintenance cycle is quite long compare to other variants of breaker. These CBs are compact and suitable for indoor and outdoor applications. Most common example for indoor application is indoor GIS system. But SF6 circuit breakers have some disadvantages. SF6 is a greenhouse gas, requiring strict handling regulations. There are many countries where use of SF6 gas is band.

Vacuum Circuit Breaker (VCB)

A vacuum circuit breaker uses a vacuum chamber to extinguish the arc. For medium-voltage (33kV – 66kV) levels vacuum circuit breakers are mostly used. There is no SF6 like gas emissions hence it is more eco-friendly. VCBs are not bulky like air circuit breakers and not dirty like oil circuit breakers. VCBs have longer lifespan than an oil circuit breaker of same voltage level. But unfortunately this vacuum technology is not suitable for extra high voltage (EHV) applications.

Oil Circuit Breaker (OCB)

An oil circuit breaker uses insulating oil to quench the arc. This is the older technology, now oil circuit breakers are largely replaced by SF6 and vacuum breakers. Previously OCBs were used for both medium and high voltage systems. The first disadvantage of oil circuit breaker is the risk of fire hazard due to flammable oil. Due to arcing between opening contacts oil gets carbonized hence OCBs require regular oil replacement. Bulky size of OCBs is the second main disadvantage.