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. The transmission systems use HVCBs commonly at voltage levels ranging from 33 kV up to 800 kV and even above.
Why is a High Voltage Circuit Breaker important?
High-voltage transmission networks carry large quantities of electrical energy over long distances. Therefore, 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 the faulty part of the system within milliseconds to prevent these issues and ensure the continuous operation of the power grid.
How Does a High Voltage Circuit Breaker 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 a relay detects an abnormal condition, it sends a trip signal to the circuit breaker. This signal typically involves sending a DC voltage to the tripping coil of the circuit breaker. Hence, the tripping coil displaces a plunger. Then displacement of the plunger triggers the circuit breaker 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. The breaker must extinguish the arc quickly to stop the current flow. In AC systems, the current naturally crosses zero at the end of every half-cycle. Different high voltage circuit breakers employ different arc extinction methods to prevent the arc from reigniting after zero crossings. This principle, known as the zero current crossing method, is the primary approach for arc quenching of high-voltage circuit breakers. Common arc extinction media include air, oil, SF6 gas, and vacuum.
Types of High Voltage Circuit Breaker
Power utilities use different high voltage circuit breakers based on voltage levels, applications, and environmental conditions. For example, the main types include Air Circuit Breakers, SF6 Circuit Breakers, Vacuum Circuit Breakers, and Oil Circuit Breakers.
SF6 Circuit Breaker
This type of CB uses sulfur hexafluoride gas (SF6 gas) as an insulating and arc-quenching medium. Power utilities use SF6 circuit breakers most widely for 132kV to 400kV, and even for higher voltage systems. Because these have excellent arc-quenching properties. SF6 CBs demand significantly lower maintenance compared to oil and air circuit breakers. This breaker requires a longer maintenance cycle compared to other variants. These CBs are compact and suitable for indoor and outdoor applications. The most common example of an indoor application is an indoor GIS. But SF6 circuit breakers have some disadvantages. SF6 is a greenhouse gas, requiring strict handling regulations. Therefore, many countries bann the use of SF6 gas.
Vacuum Circuit Breaker (VCB)
A vacuum circuit breaker uses a vacuum chamber to extinguish the arc. For medium-voltage (33kV – 66kV) levels, most of the utilities use vacuum circuit breakers. There is no SF6 like gas emission. Hence, it is more eco-friendly. VCBs are not bulky like air circuit breakers and not dirty like oil circuit breakers. VCBs have a longer lifespan than an oil circuit breaker of the same voltage level. But unfortunately, this vacuum technology is not suitable for extra high voltage (EHV) applications.
Oil Circuit Breaker (OCB)
There are two types of oil circuit breaker. These are bulk oil circuit breaker and minimum oil circuit breaker. An oil circuit breaker uses insulating oil to quench the arc. This is the older technology. Nowadays, SF6 and vacuum breakers are replacing oil circuit breakers. Power utilities previously used OCBs for both medium and high-voltage systems. The first disadvantage of an oil circuit breaker is the risk of fire hazard due to flammable oil. Arcing carbonizes oil. Hence, OCBs require regular oil replacement. The bulky size of OCBs is the second main disadvantage.
Hybrid Circuit Breaker
This is a combination of SF6, vacuum technology. Here, mainly, the arc quenching is performed in a vacuum chamber. We also call this vacuum chamber a vacuum interrupter (VI) or vacuum bottle. SF6 gas insulates the conducting portions of the CB. 33KV or 66KV modern gas insulated systems use this technology. The hybrid system uses a lower quantity of SF6 greenhouse gas. Since it does not require SF6 gas for arc quenching. This improves the compactness of GIS. The main disadvantage of this technology is that, due to the presence of vacuum interrupting units, hybrid circuit breakers are not suitable for 132KV and above systems.
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 blast of air lengthens the arc. As a result, the resistance of the arc increases. This increased arc resistance leads to its extinguishment. This type of circuit breaker has become obsolete for high voltage levels. Although we commonly use air circuit breakers at low voltage levels. However, some power utilities are still using high-voltage air circuit breakers, only in older substations. Now, SF6 and vacuum breakers have already replaced most of these high-voltage air breakers. This type of CB requires high maintenance and generates significant noise during operation.