In a low voltage circuit breaker, using air or Arc Chutes is sufficient to extinguish an arc. In the medium voltage range, a different technology needs to be used. The main technology used today is the Vacuum Interrupter.
Vacuum Interrupter Technology
The vacuum interrupter (VI) was briefly discussed in Module 5, Fundamentals of Circuit Breakers.
Basically, the vacuum interrupter is a pair of separable contacts (called "primary contacts") enclosed in a vacuum-tight envelope. The Envelope itself is a ceramic material, with a metal end plate brazed to each end. The metal plates seal the ends and provide support for the parts inside.
Of the two contacts (also called "electrodes") inside, one is fixed. The other is movable, through a bellows-type connection. Various shields inside the envelope provide different types of protection to interrupter parts.
depicts the important arcing and interruption phenomena within a vacuum.
When the circuit breaker is closed, the contacts within the interrupter touch, allowing current to flow.
When a fault occurs and interruption is required, the contacts are quickly separated and an arc forms. An arc is formed because the voltage tries to keep the current moving.
Enclosing Contacts in a Vacuum
The arc burns in the metal vapor evaporated from hot spots on the contact surfaces. This metal vapor continuously leaves the contact region and recondenses on the contact surfaces and surrounding metal shield, which protects the ceramic envelope.
At Current Zero, the arc extinguishes, contact vapor production stops, and the original vacuum condition is restored. Current zero is a point in the AC current sine wave where the value is zero.
The vacuum in the envelope is considered a Dielectric. The Dielectric Strength is the maximum voltage the dielectric can withstand without breaking down. The Transient Recovery Voltage (TRV) is the most severe waveform the interrupter will have to withstand. This is why the speed of the dielectric recovery and the strength of the dielectric inside the interrupter are critical issues for successful circuit interruption. If the dielectric does not reach sufficient strength fast enough, the arc will re-ignite.
Vacuum interrupters for circuit breaker duty must be capable of interrupting currents of 12 to 50 kA (and up), at voltages up to 38 kV
Rear View of Circuit Breaker with
Vacuum Interrupters Installed (One Per Phase)
Spiral Contacts
A newer technology is now being used in the vacuum interrupter. It involves using spiral-shaped copper-chrome contacts inside the vacuum tube. They provide a self-induced magnetic effect that moves the arc root around the contact periphery. This very efficient arc control method prevents hot spots, minimizing contact erosion.
Spiral Contacts
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SF6 Circuit Breaker
Vacuum Interrupter Technology
The vacuum interrupter (VI) was briefly discussed in Module 5, Fundamentals of Circuit Breakers.
Basically, the vacuum interrupter is a pair of separable contacts (called "primary contacts") enclosed in a vacuum-tight envelope. The Envelope itself is a ceramic material, with a metal end plate brazed to each end. The metal plates seal the ends and provide support for the parts inside.
Of the two contacts (also called "electrodes") inside, one is fixed. The other is movable, through a bellows-type connection. Various shields inside the envelope provide different types of protection to interrupter parts.
depicts the important arcing and interruption phenomena within a vacuum.
When the circuit breaker is closed, the contacts within the interrupter touch, allowing current to flow.
When a fault occurs and interruption is required, the contacts are quickly separated and an arc forms. An arc is formed because the voltage tries to keep the current moving.
Enclosing Contacts in a Vacuum
The arc burns in the metal vapor evaporated from hot spots on the contact surfaces. This metal vapor continuously leaves the contact region and recondenses on the contact surfaces and surrounding metal shield, which protects the ceramic envelope.
At Current Zero, the arc extinguishes, contact vapor production stops, and the original vacuum condition is restored. Current zero is a point in the AC current sine wave where the value is zero.
The vacuum in the envelope is considered a Dielectric. The Dielectric Strength is the maximum voltage the dielectric can withstand without breaking down. The Transient Recovery Voltage (TRV) is the most severe waveform the interrupter will have to withstand. This is why the speed of the dielectric recovery and the strength of the dielectric inside the interrupter are critical issues for successful circuit interruption. If the dielectric does not reach sufficient strength fast enough, the arc will re-ignite.
Vacuum interrupters for circuit breaker duty must be capable of interrupting currents of 12 to 50 kA (and up), at voltages up to 38 kV
Rear View of Circuit Breaker with
Vacuum Interrupters Installed (One Per Phase)
Spiral Contacts
A newer technology is now being used in the vacuum interrupter. It involves using spiral-shaped copper-chrome contacts inside the vacuum tube. They provide a self-induced magnetic effect that moves the arc root around the contact periphery. This very efficient arc control method prevents hot spots, minimizing contact erosion.
Spiral Contacts
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