What type of circuit breaker is generally used for the 36KV class ?
Ans: The 36KV class circuit breaker is normally an outdoor type Vacuum Circuit Breaker (VCB).
Why is VCB Preferred in 33KV Outdoor Systems Over SF6 Circuit Breakers?
Ans: VCBs are more economical for 33KV systems compared to SF6 circuit breakers due to simpler construction and reduced maintenance costs. VCBs do not use SF6 gas, which is a potent greenhouse gas. This makes VCBs an environmentally friendly option for 33KV systems. VCBs require less maintenance compared to SF6 CBs as they have fewer moving parts and do not require gas monitoring or refilling. VCBs are generally more compact and lightweight than SF6 CBs, making them easier to install and maintain in outdoor environments. The vacuum interrupter in VCBs provides also efficient and reliable arc quenching for 33KV systems with frequent switching operations. SF6 CBs require monitoring for gas leakage and pressure, while VCBs eliminate this concern entirely. The design of VCBs is relatively simpler with no need for low gas pressure alarm and gas pressure lockout arrangement in the control scheme and also it does not require any gas handling equipment, thereby reducing manufacturing and installation complexity hence it reduces associated costs.
What is the short circuit capacity of the vacuum bottle used in the 36KV VCB?
Ans: The vacuum bottle should have a short circuit capacity of 31.5KA for 3 seconds. In some utilities, 25KA for 3 seconds is also adopted.
What is the design life of the vacuum bottle used in the 36KV VCB?
Ans: Design life of 100 operations at the rated short circuit level.
What is life curve of a vacuum interrupter?
Ans: The vacuum interrupters are rated for 100 operations at the rated short-circuit level. However, in practice, not all faults interrupted by the vacuum interrupter occur at this maximum level. The life curve indicates the number of safe interruptions the vacuum interrupter can perform at various fault levels. Typically, the x-axis (horizontal) denotes the number of interrupting operations performed by the vacuum interrupter at different current levels, while the y-axis (vertical) indicates the magnitude of the interrupted current, ranging from normal load currents to maximum short-circuit currents. The number of operations at maximum short-circuit current is minimal, increasing as the current level decreases. Therefore, assessing the lifespan of a vacuum interrupter depends not only on the total number of fault interruptions but also on the fault levels interrupted.
Explain the duty cycle of the 36KV Vacuum Circuit Breaker.
Ans: The duty cycle O-0.3s-CO-3min-CO specifies the sequence and timing of operations a circuit breaker can perform, particularly during auto-reclosing scenarios.
O (Open Operation): The circuit breaker opens in response to a fault, interrupting the current flow.
0.3s (0.3-Second Interval): A brief pause allows transient faults to clear and the arc-quenching medium to regain its insulating properties.
CO (Close-Open Operation): The breaker closes to restore the circuit. If the fault persists, it immediately opens again to protect the system.
3min (3-Minute Interval): A longer pause permits system stabilization and allows the breaker’s operating mechanism to reset and recharge (spring charging).
CO (Second Close-Open Operation): The breaker attempts another close operation. If the fault remains, it opens again and may lock out to prevent further automatic operations.
This sequence ensures rapid restoration for transient faults while safeguarding the system from persistent issues. The initial 0.3-second interval addresses temporary faults, and the subsequent 3-minute interval provides time for more serious issues to be resolved before another reclosure attempt.
How does a vacuum circuit breaker work?
Ans: A vacuum circuit breaker (VCB) works by interrupting the current flow using vacuum as the arc-quenching medium. When the breaker operates, the moving contact separates from the fixed contact inside a sealed vacuum interrupter chamber. This separation creates an arc due to the ionization of metal vapors from the contacts. Actually, in vacuum chamber the metal contact surface easily be vaporized due to zero pressure of vacuum. As soon as the current passes through zero crossing, the free electrons and ions recombine, and the arc quickly collapses. The metal vapors condense back onto the contact surfaces. But the chance of re-ignition of arc is minimum as there is no more metal vapor left in the contact gap during next half cycle of the current. This quick arc extinction makes VCBs makes it so popular for 36KV and lower voltage system.
Why is a vacuum circuit breaker not used for very high voltage levels?
Ans: The vacuum gap between contacts must withstand the voltage across it after interruption. For very high voltages (e.g., 132 kV or higher), the required gap distance increases significantly because dielectric strength is proportional to the contact gap. So, at very high voltage levels, the gap size becomes very large (several centimeters), making the interrupter design bulky. The large contact gap requires a long travel distance for the moving contacts. Covering this long travel distance within the first 2 to 3 cycles of current is also a challenge. Because of this, making a bulky vacuum interrupter is impractical.
Although, theoretically, in high-voltage applications, the above disadvantages can be overcome by connecting multiple vacuum interrupters in series, but coordinating the simultaneous operation of these interrupters is complex and can compromise reliability.
Vacuum has excellent dielectric properties at medium voltage levels, but its strength decreases at higher voltages. This happens due to residual gas particles, surface imperfections on electrodes, and a higher risk of partial discharges at high voltages.
VCBs interrupt current by extinguishing the arc in a vacuum, which works well for moderate fault currents up to 40 to 63 kA. But in very high voltage systems, fault currents can be much higher (around 80–100 kA). At the chopping of these level of high fault current can create a very high transient voltage across the contacts. The vacuum interrupter struggles to quench the arc and withstand the steep TRV at these levels, increasing the risk of restrike or failure to finally interrupt.
Is a vacuum circuit breaker not suitable for switching a capacitor bank?
Ans: In short, VCBs can be used for charging a smaller size of capacitor bank, but they are not ideal and can face significant challenges, particularly for a large capacitor bank. Charging a capacitor bank means closing a circuit breaker to connect the bank to a live bus (source). During closing of VCB, a high transient current flows into the capacitor as it charges from zero to the system voltage. This is called inrush current. The peak inrush current may be 10 to 20 times of the steady-state current. This current change, potentially causes over voltages or restrikes. Also, in a vacuum, this high inrush current creates intense localized heating and pre arcing (before the contacts fully close). Repeated operations erode the contacts faster than in SF₆ breaker. The switching also involves de-energizing a capacitor bank. When a VCB disconnects a charged capacitor bank, the trapped charge in the bank often generates a high transient voltage (up to 2–2.5 times the peak system voltage) across the contacts. A VCB has a higher risk of restrike (re-ignition of the arc) at this transient voltage level compared to a SF₆ breaker. Because, the dielectric strength of vacuum can be broken down if metal vapor from the previous arc persists.
Why is vacuum used as the dielectric medium?
Ans: Vacuum has an outstanding dielectric strength. The dielectric strength of vacuum is about 20 to 30 kV/mm. It is far greater than that of air which is 3 kV/mm. The dielectric strength of vacuum can be comparable to the dielectric strength of SF₆ gas which is about 8 to 10 kV/mm at 1 bar.
Ideally, in a vacuum, there are no gas molecules to ionize and create free charge carriers, preventing electrical breakdown. This allows VCBs to insulate effectively across small contact gaps (e.g., 8 mm for 12 kV systems). The small contact gap makes a VCB compact in size and requires only a small mechanical movement of the moving contacts. Hence, the VCB mechanism is less complex and consumes less energy for operation.
When a VCB interrupts current, an arc forms between the separating contacts due to metal vapor from the contacts themselves. Here, it is to be remembered that due to vacuum pressure, vaporization occurs easily on the contact surface. In a vacuum, there is no surrounding gas to sustain ionization, so the arc extinguishes quickly as the vapor diffuses and condenses, typically at the first one or two current zero crossings.
Vacuum interrupters are sealed for life. This eliminates the need for gas refilling or oil maintenance, unlike SF₆ or oil-based breakers. This reduces operational costs and downtime.
Unlike SF₆ (a potent greenhouse gas) or oil (which poses fire and explosion risks), vacuum is non-toxic, non-flammable, and environmentally friendly.
What is the arc extinction process in a vacuum circuit breaker?
Ans: In a vacuum circuit breaker (VCB), arc extinction works differently compared to other breakers because it takes advantage of the unique properties of vacuum to interrupt current flow. Actually, when the moving and fixed contacts detach, their entire surfaces do not separate simultaneously. As per basic physics, during separation, the contact surface area gradually reduces before the contacts fully separate. As the contact area decreases, the current density increases at that point, creating a hotspot. At this hotspot, the contact metal vaporizes. So, when the breaker contacts separate, an arc forms due to the ionization of metal vapor released from the hotspot on the contact surface. This arc is essentially a plasma bridge that temporarily sustains current flow, even after the contacts have parted.
The arc in a VCB depends only on metal vapor from the contacts. As the AC current naturally drops to zero during zero-crossing, the energy that maintains the arc reduces, leading to a decrease in metal vapor production. Without a fresh supply of ionized particles, the arc dies out naturally. Also, at that time, the metal vapor gets condensed. During the next cycle, the contacts have already separated, so no hotspot is created again, resulting in an insufficient amount of metal vapor in the gap. Hence, the chance of re-ignition is minimal. The arc gets extinguished completely after a few cycles. The whole process happens in microseconds.
How does contact material selection affect VCB performance?
Ans: In a vacuum, the arc is formed solely due to the metal vapor generated from the contact surface during operation. It goes without saying that excessive arcing is undesirable, but the absence of an arc can also be problematic. Optimizing vapor production is the key factor in selecting the appropriate metal for the contact.
Pure copper has a lower boiling point and higher vapor pressure, so it generates more metal vapor when heated by the arc. This can keep the arc alive longer, delaying extinction and reducing interruption performance, especially at high currents. That’s why pure copper isn’t used as a contact material in VCBs.
Chromium in CuCr ensures a controlled release of metal vapor — just enough to sustain the arc until current zero but not enough to linger. This makes chromium-copper (CuCr) the preferred choice.
Adding zirconium to chromium-copper (ZiCrCu) increases the melting point and lowers vapor pressure. This can reduce vapor production too much, making the arc unstable or harder to sustain until zero-crossing, increasing the risk of interruption failure. That’s why ZiCrCu is not suitable for VCB contacts, but it is widely used in other types of circuit breakers and certain special isolators.