Jawaharlal Nehru Technological University Hyderabad B.Tech III SWITCH GEAR AND PROTECTION Previous Question Paper (January/February - 2021 - R16)

The Oil Circuit Breaker (OCB) uses oil for both insulation and arc quenching. While robust, it carries a significant fire risk and requires frequent maintenance due to carbonization of oil. Air Blast Circuit Breakers (ABCB) utilize high-pressure air to blast the arc away, making them suitable for rapid reclosing in high-voltage applications, though they require complex auxiliary air compressor systems.

SF6 Circuit Breakers utilize Sulfur Hexafluoride gas, which has superior electronegative properties, allowing for excellent arc extinction and high dielectric strength. They are the preferred choice for EHV and UHV systems due to their compact design and reliability. Vacuum Circuit Breakers (VCB) extinguish the arc in a vacuum chamber. Because the arc is interrupted at the first current zero and the dielectric strength recovers almost instantaneously, they are ideal for frequent switching operations in medium-voltage distribution networks with minimal maintenance requirements.

Resistance switching is a technique used in circuit breakers to limit the Restriking Voltage and the Rate of Rise of Restriking Voltage (RRRV). When a circuit breaker opens, a resistor is connected in parallel with the contact gap. This resistor dampens the high-frequency oscillations of the restriking voltage, thereby reducing the peak voltage and the severity of the transient recovery voltage across the contacts.

To derive the expression for the critical damping resistance, consider the circuit breaker contact gap as a capacitance C in parallel with a resistance R, connected to an inductance L representing the system. The voltage across the capacitor is governed by the differential equation:

The characteristic equation is given by:

The roots of this equation are:

For the system to be critically damped (which prevents oscillations and minimizes RRRV), the term under the square root must be zero:

Solving for R:

Thus, the value of the resistance required to achieve critical damping is R = 0.5 * √(L/C). Inserting this resistance ensures that the restriking voltage does not overshoot significantly, protecting the circuit breaker insulation.