Resistance Switching in a Circuit Breaker

Edwiin

Field: Power switch

China

Resistance Switching

Resistance switching refers to the practice of connecting a fixed resistor in parallel with the contact gap or arc of a circuit breaker. This technique is applied in circuit breakers with high post-arc resistance in the contact space, primarily to mitigate re-striking voltages and transient voltage surges.

Severe voltage fluctuations in power systems arise from two main scenarios: interrupting low-magnitude inductive currents and breaking capacitive currents. Such overvoltages pose risks to system operation but can be effectively managed through resistance switching—achieved by connecting a resistor across the breaker contacts.

The underlying principle involves the parallel resistor diverting a portion of current during interruption, thereby limiting the rate of current change (di/dt) and suppressing the rise of transient recovery voltage. This not only reduces the probability of arc re-ignition but also dissipates arc energy more efficiently. Resistance switching is particularly critical in extra-high-voltage (EHV) systems for applications sensitive to switching overvoltages, such as de-energizing unloaded transmission lines or switching capacitor banks.

When a fault occurs, the circuit breaker contacts open, initiating an arc between them. As the arc is shunted by resistance R, a fraction of the arc current diverts through the resistor, reducing the arc current and accelerating the deionization rate of the arc channel.

This triggers a self-reinforcing cycle: as arc resistance increases, more current flows through shunt resistor R, further starving the arc of energy. This process continues until the current drops below the critical threshold for arc sustenance (as depicted in the figure below), at which point the arc extinguishes and the circuit breaker successfully interrupts the circuit.

The mechanism hinges on the shunt resistor dynamically regulating current distribution, forcing the arc into a vicious cycle of "current decay → accelerated deionization → rising arc resistance." This enables rapid recovery of dielectric strength in the arc channel—often before the current zero-crossing—making it particularly effective for suppressing high-frequency re-ignition overvoltages. Such functionality is critical in EHV circuit breakers during capacitive current interruption or small inductive current breaking.

Alternatively, the resistance can be automatically engaged by transferring the arc from the main contacts to the probe contacts— as seen in axial blast circuit breakers—with this action occurring in an extremely short time. By substituting the arc path with a metallic pathway, the current flowing through the resistance is limited, allowing for easy interruption.

The shunt resistor also plays a critical role in dampening the oscillatory growth of restriking voltage transients. Mathematically, it can be proven that the natural frequency (fn) of oscillations in the circuit shown is governed by: introducing a resistive element enhances the circuit's damping characteristics, reducing oscillation amplitude and retarding voltage rise rates. This is analogous to incorporating a dissipative branch into an LC oscillatory loop, transforming undamped oscillations into decaying ones and significantly improving breaker interruption stability.

In axial blast configurations, the rapid arc transfer ensures the resistor engages before current zero, providing damping control at the onset of the transient process. This design is particularly suited for EHV applications requiring switching overvoltage limitation, as the synergistic effect of resistance and arc enables ordered dissipation of electromagnetic energy during interruption.

Summary of Resistance Switching Functions

In summary, a resistor across circuit breaker contacts can perform one or more of the following functions:

Reduces RRRV (Rate of Rise of Restriking Voltage) on the Circuit Breaker

By diverting arc current and accelerating arc channel deionization, the resistor suppresses the rate of transient recovery voltage (TRV) rise, easing the dielectric strength recovery burden on the breaker interrupter.

Mitigates High-Frequency Restriking Voltage Transients during Inductive/Capacitive Load Switching

When interrupting inductive currents (e.g., unloaded transformers) or capacitive currents (e.g., charging cables), the shunt resistor limits oscillatory overvoltage amplitudes through energy dissipation, preventing insulation breakdown risks.

Equalizes TRV Distribution in Multi-Break Circuit Breakers

In breakers with multiple interrupting gaps, the resistor ensures uniform TRV distribution across contact gaps via voltage division, avoiding re-ignition due to voltage concentration in any single gap.

Scenarios Where Resistance Switching Is Unnecessary

Conventional circuit breakers with low post-arc resistance in the contact space (e.g., medium/low-voltage air breakers) require no additional shunt resistors. Their arc channels naturally deionize rapidly enough to meet interrupting requirements without external resistance.

Technical Principle Analysis

The core value of resistance switching lies in its synergistic mechanism of "impedance matching-energy dissipation-damping oscillation," which controls switching transients within equipment withstand limits. This technology is particularly critical in EHV systems (110kV and above), effectively addressing: