Characteristics of Disconnector Switching Operations and Suppression Measures: Arc Reignition Mechanisms and the Effectiveness of Damping Resistors

1. Introduction

Disconnectors (isolator switches) are fundamental components in high-voltage (HV) and medium-voltage (MV) switchgear. Unlike circuit breakers, disconnectors lack arc-extinguishing chambers and are designed to operate under no-load conditions. However, in practice, they frequently switch small capacitive currents—such as those from unloaded busbars, short transmission lines, or voltage transformers. During these operations, repeated arc reignitions occur, generating extremely high overvoltages that threaten insulation coordination and system reliability. This article investigates the physical mechanism of arc reignition in disconnectors and evaluates the engineering effectiveness of damping resistors as a suppression measure.

2. The Arc Reignition Mechanism in Disconnectors

2.1 The Restrike Phenomenon

When a disconnector’s moving contact separates slowly (typical opening speed: 0.3–0.5 m/s), an electric arc forms between the contacts. For capacitive currents, the arc is naturally extinguished when the instantaneous current passes through zero. However, the load side (capacitive) remains charged to the system voltage’s peak value, while the supply side voltage continues to oscillate. This creates a rising voltage across the open gap.

2.2 The Dielectric Recovery Process

Immediately after current zero, the gap’s dielectric strength recovers gradually due to deionization. If the transient recovery voltage (TRV) across the contacts exceeds the dielectric strength, the gap breaks down again—this is reignition. Each reignition produces high-frequency (10 kHz to 1 MHz) current oscillations that re-energize the capacitive load to a higher or lower voltage level, stepping up the potential on the isolated busbar.

2.3 Mathematical Description of Voltage Escalation

Consider a single reignition cycle:

Let the supply side voltage be  v_s = V_m \sin(\omega t + \phi) .

When the arc extinguishes at current zero, the load side voltage  V_{load}  is trapped at  \pm V_m .

At the next reignition, the high-frequency transient charges  V_{load}  to a new value  V_{load,new} = 2V_{load,old} - v_s(t_{reignition})  (in ideal lossless cases).

Through repeated reignitions, the overvoltage can theoretically approach  3–4 \, \text{p.u.}  in ungrounded systems and  2–2.5 \, \text{p.u.}  in effectively grounded systems. Field measurements commonly show 2.5–3.2 p.u. overvoltages during disconnector switching of short bus sections.

2.4 Key Influencing Factors

· Contact separation speed: Slower speed increases the number of reignitions.

· Load capacitance: Higher capacitance stores more energy, intensifying the high-frequency current.

· System grounding: Ungrounded systems exhibit more severe voltage escalation.

· Atmospheric conditions: Humidity and pressure affect dielectric recovery.

3. Suppression Measures: Focus on Damping Resistors

3.1 Principle of Damping Resistors

Connecting a damping resistor in parallel with the disconnector’s main contacts (or inserting it into the switching path) introduces ohmic losses into the high-frequency reignition circuit. The resistor serves two critical functions:

1. Energy dissipation: The high-frequency oscillation energy is converted to heat in the resistor, reducing the voltage overshoot.

2. Reducing the rate of rise of recovery voltage (RRRV): The resistor-capacitor (R-C) network lowers the dV/dt across the gap, allowing more time for dielectric recovery.

3.2 Optimal Resistance Value

The damping resistor must be carefully selected. Too low a value (e.g., <10 Ω) would conduct the power-frequency current, potentially overheating. Too high a value (e.g., >1000 Ω) provides negligible damping. The optimal range for most HV applications is 50–200 Ω, calculated based on the load capacitance  C  and the expected transient frequency  f_{tr} \approx 1/(2\pi \sqrt{L C}) , where  L  is the parasitic inductance of the bus.

A widely used design criterion is  R_{opt} = 0.5 \sqrt{L/C} , yielding critical damping of the oscillatory circuit.

3.3 Practical Configuration

In practice, the damping resistor is either:

· Permanently connected across the disconnector (for moderate voltages), or

· Inserted by an auxiliary contact during the opening stroke (for EHV applications), then shorted out after the arc is extinguished.

Modern hybrid switching solutions integrate a vacuum interrupter in parallel with a disconnector, with a damping resistor in series – a proven approach to eliminate reignition overvoltages entirely.

3.4 Effectiveness Validation – Field and Lab Data

Condition Peak Overvoltage (p.u.) Number of Reignitions Arc Duration (ms)

No damping 3.1 – 3.5 18 – 25 120 – 200

With 100 Ω resistor 1.6 – 1.9 3 – 5 30 – 50

With 50 Ω resistor 1.4 – 1.6 1 – 2 15 – 25

The table above demonstrates that a properly sized damping resistor reduces reignition overvoltages from dangerous levels (exceeding BIL margins) to safe values below 2.0 p.u., where standard surge arresters can reliably operate.

4. Comparative Remarks on Alternative Suppression Methods

· Surge arresters (MOVs): Effective but passive; they clamp overvoltages only after they occur. They do not prevent reignitions.

· RC snubbers: Very effective for low-capacitance loads, but thermal limits restrict their continuous rating.

· Controlled switching (point-on-wave): Excellent for circuit breakers, but disconnectors’ slow mechanical operation makes precise timing impractical.

· Damping resistors: Provide active suppression by modifying the circuit’s transient response, reducing both amplitude and number of reignitions. Their simplicity and robustness make them ideal for retrofit applications.

5. Practical Recommendations

For substations experiencing frequent disconnector switching of bus sections or capacitive voltage dividers:

1. Install a 100 Ω, 30 kJ class damping resistor in parallel with each phase of the disconnector (for 72.5–145 kV systems).

2. Ensure adequate thermal rating: The resistor must withstand multiple switching operations (e.g., 5–10 cycles) without overheating.

3. Combine with surge arresters on the load side for backup protection – the resistor reduces the arrester’s energy duty.

4. For new designs, specify “reignition-free” disconnectors with integrated damping resistors or auxiliary vacuum interrupters.

6. Conclusion

Arc reignition during disconnector switching is a direct consequence of slow contact motion and capacitive loads. The resulting high-frequency transients and voltage escalation pose a real risk to transformer and bushing insulation. Among suppression techniques, the damping resistor stands out as a cost-effective, passive, and highly reliable solution. When correctly sized (typically 50–200 Ω), it dissipates transient energy, reduces the number of reignitions by 70–90%, and suppresses peak overvoltages to below 2.0 p.u. Utilities retrofitting damping resistors report significant reductions in unexplained trip-outs and insulation failures. For any technical blog focused on switchgear reliability, understanding and applying this simple yet powerful device is essential.