Why Does Sparking Occur During Transformer Impulse Closing Tests?

Sparking or arcing during transformer impulse closing tests is typically associated with the following physical phenomena and operational conditions. The causes are analyzed below under different scenarios:

 

1. Transient Process During Closing Instant: Current Surge & Electromagnetic Energy Release

Current Inrush: During impulse closing, the transformer windings are abruptly energized, causing current to surge from zero to a steady-state value. The peak inrush current can reach 6–8 times the rated current. Such high-frequency, high-amplitude current changes may induce sparks at:

Switch Contact Gaps: Mechanical switches (e.g., circuit breakers) may generate brief arcs due to contact vibration, excessive contact resistance, or localized high temperatures causing metal vapor ionization during closure.

Loose Terminal Connections: High current through oxidized or poorly connected terminals can rapidly increase contact resistance, leading to sparks.

Case Example: Aged circuit breakers or insufficient closing speed may result in asynchronous contact closure, accompanied by sparks and popping sounds.

 

2. Insulation Defects or Partial Discharge

Internal Weak Insulation: Insulation flaws (e.g., aged oil, bubbles, contaminants) create localized electric field concentrations under impulse voltage, triggering partial discharges. Weak sparks may be observed at bushings or tank joints, accompanied by hissing sounds, ozone odor, or infrared/ultrasonic signals. Severe cases may escalate to insulation breakdown.

Surface Tracking: Contaminated or moistened bushings can form leakage current paths along surfaces, causing creeping discharge sparks. Remedial action: Clean or replace insulation components.

 

3. Electromagnetic Induction & Counter Electromotive Force (EMF)

Secondary Circuit Interruption: Accidentally disconnecting secondary-side loads (e.g., meters) during testing induces high-voltage EMF in the open circuit due to abrupt primary current changes. Sparking may occur at gaps or faulty contacts.

Formula: 𝑉=𝐿⋅𝑑𝑖𝑑𝑡V=L⋅dtdi​ (Higher 𝑑𝑖𝑑𝑡dtdi​ increases induced voltage).

Case Example: Open-circuiting current transformer (CT) secondary terminals may generate kilovolt-level EMF, causing arcing and equipment damage.

 

4. Poor Grounding or Floating Potentials

Faulty Grounding: Loose neutral-point or tank grounding leads to charge accumulation and floating potentials, resulting in ground discharge (common in non-effectively grounded systems, e.g., IT systems). Verify grounding resistance (≤4 Ω).

Capacitive Coupling: Distributed capacitance between windings or windings-to-ground under impulse voltage may generate transient currents. Impedance mismatches can induce localized discharges.

 

5. Differentiating Normal Phenomena vs. Faults

Normal Transient Sparks: Millisecond-scale minor arcs (e.g., during switch closure) without sustained discharge or abnormal vibrations; no action needed.

Fault Indicators: Immediate shutdown required if sparks are accompanied by:

Continuous discharge sounds/burning odors

Protective relay activation (e.g., differential, Buchholz)

Abnormal dissolved gas analysis (DGA), e.g., elevated 𝐶2𝐻2C2​H2​ (>1 μL/L).

 

Recommendations

Switchgear Inspection: Ensure clean contacts, proper alignment, and correct operating speed for circuit breakers/disconnectors.

Insulation Testing: Perform pre/post-test diagnostics (winding resistance, insulation resistance, tanδ, partial discharge).

Grounding Verification: Measure neutral-point and tank grounding resistance to eliminate floating potentials.

Environmental Control: Maintain dry test conditions (humidity <80%) and avoid contamination.