MV Capacitor Interview Question and Answer

 

1️⃣ Principle of a Shunt Capacitor & Applications

Answer:
A shunt capacitor is connected in parallel with a power system to improve power factor and voltage stability. It works by supplying reactive power (leading VARs) to balance the lagging reactive power of inductive loads.

  • Principle: It stores energy in an electric field and releases it, thereby compensating for reactive power.

  • Applications:

    • Power factor correction in industries.

    • Voltage support in transmission/distribution systems.

    • Reduction of line losses.

    • Increasing system capacity without adding new generation.

2️⃣ Routine & Type Tests for MV Capacitors

Routine Tests (every unit, as per IEC 60871 / IS 2834):

  • Capacitance measurement.

  • Tan δ (dissipation factor) measurement.

  • Insulation resistance test.

  • AC withstand voltage test between terminals.

  • AC withstand voltage test between terminals and container.

Type Tests (on prototype or selected sample):

  • Thermal stability test.

  • Discharge test.

  • Partial discharge measurement.

  • Impulse voltage test (lightning impulse).

  • Endurance test (prolonged operation under rated conditions).

  • Temperature rise test.

3️⃣ Tan δ (Dissipation Factor) Test & Significance

Answer:

  • Principle: When AC voltage is applied to a capacitor, ideally the current leads by 90°. In real capacitors, dielectric losses cause the current to lead by slightly less. The angle difference (δ) represents dielectric losses, and Tan δ = Power loss / Reactive power.

  • Significance:

    • Indicates dielectric health of the capacitor.

    • Low Tan δ = good insulation; high Tan δ = deterioration, moisture ingress, or partial discharge inception.

    • Helps in preventive maintenance by identifying weak units before failure.

4️⃣ Ensuring Safety During HV Testing in the Lab

Answer:

  • Grounding & Earthing: Ensure proper earthing of test set, specimen, and measuring instruments.

  • Interlocks: HV test areas must have door interlocks to cut off supply if opened.

  • Barriers & Signs: Isolate HV zone with barriers, warning lamps, and danger signs.

  • Remote Operation: Operate HV equipment from control room with observation windows or CCTV.

  • Discharge Rod: After each test, discharge capacitors fully with earthing rod before touching.

  • PPE: Use insulating gloves, mats, safety shoes.

  • Training & Procedures: Strict adherence to SOPs, only trained personnel allowed.

5️⃣ Common Failure Modes in Capacitors & Investigation

Failure Modes:

  • Dielectric breakdown: due to overvoltage, poor insulation, or ageing.

  • Partial discharges: from voids, moisture, or poor impregnation.

  • Overheating: due to harmonics, high ambient temperature, or poor cooling.

  • Explosion/bursting: due to internal arcing or pressure buildup.

  • Loss of capacitance: caused by open connections or element failure.

Investigation Approach:

  • Visual inspection: swelling, leakage, ruptured case.

  • Electrical checks: capacitance, Tan δ, insulation resistance.

  • Failure analysis: check test history, loading conditions, harmonic content.

  • Root Cause Tools: 5-Why, Fishbone diagram, correlation with operating conditions.


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