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.

Courses Joining Link

https://tipsengineerzoneelectricalenggcourses.blogspot.com/2023/08/list-of-all-online-engineering-courses.html