Power Factor and Correction Interview Questions

Q1. What is power factor and why does it matter in industrial installations?

Power factor is the ratio of real power (kW) to apparent power (kVA), representing how effectively current is being converted to useful work. In a 415 V induction motor drawing 50 A, a PF of 0.7 means only 70% of the supplied power does real work, causing higher current and cable losses. Utilities in India levy PF penalty charges below 0.95, making correction directly profitable for industries.

Follow-up: How would you calculate the capacitor bank size needed to improve PF from 0.75 to 0.95 for a 100 kW load?

Q2. What causes a lagging power factor in industrial loads?

Lagging power factor is caused by inductive loads that draw magnetizing current, making current lag behind voltage. A 3-phase induction motor running at no load can have a PF as low as 0.2 because almost all current is reactive magnetizing current. The larger the inductive reactance relative to resistance, the worse the lagging PF.

Follow-up: Can a load ever have a leading power factor, and in what situation does that occur?

Q3. What is reactive power and how is it different from real power?

Reactive power (VAR) is the power oscillating between the source and the magnetic or electric field of a load without being consumed, while real power (W) is irreversibly converted to work or heat. A 10 kVAR capacitor bank absorbs reactive power from the grid, reducing the generator's burden without reducing the load's mechanical output. Reactive power cannot do useful work but it must be supplied by the source, increasing current and losses.

Follow-up: What happens to the system voltage if reactive power compensation is excessive?

Q4. How does a capacitor bank correct power factor?

A capacitor bank supplies leading reactive current locally to the load, reducing the lagging reactive current that must be drawn from the grid. A 50 kVAR capacitor bank installed at the terminals of an induction motor reduces the reactive current component from the supply by up to 50 kVAR, lowering total supply current. This reduces cable losses, improves terminal voltage, and avoids utility PF penalty tariffs.

Follow-up: Should capacitor banks be installed at the main bus or at individual motor terminals, and why?

Q5. What is the difference between fixed and automatic power factor correction panels?

Fixed capacitor banks provide constant reactive compensation regardless of load variation, while automatic APFC panels switch capacitor steps in and out using a PF relay to maintain PF near unity under varying loads. A factory with variable load from 20% to 100% would use an APFC panel with 6–8 steps of 25 kVAR each to avoid leading PF at light load. Fixed banks are cheaper but risk over-compensation and voltage rise at light loads.

Follow-up: What is the role of a power factor relay in an APFC panel?

Q6. What happens if a system becomes over-compensated with capacitors?

Over-compensation causes leading power factor, where the load appears capacitive to the source, leading to voltage rise and potential resonance with supply inductance. In a 33 kV feeder, excessive shunt capacitance can cause voltage to exceed the rated level by 5–10%, damaging transformer insulation. Utilities penalize both lagging and leading PF below 0.95 in some tariff structures.

Follow-up: How do you protect against ferroresonance when capacitor banks are used with lightly loaded transformers?

Q7. How do you size a capacitor bank to improve power factor from 0.8 to 0.95?

Capacitor size in kVAR equals P × (tan φ1 − tan φ2), where P is real power in kW, φ1 is the original PF angle, and φ2 is the target PF angle. For a 200 kW load at 0.8 PF, tan(36.87°) = 0.75 and tan(18.19°) = 0.329, so required kVAR = 200 × (0.75 − 0.329) = 84.2 kVAR. Always select the next standard step above the calculated value, typically 90 kVAR in this case.

Follow-up: How does operating temperature affect the actual kVAR output of a capacitor bank?

Q8. What is the significance of power factor in transformer loading?

A transformer is rated in kVA, so at lower power factor a larger transformer is needed to deliver the same real power kW to the load. A 500 kVA transformer at 0.8 PF delivers only 400 kW, but at 0.95 PF it delivers 475 kW — a 19% increase in useful output from the same asset. Improving PF effectively increases the kW capacity of existing transformers and cables without capital replacement.

Follow-up: How does harmonics from VFDs affect power factor measurement and capacitor bank performance?

Q9. What is displacement power factor versus true power factor?

Displacement PF is the cosine of the phase angle between the fundamental voltage and current waveforms, while true PF accounts for harmonic distortion and equals real power divided by apparent power including all harmonics. In a VFD-fed motor system with 25% THD in current, displacement PF may read 0.95 while true PF is only 0.87. Energy meters measure true PF, so utilities see the lower value for billing purposes.

Follow-up: Why can installing standard capacitors worsen harmonic distortion instead of improving it?

Q10. How do synchronous motors and synchronous condensers help in PF correction?

A synchronous motor operating over-excited draws leading current from the supply, acting like a rotating capacitor and supplying reactive power to the grid. BHEL and large steel plants use synchronous condensers — synchronous motors running at no mechanical load — purely for reactive compensation, capable of providing 50–200 MVAR continuously. Unlike capacitor banks, their VAR output can be continuously varied by adjusting field excitation.

Follow-up: What is the advantage of a STATCOM over a synchronous condenser for dynamic reactive compensation?

Q11. What is the unity power factor condition and is it always the best operating point?

Unity PF means all current drawn from the supply is real current, with zero reactive component, minimizing supply current for a given real power load. While unity PF minimizes I²R losses in cables and transformers, operating slightly lagging at 0.98 is often preferred to provide voltage support margin in weak grids. Capacitor banks targeting exactly unity PF risk leading PF at light load, which can cause voltage instability.

Follow-up: Why do grid operators prefer distribution systems to operate at a slightly lagging power factor rather than exactly unity?

Q12. How does a low power factor increase electricity bills for industrial consumers?

Electricity boards in India apply a PF surcharge — typically 1–2% extra for every 0.01 drop below 0.90 PF — directly increasing the billed amount on top of kWh consumption. A cement plant consuming 1,00,000 kWh/month at 0.80 PF could pay 10–20% more than the same plant at 0.98 PF. Some boards also offer PF incentives, crediting 1% for every 0.01 PF above 0.99.

Follow-up: Which Indian electricity boards have the strictest PF penalty clauses in their tariff orders?

Q13. What is the effect of power factor on voltage regulation of a feeder?

Voltage regulation of a feeder worsens as power factor decreases because more reactive current flows, creating larger voltage drops across the feeder's inductive reactance. A 11 kV feeder with 0.6 Ω/km reactance carrying load at 0.75 PF has significantly higher voltage drop than the same feeder at 0.95 PF, often exceeding the ±6% limit permitted by CEA regulations. PF correction at the load end directly reduces feeder voltage drop.

Follow-up: What is the Ferranti effect and how is it related to power factor on long feeders?

Q14. What are the protective measures needed when installing large capacitor banks?

Large capacitor banks need series reactors (5–6% reactance) to limit inrush current on switching, fuses or circuit breakers rated for capacitive switching duty, and discharge resistors to drain stored charge within 1 minute after disconnection per IEC 60831. A 1 MVAR capacitor bank at 11 kV stores enough energy to cause fatal electric shock and equipment damage if not discharged before maintenance access. Overvoltage relays (59) protect against resonance over-voltage conditions.

Follow-up: What is the difference between capacitor switching duty breakers and standard circuit breakers?

Q15. How does harmonic distortion interact with power factor correction capacitors?

Capacitors have decreasing impedance at higher frequencies, so they attract harmonic currents from non-linear loads like VFDs and UPS systems, causing capacitor overheating and failure. A 100 kVAR capacitor bank near a 132 kW VFD without detuning reactors can carry 150–200% of rated current due to 5th and 7th harmonics, failing within months. Detuned harmonic filters use series reactors tuned to 189 Hz (4.7% reactor) to prevent harmonic amplification.

Follow-up: What is a harmonic filter and how does it differ from a plain capacitor bank with a reactor?