33 real Power Quality & Harmonics questions from the Electrical Design bank, as asked in Indian campus drives and tech interviews. Every question has a verified answer and an AI-tutor explanation on placd — free to start.
1. What is Total harmonic distortion?
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A.drop to 10–90% of nominal rms lasting half a cycle to one minute, usually from remote faults or large motor starting, per IEEE 1159
B.parallel resonance between PF capacitors and source inductance near f1·√(Ssc/Qc), amplifying a nearby harmonic and blowing capacitor fuses
C.short-term flicker severity from a 10-minute measurement per IEC 61000-4-15, where Pst = 1 is the conventional irritation threshold
D.ratio of the rms of all harmonic components to the fundamental, expressed as a percentage, for either voltage or current
A.Total harmonic distortion — required kVAr equals kW times (tan φ1 − tan φ2) to move from the existing to the target power factor
B.Total harmonic distortion — voltage THD limited to 8% at 1 kV and below, 5% for 1–69 kV and 2.5% for 69–161 kV at the point of common coupling
C.Total harmonic distortion — short-term flicker severity from a 10-minute measurement per IEC 61000-4-15, where Pst = 1 is the conventional irritation threshold
D.Total harmonic distortion — ratio of the rms of all harmonic components to the fundamental, expressed as a percentage, for either voltage or current
5. Which term means: "displacement PF is cos φ of the fundamental only; true PF includes distortion, so a VFD can show 0.95 displacement but 0.7 true"?
A.Displacement vs true power factor — short-term flicker severity from a 10-minute measurement per IEC 61000-4-15, where Pst = 1 is the conventional irritation threshold
B.Displacement vs true power factor — displacement PF is cos φ of the fundamental only; true PF includes distortion, so a VFD can show 0.95 displacement but 0.7 true
C.Displacement vs true power factor — series reactor of 7% (189 Hz tuning) makes the capacitor bank inductive at the 5th harmonic so it cannot resonate with the supply
D.Displacement vs true power factor — drop to 10–90% of nominal rms lasting half a cycle to one minute, usually from remote faults or large motor starting, per IEEE 1159
8. Which term means: "drop to 10–90% of nominal rms lasting half a cycle to one minute, usually from remote faults or large motor starting, per IEEE 1159"?
A.Voltage sag — two 6-pulse bridges fed by delta and star secondaries cancel the 5th and 7th, leaving the 11th and 13th as the lowest characteristic harmonics
B.Voltage sag — ratio of the rms of all harmonic components to the fundamental, expressed as a percentage, for either voltage or current
C.Voltage sag — series reactor of 7% (189 Hz tuning) makes the capacitor bank inductive at the 5th harmonic so it cannot resonate with the supply
D.Voltage sag — drop to 10–90% of nominal rms lasting half a cycle to one minute, usually from remote faults or large motor starting, per IEEE 1159
A.IEEE 519 voltage limits — series reactor of 7% (189 Hz tuning) makes the capacitor bank inductive at the 5th harmonic so it cannot resonate with the supply
B.IEEE 519 voltage limits — 3rd, 9th and 15th order zero-sequence harmonics that add in the neutral of a four-wire system and can load the neutral above phase current
C.IEEE 519 voltage limits — voltage THD limited to 8% at 1 kV and below, 5% for 1–69 kV and 2.5% for 69–161 kV at the point of common coupling
D.IEEE 519 voltage limits — ratio of the rms of all harmonic components to the fundamental, expressed as a percentage, for either voltage or current
A.IEEE 519 current TDD — current distortion limit depends on the Isc/IL ratio at the PCC, from 5% for a stiff ratio below 20 up to 20% above 1000
B.IEEE 519 current TDD — series reactor of 7% (189 Hz tuning) makes the capacitor bank inductive at the 5th harmonic so it cannot resonate with the supply
C.IEEE 519 current TDD — ratio of the rms of all harmonic components to the fundamental, expressed as a percentage, for either voltage or current
D.IEEE 519 current TDD — voltage THD limited to 8% at 1 kV and below, 5% for 1–69 kV and 2.5% for 69–161 kV at the point of common coupling
A.Capacitor bank sizing — required kVAr equals kW times (tan φ1 − tan φ2) to move from the existing to the target power factor
B.Capacitor bank sizing — drop to 10–90% of nominal rms lasting half a cycle to one minute, usually from remote faults or large motor starting, per IEEE 1159
C.Capacitor bank sizing — series reactor of 7% (189 Hz tuning) makes the capacitor bank inductive at the 5th harmonic so it cannot resonate with the supply
D.Capacitor bank sizing — voltage THD limited to 8% at 1 kV and below, 5% for 1–69 kV and 2.5% for 69–161 kV at the point of common coupling
20. Which term means: "series reactor of 7% (189 Hz tuning) makes the capacitor bank inductive at the 5th harmonic so it cannot resonate with the supply"?
A.Detuned filter reactor — ratio of the rms of all harmonic components to the fundamental, expressed as a percentage, for either voltage or current
B.Detuned filter reactor — required kVAr equals kW times (tan φ1 − tan φ2) to move from the existing to the target power factor
C.Detuned filter reactor — series reactor of 7% (189 Hz tuning) makes the capacitor bank inductive at the 5th harmonic so it cannot resonate with the supply
D.Detuned filter reactor — parallel resonance between PF capacitors and source inductance near f1·√(Ssc/Qc), amplifying a nearby harmonic and blowing capacitor fuses
23. Which term means: "parallel resonance between PF capacitors and source inductance near f1·√(Ssc/Qc), amplifying a nearby harmonic and blowing capacitor fuses"?
A.Harmonic resonance — 3rd, 9th and 15th order zero-sequence harmonics that add in the neutral of a four-wire system and can load the neutral above phase current
B.Harmonic resonance — parallel resonance between PF capacitors and source inductance near f1·√(Ssc/Qc), amplifying a nearby harmonic and blowing capacitor fuses
C.Harmonic resonance — two 6-pulse bridges fed by delta and star secondaries cancel the 5th and 7th, leaving the 11th and 13th as the lowest characteristic harmonics
D.Harmonic resonance — short-term flicker severity from a 10-minute measurement per IEC 61000-4-15, where Pst = 1 is the conventional irritation threshold
26. Which term means: "short-term flicker severity from a 10-minute measurement per IEC 61000-4-15, where Pst = 1 is the conventional irritation threshold"?
A.Flicker Pst — two 6-pulse bridges fed by delta and star secondaries cancel the 5th and 7th, leaving the 11th and 13th as the lowest characteristic harmonics
B.Flicker Pst — current distortion limit depends on the Isc/IL ratio at the PCC, from 5% for a stiff ratio below 20 up to 20% above 1000
C.Flicker Pst — series reactor of 7% (189 Hz tuning) makes the capacitor bank inductive at the 5th harmonic so it cannot resonate with the supply
D.Flicker Pst — short-term flicker severity from a 10-minute measurement per IEC 61000-4-15, where Pst = 1 is the conventional irritation threshold
29. Which term means: "3rd, 9th and 15th order zero-sequence harmonics that add in the neutral of a four-wire system and can load the neutral above phase current"?
A.Triplen harmonics — short-term flicker severity from a 10-minute measurement per IEC 61000-4-15, where Pst = 1 is the conventional irritation threshold
B.Triplen harmonics — voltage THD limited to 8% at 1 kV and below, 5% for 1–69 kV and 2.5% for 69–161 kV at the point of common coupling
C.Triplen harmonics — current distortion limit depends on the Isc/IL ratio at the PCC, from 5% for a stiff ratio below 20 up to 20% above 1000
D.Triplen harmonics — 3rd, 9th and 15th order zero-sequence harmonics that add in the neutral of a four-wire system and can load the neutral above phase current
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