EMP Electrical Systems & Power Quality 1 — Questions and Answers
Question 1: What is power factor in an electrical system?
- The ratio of real power to apparent power (Correct answer)
- The ratio of reactive power to real power
- The ratio of apparent power to reactive power
- The ratio of voltage to current
Correct answer: The ratio of real power to apparent power
Power factor is the ratio of real power (kW) to apparent power (kVA), representing how efficiently electrical power is being converted into useful work.
Question 2: What is the typical minimum acceptable power factor for commercial and industrial facilities before utility penalties apply?
- 0.50
- 0.70
- 0.85 (Correct answer)
- 0.99
Correct answer: 0.85
A power factor of 0.85 is generally considered the minimum acceptable threshold, and many utilities impose penalty charges when facility power factors fall below this level.
Question 3: Which device is most commonly installed to improve power factor in commercial and industrial electrical systems?
- Step-up transformer
- Capacitor bank (Correct answer)
- Inductor coil
- Voltage regulator
Correct answer: Capacitor bank
Capacitor banks supply reactive power locally at the facility, reducing the reactive power that must be drawn from the utility and thereby improving power factor.
Question 4: What does Total Harmonic Distortion (THD) measure in an electrical system?
- Total power loss across all distribution feeders
- The ratio of the sum of all harmonic components to the fundamental frequency component (Correct answer)
- The efficiency rating of electrical transformers under full load
- The variation in supply voltage measured over a 24-hour period
Correct answer: The ratio of the sum of all harmonic components to the fundamental frequency component
THD quantifies waveform distortion by expressing the cumulative energy of all harmonic frequencies as a ratio relative to the power at the fundamental frequency (60 Hz in the US).
Question 5: Which of the following is the most common cause of voltage sags in commercial electrical distribution systems?
- Capacitor bank switching on the distribution feeder
- Addition of voltage regulation equipment upstream
- Starting of large motors or heavy industrial equipment (Correct answer)
- Reduced impedance in distribution transformers
Correct answer: Starting of large motors or heavy industrial equipment
Starting large motors causes significant inrush current—often 6 to 10 times the normal running current—which creates a momentary voltage drop (sag) throughout the distribution system.
Question 6: What is the primary purpose of a power quality analyzer used in energy management?
- To measure and record electrical parameters such as voltage, current, and harmonics to identify power quality issues (Correct answer)
- To regulate voltage fluctuations in distribution circuits in real time
- To convert AC power to DC power for sensitive equipment
- To distribute electrical load evenly across all three phases
Correct answer: To measure and record electrical parameters such as voltage, current, and harmonics to identify power quality issues
A power quality analyzer captures and records electrical parameters—including voltage, current, frequency, harmonic distortion, and power factor—to identify, diagnose, and document power quality problems.
Question 7: What is a demand charge in utility billing for commercial and industrial customers?
- A fee assessed on reactive power consumption measured in kVAR
- The cost per kilowatt-hour of electricity consumed during a billing period
- A charge based on the highest peak power demand recorded during the billing period (Correct answer)
- A surcharge applied to electricity used during off-peak hours
Correct answer: A charge based on the highest peak power demand recorded during the billing period
A demand charge is a utility fee based on the highest average power demand (kW) recorded during any demand interval (typically 15 or 30 minutes) within the billing period.
EMP Electrical Systems & Power Quality 1
What is power factor in an electrical system?