Q.1 Which IEC standard defines the limits for voltage sags and swells in power quality monitoring?
IEC 61000-4-30
IEC 61000-4-11
IEC 61000-4-30
IEC 61000-4-9
Explanation - IEC 61000‑4‑30 specifies the voltage dip (sag) and swell limits used for power‑quality monitoring.
Correct answer is: IEC 61000-4-30
Q.2 What does the acronym THD stand for in power quality analysis?
Total Harmonic Distortion
Thermal Harmonic Deviation
Transient Harmonic Drain
Total Harmonic Drain
Explanation - THD is the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency.
Correct answer is: Total Harmonic Distortion
Q.3 Which device is commonly used to measure instantaneous voltage and current waveforms for power quality monitoring?
Oscilloscope
Power Quality Analyzer
Multimeter
Thermocouple
Explanation - A power quality analyzer captures high‑speed voltage/current data and computes parameters such as THD, flicker, and sags.
Correct answer is: Power Quality Analyzer
Q.4 According to IEEE 1159, which event is defined as a temporary reduction in RMS voltage to between 10% and 90% of nominal for 0.5 cycles to 1 minute?
Voltage Sag
Voltage Swell
Voltage Dip
Voltage Flicker
Explanation - A voltage sag (or dip) matches that description in IEEE 1159.
Correct answer is: Voltage Sag
Q.5 Which parameter best quantifies the severity of voltage flicker perceived by human eyes?
Pst (Short‑term flicker severity)
THD
VUF
Crest factor
Explanation - Pst is a statistical index used to assess short‑term flicker that can cause visual discomfort.
Correct answer is: Pst (Short‑term flicker severity)
Q.6 In a digital power quality monitor, what is the purpose of applying a Fast Fourier Transform (FFT) to the sampled waveform?
To convert the signal from time domain to frequency domain
To increase the voltage level
To filter out noise completely
To measure DC offset
Explanation - FFT decomposes the sampled waveform into its frequency components, enabling harmonic analysis.
Correct answer is: To convert the signal from time domain to frequency domain
Q.7 Which of the following is NOT a typical cause of voltage sags?
Motor starting
Lightning strike
Transformer energization
Capacitor bank switching
Explanation - Lightning strikes usually cause voltage surges or transients, not sags.
Correct answer is: Lightning strike
Q.8 What is the recommended minimum sampling rate for accurate harmonic analysis up to the 50th harmonic in a 50 Hz system?
2.5 kHz
10 kHz
5 kHz
1 kHz
Explanation - Nyquist requires at least twice the highest frequency component: 50 Hz × 50 = 2.5 kHz, so a safe margin is 5 kHz.
Correct answer is: 5 kHz
Q.9 Which metric is used to express the proportion of harmonic voltage to the fundamental voltage?
Voltage Unbalance Factor (VUF)
Total Harmonic Distortion (THD)
Crest Factor
Power Factor
Explanation - THD quantifies the harmonic content relative to the fundamental component.
Correct answer is: Total Harmonic Distortion (THD)
Q.10 A power quality monitoring system logs data for later analysis. Which type of monitoring does this represent?
Real‑time monitoring
Offline (post‑event) monitoring
Predictive monitoring
Preventive monitoring
Explanation - Data is stored and examined after the event, unlike real‑time systems that act immediately.
Correct answer is: Offline (post‑event) monitoring
Q.11 Which of the following devices can actively inject compensating currents to mitigate harmonics?
Passive harmonic filter
Active Power Filter (APF)
Surge arrester
Circuit breaker
Explanation - APFs generate currents that cancel harmonic components, providing active compensation.
Correct answer is: Active Power Filter (APF)
Q.12 In IEC 61000‑4‑30, what does the term ‘event‑based recording’ refer to?
Recording only when a predefined threshold is exceeded
Continuous recording at fixed intervals
Recording after a power outage
Recording only during peak load
Explanation - Event‑based recording captures data only when a power‑quality event meets set criteria.
Correct answer is: Recording only when a predefined threshold is exceeded
Q.13 Which parameter is most useful for assessing the impact of voltage unbalance on three‑phase motors?
Voltage Unbalance Factor (VUF)
THD
Flicker severity (Pst)
Crest factor
Explanation - VUF measures the degree of voltage imbalance, directly affecting motor heating and torque.
Correct answer is: Voltage Unbalance Factor (VUF)
Q.14 What is the typical duration range considered for a voltage interruption in power quality standards?
0 ms to 1 ms
1 ms to 0.1 s
0.1 s to 1 min
1 min to 10 min
Explanation - Interruptions are defined as voltage drops to zero lasting from 0.1 s up to 1 minute.
Correct answer is: 0.1 s to 1 min
Q.15 Which of the following is a common cause of high‑frequency transients in power systems?
Capacitor switching
Motor overload
Transformer energization
Line-to-ground fault
Explanation - Sudden switching of large capacitors generates high‑frequency ringing transients.
Correct answer is: Capacitor switching
Q.16 In a power quality monitor, what does the acronym RMS stand for?
Root Mean Square
Random Modulation Signal
Real Minimum Sum
Resistance Measurement Standard
Explanation - RMS is the effective value of a varying voltage or current.
Correct answer is: Root Mean Square
Q.17 Which monitoring method can detect power quality events that occur for only a few milliseconds?
Slow‑log data acquisition (1 s interval)
High‑speed transient recording (≥10 kS/s)
Daily averaged readings
Manual meter reading
Explanation - High‑speed sampling captures short‑duration events that slower methods miss.
Correct answer is: High‑speed transient recording (≥10 kS/s)
Q.18 Which parameter is most directly related to the heating effect of harmonics in electrical equipment?
Total Harmonic Distortion (THD)
Voltage flicker (Pst)
Crest factor
Power factor
Explanation - Higher THD increases RMS heating in conductors and equipment.
Correct answer is: Total Harmonic Distortion (THD)
Q.19 What does the term ‘swell’ describe in power quality terminology?
A temporary increase in RMS voltage above nominal
A temporary decrease in RMS voltage below nominal
A rapid voltage oscillation
A long‑term voltage drift
Explanation - Voltage swell is a short‑duration rise in voltage magnitude, opposite of a sag.
Correct answer is: A temporary increase in RMS voltage above nominal
Q.20 Which of the following devices is primarily used to protect against over‑voltage transients?
Surge arrester
Harmonic filter
Voltage regulator
Breaker
Explanation - Surge arresters clamp high‑voltage spikes to safe levels.
Correct answer is: Surge arrester
Q.21 In a three‑phase system, which quantity is used to calculate voltage unbalance according to IEC 61000‑4‑30?
Maximum deviation of any phase from the average
Sum of all phase voltages
Difference between the highest and lowest phase voltage
Phase sequence angle
Explanation - IEC 61000‑4‑30 defines voltage unbalance as the greatest deviation of a phase voltage from the average of the three phases.
Correct answer is: Maximum deviation of any phase from the average
Q.22 Which type of power quality monitor typically communicates data using Modbus TCP/IP?
Standalone handheld meter
Networked remote monitor
Analog panel meter
Portable oscilloscope
Explanation - Remote monitors often support Modbus TCP/IP for SCADA integration.
Correct answer is: Networked remote monitor
Q.23 What does the term ‘cumulative distribution function (CDF)’ represent in power quality statistics?
Probability that a parameter exceeds a certain value
Average value of a parameter over time
Maximum observed value of a parameter
Rate of change of a parameter
Explanation - CDF shows the probability that a measured value is less than or equal to a given threshold.
Correct answer is: Probability that a parameter exceeds a certain value
Q.24 Which metric is used to evaluate the effect of voltage fluctuations on sensitive electronic equipment?
Short‑term flicker severity (Pst)
Total Harmonic Distortion (THD)
Voltage Unbalance Factor (VUF)
Crest factor
Explanation - Pst quantifies the perceptible effect of rapid voltage changes on equipment.
Correct answer is: Short‑term flicker severity (Pst)
Q.25 A power quality monitor reports a THD of 8 % at the point of common coupling (PCC). According to IEC 61000‑2‑2, is this level acceptable for most industrial loads?
Yes, it is below the 10 % limit
No, it exceeds the 5 % limit
Yes, because any THD below 15 % is acceptable
No, because any THD above 3 % is unacceptable
Explanation - IEC 61000‑2‑2 recommends a THD limit of 10 % for most industrial applications.
Correct answer is: Yes, it is below the 10 % limit
Q.26 Which of the following statements about voltage flicker is true?
Flicker is caused only by voltage sags.
Flicker is a statistical measure of voltage fluctuations over time.
Flicker is measured in percent harmonics.
Flicker only affects lighting loads.
Explanation - Flicker quantifies the frequency and magnitude of voltage changes that cause visual disturbance.
Correct answer is: Flicker is a statistical measure of voltage fluctuations over time.
Q.27 What is the purpose of a ‘phase‑locked loop (PLL)’ in a digital power quality analyzer?
To synchronize the sampling clock with the fundamental frequency
To increase the voltage amplitude
To filter out low‑frequency noise
To convert AC to DC
Explanation - PLL ensures that the analyzer tracks the true line frequency for accurate RMS and harmonic calculations.
Correct answer is: To synchronize the sampling clock with the fundamental frequency
Q.28 Which data storage format is commonly used by power quality monitors for easy post‑processing?
CSV (Comma‑Separated Values)
PDF
JPEG
DOCX
Explanation - CSV files can be imported into spreadsheets and analysis tools for further processing.
Correct answer is: CSV (Comma‑Separated Values)
Q.29 What is the typical threshold for considering a voltage dip as a ‘severe sag’ in IEEE 1159?
Voltage drop to 0.9 pu
Voltage drop to 0.5 pu
Voltage drop to 0.2 pu
Voltage drop to 0.1 pu
Explanation - IEEE 1159 classifies a sag to 0.5 pu or lower as a severe sag.
Correct answer is: Voltage drop to 0.5 pu
Q.30 In power quality monitoring, what does the term ‘event‑duration histogram’ illustrate?
Frequency of events versus their durations
Amplitude of voltage versus time
Harmonic order distribution
Power factor variation
Explanation - A histogram shows how many events occurred within specific duration bins.
Correct answer is: Frequency of events versus their durations
Q.31 Which component in a power quality monitor converts analog voltage signals to digital values?
Analog‑to‑Digital Converter (ADC)
Digital‑to‑Analog Converter (DAC)
Transformer
Relay
Explanation - The ADC samples analog waveforms and provides digital data for processing.
Correct answer is: Analog‑to‑Digital Converter (ADC)
Q.32 A power quality monitor indicates a voltage unbalance factor (VUF) of 3 %. According to IEC 61000‑2‑2, is this acceptable for most distribution networks?
Yes, because it is below the 5 % limit
No, because any VUF above 1 % is unacceptable
Yes, because VUF is not regulated
No, because the limit is 2 %
Explanation - IEC 61000‑2‑2 recommends a VUF limit of 5 % for most networks.
Correct answer is: Yes, because it is below the 5 % limit
Q.33 Which of the following best describes a ‘real‑time’ power quality monitoring system?
It records data continuously and provides instant alerts for events.
It stores data for analysis after a month.
It only measures voltage once per hour.
It requires manual data entry.
Explanation - Real‑time systems detect and often act on power‑quality disturbances as they happen.
Correct answer is: It records data continuously and provides instant alerts for events.
Q.34 Which of the following is a typical advantage of using a cloud‑based power quality monitoring platform?
Remote access to data from any location
Elimination of the need for sensors
Zero cost for all users
Automatic correction of power disturbances
Explanation - Cloud platforms enable centralized data storage and remote visualization.
Correct answer is: Remote access to data from any location
Q.35 What is the main purpose of a ‘neutral‑to‑ground’ voltage monitor in power quality analysis?
Detect stray voltages that may cause equipment damage
Measure line‑to‑line voltage
Control motor speed
Regulate frequency
Explanation - Monitoring neutral‑to‑ground voltage helps identify ground faults and leakage currents.
Correct answer is: Detect stray voltages that may cause equipment damage
Q.36 Which method is commonly used to reduce the impact of voltage sags on critical loads?
Installation of Uninterruptible Power Supplies (UPS)
Adding series reactors
Increasing line length
Using larger fuses
Explanation - UPS devices provide instant backup power, maintaining voltage during sags.
Correct answer is: Installation of Uninterruptible Power Supplies (UPS)
Q.37 In a harmonic spectrum, which order is typically the most dominant after the fundamental in a three‑phase system with nonlinear loads?
3rd harmonic
5th harmonic
7th harmonic
9th harmonic
Explanation - Odd‑order, non‑multiples of three (5th, 7th, 11th, etc.) are usually the strongest in three‑phase systems.
Correct answer is: 5th harmonic
Q.38 What does the term ‘crest factor’ represent in the context of voltage waveforms?
Ratio of peak value to RMS value
Ratio of RMS to average value
Difference between peak and trough values
Sum of all harmonic amplitudes
Explanation - Crest factor = Vpeak / Vrms; it indicates the peaking of a waveform.
Correct answer is: Ratio of peak value to RMS value
Q.39 A power quality monitor shows a 30 % voltage sag lasting 0.2 s. Which of the following devices is most likely to trigger an alarm?
Sags detector with a 10 % threshold and 0.1 s minimum duration
Harmonic monitor set to 50 % THD
Flicker meter with Pst > 1.0
Voltage unbalance logger
Explanation - The sag exceeds both magnitude and duration thresholds, causing an alarm.
Correct answer is: Sags detector with a 10 % threshold and 0.1 s minimum duration
Q.40 Which sampling technique minimizes aliasing when measuring high‑frequency transients?
Using an anti‑aliasing filter before the ADC
Sampling at 1 kS/s regardless of signal content
Averaging multiple samples
Increasing the voltage level
Explanation - Anti‑aliasing filters attenuate frequencies above half the sampling rate.
Correct answer is: Using an anti‑aliasing filter before the ADC
Q.41 In the context of power quality, what does the abbreviation ‘PQ’ stand for?
Power Quality
Peak Quantity
Phase Quantity
Power Quotient
Explanation - ‘PQ’ is the standard abbreviation for Power Quality.
Correct answer is: Power Quality
Q.42 Which IEC standard defines the measurement methods for voltage sags, swells, and interruptions?
IEC 61000‑4‑30
IEC 61000‑4‑11
IEC 61000‑4‑34
IEC 61000‑2‑2
Explanation - IEC 61000‑4‑30 specifies measurement techniques for these power‑quality events.
Correct answer is: IEC 61000‑4‑30
Q.43 What is the effect of a balanced three‑phase system on the zero‑sequence component of the current?
Zero‑sequence current is zero
Zero‑sequence current is maximum
Zero‑sequence current equals line current
Zero‑sequence current depends on load type
Explanation - In a perfectly balanced system, the sum of the three phase currents is zero, eliminating zero‑sequence components.
Correct answer is: Zero‑sequence current is zero
Q.44 A power quality monitor uses a 16‑bit ADC with a full‑scale range of ±400 V. What is the voltage resolution of the converter?
12.2 mV
6.1 mV
24.4 mV
0.61 mV
Explanation - Resolution = (Full‑scale range) / (2^bits) = 800 V / 65536 ≈ 12.2 mV.
Correct answer is: 12.2 mV
Q.45 Which parameter would you monitor to detect a possible resonance condition in a distribution network?
Amplified harmonic at a specific frequency
Voltage magnitude only
Frequency deviation
Power factor lagging
Explanation - Resonance manifests as a large increase in a particular harmonic amplitude.
Correct answer is: Amplified harmonic at a specific frequency
Q.46 Which type of filter is best suited to attenuate high‑order harmonics while preserving the fundamental frequency?
Low‑pass passive filter
Band‑stop filter
High‑pass filter
All‑pass filter
Explanation - A low‑pass filter passes the fundamental and blocks higher‑order harmonics.
Correct answer is: Low‑pass passive filter
Q.47 What does the term ‘interharmonic’ refer to in a power quality spectrum?
A frequency component that is not an integer multiple of the fundamental
A harmonic that is exactly twice the fundamental
A DC offset in the waveform
A measurement error
Explanation - Interharmonics lie between the integer harmonic frequencies.
Correct answer is: A frequency component that is not an integer multiple of the fundamental
Q.48 In a power quality report, the parameter ‘Pst = 2.5’ indicates:
Severe flicker likely to cause visual discomfort
Excellent voltage stability
Low harmonic distortion
High voltage unbalance
Explanation - Pst values above 1.0 are considered perceptible; 2.5 is classified as severe.
Correct answer is: Severe flicker likely to cause visual discomfort
Q.49 Which of the following is a primary benefit of installing a STATCOM for power quality improvement?
Dynamic reactive power compensation
Permanent reduction of all harmonics
Elimination of voltage sags
Increasing line resistance
Explanation - STATCOMs provide fast reactive power support, improving voltage stability.
Correct answer is: Dynamic reactive power compensation
Q.50 Which term describes the ratio of the real power (kW) to the apparent power (kVA) in an AC system?
Power factor
Crest factor
THD
Flicker index
Explanation - Power factor = P / S, indicating the phase relationship between voltage and current.
Correct answer is: Power factor
Q.51 A power quality monitor shows a repetitive voltage dip every 20 ms in a 50 Hz system. What is the most likely cause?
Cycling load such as a compressor
Lightning strike
Single‑phase to three‑phase conversion error
Transformer failure
Explanation - A 20 ms period corresponds to 50 Hz/2, typical of periodic motor starting currents.
Correct answer is: Cycling load such as a compressor
Q.52 Which of the following is NOT a typical output of a power quality monitoring software?
Harmonic spectrum plot
Voltage sag duration histogram
Real‑time video streaming
Cumulative flicker index (Plt)
Explanation - Power quality software focuses on electrical data, not video streams.
Correct answer is: Real‑time video streaming
Q.53 In a three‑phase four‑wire system, a neutral current of 30 A is observed while line currents are balanced at 100 A. Which condition could cause this?
Unbalanced single‑phase loads
Perfectly balanced three‑phase loads
Presence of only resistive loads
Zero‑sequence harmonic currents only
Explanation - Unequal single‑phase loads cause neutral current even when line currents are balanced.
Correct answer is: Unbalanced single‑phase loads
Q.54 Which measurement technique is used to detect transient over‑voltages that last less than 1 ms?
High‑speed transient recorder (≥1 MS/s)
Hourly averaged RMS monitor
Daily energy meter
Power factor meter
Explanation - Fast sampling is required to capture sub‑millisecond transients.
Correct answer is: High‑speed transient recorder (≥1 MS/s)
Q.55 The IEC 61850 protocol is primarily used for:
Communication between intelligent electronic devices (IEDs) in substations
Measuring harmonic distortion
Controlling motor speed
Generating power
Explanation - IEC 61850 defines data models and communication services for substation automation.
Correct answer is: Communication between intelligent electronic devices (IEDs) in substations
Q.56 What does the term ‘RMS jitter’ refer to in the context of power quality monitoring?
Variation in the sampling instant of the RMS measurement
Fluctuation of the voltage frequency
Random noise in the measured waveform
Change in the crest factor
Explanation - RMS jitter describes timing variations that affect RMS accuracy.
Correct answer is: Variation in the sampling instant of the RMS measurement
Q.57 Which of the following actions is most effective for reducing voltage flicker caused by large lighting loads?
Installing a static VAR compensator (SVC)
Adding series resistors
Increasing line length
Using a larger breaker
Explanation - SVCs can rapidly inject or absorb reactive power, stabilizing voltage during load changes.
Correct answer is: Installing a static VAR compensator (SVC)
Q.58 A power quality monitor records a 10 % voltage swell lasting 0.4 s. According to IEC 61000‑4‑30, into which category does this event fall?
Minor swell
Moderate swell
Severe swell
Transient spike
Explanation - IEC 61000‑4‑30 classifies swells of 0.1–0.2 pu as minor; 10 % (0.1 pu) matches this range.
Correct answer is: Minor swell
Q.59 Which component of a power quality monitor is responsible for storing captured event data for later retrieval?
Memory buffer (e.g., SD card)
Voltage transformer
Current shunt
Relay contact
Explanation - Event data is written to internal memory or external storage devices.
Correct answer is: Memory buffer (e.g., SD card)
Q.60 In the context of power quality, what does the abbreviation ‘VUF’ stand for?
Voltage Unbalance Factor
Voltage Under Frequency
Variable Utilization Factor
Voltage Unity Factor
Explanation - VUF quantifies the degree of voltage unbalance in a three‑phase system.
Correct answer is: Voltage Unbalance Factor
Q.61 Which type of power quality disturbance is most likely to cause data loss in digital communication equipment?
Voltage sags
Voltage swells
High‑frequency transients
Voltage unbalance
Explanation - Fast transients can upset sensitive digital circuits and cause data errors.
Correct answer is: High‑frequency transients
Q.62 A power quality analyzer displays a crest factor of 1.8 for a particular voltage waveform. What does this indicate?
The peak voltage is 1.8 times the RMS voltage
The waveform is highly distorted
The total harmonic distortion is 1.8 %
The frequency is 1.8 Hz
Explanation - Crest factor = Vpeak / Vrms; a value of 1.8 is typical for a slightly peaky waveform.
Correct answer is: The peak voltage is 1.8 times the RMS voltage
Q.63 Which power quality index combines the effects of voltage sags, swells, and interruptions into a single value?
Power Quality Index (PQI)
Total Harmonic Distortion (THD)
Flicker Severity (Pst)
Voltage Unbalance Factor (VUF)
Explanation - PQI aggregates multiple power‑quality event metrics into a composite score.
Correct answer is: Power Quality Index (PQI)
Q.64 Which of the following statements about the IEEE 519 standard is correct?
It defines limits for voltage sags and swells.
It provides guidelines for harmonic control in power systems.
It specifies communication protocols for power quality monitors.
It mandates the use of active filters in all facilities.
Explanation - IEEE 519 focuses on harmonic limits and mitigation techniques.
Correct answer is: It provides guidelines for harmonic control in power systems.
Q.65 When configuring a power quality monitor, why is it important to set appropriate trigger thresholds?
To avoid missing important events while reducing unnecessary data storage
To increase the voltage level of the system
To change the system frequency
To calibrate the power factor
Explanation - Proper thresholds ensure relevant events are captured without overwhelming storage with irrelevant data.
Correct answer is: To avoid missing important events while reducing unnecessary data storage
Q.66 Which of the following best describes a ‘frequency dip’ in power quality monitoring?
A temporary reduction in system frequency below nominal
A sudden increase in voltage magnitude
A rapid change in harmonic content
A sustained over‑voltage condition
Explanation - Frequency dip refers to a short‑term drop in system frequency.
Correct answer is: A temporary reduction in system frequency below nominal
Q.67 A power quality monitor uses a moving‑average filter on the RMS voltage calculation. What is the main effect of this filter?
It smooths out rapid voltage changes, reducing sensitivity to short events
It amplifies high‑frequency harmonics
It converts AC to DC
It increases the sampling rate
Explanation - Moving‑average filters average values over a window, dampening fast fluctuations.
Correct answer is: It smooths out rapid voltage changes, reducing sensitivity to short events
Q.68 Which of the following is a common cause of interharmonics in a power system?
Cycloconverter operation
Resistive heating
Linear loads
Balanced three‑phase motors
Explanation - Cycloconverters generate frequencies that are not integer multiples of the fundamental.
Correct answer is: Cycloconverter operation
Q.69 In a power quality report, a ‘cumulative flicker index (Plt)’ of 0.9 is shown. What does this indicate about the long‑term flicker exposure?
The exposure is close to the limit of perceptibility
The system has no flicker problems
Voltage distortion is high
Harmonic distortion is negligible
Explanation - Plt values approaching 1.0 indicate significant long‑term flicker, potentially noticeable.
Correct answer is: The exposure is close to the limit of perceptibility
Q.70 Which measurement unit is used for reporting voltage harmonic amplitude?
Percent of fundamental (%)
Volts per hour (V/h)
Amperes (A)
Hertz (Hz)
Explanation - Harmonic amplitudes are typically expressed as a percentage of the fundamental RMS voltage.
Correct answer is: Percent of fundamental (%)
Q.71 What is the primary benefit of using a ‘distributed’ power quality monitoring architecture?
Higher spatial resolution of events across the network
Elimination of all power‑quality problems
Reduced need for any sensors
Automatic correction of voltage sags
Explanation - Distributed monitors provide localized data, helping pinpoint disturbance origins.
Correct answer is: Higher spatial resolution of events across the network
Q.72 Which of the following best explains why a high‑order harmonic (e.g., 31st) is often less concerning than lower‑order harmonics?
Higher‑order harmonics have less energy and are more easily filtered by system impedance
They cause more heating than low‑order harmonics
They increase voltage sags significantly
They are not measured by any standard
Explanation - System impedance rises with frequency, attenuating higher‑order harmonics.
Correct answer is: Higher‑order harmonics have less energy and are more easily filtered by system impedance
Q.73 Which IEC standard deals specifically with the measurement of voltage fluctuations (flicker)?
IEC 61000‑4‑15
IEC 61000‑4‑30
IEC 61000‑4‑11
IEC 61000‑2‑2
Explanation - IEC 61000‑4‑15 defines measurement methods for flicker (voltage fluctuations).
Correct answer is: IEC 61000‑4‑15
Q.74 A power quality monitoring system uses a 1 kS/s sampling rate. What is the highest frequency component it can accurately capture according to the Nyquist theorem?
500 Hz
1 kHz
250 Hz
2 kHz
Explanation - Nyquist limit = half the sampling rate; thus 1 kS/s → 500 Hz maximum.
Correct answer is: 500 Hz
Q.75 Which of the following power quality events is typically measured in terms of its “depth” rather than its magnitude?
Voltage sag
Harmonic distortion
Flicker
Frequency deviation
Explanation - Depth refers to how far the voltage drops below nominal during a sag.
Correct answer is: Voltage sag
Q.76 What is the typical frequency range for power quality transients that are considered “high‑frequency”?
Above 2 kHz
Below 50 Hz
Between 50 Hz and 60 Hz
Exactly 60 Hz
Explanation - High‑frequency transients are generally defined as those >2 kHz.
Correct answer is: Above 2 kHz
Q.77 Which of the following is the most suitable metric for assessing the impact of voltage unbalance on a three‑phase induction motor?
Voltage Unbalance Factor (VUF)
Total Harmonic Distortion (THD)
Crest factor
Frequency deviation
Explanation - VUF directly correlates with motor heating and torque ripple caused by unbalance.
Correct answer is: Voltage Unbalance Factor (VUF)
Q.78 A power quality monitor logs a 0.3 % THD at the PCC. According to IEEE 519, is this level acceptable for a sensitive data center?
Yes, because it is well below the typical 5 % limit for sensitive loads
No, because any THD above 0.1 % is unacceptable for data centers
Yes, because data centers are not affected by harmonics
No, because the limit is 0.2 %
Explanation - IEEE 519 allows up to 5 % THD for most sensitive equipment; 0.3 % is comfortably within limits.
Correct answer is: Yes, because it is well below the typical 5 % limit for sensitive loads
Q.79 Which technology can provide both voltage regulation and harmonic filtering in a single unit?
Hybrid active‑passive filter
Series reactor
Surge arrester
Phase‑shifting transformer
Explanation - Hybrid filters combine active filtering for low‑order harmonics with passive components for higher orders, also offering voltage regulation.
Correct answer is: Hybrid active‑passive filter
Q.80 In the context of power quality monitoring, what does the term ‘event‑based logging’ imply?
Data is recorded only when a predefined power‑quality event occurs
Continuous logging at a fixed interval
Logging after the power system is shut down
Recording only voltage magnitude
Explanation - Event‑based logging saves storage by capturing only significant disturbances.
Correct answer is: Data is recorded only when a predefined power‑quality event occurs
Q.81 Which of the following devices is specifically designed to mitigate voltage flicker caused by large, rapidly changing loads?
Static VAR Compensator (SVC)
Over‑current relay
Circuit breaker
Voltage transformer
Explanation - SVCs can quickly inject or absorb reactive power, reducing voltage fluctuations that cause flicker.
Correct answer is: Static VAR Compensator (SVC)
Q.82 A power quality analyzer shows a harmonic at 250 Hz with an amplitude of 15 % of the fundamental. Which harmonic order does this correspond to in a 50 Hz system?
5th harmonic
3rd harmonic
7th harmonic
9th harmonic
Explanation - 250 Hz / 50 Hz = 5, so it is the 5th harmonic.
Correct answer is: 5th harmonic
Q.83 Which parameter is most directly used to assess the risk of equipment overheating due to harmonic currents?
Total Harmonic Distortion (THD)
Crest factor
Voltage sag depth
Frequency deviation
Explanation - Higher THD means more RMS harmonic current, leading to additional heating.
Correct answer is: Total Harmonic Distortion (THD)
Q.84 Which of the following is a key advantage of using a cloud‑based dashboard for power quality data visualization?
Access from multiple locations and devices
Elimination of the need for any on‑site sensors
Automatic correction of power quality events
Zero latency in data transmission
Explanation - Cloud dashboards provide remote, web‑based access to real‑time and historic data.
Correct answer is: Access from multiple locations and devices
Q.85 What is the primary reason for applying a low‑pass filter before performing an FFT on a sampled voltage waveform?
To prevent aliasing of higher‑frequency components
To increase the fundamental frequency
To convert the waveform to DC
To amplify the harmonic content
Explanation - Low‑pass filtering removes frequencies above the Nyquist limit, avoiding aliasing in the FFT.
Correct answer is: To prevent aliasing of higher‑frequency components
Q.86 In a three‑phase system, a voltage unbalance factor (VUF) of 6 % is measured. According to IEC 61000‑2‑2, how should this be classified?
Above the recommended limit; corrective action needed
Within acceptable limits
Irrelevant for power quality
Indicative of a frequency problem
Explanation - IEC 61000‑2‑2 recommends VUF ≤5 %; 6 % exceeds the guideline.
Correct answer is: Above the recommended limit; corrective action needed
Q.87 Which of the following best describes a ‘transient over‑voltage’ event?
A short‑duration voltage spike lasting a few microseconds to milliseconds
A permanent increase in voltage level
A slow rise in voltage over several minutes
A drop in voltage below nominal
Explanation - Transient over‑voltages are brief spikes often caused by lightning or switching.
Correct answer is: A short‑duration voltage spike lasting a few microseconds to milliseconds
Q.88 Which monitoring technique can provide a statistical view of flicker over a 24‑hour period?
Cumulative flicker index (Plt) calculation
Instantaneous THD measurement
Voltage sag depth histogram
Frequency deviation plot
Explanation - Plt aggregates short‑term flicker measurements over a longer period.
Correct answer is: Cumulative flicker index (Plt) calculation
Q.89 A power quality monitor is configured to trigger an alarm when THD exceeds 5 % for more than 10 seconds. Which of the following events will generate an alarm?
THD rises to 6 % and stays for 12 seconds
THD rises to 7 % for 5 seconds
THD stays at 4 % for 20 seconds
THD briefly spikes to 5.5 % for 2 seconds
Explanation - Both the magnitude (>5 %) and duration (>10 s) thresholds are met.
Correct answer is: THD rises to 6 % and stays for 12 seconds
Q.90 Which of the following is the most accurate description of a ‘harmonic spectrum’?
A plot showing the amplitude of each harmonic order present in a waveform
A time‑domain representation of voltage
A measurement of frequency deviation
A calculation of RMS voltage only
Explanation - A harmonic spectrum displays the magnitude of each harmonic component versus its order.
Correct answer is: A plot showing the amplitude of each harmonic order present in a waveform
Q.91 Which of the following devices can be used to inject a compensating voltage to counteract a sag in real time?
Dynamic Voltage Restorer (DVR)
Static VAR Compensator (SVC)
Surge arrester
Transformer
Explanation - DVRs generate a voltage waveform that adds to the supply, mitigating sags instantly.
Correct answer is: Dynamic Voltage Restorer (DVR)
Q.92 In a power quality monitoring system, what is the purpose of a ‘trigger level’ setting?
To define the threshold at which an event is recorded
To set the maximum voltage the system can handle
To adjust the sampling frequency
To calibrate the power factor
Explanation - The trigger level determines when the monitor starts logging an event.
Correct answer is: To define the threshold at which an event is recorded
Q.93 Which of the following best describes the term ‘power quality monitoring’?
The continuous observation and analysis of electrical parameters to detect deviations from ideal sinusoidal conditions
The process of increasing power generation capacity
A method for measuring only energy consumption
A technique for designing transformers
Explanation - Power quality monitoring focuses on detecting and analyzing disturbances such as sags, swells, harmonics, and flicker.
Correct answer is: The continuous observation and analysis of electrical parameters to detect deviations from ideal sinusoidal conditions
Q.94 Which of the following is a typical consequence of prolonged voltage unbalance on three‑phase motors?
Increased heating and reduced efficiency
Improved torque ripple
Reduced harmonic distortion
Higher power factor
Explanation - Unbalance causes uneven magnetic fields, leading to extra heating and lower efficiency.
Correct answer is: Increased heating and reduced efficiency
Q.95 A power quality monitor records a voltage sag of 50 % depth lasting 0.3 s. According to IEEE 1159, what classification does this event receive?
Severe sag
Moderate sag
Minor sag
Transient spike
Explanation - IEEE 1159 defines a sag to 0.5 pu (50 %) or lower as a severe sag.
Correct answer is: Severe sag
Q.96 Which of the following parameters is most useful for assessing the effect of voltage sags on sensitive electronic equipment?
Sag depth and duration
Total Harmonic Distortion (THD)
Crest factor
Frequency deviation
Explanation - Both the magnitude of the voltage drop and how long it lasts determine the impact on electronics.
Correct answer is: Sag depth and duration
Q.97 In a power quality monitoring system, which of the following is NOT typically a configurable alarm parameter?
Voltage sag depth
Harmonic order limit
Temperature of the monitor
Flicker severity (Pst)
Explanation - Alarms are set for power‑quality parameters; monitor temperature is usually a status indicator, not an alarm trigger.
Correct answer is: Temperature of the monitor
Q.98 Which of the following statements about the sampling theorem (Nyquist) is correct for power quality monitoring?
The sampling frequency must be at least twice the highest frequency component to be measured
Sampling frequency can be lower than the fundamental frequency
Higher sampling rates always reduce measurement accuracy
Nyquist theorem does not apply to voltage measurements
Explanation - Nyquist theorem ensures that frequencies up to half the sampling rate can be reconstructed without aliasing.
Correct answer is: The sampling frequency must be at least twice the highest frequency component to be measured
Q.99 What is the most common method to reduce the effect of high‑frequency transients on sensitive equipment?
Installing surge protection devices (SPDs)
Adding inductive reactors
Increasing line impedance
Using larger transformers
Explanation - SPDs clamp voltage spikes, protecting equipment from fast transients.
Correct answer is: Installing surge protection devices (SPDs)
Q.100 Which power quality metric combines the effect of voltage magnitude variations and duration into a single value?
Voltage dip index
Cumulative voltage deviation (CVD)
Power Quality Index (PQI)
Frequency deviation index
Explanation - PQI aggregates magnitude and duration of various events into one comprehensive score.
Correct answer is: Power Quality Index (PQI)
Q.101 A power quality monitor reports a frequency deviation of +0.3 Hz for a 50 Hz system. What is the percentage deviation?
0.6 %
3 %
0.03 %
6 %
Explanation - Percentage deviation = (0.3 Hz / 50 Hz) × 100 % = 0.6 %.
Correct answer is: 0.6 %
Q.102 Which of the following is an advantage of using a high‑resolution (24‑bit) ADC in a power quality analyzer?
Improved ability to detect low‑amplitude disturbances
Reduced need for anti‑aliasing filters
Faster sampling speed
Lower power consumption
Explanation - Higher resolution provides finer voltage steps, allowing detection of small deviations.
Correct answer is: Improved ability to detect low‑amplitude disturbances
Q.103 In a power quality report, the term ‘duration histogram’ is most closely associated with which type of event?
Voltage sag
Total Harmonic Distortion
Frequency deviation
Power factor
Explanation - Duration histograms display how long sags (or other events) persisted across occurrences.
Correct answer is: Voltage sag
Q.104 Which of the following devices can provide both voltage regulation and harmonic mitigation in a single solution?
Hybrid active‑passive filter
Series capacitor
Standard surge arrester
Phase‑shifting transformer
Explanation - Hybrid filters combine active compensation (voltage regulation) with passive filtering (harmonics).
Correct answer is: Hybrid active‑passive filter
Q.105 A power quality analyzer uses a 1 MS/s sampling rate. According to Nyquist, what is the highest frequency that can be accurately measured?
500 kHz
1 MHz
250 kHz
2 MHz
Explanation - Nyquist limit is half the sampling rate; 1 MS/s → 500 kHz maximum.
Correct answer is: 500 kHz
Q.106 Which IEC standard defines the measurement methods for voltage flicker (short‑term and long‑term)?
IEC 61000‑4‑15
IEC 61000‑4‑30
IEC 61000‑4‑11
IEC 61000‑2‑2
Explanation - IEC 61000‑4‑15 specifies the procedure for measuring flicker severity (Pst, Plt).
Correct answer is: IEC 61000‑4‑15
Q.107 What does the abbreviation ‘PDF’ stand for when referring to a power quality report format?
Portable Document Format
Power Distortion Factor
Peak Demand Factor
Phase Difference Frequency
Explanation - PDF is a common file format for sharing readable reports.
Correct answer is: Portable Document Format
Q.108 Which of the following is the most effective method to reduce low‑order harmonics caused by nonlinear loads?
Installing an active power filter
Adding a series inductor
Increasing the cable size
Using a larger transformer
Explanation - Active filters can target and cancel specific low‑order harmonics.
Correct answer is: Installing an active power filter
Q.109 In power quality monitoring, what does the term ‘RMS stability’ refer to?
The constancy of RMS values over a defined period
The ability to change frequency rapidly
The variation of peak voltage
The stability of the power factor
Explanation - RMS stability indicates how steady the effective voltage or current remains.
Correct answer is: The constancy of RMS values over a defined period
Q.110 Which of the following is a primary benefit of using a ‘ring buffer’ in a power quality monitor?
Continuous data capture with limited memory usage
Automatic harmonic correction
Increasing voltage amplitude
Reducing sampling rate
Explanation - A ring buffer overwrites old data, ensuring recent events are always stored without exceeding memory.
Correct answer is: Continuous data capture with limited memory usage
Q.111 A power quality monitor logs a voltage dip of 20 % for 0.08 s. According to IEC 61000‑4‑30, will this event be recorded as a sag?
No, because the duration is shorter than the 0.1 s minimum
Yes, because the depth exceeds 10 %
No, because the depth is less than 30 %
Yes, because any voltage drop is recorded
Explanation - IEC 61000‑4‑30 requires a minimum duration of 0.1 s for a sag event.
Correct answer is: No, because the duration is shorter than the 0.1 s minimum
Q.112 Which of the following is the most appropriate metric to evaluate the effect of voltage unbalance on a three‑phase transformer?
Voltage Unbalance Factor (VUF)
Total Harmonic Distortion (THD)
Crest factor
Frequency deviation
Explanation - VUF directly measures how unbalanced the phase voltages are, influencing transformer heating.
Correct answer is: Voltage Unbalance Factor (VUF)
Q.113 Which device is commonly employed to compensate for both voltage sags and harmonics in real time?
Dynamic Voltage Restorer (DVR) with active filtering
Series reactor
Surge arrester
Phase‑shifting transformer
Explanation - A DVR can inject a compensating voltage while integrated active filters address harmonics.
Correct answer is: Dynamic Voltage Restorer (DVR) with active filtering
Q.114 In a power quality monitoring system, which of the following best describes a ‘log‑interval’ setting?
The fixed time between successive stored measurements when no event occurs
The duration of a voltage sag
The frequency of the mains supply
The number of harmonics to calculate
Explanation - Log‑interval determines how often data is recorded during normal operation.
Correct answer is: The fixed time between successive stored measurements when no event occurs
Q.115 What is the typical effect of a high‑order harmonic (e.g., 31st) on transformer insulation?
Minimal impact due to high frequency attenuation
Severe heating
Increase in core losses
Reduction of voltage rating
Explanation - Transformers' impedance rises with frequency, so high‑order harmonics are largely attenuated.
Correct answer is: Minimal impact due to high frequency attenuation
Q.116 Which of the following statements about the ‘Crest factor’ is correct?
A higher crest factor indicates a waveform with higher peaks relative to its RMS value
Crest factor measures the total harmonic distortion
Crest factor is only applicable to DC signals
A lower crest factor means more harmonic content
Explanation - Crest factor = peak / RMS; larger peaks raise the factor.
Correct answer is: A higher crest factor indicates a waveform with higher peaks relative to its RMS value
Q.117 Which power quality standard sets limits for voltage unbalance in low‑voltage networks?
IEC 61000‑2‑2
IEEE 1159
IEEE 519
IEC 61850
Explanation - IEC 61000‑2‑2 provides guidelines for voltage unbalance limits.
Correct answer is: IEC 61000‑2‑2
Q.118 A power quality monitor records a THD of 12 % on the supply side and 4 % on the load side. What could explain the reduction?
Presence of passive harmonic filters at the load
Increase in line impedance
Higher load current
Lower supply voltage
Explanation - Filters attenuate harmonic currents, reducing THD seen by the load.
Correct answer is: Presence of passive harmonic filters at the load
Q.119 Which of the following is the most appropriate metric to evaluate the cumulative effect of short‑duration voltage sags over a month?
Voltage Sag Severity Index (VSSI)
Total Harmonic Distortion (THD)
Power Factor
Frequency Deviation Index
Explanation - VSSI aggregates sag depth and duration over a longer period to assess overall impact.
Correct answer is: Voltage Sag Severity Index (VSSI)
Q.120 In power quality monitoring, what does a ‘zero‑sequence component’ refer to?
A component that is identical in all three phases and returns via the neutral
The fundamental frequency component
A high‑frequency transient
The difference between line currents
Explanation - Zero‑sequence components are in phase in all three phases and flow through the neutral.
Correct answer is: A component that is identical in all three phases and returns via the neutral
Q.121 Which of the following techniques can be used to improve the accuracy of RMS voltage measurement during fast transients?
Applying a true‑RMS algorithm with a high sampling rate
Using a simple average‑value meter
Measuring only the peak voltage
Increasing the transformer ratio
Explanation - True‑RMS algorithms calculate the effective value even under distorted conditions, provided the sampling rate captures the waveform details.
Correct answer is: Applying a true‑RMS algorithm with a high sampling rate
Q.122 Which power quality event is most likely to cause data corruption in digital communication lines?
High‑frequency transients
Voltage swells
Low‑frequency sags
Voltage unbalance
Explanation - Fast transients can upset the timing and logic of digital circuits, leading to errors.
Correct answer is: High‑frequency transients
Q.123 A power quality monitor provides a histogram of flicker events. Which axis typically represents the flicker severity?
X‑axis
Y‑axis
Both axes equally
Neither axis; flicker is not histogrammed
Explanation - Histograms generally plot severity intervals on the X‑axis and count of events on the Y‑axis.
Correct answer is: X‑axis
Q.124 Which of the following is a common source of interharmonic distortion?
Cycloconverter operation
Resistive heating
Linear motor drives
Balanced three‑phase loads
Explanation - Cycloconverters generate frequencies that are not integer multiples of the fundamental, leading to interharmonics.
Correct answer is: Cycloconverter operation
Q.125 In a power quality monitor, what is the purpose of a ‘pre‑trigger buffer’?
To capture waveform data a short time before an event is detected
To increase the voltage level
To store only post‑event data
To filter out low‑frequency components
Explanation - Pre‑trigger buffers preserve a few milliseconds of data before the trigger, providing context for the event.
Correct answer is: To capture waveform data a short time before an event is detected
Q.126 Which of the following standards specifically addresses power quality monitoring in residential low‑voltage networks?
IEC 61000‑4‑30
IEEE 1159‑1
IEC 61850
IEEE 519‑2
Explanation - IEC 61000‑4‑30 provides measurement methods for low‑voltage power quality monitoring.
Correct answer is: IEC 61000‑4‑30
Q.127 A power quality monitor reports a cumulative flicker index (Plt) of 1.2 over a 24‑hour period. According to IEC 61000‑4‑15, what does this indicate?
Excessive flicker; corrective measures required
Acceptable flicker level
No flicker present
Measurement error
Explanation - IEC 61000‑4‑15 recommends Plt ≤ 1.0; a value of 1.2 exceeds the limit.
Correct answer is: Excessive flicker; corrective measures required
Q.128 Which of the following is the primary reason to use a high‑resolution ADC (e.g., 24‑bit) in a power quality analyzer?
To detect very low‑level voltage disturbances
To increase the sampling rate beyond 10 MS/s
To reduce the need for anti‑aliasing filters
To lower the instrument's power consumption
Explanation - Higher bit depth provides finer voltage steps, allowing detection of minute variations.
Correct answer is: To detect very low‑level voltage disturbances
Q.129 Which of the following parameters would you monitor to assess the impact of a large motor starting on the supply voltage?
Voltage sag depth and duration
Total Harmonic Distortion (THD)
Frequency deviation
Crest factor
Explanation - Motor starting typically causes temporary voltage drops (sags).
Correct answer is: Voltage sag depth and duration
Q.130 In power quality monitoring, what does the term ‘continuous monitoring’ imply?
The system records data without interruption, often in real‑time
The system only records when an alarm is triggered
The system measures only once per day
The system only monitors frequency
Explanation - Continuous monitoring ensures all events are captured as they occur.
Correct answer is: The system records data without interruption, often in real‑time
Q.131 Which of the following is the most common cause of voltage flicker in industrial plants?
Large, rapidly varying loads such as arc furnaces or welding machines
Balanced three‑phase motor operation
Steady resistive heating
Constant lighting loads
Explanation - Fast load changes cause voltage fluctuations perceived as flicker.
Correct answer is: Large, rapidly varying loads such as arc furnaces or welding machines
Q.132 A power quality monitor is set to log events when voltage deviates by more than ±5 % for longer than 0.2 s. Which of the following occurrences will be recorded?
A 6 % voltage swell lasting 0.25 s
A 3 % voltage dip lasting 0.5 s
A 10 % voltage sag lasting 0.1 s
A 2 % voltage fluctuation lasting 1 s
Explanation - Both magnitude (>5 %) and duration (>0.2 s) thresholds are satisfied.
Correct answer is: A 6 % voltage swell lasting 0.25 s
Q.133 Which IEC standard defines the measurement method for voltage unbalance in three‑phase systems?
IEC 61000‑4‑30
IEC 61000‑2‑2
IEC 61000‑4‑15
IEC 61000‑4‑11
Explanation - IEC 61000‑2‑2 provides definitions and limits for voltage unbalance.
Correct answer is: IEC 61000‑2‑2
Q.134 Which of the following best describes the function of a ‘dynamic voltage restorer (DVR)’?
To inject a compensating voltage during sags and swells, restoring the waveform
To filter high‑frequency harmonics only
To limit fault currents
To increase the system frequency
Explanation - DVRs generate a voltage that adds to the supply, correcting sags/swells in real time.
Correct answer is: To inject a compensating voltage during sags and swells, restoring the waveform