Q.1 What is the primary cause of partial discharge in high‑voltage cables?
Excessive mechanical stress
Electrical insulation defects
Thermal expansion of conductors
Ambient humidity only
Explanation - Partial discharge occurs when there is a breakdown within the insulation due to defects such as voids, cracks or moisture, which create localized high electric fields.
Correct answer is: Electrical insulation defects
Q.2 Which of the following is a typical frequency range for partial discharge pulses?
0 – 10 kHz
10 – 100 kHz
100 – 500 kHz
1 – 10 MHz
Explanation - PD pulses are broadband, typically ranging from a few MHz to several tens of MHz, often extending into the GHz region depending on the system.
Correct answer is: 1 – 10 MHz
Q.3 Which measurement technique is most suitable for detecting PD in underground cables?
Acoustic Emission
Electrostatic Field Meter
Ground‑Potential Gradient
Ultrasonic Scanning
Explanation - Ground‑Potential Gradient (GPG) sensors measure the voltage variations on the cable’s shield, making it effective for buried cable PD detection.
Correct answer is: Ground‑Potential Gradient
Q.4 The term 'PD activity' refers to:
The average PD pulse energy per second
The maximum PD pulse amplitude
The total number of PD pulses per unit time
The PD pulse rise time
Explanation - PD activity is quantified as the count of discharge events per unit of time, providing a measure of the severity of insulation degradation.
Correct answer is: The total number of PD pulses per unit time
Q.5 Which type of PD is most likely to occur at the interface of a dielectric with a small void?
Surface PD
Volume PD
Cavitation PD
Corona PD
Explanation - Volume PD originates within the bulk of the dielectric where a void concentrates the electric field, causing localized breakdown inside the insulation.
Correct answer is: Volume PD
Q.6 In a voltage‑time curve of a partial discharge event, the slope of the rise indicates:
Pulse amplitude
Pulse duration
Rise time
Decay time
Explanation - The slope during the initial voltage jump is inversely proportional to the rise time of the PD pulse.
Correct answer is: Rise time
Q.7 Which instrument is commonly used for PD detection in overhead transmission lines?
High‑frequency Current Transformer (HFCT)
Insulation Resistance Tester
Capacitance Meter
Thermocouple
Explanation - HFCTs detect the high‑frequency components of the current that accompany PD events, making them suitable for overhead line diagnostics.
Correct answer is: High‑frequency Current Transformer (HFCT)
Q.8 The dielectric strength of XLPE insulation is typically:
1 – 5 kV/mm
10 – 20 kV/mm
30 – 40 kV/mm
50 – 60 kV/mm
Explanation - XLPE (cross‑linked polyethylene) generally has a dielectric strength between 10 and 20 kV/mm, higher than conventional PVC.
Correct answer is: 10 – 20 kV/mm
Q.9 Which of the following is a non‑intrusive PD monitoring method?
Direct Contact Probes
Cable Testers with Probes
Electro‑Capacitance Probes
Surface Potential Sensors
Explanation - Surface potential sensors can be mounted externally to measure changes in surface voltage without contacting the cable.
Correct answer is: Surface Potential Sensors
Q.10 Partial discharge can lead to which of the following insulation failures?
Thermal runaway
Electrolysis
Chemical aging
Physical cracking
Explanation - Repeated PD causes localized heating and mechanical stresses, eventually leading to cracks and void formation in the insulation.
Correct answer is: Physical cracking
Q.11 The PD index is a dimensionless quantity calculated as:
Total discharge energy divided by the voltage
Pulse amplitude divided by rise time
Pulse amplitude multiplied by pulse width
Number of pulses per hour divided by voltage
Explanation - The PD index normalizes discharge activity by the applied voltage, enabling comparisons across different test conditions.
Correct answer is: Number of pulses per hour divided by voltage
Q.12 Which component is most susceptible to PD during the manufacturing process of power transformers?
Core laminations
Windings insulation
Cooling oil
Cooling fan
Explanation - Windings insulation can contain air gaps or moisture introduced during winding, making it prone to early PD.
Correct answer is: Windings insulation
Q.13 Which frequency range is generally filtered out in PD detection to suppress noise?
Below 1 kHz
1 – 10 kHz
10 – 100 kHz
Above 1 MHz
Explanation - Low‑frequency noise (e.g., power line interference) is typically filtered out, focusing on the PD’s high‑frequency content.
Correct answer is: Below 1 kHz
Q.14 What is the typical shape of a PD pulse in the time domain?
Sinusoidal wave
Sharp spike with exponential decay
Flat top rectangular pulse
Triangular ramp
Explanation - PD pulses are characterized by an abrupt rise followed by a rapid decay, resembling a short impulse.
Correct answer is: Sharp spike with exponential decay
Q.15 Which material property of insulation directly influences its susceptibility to PD?
Color
Density
Dielectric constant
Magnetic permeability
Explanation - A higher dielectric constant increases local field distortion, making the material more prone to partial discharge.
Correct answer is: Dielectric constant
Q.16 The 'PD inception voltage' is defined as:
Voltage at which the first PD event occurs
Maximum voltage the cable can withstand
Voltage at which insulation fails permanently
Voltage at which dielectric constant doubles
Explanation - Inception voltage marks the onset of partial discharge activity in the insulation system.
Correct answer is: Voltage at which the first PD event occurs
Q.17 Which diagnostic tool uses a resonant tank circuit to detect PD?
Capacitance Meter
Resonant Circuit Detector
Ultrasonic Scanner
Thermal Camera
Explanation - Resonant circuit detectors amplify the high‑frequency components of PD via a tuned circuit.
Correct answer is: Resonant Circuit Detector
Q.18 Partial discharge can be accelerated by which environmental factor?
Low temperatures
High humidity
Low pressure
High light intensity
Explanation - Moisture infiltrates insulation voids, reducing dielectric strength and facilitating PD.
Correct answer is: High humidity
Q.19 The PD pulse energy is often expressed in:
Watts
Joules
Volts
Amps
Explanation - Energy of a PD event is calculated by integrating the power over the pulse duration, measured in Joules.
Correct answer is: Joules
Q.20 Which of the following is a sign of an impending insulation failure due to PD?
Increasing PD activity
Decreasing temperature
Stable voltage profile
Uniform material density
Explanation - A rise in PD count or energy indicates that the insulation is deteriorating and may soon fail.
Correct answer is: Increasing PD activity
Q.21 Which component is NOT typically monitored for PD in high‑voltage switchgear?
Bushings
Gapped spacers
Neutral grounding
Control panels
Explanation - Control panels are usually not part of the insulation system; PD monitoring focuses on physical components like bushings and spacers.
Correct answer is: Control panels
Q.22 What is the main advantage of using a fiber‑optic PD sensor?
High sensitivity to low voltage
Direct contact measurement
Long‑range detection without signal loss
Low cost compared to metal probes
Explanation - Fiber‑optic sensors can transmit PD signals over long distances with minimal attenuation, useful for underground or buried cables.
Correct answer is: Long‑range detection without signal loss
Q.23 Which of these is a typical PD source in power transformer oil?
Micro‑bubbles
Sodium salts
Thermal cracks
All of the above
Explanation - Micro‑bubbles trapped in oil create voids that act as PD initiation sites.
Correct answer is: Micro‑bubbles
Q.24 PD detection via acoustic emission requires:
High‑frequency sound detectors
Ultrasound transducers
Thermal imaging
Optical sensors
Explanation - Acoustic emission sensors detect the acoustic waves generated by PD, which are in the ultrasonic range.
Correct answer is: Ultrasound transducers
Q.25 Which parameter is NOT used to quantify PD severity?
PD activity
PD energy
PD inception voltage
Capacitance
Explanation - Capacitance is a property of the cable, not a measure of PD severity.
Correct answer is: Capacitance
Q.26 Which insulation type is least susceptible to PD?
PVC
XLPE
Silicone rubber
EPR
Explanation - Silicone rubber has excellent dielectric properties and is less prone to void formation, reducing PD risk.
Correct answer is: Silicone rubber
Q.27 In a PD time‑of‑flight measurement, a longer distance to the PD source will:
Increase the rise time
Decrease the pulse amplitude
Shorten the propagation time
Increase the decay time
Explanation - A longer distance actually increases the travel time, so a longer time‑of‑flight indicates a farther source.
Correct answer is: Shorten the propagation time
Q.28 Which of the following is a common PD test voltage for cable insulation?
0.5 × Rated Voltage
1.0 × Rated Voltage
1.5 × Rated Voltage
2.0 × Rated Voltage
Explanation - Testing at 1.5× the rated voltage helps to stress the insulation and reveal latent PD activity.
Correct answer is: 1.5 × Rated Voltage
Q.29 A PD event that produces a long‑lasting high‑frequency current is known as:
Pulsed PD
Continuous PD
High‑frequency PD
Low‑frequency PD
Explanation - High‑frequency PD emits a sustained high‑frequency current trace after the initial discharge.
Correct answer is: High‑frequency PD
Q.30 What is the main effect of partial discharge on insulation dielectric loss?
Decrease
Increase
No effect
Oscillate periodically
Explanation - PD generates ionization and heating, increasing the dielectric loss factor of the insulation.
Correct answer is: Increase
Q.31 Which type of PD is typically observed at the outer surface of a cable sheath?
Surface PD
Volume PD
Corona PD
Cavitation PD
Explanation - Surface PD occurs along the external sheath where the electric field concentrates due to surface imperfections.
Correct answer is: Surface PD
Q.32 The 'PD energy density' is expressed in:
J/m
J/cm²
W/m³
V/m
Explanation - Energy density measures the energy per unit area of the insulation, helping to assess localized damage.
Correct answer is: J/cm²
Q.33 Which of the following is a direct consequence of repeated PD on cable insulation?
Reduced dielectric constant
Increased tensile strength
Improved thermal stability
Higher electrical conductivity
Explanation - PD can degrade insulation, leading to a drop in dielectric constant and loss of insulating capability.
Correct answer is: Reduced dielectric constant
Q.34 What is the purpose of a 'PD suppression' system?
To detect PD events
To eliminate PD sources
To increase voltage levels
To reduce cable length
Explanation - PD suppression systems mitigate the effects of PD by removing moisture, cleaning surfaces, or applying high‑voltage conditioning.
Correct answer is: To eliminate PD sources
Q.35 Which instrument measures the PD waveform in the frequency domain?
Spectrum Analyzer
Oscilloscope
High‑frequency Current Transformer
Multimeter
Explanation - A spectrum analyzer decomposes the PD pulse into its frequency components, revealing characteristic peaks.
Correct answer is: Spectrum Analyzer
Q.36 Which of these is a typical PD measurement unit?
Vpp
μJ
Hz
A
Explanation - PD energy is often reported in microjoules (μJ) to represent the small energy of individual discharge events.
Correct answer is: μJ
Q.37 In a PD test, what does the 'inception voltage' threshold indicate?
Maximum voltage the insulation can handle
Minimum voltage for PD to start
Voltage at which PD stops
Voltage of the test equipment
Explanation - Inception voltage is the lowest applied voltage that initiates detectable PD activity in the insulation.
Correct answer is: Minimum voltage for PD to start
Q.38 Which technique can be used to locate the PD source along a cable?
Time‑of‑flight analysis
Direct voltage measurement
Capacitance mapping
Temperature profiling
Explanation - By measuring the travel time of PD pulses, one can determine the distance to the source.
Correct answer is: Time‑of‑flight analysis
Q.39 Partial discharge can cause which of the following in transformers?
Oil degradation
Core magnetization changes
Windings mechanical distortion
All of the above
Explanation - PD in transformers leads to oil oxidation, magnetic core changes, and mechanical stress on windings.
Correct answer is: All of the above
Q.40 The PD detection signal is usually:
Low‑frequency and continuous
High‑frequency and pulsed
Steady DC
Constant amplitude
Explanation - PD produces brief, high‑frequency pulses superimposed on the AC power frequency.
Correct answer is: High‑frequency and pulsed
Q.41 Which of the following is a major challenge in PD measurement?
Noise from power lines
High cost of sensors
Difficulty in data interpretation
All of the above
Explanation - Noise, cost, and complex data analysis all present obstacles to accurate PD diagnostics.
Correct answer is: All of the above
Q.42 Which of the following is NOT a typical PD source in overhead lines?
Corrosion at terminals
Wind-induced sag
Insulation defects
High ambient temperature
Explanation - While sag can affect voltage levels, it does not directly cause PD; corrosion, insulation defects, and temperature do.
Correct answer is: Wind-induced sag
Q.43 A PD test at 1.5× rated voltage is used to:
Verify cable insulation integrity
Determine the cable's mechanical strength
Measure the cable's thermal conductivity
Check the cable's shielding effectiveness
Explanation - The test stresses insulation to uncover hidden PD activity, indicating potential failures.
Correct answer is: Verify cable insulation integrity
Q.44 Which of the following is a PD detection method based on optical signals?
Fiber‑optic PD sensor
Infrared thermography
Laser Doppler velocimetry
Visible light spectrometer
Explanation - Fiber‑optic sensors convert high‑frequency electrical events into optical signals for remote detection.
Correct answer is: Fiber‑optic PD sensor
Q.45 The PD pulse width is typically:
Several milliseconds
A few microseconds
Nanoseconds
Seconds
Explanation - PD pulses are extremely short, often on the order of nanoseconds to tens of nanoseconds.
Correct answer is: Nanoseconds
Q.46 Which of these is a safety hazard of uncontrolled PD?
Thermal runaway
Arcing
Electrostatic discharge
All of the above
Explanation - Uncontrolled PD can generate heat, arcing, and static, posing significant safety risks.
Correct answer is: All of the above
Q.47 The 'PD decay time constant' indicates:
The time for PD to cease
The rate at which the PD pulse decays
The interval between PD events
The rise time of the PD pulse
Explanation - It reflects how quickly the discharge current returns to baseline after the pulse.
Correct answer is: The rate at which the PD pulse decays
Q.48 Which component of a transformer is most susceptible to PD?
Core laminations
Windings insulation
Cooling fan
Transformer housing
Explanation - Windings insulation is the most critical and vulnerable part for PD initiation.
Correct answer is: Windings insulation
Q.49 Which type of PD is typically observed in the gaps between layers in a cable?
Surface PD
Cavitation PD
Volume PD
Corona PD
Explanation - Gaps between layers create voids where volume PD can occur.
Correct answer is: Volume PD
Q.50 Which of the following is a typical PD monitoring frequency band?
0 – 50 kHz
100 kHz – 3 MHz
3 – 10 MHz
10 – 100 MHz
Explanation - PD detection often focuses on the 3–10 MHz band where most discharge energy resides.
Correct answer is: 3 – 10 MHz
Q.51 What is a common symptom of PD in power cables?
Visible sparks
Hissing noise
Fluctuating voltage
All of the above
Explanation - Hissing is a typical audible symptom of PD, often heard near the source.
Correct answer is: Hissing noise
Q.52 Which measurement technique uses a 'coil probe' to detect PD?
High‑frequency Current Transformer
Capacitive probe
Resonant tank
Surface potential probe
Explanation - HFCTs are coil‑based sensors that sense the high‑frequency magnetic field of PD events.
Correct answer is: High‑frequency Current Transformer
Q.53 The dielectric breakdown voltage of a typical XLPE cable is:
50–70 kV/mm
20–40 kV/mm
10–20 kV/mm
5–10 kV/mm
Explanation - XLPE typically has a breakdown strength between 10 and 20 kV/mm.
Correct answer is: 10–20 kV/mm
Q.54 Partial discharge can be monitored in real‑time using:
Data loggers
Spectral analysis software
High‑speed oscilloscopes
All of the above
Explanation - Real‑time PD monitoring integrates data acquisition, spectral analysis, and high‑speed recording.
Correct answer is: All of the above
Q.55 Which of the following is a typical PD source in gas‑insulated switchgear?
Gas leakage
Electrodes surface defects
Temperature gradient
All of the above
Explanation - Gas leakage, electrode defects, and temperature differences can all initiate PD in gas‑insulated systems.
Correct answer is: All of the above
Q.56 The PD energy of an individual discharge event is typically:
From picojoules to nanjoules
From millijoules to joules
From kilojoules
From megajoules
Explanation - PD events are small, usually delivering energy in the picojoule to nanjoule range.
Correct answer is: From picojoules to nanjoules
Q.57 Which of the following is an indicator of 'soft' PD?
Large amplitude pulses
Small amplitude pulses
Long duration pulses
No detectable pulses
Explanation - Soft PD refers to low‑energy, small‑amplitude discharge events that are often early signs of insulation degradation.
Correct answer is: Small amplitude pulses
Q.58 The term 'PD current' refers to:
Current flowing through the entire cable
Current generated during PD events
DC current of the system
Leakage current of the insulation
Explanation - PD current is the short, high‑frequency current produced by a partial discharge event.
Correct answer is: Current generated during PD events
Q.59 Which of the following is a typical PD detection equipment?
Acoustic emission detector
Thermal camera
Spectrometer
All of the above
Explanation - Acoustic emission detectors sense the high‑frequency sound generated by PD events.
Correct answer is: Acoustic emission detector
Q.60 The PD detection system often uses a 'band‑pass filter' to:
Amplify low‑frequency noise
Suppress high‑frequency noise
Isolate the PD frequency band
Convert voltage to current
Explanation - Band‑pass filters allow only the PD frequency components to pass while rejecting unwanted signals.
Correct answer is: Isolate the PD frequency band
Q.61 In the context of PD, 'inception voltage' is:
The minimum voltage to start PD
The maximum voltage before insulation fails
The voltage at which PD stops
The average operating voltage
Explanation - Inception voltage is the threshold at which PD becomes detectable in the insulation.
Correct answer is: The minimum voltage to start PD
Q.62 Which of the following is a typical PD source in high‑voltage cables?
Air bubbles
Moisture ingress
Thermal cracks
All of the above
Explanation - All these factors can create voids or defects that initiate PD.
Correct answer is: All of the above
Q.63 The primary effect of PD on transformer oil is:
Increase in resistivity
Decrease in dielectric strength
Increase in viscosity
All of the above
Explanation - PD generates ionization, causing the oil’s dielectric strength to drop.
Correct answer is: Decrease in dielectric strength
Q.64 Which of the following best describes a 'hard' PD event?
High‑energy, high‑amplitude pulses
Low‑energy, small‑amplitude pulses
Continuous discharge
Non‑electrical discharge
Explanation - Hard PD refers to large energy pulses that can cause visible sparks or damage.
Correct answer is: High‑energy, high‑amplitude pulses
Q.65 Which diagnostic technique uses a 'coil probe' to sense magnetic fields induced by PD?
High‑frequency Current Transformer
Capacitive probe
Resonant circuit
Optical sensor
Explanation - HFCTs use coil geometry to detect transient magnetic fields from PD events.
Correct answer is: High‑frequency Current Transformer
Q.66 The PD pulse energy is calculated by integrating the power over the:
Entire test duration
Rise time only
Pulse duration
Decay time only
Explanation - Energy is derived from the product of power and the duration of each discharge pulse.
Correct answer is: Pulse duration
Q.67 What is the main purpose of PD suppression in cables?
To increase cable length
To eliminate PD sources and extend service life
To reduce cable cost
To improve mechanical flexibility
Explanation - PD suppression mitigates initiation sites, thereby prolonging insulation life.
Correct answer is: To eliminate PD sources and extend service life
Q.68 The frequency of PD pulses can be influenced by:
Applied voltage amplitude
Insulation thickness
Cable length
Ambient temperature
Explanation - Higher applied voltages can increase the frequency of PD events due to stronger electric fields.
Correct answer is: Applied voltage amplitude
Q.69 Which of the following is a typical PD diagnostic in underground cables?
Ground‑Potential Gradient
Capacitance Measurement
Voltage‑Divider Test
Thermal Imaging
Explanation - GPG sensors detect voltage variations along the cable shield, effectively diagnosing underground PD.
Correct answer is: Ground‑Potential Gradient
Q.70 Which of these is a characteristic of a 'volume' PD event?
Occurs on the external surface
Occurs within a void in the insulation
Produces a low‑frequency signal
Has no high‑frequency component
Explanation - Volume PD originates inside the bulk insulation, often in voids.
Correct answer is: Occurs within a void in the insulation
Q.71 A 'surface' PD is mainly caused by:
Void within the dielectric
Contamination on the insulation surface
High temperature
Mechanical strain
Explanation - Surface contamination lowers the surface dielectric strength, triggering surface PD.
Correct answer is: Contamination on the insulation surface
Q.72 The 'PD activity factor' is used to compare:
Voltage levels between cables
PD frequency between tests
Pulse amplitude between measurements
Cable lengths
Explanation - The activity factor normalizes PD count relative to voltage, allowing comparison of different conditions.
Correct answer is: PD frequency between tests
Q.73 Which of the following is a typical PD detection equipment for overhead lines?
High‑frequency Current Transformer
Optical fiber sensor
Thermocouple
Ultrasonic scanner
Explanation - HFCTs capture the high‑frequency magnetic field signatures of PD in overhead lines.
Correct answer is: High‑frequency Current Transformer
Q.74 Which of the following is NOT a typical PD source in high‑voltage equipment?
Insulation defects
Gas leaks
Mechanical vibration
Corrosion at terminations
Explanation - While vibration can degrade insulation, it does not directly cause PD initiation.
Correct answer is: Mechanical vibration
Q.75 Which technique is used to convert the PD voltage pulse into a measurable current signal?
Capacitive coupling
Inductive coupling
Resistive divider
Optical conversion
Explanation - Inductive coupling captures the high‑frequency magnetic flux and converts it into a current trace.
Correct answer is: Inductive coupling
Q.76 What is the main advantage of using an optical PD sensor?
High electrical conductivity
Long‑distance signal transmission
Low cost
Direct contact measurement
Explanation - Optical sensors can transmit PD signals over many meters with minimal loss.
Correct answer is: Long‑distance signal transmission
Q.77 Which of the following PD types occurs when the electric field concentrates at a sharp point?
Surface PD
Volume PD
Corona PD
Cavitation PD
Explanation - Corona discharge initiates at sharp points where the field exceeds the breakdown threshold.
Correct answer is: Corona PD
Q.78 Which of these is a typical symptom of high PD activity in power lines?
High current spikes
Voltage flicker
Visible sparking
All of the above
Explanation - High PD activity can cause voltage and current disturbances and even visible sparks.
Correct answer is: All of the above
Q.79 The PD measurement system often uses a 'time‑domain reflectometer' to:
Measure cable capacitance
Detect PD source location
Test insulation resistance
Analyze frequency spectrum
Explanation - TDR can locate the position of PD by reflecting the pulse back from the source.
Correct answer is: Detect PD source location
Q.80 Which of the following is a key parameter in the PD energy spectrum?
Peak voltage
Frequency content
Pulse width
All of the above
Explanation - The energy spectrum shows how energy is distributed across frequencies of the PD pulse.
Correct answer is: Frequency content
Q.81 Which of these is a major cause of PD in composite cable insulations?
Moisture absorption
Thermal expansion mismatch
Mechanical abrasion
All of the above
Explanation - All these factors can create voids or defects, leading to PD.
Correct answer is: All of the above
Q.82 In a PD test, the 'voltage hold‑off' level is:
Voltage at which the cable stops conducting
Maximum applied voltage during the test
Minimum voltage to start PD
Voltage at which PD stops
Explanation - Voltage hold‑off refers to the maximum voltage that the insulation can withstand without breaking down during the test.
Correct answer is: Maximum applied voltage during the test
Q.83 Which of the following best describes the PD current waveform?
Continuous sinusoidal
Pulse‑like high‑frequency burst
DC steady
Low‑frequency square wave
Explanation - PD current is a short, high‑frequency burst superimposed on the power frequency.
Correct answer is: Pulse‑like high‑frequency burst
Q.84 The 'PD energy spectrum' is often used to:
Identify the type of PD
Measure cable length
Calculate insulation resistance
Determine voltage level
Explanation - Different PD types show distinct spectral signatures.
Correct answer is: Identify the type of PD
Q.85 Which of the following is a typical PD detection technique for power transformers?
High‑frequency current transformer
Acoustic emission sensor
Thermography
All of the above
Explanation - Transformers use multiple detection methods to identify PD.
Correct answer is: All of the above
Q.86 In PD diagnostics, the 'rise time' of the pulse indicates:
Energy of the discharge
Size of the discharge region
Frequency of PD events
None of the above
Explanation - Shorter rise times typically correspond to higher energy pulses.
Correct answer is: Energy of the discharge
Q.87 Which of the following is a primary PD source in underground cables?
Air bubbles
Water ingress
Mechanical stress
All of the above
Explanation - All these factors can create voids that initiate PD.
Correct answer is: All of the above
Q.88 What is the main function of a 'band‑stop filter' in a PD detection system?
Amplify PD signals
Suppress unwanted frequency components
Convert voltage to current
Measure cable length
Explanation - Band‑stop filters remove known noise frequencies to isolate the PD signals.
Correct answer is: Suppress unwanted frequency components
Q.89 Which of the following is a typical PD measurement unit?
Vpp
μJ
Hz
A
Explanation - PD energy is commonly expressed in microjoules (μJ).
Correct answer is: μJ
Q.90 The term 'PD activity' refers to:
Maximum pulse amplitude
Number of PD events per time unit
Voltage rise time
Decay time
Explanation - PD activity is measured by counting the pulses within a specific time interval.
Correct answer is: Number of PD events per time unit
Q.91 Which of these is an indicator of impending cable failure due to PD?
Increasing PD activity
Stable temperature
No PD detected
High voltage hold‑off
Explanation - A rise in PD count signals degradation leading to failure.
Correct answer is: Increasing PD activity
Q.92 Which diagnostic technique uses a 'coil probe' to detect PD in power lines?
High‑frequency Current Transformer
Capacitive probe
Resonant tank
Optical sensor
Explanation - HFCTs capture the transient magnetic fields of PD events using a coil.
Correct answer is: High‑frequency Current Transformer
Q.93 The PD energy is calculated by integrating the current over the:
Entire test duration
Pulse duration
Voltage rise
Decay time
Explanation - Energy is derived from the integral of current squared times resistance during the pulse.
Correct answer is: Pulse duration
Q.94 Which of the following is a typical PD detection method for gas‑insulated switchgear?
Thermography
Acoustic emission sensor
PD suppression coil
All of the above
Explanation - Multiple methods are used to monitor PD in gas‑insulated systems.
Correct answer is: All of the above
Q.95 The main challenge in measuring PD in underground cables is:
Noise from power lines
Signal attenuation in shield
High temperature
Low voltage levels
Explanation - The cable shield can dampen the high‑frequency components, making PD detection difficult.
Correct answer is: Signal attenuation in shield
Q.96 Which of these is not a typical PD source in high‑voltage equipment?
Corrosion
Moisture ingress
Mechanical vibration
Thermal expansion
Explanation - Vibration can cause mechanical stress but does not directly initiate PD.
Correct answer is: Mechanical vibration
Q.97 Which of the following is a PD detection method that relies on electromagnetic radiation?
PD suppression coil
Electro‑Capacitance Probe
High‑frequency Current Transformer
None of the above
Explanation - HFCTs detect the magnetic field of PD, which is a form of electromagnetic radiation.
Correct answer is: High‑frequency Current Transformer
Q.98 Partial discharge can lead to which of the following in transformers?
Oil degradation
Core magnetization change
Windings mechanical distortion
All of the above
Explanation - PD causes oil oxidation, core changes, and mechanical damage.
Correct answer is: All of the above
Q.99 The PD activity factor is calculated as:
PD count divided by applied voltage
PD count multiplied by voltage
Voltage divided by PD count
Voltage added to PD count
Explanation - Activity factor normalizes PD count per unit of applied voltage.
Correct answer is: PD count divided by applied voltage
Q.100 Which of the following is a typical symptom of high PD activity in power cables?
Visible sparks
Hissing noise
Voltage flicker
All of the above
Explanation - High PD activity can cause sparks, noise, and voltage disturbances.
Correct answer is: All of the above
Q.101 Which of the following is NOT a typical PD detection equipment?
High‑frequency Current Transformer
Capacitance meter
Acoustic emission detector
All of the above
Explanation - Capacitance meters measure dielectric constant but do not detect PD pulses.
Correct answer is: Capacitance meter
Q.102 The PD pulse shape in the time domain is usually described as:
Sharp spike with exponential decay
Smooth sinusoid
Rectangular pulse
Triangular wave
Explanation - PD pulses are short, sharp spikes followed by exponential decay.
Correct answer is: Sharp spike with exponential decay
Q.103 The main effect of PD on the dielectric loss factor is to:
Decrease it
Increase it
Keep it constant
Fluctuate randomly
Explanation - PD increases dielectric loss due to ionization and localized heating.
Correct answer is: Increase it
Q.104 Which of these is a common PD detection method in high‑voltage cables?
Ground‑Potential Gradient
Thermal imaging
Capacitance mapping
All of the above
Explanation - GPG sensors are widely used for buried cable PD detection.
Correct answer is: Ground‑Potential Gradient
Q.105 PD suppression techniques can include:
Moisture removal
Electro‑static conditioning
Thermal cycling
All of the above
Explanation - All listed methods help mitigate PD sources.
Correct answer is: All of the above
Q.106 The PD detection signal is typically:
Low‑frequency sinusoid
High‑frequency burst
Constant DC
Square wave
Explanation - PD produces a high‑frequency, short‑duration burst superimposed on the AC waveform.
Correct answer is: High‑frequency burst
Q.107 In PD testing, the 'inception voltage' is:
Minimum voltage to start PD
Maximum voltage before insulation fails
Voltage at which PD stops
Average operating voltage
Explanation - Inception voltage is the threshold voltage at which PD becomes detectable.
Correct answer is: Minimum voltage to start PD
Q.108 Which of the following is a key parameter in the PD energy spectrum?
Peak voltage
Frequency content
Pulse width
All of the above
Explanation - The energy spectrum shows how energy is distributed across the frequency components of the PD pulse.
Correct answer is: Frequency content
Q.109 The main advantage of using a 'fiber‑optic' PD sensor is:
Low cost
Direct contact measurement
Long‑range signal transmission
High mechanical strength
Explanation - Fiber‑optic sensors can transmit PD signals over long distances with minimal loss.
Correct answer is: Long‑range signal transmission
Q.110 A PD event that produces a long‑lasting high‑frequency current is known as:
Soft PD
Hard PD
High‑frequency PD
Low‑frequency PD
Explanation - High‑frequency PD emits a sustained high‑frequency current trace after the initial discharge.
Correct answer is: High‑frequency PD
Q.111 Which of the following is a typical PD detection technique for overhead power lines?
High‑frequency Current Transformer
Ground‑Potential Gradient
Capacitance meter
Thermocouple
Explanation - HFCTs capture the transient magnetic fields produced by PD in overhead lines.
Correct answer is: High‑frequency Current Transformer
Q.112 The PD energy of an individual discharge event is typically:
Picojoules to nanjoules
Millijoules to joules
Kilojoules
Megajoules
Explanation - PD events are small, typically delivering energy in the picojoule to nanjoule range.
Correct answer is: Picojoules to nanjoules
Q.113 Which of the following is a typical PD source in underground cables?
Air bubbles
Water ingress
Mechanical stress
All of the above
Explanation - All these factors can create voids that initiate partial discharge.
Correct answer is: All of the above
Q.114 Which of the following best describes a 'volume' PD event?
Occurs on the external surface
Occurs within a void in the insulation
Occurs in the metal sheath
Occurs in the conductor core
Explanation - Volume PD originates inside the bulk insulation, often in voids.
Correct answer is: Occurs within a void in the insulation
Q.115 The PD detection system often uses a 'band‑pass filter' to:
Amplify low‑frequency noise
Suppress high‑frequency noise
Isolate the PD frequency band
Convert voltage to current
Explanation - Band‑pass filters allow only the PD frequency components to pass while rejecting unwanted signals.
Correct answer is: Isolate the PD frequency band
Q.116 Which of the following is a typical PD detection equipment for underground cables?
High‑frequency Current Transformer
Ground‑Potential Gradient sensor
Capacitance meter
Thermal camera
Explanation - GPG sensors detect voltage variations along the cable shield, effectively diagnosing underground PD.
Correct answer is: Ground‑Potential Gradient sensor
Q.117 The primary effect of PD on the dielectric loss factor is to:
Decrease it
Increase it
Keep it constant
Fluctuate randomly
Explanation - PD increases dielectric loss due to ionization and localized heating.
Correct answer is: Increase it
Q.118 Which of the following is NOT a typical PD source in high‑voltage equipment?
Insulation defects
Gas leaks
Mechanical vibration
Corrosion at terminations
Explanation - While vibration can degrade insulation, it does not directly cause PD initiation.
Correct answer is: Mechanical vibration
Q.119 In a PD test, the 'voltage hold‑off' level is:
Voltage at which the cable stops conducting
Maximum applied voltage during the test
Minimum voltage to start PD
Voltage at which PD stops
Explanation - Voltage hold‑off refers to the maximum voltage that the insulation can withstand without breaking down during the test.
Correct answer is: Maximum applied voltage during the test
Q.120 Which of the following PD types occurs when the electric field concentrates at a sharp point?
Surface PD
Volume PD
Corona PD
Cavitation PD
Explanation - Corona discharge initiates at sharp points where the field exceeds the breakdown threshold.
Correct answer is: Corona PD
Q.121 The PD detection signal is typically:
Low‑frequency sinusoid
High‑frequency burst
Constant DC
Square wave
Explanation - PD produces a high‑frequency, short‑duration burst superimposed on the AC waveform.
Correct answer is: High‑frequency burst
Q.122 Which of the following is a key parameter in the PD energy spectrum?
Peak voltage
Frequency content
Pulse width
All of the above
Explanation - The energy spectrum shows how energy is distributed across the frequency components of the PD pulse.
Correct answer is: Frequency content
Q.123 Partial discharge can lead to which of the following in transformers?
Oil degradation
Core magnetization change
Windings mechanical distortion
All of the above
Explanation - PD causes oil oxidation, core changes, and mechanical damage.
Correct answer is: All of the above
Q.124 The main challenge in measuring PD in underground cables is:
Noise from power lines
Signal attenuation in shield
High temperature
Low voltage levels
Explanation - The cable shield can dampen the high‑frequency components, making PD detection difficult.
Correct answer is: Signal attenuation in shield
Q.125 Which of the following is a typical PD detection method for gas‑insulated switchgear?
Thermography
Acoustic emission sensor
PD suppression coil
All of the above
Explanation - Multiple methods are used to monitor PD in gas‑insulated systems.
Correct answer is: All of the above