Q.1 What is the primary purpose of insulation coordination in high‑voltage substations?
To reduce the cost of insulation materials
To ensure the insulation withstands the maximum prospective over‑voltage
To increase the operating temperature of the equipment
To simplify the maintenance schedule
Explanation - Insulation coordination ensures that the insulation of all equipment can handle the highest prospective over‑voltage that may appear during normal operation or fault conditions, thereby preventing insulation failure.
Correct answer is: To ensure the insulation withstands the maximum prospective over‑voltage
Q.2 Which IEC standard specifically provides the methodology for high‑voltage insulation coordination?
IEC 60076
IEC 60814
IEC 60214
IEC 61000
Explanation - IEC 60814 outlines the detailed methodology for coordination of insulation levels in high‑voltage networks.
Correct answer is: IEC 60814
Q.3 In insulation coordination, the ‘design level’ is defined as:
The voltage the system will never exceed
The highest voltage the insulation is expected to withstand with a safety margin
The average operating voltage of the transformer
The lowest voltage in the network
Explanation - Design level is the maximum prospective over‑voltage the insulation must resist, including a safety margin for uncertainties.
Correct answer is: The highest voltage the insulation is expected to withstand with a safety margin
Q.4 Which factor is NOT considered in the prospective over‑voltage calculation?
Transient over‑voltages from switching operations
Dielectric strength of the insulation material
Breakdown voltage of the insulation at room temperature
Load‑interruption over‑voltages
Explanation - Dielectric strength is an inherent material property, not a factor in calculating prospective over‑voltages; it is used to verify design level after calculation.
Correct answer is: Dielectric strength of the insulation material
Q.5 Which of the following best describes the ‘time factor’ in insulation coordination?
The time it takes to repair damaged insulation
A multiplier that accounts for the duration of the over‑voltage
The time before a transformer can be energized
The interval between maintenance checks
Explanation - Time factor is used to reduce the prospective over‑voltage for short‑duration transients, reflecting the lower risk of breakdown for brief over‑voltage events.
Correct answer is: A multiplier that accounts for the duration of the over‑voltage
Q.6 What is the typical maximum permissible over‑voltage for a 150 kV system under normal operating conditions?
1.2 kV per kV of rated voltage
0.9 kV per kV of rated voltage
1.5 kV per kV of rated voltage
2.0 kV per kV of rated voltage
Explanation - Normal operating over‑voltage is generally limited to 1.2 kV per kV of rated system voltage.
Correct answer is: 1.2 kV per kV of rated voltage
Q.7 Which of the following is NOT a source of transient over‑voltage?
Switch‑on of a transmission line
Short‑circuit on a downstream circuit
Voltage step from a transformer tap change
Atmospheric lightning strike
Explanation - Tap changes usually produce controlled voltage steps, not transient over‑voltages. Transients arise from switching, faults, or lightning.
Correct answer is: Voltage step from a transformer tap change
Q.8 Which insulation level is usually used for a 500 kV transformer secondary winding?
600 kV
800 kV
1000 kV
1200 kV
Explanation - Insulation levels are typically selected to exceed the design level; for a 500 kV system, an 800 kV insulation level is common.
Correct answer is: 800 kV
Q.9 The term 'voltage stress' refers to:
The physical stress on the transformer core
The difference between prospective over‑voltage and the rated voltage
The maximum rated voltage of a switchgear
The mechanical load on a tower conductor
Explanation - Voltage stress is the extra voltage beyond the rated value that the insulation must endure.
Correct answer is: The difference between prospective over‑voltage and the rated voltage
Q.10 Which phenomenon is the main cause of insulation aging at elevated temperatures?
Corona discharge
Dielectric heating
Mechanical vibration
Electromagnetic interference
Explanation - Elevated temperatures increase dielectric heating, accelerating the breakdown of insulation materials.
Correct answer is: Dielectric heating
Q.11 When performing insulation coordination, the worst‑case prospective over‑voltage is typically determined by:
Summing all individual over‑voltages linearly
Taking the maximum of all transient over‑voltage values
Using the root‑sum‑square (RSS) method for multiple sources
Multiplying the maximum prospective over‑voltage by 2
Explanation - RSS is the correct method for combining multiple transient over‑voltages to find the worst‑case scenario.
Correct answer is: Using the root‑sum‑square (RSS) method for multiple sources
Q.12 The term 'design voltage' in the context of a transformer winding refers to:
The nominal voltage at which the transformer operates
The highest voltage the winding can physically withstand
The voltage at which insulation coordination is performed
The average voltage during normal operation
Explanation - Design voltage is the transformer’s rated operating voltage.
Correct answer is: The nominal voltage at which the transformer operates
Q.13 Which of the following is an example of a 'long‑duration' over‑voltage?
A switching event lasting 5 ms
A lightning strike lasting 500 µs
A 10‑s over‑voltage during a fault
A 20‑µs surge from a capacitor bank
Explanation - Long‑duration over‑voltages last seconds or more, unlike millisecond or microsecond transients.
Correct answer is: A 10‑s over‑voltage during a fault
Q.14 The primary goal of applying a safety factor to the design voltage is to:
Increase the rated current of the transformer
Compensate for uncertainties in future operating conditions
Reduce the cost of insulation
Extend the life of the switchgear
Explanation - Safety factors account for unknowns such as future load changes or unexpected faults.
Correct answer is: Compensate for uncertainties in future operating conditions
Q.15 Which of these is NOT a typical component used in high‑voltage insulation coordination studies?
Transient over‑voltage generator
Statistical data on fault frequency
Load flow solver
Grounding resistance database
Explanation - Fault frequency data are used for reliability studies, not directly for insulation coordination.
Correct answer is: Statistical data on fault frequency
Q.16 The term 'breakdown voltage' is defined as:
The voltage at which a conductor starts to conduct
The voltage at which the insulation material fails permanently
The maximum voltage a transformer can handle
The voltage at which the system will trip
Explanation - Breakdown voltage is the threshold at which insulation permanently fails due to dielectric breakdown.
Correct answer is: The voltage at which the insulation material fails permanently
Q.17 Which of the following best explains the concept of 'voltage margin'?
The extra voltage the system can handle before failure
The voltage drop across a conductor
The difference between rated and design voltage
The voltage lost during switching
Explanation - Voltage margin is the buffer between design level and actual prospective over‑voltage.
Correct answer is: The extra voltage the system can handle before failure
Q.18 In a 230 kV system, a normal operating over‑voltage is typically:
276 kV
274 kV
250 kV
260 kV
Explanation - Normal over‑voltage is usually rated at 1.2 times the nominal voltage (1.2 × 230 kV = 276 kV).
Correct answer is: 276 kV
Q.19 Which of the following best describes a 'potential rise' event?
An increase in voltage due to a fault
A decrease in voltage caused by line sag
A change in phase angle
A transient spike from a capacitor bank
Explanation - Potential rise is a voltage increase caused by faults or switching operations.
Correct answer is: An increase in voltage due to a fault
Q.20 What is the typical safety margin added to the design level for a 110 kV substation?
10 kV
20 kV
30 kV
40 kV
Explanation - A safety margin of about 20 kV is commonly used for 110 kV systems.
Correct answer is: 20 kV
Q.21 Which of the following best explains the term 'voltage surge'?
A sustained voltage increase over minutes
A rapid voltage increase that lasts microseconds to milliseconds
The maximum voltage during a transformer tap change
The voltage drop across a breaker during operation
Explanation - Voltage surges are high‑frequency, short‑duration voltage spikes.
Correct answer is: A rapid voltage increase that lasts microseconds to milliseconds
Q.22 Which factor is most critical in determining the corona inception voltage?
Conductor diameter
Ambient temperature
Conductor material
Conductor surface condition
Explanation - Surface imperfections, roughness, or contamination reduce the corona inception voltage.
Correct answer is: Conductor surface condition
Q.23 What is the main reason for using a 'time factor' of 1 in steady‑state over‑voltage calculations?
The over‑voltage persists for a long time
The over‑voltage is instantaneous
The over‑voltage is short‑duration
The over‑voltage is unknown
Explanation - Time factor 1 corresponds to steady‑state conditions where the over‑voltage lasts indefinitely.
Correct answer is: The over‑voltage persists for a long time
Q.24 In insulation coordination, the term 'probability of failure' refers to:
The chance that a breaker will trip during a fault
The likelihood that insulation will fail under a specific over‑voltage
The probability of a voltage spike occurring
The chance that a transformer will overheat
Explanation - Probability of failure is a statistical measure of insulation reliability under defined stress conditions.
Correct answer is: The likelihood that insulation will fail under a specific over‑voltage
Q.25 Which of the following is a common practice to reduce over‑voltage levels on a transmission line?
Installing additional reactors at the line ends
Shortening the line length
Using thicker conductors
Adding more insulators
Explanation - Shunt reactors help dampen transient over‑voltages by providing inductive impedance.
Correct answer is: Installing additional reactors at the line ends
Q.26 A 400 kV substation has a design voltage of 500 kV. What is its voltage margin?
50 kV
100 kV
80 kV
120 kV
Explanation - Voltage margin = design voltage – rated voltage = 500 kV – 400 kV = 100 kV.
Correct answer is: 100 kV
Q.27 Which type of over‑voltage is the most dangerous for insulation materials?
Normal operating over‑voltage
Transient over‑voltage from switching
Long‑duration over‑voltage from a fault
Voltage step from tap changing
Explanation - Long‑duration over‑voltages can cause substantial heating and dielectric degradation.
Correct answer is: Long‑duration over‑voltage from a fault
Q.28 The calculation of prospective over‑voltage for a line involves:
The line length only
The line length and the load current only
The line length, the load current, and the fault level
The line length, the load current, and the fault level, plus a safety margin
Explanation - Prospective over‑voltage depends on multiple parameters and requires a safety margin for design.
Correct answer is: The line length, the load current, and the fault level, plus a safety margin
Q.29 Which of the following best represents the 'time factor' for a 20 ms switching transient?
1.0
0.5
0.1
0.01
Explanation - Short transients typically have a time factor around 0.1 to reflect reduced risk.
Correct answer is: 0.1
Q.30 Why are high‑voltage insulators often made from porcelain?
They have excellent mechanical strength
They provide good dielectric properties and are easy to manufacture
They are inexpensive compared to other materials
They are transparent
Explanation - Porcelain offers high dielectric strength, low loss, and ease of casting.
Correct answer is: They provide good dielectric properties and are easy to manufacture
Q.31 Which of the following is a primary cause of insulation aging in transformer oil?
Mechanical vibration
Low dielectric constant
Oxidation of the oil
High mechanical tension
Explanation - Oxidation reduces the dielectric strength of transformer oil over time.
Correct answer is: Oxidation of the oil
Q.32 The term 'voltage level' in the context of high‑voltage networks refers to:
The maximum rated voltage of equipment
The nominal voltage of a line
The design voltage of a transformer
The voltage drop across a conductor
Explanation - Voltage level denotes the operating voltage category of the network (e.g., 400 kV, 230 kV).
Correct answer is: The nominal voltage of a line
Q.33 Which of the following is NOT a recommended practice for reducing corona over a high‑voltage line?
Increasing conductor diameter
Applying a surface coating to conductors
Adding intermediate insulators
Shortening the line
Explanation - Line length has little influence on corona; diameter and surface treatments are more effective.
Correct answer is: Shortening the line
Q.34 What is the typical frequency of normal operating over‑voltage events?
Less than 1 per hour
Between 1 and 5 per hour
Between 5 and 10 per hour
More than 10 per hour
Explanation - Normal operating over‑voltages are rare, often less than once per hour.
Correct answer is: Less than 1 per hour
Q.35 In high‑voltage engineering, the 'breakdown voltage' is measured at:
Room temperature
Operating temperature
Absolute zero
Atmospheric pressure at sea level
Explanation - Breakdown voltage varies with temperature; it is typically specified at the expected operating temperature.
Correct answer is: Operating temperature
Q.36 Which of the following best defines 'insulation coordination level'?
The voltage level at which equipment operates
The selected design level for all network components
The voltage difference between equipment and ground
The maximum permissible over‑voltage on a system
Explanation - Insulation coordination level is the target design voltage that ensures all equipment meets coordination requirements.
Correct answer is: The selected design level for all network components
Q.37 Which of the following is a typical step in the insulation coordination process?
Selecting the largest possible voltage rating
Calculating the prospective over‑voltage for each component
Installing the lowest cost insulation
Measuring the current through each conductor
Explanation - Prospective over‑voltage calculation is essential for determining design levels.
Correct answer is: Calculating the prospective over‑voltage for each component
Q.38 A 300 kV line is designed with a safety margin of 25 kV. What is its design voltage?
325 kV
3250 kV
300 kV
350 kV
Explanation - Design voltage = rated voltage + safety margin = 300 + 25 = 325 kV.
Correct answer is: 325 kV
Q.39 Which of the following is a common method for measuring dielectric strength of a transformer oil?
Megger test
Dissolved gas analysis
Partial discharge monitoring
Oil viscosity test
Explanation - Megger (high‑voltage) test applies a high voltage to the oil to evaluate its dielectric strength.
Correct answer is: Megger test
Q.40 Why is it important to consider the 'time factor' for lightning induced surges?
Lightning surges are long‑duration events
Lightning surges are short‑duration but high‑energy events
Lightning surges are constant
Lightning surges are irrelevant to insulation coordination
Explanation - Lightning surges are short but powerful; time factors help estimate their effect on insulation.
Correct answer is: Lightning surges are short‑duration but high‑energy events
Q.41 The 'maximum prospective over‑voltage' is calculated using:
Only the line length
Line length, load, and fault level
Load current only
Fault level only
Explanation - All three parameters influence the prospective over‑voltage magnitude.
Correct answer is: Line length, load, and fault level
Q.42 What does the 'voltage rise factor' account for in over‑voltage calculations?
The rate at which voltage increases during a fault
The voltage drop across a breaker
The change in voltage due to line sag
The voltage difference between phases
Explanation - Voltage rise factor models how quickly the voltage rises in a fault condition.
Correct answer is: The rate at which voltage increases during a fault
Q.43 Which of the following is NOT considered a 'steady‑state' over‑voltage source?
Load‑interruption voltage
Line‑to‑ground fault
Normal operating voltage
Permanent over‑voltage due to long‑term load changes
Explanation - Line‑to‑ground faults are transient; steady‑state over‑voltages persist without fault events.
Correct answer is: Line‑to‑ground fault
Q.44 Which of the following parameters is NOT used to calculate the time factor for a transient over‑voltage?
Duration of the over‑voltage
Amplitude of the over‑voltage
Time constant of the network
Safety margin of the insulation
Explanation - The safety margin is applied after calculating the design level, not within the time factor.
Correct answer is: Safety margin of the insulation
Q.45 A high‑voltage transformer has a design voltage of 735 kV. What is a suitable insulation level for this transformer?
800 kV
1000 kV
750 kV
650 kV
Explanation - Insulation levels are chosen to exceed the design voltage; 1000 kV is common for 735 kV equipment.
Correct answer is: 1000 kV
Q.46 Which of the following is a direct consequence of inadequate insulation coordination?
Reduced system reliability
Higher power losses
Shorter line length
Lower operational voltage
Explanation - Insulation failures due to over‑voltages cause equipment damage and system outages.
Correct answer is: Reduced system reliability
Q.47 Which type of over‑voltage is typically associated with a line‑to‑line fault?
Long‑duration over‑voltage
Short‑duration over‑voltage
Normal operating over‑voltage
Transient over‑voltage from switching
Explanation - Line‑to‑line faults create brief over‑voltage spikes that decay quickly.
Correct answer is: Short‑duration over‑voltage
Q.48 Which of the following is a key factor determining the 'design voltage' of a transformer?
The transformer's size
The transformer's core material
The transformer's rated capacity and operating voltage
The transformer's cooling method
Explanation - Design voltage depends on the rated voltage and the power level it must handle.
Correct answer is: The transformer's rated capacity and operating voltage
Q.49 What is the purpose of applying a 'time factor' of 0.2 in a 200 µs surge calculation?
To account for a very short duration over‑voltage
To double the prospective over‑voltage
To reduce the safety margin
To increase the design voltage
Explanation - A time factor of 0.2 reduces the prospective over‑voltage for brief surges.
Correct answer is: To account for a very short duration over‑voltage
Q.50 Which of the following best describes a 'partial discharge' in insulation?
A complete breakdown of insulation
A localized, intermittent discharge within the insulation material
A discharge between conductors and ground
A voltage drop across a transformer winding
Explanation - Partial discharge is a small, localized event that can indicate insulation defects.
Correct answer is: A localized, intermittent discharge within the insulation material
Q.51 Which of the following is a recommended measure to reduce the probability of a dielectric breakdown?
Increasing the temperature of the insulation
Using lower voltage ratings
Operating at higher load currents
Increasing the frequency of the supply
Explanation - Operating at lower voltages reduces the stress on insulation, lowering breakdown risk.
Correct answer is: Using lower voltage ratings
Q.52 In a 230 kV network, the prospective over‑voltage due to a fault is calculated to be 350 kV. If the design level is set at 380 kV, what is the voltage margin?
30 kV
20 kV
50 kV
10 kV
Explanation - Margin = design level – prospective over‑voltage = 380 – 350 = 30 kV.
Correct answer is: 30 kV
Q.53 Which of the following is a typical application of a shunt reactor in high‑voltage networks?
Reducing the voltage rise during switching operations
Increasing the current carrying capacity of a line
Providing additional grounding
Storing energy during peak demand
Explanation - Shunt reactors provide inductive impedance that limits voltage spikes during switching.
Correct answer is: Reducing the voltage rise during switching operations
Q.54 The 'breakdown voltage' of a dielectric material is measured under:
High temperature
Low temperature
Ambient temperature
Vacuum conditions
Explanation - Breakdown voltage is usually specified at ambient (ambient temperature) conditions.
Correct answer is: Ambient temperature
Q.55 Which of the following best defines 'voltage spike'?
A sustained high voltage level
A rapid increase in voltage that lasts microseconds
The voltage drop across a transformer core
The difference between two phase voltages
Explanation - Voltage spikes are short‑duration, high‑amplitude events.
Correct answer is: A rapid increase in voltage that lasts microseconds
Q.56 Which of the following is an effect of increased ambient temperature on high‑voltage insulation?
Increase in dielectric strength
Reduction in dielectric strength
No change in dielectric strength
Improved corona performance
Explanation - Higher temperatures typically lower the dielectric strength of insulation materials.
Correct answer is: Reduction in dielectric strength
Q.57 Which of the following is a common cause of corona discharge?
Large conductor radius
Smooth conductor surface
High humidity
High voltage gradients at the conductor surface
Explanation - Corona occurs when the electric field exceeds the breakdown strength of the surrounding air.
Correct answer is: High voltage gradients at the conductor surface
Q.58 What is the main objective of applying a safety margin to the design voltage?
To reduce the cost of insulation
To allow for future expansion
To compensate for uncertainties in operating conditions
To increase the maximum current rating
Explanation - Safety margins accommodate unforeseen variations and ensure reliability.
Correct answer is: To compensate for uncertainties in operating conditions
Q.59 The 'maximum prospective over‑voltage' for a 400 kV line is calculated to be 450 kV. What should be the minimum design voltage?
450 kV
480 kV
500 kV
600 kV
Explanation - The design voltage should exceed the maximum prospective over‑voltage by a safety margin (e.g., 10‑15%).
Correct answer is: 500 kV
Q.60 Which of the following best describes a 'voltage rise' during a fault?
A drop in voltage
An increase in voltage that persists for several minutes
A rapid increase in voltage that decays quickly
A constant voltage level
Explanation - Voltage rise occurs quickly and then diminishes as the fault clears.
Correct answer is: A rapid increase in voltage that decays quickly
Q.61 Which of the following is a key factor in determining the time factor for a lightning strike?
Duration of the surge
Amplitude of the surge
Location of the strike
Frequency of the supply
Explanation - Time factor depends on how long the over‑voltage lasts during a lightning event.
Correct answer is: Duration of the surge
Q.62 The 'voltage rating' of an insulator is primarily based on:
Its mechanical strength
Its thermal conductivity
Its dielectric strength and safety factor
Its color
Explanation - Voltage rating reflects the maximum voltage the insulator can handle safely.
Correct answer is: Its dielectric strength and safety factor
Q.63 Which of the following is a common method for measuring corona inception voltage?
Using a high‑voltage test system on a conductor
Measuring the temperature rise on a transformer core
Monitoring partial discharge signals
Measuring oil conductivity
Explanation - Corona inception voltage is measured by gradually increasing voltage on a conductor until corona starts.
Correct answer is: Using a high‑voltage test system on a conductor
Q.64 In insulation coordination, a 'voltage margin' is often expressed as:
Absolute voltage value
Percentage of the rated voltage
Time duration
Temperature difference
Explanation - Voltage margins are typically quoted as a percentage of the nominal system voltage.
Correct answer is: Percentage of the rated voltage
Q.65 Which of the following is a consequence of a high time factor for a transient over‑voltage?
Lower prospective over‑voltage
Higher prospective over‑voltage
No effect on over‑voltage
Increased probability of failure
Explanation - A higher time factor means the over‑voltage is considered to last longer, increasing the effective stress.
Correct answer is: Higher prospective over‑voltage
Q.66 Which of the following is a typical design criterion for high‑voltage insulation coordination?
The color of the insulator
The shape of the transformer core
The maximum prospective over‑voltage plus safety factor
The number of phases in the network
Explanation - Design criteria incorporate prospective over‑voltage with added safety margins.
Correct answer is: The maximum prospective over‑voltage plus safety factor
Q.67 Which of the following best explains why a 'design level' is higher than the 'rated level' of equipment?
To account for manufacturing tolerances
To ensure equipment can handle peak over‑voltages
To reduce the cost of materials
To increase the efficiency of the system
Explanation - Design level is higher to accommodate peak over‑voltage events beyond normal operation.
Correct answer is: To ensure equipment can handle peak over‑voltages
Q.68 Which of the following is a key parameter in the calculation of prospective over‑voltage from a switching event?
The line length
The load current
The switching time constant
All of the above
Explanation - Prospective over‑voltage depends on line length, load, and switching characteristics.
Correct answer is: All of the above
Q.69 The 'probability of failure' curve for a dielectric material is typically expressed as:
Voltage versus time
Temperature versus time
Probability versus voltage
Voltage versus temperature
Explanation - The probability of failure is plotted against voltage to show reliability over stress levels.
Correct answer is: Probability versus voltage
Q.70 What does the 'corona factor' account for in insulation coordination?
The effect of moisture on insulation
The influence of conductor diameter
The occurrence of corona discharge in air
The thermal conductivity of the insulator
Explanation - Corona factor adjusts for the voltage reduction due to corona discharge around conductors.
Correct answer is: The occurrence of corona discharge in air
Q.71 Which of the following is a typical characteristic of a 'steady‑state' over‑voltage?
Short duration (microseconds)
Long duration (seconds to minutes)
Sudden onset
Rapid decay
Explanation - Steady‑state over‑voltages persist for extended periods.
Correct answer is: Long duration (seconds to minutes)
Q.72 Which of the following is NOT a typical method for reducing corona over‑voltage on a line?
Increasing conductor spacing
Adding shunt reactors
Using a smaller conductor diameter
Applying a corona ring on the conductors
Explanation - Smaller diameter increases the electric field, worsening corona.
Correct answer is: Using a smaller conductor diameter
Q.73 Which of the following best describes the 'voltage rise factor' used in fault over‑voltage calculations?
The ratio of fault current to normal current
The ratio of prospective over‑voltage to rated voltage
The rate at which voltage increases during a fault
The time constant of the network
Explanation - Voltage rise factor quantifies how quickly the voltage rises during fault conditions.
Correct answer is: The rate at which voltage increases during a fault
Q.74 In the context of high‑voltage equipment, 'insulation coordination' primarily helps to:
Increase the operating voltage
Reduce the cost of transformers
Ensure that insulation withstands maximum prospective over‑voltages
Decrease the physical size of equipment
Explanation - Insulation coordination guarantees that equipment insulation can handle all potential over‑voltage events.
Correct answer is: Ensure that insulation withstands maximum prospective over‑voltages
Q.75 Which of the following is a factor that increases the probability of insulation failure?
Operating at lower temperatures
Using high‑quality insulating materials
Operating under high humidity
Operating at reduced load levels
Explanation - High humidity can reduce insulation dielectric strength and promote partial discharge.
Correct answer is: Operating under high humidity
Q.76 Which of the following best defines the 'maximum prospective over‑voltage' (MPOV)?
The highest voltage that the system will ever experience
The maximum calculated over‑voltage from all potential sources under normal conditions
The voltage applied during a fault
The rated voltage of the equipment
Explanation - MPOV is the worst‑case over‑voltage that the system may encounter, used in design.
Correct answer is: The maximum calculated over‑voltage from all potential sources under normal conditions
Q.77 Which of the following is NOT an over‑voltage source considered in high‑voltage insulation coordination?
Switching over‑voltage
Lightning over‑voltage
Thermal over‑voltage
Load‑interruption over‑voltage
Explanation - Thermal changes affect insulation properties but are not over‑voltage sources.
Correct answer is: Thermal over‑voltage
Q.78 Which of the following is a typical safety factor applied in insulation coordination?
0.5
1.0
1.2
2.0
Explanation - A safety factor of 1.2 is common to provide a margin over the calculated prospective over‑voltage.
Correct answer is: 1.2
Q.79 What is the most common material used for high‑voltage insulators?
Steel
Concrete
Porcelain
Aluminum
Explanation - Porcelain has high dielectric strength and is widely used in HV insulators.
Correct answer is: Porcelain
Q.80 Which of the following is the primary cause of partial discharge in HV systems?
Insulation degradation
Mechanical vibration
Over‑voltage exceeding the dielectric strength
Low operating temperatures
Explanation - Partial discharge occurs when over‑voltage surpasses the insulation’s dielectric strength.
Correct answer is: Over‑voltage exceeding the dielectric strength
Q.81 Which of the following best describes a 'steady‑state over‑voltage'?
A transient over‑voltage lasting less than a millisecond
A voltage rise that persists for seconds to minutes
A voltage drop below the rated value
A sudden voltage spike during switching
Explanation - Steady‑state over‑voltages are prolonged and maintain for extended periods.
Correct answer is: A voltage rise that persists for seconds to minutes
Q.82 Which of the following best explains the concept of 'insulation coordination level'?
The voltage level at which a transformer operates
The design level chosen for a network
The maximum voltage the network can handle
The voltage difference between phases
Explanation - Insulation coordination level refers to the chosen design voltage ensuring all equipment meets coordination requirements.
Correct answer is: The design level chosen for a network
Q.83 Which of the following is a typical step in determining the voltage margin for a 400 kV line?
Adding the prospective over‑voltage to the rated voltage
Subtracting the safety factor from the prospective over‑voltage
Subtracting the prospective over‑voltage from the design voltage
Multiplying the prospective over‑voltage by the safety factor
Explanation - Voltage margin = design voltage – prospective over‑voltage.
Correct answer is: Subtracting the prospective over‑voltage from the design voltage
Q.84 Which of the following best defines 'design voltage' for a transformer?
The maximum voltage it can handle
The minimum voltage it can handle
The nominal operating voltage plus a safety margin
The rated power level
Explanation - Design voltage includes the nominal voltage and an added safety margin.
Correct answer is: The nominal operating voltage plus a safety margin
Q.85 Which of the following best explains the purpose of a time factor of 0.5 in a transient over‑voltage calculation?
To increase the prospective over‑voltage by 50%
To reduce the prospective over‑voltage by 50%
To double the safety margin
To halve the design voltage
Explanation - A time factor less than 1 reduces the prospective over‑voltage for short‑duration events.
Correct answer is: To reduce the prospective over‑voltage by 50%
Q.86 Which of the following is a major contributor to high‑voltage insulation breakdown?
High humidity
Low temperature
Short line lengths
Low load currents
Explanation - High humidity can decrease dielectric strength and promote partial discharge leading to breakdown.
Correct answer is: High humidity
Q.87 Which of the following best describes the relationship between corona inception voltage (CIV) and conductor surface condition?
A smoother surface increases the CIV
A rougher surface increases the CIV
Surface condition has no effect on CIV
CIV is independent of conductor geometry
Explanation - Smoother surfaces reduce electric field concentration, raising the CIV.
Correct answer is: A smoother surface increases the CIV
Q.88 In the context of high‑voltage systems, what does 'voltage rise factor' represent?
The maximum permissible voltage difference between phases
The rate of voltage increase during a fault event
The ratio of the rated voltage to the design voltage
The time it takes for a voltage to decay after a fault
Explanation - Voltage rise factor indicates how quickly the voltage rises during fault conditions.
Correct answer is: The rate of voltage increase during a fault event
Q.89 Which of the following is a typical maximum prospective over‑voltage for a 230 kV system during a line‑to‑line fault?
260 kV
300 kV
350 kV
400 kV
Explanation - During a line‑to‑line fault, prospective over‑voltages can reach around 350 kV for a 230 kV system.
Correct answer is: 350 kV
Q.90 Which of the following best explains why insulation coordination is essential for high‑voltage substations?
To reduce the physical size of transformers
To ensure insulation can handle prospective over‑voltages
To increase the speed of fault clearing
To lower the cost of switching gear
Explanation - Proper coordination guarantees that insulation will not fail when subjected to over‑voltage events.
Correct answer is: To ensure insulation can handle prospective over‑voltages
Q.91 Which of the following is NOT a typical over‑voltage source considered in the design of high‑voltage equipment?
Switching over‑voltage
Load‑interruption over‑voltage
Short‑circuit over‑voltage
Ambient temperature over‑voltage
Explanation - Ambient temperature is not an over‑voltage source but affects dielectric properties.
Correct answer is: Ambient temperature over‑voltage
Q.92 The 'time factor' for a 100 µs surge is typically:
0.5
1.0
0.1
0.01
Explanation - Short surges have a time factor around 0.1 to reduce the effective over‑voltage.
Correct answer is: 0.1
Q.93 What is the main advantage of using a time factor in high‑voltage insulation coordination?
It simplifies the calculation of prospective over‑voltage
It allows for the use of lower voltage ratings
It reduces the size of the insulators
It eliminates the need for safety margins
Explanation - Time factors reduce the prospective over‑voltage for short transients, permitting lower insulation levels.
Correct answer is: It allows for the use of lower voltage ratings
Q.94 Which of the following is a key parameter in the calculation of the maximum prospective over‑voltage (MPOV) for a transmission line?
The conductor diameter
The line length
The insulation material
The color of the insulator
Explanation - Line length influences the prospective over‑voltage as longer lines can experience higher voltage rises.
Correct answer is: The line length
Q.95 Which of the following is NOT a typical consideration when selecting the insulation level for a 400 kV transformer?
The design voltage
The prospective over‑voltage of the network
The color of the transformer oil
The safety factor applied
Explanation - The color of transformer oil has no effect on the insulation level selection.
Correct answer is: The color of the transformer oil
Q.96 Which of the following best defines a 'probability of failure' for a dielectric material?
The likelihood that the material will conduct electricity
The chance of dielectric breakdown under a specific voltage
The probability that the material will melt
The chance that the material will expand
Explanation - Probability of failure quantifies the risk of breakdown at given voltage levels.
Correct answer is: The chance of dielectric breakdown under a specific voltage
Q.97 Which of the following is a typical source of a 'transient over‑voltage' in a high‑voltage network?
Switching of a circuit breaker
Long‑duration load change
Thermal expansion of conductors
Constant operating voltage
Explanation - Switching events produce transient over‑voltages that are short in duration.
Correct answer is: Switching of a circuit breaker
Q.98 Which of the following is NOT a typical method to reduce corona over a high‑voltage line?
Increasing conductor diameter
Adding shunt reactors
Applying a corona ring
Shortening the line length
Explanation - Line length has minimal impact on corona; conductor diameter and surface treatments are more effective.
Correct answer is: Shortening the line length
Q.99 What does the term 'breakdown voltage' represent in the context of high‑voltage insulation?
The maximum voltage the system can handle before failure
The voltage at which the material permanently fails
The rated operating voltage
The voltage drop across a breaker
Explanation - Breakdown voltage is the point where dielectric failure occurs, leading to permanent damage.
Correct answer is: The voltage at which the material permanently fails
Q.100 Which of the following is a common factor considered in the calculation of the voltage margin?
The color of the insulator
The duration of the prospective over‑voltage
The length of the transformer core
The shape of the busbar
Explanation - Time factor and duration influence the calculation of voltage margin.
Correct answer is: The duration of the prospective over‑voltage