A coaxial insulation has a 1 cm inner radius, 5 cm outer radius and 100 kV. Estimate field at inner surface.
A 1.24 MV/m
B 3.11 MV/m
C 12.4 MV/m
D 6.22 MV/m
Answer: D. 6.22 MV/m. For a coaxial geometry, E(r) equals V divided by r ln(b/a); at r 0.01 m, b/a 5, and 100 kV, it is about 6.22 MV/m.
An insulation has C 1 nF, V 100 kV rms, f 50 Hz and tanΞ΄ 0.005. Estimate dielectric loss.
A 1.57 W
B 31.4 W
C 15.7 W
D 157 W
Answer: C. 15.7 W. Dielectric loss is approximately omega C V squared tan delta; using 2Ο times 50 Hz gives about 15.7 W.
What are the two time labels in a 1.2/50 Β΅s impulse?
A RMS and peak values
B Front and half-value times
C Frequency and phase
D Rise and switching rates
Answer: B. Front and half-value times. The lightning impulse notation gives a virtual front time of 1.2 microseconds and a time to half value of 50 microseconds.
Four ideal Marx stages each charge to 50 kV. Find open-circuit impulse voltage.
A 50 kV
B 100 kV
C 200 kV
D 250 kV
Answer: C. 200 kV. In the ideal series discharge, four stage voltages add to four times 50 kV, or 200 kV.
A 1000:1 divider produces 75 V at its output. Estimate input voltage.
A 7.5 kV
B 75 kV
C 50 kV
D 750 kV
Answer: B. 75 kV. The input is 1000 times the output: 75,000 V, or 75 kV.
Why correct sphere-gap voltage for air density?
A Copper resistance depends on density
B Air breakdown depends on density
C Battery capacity depends on altitude
D Gap radius changes with frequency
Answer: B. Air breakdown depends on density. Gas discharge strength varies with air density, which depends mainly on pressure and temperature.
How does increasing conductor radius affect surface field at fixed voltage and spacing?
A It generally lowers surface field
B It always raises surface field
C It leaves field unchanged
D It reverses field direction
Answer: A. It generally lowers surface field. A larger conductor radius reduces field concentration at its surface in common transmission-line geometries.
What is the key risk from a cavity in solid insulation?
A Higher thermal conductivity
B Local partial discharge
C Lower electric stress
D Automatic field grading
Answer: B. Local partial discharge. An air or gas-filled cavity can experience high local electric stress and repeated partial discharge, degrading insulation.
What does tanΞ΄ compare in a dielectric?
A Active and reactive loss
B Voltage and frequency
C Current and resistance
D Peak and average field
Answer: A. Active and reactive loss. Tan delta is related to the ratio of active dielectric loss to reactive capacitive power.
If test frequency doubles with C, V and tanΞ΄ fixed, what happens to dielectric loss?
A It halves
B It stays fixed
C It becomes zero
D It doubles
Answer: D. It doubles. The loss expression contains angular frequency directly, so doubling frequency doubles loss under the stated assumptions.
Why can bundled transmission conductors reduce corona?
A They remove line capacitance
B They reduce phase count
C They lower surface field
D They eliminate lightning
Answer: C. They lower surface field. A bundle has a larger effective radius, lowering electric-field stress at each conductor surface.
Where does a grading ring reduce electric stress?
A Inside the turbine hub
B At the cable neutral only
C Near insulator energized end
D On the battery case
Answer: C. Near insulator energized end. A grading ring controls field concentration near the high-voltage end of an insulator string.
What can PD location help identify?
A Insulation defect position
B System frequency only
C Load power factor only
D Cable conductor material
Answer: A. Insulation defect position. Locating partial discharge helps identify the position of an insulation defect in cables, transformers or GIS.
Why use a cascade transformer for a high AC test voltage?
A Add secondary stage voltages
B Multiply test frequency
C Create impulse current only
D Remove all insulation
Answer: A. Add secondary stage voltages. Cascade stages are connected so their secondary voltages add while each stage is insulated for its potential.
At series resonance, what does the HV source mainly supply?
A Full reactive power
B Only DC current
C No current at all
D Circuit losses
Answer: D. Circuit losses. At resonance, source and load reactances cancel, so the supply mainly provides real losses while a large voltage appears across the test object.
What is a Cockcroft-Walton circuit output polarity relative to its input?
A Depends on diode orientation
B Always zero
C Always alternating
D Always ground
Answer: A. Depends on diode orientation. The multiplier's output polarity is set by the orientation of its diodes and capacitors.
Which element in an impulse generator most directly shapes the wavefront?
A Grounding rod
B Test-object label
C Front resistor
D Oil gauge
Answer: C. Front resistor. The front resistor and effective capacitances strongly influence the impulse rise time.
Why can nearby grounded objects affect a sphere-gap reading?
A They change air pressure
B They distort the electric field
C They change supply frequency
D They reduce sphere diameter
Answer: B. They distort the electric field. Nearby objects alter the field distribution and can change the disruptive-discharge voltage of the gap.
What does a surge arrester's residual voltage represent?
A Voltage during surge conduction
B Normal open-circuit voltage
C Battery terminal voltage
D DC ripple magnitude
Answer: A. Voltage during surge conduction. Residual voltage is the voltage appearing across an arrester while it conducts a specified surge current.
What should equipment withstand exceed in insulation coordination?
A Normal voltage only
B Arrester leakage current
C Protective level with margin
D Ground resistance
Answer: C. Protective level with margin. Insulation coordination maintains a suitable margin between the arrester protective level and equipment withstand strength.
How can moisture affect many insulating materials?
A Remove electric field
B Increase leakage and loss
C Raise insulation resistance always
D Stop all partial discharge
Answer: B. Increase leakage and loss. Moisture can increase conductivity and dielectric loss and accelerate ageing or surface discharge.
What can polluted wet insulator surfaces lead to?
A Higher dielectric strength
B Lower surface current
C Zero discharge risk
D Leakage heating and flashover
Answer: D. Leakage heating and flashover. Contamination plus moisture creates leakage current, dry-band arcing and possible surface flashover.
What is electrical treeing in solid insulation?
A Mechanical blade wear
B Oil flow pattern
C Grounding electrode corrosion
D Branched discharge degradation
Answer: D. Branched discharge degradation. Treeing is progressive branched damage formed by electrical activity within a solid dielectric.
What is a divider's scale factor?
A Input divided by output
B Output divided by frequency
C Current divided by voltage
D Power divided by time
Answer: A. Input divided by output. A divider scale factor is the ratio of its high-voltage input to its measured low-voltage output.
Why is a voltage divider's frequency response important for impulses?
A Raise test-object capacitance
B Set atmospheric pressure
C Increase sphere gap
D Preserve waveform shape
Answer: D. Preserve waveform shape. The divider and measurement cable must have adequate bandwidth and compensation to reproduce the impulse waveform accurately.