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HV exit exam questions and answers — Set 13 — High Voltage Safety Officer & Switchgear Operations

Set 13 of the High Voltage Safety Officer & Switchgear Operations (HV) exit exam bank — 30 of 428 questions, with the correct option marked and a one-line explanation for every one. Read it end to end, jump to a single set, or search it — then sit the same bank as a timed paper. The real DG Shipping exit exam is 30 marks with 15 to pass.

These are the bank's original groupings. Practising deals the whole bank out again every time you reshuffle, so a practice “Set 3” is a different 30 questions each time — the set numbers below are the source file's, and are only used here so the key stays navigable. Options are listed in their original order with fixed letters; when you practise, those move too.

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Set 13 — 30 HV exit exam questions with answers

1. The maximum output frequency of a cycloconverter is practically limited to approximately:

  • A. Twice the supply frequency
  • B. One third to about 40% of the supply frequency
  • C. Ten times the supply frequency
  • D. Exactly the supply frequency
Answer: B — One third to about 40% of the supply frequency
Because the output is synthesised from portions of the input waveform, waveform quality degrades as output frequency rises. Cycloconverters are therefore limited to roughly one third to 40% of supply frequency, adequate for low-speed propulsion motors.

2. In a synchroconverter (load commutated inverter, LCI) drive, the inverter thyristors are commutated mainly by:

  • A. Forced commutation from IGBTs
  • B. The ship's emergency battery
  • C. The back-EMF of the synchronous motor
  • D. A large DC link capacitor
Answer: C — The back-EMF of the synchronous motor
An LCI uses a current-source DC link (inductor) and a thyristor inverter whose commutation relies on the leading-power-factor back-EMF of the over-excited synchronous motor. At very low speed this EMF is insufficient, so a pulsed start mode is used.

3. A pulse-width modulated (PWM) voltage source inverter drive is characterised by:

  • A. No rectifier stage
  • B. Direct AC-AC conversion
  • C. A DC link capacitor and fast switching devices such as IGBTs in the inverter
  • D. A DC link inductor and naturally commutated thyristors
Answer: C — A DC link capacitor and fast switching devices such as IGBTs in the inverter
A VSI holds the DC link voltage constant with a capacitor bank, and the IGBT inverter switches at high frequency to build a variable voltage and frequency output. It is widely used for induction motor propulsion and thruster drives.

4. A current source inverter (CSI) drive is identified by:

  • A. A large DC link capacitor
  • B. A large DC link inductor that keeps the DC link current nearly constant
  • C. Use of a cycloconverter
  • D. No DC link at all
Answer: B — A large DC link inductor that keeps the DC link current nearly constant
In a CSI, the DC link reactor smooths the current rather than voltage, and the inverter switches this current between motor phases. The LCI used for synchronous propulsion motors is a type of current source inverter.

5. Why is the voltage-to-frequency (V/f) ratio kept approximately constant when controlling the speed of an induction motor with a frequency converter?

  • A. To allow the motor to run without a supply
  • B. To increase slip to 100%
  • C. To keep the motor current at zero
  • D. To maintain constant magnetic flux in the motor so it can develop rated torque without saturating
Answer: D — To maintain constant magnetic flux in the motor so it can develop rated torque without saturating
Air-gap flux is proportional to V/f. Reducing frequency without reducing voltage would saturate the core and cause high magnetising current, while too low a voltage would weaken torque, so the drive varies both together.

6. Field-oriented (vector) control of an induction motor drive works by:

  • A. Switching the motor between star and delta
  • B. Varying only the voltage while frequency is fixed
  • C. Separately controlling the flux-producing and torque-producing components of stator current
  • D. Inserting resistance in the rotor circuit
Answer: C — Separately controlling the flux-producing and torque-producing components of stator current
Vector control resolves the stator current into components aligned with and perpendicular to the rotor flux. Controlling them independently gives dynamic performance similar to a separately excited DC motor.

7. Direct torque control (DTC) in a motor drive differs from conventional PWM vector control because it:

  • A. Controls motor torque and flux directly via hysteresis comparators and a switching table, without a separate modulator
  • B. Uses a mechanical gearbox
  • C. Requires a DC motor
  • D. Controls only the motor voltage magnitude
Answer: A — Controls motor torque and flux directly via hysteresis comparators and a switching table, without a separate modulator
DTC estimates stator flux and torque from measured voltages and currents and selects inverter switching states directly to keep them within hysteresis bands. This gives very fast torque response without a PWM modulator.

8. The synchronous speed of a 6-pole induction motor supplied at 60 Hz is:

  • A. 1,800 rpm
  • B. 900 rpm
  • C. 1,200 rpm
  • D. 3,600 rpm
Answer: C — 1,200 rpm
Synchronous speed Ns = 120f/P = 120 x 60 / 6 = 1,200 rpm. The induction motor rotor runs slightly below this speed by the slip needed to induce rotor current.

9. Large HV motors such as bow thrusters are often started by auto-transformer, soft starter or frequency converter rather than direct-on-line mainly to:

  • A. Reverse the motor direction
  • B. Increase the starting current
  • C. Reduce the motor's rated power
  • D. Limit the large starting current and the resulting voltage dip on the switchboard
Answer: D — Limit the large starting current and the resulting voltage dip on the switchboard
Direct-on-line starting draws several times full-load current, which on a ship's limited generating capacity can cause severe voltage dips and possible blackout. Reduced-voltage or converter starting limits this inrush.

10. Energising a large HV/LV transformer can cause a magnetising inrush current several times rated current. Transformer differential protection avoids tripping on this by:

  • A. Increasing the system frequency
  • B. Harmonic (second harmonic) restraint
  • C. Earthing the transformer neutral solidly
  • D. Removing the CTs during energisation
Answer: B — Harmonic (second harmonic) restraint
Magnetising inrush is rich in second harmonic current, while internal fault current is not. Differential relays measure this harmonic content and restrain operation during energisation, preventing nuisance tripping.

11. HV/LV distribution transformers on board ships are most commonly of which type?

  • A. Oil-filled transformers with conservator tanks
  • B. Dry-type (cast resin or air-insulated) transformers
  • C. Auto-transformers without isolation
  • D. Rotating transformers
Answer: B — Dry-type (cast resin or air-insulated) transformers
Dry-type transformers avoid the fire risk and leakage problems of oil in machinery spaces and require less maintenance. They are usually air-cooled, with winding temperature sensors for alarm and trip.

12. Before working inside a variable frequency drive cabinet after isolation, why must the specified waiting time be observed?

  • A. The DC link capacitors may retain a dangerous charge for several minutes
  • B. The fans must cool down
  • C. The PMS needs to reset
  • D. The cabinet door interlock opens only after an hour
Answer: A — The DC link capacitors may retain a dangerous charge for several minutes
DC link capacitors store considerable energy and discharge through bleed resistors over a period stated by the manufacturer. The DC link voltage must be confirmed dead with a suitable tester before touching any components.

13. Large HV synchronous motors and generators on board are usually excited by a brushless system in which:

  • A. Carbon brushes feed DC from batteries directly to the rotor
  • B. Permanent magnets alone provide excitation, which cannot be adjusted
  • C. The stator is fed with DC
  • D. An AC exciter and rotating diode rectifier on the shaft supply DC to the main field
Answer: D — An AC exciter and rotating diode rectifier on the shaft supply DC to the main field
In brushless excitation, an AC exciter's armature on the rotor feeds a rotating rectifier which supplies the main field winding. This eliminates brushes and slip rings and their maintenance.

14. Running an HV switchboard with the bus tie breaker open (split bus) during critical operations is done mainly to:

  • A. Reduce the number of generators needed
  • B. Prevent a single fault from blacking out the entire power plant
  • C. Increase fault level on each section
  • D. Allow closing generators without synchronising
Answer: B — Prevent a single fault from blacking out the entire power plant
Splitting the main switchboard into independent sections limits the effect of a short circuit or generator failure to one section. This redundancy is applied, for example, on DP vessels and during manoeuvring.

15. When a VSI drive decelerates a motor with high inertia, energy flows back into the DC link. If there is no regenerative front end, this energy is commonly dissipated by:

  • A. The AVR
  • B. The motor anti-condensation heater
  • C. A braking chopper and braking resistor
  • D. Opening the main breaker
Answer: C — A braking chopper and braking resistor
Regenerated energy raises the DC link voltage. A braking chopper switches a resistor across the DC link to dissipate the energy as heat; otherwise the drive trips on DC overvoltage. Active front ends can return the energy to the network.

16. HV motor stator coils are normally insulated using form-wound coils impregnated by:

  • A. Paraffin wax
  • B. Simple varnish dipping without vacuum
  • C. Vacuum pressure impregnation (VPI) with resin
  • D. Paper wrapped with no impregnation
Answer: C — Vacuum pressure impregnation (VPI) with resin
Form-wound HV coils are taped with mica-based insulation and vacuum pressure impregnated with epoxy or polyester resin. VPI removes voids that would otherwise cause partial discharge and gives good thermal and mechanical strength.

17. A synchronous motor that is over-excited operates at:

  • A. Zero speed
  • B. Leading power factor, supplying reactive power to the system
  • C. Lagging power factor, drawing reactive power
  • D. Speed above synchronous speed
Answer: B — Leading power factor, supplying reactive power to the system
Increasing a synchronous motor's field current beyond the normal value makes it supply reactive power, so it draws leading current. This property is used to support system voltage and for LCI commutation.

18. How is the vacuum integrity of a vacuum circuit breaker interrupter usually checked during maintenance?

  • A. By listening for gas leaks
  • B. By weighing the bottle
  • C. By applying a specified high test voltage across the open contacts and checking that no breakdown occurs
  • D. By measuring the contact resistance only
Answer: C — By applying a specified high test voltage across the open contacts and checking that no breakdown occurs
A healthy vacuum withstands a high voltage across the open contact gap. If the vacuum has deteriorated, the gap breaks down at the test voltage. This test is done with a vacuum tester or HV test set per the manufacturer's procedure.

19. Contact erosion in a vacuum circuit breaker cannot be seen directly. It is usually assessed by:

  • A. Measuring the insulation resistance of the control circuit
  • B. Opening the vacuum bottle
  • C. Counting the number of shutters
  • D. Checking the contact wear indicator or gap measurement on the moving contact mechanism against manufacturer's limits
Answer: D — Checking the contact wear indicator or gap measurement on the moving contact mechanism against manufacturer's limits
As the contacts erode, the moving contact travels further, reducing the gap at a wear mark or spring setting. Gap gauging against the manufacturer's limits shows when the interrupter must be replaced.

20. Main contact resistance of an HV circuit breaker is measured using:

  • A. A micro-ohmmeter injecting a high DC current through the closed contacts
  • B. A clamp meter
  • C. An analogue multimeter on the ohms range
  • D. An insulation resistance tester at 5 kV
Answer: A — A micro-ohmmeter injecting a high DC current through the closed contacts
Contact resistance is in the micro-ohm range. A micro-ohmmeter (ductor) passes a substantial DC current and measures the voltage drop, giving an accurate value; high readings indicate worn, dirty or poorly aligned contacts.

21. A withdrawable HV circuit breaker is in the 'test' position when:

  • A. It is fully connected to the busbar and closed
  • B. Its earthing switch is closed and the breaker is closed on load
  • C. Its main contacts are disconnected from the busbar and cable, but the auxiliary/control circuits are connected so its operation can be tested
  • D. It is removed from the switchboard
Answer: C — Its main contacts are disconnected from the busbar and cable, but the auxiliary/control circuits are connected so its operation can be tested
The test position separates the main circuits while keeping secondary plugs connected. The breaker can then be closed, opened and tripped to check mechanism, protection and interlocks without energising the HV circuit.

22. Racking an HV circuit breaker in or out is mechanically prevented unless:

  • A. The busbar earthing switch is closed
  • B. The cable is energised
  • C. The control supply is switched off
  • D. The breaker is in the open position
Answer: D — The breaker is in the open position
An interlock prevents a closed breaker from being moved between service and test or isolated positions, because separating the plug-in contacts under load would draw a dangerous arc. The breaker must be open first.

23. Thermographic (infrared) surveys of HV switchboards are carried out to detect:

  • A. Vacuum loss in VCBs
  • B. Overheating connections and components while the equipment is on load
  • C. Earth faults on de-energised cables
  • D. Insulation resistance values
Answer: B — Overheating connections and components while the equipment is on load
Loose or corroded busbar and cable connections produce local heating under load. Infrared surveys through inspection windows identify hot spots without opening live compartments, allowing repair before failure or fire.

24. An earth fault alarm appears on an HV system with high-resistance neutral earthing. The first step in remedial action is to:

  • A. Ignore the alarm until the next port
  • B. Remove the neutral earthing resistor
  • C. Identify the faulty feeder using earth fault relay indications and the switching plan, then isolate it in a controlled manner
  • D. Immediately open all generator breakers
Answer: C — Identify the faulty feeder using earth fault relay indications and the switching plan, then isolate it in a controlled manner
Earth fault relays on each feeder indicate where the fault is. The Authorised Person then plans the isolation of that circuit, transferring loads if necessary, so the faulty equipment can be safely tested and repaired without blacking out the ship.

25. When isolating an HV feeder using a withdrawable circuit breaker with an integral earthing switch, the correct sequence is:

  • A. Rack out the closed breaker, then open it
  • B. Open the breaker, rack it to isolated position and lock the shutters, prove dead, then close the earthing switch
  • C. Prove dead after closing the earthing switch only
  • D. Close the earthing switch, then open the breaker
Answer: B — Open the breaker, rack it to isolated position and lock the shutters, prove dead, then close the earthing switch
The breaker is opened, then withdrawn to the isolated position with shutters locked. The cable side is proved dead before the earthing switch is closed and locked, applying the circuit main earth.

26. Before closing an HV earthing switch on a circuit, it is essential to prove it dead because:

  • A. Closing an earth onto a live circuit causes a severe short circuit and arc flash
  • B. Earthing switches are only rated for live closing
  • C. The earthing switch will not close otherwise
  • D. The PMS requires it
Answer: A — Closing an earth onto a live circuit causes a severe short circuit and arc flash
Earthing a live conductor creates a bolted fault with very high fault current. Although earthing switches have a fault-making rating, proving dead and interlocks protect the operator from this danger.

27. An SF6 circuit breaker gives a 'low gas density' alarm followed by a lockout. The purpose of the lockout is to:

  • A. Increase the gas pressure automatically
  • B. Bypass the protection relays
  • C. Prevent the breaker from operating when it cannot interrupt current safely
  • D. Allow the breaker to operate faster
Answer: C — Prevent the breaker from operating when it cannot interrupt current safely
The dielectric strength and arc-quenching ability of SF6 depend on gas density. Below the lockout level the breaker may fail to interrupt a fault, so operation is blocked and the gas leak must be repaired and the gas refilled.

28. When opening an SF6 circuit breaker compartment that has interrupted faults, personnel should use PPE and ventilation because:

  • A. Arced SF6 decomposes into toxic and corrosive by-products and powders
  • B. Pure SF6 is explosive
  • C. SF6 is radioactive
  • D. SF6 is lighter than air and causes frostbite
Answer: A — Arced SF6 decomposes into toxic and corrosive by-products and powders
Pure SF6 is non-toxic, but arcing produces decomposition products such as sulphur fluorides and metal fluoride powders that are toxic and corrosive. Gas should be recovered and protective equipment worn while handling interrupter components.

29. During routine maintenance of HV switchgear, insulating surfaces such as bushings and spouts should be:

  • A. Greased heavily with ordinary grease
  • B. Cleaned with lint-free cloth and approved cleaner, and inspected for tracking marks
  • C. Sprayed with water
  • D. Painted with conductive paint
Answer: B — Cleaned with lint-free cloth and approved cleaner, and inspected for tracking marks
Dust, salt and moisture on insulation surfaces create leakage paths leading to tracking and flashover. Cleaning with approved materials and checking for carbon tracking are essential parts of HV switchgear maintenance.

30. On an HV circuit breaker with a spring-operated mechanism, the closing spring is normally:

  • A. Not used; the breaker closes by gravity
  • B. Charged only by the protection relay
  • C. Charged by the arc energy
  • D. Charged by a motor and released to close the breaker, which in turn charges the opening spring
Answer: D — Charged by a motor and released to close the breaker, which in turn charges the opening spring
A motor charges the closing spring, with manual charging available. Releasing it closes the contacts quickly and charges the opening spring, so the breaker is always ready to trip after closing.

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