Marine Electrical, Electronic and Control Engineering
Marine Electrical, Electronic and Control Engineering
The exam checks generation, distribution, protection and the control loops behind alarms, not the ability to redraw wiring diagrams from memory.
Generation and parallel operation
- Synchronise on voltage, frequency, phase sequence and phase angle before closing the breaker.
- kW load is shared by the governor and speed droop; kVAR and power factor by the AVR and excitation. Using the governor to correct a power factor is a classic error.
- Protection: overcurrent, reverse power (a few per cent of rating with a time delay), preferential tripping of non-essential loads before the main breaker opens, earth-fault indication on an insulated system.
Insulation and emergency power
- Insulation resistance is measured with the circuit isolated and discharged; a common bench mark is at least 1 MOhm per kV of rated voltage plus 1 MOhm, so a 440 V motor should read above about 1.44 MOhm.
- Emergency generator: automatic start and connection to the emergency switchboard within 30 seconds, carrying the loads listed in SOLAS II-1.
Worked example
A 45 kW, 440 V three-phase motor, power factor 0.86, efficiency 0.91. I = P / (sqrt3 x V x pf x efficiency) = 45 000 / (1.732 x 440 x 0.86 x 0.91) = 45 000 / 596.9, about 75.4 A. The thermal overload is set near this full-load current and corrected for ambient and starting duty; the breaker is rated above it and the cable must carry it continuously.
Control
- A PID loop with too wide a proportional band reacts slowly; too narrow a band hunts. Integral removes offset, derivative damps overshoot. Know which term you are changing and record the change.
Traps
Closing the breaker with phase sequence unchecked; correcting kW with the AVR; treating the first earth fault on an insulated system as a trip; resetting a tripped starter before finding the cause.