Electrical, Electronic and Control Engineering
This domain tests generation, distribution and control of ship's power: paralleling, protection, insulation, alarms and the automatic systems that keep the plant running after a fault.
Rules and concepts
- Parallel operation: generators share load through governor droop and excitation. Speed droop sets kW sharing; AVR and voltage droop set reactive (kVAr) sharing.
- Protection: discrimination so a fault trips only the smallest breaker; short-circuit, overload, reverse power, under/over voltage and frequency protection.
- Insulation: measure with a 500 V megohmmeter on a dead, isolated, discharged circuit, and record temperature and time with the reading.
- Automation: 4-20 mA loops, fail-safe design, and the 30 s emergency generator start and connection required by SOLAS II-1/43.
Worked example
A 50 Hz generator has 4 % speed droop: 50 Hz at no load, 48 Hz at rated load. At 60 % load, frequency = 50 - 0.6 x 2 = 48.8 Hz. If the second generator takes more than its share, the droop or speed settings differ. Adjust the speed setting for kW, never the AVR.
Traps
Adjusting voltage to change kW sharing; insulation testing on a live or undischarged circuit; trusting an alarm without checking the sensor; forgetting that reverse-power protection saves the prime mover after a breaker trip.