Engines and Generators
This domain tests reciprocating-engine operation and generator control. The exam expects you to explain governor droop, synchronization, excitation, power factor, load sharing and the protective relays that prevent damage or islanding.
Engines and governors
- Four-stroke engines use intake, compression, power and exhaust strokes. Compression ignition relies on high compression temperature; spark ignition uses a timed spark.
- Speed droop lets paralleled generators share load predictably. Droop is the percentage speed rise from no load to full load. Isochronous control holds constant speed and is used for isolated or single-unit operation, not usually for unlimited parallel load sharing.
- Mechanical, hydraulic and electronic governors position fuel racks or throttle valves. Load sharing must account for droop setting and machine rating.
Generators and protection
- Synchronization requires matching voltage, frequency, phase angle and phase sequence. Closing a breaker out of phase causes large currents and mechanical stress.
- The automatic voltage regulator controls excitation and terminal voltage. Increasing excitation raises reactive power output and power factor moves toward lagging; decreasing excitation can lead to leading power factor and instability.
- Generator protection includes overcurrent, differential, loss-of-field, reverse-power, over/under-frequency, overvoltage and stator earth fault. Reverse-power protection prevents motoring when prime-mover input is lost.
Worked example
A 1,000 kW generator has 4% droop and a no-load frequency of 60 Hz. Full-load frequency is 60 - 0.04 x 60 = 57.6 Hz. At 500 kW, the frequency is 60 - 0.04 x 60 x (500/1,000) = 60 - 1.2 = 58.8 Hz. A second generator with the same droop will share load according to its rating and speed setting; changing the speed setpoint shifts load between machines without changing the system frequency much.
Traps that cost marks
Confusing kW with kVA or forgetting power factor; synchronizing on voltage and frequency but ignoring phase angle; running a generator at leading power factor without checking stability limits; and assuming reverse-power protection is the same as overcurrent protection. Also watch governor droop direction: too little droop can cause load hunting between units.
Key points: droop allows stable parallel load sharing; synchronization requires voltage, frequency, phase angle and sequence; AVR controls voltage and reactive power; reverse-power protection prevents motoring; kW and kVA differ by power factor.