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AC Generation and Distribution
The three generators are driven at a governed speed which ensures a nominal 400 Hz output. During normal operation, the main generators supply power to their respective AC main busbars (Nos.1 and 2) through associated circuit breakers (GCBs).
Each main AC busbar supplies its own distribution system. If one generator becomes inoperative or disconnected from its busbar, both busbars are automatically connected together by a split system breaker (SSB), and one generator supplies both No.1 and No.2 busbars. If the APU is running, the SSB remains open and the APU generator automatically substitutes for the offline generator through an associated GCB (No.1A or No.2A). If the APU is started after a main generator failure, the SSB is automatically opened before the APU generator comes on-line. Fault protection facilities protect each channel from the following faults:
- Over-voltage
- Voltage instability (stability protection)
- Differential current faults (differential protection)
- Under/Over-frequency
- Under-voltage
- Over-current
The fault protection circuits automatically disconnect the respective generator from its busbar by opening the associated GCB when any, or a combination of any, of faults 1 to 5 are sensed. If an overcurrent fault is sensed by No.1 or No.2 generator, the alternative generator is prevented from taking over the defective busbar by automatic locking-out of the SSB. In addition, the accompanying under-voltage and/or under-frequency fault opens the associated GCB. In the case of the APU generator, No.1A or No.2A GCB will close, and on sensing the fault both GCBs will trip automatically.
For under-frequency/over-frequency and under-voltage faults, without overcurrent, a generator will automatically come on-line when the fault clears.
The under-frequency and under-voltage conditions automatically effect generator disconnection from its busbar during engine shutdown.
To reinstate a generator after over-voltage, stability protection, differential protection or over-current faults, it is necessary to select RESET on an associated generator control (GEN) switch. A selection of TRIP, on the generator control switch, disconnects the associated generator from its busbar. A reset selection is then necessary to reinstate the generator.
Amber generator failure (GEN FAIL) warning lights (one for each generator) give a visible indication when the associated generator is disconnected from its busbar, but the supply circuit of the APU GEN FAIL warning light is so arranged that an APU generator failure will not be indicated when both main generators are on-line.
A red BUS FAIL warning light is provided for each AC main busbar to provide a visible warning if the associated busbar voltage falls below a pre-determined value. The lights are connected to the master warning system.
KVA meters (one for each generator) indicate the total power supplied by the associated generator, and a frequency/voltage selector switch with five positions (EXT, GEN 1, APU GEN, GEN 2 and ESS) is provided to monitor the selected supply.
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