Transfers, and losing it all
Power changes hands in three situations:
Power transfer between supplies — happens at every start and stop: external power hands over to the APU, the APU to the engine generators, and back again after landing. In a split system the transfer is break-before-make: the bus is disconnected from one source before the next connects, and the cabin sees the blink.
Transfer after a supply failure — the distribution lesson's logic run automatically: the dead source's contactor opens, the tie closes, and the surviving sources pick up the buses, shedding load if they must.
Loss of all normal AC supplies — the case the emergency layer exists for: with every generator gone, the battery feeds the essential DC buses directly and the essential AC bus through the static inverter, the RAT swings out to add its emergency generation, and everything outside the essential core goes dark. The aircraft flies on instruments and radios sized to the batteries lesson's endurance arithmetic.
The AC buses carry the aircraft's muscle. Typical AC consumers: the galleys and their ovens, the large fans of the air-conditioning system, electric hydraulic pumps and fuel boost pumps, the ice-protection heaters, cabin lighting and the passenger entertainment system.
Paralleling AC generators
DC sources can be connected together with little ceremony (matched voltages are enough); AC sources cannot, because two alternating voltages that disagree fight each other with equalising currents limited only by the machines' internals. Before two AC generators may be paralleled, three things must match:
Voltage — equal output voltages, or the higher machine drives current into the lower one.
Frequency — equal frequencies, or the machines slide continuously in and out of step, exchanging surging currents.
Phase — the same phase sequence and the waves in step at the moment of connection: closing the contactor with the waves opposed is electrically the same as short-circuiting two sources into each other.
Total power consumed in an AC system is measured in volt-amperes (VA). The fundamentals lesson noted that some AC consumers shift their current out of phase with the voltage; when that happens, volts times amperes no longer all becomes useful work, but the generator must still supply the full product. So AC generation is rated and loaded in volt-amperes (in practice kVA, thousands of them): the honest measure of what the machine must carry, whatever fraction ends up as work.
The control chain
Between each generator and its bus runs a chain of control, monitoring and annunciation (annunciation: the lights and messages that tell the crew), and the syllabus asks for its members by name:
GCU (generator control unit) — the generator's brain: it monitors the output, regulates voltage through the field current, and provides the network protection this lesson's next section describes.
Exciter contactor (breaker/relay) — controls the generator's exciter field: open it and the machine, still spinning, produces nothing.
Generator contactor (breaker/relay) — connects the generator's output to the network: the switch between the machine and its bus.
Bus-tie contactor (breaker/relay) — connects busbars together, the re-feeding path of every failure story so far.
Generator switch — the flight-deck switch giving the crew manual control of the exciter contactor: switching a generator OFF de-excites it.
IDG/CSD disconnect switch — the guarded flight-deck switch of the generation lesson, mechanically uncoupling the drive: the one-way decision.
Bus-tie switch — the flight-deck's say over the ties, deliberately minimal: AUTO and OFF positions only. In AUTO the system manages the ties itself; OFF exists to isolate, never to force a connection the logic would refuse.
The load management system
The electrical load management system runs the network in normal and degraded modes, on the ground and in flight, doing with the pieces above what the crew could never do fast enough, through three functions:
Distribution — routes sources to buses along the priority sequence, opening and closing generator and bus-tie contactors as sources come and go.
Monitoring — watches every generator's voltage, frequency and load, and annunciates what the crew must know.
Protection — acts through the same contactors when the numbers go wrong, in three ways:
Incorrect voltage — over- or undervoltage: the GCU de-excites the machine (exciter contactor) and disconnects it (generator contactor) before consumers are damaged or starved.
Incorrect frequency — over- or underfrequency, usually a drive running away or dying: the generator is disconnected before the frequency-sensitive consumers suffer.
Differential fault — the current leaving the generator is compared with the current arriving at the bus; any difference means current is escaping between them, a short in the feeder cables, and the machine is de-excited and disconnected instantly, because a feeder fault is fed by the generator with nothing else to limit it.
In degraded modes the same functions keep working with less: a failed generator's bus is re-fed along the priority list, shedding trims the load to the surviving sources, and on the ground the same logic runs the network from external power or the APU with the engines dark.
Watching the batteries
The aircraft batteries must be monitored because everything in this module ends with them: the battery is the reserve every failure story falls back on, and a reserve is only real if it is charged, healthy and cool. The system (and the crew) watch the battery's voltage, its charge or discharge current, and its temperature. Temperature monitoring matters most for nickel-cadmium and lithium batteries: both are the chemistries of the batteries lesson's thermal runaway, and a rising battery temperature is the earliest warning that charging has begun to feed on itself: caught warm, a NiCd is disconnected and cooled; caught late, it is an emergency.
The battery ammeter is read like a story: it shows the current into or out of the battery, and each indication has a meaning. A healthy battery after an engine start shows a high charge current that tapers steadily toward zero as the start's withdrawal is repaid. A near-zero reading in cruise is a full battery floating on the bus: held at bus voltage, neither taking nor giving current. A discharge reading in flight means the battery is feeding the aircraft, which with generators running is itself an abnormality to chase. And a heavy charge current that refuses to taper is the sinister one: the battery is swallowing energy without filling, the signature of a failing battery or the beginning of an overcharge runaway, and with a NiCd it belongs together with the temperature gauge.