AC Electrics -Alternators — Page 210, Lesson 223

I want to walk you through the Generator Control Unit, or GCU, which is the brain behind a modern generator control system. The GCU houses all the circuitry that provides many functions of power control and protection. A typical GCU will monitor the generator's output and provide voltage regulation by controlling the exciter field current — that's the current flowing through the field windings of the exciter, which in turn controls the main generator's output voltage. The GCU also contains protection circuitry that monitors for several fault conditions: overvoltage, overcurrent, frequency errors, incorrect phase sequence, and differential current. Differential current protection compares the current flowing into a winding with the current flowing out; if they don't match, it indicates a fault inside the generator. Each generator on the aircraft has its own GCU, and these GCUs may work as a team with the BPCU — the Bus Power Control Unit — in controlling fault isolation switching, meaning they coordinate to isolate a faulty generator from the system. Inside the GCU, you may also find an Exciter Control Relay, which is also known as a Generator Control Relay or a Generator Field Relay. This relay controls the exciter field current supply to the generator field. If a dangerous fault occurs — specifically overexcitation or overvoltage — the fault protection circuit inside the GCU will open this exciter control relay. When that relay opens, the generator output falls to a residual value, making it safe. Residual value is the small voltage a generator produces from its own permanent magnetism when the field current is cut off. At the same time, the GCU will also open the Generator Circuit Breaker, or GCB, to disconnect the generator from its bus bar. In a paralleled system, power would be maintained to that generator's bus bar from the other generators through the BTB — the Bus Tie Breaker. Now let's move on to emergency supplies. In the unlikely event that some or all of the engine-driven AC power generation systems on the aircraft fail, alternative sources must be available. The excerpt lists four alternative means of providing AC power: the Ram Air Turbine, the Auxiliary Power Unit, a Static Inverter, and a Hydraulic Motor driven generator. Let's start with the Ram Air Turbine, abbreviated RAT or sometimes ELRAT. When lowered into the slipstream of an aircraft in flight, the RAT will produce an emergency source of AC power. Its output is controlled at a nominal 115 volts, 200 volts, 400 hertz, three-phase. That's a standard aircraft AC power specification: 115 volts phase-to-neutral, 200 volts phase-to-phase, at 400 Hz, three-phase. This gives limited operation only of flight instrument and radio services in the event of total alternator failure. The excerpt notes that RATs driving an electrical generator have been largely replaced by RATs driving a hydraulic pump, because modern aircraft are more dependent on hydraulic power to operate the primary flying controls in an emergency. Next is the Auxiliary Power Unit, or APU. The APU is usually a small gas turbine engine mounted in the aircraft tailcone. It runs at a constant speed and has its own protection devices for conditions like a fire, low oil pressure, high oil temperature, overspeed, or overheat. The APU can be used to drive a 115 volt, 200 volt, 400 Hz, three-phase alternator for ground servicing supplies — that means powering the aircraft on the ground without the main engines running — or, in some aircraft, for emergency supplies in the air. However, the APU alternator cannot be paralleled with the engine-driven alternators. It will only supply power to the bus bars when no other source is feeding them. That's a critical operational limitation: the APU takes over only when all other sources are off the bus.