AC Electrics -Practical Aircraft Systems — Page 222, Lesson 243

I want to walk you through a parallel bus bar system for AC electrics — this is the kind of architecture you’ll find on multi-engine turbine aircraft. Let’s start with the big picture. Figure 13.5 illustrates a four-generator paralleled system. That means we have four alternators — often driven by each engine plus an APU — that can all work together or in selected combinations. The system allows various combinations of alternator operation, so you’re not stuck with a single configuration. Operation begins with the excitation of the alternator field. Excitation means we apply a DC current to the field winding inside the alternator to create a magnetic field. That field is what lets the alternator generate AC voltage. The system waits until that alternator’s output comes within the required limits — voltage, frequency, and phase — before anything else can happen. Only then can the Generator Circuit Breaker, or GCB, operate. When the GCB closes, it connects its associated alternator to its Load Bus Bar. The Load Bus Bar is the distribution point that feeds electrical loads — things like avionics, lights, pumps. Once the GCB has closed, it will remain closed during all normal circuit functioning. It doesn’t cycle open and closed in normal operation; it stays latched. Now, the Bus Tie Breakers — BTBs — are normally closed. That means the path from each Load Bus Bar to a common synchronizing bus bar is already made. So when the GCB closes, it effectively connects the alternator to the Synchronizing Bus Bar. The synchronizing bus bar is the backbone that lets multiple alternators run in parallel. If the other alternator of a pair — for example, alternators 1 and 2 are a pair, and alternators 3 and 4 are another pair — now comes online, it too will be joined in parallel to the synchronizing bus bar. But only once its voltage, frequency, and phase sequence have been satisfied, allowing its GCB to close. So the system checks that the incoming alternator matches the bus in all three respects before it ties in. In this system there are two synchronizing bus bars. They can be combined or isolated by the Split System Breaker, or SSB, depending on the flight phase or other system requirement. Keeping the synchronizing bus bars isolated from each other allows the alternators to operate as two paralleled pairs. That’s a requirement, for example, during a dual autopilot autoland — you want the two autopilots to have totally separate power supplies so a single electrical fault can’t take out both. Now let’s talk about failures. If a single alternator fails in a system like Figure 13.5, opening of its associated GCB will allow its paired alternator — the other one on the same synchronizing bus bar — to feed the loads of both alternators. But that places a larger load on that one alternator than is being carried by the pair on the other synchronizing bus bar. So you get an imbalance. Closure of the SSB would bring all three remaining alternators into parallel operation, thus sharing the total aircraft load between them. That balances the load across all serviceable generators. Failures are not always that simple, though. If there was an earth fault on a load bus bar — meaning a short circuit to the aircraft structure — opening the associated GCB would do little to help. The other alternators would now be attempting to feed that earth fault through the still-closed bus tie breakers. So instead, operation of the Bus Tie Breaker associated with the faulty bus bar would prevent the serviceable alternators from being affected by the fault. Then the earth fault could be totally isolated by opening the GCB of the alternator feeding it. So the sequence is: open the BTB to isolate the faulty bus from the synchronizing bus, then open the GCB to disconnect the alternator from the faulty bus. That’s the core of a parallel bus bar system — how alternators come online, how they share load, and how you isolate faults without losing all electrical power.