AC Electrics - Alternators — Page 201, Lesson 214

I want to walk you through the layout of a paralleled AC electrical system — specifically a three-generator paralleled system. This is how large aircraft manage to run multiple engine-driven alternators together, sharing the electrical load. Let's start with the bus bar arrangement. For each of the three phases of the generator output, there is a separate bus bar. So for Generator No. 1, its bus bar — called Gen Bus 1 — is actually made up of three separate bus bars: one for phase A, one for phase B, and one for phase C. Each generator connects to its own bus bar through a three-phase circuit breaker called the Generator Circuit Breaker, or GCB. The GCB can be operated automatically or controlled from the flight deck. All the electrical loads of the aircraft are shared between the three generator bus bars. Now, to operate the generators in parallel, they are connected through their respective generator bus bars to a synchronizing bus bar via a Bus Tie Breaker, or BTB. A Bus Tie Breaker is also a three-phase circuit breaker, and like the GCB, it can be controlled automatically or manually from the flight deck. The synchronizing bus bar is a key component here. It takes no electrical loads at all — its only purpose is to allow the engine-driven generators to be operated in parallel. Additionally, ground power or power from the APU generator can be connected into the synchronizing bus. From there, that external power can be fed to the load bus bars through the Bus Tie Breakers, but only when the engine generators are not operating and their GCBs are open. Now let's talk about real load sharing. The Load Controller controls the basic frequency of the AC generator, which is 400 Hz. After the generators are paralleled, the load controllers work together to evenly share the real load. They do this by increasing the torque input to the lower-speeding alternator's drive and decreasing the torque input to the higher-speeding alternator's drive. This ensures each alternator takes an equal share of the load. To sense how the real load is distributed, current transformers are placed at the output of each of the paralleled alternators. They measure the real load distribution so the load controllers know how to adjust the torque inputs accordingly. So to summarise: you have three generators, each with its own GCB and three-phase bus bar. They connect through BTBs to a synchronizing bus that carries no load itself — it's purely for paralleling. Ground or APU power can also enter through that synchronizing bus when the engine generators are offline. And once paralleled, the load controllers balance the real load by adjusting torque based on feedback from current transformers.