Originally Posted by
tdracer
The 777 can sync the AC buses when needed.
The 777 does parallel generators. But only for the duration of a power transfer (shutting down or spinning up a generator). The IDGs (and APU controller) are equipped with a speed control input that bring two sources into sync by throttling one up or down. Once synced, the new source is paralleled with the system by closing the proper breaker. Once the new source is on line, a tie breaker is opened to isolate the two sources and their respective loads. Buses do not operate continuously tied together.
On the various 747 models, the generators did remain in parallel continuously, each feeding into the four main buses tied together. 747-400 buses will revert to isolated operation for certain autoland configurations requiring isolated and redundant autopilot channels. On 757 and 767, the electrical systems remain isolated. All transfers are break before make, resulting in a short but annoying power interruption.
During normal operation, the 747 is the only model where an electrical fault can be 'seen' momentarily by all buses. Until the protection systems act to isolate the faulted section. Because of this parallel operation and the need to identify the location of and isolate a fault, the 747 has perhaps the most complex protection system. The heart of this system is something called differential protection, which can identify where in the system a fault has occurred and open the correct breakers to isolate it. All other models have much simpler (if any) differential protection systems and may trip more breakers than needed upon sensing fault currents passing through.
Due to the isolated (at most times) operation of the 777 electrical system, no single fault should affect more than the half of the system (left or right) where it occurs. So this particular incident is troubling. One possible cause is that (for some reason) one of the generator control units failed to transfer load to its generator upon engine start. Leaving the left and right main buses powered from one generator through a bus tie breaker. A subsequent fault, or even an overload condition due to one generator available resulted in the good generator tripping off line. Leaving only standby (battery and RAT) sources to power critical loads.