Synchronising generators

[Carpass]

On a multi engined aircraft how does a GCU know when the generator it's controlling/monitoring is phase synchronised with the others, and ready to come on line? Is there a seperate synchronising unit which watches all the generators? I've heard that on the DC10, you sometimes have to advance or retard the throttles to get the generators matched. Any help much appreciated thanks.

[Bally Heck]

Carpass

On most modern aircraft the AC generators are not synchronised. They each power an independant bus, each of which powers independant systems which suffice to operate the aircraft in the event of a bus failure.

When a generator is brought on line, it's respective bus is momentarily unpowered during the transfer.

Typically a two engines aircraft will have a left bus, a right bus and an APU bus with tie breakers isolating them.

When the APU is on line and both engines shut down, both left and right tie breakers are closed allowing the APU to supply both buses.

When the right engine generator is brought on line, the right bus tie breaker opens and the generator breaker closes. Ditto with the left engine.

If a generator fails with the APU off line, both bus tie breakers will close allowing the good generator to supply both buses (with some automatic load shedding.

If a bus suffers a short then the other bus will supply it's own customers in isolation.

Don't know about older types such as the DC10 but I would be surprised if it was much different.

Incidentally, the generators are driven through a constant speed unit which keeps them spinning at a pretty constant speed to maintain 400hz so I doubt changing engine revs would have much effect.
:cool:

[peterl]

When I used to work on the AWAC it had eight generators, all could be (and usually were)connected to a synch bus. The unit controlling the synch bus (and the eight GCUs) is a BPCU (bus power control unit), one downside of having all the generators on a single bus is that of load sharing, so the bus is protected by both real and reactive load sharing circuits.
On older Boeing 707s and the like then the flight engineer could control the phase of each generator relative to each other before they were connected to the synch bus. :D

[Cornish Jack]

Carpass
I have a feeling that Bally Heck's reply related to modern twins. The situation with modern four jets e.g. 744, is that the gennies are synchronised and connected in parallel to a Synchronous Bus-bar. The synchronisation is taken care of automatically and the Bus-tie breakers (BTBs) look after the connection/disconnection. The Synch bus itself can be split to allow the two electrical halves of the aircraft to operate independently or can be re-connected to allow full load sharing and (in the case of Autoland) fail safe redundancy for Cat 3 operations. That same synch bus splitting allows the two (unsynchronised) APU gennies (or ground supplies) to power their individual halves of the aircraft without having to be concerned with synchronising.
The actual sychronisation is taken care of by the GCU which is being fed a reference frequency from the #1 BCU. The GCU compares the reference frequency to it's own genny and sends a speed adjustment signal to the IDG. Once the frequency is correct, the GCU allows the BTB to close to connect the individual AC bus to the synch bus.

Have just re-read all that and I'm starting to feel confused. :D I think I'd better go and lie down :D

[Bally Heck]

Cornish Jack

A thousand pardons. I was indeed referring to twin jets. I stand corrected. :rolleyes:

[Dan Winterland]

I'm old enough to remeber flying an aircraft where the gennies had to be manually synched. A light on the elec panel glowed on and off as the phases matched. When it was out, you pressed a button and hey presto - you had gennie synch!