OPERATIONS MANUAL

Electrical
Bus Tie System: (continued)
However, only IDGs can be synchronized permanently with each other.
Synchronization among external or APU generators is not possible, and
they cannot be permanently synchronized with IDGs. When connecting
a generator that is not synchronizable with the already connected
generators, the bus control units (BCUs) command certain bus breakers
to open. For this purpose the BCUs apply a programmed schedule for
every possible combination of generator connections. BCU 1 normally
commands the system components on the left half of the synchronous bus,
and BCU 2 those on the right half. Either BCU can open the SSB, but only
BCU 2 can close it.
The BCUs accomplish a no break power transfer (NBPT) when power is
transferred between IDGs, external or APU generators; like, for example,
when an APU generator connects while IDGs are already connected, or
when an APU generator connects while an external generator is already
connected. Only when the generators are precisely synchronized—which
may take several seconds—, the system logic will command the respective
breaker to close, and the other breaker to open. Power is interrupted only
when connecting two APU generators or two external generators while the
IDGs are not in use.
In addition to the four AC busses and their tie system, the aircraft also
incorporates four DC busses and a DC tie bus. Each AC bus powers a
transformer rectifier unit (TRU) whose 28 V DC output is routed to
a DC bus. A DC isolation relay (DCIR) connects the DC bus to the
DC tie bus so that when the DC bus receives no power from its TRU,
the other DC busses keep powering the affected DC bus. The controller
that commands the BTB of the AC bus also commands the DCIR of the
associated DC bus. So when the BTB opens in normal operations, the
respective DCIR opens as well.
For autoland, the system logic opens the BTBs and DCIRs of busses 1, 2,
and 3. Each of these three busses powers a different set of flight systems.
When the three busses are isolated from each other, the three sets of flight
systems are isolated also. This setting provides a higher safety level as a
fault in one system will not propagate to the others.—Should one of the
three GCBs open, the respective BTB will close, and IDG 4 will power the
affected bus; this will decrease the safety level because, should yet another
failure occur, no more backup power will be available. But this degradation
will still allow autoland with three independent, operative busses. Only
when two or more GCBs open, all BTBs and DCIRs will close, and the
autoland function will disengage.
— Page 185 —
For preview only. Not for sale. Many pages are intentionally removed.