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Parallel electrical systems, the synchronising bus and ATPL theory’s wisdom thereto

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Parallel electrical systems, the synchronising bus and ATPL theory’s wisdom thereto

Old 25th Apr 2016, 11:55
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Parallel electrical systems, the synchronising bus and ATPL theory’s wisdom thereto

Hello there

I’m currently studying for my ATPL theory exam in Aircraft General Knowledge and have for some time been quite confused as to the function of a synchronising bus in a parallel electrical system.

According to both my textbook (PadPilot: Aircraft General Knowledge – Electrics) and my instructor, a synchronising bus cannot carry a load. The textbook reads, “The purpose of a synchronising bus is purely to allow for a parallel connection between the four AC buses, thus no loads are connected to either of the synchronising buses”.

I cannot see that this could be correct; even by the logic of their schematic (attached below). It illustrates the APU’s generator circuit breakers as being connected to the synchronising buses; were it the case that the synchronising bus could not carry a load, the APU generator would be rendered useless.



My other cause for cynicism is that I believe that the electrical system illustrated, despite being drawn as an overlay of a BAe 146, is in fact that of a 747. It certainly is not that of the Bae 146, which has generators only on the numbers 1 and 4 engines. For example, the term chosen for the circuit breaker which divides the synchronising buses, “Split System Bus”, is one I’ve seen used only by Boeing for the 747’s electrical system. Beneath is a schematic of the 747-400’s electrical system.



Furthermore, in describing system logic, as an example of a situation which requires bus isolation, PadPilot states that “Generally the BTB will be closed unless particular conditions exist, the requirement for redundancy and individual power sources during an autoland is one”. I know this to be identical logic to that of the 747-400. Its FCOM states (Chapter 6.20.8) “During automatic ILS approach, AC and DC busses 1, 2, and 3 are isolated from the synchronous bus to provide independent power sources for the three autopilots. AC bus 4 continues to power the synchronous bus”.

If this is in fact a 747’s electrical system, I must say that I’m particularly confused as to why the textbook states that the synchronising bus cannot supply a load. In the case of the 747-400, this is most definitely not true. The FCOM states (Chapter 6.20.4) “If the IDG is not able to maintain acceptable power quality, the GCB opens and the BTB closes to provide power from the synchronous bus”.

The reason as to why I’m going to such lengths in asking about this is that between my two databases of exam questions, I have two different answers to the same question. Please see below.




Whilst I am quite happy to learn logic which is to my mind is flawed and accept that I’ve probably just misunderstood something along the way, in this case it would seem that is not an option. My question is, what is the basis of the notion that a parallel, constant frequency AC electrical system cannot supply loads from its synchronising bus? Was this historically the case or is the 747 simply something of an exception to the rule? I certainly doubt that it is the latter, given that the MD-11’s parallel system follows fairly similar logic to that which I’ve stated above.

Additionally, is anyone aware of the CAA’s current thinking? They appear surprisingly fickle in spite of their legal omniscience, and that applies to more questions than just this one.

Finally, as point of personal interest, I’ll ask about no-break power transfer. PadPilot states that “On some aeroplanes [synchronising voltage, phase and frequency] allows for no-break power transfer between various sources, unlike for the split busbar system…” I fully understand the reason for stating that the feature is unavailable to aircraft with split bus electrical systems, although it is not the case. Several split bus systems integrate the feature including that of the A330, whose FCOM states (Chapter 1.24.10) “No break power transfer… This function avoids busbar power interruption during supply source transfer on ground in normal configuration.”

In the case of the parallel system, this feature is clearly necessitated to allow for sources to be paralleled, however I’m unsure as to its benefit in a split bus system. Whilst it could in some circumstances eliminate the need for hold-up voltage fed to computers heavily reliant on volatile memory, such as Display Electronic Units during power transfer, it would not do so at all times. The FCOM states that “If synchronisation is not achieved within allowed time, transfer is performed anyway…” Therefore, a continuous DC power source and the wiring which goes with it is still required to such components. Given the complexity of the logic required of the Electrical Contactor Management Units which allow for this, it seems a significant engineering challenge for the sole benefit of avoiding flickering of the cabin lights during start-up. I'd love to know what the real world benefits are.

Kind regards

Charlie Reed
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Old 26th Apr 2016, 21:46
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It's late am tired, so, yes that is a 747 electrical schematic.
The 2 same questions, are NOT the same!
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Old 26th Apr 2016, 23:21
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Hi there,

Granted they're not identical, but I fail to see how that nullifies my point, given it's regarding BTB logic in the event of an isolated generator. This is the situation in both questions. If however this is the case, would it be possible for you to explain why?

Regardless, here is an example of an identical questions with the alternative answer:



Kind regards

Charlie
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Old 27th Apr 2016, 10:37
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1. Saying that there are no loads CONNECTED to the sync bus is NOT the same as saying that the sync bus cannot carry any loads.
2. The questions are most definitely not the same. Think about what would cause over and under excitation and you might have your answer.
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Old 27th Apr 2016, 11:58
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I can see that either fault could have been caused by a failure of the voltage regulator. I suppose that an over excitation could have also been caused by something more hazardous such as CSDU overspeed. Both questions state however that it is a persistent fault. Since the over-excitation could pose a fire risk, I believe it would be necessary to first trip the ECB and then trip the GCB to prevent reverse current flow from the busbar. This may follow CSD disconnect.

In the case of the under-excitation fault, I think that the GCB would be tripped first and if the fault persisted, as it is supposed to have done, the ECB would be tripped.

In both cases, reactive (and in the event of a CSD overspeed, real loads too) loads would become unbalanced, requiring isolation of the AC bus from the synchronising bus by the BTB. However, once the GCB is opened, which would seem to be the case in both scenarios, I don't see why this would be necessary, so long as the fault is not with the busbar itself. Is the correct answer suggesting that the protection device only opens the BTB transiently?

Furthermore, is the question I posted in my previous response, about a "top excitation limit fault" not asking the same thing as the second question in my original post about an "over excitation limit fault"?

Anyway, thank you for your help, but I'm really struggling to understand this and since it would seem that I've still got it wrong, I'd really appreciate if you could explain it for me.

Kind regards

Charlie
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Old 27th Apr 2016, 12:26
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Agree with Mono.

The tie bus links bus bars together to facilitate load sharing, APU power supply etc.

Generally, the aircraft systems are supplied via circuit breakers from their respective bus bars, not from a tie bus,
this allows the hierarchy of supply in the event of failure, i.e. Bus 1, Essential Bus, Battery Bus, Hot Battery Bus…
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Old 5th May 2016, 00:16
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Good question which I think you have made overly complex. I am an Avionics engineer and occasionally was roped in to teach the new pilots to our airline about aircraft systems. Unfortunately they simplify things a bit for the pilot course to the point where a smart pilot would figure out things didn't quite make sense which is where I came in.

Without looking to hard at your diagrams or multichoice questions I will just say this.

A synchronization bus usually is a simple wire running between the voltage regulators to make them share the load and stay in phase.
A bus tie connects the appropriate busses together to supply aircraft system loads.
If the voltage regulator cannot control the generator its connected to within limits it prevents the generator from coming online, being connected to the buss and tied to others.
On the machines I worked on the fault would read, " gen light wont go out, bus tie wont work", no circuit breakers opened and the mel was invoked.
Good luck and remember the diagrams they are feeding you are simplified, often they don't make sense if you look real hard at them. Try the Maintenance manual for the true story
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