Electrical system question

[Guest bruchpilot]

Hello there,the other day I had a BUS 3 Isolation failure. One of the fuel pumps went inop. Is that for load shedding?As I understand it however, the isolation is just for providing a connection, should one generator fail. However if the power source is still working, everything should work without a difference. Where is my fault?GreetingsEdgar

[Guest Ricardo Sevarant]

I would have to read up on the 747 electrical system to be able to answer your question completely. In general, if a bus tie switch in the normal (AUTO) position isolates automatically, it would mean a fault is detected in the associated bus that could potentially overload or short another bus. To prevent this, the affected bus is isolated, or cut off from the rest of the system. I'll try isolate BUS 3 on my sim and see what happens.

[Guest Ricardo Sevarant]

Ok, I went into the sim and programmed a BUS 3 isolation failure and had the same results as you. The PMDG systems manual describes the following:Load Shedding: In the event that ACpower availability decreases due to engineor generator failure, the ELCUs reduce ACpower load requirements by shutting downthe galley buses until AC power availabilityincreases, or until the AC load has beenreduced to a level sustainable with thecurrent supply.During load shedding, associated EICASalert messages and illumination of the utilityswitch OFF lights are inhibited. However,the following EICAS advisory messagesmay be displayed in the order showndepending upon fuel system configurationand the extent of load shedding:

[Guest bruchpilot]

I found the solution myself:AOM says:"If power on an AC bus is unacceptable, the related BTB opens and the bus disconnects from the synchronous bus. However, the AC bus remains powered by its IDG. 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."So what this failure actually simulates is a bad power quality. So load shedding (3 generators still providing power) occurs. So, Ricardo, the No 3 IDG is still working (obviously not properly but working) but *not* online. GreetingsEdgar

[Guest bruchpilot]

There is only one weak point in my arguments:The AOM says:"The EICAS message ELEC BUS ISLN displays if the BTB is open."...Any ideas?Edgar

[Guest bruchpilot]

Now that I'm thinking about it all makes no sense (e.g. GCB is still closed in this scenario). Maybe someone who really knows it can give an advice.Thanks!Edgar

[Guest Ricardo Sevarant]

I'll play with the sim again and initiate a number 3 engine IDG failure to see what happens to the number 3 BUS TIE switch. I would think that an IDG failure would normally cause the associated BUS TIE to close, thereby powering the afficted bus from the combined synchronous system bus. I don't think a GFC and/or IDG failure would by itself cause an isolation fault, but I may be wrong. My understanding of the BUS TIE logic is that it isolates only when unacceptable power quality throughout the associated bus is detected, not just when an IDG unit or generator field control unit fails. I don't think a simple generator failure would cause an overload of the rest of the synchronous bus and would not therefore require a bus isolation. Rather, some kind of electrical short or other potential overload in the bus would govern the BTB to trip. Big emphasis on the "I think" here :)Time to test out my copious (sic) 747 knowledge.

[Guest Ricardo Sevarant]

Ok, in the PMDG sim an IDG failure/disconnect does not cause a BUS TIE isolation on the associated bus. I believe this is realistic. The BUS TIE closes and the afflicted bus becomes automatically powered by the synchronous bus. Shutting down the generator field control switch has the same effect. So, load shedding in the PMDG sim occurs when a bus is automatically isolated due to specific electrical fault(s) in the associated bus. Why does load shedding occur in this particular scenario and not when a single IDG or GFC unit fail? I would guess that 747 electrical system logic dictates that an isolated system bus (or in particular, the number 3 electrical bus), could somehow potentially cause a synchronous bus overload, whereas a single generator failure with a non-isolated bus does not. We need help from Q (the Wizard from Oz), master of all things mechanical.

[Guest bruchpilot]

Ok, I played around with it also. It is not necessary to use the failure. Clicking on the Bus Tie Button has the same effect.The question is: What are we seing there? Is that load shedding? If so: - Why is the right Utility OFF light on? It shouldn't be.- Why isn't the Galley removed but the galley and the utility bus at the same time?It must be something different.Looks like we need professional help ;-)GreetingsEdgar

[Guest D17S]

Good thinking, Edgar SO, now we need clarification about this

[Guest bruchpilot]

>So, I

[Guest D17S]

Yea, me too. I think a buss iso fault trip is going to be related to a buss problem, not a generator problem. I have all the 741/2 757-67 books at work and I'll have a look this weekend. The philosophy's got to be about the same.

[cowpatz 243]

>Hello there,>>the other day I had a BUS 3 Isolation failure. One of the fuel>pumps went inop. Is that for load shedding?Yes it is for load shedding. The main busses normally connect to the synchronous bus if the power is of acceptable quality. If one of the busses is of poor quality then the bus tie breaker will open and remove the bus from the synchronous bus. Each Bus powers a utility and galley bus. You will not notice a galley power bus not being powered but you will notice the utilty bus not being powered by way of the fuel pump message. Have a look at the elec synoptic display when it happens and it will show on there. You will also most likely have a BUS ISOL amber light on the overhead panel. You can try to isolate the fault (if it happens again) by opening the Gen control breaker and then reset the BUS ISL breaker. If there is not a problem on BUS 3 then the synchronous bus should power BUS 3. If it wont then there is a problem with the bus.>As I understand it however, the isolation is just for>providing a connection, should one generator fail. However if>the power source is still working, everything should work>without a difference. Where is my fault?It is a 2 way connection. Bad power is prevented from getting to the Synch bus or in the case of a gen fail then the synch bus can power the lost bus. I am picking the problem is a bus fault. If it were a GEN fault then it is most likely that the GEN CONT BREAKER would open.CheersSteve

[Guest bruchpilot]

>You can try to isolate the fault (if it happens again) by>opening the Gen control breaker and then reset the BUS ISL>breaker. If there is not a problem on BUS 3 then the>synchronous bus should power BUS 3. If it wont then there is a>problem with the bus.Sounds good. But then you would have BUS 3 unpowered for a moment. The manual says you have to try once to reset the BTB.So is it a bug, that the right utility BUS light comes on? I doubt it.Maybe someone of the developers can help us out.GreetingsEdgar

[Qavion 19]

"We need help from Q (the Wizard from Oz), master of all things mechanical."Unfortunately, what Edgar is describing to me, doesn't make any sense. If your GCB is closed and your BTB is open, there should still be power on the bus....and therefore no loadshedding should occur.Actually, I wasn't even aware loadshedding was modelled in PMDG. Loadshedding on the 744 is primarily based on current sensing. Mostly likely too complex to be modelled (?)Cheers.Q>