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GogonAger
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Multiple external power switches

Sat Aug 01, 2020 8:27 pm

Hi. As an enthusiast I was reading through this forum for a while now and learned a ton of interesting facts i could not even imagine.
ow its time to ask a question myself.

I noticed that on the electrical panel of some airliners there are more then just one switch for the external power. E.g. on the 777 you can spot two of them (labeled "Primary/Secondary") if I remember correctly.
I was wondering why that is an whether the two switches resemble two physical plugs or are both "connected" to the same and just providing power to diffrent systems of the A/C?

Sure some of you out here know the answer and I am existed to learn about another detail that really was intriguing me the last days!
 
unimproved
Posts: 260
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Re: Multiple external power switches

Sat Aug 01, 2020 9:27 pm

They are different plugs since a single one can only carry limited current.

IIRC on the A330 it's automatic, 777 manual but uses load shedding, and the 747 has some systems such as cargo only on secondary. 787 has 3 power supplies since engine start is electric too (dry motoring is possible with 2, but you'll be sitting in the dark).
 
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Starlionblue
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Re: Multiple external power switches

Sun Aug 02, 2020 1:03 am

Type dependent but I wager most widebodies have at least two connections. In contrast, the A320 has only one, but its power draw is going to be lower than, say, an A330.

At some ports, you'll only get one ground power connection. As unimproved says that just means you have less power available. On the A330, the ground connectors are 90 kVA while the APU and engine generators are 115 kVA.

You can see here that there are two sockets, one of which has a cable plugged in at the moment.

Image

Here's an example with both sockets in use. This seems to be at an airshow so these are some sort of mobile power units. At most big airports, ground power is a fixed installation at each gate.

Image
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
FGITD
Posts: 1681
Joined: Wed Jun 15, 2016 1:44 pm

Re: Multiple external power switches

Sun Aug 02, 2020 3:08 am

Starlionblue wrote:
Here's an example with both sockets in use. This seems to be at an airshow so these are some sort of mobile power units. At most big airports, ground power is a fixed installation at each gate.

Image


I'm surprised by the size of that GPU. Seems small for what looks to be a 777. Also hate the way it's attached. No hooks or weight relief! Good way to yank a plug right out of the airplane. At least Boeing finally figured out to put the plugs on the left side with the 787...

I don't know too much beyond what's been said already. I've worked with engineers who used limited power availability to do things like get the airplane loaded, keeping above wing dark, then swapping over for the crew and pax.
 
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Starlionblue
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Re: Multiple external power switches

Sun Aug 02, 2020 3:24 am

FGITD wrote:
Starlionblue wrote:
Here's an example with both sockets in use. This seems to be at an airshow so these are some sort of mobile power units. At most big airports, ground power is a fixed installation at each gate.

Image


I'm surprised by the size of that GPU. Seems small for what looks to be a 777. Also hate the way it's attached. No hooks or weight relief! Good way to yank a plug right out of the airplane. At least Boeing finally figured out to put the plugs on the left side with the 787...

I don't know too much beyond what's been said already. I've worked with engineers who used limited power availability to do things like get the airplane loaded, keeping above wing dark, then swapping over for the crew and pax.


There seem to be two leads going to a box which can be glimpsed on the left. Maybe that's the actual GPU?
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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77west
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Re: Multiple external power switches

Sun Aug 02, 2020 3:52 am

Starlionblue wrote:
FGITD wrote:
Starlionblue wrote:
Here's an example with both sockets in use. This seems to be at an airshow so these are some sort of mobile power units. At most big airports, ground power is a fixed installation at each gate.

Image


I'm surprised by the size of that GPU. Seems small for what looks to be a 777. Also hate the way it's attached. No hooks or weight relief! Good way to yank a plug right out of the airplane. At least Boeing finally figured out to put the plugs on the left side with the 787...

I don't know too much beyond what's been said already. I've worked with engineers who used limited power availability to do things like get the airplane loaded, keeping above wing dark, then swapping over for the crew and pax.


There seem to be two leads going to a box which can be glimpsed on the left. Maybe that's the actual GPU?


I think you are right, the boxes visible may be some sort of UPS/power conditioner/converter rather than the main GPU itself.
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mmo
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Re: Multiple external power switches

Sun Aug 02, 2020 6:47 am

Not having flown an Airbus wide-body, I can't comment on their philosophy. But, Boeing generally has several busses that can power the aircraft. There is the normal AC power bus which when powered will power the entire aircraft. There is also the service bus which allows the aircraft to be powered in selected areas, for example, cargo doors, cargo loading system, a/c plugs in the cabin, lights in the cabin and the refueling/defueling system. The cockpit is unpowered in this configuration. IIRC, on the 747F/744F there is also the cargo handling bus which is a separate receptacle and IIRC, the 777 also has this option. The 787 likes to have two GPUs as the draw for the power is so great. If you use a portable A/C unit, you can get away with just one GPU.
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Starlionblue
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Re: Multiple external power switches

Sun Aug 02, 2020 8:33 am

mmo wrote:
Not having flown an Airbus wide-body, I can't comment on their philosophy. But, Boeing generally has several busses that can power the aircraft. There is the normal AC power bus which when powered will power the entire aircraft. There is also the service bus which allows the aircraft to be powered in selected areas, for example, cargo doors, cargo loading system, a/c plugs in the cabin, lights in the cabin and the refueling/defueling system. The cockpit is unpowered in this configuration. IIRC, on the 747F/744F there is also the cargo handling bus which is a separate receptacle and IIRC, the 777 also has this option. The 787 likes to have two GPUs as the draw for the power is so great. If you use a portable A/C unit, you can get away with just one GPU.


On the A330/A350, there's a switch that powers off the flight stuff while keeping the services operating. There is no separate external power receptacle for ground services.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
GogonAger
Topic Author
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Re: Multiple external power switches

Sun Aug 02, 2020 10:51 am

Thank you all! That totally answers my question and was interesting to hear.
 
unimproved
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Re: Multiple external power switches

Sun Aug 02, 2020 1:40 pm

FGITD wrote:
Starlionblue wrote:
Here's an example with both sockets in use. This seems to be at an airshow so these are some sort of mobile power units. At most big airports, ground power is a fixed installation at each gate.

Image


I'm surprised by the size of that GPU. Seems small for what looks to be a 777. Also hate the way it's attached. No hooks or weight relief! Good way to yank a plug right out of the airplane. At least Boeing finally figured out to put the plugs on the left side with the 787...

I don't know too much beyond what's been said already. I've worked with engineers who used limited power availability to do things like get the airplane loaded, keeping above wing dark, then swapping over for the crew and pax.

That is a converter and not a GPU, turning 3 phase 380v into 115v 400hz. If you look closely you can see two green hooks on the bottom of the plug which are the weight relief straps on the 777.

mmo wrote:
Not having flown an Airbus wide-body, I can't comment on their philosophy. But, Boeing generally has several busses that can power the aircraft. There is the normal AC power bus which when powered will power the entire aircraft. There is also the service bus which allows the aircraft to be powered in selected areas, for example, cargo doors, cargo loading system, a/c plugs in the cabin, lights in the cabin and the refueling/defueling system. The cockpit is unpowered in this configuration. IIRC, on the 747F/744F there is also the cargo handling bus which is a separate receptacle and IIRC, the 777 also has this option. The 787 likes to have two GPUs as the draw for the power is so great. If you use a portable A/C unit, you can get away with just one GPU.

The ground service bus feeds from normal external power and is mostly used for overnight cleaning and other actions where you only need lights and doors. It's a cabin attendant switch near door 12.

The 747 still has buses and main deck cargo is only powered by secondary external. The 777 has ELMS which can shed individual components when there isn't enough power such as running multiple hydraulic pumps on one GPU.
 
Lpbri
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Re: Multiple external power switches

Sun Aug 02, 2020 4:49 pm

For the 777, the primary external power connects to the P200 bus, which is the main right hand AC bus. Secondary power connects directly to the P300 bus, which is Auxililary/Standby power. Everything is connected by bus tie breakers controlled by ELMS ( electric load management system) There are 2 power connectors because the power requirements. You can power most of the airplane with just primary power connected, but it is not recommended. Secondary power can only be applied if primary power is connected.
 
FGITD
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Re: Multiple external power switches

Sun Aug 02, 2020 5:18 pm

unimproved wrote:
FGITD wrote:
Starlionblue wrote:
Here's an example with both sockets in use. This seems to be at an airshow so these are some sort of mobile power units. At most big airports, ground power is a fixed installation at each gate.

Image


I'm surprised by the size of that GPU. Seems small for what looks to be a 777. Also hate the way it's attached. No hooks or weight relief! Good way to yank a plug right out of the airplane. At least Boeing finally figured out to put the plugs on the left side with the 787...

I don't know too much beyond what's been said already. I've worked with engineers who used limited power availability to do things like get the airplane loaded, keeping above wing dark, then swapping over for the crew and pax.

That is a converter and not a GPU, turning 3 phase 380v into 115v 400hz. If you look closely you can see two green hooks on the bottom of the plug which are the weight relief straps on the 777.


Ah ok, well thank you for the explanation. Never worked with a converter so it’s new to me! Interesting technology

Also didn’t realize I could enlarge the image, and right you are, there are the straps.
 
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Horstroad
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Re: Multiple external power switches

Tue Aug 04, 2020 7:03 pm

Lpbri wrote:
Secondary power can only be applied if primary power is connected.

I'm not sure about that. From the top of my head I remember that the bus power control unit can get powered by the P200 bus or battery power. To get power directly from the secondary power receptacle to the BPCU the bus tie relays need to be closed (which are controlled by the BPCU), so with secondary external power alone you are right. But I think when you switch on the battery first, the BPCU gets powered and the bus tie relays can close which would enable you to power the aircraft with only secondary external power. You wouldn't have ground handling power though, as this is connected only to primary external power or the APU... so no cargo doors, now engine cowlings, no refueling...
I have to check on that later, never tried it. I don't have much time right now

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