11.06 Electrical Power Flashcards
What are the usual four main categories that aircraft electrical power systems are required to supply?
- Electrical heating,
- Lighting,
- To produce mechanical power,
- Electronic devices
Approximately, how much power does the average two engine aircraft require?
100kw
What two components are the electrical power system divided up into?
- generation
- distribution
How are power generation and distribution further divided?
- AC subsystem and
- DC subsystem
What are the five different sources of AC on modern jet aircraft?
- engine driven generators
- APU generator
- External power
- Emergency generator
- Static inverter
What is the normal supplied AC voltage and frequency?
- 115V
- 400 Hz
What other function does the AC system have?
Provides the input power for DC generation
How is AC power used for DC generation?
Transformer Rectifier Units (TRU) - Several identical TRU’s convert the 3 phase 115 volts AC power from the AC distribution system into 28 volts DC
What voltage does the aircraft battery normally supply?
24V DC`
On what type of aircraft would DC be the primary power supply?
Propeller aircraft - 28V DC generators
What does the abbreviation VFG stand for?
Variable Frequency Generator - 115V AC
What are the three levels of distribution of the electrical power supply?
- Main power distribution
- Essential power distribution
- Emergency power distribution
In the case of a total power loss of normal sources, what ill supply electrical power?
Emergency generator or battery
Where will the important electrical power system information be displayed?
ECAM or EICAS
- Power distribution pages
What is the purpose of power source switching?
connects the power sources to the busses
On many two engine aircraft (airbus) what is the function of bus tie switching?
provides a connection between the two busses and part
of the power source switching.
What is the purpose of an electrical bus?
A bus connects the power source to all the electrical consumers which use the same type of electrical power.
In reality, how is an AC bus constructed?
A bus consists only of a low resistance electrical connection between all the wires that lead to the different consumers and the wires from the power source and bus tie switching
What are the four AC consumer groups?
- Single phase consumers
• Voltage different from 115 volts
• 3 phase consumers
• Large loads
What consumer group usually consumes the most power?
Large loads
How can the reliability of the power relays be improved during switching operations?
‘Load shedding’ - Large loads must be switched off before any power source switching on/off on the ground.
How can load shedding be achieved?
- manually via a pushbutton or
- automatically
What components perform the power source switching and how are they controlled?
- Generator control units (GCU, generator power sources)
- Ground power control units (GPCU, external power sources)
When does power source switching take place?
Power source switching always occurs when an existing power source is lost or when a new power source becomes available
How is power source switching achieved?
- manually, via control panel
- or automatically by GCU
When will automatic power switching occur?
- Electrical control in auto
- Power ready condition
What does the power ready condition signify?
means that the voltage and frequency are OK and no failures have been detected by the GCU
On a two engine aircraft, what is the normal configuration of the power sources during flight?
- both engine generators working
- Power source switching, connects both engine generators to their corresponding main AC busses.
- Both bus tie switch power relays are open, meaning that the power sources work in isolation from each other. This is known as ‘isolated’ operation.
- Generator number 1 supplies the loads of the left part of the electrical power distribution and generator number 2 supplies the loads of the right part
What would happen on a two engine aircraft, with normal configuration of power sources, if one generator failed?
- the bus tie switching automatically closes both bus tie power relays
- the remaining generator supplies the complete electrical power distribution
On certain aircraft, what action would allow the reconnection of the large loads after load shed?
most civil aircraft types can run the APU and its generator as an alternate power source during flight.
What manual action is available with bus tie switching and why?
- to inhibit the bus tie switching from closing the bus tie power relays
- The splitting of the electrical power distribution (buses) allows an easier isolation of a smoke source.
What is the result of selecting the bus tie push to off?
both bus tie power relays open and power distribution between busses is no longer possible
What is the normal condition of the bus tie power relays in a paralleled circuit?
- Closed
What must be carried out prior to bringing the first generator on line in a parallel system?
Manual load shedding of all the large loads
What condition must exist prior to generators being paralleled?
Parallel operation’ is only allowed when both generators are synchronized, that means they have the same:-
• Voltage,
• Frequency,
• Phase angle
What are the two advantages of parallel operation?
- All generators share the same part of the total load
* There is no power interruption when one or several generators are switched off
What two conditions would cause automatic bus tie switching in normal flight?
- when a serious problem in the distribution (e.g. short circuit) is detected,
- or during automatic landing when the redundant avionic equipment needs isolated power supplies.
What component monitors ground power condition?
Ground Power Control Unit (GPCU)
How is ground power brought on line?
Ground power source switching must always be done manually
Which relays will operate on selection of the ground power push button?
- external power relay is closed by the Ground Power Control Unit (GPCU),
- Both bus tie power relays are closed automatically by the bus tie switching
What two different sources of achieving ground power are available?
- APU generators
- Ground power
What problem can arise on multi-engine aircraft when two or more GPU’s are connected?
As multiple external power sources are not synchronized, parallel operation is not possible
What component is utilised to allow multiple GPU’s to be connected?
The Split System Breaker (SSB)
When switching from one isolated power source to another, what could possibly occur?
A loss of power (break power)
How is the problem of switching between non-paralleling power sources overcome on some aircraft?
‘No Break Power Transfer’ (NBPT).
Where does the supply for the main DC bus come from?
The corresponding AC bus via the TRU
Why does DC bus isolation occur on selection of automatic landing?
bus isolation occurs when an automatic landing is started and is necessary to prevent a single failure (like a short circuit) affecting all systems at once
On an aircraft equipped with an EFCS, when would the DC buses be connected together?
If one of the TRU’s fail. Ensure DC buses are always powered
Why is the DC buses being connected together in an EFCS dangerous and how can it be resolved?
- the two Electrical Flight Control Systems share the same power source, this is not a safe situation
- is solved by adding an additional ‘Essential’ power distribution channel.
What is the power supply for the AC essential bus?
- main AC bus 1
- alternate AC bus 2
What is a transfer bus?
the bus is automatically transferred from the normal power source to the alternate power source by the switching
How long must the aircraft battery supply electrical power in the case of loss of normal power supplies?
30 minutes
What additional item must EROP aircraft also have?
An additional emergency generator
What must be done to all battery switches when the aircraft is parked and why?
- All battery switches must be switched into the ‘off’ position during the time the aircraft is parked.
- This prevents a discharge of the battery by the emergency equipment.
Which bus, if any, will still be powered even with the battery switches off?
Hot battery bus
What two functions could a RAT be used for?
- The RAT either generates the hydraulic power needed for the hydraulically driven emergency generator
- or drives the emergency generator directly
If the RAT ‘manual on’ pushbutton is selected on the ground, what, if anything, would happen?
- same effect as automatic function
- RAT deploys
What are the two types of power relay?
- Contactors and
- Breakers
What is the main difference between contactors and breakers?
Breaker has two advantages over a contactor:
- The coil can be smaller, because no heat is produced in the coil during the closed state.
- The breaker stays closed when no electrical power is available
What components are used to protect the distribution circuits?
Fuses and circuit breakers
What are the symbols for a fuse and a circuit breaker?
- fuse = S with two blobs at either end
- circuit breaker = little bowl with a handle on it