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
How does a circuit breaker work?
- When the actual current exceeds the maximum permitted current, the temperature of the bimetallic increases and starts bending.
- This triggers a release mechanism, which pops out the control knob and shows the white band.
- The electrical circuit is opened by a spring pushing the bimetallic off the contacts.
What affects the speed at which the CB trips?
- high current
- Short circuit is fastest trip
What does the term trip free type mean?
meaning they must be able to open an overloaded circuit even if the control knob mechanism is stuck in (trapped/held in)
What is the purpose of green coloured circuit breakers?
- these are mainly for essential systems and are connected to a circuit breaker monitoring system.
- This system informs the pilot via the ECAM display when a connected circuit breaker has tripped
What are SSPC’s?
- Solid state power controllers
- Electronic circuit breakers
What is an RCCB?
- Remote control circuit breakers
- installed near the consumers to save weight (wire) and space in the cockpit
Where would an RCCB be situated and why?
- installed near the consumers to save weight (wire) and space in the cockpit
On an RCCB, what is the purpose of the control CB?
- The control C/B opens on the Flight Deck when the RCCB opens
- allows a tripped RCCB to be reset by the crew.
- Additionally, the RCCB can be used like a manual ‘on/off’ switch.
What is a programmable RCCB?
- measures the actual current with a current transformer or
hall sensor inside the RCCB. - An RCCB module (RCCBM) computer compares this value with a stored nominal value and opens the contactor inside the RCCB, when the nominal current is exceeded.
What may happen to a current transformer if the GCU was removed during normal operation?
- the current transformer will be damaged
- because the primary current magnetic field is no longer
compensated for and leads to high voltage and heat generation in the Current transformer
How many pole pairs would a generator need to give 400Hz from an RPM of 6000rpm?
4 pole pairs
(400 (frequency) x 60) / 4 (pole pairs) = 6000rpm
What is the phase shift in a three phase generator?
120 degrees
In a three phase generator, what are the line to neutral and line to line voltage values?
- Line to neutral = 115v
- line to line = 200v
What is the advantage of the 3 phase system with regard to the neutral line?
to have 2 different voltage levels available without additional equipment.
What component is used to gain a constant output frequency?
A constant speed drive (CSD)
How is voltage regulation achieved?
by an exciter current which influences the strength of the magnetic field
What type are most aircraft generators?
brushless generators
Where does the GCU (generator control unit) receive its power from?
PMG Permananet magnet generator
How does the cooling method affect the load limit of generators?
The better the cooling conditions the higher the load current limit
What are the basic components of a static inverter?
All Static Inverters consist of a
• Transformer
• Switch.
What component of a static inverter ensures correct timing of the switching?
A beat generator closes the solid state switches at the correct time to get the correct output frequency
What is the normal operating mode of the CSD (constant speed drive)
Under drive operation (engine running at speed which needs a speed reduction in the CSD)
Why is CSD straight drive operation only allowed for short periods?
The oil inside the motor/pump unit is trapped and cannot return to the oil circuit for cooling
What component of the CSD transfers input torque to the output?
The differential gear
In a CSD, what component adds or subtracts the speed of the output?
The differential gear
What are the two different types of speed control that may be used with a CSD?
- Electromechanically (frequency control circuit) or
* Mechanically. (flyweight governor)
When can an adjustment of the frequency via the CSD be carried out?
- Only when the engine is stopped
- one full turn corresponds to 3 hz
What is the purpose of the CSD oil reservoir?
- Ensures that enough oil is available for the user in all attitude conditions
- oil used for cooling and lubrication
When would the CSD oil pressure switch send a low pressure signal to the monitoring circuit?
when the charge pressure decreases below 50% of the normal pressure
What are the two CSD temperature monitor indications available?
- Absolute oil temperature and,
* Rise temperature.
If the CSD absolute temperature is high and the rise temperature lower than normal, what does this indicate?
Cooling is not sufficient
What action should be carried out for a CSD overtemp or low pressure occurrence?
manual disconnect of the CSD must be carried out
What could be the consequence of attempting CSD disconnect at low input speed?
If the input speed is too low, the centrifugal force cannot open the clutch completely and this can lead to clutch damage, such as broken teeth
What is the minimum speed for a disconnect attempt?
a disconnect must only be performed when the engine is
running above or at ‘idle speed’
What is the only reliable indication of a successful disconnect?
The only reliable indication of a successful disconnect is the voltage and frequency show zero (0).
What does the CSD DISC caption indicate?
The ‘DISC’ caption on the ECAM only shows that the disconnect has been performed, but does not indicate whether it was successfully completed
When and how should the constant speed drive reset be performed?
- Prior to engine restart to prevent damage to clutch
- The CSD reset is performed by pulling the ring on the CSD housing, when the engine is completely stopped.
What physical indication will you get that the CSD worm drive is reset into position?
A physical click
How is the function of the GCU divided?
- Regulation,
- Protection,
- Switching.
How does the APU GCU differ from the main generator GCU?
has the same functions as the engine driven generator GCU, but is always limited to ‘isolated’ operation.
What function does the CB on the front of the GCU serve?
It protects the external circuit against short circuits
What regulation is applied to generators working in isolated operation?
- Voltage regulation
- Frequency regulation.
The purpose of these regulation functions is to control the corresponding generator output parameter to the specified values.
Where is the voltage measured for generator regulation and why is it located there?
- “Point of Regulation” (POR).
- POR is located at the generator side of the generator power relay
- allows the measurement of the generator output voltage before the generator is switched to the busses.
What would be the output at the generator terminals?
120V
Where is generator frequency measurement taken from?
The frequency regulation loop
What is the main task of regulation in paralleled systems?
- to ensure all generators share the same load
- they must have the same voltage, speed and torque to share the same load
What are the two components of AC load?
- Real load
- Reactive load
Which type of AC load is controlled by voltage regulation?
Reactive load
What is the circuit used for AC load regulation called?
The load division loop
What are the three components of the load division loop?
- One current transformer for each generator which measures the individual load current in one of the three phases at the Point of Regulation (POR).
- Closed loop circuit - that creates the average current out of the individual currents.
- Auxiliary contact of each generator and bus tie power relay.
When working in parallel, what are the positions of the power relay, bus tie relay and auxiliary contacts?
- Generator Power Relay (GPR) CLOSED
- Bus Tie Power Relay COSED
- Corresponding auxiliary contacts are OPEN
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113
When successfully paralleled, what input does the GCU receive from the load division loop?
no input signal is provided to the GCU’s
In a paralleled system of four generators, what is the result of one generator taking more load than the others?
GCU 1 gets a decreased signal for its regulation circuits and the other 3 GCU’s get a smaller increased signal until the normal situation is reached
How is electrical control of the flyweights performed in a purely mechanical frequency regulation circuit?
by using permanent magnets as fly weights, which can be
influenced by the magnetic field of a trim coil. The current for the trim coil comes either from the;
• GCU
• Load controller - a separate component in older aircraft types
In a paralleled system, why doesn’t adjusting the CSD torque of an individual generator affect the speed and frequency?
- When the generator works in parallel operation, this reduces the share of the real load.
- However, the speed and frequency are not affected as long as the other generators run with the same speed
Apart from voltage and frequency synchronisation, what other parameter needs to be synchronised prior to paralleling?
Phase angle
How is phase angle synchronisation achieved?
- This is performed by a synchronization signal to the frequency regulation,
- derived from the voltage differences of the new power source and the already active power source supplying the main AC bus
Which two switching functions are performed by all GCU’s?
- Switching that controls the Generator Control Relay (GCR) and
- Switching of the Generator Power Relay (GPR)
What condition allows the switching of the first generator to the system?
Dead bus (aircraft bus has now power)
What conditions will remove the power ready signal?
This happens when;
• the GCR is tripped
• an under speed condition is detected by the protection circuit.
What are the five main AC generation protection circuits found in GCU’s?
- Over voltage,
- Over frequency,
- Under Voltage,
- Under frequency,
- Under speed
What are the three main AC distribution protection circuits in the GCU?
- Overload,
- Open phase,
- Differential Protection
What is an overvoltage?
Over voltage means that the voltage at the Point of Regulation (POR) exceeds 130 volt
What feature is built in to the overvoltage protection circuit to increase its effectiveness?
- automatic trip function with an inverse time delay.
- This means that the higher the over voltage, the shorter the time delay
What causes an over frequency fault?
when the generator speed is too high due to a faulty generator drive speed regulation.
Apart from faulty regulation, or generator, what other conditions can cause an under-voltage?
- an under voltage can also be caused by an under speed or overload
- therefore these conditions inhibit the automatic trip.
What condition will inhibit the automatic trip function of the under frequency?
- If the input speed is too low, this will also cause an under frequency condition,
- therefore, the automatic trip is inhibited when under speed is present.
When is an underspeed most likely to occur?
engine or APU shut down
Which other protection circuit is blocked by the overload protection circuit if it detects a fault?
- A high current drops the voltage at the POR,
- therefore, the under voltage protection circuit is blocked.
If an open phase is detected, what action is carried out by the protection circuit?
- The Generator Control Relay (GCR) and the Generator Power Relay (GPR) are tripped,
- because three phase consumers cannot work properly with only 2 phases
What is the trip time delay if a differential fault is detected?
No time delay due to dangerous consequences of a short circuit
Why are voltage and frequency faults not directly detected by the respective protection circuits in a parallel circuit?
- because the values of the faulty generator will not change,
- as it is kept at nearly normal level by the working generators
How is the system protected from a large load imbalance?
by the ‘load protection loop’
What action will the load protection loop initiate on detecting a large load imbalance?
- the Bus Tie Power Relay is tripped automatically and terminates the parallel operation of the problem generator when a certain difference in current is exceeded.
- generator works in ‘isolated’ operation.
How are the windings in a TRU arranged?
- 1st set star star windings
- 2nd set delta windings
- The combination of a ‘star’ and ‘delta’ secondary windings produces a nearly ideal DC voltage without the ripples which normally remain after rectifying
What two protection circuits does the TRU have for failure detection?
- Low current detection
- Overheat detection
When can a tripped TRU be switched back on?
- The reset signal can only be carried out on the ground after successful trouble shooting
- performed via the Central Maintenance Computer System (CMCS) or a reset push button in the electric compartment
What types of batteries are used in aircraft?
Nickel Cadmium (Ni-Cd)
What are the two main advantages of ni-cad batteries over lead-acid?
The 2 main advantages are;
• Less maintenance costs.
• Low temperatures performance.
During charging, what voltage do the individual cells reach in a battery?
1.5V (normally 1.2)
On average, how often are workshop level battery condition checks carried out?
every 2000 operating hours
which battery parameters are usually monitored in service?
- Voltage
* Current.
What are the two charging methods used by the battery charger?
- Constant potential or constant voltage charging method and
* Constant current charging method.
Which battery charging method is the safer and why?
- constant current charging method
- no risk of thermal runaway and battery damage
What other benefit does the constant voltage charging method have?
If several TRU’s fail or are not powered, the battery charger can work like a TRU and take over the supply to the electrical consumers that are connected to the battery bus
When charging ni-cad batteries via a TRU with the constant voltage method, what situation can occur if not controlled?
thermal runaway
How can thermal runaway be avoided?
- installation of a battery charge limiter which switches off the battery contactor when fully charged or overheat is
what method of control is used with a battery charger circuit?
- To prevent the risk of a thermal runaway in the battery charging circuit that uses a battery charger, a temperature switch is installed either inside or
outside the battery.
-The bimetallic switch closes when 60°C is exceeded and switches off the battery charger
What is the purpose of pins E and F in the ground power receptacle?
Pin E - (short pin ‘E’) latch ensures that if the connection is not performed completely, the external power source will not stay ‘on’
Pin F - the DC returns to the aircraft GPCU via short pin ‘F’.
9) This illuminates the external power connected light in the aircraft, when the quality of the external power is good (also named the “available” light).
What must be carried out prior to switching on external power?
Follow the ‘check list’ to ensure that all systems are in a defined switching state, preventing an unexpected or dangerous situations
What is the towing bus and how is it supplied?
- towing bus connects navigation lights, cockpit lights and the indication of the brake pressure to power source during towing
- The battery provides 24 volts DC and 115 volts AC via a static Inverter for the AC equipment
