AC power supply systems can be split into the following categories:
¾ Frequency Wild AC Systems. These are used in small to medium sized
aeroplanes ranging from small piston engined aeroplanes to large twin engined turbo propeller aeroplanes.
¾ Constant Frequency AC Systems. These are used on jet aeroplanes, and are
either split busbar or parallel systems.
Piston-Engine Frequency Wild AC System Architecture
A typical single-engined aeroplane uses a frequency wild electrical generation system.
In this system the AC generator (alternator) is driven directly from the engine via a fan belt, so the frequency output from the generator will be dependent, and proportional to the engine speed. Before being fed to the aeroplane loads the AC is changed directly into DC, via rectifiers inside the generator.
Operation of a Piston-Engine Frequency Wild AC System
On initially switching the battery on, the busbar will be supplied with approximately 12 volts, and depending on the loads selected, the ammeter will read a discharge as a negative value. The under voltage lamp will also be illuminated indicating that the busbar is below 13.5 volts, and the output from the AC generator will be zero until the generator switch is placed in the 'ON' position. The generator field excitation will be supplied initially from the battery, but once the generator produces an output, it will become self-excited. When the generator output is approximately 14 volts DC the under-voltage warning lamp will go out and a
charging current, indicated by the ammeter reading a positive value, will flow towards the battery.
The advantages of this system over the older commutator generator method of producing DC are that:-
¾ There is no necessity for a cut out because no reverse current can flow into the AC generator.
¾ The output is taken from stationary stator windings and only a small current need to be transferred to the motor field by way of brush gear and slip rings.
¾ There is no need for a cast iron yolk to concentrate the field, so the AC generator is a lot lighter.
Fault Protection in a Piston-Engine Frequency Wild AC System
The following faults protections exist in a piston-engined frequency wild AC system:-
¾ Over-Voltage. If an over-voltage occurs (15.5 volts approximately) the voltage
regulator will break the field and lock it out, causing the under-voltage lamp to illuminate. One attempt to reset the system by switching the generator switch 'OFF' for a few seconds to break the lock, then switching it to 'ON' again may be made, which is often referred to as ‘Cycling’ the generator switch.
¾ Under-Voltage. If the generator under-volts or is switched off, the under-
voltage warning lamp will illuminate, and will be automatically extinguished when the voltage returns to normal.
¾ Overheat. Some of these systems are fitted with an overheat thermostatic
sensor. If an overheat condition occurs it is annunciated to the flight crew, who should manually switch the generator switch off, and allow it to cool. When the generator has cooled, the overheat warning annunciator will automatically reset itself.
Note that the indications on this system for an over-volt or under-volt condition are somewhat similar, so if this fault occurs, ‘cycling’ the generator switch can reset the system.
Twin-Engine Turbo-Propeller Frequency Wild AC System Architecture
In this system the AC generators are fitted directly to each engine, and unless the engines run at a constant speed, the output frequency will vary (Frequency Wild).
The output from each generator is normally 200 volt three-phase, and varies in frequency between 280 - 540 Hertz, which corresponds respectively to low and high engine RPM's. The generators in this system should not be run in parallel under any circumstance, so their AC output is normally used to feed heating elements only. This is because the elements are purely resistive, and are thus unaffected by changes in frequency. In some systems part of the frequency wild output is rectified in a Transformer Rectifier Unit (TRU), and provides an alternative DC supply. The DC supplies may also be paralleled provided that the voltages are matched.
Operation of a Twin-Engine Turbo-Propeller Frequency Wild AC System
With the engine started and running the generator is initially excited by a separate power source, ie. the battery or ground power, as shown below.
Firstly switching the generator control switch to ‘RESET’, and thus closing the field relay achieves this. When the generator is producing an output part of it is fed back through the voltage regulator, and ‘Bridge Rectifier Pack’ to provide the generator field, thus providing
self-excitation. Once the generator is operating at its regulated output voltage of 200 volts, the line-contactor will close, and the generator warning light will go out. Moving the control switch to the 'ON' position will subsequently de-excite the field relay, and will remove the
source of the initial excitation current. The generator will now be fully self-excited, and the voltage regulator will continue to adjust the field excitation for varying speed load conditions.
Fault Protection in a Twin-Engine Turbo-Propeller Frequency Wild AC System
The following faults protections exist in a twin-engined turbo-propeller frequency wild AC system:-:-
¾
Overheat. If the generator overheats due to inadequate cooling or overload, awarning light will illuminate on the flight deck, and the generator should be manually switched off.
¾
Earth-Leakage. If there is low insulation in the alternator system or loads awarning light will illuminate, and if this occurs the generator should be switched off.
¾
Under-Voltage. This fault normally uses the same warning light as that used toindicate an earth leakage fault. The system voltmeter is thus used to discriminate between an earth leakage fault, and an under-voltage fault.
¾
Over-Voltage. If an over voltage occurs a sensing circuit will automatically de-excite the generator and remove it from the busbar. One attempt is usually allowed to reset the system by cycling the control switch between ‘RESET’ and ‘RUN’.
¾
Differential Protection. This system is used to:-¾
monitor line to line faults.¾
monitor line to earth faults.¾
ensure that the output current flowing from the generator is the same as that flowing to the loads and returning to the generator.If one of the above faults exists the generator will be automatically de-excited, and will also be removed from the busbar. One reset may be attempted, but even if the system resets satisfactorily for the rest of the flight, the fault must still be reported on landing.
The Constant Frequency Split Busbar AC System
The electrical system shown below is typically used on a twin jet engine aeroplane whose AC power supply is 200 Volt 400 Hz three phase.
The power supply can be derived from four sources; two engine driven Integrated Drive Generators (IDG’s), an Auxiliary Power Unit (APU), and an external power receptacle.
These sources should never be paralleled at any time. Under normal operation the
generators will independently feed the left and right section loads of the electrical system. The loads being fed by these generators are normally indicated on ammeters fitted to each generator output. The APU is used to drive a third generator which can supply the electrical power necessary for ground operations, or act as a substitute for a failed engine-driven generator. External power can also be used instead of APU power on the ground, but not simultaneously.
Operation of a Constant Frequency Split Busbar AC System
The circuit on the opposite page is shown in the power off condition. On most aeroplanes the APU is started by an electrical starter, which is supplied from its own dedicated battery, or from the aeroplane battery. When the APU is up and running, the generator is selected by the APU generator circuit breaker (GCB) to feed No.1 and No.2 main AC bus bars. The APU generator will then supply all of the aeroplane AC requirements, and the Transformer Rectifier Units (TRU's) will supply any DC requirements.
If the No.1 engine is initially started and run up, its dedicated IDG will produce the correct output (200v 400 Hz three-phase) and it will feed the No. 1 main AC busbar. However before it can supply this busbar the APU power must be removed from the No.1 main AC busbar by opening the appropriate GCB, followed by the closing of the No.1 IDG GCB. The No.1 IDG will now feed the No.1 main AC busbar and the A.P.U. generator will continue to feed the No.2 main AC busbar. When the No.2 engine is up and running its IDG will alternatively feed the No.2 main AC busbar. The APU generator supply must however be firstly removed from the No.2 busbar before the IDG is allowed to feed it. At this point the APU is no longer needed to feed the electrical system, and is therefore shut down. Both engine driven IDG AC supplies will now operate independently of each other, and will be kept separated by the ‘Bus-Tie Breaker (BTB)’.
If one of the IDG's fails the BTB between the two systems will automatically close, and the serviceable generator will feed both of the main AC busbars. If the APU is started again it will substitute for the failed generator and the BTB will open. The main aeroplane DC supply will be maintained by two TRU's (one for each IDG), as follows.
¾ The No.1 TRU will feed the DC essential busbar. ¾ The No.2 TRU will feed the DC non- essential busbar.
The TRU's are kept independent from each other by an ‘Isolation Relay’, but if either TRU fails, the Isolation Relay between the two sides will automatically close, and the serviceable TRU will feed both busbars.
Regulation and Protection of Constant Frequency Units
Most of these systems have separate or combined solid-state regulation and protection units dedicated to each generator.
The regulator is divided into the following parts:-
¾ A speed regulator, which senses the output speed or frequency of the IDG and adjusts the IDG to give a frequency output of between 380 - 420 Hz.
¾ A voltage regulator, which regulates the output voltage to 200 volts ± 5 volt by adjusting the IDG's field excitation.
A dedicated protection unit houses the circuitry, which detects any faults occurring up to, and including the busbars. Faults within this zone usually have time delays so that any faults occurring after the busbars will have time to trip the circuit breakers, or blow the fuses.
Faults on a Constant Frequency Split Busbar AC Generator System
Some faults in a split busbar generator system will cause the IDG to de-excite and its related GCB to open, thus removing the IDG from its own busbar. These faults are as follows:-
¾ Over-Voltage. If this type of fault is allowed to persist it could cause serious
damage to cable insulation and components.
¾ Differential Protection. This type of protection monitors the following faults:-
¾ A line to line or line to earth fault, which normally occurs inside the IDG. ¾ If the current flowing to the busbar is different from the current flowing from
Differential faults are detected by current transformers, which sense an imbalance in current between the generator and the busbar. If one of the above faults exists the generator field will be automatically de-excited and the generator removed from the busbar
¾ Over-Frequency. If this fault is allowed to continue it may damage any
capacitive circuits due to high currents.
¾ Under-Frequency. This fault will cause high currents and the overheating of
any inductive circuits.
¾ Resetting. Many of the faults mentioned have a facility by which the system
can be reset. One reset only is usually allowed, ie. the system is ‘Cycled’. Other faults which might occur are:-
¾ Generator Overheat. If the generator overheats due to frictional heating or
inefficient cooling, an overheat warning will be annunciated to the flight crew. If this occurs the system should be manually switched off.
¾ IDG Disconnect (CSDU Disconnect). The oil pressure and oil temperature of
the IDG is monitored. If during a fault the oil pressure drops, accompanied with an oil temperature rise, the flight crew may elect to operate the IDG disconnect, but once this has been initiated, the system can only be manually reset on the ground with the engine stopped.
¾ Generator Bearing Failure. If an excessive clearance exists in the bearings of
the engine, or APU generators, a bearing failure warning light will illuminate on the flight deck.
Emergency Supplies
In the unlikely event that both IDG's and the APU generator fail AC can still be obtained from:-
¾ The aeroplane battery, which will automatically feed the AC essential busbar via a static inverter.
¾ A ‘Ram Air Turbine (RAT)’ can be automatically or manually dropped into the
airstream to drive an AC generator, which will produce a constant frequency output for the AC essential busbar.
If the emergency power supplies are selected it is normal to shed any non-essential loads,
eg. galleys, in order to prevent overloading the remaining generators, which is known as
‘Load Shedding’. Battery Charger
Modern aeroplanes are fitted with battery chargers that are supplied from AC power supplies. These provide a DC supply to charge a battery in the shortest possible time, within certain voltage constraints, and without causing excessive gassing.
The charger provides a DC current of between 45-50 Amps until the charge reaches completion. It will then revert to the pulse mode to prevent the battery voltage becoming excessive. Comprehensive protection circuitry is provided in the battery charger to give protection against:-
¾ Over voltage ¾ Overheating
¾ Battery disconnection
If the battery over-volts the battery charger will be automatically switched off, and can only be reset by a push-switch situated on the front of the battery charger. If the charger overheats it will be automatically shut down, but will reset itself when cooled. If the battery is disconnected the charger will not be able to be switched on.
Battery Power
The batteries will supply secondary DC power. On most aeroplanes they will also feed essential DC, and through a static inverter essential AC for a period of 30 minutes or more. Some batteries are additionally fitted in non-pressurized areas in the fuselage, and are provided with electrically heated blankets to prevent freezing.
Ground Handling Bus
The ground handling busbar is powered from either an APU generator or an external power unit. The busbar is powered automatically whenever external or APU power is available. This busbar is used mainly on the ground to power lights, and the refuelling system.
Constant Frequency Parallel AC System
The constant frequency system is almost exclusive to three and four engine jet aeroplanes, and a typical system is shown on the next page. In older systems the AC generator and the CSDU are separate items, but on modern aeroplanes the two components are combined to form an IDG. In addition to the engine-driven generators an APU drives a generator, which is capable of supplying the aeroplane with power on the ground, and at altitudes up to approximately 35,000 ft. The APU may however experience difficulties in starting at altitudes above 25,000 ft. Some aeroplanes also have emergency ram air turbines, which can be deployed in an emergency. The generators fitted on each engine and are normally run in parallel. The system does however have the following advantages and disadvantages over Split Busbar AC System:-
¾ Advantages. When operating in parallel this system:-
¾ prolongs the generator life expectancy, since each generator is normally run on part load.
¾ readily absorbs large transient loads.
¾ Disadvantages. The disadvantages of the system are that:-
¾ expensive protection circuitry is required since any single fault may propagate through the complete system.
¾ Parallel operation does not meet the requirements for totally independent supplies.
On the most aeroplanes only the engine-driven generators can normally be paralleled, but the APU or the ground power unit cannot be paralleled with the engine driven generators, or each other. Circuit interlocks will prevent this occurring in the case of incorrect system management.
Operation of a Constant Frequency Parallel AC System
Once all of the above conditions have been satisfied, a ground power available light will come on. When 'ground power' is selected, the ground power breaker (GPB) will close and allow the ground power to feed the generator busbars.
With the No.1 engine running its generator will be excited when the generator control relay (GCR) is closed, which will enable the generator to give an output (200v three phase 400 Hz.). On closing the generator switch, the external services breaker (ESB) will open, thus removing ground power, and the No.1 generator circuit breaker will close. This will allow the No.1 generator to supply the necessary aeroplane power.
With the No.2 engine running, and its generator is producing the necessary output, it can be paralleled with the No.1 generator via the synchronizing busbars by closing the No.2 generator's GCB. The following conditions however must exist before paralleling can take place between two generators the:-
¾ voltages must be within tolerance. ¾ frequencies must be within tolerance.
¾ phase displacement must be within tolerance. ¾ phase rotation must be correct.
Once all of the above conditions have been satisfied the selecting the No.2 generator switch to 'ON', will cause the GCB. to close and the No.1 and No.2 generators to run in parallel. Both generators must share the real (Watts) and reactive (VAR) loads equally, and these are monitored on individual generator Watts/VAR meters on the flight deck.
The No.3 and No.4 generators are paralleled using the same method as the No.1 and No.2. generators. When all of the generators are running the No.1 and No.3 generators will be
kept separate from the No.2 and No.4 generators by a split system breaker (SSB). If any engine driven generator fails the SSB will automatically close.
Reactive Load Sharing
Reactive load sharing is achieved by a load-sharing loop, which will automatically adjust the excitation of the paralleled generator fields simultaneously, via their individual voltage regulators.
Real Load Sharing
Real load sharing is achieved by a load-sharing loop, which adjusting the magnetic trim in the mechanical governor of the CSDU's simultaneously, via their load controllers.
Paralleling
The following methods are used to parallel AC generators:-
Manual Paralleling is an old method of paralleling generators. To facilitate this
method a lamp is fitted across the main contacts of the GCB. When both generators outputs are the same the lamp will darken and go out. When this occurs the engineer closes the on coming generators control switch. This is also known as the
Automatic Paralleling. When using the automatic paralleling method, the