Chapter 11 AC Power Generation Systems

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Chapter 11 AC Power
Generation Systems
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Chapter 11 AC Power Generation Systems
• AC supply systems vary in complexity
depending on aircraft type and electrical
requirements. There are two categories of AC
systems commonly used dependent on
whether the output frequency of the generator
is controlled or not. They are known as
frequency wild and constant frequency
systems and are fully described below.
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11.1 Frequency-Wild AC System
• 11.1.1 A Typical 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 varies (frequency-wild).
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11.1 Frequency-Wild AC System (continue)
• The output from each generator is normally
200 V three-phase and varies in frequency
between 280 and 540 Hz, which
corresponds respectively to tow and high
engine rpm.
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11.1 Frequency-Wild AC System (continue)
• 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
unaffected by changes in frequency.
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11.1 Frequency-Wild AC System (continue)
• 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.
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Figure 11-1 A Typical Frequency-Wild AC System
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11.1.3 Fault Protection in a Typical
Frequency-Wild AC System
• The following fault protections exist in a twinengine
turbo-propeller frequency-wild AC
system:
•  Overheat
• If the generator overheats due to
inadequate cooling or overload, a warning
light illuminates on the flight deck, and the
generator should be manually switched off.
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•  Earth-Leakage
• If there is low insulation in the
alternator system or loads, a warning light
illuminates. If this occurs, switch off the
generator.
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•  Under-Voltage
• This fault normally uses the same
warning light as that used to indicate an
earth leakage fault. The system voltmeter
is used to discriminate between an earth
leakage fault and an under-voltage fault.
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•  Over-Voltage
• If an over voltage occurs, a sensing
circuit automatically de-excites the
generator and removes it from the busbar.
One attempt is usually allowed to reset the
system by cycling the control switch
between RESET and RUN.
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•  Differential Protection
• This system is used to:
• ● Monitor line-to-line faults
• ● Monitor line-to-earth faults
• ● Ensure that the output current flowing
from the generator is the same as that
flowing to the loads and returning to the
generator.
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Figure 11-2 Fault Protection in a Typical Frequency-Wild AC System
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• If one of the above faults exists, the
generator is automatically de-excited and
is 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.
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11.2 Constant Frequency
Split Busbar AC System
• The following electrical system is typically
used on a twin-jet engine aircraft whose
AC power supply is 200 V 400 Hz threephase.
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11.2 Constant Frequency Split Busbar
AC System (continue)
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11.3 Constant Frequency Parallel
AC System
• Advantages:
• ● Provides a continuity of electrical supply
• ● Prolongs the generator life expectancy,
since each generator is normally run on part
load
• ● Readily absorbs large transient loads
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11.3 Constant Frequency Parallel
AC System (continue)
• Disadvantages:
• ● Expensive protection circuitry is
required since any single fault may
propagate through the complete system.
• ● Parallel operation does not meet the
requirements for totally independent
supplies.
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11.3 Constant Frequency Parallel AC System
(continue)
• The following conditions must exist before
paralleling can take place between two
generators:
• 1. Voltages must be within tolerance.
• 2. Frequencies must be within tolerance.
• 3. Phase displacement must be within
tolerance.
• 4. Phase rotation must be correct.
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11.3 Constant Frequency Parallel
AC System (continue)
generator circuit breakers (GCB) split system breaker (SSB)
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11.3.2 Reactive Load Shearing
• Reactive load sharing is achieved by a
load-sharing loop which automatically
adjusts the excitation of the paralleled
generator fields simultaneously via their
individual voltage regulators.
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11.3.2 Reactive Load Shearing (continue)
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11.3.3 Real Load Shearing
• Real load sharing is achieved by a loadsharing
loop, which adjusts the magnetic
trim in the mechanical governor of the
CSDUs simultaneously via their load
controllers.
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11.3.3 Real Load Shearing (continue)
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11.3.4 Paralleling
• 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 oncoming generator's control
switch. This is known as the lamps dark
method of paralleling.
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11.3.4 Paralleling (continue)
• Automatic Paralleling. When using the
automatic paralleling method, the
generator switch is selected to on at any
time, and once the auto paralleling circuits
sense that both generators are ready for
paralleling, the GCB automatically closes.
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• Over-Excitation (Parallel Fault) protection
devices operate whenever the excitation to
the field of one of the generator increases.
This is sensed when the over-excited
generator takes more than its share of
reactive load. The fault signal has an inverse
time function that trips the BTB of the overexcited
generator. The voltage regulator or
reactive load-sharing circuit could cause this
fault.
11.3.5 Fault Protections in A Constant Frequency
AC Parallel System (continue)
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• Over-Voltage protection devices operate whenever
the system voltage exceeds 225 V. They protect the
components in the system from damage due to
excessive voltages. This protection device operates
on an inverse time function, which means that the
magnitude of voltage determines the time in which
the offending generator is de-energised by tripping
the GCR and GCB. The GCR de-energises the field,
and the GCB trips the generator off the busbar.
11.3.5 Fault Protections in A Constant Frequency
AC Parallel System (continue)
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• Under-Excitation (Parallel Fault) protection
devices operate whenever the excitation of one of
the generator fields is reduced. This is sensed
when the under-excited generator takes less than
its share of reactive load, and a fault signal
causes the BTB to trip in a fixed time (3-5 sec).
This type of fault could be caused by a fault in the:
•  Reactive load sharing circuit
•  Generator
•  Voltage regulator
11.3.5 Fault Protections in A Constant Frequency
AC Parallel System (continue)
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• Under-Voltage protection devices operate
to prevent damage to equipment from high
currents and losses in motor loads, which
may cause over-heating and burn out.
When this device operates, it trips the
GCR and GCB in a fixed time (3-5 sec),
resulting in the shut-down of that
generator.
11.3.5 Fault Protections in A Constant Frequency
AC Parallel System (continue)
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• Differential Protection devices operate in
the same way as stated in the split-busbar
generator system. They operate if any of
the following faults exist:
•  A line-to-line or line to-earth fault
•  If the current flowing to the busbar
is different from the current flowing from
the generator
11.3.5 Fault Protections in A Constant Frequency
AC Parallel System (continue)
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11.3.5 Fault Protections in A Constant Frequency
AC Parallel System (continue)
• Instability Protection (Parallel Fault)
devices are incorporated in the system to
guard against oscillating outputs from the
generators, which may cause sensitive
equipment to malfunction or trip Off.
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• Negative Sequence Voltage Protection
devices detect any line-to-line or line-toearth
faults after the differentially
protected zone and cause all the BTBs to
trip.
11.3.5 Fault Protections in A Constant Frequency
AC Parallel System (continue)
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• Overheat warning lights illuminate if a
temperature sensor fitted in the generator
senses an overheat condition. If this
warning occurs, the pilot should operate
the GCR switch, which will Cause the GCR
and GCB to trip.
11.3.5 Fault Protections in A Constant Frequency
AC Parallel System (continue)
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• Over-speed (Over Frequency) devices
operate if a fault occurs in the CSDU,
which may cause the generator to exceed
its specified frequency limits. If an overspeed
condition occurs, it causes the GCB
to trip and puts the CSDU into under-drive.
11.3.5 Fault Protections in A Constant Frequency
AC Parallel System (continue)
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• Under-speed (Under-Frequency) of the
CSDU is sensed by an oil pressure switch
in the CSDU. This causes the GCB to trip,
removing the generator from the busbar,
and protecting the loads from an underfrequency.
11.3.5 Fault Protections in A Constant Frequency
AC Parallel System (continue)
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• Time delays are fitted in the generator
protection system to give the normal
circuit protection devices (i.e. circuit
breakers and fuses) time to operate, rather
than removing a generator from the
system.
11.3.5 Fault Protections in A Constant Frequency
AC Parallel System (continue)
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11.4 DC Power Supplies
• Primary aircraft DC power supplies are
derived from transformer rectifier units,
which are supplied from the 200 V AC
busbars. The TRUs are normally run in
parallel, although some systems have
isolation relays installed, which are
designed to separate the DC busbars
during fault conditions.
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11.4 DC Power Supplies (continue)
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11.5 Emergency Supplies
• In the unlikely event that both IDGs and the APU
generator fail, AC can still be obtained from:
•  The aircraft battery which automatically feeds the
AC essential busbar via a static inverter.
•  A Ram Air Turbine (RAT) can be automatically or
manually dropped into the airstream to drive an AC
generator, which produces a constant frequency
output for the AC essential busbar.
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11.5 Emergency Supplies (continue)
• If the emergency power supplies are
selected, it is normal to shed any nonessential
loads (e.g. galleys) in order to
prevent overloading the remaining
generators, which is known as Load
Shedding.
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11.6 Battery Charger
• Modern aircraft 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.
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11.6 Battery Charger (continue)
• The charger provides a DC current of 45-
50 Amps until the charge reaches
completion. It then reverts to the pulse
mode to prevent the battery voltage from
becoming excessive.
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11.6 Battery Charger (continue)
• Comprehensive protection circuitry is
provided in the battery charger to give
protection against:
•  Over voltage
•  Overheating
•  Battery disconnection
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11.6 Battery Charger (continue)
• If the battery over-volts, the battery
charger is 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 is automatically
shut down but resets itself when cooled.
• If the battery is disconnected, the charger
cannot be switched on.
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11.7 Battery Power
• The batteries supply secondary DC power
on most aircraft, they 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-pressurised areas in the fuselage and
are provided with electrically heated
blankets to prevent freezing.
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11.8 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.
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END OF CHAPTER 11

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