AIRCRAFT DISTRIBUTION SYSTEMS AN OVERVIEW
What is a bus bar? 2.5.1 General
>> BUSBARS BUSBARS:: Low impedance conductors in junction box or distributed panel >> CONNECTION FROM BUSBARS: BUSBARS: Carry-all functions >> BUSBAR CONSTRUCTION: • Strip of interlinked terminals (simple system) • Thick copper strips/r strips/rods ods (complex system) system)
INTRODUCTION •
Normally anyone of the THREE types of BUSBAR systems systems are found in most of the Aircraft.
1. Par aral alle lell bus bus bar bar sys systtem ems. s. 2. Sp Spli litt bus bus ba barr sy system ems. s. 3. Spl Split it pa parrall allel el bu buss bar bar sys ysttems ems..
Parallel bus bar systems •
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Normally found on THREE engined Aircr Aircraft aft like L-1011, MD-11, DC-10 and B-727. Here all the three engine driven genera generator torss are paralleled once the engines are “ON”. Normally, a third crew member, a Flight Engineer is located in the cockpit, whose job is to see that generators are synchronized and are in parallel.
Parallel bus bar systems •
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In case, an engine eng ine or a generator fails, fails, the loads are automatically taken care of. There are many electronic modules to monitor and give warning about the status of the generators.
Parallel bus bar systems Conditions for paralleling of generators:
1. Voltages must be within tolerance. 2. Frequencies must be within tolerance. 3. Phase displacemen displacementt must be within tolerance. 4. Phase rotation must be correct.
Constant Frequency Parallel AC System •
Advantages:
● Provides a continuity of electrical supply. ● Prolongs the generat generator or life expectancy, expectancy, since
each generator generator is normally run on part load. ● Readily absorbs large transient loads
Constant Frequency Parallel AC System (continue) •
Disadvantages:
● Expensive protection circuitry is required
since any single fault may propagate through the complete system. ● Par Parallel allel operation does not meet the
requirements for for totally independent supplies.
Constant Frequency Parallel AC System (continued)
generator circuit breakers (GCB ( GCB))
split system breaker (SSB (SSB))
Paralleling •
Manual Par Paralleling alleling 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 control switch. This is known as the lamps dark method of paralleling.
Paralleling (continue) •
Automatic Paralleling. Paralleling. When using the automatic paralleling method, the generat generator or 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.
Fault Protections Protections in A Constant Frequency AC Parallel System (continue) •
Over-Excitation (Parallel Fault) protection devices operate operat e whenever the ex excitation citation to the field of one of the generator generator increases. This is sensed s ensed when the overexcited generator takes more than its share of reactive load. The fault signal has an inver inverse se time ti me function that trips the BTB of the over-ex over-excited cited generator. The voltage regulator or reactive loadsharing circuit could cause this fault.
Fault Protections in A Constant Frequency AC Parallel System (continue) •
Over-Voltage protection devices operate whenever the system voltage exceeds 225 V. They protect the components in the system from damage due to excessive ex cessive voltages. This protection device operates on an inver inverse se time function function,, which means that the magnitude of voltage determines the time in which the offending generator generator is de-energised by tripping the GCR and GCB. The GCR de-energises the field, and the GCB trips t rips the th e generator off the busbar. busbar.
Fault Protections in A Constant Frequency AC Parallel System (continued) •
Under-Excitation (Parallel Fault) protection devices operate whenever the excitation of one of the generator genera tor 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 anyone of: 1. Reactive load sharing circuit 2. Generator 3. Voltage regulator
Fault Protections Protections in A Constant Frequency AC Parallel System (continued) •
Under-Voltage protection devices operate to preventt damage to equipment preven equip ment from high currents and losses in motor loads, which may cause over-heating and burn out. When this device operates, operates, it trips the GCR and GCB in in a fixed time (3-5 sec), resulting in the shut-down of that generator.
Fault Protections Protections in A Constant Frequency AC Parallel System (continued) •
Differential Protection devices operate operate in the same way as stated 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 diff erent from the current flowing from the generator.
Fault Protections Protections in A Constant Frequency AC Parallel System (continued) •
Instability Prot Protection ection (Parallel Fault) devices are incorporated in the system to guard against oscillating outputs outpu ts from the genera generator tors, s, which may cause sensitive equipment to malfunction or trip Off.
Fault Protections Protections in A Constant Frequency AC Parallel System (continued) •
Negative Sequence Voltage Protection devices detect any line-to-line or line-to-earth faults after the differentially protected zone and cause all the th e BTBs to trip.
Fault Protections Protections in A Constant Frequency AC Parallel System (continued) •
Overheat warning lights illuminate if a temperature sensor fitted in the generator senses an overheat condition. If this warning occurs, the pilot should operat operate e the GCR switch, which will Cause the GCR and GCB to trip.
Fault Protections Protections in A Constant Frequency AC Parallel System (continued) •
Over-speed (Over Frequency) Frequency) devices operate if a fault occurs in the CSDU, which may m ay cause the generator to exceed its specified frequency limits. If an over-speed condition occurs, it causes the GCB to trip and puts the CSDU into under-drive.
Fault Protections Protections in A Constant Frequency AC Parallel System (continued) •
Under-speed (Under-Frequency 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 under-frequency.
Fault Protections Protections in A Constant Frequency AC Parallel System (continued) •
Time delay del ayss 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.
Reactive Reactiv e Load Shearing Reactive load sharing is achieved by a load-sharing loop which automatically adjusts the the excitation excitation of the paralleled generator fields simultaneously simultan eously via their individual voltage regulators.
Reactive Load Shearing (continue)
Real Load Shearing •
Real load sharing is achieved by a load-sharing loop, which adjusts the magnetic trim in the mechanical governor of the CSDUs simultaneously via their load contro controllers. llers.
Real Load Shearing (continued)
SPLIT BUS BAR SYSTEM •
Normally found in twin engined Aircr Aircraft. aft. Here, the two generators never get paralleled. Hence they don’t need advanced circuits that are required for paralleling. Further, each generators can run with slightly different frequency. In case, one engine or generator fails, a bus tie breaker connects connects both the bus bars and loads are taken taken care of by a single generator.
Constant Frequency Split Busbar AC System (continued)
Engine starting sequence •
When on ground, the whole aircraft aircraft is powered by either a ground supply or by an APU. When one engine is started and the generator generat or builds up enough voltage, then that side bus is i s isolated by bus tie breaker and the other bus is powered by APU. APU. When the other engine comes ON, than APU is isolated and the aircraft departs.
Engine starting sequence •
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During take off, the galley power and utility bus are switched off to conserve energy. Incase, an engine fails during take-off, take-off, the APU (if “on”) comes into circuit automatically to take care of the loads. If the failed engine is able to start in air, then again APU is isolated and switched off.
DC Power Supplies •
Primary aircraft DC power supplies are derived from transf transformer ormer rectifier units, units, which are supplied from the 200 V AC busbars. busba rs. The TRUs are normally run in parallel parallel,, although some systems have isolation relays installed, which are designed to separate separate the DC busbars during fault conditions.
DC Power Supplies (continued)
Emergency Emerg ency Supplies •
In the unlikely event event that both IDGs and the APU generator fail, AC can still be obtained from: The aircr ai rcraft aft battery which automatically feeds the AC essential busbar via a static inverter.
A Ram Air Turbine (RA RAT T) 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 busbar..
Emergency Emerg ency Supplies Suppl ies (con (continued) tinued) •
If the emergency power supplies are selected, it is normal to shed any non-essential loads (e.g. galleys) in order to prevent prevent overloading the remaining generators, generators, which is known as Load Shedding. Shedding.
Battery Charger •
Modern aircraft are fitted with battery chargerss that are supplied from AC power charger supplies. These provide a DC supply to charge a battery in the shortest possible time, time, within certain voltage constraints, constraints, and without causing excessive gassing.
Battery Charger (continued) •
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.
Battery Charger (continued) •
Comprehensive protection circuitry is provided in the batt battery ery charger to give protection against:
Over voltage
Overheating
Battery Batt ery disconnection
Battery Charger (continued) •
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If the batt battery ery over-volts over-volts,, the batt battery ery 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 overheats,, it is automatically shut down but resets itself when cooled. If the battery is disconnected disconnected,, the charger cannot be switched on.
Battery Power •
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The batteries supply secondary DC power on most aircraft, they also feed essential DC and, through a static inverter inverter,, essential AC for a period of 30 of 30 minutes or more. more. Some batteries are additionally fitted in nonpressurized pressuriz ed areas in the fuselage and are provided with electrically heated blankets blankets to prevent freezing.
Battery charging •
Two types of battery charging are employed in workshop:
1. Constan Constantt current charging. (Both voltage & current are constant) 2. Constant voltage charging charging.. (Only voltage is constant, current varies) Of these, const constant ant voltage charging is employed in the aircraft.
Ground Handling Bus •
The ground handling busbar is powered from either an APU generator 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 in cargo bay bay and the refueling system.
SPLIT PARALLEL BUS BAR SYSTEM •
Found only in B-747-400 B -747-400 series only. only.
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May be in A-380?
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This function is somewhat a combination of both split and parallel bus bar sys systems. tems.
QUESTIONS?
