Aircraft Distribution Systems-ov3

AIRCRAFT DISTRIBUTION SYSTEMS

AN OVERVIEW

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What is a bus bar?

2.5.1 General

>> BUSBARS: Low impedance conductors in junction box or distributed panel

>> CONNECTION FROM BUSBARS: Carry-all functions

>> BUSBAR CONSTRUCTION:

• Strip of interlinked terminals (simple system)• Thick copper strips/rods (complex system)

INTRODUCTION

• Normally anyone of the THREE types of BUSBAR systems are found in most of the Aircraft.

1.Parallel bus bar systems.2.Split bus bar systems.3.Split parallel bus bar systems.

Parallel bus bar systems

• Normally found on THREE engined Aircraft like L-1011, MD-11, DC-10 and B-727.

• Here all the three engine driven generators 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.


• In case, an engine or a generator fails, the loads are automatically taken care of.

• There are many electronic modules to monitor and give warning about the status of the generators.


• Conditions for paralleling of 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.

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

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.

Constant Frequency Parallel AC System (continued)

split system breaker (SSB) generator circuit breakers (GCB)

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.

Reactive Load Shearing (continue)

Real Load Shearing (continued)

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.

Paralleling (continued)

• 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.

Fault Protections in A Constant Frequency

AC Parallel System (continued)• 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 over-excited generator. The voltage regulator or reactive load-sharing circuit could cause this fault.


AC Parallel System (continued)• 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.


AC Parallel System (continued)• 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 anyone of: 1. Reactive load sharing circuit

• 2. Generator• 3. Voltage regulator


AC Parallel System (continued)• 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.


AC Parallel System (continued)• 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.


AC Parallel System (continued)• 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.


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 BTBs to trip.


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 operate the GCR switch, which will Cause the GCR and GCB to trip.


AC Parallel System (continued)• 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 over-speed condition occurs, it causes the GCB to trip and puts the CSDU into under-drive.


AC Parallel System (continued)• 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 under-frequency.


AC Parallel System (continued)• 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.

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.

Reactive Load Shearing (continued)

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 controllers.

Real Load Shearing (continued)

SPLIT BUS BAR SYSTEM

• Normally found in twin engined Aircraft. 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 both the bus bars and loads are taken care of by a single generator.

Constant Frequency Split Busbar AC System (continued)

Engine starting sequence

• When on ground, the whole aircraft is powered by either a ground supply or by an APU. When one engine is started and the generator builds up enough voltage, then that side bus is isolated by bus tie breaker and the other bus is powered by APU. When the other engine comes ON, than APU is isolated and the aircraft departs.

Engine starting sequence

• During take off, the galley power and utility bus are switched off to conserve energy. Incase, an engine fails during 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 SUPPLY

• This topic is common to all the three types.

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.

DC Power Supplies (continue)

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.

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.

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.

Emergency Supplies (continued)

• If the emergency power supplies are selected, it is normal to shed any non-essential loads (e.g. galleys) in order to prevent overloading the remaining generators, which is known as Load Shedding.

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.

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.


• Comprehensive protection circuitry is provided in the battery charger to give protection against:

• Over voltage• Overheating• Battery disconnection


• 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.

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-pressurized areas in the fuselage and are provided with electrically heated blankets to prevent freezing.

Battery charging

• Two types of battery charging are employed in workshop:

• 1. Constant current charging.• (Both voltage & current are constant)• 2. Constant voltage charging.• (Only voltage is constant, current varies)• Of these, constant voltage charging is

employed in the aircraft.

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 in cargo bay and the refueling system.

SPLIT PARALLEL BUS BAR SYSTEM

• Found only in B-747-400 series only.• May be in A-380?• This function is somewhat a combination of

both split and parallel bus bar systems.

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Aircraft Distribution Systems-ov3