Aircraft Maintenance Engineering Series

Aircraft Electrical SystemsShahzad Khalil

shahzadkhalil@piac.aero

Module 13.5 EASA Part-66 Cat.B2

13.5 Electrical Power (ATA 24) level 313.5.1 Batteries Installation And Operation;13.5.2 DC Power Generation;13.5.3 AC Power Generation;13.5.4 Emergency Power Generation;13.5.5 Voltage Regulation;13.5.6 Power Distribution;13.5.7 Inverters, Transformers, Rectifiers;13.5.8 Circuit Protection;13.5.9 External/Ground Power.

13.5.1 Batteries Installation and Operation

An aircraft is fitted with one or two main batteries depending on its size and role. The battery is located as close as possible to its point of distribution; this is to reduce IR losses through heavy-duty cables. In smaller general aviation (GA) aircraft, the battery can be located in the engine compartment, alternatively behind the luggage compartment in the rear fuselage, see Fig. 1.1(a). On some larger GA aircraft the battery is located in the leading edge of the wing, see Fig. 1.1 (b). Other locations include the nose equipment bay on medium size helicopters (Fig. 1.1 (c) ) orattached to the external airframe , see Fig. 1.1 (d) . For larger aircraft, e.g. the Boeing 737, one battery is located in the Avionics compartment.

Batteries are installed in a dedicated box or compartment designed to retain it in position and provide ventilation. The battery compartment is usually fitted with a tray to collect any spilt electrolyte and protect the airframe. Tray material will be resistant to corrosion and non-absorbent. The structure around the battery compartment will be treated to reduce any damage from corrosion resulting from any spilt electrolyte or fumes given off during charging. Batteries must be secured to prevent them from becoming detached during aircraft maneuvers; they are a fire risk if they become detached from their tray.

Warning:

When installing batteries in the aircraft, extreme care must be taken not to directly connect (or ‘short circuit’) the terminals. This could lead to a high discharge of electrical energy causing personal harm and/or damage to the aircraft.

Key maintenance point

The battery must be secured without causing any deformation of the casing which could lead to plate buckling and internal shorting.

Battery venting

Main battery installations must be vented to allow gases to escape, and accommodate electrolyte spillage. Rubber or other non-corroding pipes are used as ventilation lines which direct the gases overboard, usually terminating at the fuselage skin. On pressurized aircraft the differential pressures between cabin and atmosphere are used to draw air through the venting system. Some installations contain traps to retain harmful gases and vapors. Figure 1.2 illustrates battery venting, acid traps and how pressurized cabin air is used to ventilate the battery.

Fig.1.1 Typical battery locations: (a) battery compartment (GA aircraft); (b) wing leadingedge (Beech King Air); (c) nose equipment bay (medium helicopter); (d) externally mounted

(small helicopter)

Fig. 1.2 Battery venting

Figure 1.2 Battery connections

Key maintenance pointRemoval of the aircraft battery can result in loss of power to any clocks that are electrically. It will usually be necessary to check and reset the clocks on the flight deck when battery power is eventually restored.

When no other power is on the airplane (B777), the hot battery bus supplies power to the static random access memory (SRAM) of the AIMS. If you disconnect the main battery with no power to the AIMS cabinets, you lose some data.

Electrical Power Introduction:

The Main parts of a typical alternating current (AC) aircraft electrical system are shown in Fig.1 and comprise the following:

Power generation Primary power distribution and protection Power conversion and energy storage Secondary power distribution and protection

Fig 1 Aircraft AC electrical system

13.5.2 DC Power Generation

DC systems use generators to develop a DC voltage to supply aircraft system loads; usually the voltage is 28 VDC but there are 270 VDC systems in being which will be described later in the chapter. The generator is controlled- the technical term is regulated – to supply 28 VDC at all times to the aircraft loads such that any tendencies for the voltage to vary or fluctuate are overcome. DC generators are self-exciting, in that they contain rotating electromagnets that generate the electrical power. The conversion to DC power is achieved by using a device called a commutator which enables the output voltage, which would appear as a simple sine wave output, to be effectively half-wave rectified and smoothed to present a steady DC voltage with a ripple imposed. In aircraft applications the generators are typically shunt-wound in which the high resistance field coils are connected in parallel with the armature as shown in Fig. 2.1

Fig. 2.1 Shunt-wound DC generator

The natural load characteristic of the shunt-wound generator is for the voltage to ‘droop’ with the increasing load current, whereas the desired characteristic is to control the output at a constant voltage – nominally 28 VDC. For this purpose a voltage regulator is used which modifies the field current to ensure that terminal voltage is maintained while the aircraft engine speed and generator loads vary. The principle of operation of the DC voltage regulator is shown in a Fig.2.2

Fig.2.2 DC Voltage Regulator

13.5.3 AC power generation

An AC system uses a generator to generate a sine wave of a given voltage and, in most cases, of a constant frequency. The construction of the alternator is simpler than that of the DC generator in that no commutator is required. Early AC generators used slip rings to pass current to/from the rotor windings; however these suffered from abrasion and pitting, especially when passing high currents at altitude. Modern AC generators work on the principle shown in Fig. 3.1.

Fig.3.1 Principle of Operation of Brushless AC Generator

This AC generator may be regarded as several machines sharing the same shaft. From left to right as viewed on the diagram they comprise:

A Permanent Magnet Generator (PMG) Exciter Generator-An excitation stator and an excitation rotor containing rotating diodes Main Generator -A Power rotor encompassed by a power stator Main Generator

The flow of power through this generator is highlighted by the dashed line. The PMG generates ‘raw’ (variable frequency, variable voltage) power sensed by the control and regulation section that is part of the generator controller. This modulates the flow of DC current into the excitation stator windings and therefore controls the voltage generated by the excitation rotor. The rotation of the excitation rotor within the field produced by the excitation stator windings is rectified by means of diodes contained within the rotor and supplies a regulated and controlled DC voltage to excite the power rotor windings. The rotating field generated by the power rotor induces an AC voltage in the power stator that may be protected and supplied to the aircraft systems. Most AC systems used on aircraft use a three-phase system, that is the alternator generates three sine waves; each phase positioned 120 degrees out of phase with the others. These phases are most often connected in a star configuration with one end of each of the phases connected

to a neutral point as shown in Fig. 3.2. In this layout the phase voltage of a standard aircraft system is 115 VAC, whereas the line voltage measured between lines is 200 VAC. The standard for aircraft frequency-controlled systems is 400 Hz. The descriptions given above outline the two primary methods of power generation used on aircraft for many years. The main advantage of AC power is that it operates at a higher voltage; 115 VAC rather than 28 VDC for the DC system. The use of a higher voltage is not an advantage in itself; in fact higher voltages require better standards of insulation. It is in the transmission of power that the advantage of higher voltage is most apparent.

Fig.3.2 Star connected 3-phase AC generator

For a given amount of power transmission, a higher voltage relates to an equivalent lower current. The lower the current the lower are losses such as voltage drops (proportional to current) and power losses (proportional to current squared). Also as current conductors are generally heavy is can be seen that the reduction in current also saves weight; a very important consideration for aircraft systems.

Modern Electrical Power Generation Types:

So far basic DC and AC power generating systems have been described. The DC system is limited by currents greater than 400 A and the constant frequency AC method using an Integrated Drive Generator

(IDG) has been mentioned. In fact there are many more power generation types in use today. A number of recent papers have identified the issues and projected the growth in aircraft electric power requirements in a civil aircraft setting, even without the advent of more-electric systems. However not only are aircraft electrical system power levels increasing but the diversity of primary power generation types is increasing.

Fig.3.3 Electrical Power Generation Types

The different types of electrical power generation currently being considered are shown in Fig.3.3. The Constant Frequency (CF) 115 VAC, three-phase, 400 Hz options are typified by the Integrated Drive Generator (IDG), variable speed constant frequency (VSCF) cycloconverter and DC link options. Variable frequency (VF) 115 VAC, three phase power generation – sometimes termed ‘frequency-wild’ – is also a more recent contender, and although a relatively inexpensive form of power generation, it has the disadvantage that some motor loads may require motor controllers. Military aircraft in the US are inclining toward 270 VDC systems. Permanent Magnet Generators (PMGs) are used to generate 28 VDC emergency electrical power for high-integrity systems. Fig.3.3. is also interesting in that it shows the disposition between generation system components located on the engine and those within the airframe.

Without being drawn into the partisan arguments regarding the pros and cons of the major types of power generation in use or being introduced today it is worth examining the main contenders:

Constant frequency using an IDG Variable frequency Variable Speed Constant Frequency (VSCF) options

Constant frequency/IDG:

Fig.3.4 Constant frequency/IDG generation

The main features of CF/IDG power are shown in Fig.3.4. In common with all the other power generation types this has to cater for a 2:1 ratio in engine speed between maximum power and ground idle. The Constant Speed Drive (CSD) in effect acts as an automatic gearbox, maintaining the generator shaft speed at a constant rpm which results in a constant frequency output of 400 Hz, usually within approximately 10 Hz or less. The drawback of the hydromechanical CSD is that it needs to be correctly maintained in terms of oil charge level and oil cleanliness. Also to maintain high reliability frequent overhauls may be necessary. That said, the IDG is used to power the majority of civil transport aircraft today as shown in Table1.

CSD

Variable frequency:

Fig.3.5 Variable frequency power generation

Variable frequency (VF) power generation as shown in Fig.3.5. It is the simplest and most reliable form of power generation. In this technique no attempt is made to nullify the effects of the 2:1 engine speed ratio and the power output, though regulated to 115 VAC, suffers a frequency variation typically from 380 to 720 Hz. This wide band VF power has an effect on frequency-sensitive aircraft loads, the most obvious being the effect on AC electric motors that are used in many aircraft systems. There can therefore be a penalty to be paid in the performance of other aircraft systems such as fuel, ECS and hydraulics. In many

cases variations in motor/pump performance may be accommodated but in the worst cases a motor controller may be needed to restore an easier control situation.VF is being widely adopted in the business jet community as their power requirements take them above the 28 VDC/12 kW limits of twin 28 VDC systems.

VSCF:

Figure 12 shows the concept of the VSCF converter. In this technique the variable frequency power produced by the generator is electronically converted by solid-state power-switching devices to constant frequency 400 Hz, 115 VAC power. Two options exist:

Fig. 3.6 VSCF power generation

DC link: In the DC link the raw power is converted to an intermediate DC power stage – the DC link – before being electronically converted to three-phase AC power. DC link technology has been used on the B737NG, MD-90 and B777 but has yet to rival the reliability of CF or VF power generation.Cycloconverter: The cycloconverter uses a different principle. Six phases are generated at relatively high frequencies in excess of 1,600 Hz and the solid-state devices switch between these multiple phases in a predetermined and carefully controlled manner. The effect is to electronically commutate the input and provide three phases of constant frequency 400 Hz power. Though this appears to be a complex technique it is in fact quite elegant and cycloconverter systems have been successfully used on military aircraft.

Table 3.1 Recent civil and military aircraft power system developments

13.5.4 Emergency Power Generation

In certain emergency conditions the typical aircraft power generation system already described may not meet all the airworthiness authority requirements and additional sources of power generation may need to be used to power the aircraft systems. The aircraft battery offers a short-term power storage capability, typically up to 30 minutes. However for longer periods of operation the battery is insufficient. The operation of twin-engined passenger aircraft on ETOPS flights now means that the aircraft has to be able to operate on one engine while up to 180 minutes from an alternative or diversion airfield. This has led to modification of some of the primary aircraft systems, including the electrical system, to ensure that sufficient integrity remains to accomplish the 180 minute diversion while still operating with acceptable safety margins. The three standard methods of providing back-up power on civil transport aircraft are:(1) Ram Air Turbine (RAT)(2) Back-up Converters(3) Permanent Magnet Generators (PMGs)

Ram Air Turbine

Fig.4.1 Ram Air TurbineThe Ram Air Turbine or RAT is deployed when most of the conventional power generation system has failed or is unavailable for some reason. The RAT is an airdriven turbine, normally stowed in the aircraft ventral or nose section that is extended either automatically or manually when the emergency commences. The passage of air over the turbine is used to power a small emergency generator of limited capacity, usually enough to power the crew’s essential flight instruments and a few other critical services – see Fig. 4.1. Typical RAT generator sizing may vary from 5 to 15 kVA depending upon the aircraft. The RAT also

powers a small hydraulic power generator for similar hydraulic system emergency power provision. Once deployed then the RAT remains extended for the duration of the flight and cannot be restowed withoutmaintenance action on the ground. The RAT is intended to furnish the crew with sufficient power to fly the aircraft while attempting to restore the primary generators or carry out a diversion to the nearest airfield. It is not intended to provide significant amounts of power for a lengthy period of operation.

Back-up converters

The requirements for ETOPS have led to the need for an additional method of back-up power supply, short of deploying the RAT that should occur in only the direst emergency. The use of back-up converters satisfies this requirement and is used on the Boeing 777. Back-up generators are driven by the same engine accessory gearbox but are quite independent of the main IDGs. Refer to Fig. 4.2. The back-up generators are VF and therefore experience significant frequencyvariation as engine speed varies. The VF supply is fed into a back-up converter which,

Fig 4.2 Simplified back-up VSCF converter system

using the DC link technique, first converts the AC power to DC by means of rectification. The converter then synthesizes three-phase 115 VAC 400 Hz power by means of sophisticated solid-state power-switching techniques. The outcome is an alternative means of AC power generation which may power some of the aircraft AC busbars; typically the 115 VAC transfer buses in the case of the Boeing 777. In this way substantial portions of the aircraft electrical system may remain powered even though some of the more sizeable loads such as the galleys and other non-essential loads may need to be shed by the Electrical Load Management System (ELMS).

Permanent Magnet Generators (PMGs)

The use of PMGs to provide emergency power has become prominent over the last decade or so. As can be seen from the description of the back-up converter above, the back-up generator hosts PMGs which may supply several hundred watts of independent generated power to the flight control DC system where the necessary conversion to 28 VDC is undertaken. It was already explained earlier in the chapter that AC generators include a PMG to bootstrap the excitation system. PMGs – also called Permanent Magnet Alternators (PMAs) – are used to provide dual independent on-engine supplies to each lane of the FADEC. As an indication of future trends it can therefore be seen that on an aircraft such as the Boeing 777 there are a total of 13 PMGs/PMAs across the aircraft critical control systems – flight control, engine control andelectrical systems. See Fig. 4.3.

Fig.4.3. Boeing 777 PMG/PMA complement

13.5.5 Voltage regulation

a) DC Voltage Regulator

DC generation is by means of shunt-wound self-exciting machines as briefly outlined above. The principle of voltage regulation is outlined in Fig. 2.2. This shows a variable resistor in series with the field winding such that variation of the resistor alters the resistance of the field winding; hence the field current and output voltage may be varied. In actual fact the regulation is required to be an automatic function that takes account of load and engine speed. The voltage regulation needs to be in accordance with the standard used to specify aircraft power generation systems, namely MIL-STD- 704D. This standard specifies the voltage at the point of regulation and the nature of the acceptable voltage drops throughout the aircraft distribution, protection and wiring system. DC systems are limited to around 400 A or 12 kW per channel maximum for two reasons:(1) The size of conductors and switchgear to carry the necessary current becomes prohibitive.(2) The brush wear on brushed DC generators becomes excessive with resulting maintenance costs if these levels are exceeded. Carbon-pile regulator: Another type of electromechanical regulator is the carbon-pile device. This type of regulator is used in generator systems with outputs in excess of 50 A and provides smoother regulation compared with the vibrating contact regulator. Carbon-pile regulators consists of a variable resistance in series with the generator’s shunt wound field coil. The variable resistance is achieved with a stack (or pile) of carbon discs (washers). These are retained by a ceramic rube that keeps the discs aligned. Figure 5.1 shows the main features of the regulator in cross-section.

Fig..5.1 Carbon pile Regulator Cross SectionThe surface of each disc is relatively rough; applying pressure to the discs creates more surface contact, thereby reducing the resistance of the pile. When pressure is reduced, the reverse process happens, and the resistance through the pile increases. Pressure is applied to the pile by a spring plate. This compression is opposed by the action of an electromagnet connected to the generator output; the strength of the electromagnet’s flux varies in proportion with generatoroutput voltage. Higher generator output increases the current in the electromagnet; this attracts the steel centre of the spring, which reduces compression on the pile, thereby increasing its resistance. Less field

current reduces the generator output voltage; the current in the voltage coil reduces electromagnetic effect and the spring compresses the pile, reducing its resistance.

The varying force applied by the electromagnet and spring thereby controls the pile’s resistance to control field current and maintains a constant generator output voltage. The regulator is contained within a cylinder (typically three inches in diameter and six inches in length) with cooling fins.

Functions of each component are as follows: Compression Screw : the means of setting up compression on the pile and compensating for

erosion of the pile during its life. Spring plate and Armature : this compresses the pile to its minimum resistance position.Voltage coil: contains a large number of turns of copper wire and, with the core screw, forms an electromagnet when connected across the generator output.Magnet core: concentrates the coil flux; it is also used for voltage adjustment during servicing.

Bi-metallic washers: providing temperature compensation.

Figure .5.2 shows the carbon-pile regulator connected into the generator’s regulating circuit. The ballast resistor has a low-temperature coefficient and minimizes the effects of temperature on the voltage coil. The trimmer resistors (in series with the ballast resistor) allow the generator output voltage to be trimmed on the aircraft. The boost resistor is normally shorted out; if the switch is opened it allows a slight increase in generator output to meet short-term increases in loading. This is achieved by temporarily reducing the current through the voltage coil. The boost resistor can either be located in the regulator and/or at a remote location for easy access during maintenance.

Fig.5.2 Carbon Pile Regulator Schematics

Electronic Voltage Regulator

There are many types and configurations of electronic voltage regulators. A representative type is illustrated in Fig. 5.3. The alternator master switch used in AC systems energizes the field relay and applies current to the base of TR 2 and the resistor network of R 1, R 2 , RV1 . This network, together with the Zener diode (Z) is used to establish the nominal operating voltage.

Current flows through the alternator’s field coil via transistors TR 2 and TR 3, allowing the generator’s output to increase. When the output reaches its specified value (14 or 28 V DC depending on the installation) Zener diode Z conducts which turns on transistor TR1, shorting out transistor TR 2 and TR 3. The generator voltage falls and Zener diode Z stops conducting, thereby turning of transistor TR 1. This turns transistors TR 2 and TR 3 back on, allowing the generator output to increase again. This operation is repeated many times per second as with

Fig.5.3 Electronic voltage regulator

the vibrating contact regulator; the difference being that electronic circuits have no moving parts and do not suffer from arcing across contacts. Diode D1 provides protection against the back e.m.f. induced in the field each time TR 3 is switched. The trimming resistor R V1 can be used to adjust the nominal voltage output of the regulator.

a) AC Voltage Regulator (GCU)The automatic voltage regulation uses a controlled d.c. current through the generator's excitor field. There are various methods used on different aircraft, we will only discuss the transistorized method.

Fig. 5.4Voltage TrimmingWith the earlier designed systems the voltage regulation is adjustable in situ; this normally applies to a carbon pile type voltage regulator which does tend to drift during service. In the later type of transistor controlled voltage regulator, in situ adjustment is not allowed. Voltage adjustment is a workshop function only. Point of RegulationWith the larger type of aircraft there is a long distance between the generator and the distribution point which can lead to a small volts drop under heavy loads. As we require a controlled voltage at the busbar, the point of regulation is chosen as near to the distribution busbar as possible but on the generator side of the GCB.Current LimitingSome systems control the generators maximum overload current. This is achieved, within the voltage regulation section of the GCU, by monitoring the load current and reducing the generator's output voltage when the load current limit is reached.

Transistorised Voltage Regulator

Voltage SensingThe voltage regulator senses three phase voltage at the point of regulation. The average of all three phases is compared to a reference value and the resultant signal is fed to a summing network. The summing circuit senses the need for increased or decreased excitation.

Fig.5.5

Field Current RegulationOutput transistors in the final stage of the voltage regulator provide a pulse width modulated signal to the excitor field by switching the PMG current on and off. Voltage amplitude is maintained constant while the on/off pulse is varied according to excitation requirements,see Fig. 5.6.

Fig.5.6

13.5.6 Power distribution

DC System:

Parallel Operation of DC Generators:

In multi-Engined aircraft each engine will be driving its own generator and in this situation it is desirable that ‘no-break’ or uninterrupted power is provided in cases of engine or generator failure. A number of sensitive aircraft instruments and navigation devices which comprise some of the electrical loads may be disturbed and may need to be restarted or re-initialized following a power interruption. In order to satisfy this requirement, generators may be paralleled to carry an equal proportion of the electrical load between them. Individual generators are controlled by means of voltage regulators that automatically compensate for variations. In the case of parallel generator operation there is a need to interlink the voltage regulators such that any unequal loading of the generators can be adjusted by means of corresponding alterations in field current.

Fig.6.1 DC generator parallel operation

This paralleling feature is more often known as an equalizing circuit and therefore provides ‘no-break’ power in the event of a major system failure. A simplified diagram showing the main elements of DC parallel operation is at Fig.6.1.

Protection functionsThe primary conditions for which protection needs to be considered in a DC system are as follows.Reverse current. In a DC system it is evident that the current should flow from the generator to the busbars and distribution systems. In a fault situation it is possible for current to flow in the reverse direction and the primary system components need to be protected from this eventuality. This is usually achieved by means of reverse current circuit-breakers or relays. These devices effectively sense reverse current and switch the generator out of circuit thus preventing any ensuing damage. Overvoltage Protection. Faults in the field excitation circuit can cause the generator to overexcite and thereby regulate the supply voltage to an erroneous overvoltage condition. This could then result in the electrical loads being subject to conditions that could cause permanent damage. Overvoltage protection senses these failure conditions and opens the line contactor taking the generator off-line.

Undervoltage Protection. In a single generator system undervoltage is a similar fault condition as the reverse current situation already described. However, in a multi-generator configuration with paralleling by means of an equalizing circuit, the situation is different. Here an undervoltage protection capability is essential as the equalizing circuit is always trying to raise the output of a lagging generator; in this situation the undervoltage protection is an integral part of the parallel load sharing function.

AC System:

There are three main types of AC distribution system architecture used on aircraft.Split bus systemParallel Bus systemFrequency wild System

Split Bus system

This is a completely isolated twin generation system, sometimes called a non-parallel system used on twin-engine aircraft, see Fig.6.2 . Primary power is based on two main AC integrated drive generators (typically 40 kVA on each engine). An APU generator (40 kVA) is used as back-up in the event of a main integrated drive generator (IDG) failure. Note that the APU is normally a constant speed device in its own right; therefore an IDG is not required. The advantage of a split-bus system is that the generators do not need to be operating at exactly the same frequency and can be running out of phase with each other. Secondary power is derived from step-down transformers to provide 26 V AC; transformer rectifier units (TRU) provide 28 V DC for the DC busbars and battery charging. Referring to Fig.6.2, the right and left generators feed their own busbars to which specific loads are connected.

Fig.6.2 Split bus system

Each generator bus is connected to a transfer bus via transfer relays. In the event of a generator failure, the remaining generator (engine or APU) supplies essential loads. Control of the system is via a number of flight compartment switches, control breakers and relays arranged to connect and disconnect the generators and busbars.

Figure 6.3 Typical electrical power control panel features

Typical control panel features for a split bus system are shown in Fig.6.3 ; the features of this panel are: Ammeters for the main generators to indicate load Current Ground power available (blue) when external power supply is connected Ground power on/off switch to select ground power onto the aircraft Transfer bus off (amber) when the transfer relay is de-energized (either normal or transfer) Bus off (amber) both respective generator circuit breakers (GCB) and bus tie breakers (BTB) open Generator bus off (blue) if the respective GCB is open APU generator bus off (blue) APU running at 95% RPM, no power from generator.

Parallel Bus System

Introduction: A.C. generators operated in parallel means that two or more a.c. generators are connected to the same busbar and share the load. To operate a.c. generators in parallel, consideration must be given to the phase relationship and the control of voltage and torque/load. Fig. 6.4 shows an a.c. distribution system with three main generators, an APU generator and provision for supplying power from an external source. The actual layout does vary between different aircraft manufacturers.

Fig 6.4. Parallel Busbar System

AC generators are operated in parallel to provide ‘no-break’ power just like DC system. However this technique only applies to constant frequency AC generation as it is impossible to parallel frequency wild or Variable Frequency (VF) AC generators. In fact many of the aircraft loads such as anti/de-icing heating elements driven by VF generators are relatively frequency insensitive and the need for ‘no-break’ power is not nearly so important. To parallel AC machines, the control task is more complex as both real and reactive (imaginary) load components have to be synchronized for effective load sharing.

Operation A.C: The normal operating configuration is with all GCBs and BTBs closed.Failure of a main generator would result appropriate GCB opening, the busbar remaining powered via the bus tie line and BTBs. The APU generator is available for ground use and in flight as a back up. It is not normal for the APU generator to be connected in parallel with the main generators as the APU normally has no CSD/IDG fitted and therefore no torque/load control is available. The external power supply is not compatible with the main generators or the APU generator as the aircraft system has no control over the voltage or torque/load.

Before understanding how parallel operation of AC generators is obtained first we will see the effect of load on AC Generator and then see how this load is balanced for parallel operation.

Effect of load on AC Generator:

No Load:With the no load condition, there is no distortion of the magnetic field as shown in Fig.1

Fig.1 No LoadResistive Load (Unity Power Factor):Any increase in the resistive load, also referred to as REAL LOAD, causes the rotating field to twist against the direction of rotation as shown in Fig. 2. As a result there will be a tendency for the speed and frequency to reduce, the induced current opposes the motion producing it (Lenz's Law), also the terminal voltage falls slightly due to the IR loss in the stator windings.An increase in the prime mover torque will regain the original speed of rotation and therefore the original frequency while an increase in the field current maintains the output voltage.

Fig.2 Real Load

Inductive Load (Zero Power Factor, Lagging): An increase in the inductive load, also referred to as REACTIVE LOAD, results in a weaker rotating field, see Fig.3.

Fig. 3 Inductive Load

The resultant effect is a considerable loss in the total field strength and a falling terminal voltage.Increasing the field current restores the terminal voltage, but the increased field current causes an increase in the rotor temperature.Capacitive Load (Zero Power Factor): Increasing the capacitive load or reactive load results in a stronger rotating field. The rotor field and the stator fields are additive giving a stronger rotating field and therefore a higher output voltage, see Fig.4. Reducing the rotating field current controls the voltage

Fig. 4 Capacitive LoadNOTE: Aircraft a.c. generation systems normally have a lagging power factor, and inductive loads.

Further Parallel Operation: Normal operation is with the main generators connected to their individual busbars by their own GCBs which in turn are connected to the tie busbar via the BTBs. In this configuration the generators are connected in parallel and are sharing the loads equally.

Fig.6.5 Parallel operation

Consider the system shown in Fig. 6.5. the APU can power all the busbars via the BTBs and the SSB (split system breaker) with the assistance of some load shedding. One external power supply can also supply all four buses, but if a second external power supply is switched on the SSB is opened to prevent parallel operation. The APU and external power are also prevented from being connected together in parallel.

The next diagram, illustrates the need for certain conditions to be satisfied for paralleling to take place.

Fig.6.6 Requirements for ParallelingThese requirements relate to:-Frequency: The frequencies on each side of the BTB must be within specified limits. Frequency difference must be less than 6 Hz.Voltage: The voltages on each side of the BTB must be within specified limits. Voltage difference must be less than 10V.Phase Angle: The phase angles on each side of the BTB must be within specified limits. Phase angle difference must be less than 90 degrees.Phase Rotation:The phase rotations on each side of the BTB must be identical.

Should two or more generators be connected in parallel with the frequency, voltage or phase angle outside the specified limits, shock loads will be imposed on the CSDs as shown in Fig. 6.7.

Fig.6.7 Production of A Voltage Error

Lamps Bright MethodFor this method we are looking at the phase relationship between the generator A phase and the tie busbar B phase. A second lamp is also shown connected between the generator B phase and the tie busbar A phase. The ideal conditions for connecting the two supplies in parallel are with the lamps bright.

Fig..6.8 LAMP BRIGHT LOCKING IN PARALLEL: When a.c. generators are connected in parallel, then they are said to be 'locked' together, let’s see how. When two generators are connected in parallel they are rarely in perfect synchronization at the instant the GCB is closed. An error voltage is produced which causes a circulating current to flow between the generators. The generator that is lagging in phase tends to be driven as a motor while the generator that is leading in phase has a retarding torque, both of these effects are caused by the circulating current. The motor and retarding effects very quickly pull both generators into synchronization with each other. This situation also applies to multiple generators operating in parallel.

Normal Parallel Connection: Prior to starting the engines we can expect either the external power or the APU generator to be supplying the busbars via the BTBs. Now, for the next stage in the operation some aircraft are automatic while others are manual. With the automatic system the generator switches are selected to the ON position prior to starting the engines. As the engine speed approaches ground idle the generator's voltage and frequency stabilize and it is ready to come on line. The external power contactor or the APU contactor is opened and the GCB closes automatically.The manual system is similar but normally a flight engineer will select the GCB closed after the engine has started, the external power contactor or the APU contactor opens and the GCB closes.The second generator is brought on line via its GCB when its GCU is satisfied that it is safe to connect in parallel. The other generators are brought on line in a similar manner.

Crash Paralleling: To connect generators in parallel when they are out of phase can cause damage to the CSD. The flight engineer or pilot can only in extreme emergency conditions connect the generators in parallel when they are not synchronized. This is achieved by first opening the BTB, closing the GCB and then reclosing the BTB, we say that the system has been 'crashed paralleled'. Remember that our 'auto-parallel' circuit only controls the GCB and not the BTB.

CSD Shock Loads:In order that two generators may operate in parallel their voltages must be in phase with each other with reference to the load. This is shown in Fig.5.6.9 for a single phase from each generator.

Fig.6.9But the two voltages are 180 degrees out of phase with respect to each other. Open the switch in Fig.6.9 and you can see that the generators' voltages must oppose each other. If an error voltage is produced, as shown in Fig.6.7, and a circulating current is produced between the generators, it is the sudden application of this circulating current that can damage or even shear the drive of the CSD.

Load Sharing:

When a load is placed on an a.c. busbar, the nature of the load will determine the power factor of the system. Any load current, whether leading or lagging, can be thought of as having two components:-

One in phase with the voltage.One in quadrature with it.

The component in phase with the voltage is termed the real load component and the quadrature component is termed the reactive load component.

If one generator is supplying a given load it has to supply both of these components in their entirety and no question of load sharing arises, Fig.5.6.10.

Fig. 6.10When two (or more) generators are supplying the same load they must share the available real and reactive loads equally. To do this the actual currents supplied by each machine must be identical and their power factors must also be identical, Fig. 6.10 (b).

Consider the case in Fig. 6.11(a) where No. 1 generator is supplying more than its fair share of the real load and yet equal sharing of reactive loading is maintained. Under these conditions, the actual current supplied by No.1 is greater than that supplied by No.2 and No.1’s power factor has moved towards unity whilst No. 2's power factor has decreased.

Fig.6.11

In Fig..6.11(b) No.1 is supplying more than its fair share of reactive load and yet equal sharing of real load is maintained. Again, the actual current supplied by No. 1 is greater than that supplied by No.2, but now No.1’s power factor has decreased whilst No. 2's has moved towards unity.The condition can arise, Fig. 6.12(a), in which No. I supplies more than its fair share of both real and reactive loading and the power factors of the two machines can remain identical. Under this condition, however, the actual current supplied by No.1 will be vastly greater than that supplied by No.2.

Fig.6.12

Fig. 6.12(b) shows that actual currents may be equal yet power factors could be different. It will be evident, therefore, that it is necessary to provide two load sharing devices:-

One to share real load.The other to share reactive load.

Real Load DivisionDivision of real load among paralleled generators becomes necessary, because it is not possible to attain exactly identical speed governor settings on all four generator constant speed drives.Therefore, in a paralleled system, the generator which has the highest speed governor setting will carry more than its share of real load. The unbalance in real load among paralleled generators isdetected by means of current transformers and a real load division loop, whereby signals proportional to the unbalance are supplied to control devices which correct the torque on the generator rotors.

The frequency of an isolated generator is determined by the initial setting of the basic speed governor on its associated constant speed drive. Since the a.c. generators are synchronous machines, two or more generators operating in parallel will be locked together with respect to frequency, whereby the frequency of the paralleled system is that of the generator which supplies the highest frequency. If the speed governor setting on one constant speed drive is higher than others in a parallel operating system, its associated generator will motor the generators with which it is paralleled. In this case the generator with the higher speed governor setting rotates at the same speed as its constant speed drive output, but since each generator is mechanically coupled to its constant speed drive through an overrunning clutch, the generators which are being motored rotate at a speed which is higher than their associated constant speed drive output speeds.

Therefore there is less transfer of energy from the constant speed drives to the generators which are being motored. Since the energy supplied to the motored generators originates from a generator with a higher speed governor setting, this generator carries more than its share of real load and the motored generators carry less than their share of real load. To equally divide real load among parallel generators, equal amounts of energy must be supplied in the form of torque on the generator rotors. Fig.5.6.13 on the next page is a typical real load control circuit.

Real Load Division

Fig 6.13 Real Load Control Circuit

Each current transformer is connected across auxiliary contacts on the associated generator's bus tie breaker and generator breaker. Should the bus tie breaker or the generator breaker be open, the auxiliary contact will short out the current transformer so that any voltage induced will cause a current to circulate within the current transformer. Should the bus tie and the generator breakers be closed, indicating that the entire a.c. power system is operating paralleled, theauxiliary contacts are opened to allow all current transformers to be connected in a series loop where the load controller error detectors are parallel loads to the current transformers. Since all current transformers are connected in series, current will flow in the series loop as a result of voltages induced in the individual current transformers.

Fig.6.14

Consider Fig. 6.13, in which we will assume that generator No. 1 is supplying too much real load and that the instantaneous current flowing from its associated current transformer is eight units in the direction shown, and that the remaining generators are 'under' loaded to the extent that their associated current transformers each produce only four units of current.

It may be shown that the current flowing in the series loop is equal to the average current produced by all four current transformers. Using the current values assigned to the circuit it is found that the current circulating in the series loop is equal to five units, or the average current produced by all four current transformers. Fig. 6.13.Since five units of current circulates in the series loop each current transformer produces an amount which differs the average loop current, that difference will flow in parallel loads connected in the circuit. The five units of current in the loop enter the series-parallelcircuit at overload generator (No.1). The current transformer connected on the feeder line of No. 1 generator produces eight units of current, but only five units enters the series-parallel circuit for generator No.2. The difference, three units of current, flows through the error detector in the loadcontroller. A potential will be developed across the error detector. The value of the pd depends upon the value of the current flowing in the detector. The polarity of the pd depends upon the direction of the current flowing in the detector. The error detector senses the value and polarity of the error voltage and adjusts the setting of the basic governor via the electromagnet and the centrifugal flyweights. The action of the centrifugal flyweights in the constant speed drivebasic speed governor is such that increased centrifugal forces on the flyweights result in proportional decreases in constant speed drive output speed. Similarly, decreased centrifugal forces on the flyweights result in increased output speed from the constant speed drive.

In the condition shown in Fig. 6.13, No. 1 load controller will adjust the No. 1 speed governor to reduce the speed of No. 1 CSD. The No.2, 3 & 4 load controllers will increase the speed of their CSDs. No. 1 generator will shed real load and Nos. 2, 3 and 4 generators will take on more real load.

Reactive Load DivisionDivision of reactive load becomes necessary because the voltage adjustment settings on the voltage regulators are never exactly identical. Therefore, the amount of excitor current supplied from each generator will never be exactly identical and in a paralleled system, the generator which receives the greatest amount of excitor current will carry more than its share of reactive load. The unbalance of reactive load is detected by current transformers placed on phase C of each generator, (Fig. 6.15), whereby signals proportional to the generator's reactive load are delivered to the respective voltage regulators. The result is a reduction of excitation to the generator which carries more than its share of the reactive load, and an increase of excitation to the generators carrying less than their share of the reactive load.

An increase of excitor field current to an isolated generator results in a proportional increase of voltage on the generator load bus, however, should two or more generators operate in parallel, anincrease of excitor field current to one generator results in only a small increase of voltage on the paralleled buses. The increased bus voltage, caused by one over excited generator, will be delivered to all loads in the paralleled system, but since the remaining generators receive their normal amounts of excitation current, they will deliver voltages which are lower than the voltage on the paralleled buses. Since there is a difference in potential between generators, thegenerator which is over excited will not only supply the normal loads, but will also supply power to the generators operating normally.

Fig. 6.15

Thus, in a paralleled system, the generators which receive their normal amounts of excitation current supply less power when one generator becomes over excited, and are carried as additional loads by the over excited generator. Since the current circulating betweenthe over excited generator and the normally excited generators lags the voltage, the over excited generator carries more than its share of reactive load, and the normally excited generators carry less than their share. Should the bus tie and generator breakers be closed, indicating thatthe entire a.c power system is operating paralleled, the auxiliary contacts are opened to allow all current transformers to be connected in a series loop where the mutual reactor primaries act as parallel loads to the current transformers. Since all current transformers are connected in series, current will flow in the series loop as a result of voltages induced in the individual current transformers. Consider Fig.6.15.

The difference, three units of current, flows through the mutual reactor (MR), shown in Fig.6.16, in the voltage regulator in such a manner that the voltage induced in the secondary winding of the mutual reactor is in phase with the reactive current carried by the generator. Since the secondary winding of the mutual reactor is in series with the voltage regulator error detector, the added voltage will appear to the error detector as an over voltage. Accordingly, the voltage regulator causes a reduced amount of excitor current to be delivered to the over excited generator proportional to the average reactive load in the entire a.c. power system.

Fig. 6.16

The current flowing in the series loop represents the average reactive load in the a.c. power system; generators No.2, 3 and 4 carry less than their share of reactive load. The current transformers for generators No.2, 3 and 4 only produce five units of current each,which is one unit less than that which flows in the series loop. The difference flows through the mutual reactors in the respective voltage regulators, thereby causing an increased amount of excitor current to be delivered to the under excited generators proportional to the average reactive load in the a.c. power system as shown in Fig.6.17 on the next page.

Fig.6.17The ability of the load division loops to distinguish between real and reactive load is a property of the sensing devices. Reactive load division is accomplished by delivering to the voltage regulator, signals which are proportional to the generator's reactive load only. The mutual reactor in the voltage regulator serves this purpose.

Split/parallel bus system

This is a flexible load distribution system for large passenger aircraft; it provides the advantages of the parallel system and maintains isolation when needed. Primary power supply features include: one IDG per engine, two APU generators and two external power connections. Asplit system breaker links left and right sides of distribution system. Any generator can supply any load busbar; any combination of generators can operate in parallel.

Frequency Wild (Variable Speed Variable Frequency) System

The system that we are going to look at is an older system but it incorporates the basic principles that we require to learn about. Fig. 6 on the next page shows the complete system but it is only the principle that we require to remember, not the whole circuit.

A.C. Frequency Wild GeneratorThe generator in Fig. 6.18 is old in its design; it dates back prior to the use of rotating diodes mounted within the rotor. This type of generator used a commutator to obtain a d.c. current from an a.c. exciter generator which then supplied the rotating d.c. field via slip rings.

Fig.6.18 Frequency Wild Generator

The obvious disadvantage of this type of generator is the use of a commutator and slip ring which causes the additional problems of brush and commutator/slip-ring wear. Later generators are of the brushless design.

Voltage Regulation

The voltage regulator that is used in our system is a carbon pile voltage regulator. The voltage regulator controls the field F1 of the d.c. exciter generator therefore reducing the workload on the voltage regulator over large changes of speed and load. On start up, the residual magnetism in the d.c. exciter will enable the exciter to provide an initial output to the rotating field. The output voltage will build up and be sensed by the sensing transformer in the regulator unit. The Output of the 'field supply' secondary of this transformer is rectified and fed via the carbon pile to field F1. The output of the 'a.c. control' secondary is rectified and fed to the volts coil of

the carbon pile regulator. The carbon pile will gradually decompress to increase the resistance of Field F1 and control the line to line voltage at a nominal 208 V.When the TRU contactor is de-energized the regulator is said to operate in the 'a.c. sensing' mode. In a frequency wild system all circuits, other than the resistive heating loads, are supplied with d.c. It is therefore necessary to provide high power main transformer rectifier units each with an output of 500 A at 28 V. When the TRU contactor is energized it is essential to maintain the busbar at 28 V. To achieve this, the regulator voltage coil is disconnected from the 'a.c. control' secondary and connected to the busbar. The regulator will now ensure that any change in busbar voltage is sensed and corrected.

Fig. 6.19

Current CompoundingCurrent compounding provides a second field F2 with excitation current proportional to the generator load current. The compounding circuit enables small carbon pile regulators to beused with high power output generators. The ratio of the compounding current to the load current is such that the compounding current is always less than the necessary excitation current. The difference is provided by the voltage regulator.

13.5.7 Inverters, Transformers, Rectifiers

Inverters:

Inverters are used to convert direct current into alternating current. The input is typically from the battery; the output can be a low voltage (26 V AC) for use in instruments, or high voltage (115 V AC single or three phase) for driving loads such as pumps. Older rotary inverter technology uses a DC motor to drive an AC generator, see Fig. 7.1. A typical rotary inverter has a four-pole compound DC motor driving a star-wound AC generator. The outputs can be single- or three phase; 26 V AC, or 115 V AC. The desired output frequency of 400 Hz is determined by the DC input voltage. Various regulation methods are employed, e.g. a trimming resistor (R v ) connected in series with the DC motor field sets the correct speed when connected to the 14 or 28 V DC supply.

Fig.7.1 Rotary inverter schematicKey point: The desired output frequency of a rotary inverter is determined by the DC input voltage.

Modern aircraft equipment is based on the static inverter; it is solid state, i.e. it has no moving parts (see Fig.7.2). The DC power supply is connected to an oscillator; this produces a low-voltage 400 Hz output. This output is stepped up to the desired AC output voltage via a transformer. The static inverter can either be used as the sole source of AC power or to supply specific equipment in the event that the main generator has failed. Alternatively they are used to provide power for passenger use, e.g. lap-top computers. The DC input voltage is applied to an oscillator that produces a sinusoidal output voltage. This output is connected to a transformer that provides the required output voltage. Frequency and voltage controls are usually integratedwithin the static inverter; it therefore has no external means of adjustment.

Fig.7.2 Static Inverter

A typical inverter used on a large commercial aircraft can produce 1 kVA. Static inverters are located in an electrical equipment bay; a remote on/off switch in the flight compartment is used to isolate the inverter if required. Figure 7.3 shows an inverter installation in a general aviation aircraft. This particular inverter has the following features:

(a)

(b)

Fig.7.3

Inverters convert 28 VDC power into 115 VAC single-phase electrical power. This is usually required in a civil application to supply Captains or First Officers instruments following an AC failure. Alternatively, under certain specific flight conditions, such as autoland, the inverter may be required to provide an alternative source of power to the flight instruments in the event of a power failure occurring during the critical autoland phase. Some years ago the inverter would have been a rotary machine with a DC motor harnessed in tandem with an AC generator. More recently the power conversion is likely to be

accomplished by means of a static inverter where the use of high-power, rapid-switching, Silicon-Controlled Rectifiers (SCRs) will synthesize the AC waveform from the DC input. Inverters are therefore a minor, though essential part of many aircraft electrical systems.

Transformer Rectifier Units (TRUs)

Transformer rectifier units (TRU) convert 115 VAC 400 Hz 3 phase power into 28 V DC; these are often used to charge batteries from AC generators. A schematic diagram for a TRU is shown in Fig.7.4.

Fig. 7.4 Transformer Rectifier Unit

The three-phase 115/200 V 400 Hz input is connected to star-wound primary windings of atransformer. The dual secondary windings are wound in star and delta configuration. Outputs from each of the secondary windings are rectified and connected to the main output terminals. A series (shunt) resistor is used to derive the current output of the TRU. Overheat warnings are provided by locating thermal switches at key points within the TRU.

Battery ChargersBattery chargers share many of the attributes of TRUs and are in fact dedicated units whose function is purely that of charging the aircraft battery. In some systems the charger may also act as a stand-by TRU providing a boosted source of DC power to the battery in certain system modes of operation. Usually, the task of the battery charger is to provide a controlled charge to the battery without overheating and for this reason battery temperature is usually closely monitored.

Transformers:Transformers are devices that convert (or transfer) electrical energy from one circuit to another through inductively coupled electrical conductors. The transformer used as a power supply source can be considered as having an input (the primary conductors, or windings) and output (the secondary conductors, or windings). A changing current in the primary windings creates a changing magnetic field; this magnetic field induces a changing voltage in the secondary

windings. By connecting a load in series with the secondary windings, current flows in the transformer. The output voltage of the transformer (secondary windings) is determined by the input voltage on the primary and ratio of turns on the primary and secondary windings. In practical applications, we convert high voltages into low voltages or vice versa; this conversion is termed step down or step up. Circuits needing only small step-up/down ratios employ auto-transformers. These are formed from single winding, tapped in a specific way to form primary and secondary windings. Referring to Fig. 6.10(a), when an alternating voltage is applied to the primary (P 1 –P 2 ) the magnetic field produces links with all turns on the windings and an EMF is induced in each turn. The output voltage is developed across the secondary turns (S 1 –S 2) which can be connected for either step-up or step-down ratios. In practice, auto-transformers are smaller in size and weight than conventional transformers. Their disadvantage is that,

since the primary and secondary windings are physically connected, a breakdown in insulation places the full primary e.m.f. onto the secondary winding. The arrangement for a three-phase auto-transformer is shown in Fig. 6.10(b). This is a star – connected step-up configuration. Primary input voltage is the 200 V AC from the aircraft alternator; multiple outputs are derived from the secondary tappings: 270, 320, 410 and 480 V AC. Applications for this type of arrangement include windscreen heating.

13.5.8 Control and Protection

Various components are used for both control and protection of the power distribution system: Current Transformers Differential Current Protection Phase Protection Breakers/Contactors Electrical Load Control Units

Current transformersThese are used to sense current for control, protection and indication applications. The primary winding is the main heavy-duty AC feeder cable being monitored; the secondary winding is contained within a housing, see Fig. 8.1.

Fig.8.1Current Transformer

The secondary windings are in the form of inductive pick-up coils. When current IP flows in the feeder cable, the corresponding magnetic field induces current IS into the secondary windings;this is the output signal that is used by a control, protection or indication device.

Differential current protectionThis circuit detects short-circuits in AC generator feeder lines or busbars; it is a method of protecting the generator from overheating and burning out.

Assuming a three-phase AC generator is installed, each phase has its own protection circuit. For illustration purposes, the circuit for a single phase is described. Two control transformers (CT) are located at either end of the distribution system, see Fig. 8.2 . CT1 is located in the negative (earthed) connection of the generator’s output. CT 2 is located at the output from the busbar performing a monitoring function in a generator control unit (GCU). If a fault were to develop between the generator and busbar, a current IF flows to ground. The net current received at the busbar is therefore the total generator output current IT

minus the fault current (IF). The fault current flows back through the earth return system through CT 1 and back into the generator; the remaining current ( IT _ IF ) flows through CT 2 and into the loads. Current transformer CT 1 therefore detects ( IT _ IF ) _ IF which is the total generator current. Current transformer CT 2 detects ( IT _ IF );

Fig.8.2 Differential current Protection

the difference between control transformer outputs is therefore IF . At a pre-determined differential current, the generator control relay (GCR) is automatically tripped by the GCU and this opens the generator field.

Phase protection (Merz Price circuit)This circuit protects against faults between phases, or from individual phase to ground faults. Connections are shown for protection of a single phase in Fig. 8.3; a three-phase system would require the same circuit per phase. Two current transformers (CT) are located at each end of the feeder distribution line:

CT 1 monitors the current output from the generatorCT 2 monitors the current into the distribution system.

Fig.8.3 Merz Price circuit

Secondary windings of each current transformer are connected via two relay coils; these windings are formed in the opposite direction. When current flows through the feeder, there is equal current in both coils; the induced EMF is balanced, so no current flows. If a fault develops in the feeder line, current CT 1 flows (but not CT 2), thereby creating an unbalanced condition.Current flow in either of the coils opens the contacts and disconnects the feeder line at both ends.

Breakers/contactorsBreakers (sometimes referred to as contactors) are used in power generation systems for connecting feeder lines to busbars and for interconnecting various busbars. Unlike conventional circuit-breakers, these devices can be tripped on or off remotely.

Fig.8.4 Generator BreakerReferring to Fig. 8.4, they have several heavy-duty main contacts to switch power and a number of auxiliary contacts for the control of other circuits, e.g. warning lights, relays etc. The breaker is closed by an external control switch via contacts A; the coil remains energized via contacts B to ground. With the coil energized, the main and auxiliary contacts are closed and the spring is compressed. Contacts A latch the breaker closed, assisted by the permanent magnet. When a trip signal is applied (either by a fault condition or manual selection) current flows to ground in the opposite direction. The spring assists the reversed electromagnetic field and this breaks the permanent magnet latch. A Zener diode suppresses arcing of coil current across the contacts. An electrical power breaker installation is shown in Fig. 8.5

Fig 8.5 Electrical power breaker installation

Fig 8.6 Generator three-phase output and neutral wires

Fig 8.7 Typical power distributionElectronic Load Control Units (ELCUs)/Smart Contactors

Higher power aircraft loads are increasingly switched from the primary aircraft busbars by using Electronic Load Control Units (ELCUs) or ‘smart contactors’ for load protection. Like contactors these are used where normal rated currents are greater than 20 A per phase, i.e. for loads of around 7 kVA or greater. The ELCU has in-built current sensing coils that enable the current of all three phases to be measured. Associated electronics allow the device trip characteristics to be more closely matched to those of the load. Typical protection characteristics embodied within the electronics are I2t, modified I2t and differential current protection.

13.5.9 External/Ground Power.

The aircraft battery provides an autonomous means of starting the engine. Certain types of operation, e.g. cold weather and repeated starts, could lead to excessive demands, resulting in a battery that is not fully charged. An external power supply system schematic (as illustrated in Fig. 9.1 ) provides power to the aircraft, even when the battery is flat, or not installed.

Fig.9.1 External power supply schematic Fig.9.2External ground power from a battery pack

External power can be from a ground power unit or simply from a battery pack as shown in Fig. 9.2. On larger aircraft installations, a connector with three sockets supplies external power. These sockets connect with three pins on the aircraft fuselage as shown in Fig. 9.3. Different-size pins are used on the connectors to prevent a reverse polarity voltage being applied.Some aircraft installations have a ground power relay . Power can only be supplied into the aircraft via the main pins when the third (shorter) pin makes contact. The third pin is used to energize the ground power relay; this additional relay prevents arcing on the power connector as illustrated in Fig. 8.9 . External AC power is applied in a similar way, except that the three phases have to be connected via individual circuits.

Fig. 9.3 External ground power 3-pin connector

Fig. 9.4 External ground power 3-pin connector schematic