11.6 Electrical Power

• Electrical power is vital in modern aircraft for heating, lighting, providing mechanical power, and operating electronic devices.

Electrical Power Uses

• Electrical Heating: Used in galleys for ovens and anti-ice systems for probes and windows.
• Lighting: Used in the cockpit, cabin, and externally for visibility.
• Mechanical Power: Increasingly used instead of hydraulics, requiring substantial electrical energy.
• Electronic Devices: Essential and non-essential systems, ranging from cockpit displays to in-flight entertainment, rely on electricity.

Electrical Power System

• Consists of generation and distribution subsystems.
• Both subsystems are split into AC and DC components.

DC Power Sources

• DC batteries, external DC power, and DC generators are used.
• Aircraft batteries mainly supply emergency power and APU starter motor power.
• The main battery is typically a nickel-cadmium type, providing 24 Vdc.

Aircraft Battery Functions

• Supplies power for engine starting.
• Powers essential systems when ground power is unavailable.
• Acts as a buffer against transient loads, stabilizing the DC bus bar.

Battery Design and Installation

• Batteries are high-performance, with thin plates in a semi-sealed container with vents.
• Located in ventilated compartments as close as possible to the main battery buses to minimize voltage drops.

Battery Venting

• Various venting methods are used to remove harmful gases, with acid traps fitted on lead-acid types to neutralize fumes.

Battery Connectors

• Modern aircraft use quick-release connectors, often secured with copper lockwire for easy disconnection in emergencies.

Battery Trays

• Secure the batteries and collect electrolyte spillage.
• Made from electrolyte-proof material and treated to prevent corrosion.

Types of Batteries

• The two main types fitted to aircraft are Ni-Cd and lead-acid.
• Ni-Cd batteries are more common.

Ni-Cd Batteries

• Cells produce approximately 1.2 V each; a 24 V battery has cells connected in series.
• During charging, individual cell voltage rises to 1.5 V, and during discharge, it drops to 1.0 V.

Battery Maintenance

• Crucial for reliable operation.
• Requires regular workshop checks every 2000 hours.
• Line maintenance must avoid overheating during APU starts and discharging below 22 V.

Thermal Runaway

• A major concern, especially with Ni-Cd batteries.
• Strict maintenance procedures are essential to prevent it.
• Causes include separator damage, unbalanced cells, internal shorts, and incorrect electrolyte levels.

Battery Charging Methods

• Two basic methods: constant voltage and constant current.
• Constant voltage is primarily for lead-acid, while constant current is for Ni-Cd.

Constant Voltage Method

• Voltage is kept constant, with current decreasing as the battery charges.

Constant Current Method

• The current rate is constant, requiring careful monitoring to prevent overcharging.
• A battery charger is present between the DC bus bar and the battery.

Battery Testing

• Capacity testing is performed to ensure batteries can supply essential services for 30 minutes in case of power failure and require a capacity of 80%.
• Battery capacity is measured in Ampere hours (Ah).

Battery Charging Systems

• Ni-Cd batteries are recharged using 28 V from DC or separate charging units.

Temperature Monitoring

• During battery charging the batteries temperature is constantly monitored.
• The battery charger disconnects if overheating occurs.

Practical Battery Systems

• Batteries are connected to the battery bus, which powers the essential services bus in emergencies.
• Direct connections ensure constant power availability.

Charging from Ground Power

• Possible but limited due to the risk of overcharging: protection systems are included.

Battery Control Panel

• Includes switches for battery master, main battery, and auxiliary battery.

Battery indications

• Includes voltage and current, displayed on electronic displays or analogue meters.

Battery Removal and Installation

• Follow maintenance manual procedures.
• Always disconnecting the negative lead first during removal and connecting it last during installation.

Battery Precautions

• Wear PPE when handling lead-acid batteries due to the risk of sulphuric acid exposure.

Lithium-ion Batteries

• Used on aircraft like the Boeing 787.
• Consist of multiple cells, delivering high power.
• Benefits include high voltage, quick recharge, light weight, and no memory degradation.

Lithium-ion Charging

• Involves constant current, balancing, and constant voltage phases.
• Fail-safe circuitry disconnects the battery if voltage is outside the safe range.

Lithium-ion Safety

• Overheating/exceeding voltage limits can cause thermal runaway and cell rupture.
• Extinguish fires with halon or water. Store batteries in stainless steel enclosures with vent tubes.

DC Power Generation Sources

• DC Battery, External DC Power and DC Generators.

DC Generator Design

• Aircraft DC generators are usually shunt field self-excited machines with inter-poles and compensating windings.

DC Generator Requirements

• Constant voltage within limits across varying engine speeds and loads for load sharing with other generators. Achieved with voltage regulators.

DC Starter Generator

• DC starter-generators use electrical power to start engines to save weight.

DC Generator Ratings

• DC Generators are rated in Volts and Amps (e.g., 28 Vdc 100 to 200 A). Seldom rated in Watts. Their output is relatively low compared to AC.

Generator Attachment Methods

• Methods include flange mounting and manacle rings.

DC from AC

• Achieved via Transformer Rectifier Units (TRU's).

Non-Paralleled Systems

• Many aircraft use non-paralleled systems because of their complexity.

Single Engine Systems

• The generator supplies only one busbar in series, and all loads are in parallel. The busbar is supplied by the main battery until the generator comes online.

Twin-Engine DC Systems with DC Generators

• Each generator supplies its busbar, kept separate by a Bus Tie Breaker (BTB). In case of generator failure, the BTB closes to connect the busbars.

Twin-Engine DC Systems with AC Generators

• AC-powered aircraft use TRUs to supply DC power. The DC essential services BB is fed from either TRUs or the battery.

Modern Non-paralleled System

• AC generators supply AC buses, which then supply DC buses via TRUs. The RAT generator powers flight instrument buses upon generator failure.

Paralleled Systems

• Multiple-engine aircraft often use parallel power distribution where generators connect and power the DC loads together.

Paralleled System Advantage

• In the event of generator failure, buses are already connected, and the defective generator can be isolated.

Paralleled System Components

• BTBs (Bus Tie Breakers) are used to connect buses while GBs (Generator Breakers) connect and disconnect generators.

Paralleled System Operation

• Generators are connected to the battery bus and controlled by Generator Control Units (GCUs).

Main DC Power in AC Systems

• Main DC buses are powered by corresponding AC buses via TRUs.

Main DC Power Isolated Operation

• The main DC power distribution must always work in isolated operation on some aircraft. It's only closed if a TRU fails.

Essential DC Power Distribution

• Consists of AC and DC parts, supplying systems necessary for safe operation. Ensures no-break power to essential services.

Starter Generator Circuit

• Starts as a compound motor, then converts to a generator.

DC Power System Panel

• Displays important system data, including battery status (voltage and current), TRU voltages, and loadings.

Electrical Systems Control Panel

• Includes switches for manual selection and monitoring. Each generator has its control switch with On and ‘Off/Reset’.

Alternating Current (AC) Main Function

• The main function of an aircraft electrical system is to generate, regulate, and distribute power throughout the aircraft.

AC Power Sources

• AC generators, static inverters, and rotary inverters.

Most AC Electrical Power System

• Consists of a three-phase 400 Hz, 115/200 V, generating and distribution system.

Rotating Field Alternator

• The armature is stationary, and the field is rotating.

Factors that Boost Voltage

• Increasing the rotational speed of the rotor. Output can also be boosted by adding more rotor windings and increasing the rotating magnetic field strength.

Frequency

• Directly proportional to the rotational speed of the rotor and the number of pairs of rotating poles.

Most Modern Aircraft Powered by

• Constant frequency systems.

Constant Speed Drive

• Most modern civil aircraft use three-phase brushless generators, due to their obvious advantages.

CSD Consists Essentially

• Consists essentially of a hydraulic transmission with mechanical controls governing the output rotation speed.

Speed Sensor Function

• The speed sensor sends a signal to the GCU to vary the voltage and frequency accordingly until the CSDU adjusts the rotational speed of the generator rotor to its normal speed.

Oil Circuit Function

• (internal) oil is used as the hydraulic fluid for the motor-pump unit (external) oil is cooled by an oil cooler inside of the plane

The CSD is Monitored

• By circuits that alert the flight crew automatically if an operating parameter deviates from its normal range and by indications on the displays (ECAM or EICAS), that allows reading of CSD temperatures.

The CSD Under-speed

• It activates an automatic generator switching.

The Disconnect

• Decouples the input shaft from the input spline shaft in the event of a transmission malfunction.

Sources of AC

• AC generators and Direct Current (DC) Inverter

GCU

• Different manufacturers use different names for the GCU, such as Control and Protection Unit (CPU) or Control and Supervisory Unit (CSU).

AC Generation Systems

• Non-parallel, parallel, and frequency wild.

The Generator Control Unit (GCU)

• Is the major component used to perform these functions, although this differs from manufacturer to manufacturer.

Non-Parallel Systems

• Less complex than parallel systems because generator outputs are never connected to the same bus bars.

In Static Inverter System

• In normal operation, the AC power supply is providing via the static inverter.

Parallel Systems

• Phase faults stress the operated devices, shortening their lifespan, causing improper operation, and even bringing catastrophic failure.

Paralelling Requirements

• The frequency difference must be less than 6 Hz, Voltage side of the bus tie breaker must be less than 10 V, The phase angle difference must be less than 90°, The phase rotation of polyphase generators must be identical, generators must share the load on the tie bus within specified limits.

The Bus Power Control Unit (BPCU)

• Also receives data from the Generator Control Unit (GCU), various bus ties, and circuit breakers of the system.

Current Transformer (CT)

• There is no physical connection between the cables and the transformer.

To Detect Short Circuits in Areas Where Circuit Breakers are Not Usable

• The differential protection circuit is used.

The Neutral Point

• Current transformers are located inside modern generators.

It is Not Possible to Attain Identical Speed Governor Settings

• In real Load Sharing.

In the Example Shown

• The unbalance of the reactive load is detected by current transformers placed on phase C of each generator

Synoptic Display

• Indicates power flow from IDG, APU, backup generator, and external power. Shows bus status, BTB positions, and battery info.

Maintenance Pages

• Show AC/DC generation voltage, frequency, load, generator oil information and FBW output.

VSCF

• These AC generation systems eliminate the need to use a CSD unit on systems using frequency, thus saving weight.

Emergency Power Requirements

• Are laid down in CS25 and require alternate electrical backup systems in the event of loss of normal electrical power.

Introduction

• Aircraft are fitted with emergency electrical power systems that are completely independent of the normal electrical power generating system.

Emergency Power Switching

• Is fully automatic during flight while manual is always possible.

Aircraft with Two Engines and DC Generators

• Would normally have two inverters that provide the AC power supply 26 Vac and/or 115 Vac using separate buses.

Some Aircraft Have Auxiliary Power Units (APUs)

• The APU can be used to start the main engines and for pre-flight preparation of aircraft.

Hydraulically Driven Electrical Generating System Consists Of

• Constant Speed Motor/Generator (CSM/G) and Generator Control Unit (GCU).

Revision of Principles

• Factors governing the magnitude of the induced EMF:
• The strength of the magnetic field
• The number of turns in the coil or the length of the wire in the magnetic field
• The rate of change in flux

Manual Voltage Control System

• It consisted of a variable resistor wired in parallel across the generator output, a rheostat, and connected to the field circuit of the generator.

Vibrating Contactor Function

• Is very common in older DC-powered aircraft because it's cheap and effective.

When the Generator is Started And Below its Predetermined Level

• The spring-biased contacts are closed.

To Overcome of Simple Vibrating Connection

• To have a more stable voltage output

Today, Most Modern Voltage Regulation Systems are

• Solid-state, also known as transistorised.

To Limit the Current Demanded from the Generator

• The purpose of the current limiter

Purpose of Reverse Current Function

• Firstly, it places the generator online once the generator output has risen above a predetermined level. Secondly, it removes the generator from the bus bar if the generator output falls below that of the battery.

The Generator Must be

• Automatically connected to the busbar as soon as its output voltage rises to the required value.