Aircraft Electrical Systems: A320/A321 Briefing

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Aircraft Electrical Systems: A320/A321 Briefing Document

1. Introduction to the A320/A321 Electrical Power System

The A320/A321 aircraft utilizes a sophisticated electrical power system designed for reliability and redundancy. It primarily consists of a "three-phase 115/200 V 400 Hz constant-frequency AC system and a 28 V DC system." Electrical transients are managed to be acceptable for equipment, with commercial supply having secondary priority.

The system is designed for both normal and abnormal operations, ensuring continuous power supply to critical aircraft systems.

2. Generation of Electrical Power

2.1. AC Generators

The primary source of AC power comes from two engine-driven generators (GEN 1 and GEN 2), each capable of supplying 90 kVA of three-phase 115/200 V 400 Hz power. Two Generator Control Units (GCU) manage these generators, performing the following functions:

  • "Control the frequency and voltage of the generator output."
  • "Protect the network by controlling the associated generator line contactor."

2.2. APU Generator and External Power

  • APU Generator (APU GEN): A third generator, driven by the Auxiliary Power Unit (APU), can provide the same output as an engine generator. It can replace either or both main engine generators and connects to the ground power connector for nosewheel ground power.
  • External Power (EXT PWR): An external ground power unit can supply the complete system. External power has priority over the APU generator.

Both the APU Generator Control Unit (GPCU) and the Ground and Auxiliary Power Unit Control Unit (GAPCU) control the APU generator. The GAPCU "Regulates, via the APU Electronic Control Box, the frequency and voltage of the APU generator" and "Protects the network by controlling the external power contactor and the APU generator line contactor."

2.3. Emergency Generator

An emergency generator, identified by a "blue hydraulic circuit," automatically supplies emergency AC power to the aircraft electrical system if all main generators fail. This generator provides 5 kVA of three-phase 115 and 200 V 400 Hz power. The Emergency Generator Control Unit (GCU) performs similar functions to the main GCUs, including keeping "the emergency generator at a constant speed," controlling "the generator's output voltage," and protecting "the network by controlling the emergency generator line contactor."

2.4. Static Inverter

A static inverter converts DC power from Battery 1 into one KVA of single-phase 115 V 400 Hz AC power, which is then supplied to the AC essential bus. This occurs when the aircraft speed is above 50 kt, the inverter is activated, and if the batteries are supplying electrical power to the aircraft with BAT 1 and BAT 2 pushbuttons on.

2.5. DC Generation

DC power is primarily supplied by three main transformer rectifiers (TR 1 and TR 2, and TR Entertainment), which convert the aircraft's AC electrical system to 200 A of DC current. An "ESSENTIAL Transformer Rectifier (ESS TR)" provides the essential DC circuit in the event of failure of an engine and APU generator, or failure of TR 1 or TR 2. The TR Entertainment powers the DC Entertainment bus bar.

2.6. Batteries

The A320/A321 uses two main batteries, each with a normal capacity of 23 Ah, permanently connected to the two hot buses. Each battery has a Battery Charge Limiter (BCL) that monitors battery charging and controls its contactor. The batteries are generally disconnected in normal configuration.

3. Circuit Breakers (C/Bs)

The aircraft has two types of circuit breakers:

  • Monitored (green): When open for more than 1 minute, these C/Bs trigger a caution on the ECAM.
  • Non-monitored (black).

Wing Tip Brake (WTB) C/Bs have red caps to prevent them from being reset. The ECAM displays the location of a tripped C/B. Flight crew can clear an ECAM C/B TRIPPEd caution by pressing the EMER CANC pb. If the C/B remains pulled, additional tripped C/Bs will not trigger a caution.

4. Normal Configuration and Operations

4.1. In Flight

Each engine-driven generator supplies its associated AC BUS (1 and 2). TR 1 normally supplies DC BUS 1, DC BAT BUS, and DC ESS BUS. TR 2 normally supplies DC BUS 2. The two batteries supply the DC BAT BUS.

4.2. On Ground

On the ground, either the APU generator or external power supplies the complete system. When external power is required, personnel must select "the MAIN T BUS switch in the forward entrance area."

5. Abnormal Configurations

The system incorporates automatic reconfigurations to maintain power supply during various failures:

5.1. Failure of One Engine Generator

The system automatically replaces a failed generator with:

  • APU GEN, if available.
  • Other engine generator. The "Galley Load Automatic Shedding" function automatically sheds the galley load and the DC BUS Entertainment.

5.2. Failure of AC BUS 1

AC BUS 2 can supply AC ESS BUS and ESS TR can supply DC ESS BUS, with automatic AC ESS FEED pb switching. DC BUS 2 supplies DC BUS 1 and DC BAT BUS automatically after 5 seconds.

5.3. Failure of One TR

The contactor of the faulty TR opens automatically, due to overheat or minimum current. The other TR automatically replaces the faulty one, with the ESS TR supplying the DC ESS BUS.

5.4. Failure of TR 1 and TR 2

If both TR 1 and TR 2 are lost, "DC BUS 1, DC BUS 2, and DC BAT BUS are lost. The DC ESS BUS is supplied by the ESS TR."

5.5. Emergency Generation after Loss of All Main Generators

If both AC BUS 1 and AC BUS 2 are lost and the aircraft speed is above 100 kt, the Ram Air Turbine (RAT) extends automatically, powering the blue hydraulic system, which drives the emergency generator via a hydraulic motor. This generator supplies the AC ESS BUS and the DC ESS BUS via the ESS TR. When the RAT stalls, the blue hydraulic system no longer supplies the emergency generator. The emergency generation network automatically transfers to the batteries and static inverter. The system automatically sheds the AC SHED ESS and DC SHED ESS buses.

On ground:

  • Below 100 kt, the DC BAT BUS is automatically connected to the batteries.
  • Below 50 kt, the AC ESS BUS is automatically shed, leading to the loss of all display units.

Notes:

  1. "During RAT extension and emergency generator coupling (about 8 s), the batteries power the emergency generation network."
  2. "On ground, if only the batteries are powering the emergency generation network, APU start is only available when the speed is below 100 kt."

5.6. Flight with Batteries Only

In this scenario, the batteries are the sole source of electrical power. The system sheds the AC ESS BUS and DC ESS BUS automatically.

5.7. On Ground, Batteries Only (Speed < 50 kt)

Similar to flight with batteries only, the batteries power essential systems.

5.8. Smoke Configuration

In smoke configuration, the main busbars are shed. The electrical distribution is similar to the emergency electrical configuration (loss of main generators), except "the fuel pumps are connected upstream of the GEN 1 line connector." Approximately 75% of electrical equipment is shed, with remaining power supplied via circuit breakers on the overhead panel.

6. Controls and Indicators

6.1. Overhead Panel

The overhead panel contains various pushbuttons and indicators for monitoring and controlling the electrical system, including:

  • BAT 1(2) ind and pb-sw: Displays battery voltage and controls the battery charge limiter.
  • IDG 1(2) pb-sw (GUARDED): Disconnects the Integrated Drive Generator (IDG) from its driveshaft.
  • GEN 1(2) pb-sw: Energizes/de-energizes the generator field and closes/opens line contactors.
  • APU GEN pb-sw: Energizes/de-energizes the APU generator field and closes/opens line contactors.
  • BUS TIE pb: Automatically opens or closes to maintain power supply to AC BUS 1 and 2.
  • AC ESS FEED pb: Manually or automatically selects AC BUS 1 or 2 to supply the AC ESS BUS.
  • GALLEY pb OR GAL Y & CAB pb: Controls power to the main and secondary galleys, in-seat power supply, and In-Flight Entertainment (IFE) system. "The main galley, the in-seat power supply and the IFE systems are automatically shed: In flight: When only one generator is operating. On ground: When only one generator is operating. (All galleys are available when the APU GEN or EXT PWR is supplying power.)"
  • EXT PWR pb: Indicates external power availability and closes/opens the external power line contactor.
  • COMMERCIAL pb: Supplies or switches off commercial electrical loads (cabin and cargo lights, galley, passenger entertainment system, etc.).
  • EMER ELEC PWR (Overhead Panel Cont'd): Includes the EMER GEN TEST pb, RAT & EMER GEN pb (MAN ON and AUTO guarded), FAULT light, and EMER GEN TEST pb. The "MAN ON pb (guarded)" selects manual RAT extension and emergency generator connection when AC BUS 1 and 2 are not supplied, and aircraft speed is greater than 100 kt.

6.2. External Power Panel

This panel features "EXT PWR NOT IN USE" and "EXT PWR AVAIL" lights, indicating whether ground power is not supplying the network or if external power is available with correct voltage.

6.3. Forward Cabin (MAINT BUS SW)

This switch allows personnel to energize electrical circuits for ground servicing without energizing the entire aircraft electrical system. It energizes the AC and DC GND/FLT buses, powering various loads like passenger compartment lighting, galley lighting, lavatory lighting, vacuum cleaner sockets, and refueling systems.

6.4. ECAM ELEC Page

The ECAM ELEC page provides a visual representation of the electrical system's status, including:

  • Battery indications (BAT 1(2)): Voltage and current. Normal voltage is 28V. Amber indications if voltage is below 25V or above 31V, or in case of a BAT FAULT alert.
  • DC BAT indication: Normal or abnormal voltage (below 25V).
  • Bus bar indication (AC 1, AC ESS, AC 2): Indicates if a bus is powered or shed.
  • TR 1(2) indication: Voltage and current. Normal voltage is 28V, normal current is 150A. Amber indications if voltage is below 25V or above 34V, or if current is below 5A.
  • ESS TR indication: Normal operation (28V, 130A) or contactor open.
  • EMER GEN indication: Normal operation (116V, 400Hz). Amber indications for voltage below 110V or above 120V, or frequency below 390Hz or above 410Hz.
  • GEN 1(2) indications: Status (OFF/ON), load (%), voltage (116V), and frequency (400Hz). Amber indications for load above 100%, voltage below 110V or above 120V, or frequency below 390Hz or above 410Hz.
  • APU GEN indications: Same as GEN 1(2).
  • EXT PWR indications: Availability, voltage (116V), and frequency (400Hz). Amber indications for voltage below 110V or above 120V, or frequency below 390Hz or above 410Hz.
  • STAT INV indications: Voltage (115V) and frequency (400Hz). Amber indications for voltage below 110V or above 120V, or frequency below 390Hz or above 410Hz.
  • IDG indications: Temperature, oil pressure, and disconnection status. Amber for oil outlet temperature overheat (above 180°C), low oil pressure, or IDG disconnection.
  • GALLEY SHED indication: Appears when the GALLEY pb-sw is OFF, or when the main galleys are shed due to single generator operation.
  • RISE indication: Appears when there is a significant temperature difference between the IDG inlet and outlet.

6.5. Memo Display

The "EMER GEN" memo appears in green when the emergency generator is running.

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