Airbus A350 AC Normal Generation Description: ATA 24 Architecture, VFGs, ATUs, APU SG & External Power

1. Airbus A350 AC Normal Generation Overview & Network Architecture

The primary function of the Airbus A350 AC normal generation system is to supply electrical power to the users on ground or in flight through four sources:

• External power from Ground Power Unit(s) (GPU(s))
• Auto Transformer Units (ATUs)
• APU generator
• Variable Frequency Generators (VFGs)

As illustrated in the technical training manual overview (Page 2), the electrical architecture is organized into two symmetrical networks: Electrical Network Side 1 and Electrical Network Side 2.

Under normal engine operation, mechanical power from each Rolls-Royce Trent XWB engine accessory gearbox is converted into 230VAC electrical power by four engine-mounted Variable Frequency Generators (VFGs):

• Engine 1 operates two generators: VFG 1A and VFG 1B, which supply Electrical Network Side 1.
• Engine 2 operates two generators: VFG 2B and VFG 2A, which supply Electrical Network Side 2.

Auxiliary power generation is provided by the APU Starter Generator (SG), which supplies 230VAC electrical power to the network when the APU is available. Ground electrical power is supplied through two dedicated External Power Receptacles connected to Ground Power Units (GPUs).

Network Voltage Distribution Layers

The overview schematic defines three distinct voltage distribution levels within the aircraft network:

• 230VAC: Supplied directly by the VFGs and the APU Starter Generator, or stepped up from 115VAC by the ATUs during GPU ground operation.
• 115VAC: Derived from the 230VAC busbars via Auto Transformer Units (ATUs) during flight and APU operations, or supplied directly from GPUs on the ground.
• 28VDC: Converted from AC power through Transformer Rectifiers (TR1 on Side 1, TR2 on Side 2) and backed up by aircraft batteries (BAT1 on Side 1, BAT2 on Side 2).

2. External Power: GPUs, Receptacles & EPDC Management

Two External Power Receptacles (EXT PWR RCPTs) are available to connect one or two 115VAC 400Hz three-phases GPUs and to supply the aircraft electrical network on ground.

The GPUs can supply all of the aircraft electrical network through the transfer circuit. The training manual establishes the following operating rules:

• Minimum GPU Rating: The minimum rating of each GPU must be 90 kVA.
• Two-GPU Operation: Both GPUs can be connected simultaneously to supply the entire aircraft electrical network.
• Single-GPU Operation & Automatic Load Shedding: If only one GPU is used, it can supply all of the aircraft electrical network, but automatic shedding inhibits some commercial (cabin) loads.
• Contactor Control: The Electrical Power Distribution Centers (EPDCs) monitor the GPUs and control the External Power Line Contactors (EPLCs).

EPDC Components and Responsibilities

The EPDC1 (Side 1) and EPDC2 (Side 2) manage and control their related external power supply. Each EPDC includes the following components:

• External Power Control Units (EPCU1 in EPDC1 and EPCU2 in EPDC2):Perform GPU monitoring and electrical protections.
• Electrical Network Management Units (ENMU1A and ENMU1B in EPDC1, ENMU2A and ENMU2B in EPDC2):Host the Electrical Network Management Functions (ENMFs) applications. Their responsibilities include:
  • Connection and disconnection of the EPLCs.
  • Management of the network configuration in relation with the availability of the power sources and with the load consumption.
• Electrical Distribution Monitoring Units (EDMU1A and EDMU1B in EPDC1, EDMU2A and EDMU2B in EPDC2):Responsible for:
  • Load shedding commands.
  • External power parameters transmission.
• CAN Interface: A Controller Area Network (CAN) interface between ENMU/ENMF is used for Built-In Test Equipment (BITE) testing.

External Power Monitoring and Connection Sequence

The EXT 1 (2) P/BSWs control the connection and disconnection of the GPUs to/from the aircraft electrical network:

1. When a GPU is available and connected to an EXT PWR RCPT, the related EPCU monitors the parameters of the electrical power supply using the Point of Regulation (POR) for voltage/frequency and Current Transformers (CT 1 and CT 2) for current.
2. If the EPCU finds no fault conditions, it sends discrete signals to the related EXT P/BSW (illuminating the green AVAIL legend) and to the external power panel to show that the external power supply is available.
3. When the related EXT P/BSW is pushed, the EPLC closes (illuminating the blue ON legend), and the GPU supplies 115VAC to the aircraft electrical network.

External Power Panel Indicators

The external power panel contains a LIGHT TEST pushbutton and status annunciator lights for both external power circuits:

• EXT PWR 1 AVAIL / EXT PWR 2 AVAIL: Illuminates amber when the GPU is connected and electrical parameters meet required tolerances.
• EXT PWR 1 NOT IN USE / EXT PWR 2 NOT IN USE: Illuminates white to indicate the GPU is available but its EPLC is not closed.

EPCU Electrical Protections

The EPCU provides protection of the external power supply for:

• Over/undervoltage on any phase at the Point of Regulation (POR)
• Open neutral
• Over/underfrequency
• Incorrect phase frequency (indicated as incorrect phase sequence in the diagram)
• Open phase
• Overcurrent

If one of these malfunctions occurs, the ENMU opens the applicable EPLC to isolate the aircraft electrical network from the GPU.

External Power Avionics Interfaces

The external power system communicates with cockpit displays and load management systems through digital networks:

• Cockpit Information: The EPCU gives the status and parameters of the external power supply to the EDMUs. The EDMUs send these data to the Control and Display System (CDS) through the AFDX network. This information is shown on the ELEC AC page of the Electronic Centralized Aircraft Monitor System Display (ECAM SD).
• Automatic Load Shedding: For overload protection through automatic load shedding, the EDMUs have an interface, through the AFDX network, with the Electrical Load Management Function (ELMF) hosted in two CPIOMs.

3. Auto Transformer Units (ATUs): Reversible Bidirectional Power Conversion

There are four identical and interchangeable main ATUs: ATU1A and ATU1B on Side 1, and ATU2B and ATU2A on Side 2. The primary function of the ATUs is to change 230VAC into 115VAC or 115VAC into 230VAC.

Properties of the ATUs

• Power: 60 kVA
• Frequency range: from 360Hz to 800Hz
• Frequency behavior: The output frequency remains the same as the input frequency.
• Interchangeability: All four main units are identical and interchangeable.

GPU Generation Configuration (Step-Up Mode on Ground)

In the ground configuration, when the GPUs supply the aircraft electrical network with 115VAC:

• The 115VAC busbar AC 1A supplies the ATU1A.
• The 115VAC busbar AC 1B supplies the ATU1B.
• The 115VAC busbar AC 2B supplies the ATU2B.
• The 115VAC busbar AC 2A supplies the ATU2A.

The ATUs then supply 230VAC voltage to the main AC busbars:

• The ATU1A supplies the 230VAC busbar AC 1A.
• The ATU1B supplies the 230VAC busbar AC 1B.
• The ATU2B supplies the 230VAC busbar AC 2B.
• The ATU2A supplies the 230VAC busbar AC 2A.

VFG/APU Configuration (Normal Step-Down Mode in Flight or Engine Run)

In normal configuration, when the VFGs or the APU generator supply 230VAC to the aircraft electrical network:

• The 230VAC busbar AC 1A supplies the ATU1A.
• The 230VAC busbar AC 1B supplies the ATU1B.
• The 230VAC busbar AC 2B supplies the ATU2B.
• The 230VAC busbar AC 2A supplies the ATU2A.

The ATUs supply 115VAC voltage to the low-voltage AC busbars:

• The ATU1A supplies the 115VAC busbar AC 1A.
• The ATU1B supplies the 115VAC busbar AC 1B.
• The ATU2B supplies the 115VAC busbar AC 2B.
• The ATU2A supplies the 115VAC busbar AC 2A.

4. Auxiliary Generation: APU Starter Generator (SG) & SGCU Management

The Auxiliary Power Unit (APU) has an equipment: the Starter Generator (SG). The SG has a start function and an electrical power-generation function. In the Generate mode (auxiliary generation), the SG supplies power to the aircraft electrical network when the APU is available.

The SG is part of the APU system. A detailed description of the SG is given in the related ATA 49 chapter.

APU SG Description & Properties

When the SG is in the Generate mode, the SG (as the VFGs) includes three generators on the same shaft in the same housing:

• A Permanent Magnet Generator (PMG)
• An exciter generator
• A main generator

The properties of the SG in Generate mode are:

• Maximum continuous power: 150 kVA on ground
• Phases: Three phases
• Voltage: 230VAC
• Frequency: 400Hz constant frequency

APU SG Management and SGCU Operation

The Starter Generator Control Unit (SGCU) manages the regulation and protection for the Generate mode of the APU SG:

• Voltage Regulation: Voltage regulation is done by adjustment of the current supplied to the exciter generator, according to current and voltage values sensed by the Current Transformers (CTs) and the Point of Regulation (POR).
• Power Ready & Line Contactor Authority: When the electrical parameters are correct, the SGCU sends the Power Ready signal to the ENMU. Then, the SGCU supplies 28VDC to the Auxiliary Generator Line Contactor (AGLC), and the Electrical Network Management Function (ENMF) hosted in the ENMU has full authority on the AGLC.
• SGCU Power Supply: The SGCU is energized from the aircraft electrical network when the SG is not in operation, or from the PMG of the SG when the SG is in operation.

Manual Cockpit Controls

Manual control of the SG is accomplished through two cockpit switches:

• APU GEN P/BSW (on ELEC panel): For the SG Generate mode, has two functions:
  1. Electrical connection/disconnection to/from the aircraft main electrical network.
  2. Reset of the SGCU protection function.
  It features FAULT and OFF legends.
• APU FIRE P/BSW (on FIRE panel): De-energizes and electrically disconnects the SG in the Generate mode.

APU SG Protection Functions

The SGCU includes the following protection functions:

• Over/undervoltage
• Over/underfrequency
• Overcurrent
• Feeder differential current (feeder differential protection)
• Open cable

APU SG Digital Avionics Interfaces

• ELMF Interface: The SGCU has an ARINC 429 interface with the Electrical Load Management Function (ELMF) hosted in two CPIOMs. The ELMF gives overload protection by automatic commercial-load shedding.
• CDS & ECAM Interface: The SGCU has an ARINC 429 interface with the AFDX network through CRDCs to send the status and parameters of the SG in the Generate mode to the CDS. The related data is shown on the ELEC AC and APU pages of the ECAM SD.
• APU ECB Interface: The SGCU has a CAN interface with the Electronic Control Box (ECB) of the APU to receive APU ready (N>95%) data.

5. Main AC Generation: Variable Frequency Generators (VFGs) & GCU / OPU Control

The AC main generation operates in normal configuration, in flight or on ground when the engines are in operation. Four Variable Frequency Generators (VFGs) change engine mechanical power into 230VAC electrical power.

Each engine gearbox operates two VFGs (VFG1A and VFG1B on Engine 1; VFG2B and VFG2A on Engine 2).

Properties of the VFGs

• Maximum continuous power: 100 kVA
• Phases: Three phases
• Voltage: 230VAC
• Frequency range: from 360Hz to 800Hz depending on the N3 rotor speed from idle to max take-off thrust.
• Interchangeability: All VFGs are identical and interchangeable.

VFG Installation: V-Band Clamp Assembly

Each VFG is installed on the engine gearbox with a V-band clamp fitting. The V-band clamp assembly consists of:

• A V-band clamp
• A V-band flange
• A tension bolt
• Mounting bolts

The V-band clamp assembly is attached to the VFG mounting integral flange.

VFG Disconnection and Reconnection Mechanisms

The VFG includes two separate disconnection mechanisms:

1. Manual Mechanical Disconnection (DRIVE P/BSW)

The DRIVE P/BSW controls the mechanical disconnection of the VFG. The FAULT legend of the DRIVE P/BSW comes on when the VFG is mechanically defective.

To disconnect the generator:

1. The cockpit crew must lift the safety guard.
2. The crew pushes the DRIVE P/BSW to energize the VFG-disconnect internal solenoid.
3. The defective VFG rotor is then mechanically disconnected from the engine accessory gearbox.

Reconnection: If the troubleshooting is satisfactory, maintenance personnel can re-connect the VFG to the gearbox using the related reset handle.

2. Automatic Thermal Disconnection

The VFG also has an automatic thermal-disconnection mechanism that operates when overheating occurs (shown in the diagram as a thermal metal alloy device).

Important Maintenance Limitation: It is not possible to do a reset after a thermal disconnection. It is necessary to remove the defective VFG and send it to the applicable workshop.

Internal VFG Construction & Management (GCU and OPU)

The VFG includes three generators on the same shaft in the same housing:

• A Permanent Magnet Generator (PMG)
• An exciter generator
• A main generator

Each Generator Control Unit (GCU) and Overvoltage Protection Unit (OPU) manage the regulation and protection of their related VFG:

• Voltage Regulation: Done by adjustment of the current supplied to the exciter generator, according to current and voltage values sensed by the Current Transformers (CTs) and the Point of Regulation (POR).
• GLC Closing Order: When electrical parameters are correct, the GCU gives the order to the OPU to close the Generator Line Contactor (GLC). The GCU sends a Power Ready signal to the ENMF (hosted in the ENMU), which gives the Ground/Open signal to the GLC. The GCU (through the OPU) has full authority on the GLC.
• GCU Power Supply: The GCU is electrically supplied from the aircraft electrical network when the VFG does not operate. When the VFG operates, the PMG supplies the GCU.

GCU Protection Functions & OPU Overvoltage Redundancy

The GCU provides the following protection functions:

• Over/undervoltage
• Over/underfrequency
• Overcurrent
• Feeder differential protection
• Open cable

Overvoltage Protection Redundancy: When the GCU is not available, the OPU supplies overvoltage protection redundancy. When an overvoltage occurs while the GCU is not available, the OPU opens the related GLC and de-energizes the applicable VFG.

Manual Cockpit Controls for the VFGs

• DRIVE P/BSW: Guarded switch controlling mechanical disconnection of the VFG via internal solenoid.
• GEN P/BSW (on ELEC panel): Controls electrical connection/disconnection of the VFG to/from the aircraft main electrical network, and resets the GCU protection function. Displays FAULT and OFF legends.
• ENG 1 (2) FIRE P/BSWs (on FIRE panel): De-energizes and electrically disconnects the related VFG.

VFG Digital Avionics Interfaces

• CPIOM ELMF Interface: Each GCU has an ARINC 429 interface with the ELMF hosted in two CPIOMs, providing overload protection by automatic commercial-load shedding.
• CDS & ECAM Interface: Each GCU has an ARINC 429 interface with the AFDX network through CRDCs to send the status and parameters of the related VFG to the CDS. Telemetry is displayed on the ELEC AC page of the ECAM SD.
• ENMU / EDMU CAN Interface: The ENMUs have a CAN interface with their related EDMUs that gives the AFDX network interface with aircraft systems. Side 1 is shown in detail, and Side 2 is symmetrical.

6. Comparative Technical Specification Matrix of A350 AC Generation Sources

The following engineering matrix compares all electrical ratings, voltage levels, frequency ranges, and control units documented across the 16 pages of Chapter 24 of the A350 Technical Training Manual:

7. Source Fidelity & System Scope Boundaries

In strict compliance with source-grounding instructions, this article is confined exclusively to the factual content contained in the A350 Technical Training Manual — Maintenance Course - T1+T2 - RR Trent XWB, Chapter 24 Electrical Power, AC Normal Generation Description (2/3):

• ATA 49 Cross-Reference: As noted on page 11 of the source document, a detailed description of the Starter Generator as part of the APU system is given in the related ATA 49 chapter.
• Emergency Power Scope: While the Ram Air Turbine (RAT Module) and emergency transformer rectifiers (EMER TR1, EMER TR2) are shown on the overview diagram (Page 2), their detailed deployment and operational mechanisms are outside the scope of the AC Normal Generation Description (2/3) module.
• Training Context: This material is based on the Maintenance Course T1+T2 for Rolls-Royce Trent XWB engines. It is provided for educational and technical understanding and does not supersede official Airbus Aircraft Maintenance Manuals (AMM).