1. Airbus A350 AC Normal Generation Overview & Dual-Network Architecture
The primary function of the Airbus A350 AC normal generation system is to supply electrical power to users on the ground or in flight through four dedicated sources defined in the technical training manual:
- External power from Ground Power Unit(s) (GPU(s))
- Auto Transformer Units (ATUs)
- APU generator (Starter Generator)
- Variable Frequency Generators (VFGs)
The electrical architecture is divided into two symmetrical networks: Electrical Network Side 1 and Electrical Network Side 2. In normal engine operation, mechanical power from each Rolls-Royce Trent XWB accessory gearbox is converted directly into 230VAC electrical energy by four engine-driven Variable Frequency Generators (VFGs). Engine 1 drives VFG 1A and VFG 1B, which supply Electrical Network Side 1. Engine 2 drives VFG 2B and VFG 2A, which supply Electrical Network Side 2.
Auxiliary generation is provided on ground or in flight by the APU Starter Generator (SG), which produces 230VAC at a constant frequency of 400Hz. On the ground, external electrical power can be supplied via two dedicated External Power Receptacles (EXT PWR RCPT 1 and EXT PWR RCPT 2) connected to 115VAC 400Hz three-phase Ground Power Units (GPUs). Emergency AC power is backed up by the RAT Module (Ram Air Turbine), while DC energy conversion is handled through Transformer Rectifiers (TR1, TR2, EMER TR1, EMER TR2) and dedicated batteries (BAT 1, BAT 2, EMER BAT 1, and EMER BAT 2).
2. External Power Management: GPUs, EPDC Architecture & Receptacle Logic
On the ground, electrical power is supplied to the A350 through two External Power Receptacles (EXT PWR RCPTs). These receptacles connect one or two 115VAC 400Hz three-phases GPUs directly to the aircraft network.
GPU Power Ratings & Load Shedding Rules
The technical training manual establishes strict power ratings and operational constraints for ground power utilization:
- Minimum Power Rating: The minimum rating of each GPU must be 90 kVA.
- Dual GPU Operation: When two GPUs are connected, each GPU feeds its respective network side (GPU 1 to Side 1; GPU 2 to Side 2) through External Power Line Contactors (EPLC1 and EPLC2), powering all aircraft systems and commercial loads without restriction.
- Single GPU Operation & Automatic Shedding: If only one GPU is used, it can supply all of the aircraft electrical network through the transfer circuit; however, the automatic shedding mechanism inhibits some commercial (cabin) loads to protect the single ground power source from overload.
EPDC Components & Internal Controllers
The management and control of external power is executed by the Electrical Power Distribution Centers (EPDC1 and EPDC2). EPDC1 controls Side 1 external power, while EPDC2 controls Side 2 external power. Each EPDC incorporates three dedicated sub-units:
- External Power Control Units (EPCU1 in EPDC1, EPCU2 in EPDC2):Responsible for continuous GPU electrical monitoring and the execution of external power protections.
- Electrical Network Management Units (ENMU1A and ENMU1B in EPDC1, ENMU2A and ENMU2B in EPDC2):These units host the Electrical Network Management Functions (ENMFs) applications. Their core responsibilities include the connection and disconnection of the EPLCs, and dynamic management of the network configuration in relation to the availability of power sources and load consumption. A CAN interface between ENMU and ENMF is used for Built-In Test Equipment (BITE) testing.
- Electrical Distribution Monitoring Units (EDMU1A and EDMU1B in EPDC1, EDMU2A and EDMU2B in EPDC2):These units transmit external power parameter telemetry and issue load shedding commands in coordination with the Electrical Load Management Function (ELMF).
EPCU Electrical Protection Functions
The EPCU continuously inspects incoming GPU electrical parameters at the Point of Regulation (POR) and via Current Transformers (CT 1 and CT 2). The EPCU provides automatic protection against six specific abnormal conditions:
- Over/undervoltage on any phase at the Point of Regulation (POR)
- Open neutral
- Over/underfrequency (monitored around the nominal 400Hz baseline)
- Incorrect phase frequency / sequence
- Open phase
- Overcurrent (sensed through CT 1 and CT 2)
Contactor Isolation Action: If one of these malfunctions occurs, the ENMU opens the applicable External Power Line Contactor (EPLC) to isolate the aircraft electrical network from the defective GPU immediately.
Cockpit and Ramp Control Interfaces
External power connection and monitoring are coordinated across two distinct human-machine interfaces:
- Cockpit EXT 1 (2) Pushbutton Switches (P/BSWs): Located on overhead panel 225VM. When a GPU is connected to an EXT PWR RCPT and the EPCU confirms no fault conditions exist, the EPCU sends discrete signals to illuminate the green AVAIL legend on the cockpit EXT P/BSW. When the flight crew pushes the switch, the EPLC closes, the blue ON legend illuminates, and the GPU supplies 115VAC to the aircraft network.
- External Power Panel 925VU: Installed on the fuselage aft of the nose landing gear bay. It features a LIGHT TEST pushbutton and two pairs of annunciator lights:
- Amber AVAIL: Indicates the GPU is connected, its electrical parameters are correct, and power is available.
- White NOT IN USE: Indicates the GPU is plugged in and available, but its EPLC is open and it is not currently supplying the network.
Avionics Telemetry & Automatic Load Shedding (ELMF)
For flight deck monitoring, the EPCU transfers the status and electrical parameters of the external power supply to the EDMUs. The EDMUs transmit this telemetry to the Control and Display System (CDS) across the AFDX network. The data is displayed in real time on the ELEC AC page of the ECAM System Display (SD).
For overload protection through automatic load shedding, the EDMUs interface through the AFDX network with the Electrical Load Management Function (ELMF), which is hosted in two Core Processing and Input/Output Modules (CPIOMs). When electrical demand exceeds threshold capacity (such as single-GPU operations), the ELMF automatically sheds non-essential commercial loads to protect primary flight deck and avionics systems.
3. Auto Transformer Units (ATUs): Reversible Bidirectional Power Conversion
The Airbus A350 employs four identical and interchangeable main ATUs: ATU 1A, ATU 1B, ATU 2B, and ATU 2A. The primary function of the ATUs is bidirectional voltage transformation: to change 230VAC into 115VAC or 115VAC into 230VAC.
Hardware Properties of the ATUs
The technical training manual specifies the following baseline electrical parameters for each of the four main ATUs:
- Continuous Power Rating: 60 kVA per unit
- Frequency Range: 360 Hz to 800 Hz
- Frequency Conservation: The output frequency remains exactly the same as the input frequency (variable in flight, constant on ground).
- Physical Interchangeability: All four main units are identical and fully line-interchangeable.
Configuration 1: GPU Ground Configuration (Step-Up Mode)
In the ground configuration, when Ground Power Units supply 115VAC to the aircraft electrical network:
- The 115VAC busbar AC 1A supplies ATU 1A.
- The 115VAC busbar AC 1B supplies ATU 1B.
- The 115VAC busbar AC 2B supplies ATU 2B.
- The 115VAC busbar AC 2A supplies ATU 2A.
Acting in step-up mode, the ATUs supply 230VAC voltage to energize the high-voltage main busbars:
- ATU 1A supplies the 230VAC busbar AC 1A.
- ATU 1B supplies the 230VAC busbar AC 1B.
- ATU 2B supplies the 230VAC busbar AC 2B.
- ATU 2A supplies the 230VAC busbar AC 2A.
Configuration 2: Normal VFG / APU Configuration (Step-Down Mode)
In the normal flight or engine-running configuration, when the Variable Frequency Generators (VFGs) or the APU generator supply 230VAC to the aircraft network:
- The 230VAC busbar AC 1A supplies ATU 1A.
- The 230VAC busbar AC 1B supplies ATU 1B.
- The 230VAC busbar AC 2B supplies ATU 2B.
- The 230VAC busbar AC 2A supplies ATU 2A.
Acting in step-down mode, the ATUs supply 115VAC voltage to energize the low-voltage distribution busbars:
- ATU 1A supplies the 115VAC busbar AC 1A.
- ATU 1B supplies the 115VAC busbar AC 1B.
- ATU 2B supplies the 115VAC busbar AC 2B.
- ATU 2A supplies the 115VAC busbar AC 2A.
4. Auxiliary Generation: APU Starter Generator (SG) & SGCU Control Logic
The auxiliary power unit contains dual-purpose equipment: the Starter Generator (SG). The SG has a start function (driving turbine rotation for engine start) and an electrical power-generation function. In the Generate mode (auxiliary generation), the SG supplies power to the aircraft electrical network whenever the APU is available.
Internal Construction of the APU Starter Generator
When the SG is in the Generate mode, it incorporates three distinct generators on the same shaft within the same housing:
- Permanent Magnet Generator (PMG): Provides independent excitation power.
- Exciter Generator: Regulates the magnetic field based on control current adjustments.
- Main Generator: Delivers the primary 230VAC power output to the contactor network.
The properties of the APU Starter Generator in Generate mode include:
- Maximum Continuous Power: 150 kVA on ground
- Phase Output: Three phases
- Voltage: 230VAC
- Frequency: 400Hz constant frequency (governed by APU turbine speed control)
SGCU Regulation & Contactor Authority
The Starter Generator Control Unit (SGCU) manages voltage regulation and electrical protection for the Generate mode of the APU SG:
- Voltage Regulation: Executed by continuously adjusting the current supplied to the exciter generator according to real-time current and voltage values sensed by Current Transformers (CTs) and the Point of Regulation (POR).
- Power Supply to SGCU: The SGCU is energized from the aircraft electrical network when the SG is not in operation, or directly from the PMG of the SG once the machine is rotating and in operation.
- Contactor Authority (AGLC): When electrical parameters are confirmed within limits, the SGCU sends a Power Ready signal to the ENMU. Then, the SGCU supplies 28VDC to the Auxiliary Generator Line Contactor (AGLC). The ENMF hosted in the ENMU has full authority on the AGLC to connect APU power to the main network.
SGCU Protection Functions
The SGCU monitors the auxiliary generation feeder line and provides automatic protective trips for:
- Over/undervoltage
- Over/underfrequency
- Overcurrent
- Feeder differential current
- Open cable
Flight Deck Controls & Avionics Digital Bus Interfaces
Cockpit control and digital integration of the APU generator comprise several dedicated channels:
- APU GEN P/BSW: Located on the overhead electrical panel. For SG Generate mode, it serves two functions: electrical connection/disconnection to/from the aircraft main electrical network, and manual reset of the SGCU protection function.
- APU FIRE P/BSW: Located on the overhead fire panel. Pushing the switch de-energizes and electrically disconnects the SG in Generate mode.
- CAN Interface to ECB: The SGCU has a CAN interface with the Electronic Control Box (ECB) of the APU to receive the APU ready status signal when turbine speed exceeds 95% (N > 95%).
- ARINC 429 Interface to ELMF: Communicates with the Electrical Load Management Function hosted in two CPIOMs to trigger automatic commercial-load shedding if total aircraft load threatens auxiliary generator capacity.
- ARINC 429 Interface via CRDCs to AFDX & CDS: Transmits generator parameters and status through Common Remote Data Concentrators (CRDCs) onto the AFDX network to the CDS, displaying synoptic data on the ELEC AC and APU pages of the ECAM System Display (SD).
5. Main AC Generation: Variable Frequency Generators (VFGs) & Mechanical Drives
The AC main generation operates in normal configuration, in flight or on the ground when the engines are operating. Four Variable Frequency Generators (VFGs) change engine mechanical power directly into 230VAC electrical power.
VFG Allocation & Technical Specifications
Each Rolls-Royce Trent XWB engine accessory gearbox operates two VFGs (Engine 1 drives VFG 1A and 1B; Engine 2 drives VFG 2B and 2A). Key technical parameters specified in the source document include:
- Continuous Power Rating: 100 kVA maximum continuous power per VFG
- Phases & Voltage: Three phases, 230VAC
- Operating Frequency Range: 360 Hz to 800 Hz, directly proportional to the engine high-pressure spool speed (N3 rotor speed) from engine idle to maximum take-off thrust.
- Interchangeability: All four VFGs are completely identical and interchangeable across all engine positions.
Mechanical Installation: V-Band Clamp Assembly
Each VFG is installed on the engine accessory gearbox mounting pad using a specialized V-band clamp fitting. The V-band clamp assembly consists of four precision components:
- A V-band clamp
- A V-band flange
- A tension bolt
- Mounting bolts
The V-band clamp assembly attaches directly to the VFG mounting integral flange to secure the generator to the gearbox pad.
VFG Disconnection & Reconnection Mechanisms
The A350 VFG incorporates two independent mechanical disconnection mechanisms to protect the engine accessory gearbox:
1. Manual Mechanical Disconnection (DRIVE P/BSW)
The guarded DRIVE P/BSW on the cockpit overhead panel controls the manual mechanical disconnection of the VFG. When the VFG becomes mechanically defective, the amber FAULT legend of the DRIVE P/BSW illuminates. The cockpit crew lifts the red safety guard and pushes the switch. This energizes the VFG-disconnect internal electric actuating solenoid, causing the defective VFG rotor to mechanically disconnect from the engine accessory gearbox (illuminating the amber DISC legend).
Maintenance Reconnection: If ground troubleshooting confirms satisfactory conditions, maintenance personnel can manually re-connect the VFG to the gearbox using the dedicated reset handle on the generator casing.
2. Automatic Thermal Disconnection (Eutectic Alloy)
The VFG also features an automatic thermal-disconnection mechanism that triggers autonomously if severe internal overheating occurs. A thermal device (eutectic metal alloy pin) melts under excessive friction or bearing heat, decoupling the drive shaft.
Internal Generator Construction & GCU / OPU Dual Management
Like the APU Starter Generator, each VFG contains three machines on the same shaft in the same casing: a Permanent Magnet Generator (PMG), an exciter generator, and a main generator. Control, voltage regulation, and system protection are managed by two dedicated line-replaceable units for each VFG:
- Generator Control Unit (GCU): Regulates output voltage by adjusting exciter generator field current based on values sensed at Current Transformers (CTs) and Point of Regulation (POR) sensors. It is powered by the aircraft electrical network when stopped, and self-powered by the internal PMG once rotating.
- Contactor Authority (GLC): When electrical parameters are verified correct, the GCU orders the Overvoltage Protection Unit (OPU) to close the Generator Line Contactor (GLC). The GCU transmits a Power Ready signal to the ENMF (hosted in the ENMU), which provides the Ground/Open signal to the GLC. The GCU (through the OPU) retains full authority over the GLC.
- GCU Protection Functions: The GCU protects against over/undervoltage, over/underfrequency, overcurrent, feeder differential protection, and open cable.
- Overvoltage Protection Unit (OPU) Redundancy: The OPU provides autonomous hardware 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.”
VFG Cockpit Controls & Avionics Architecture
Manual flight deck management of the main generation system is governed through three cockpit switchgear assemblies:
- DRIVE P/BSW: Guarded switch for mechanical rotor disconnection (FAULT / DISC).
- GEN P/BSW: Electrical connection and disconnection to/from the main network and reset of the GCU protection function (displays FAULT and OFF legends).
- ENG 1 (2) FIRE P/BSWs: Pushing the fire switch de-energizes and electrically disconnects the related VFGs on that engine.
For avionics reporting, each GCU communicates via ARINC 429 to the ELMF hosted in two CPIOMs for automatic commercial-load shedding during electrical overload. Each GCU also connects via ARINC 429 through Common Remote Data Concentrators (CRDCs) to the AFDX network to provide telemetry to the CDS for display on the ELEC AC page of the ECAM System Display (SD). The ENMUs have a CAN interface with their related EDMUs that provides the AFDX network interface with aircraft systems.
6. Comparative Technical Specification Matrix of A350 AC Generation Sources
The following engineering matrix compiles all technical parameters, voltage thresholds, power ratings, and control authorities explicitly documented in Chapter 24 of the A350 Technical Training Manual:
| Parameter / Function | External Power (GPUs) | Auto Transformer Units (ATUs) | APU Starter Generator (SG) | Variable Frequency Gen (VFGs) |
|---|---|---|---|---|
| Equipment Quantity | 2 Receptacles (1 or 2 GPUs) | 4 Identical & Interchangeable | 1 Starter Generator | 4 (2 per engine gearbox) |
| Power Rating | ≥ 90 kVA minimum per GPU | 60 kVA per ATU | 150 kVA continuous on ground | 100 kVA continuous per VFG |
| Nominal Voltage | 115VAC (3-phase) | 230VAC ⇄ 115VAC (Bidirectional) | 230VAC (3-phase) | 230VAC (3-phase) |
| Operating Frequency | 400 Hz constant frequency | 360 Hz to 800 Hz (Pass-through) | 400 Hz constant frequency | 360 Hz to 800 Hz (Tied to N3 rotor) |
| Dedicated Controller | EPCU (in EPDC1 & EPDC2) | Static passive magnetic device | SGCU (Starter Gen Control Unit) | GCU + OPU (for each VFG) |
| Main Line Contactor | EPLC 1 & EPLC 2 | ATULC 1A, 1B, 2B, 2A | AGLC (Aux Gen Line Contactor) | GLC 1A, 1B, 2B, 2A |
| Overvoltage Redundancy | EPCU trips EPLC via ENMU | N/A | SGCU internal protection logic | Dedicated hardware OPU trips GLC |
| Load Shedding Logic | Single GPU: Inhibit commercial loads | Transparent to shedding | ARINC 429 to CPIOM ELMF | ARINC 429 to CPIOM ELMF |
| Disconnection Means | EXT P/BSW or plug removal | Bus contactor isolation | APU GEN P/BSW & FIRE switch | Manual DRIVE switch / Thermal pin |
| ECAM Page Display | ELEC AC page | ELEC AC page | ELEC AC page & APU page | ELEC AC page |
7. Source Fidelity & System Scope Boundaries
In strict accordance with technical documentation guidelines, the factual scope of this article is confined to the contents of 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, internal aerodynamic, mechanical, and fuel control details of the auxiliary power unit gas turbine engine are formally cataloged in the related ATA Chapter 49 manual.
- Emergency Generation Scope: The Ram Air Turbine (RAT Module) and emergency transformer rectifiers (EMER TR) are shown in the overall network topology (Pages 2 and 4), but their detailed deployment mechanisms and hydraulic stowage procedures belong to specialized emergency electrical/hydraulic sections (ATA 24/29).
- Approved Maintenance Notice: This article is published exclusively for engineering education and supplementary maintenance analysis. It does not supersede approved Airbus Aircraft Maintenance Manuals (AMM), Illustrated Parts Catalogs (IPC), or Troubleshooting Manuals (TSM).