Fundamentals of Aircraft Electrical Systems: Components and Operation
To understand aircraft electrical systems, you need to know that Aircraft have tonnes of systems that need electrical power to operate. Everything from navigation to flight control to even oxygen systems is powered by the aircraft’s electrical system, making it one of the most crucial systems in aircraft today.
In this post, I will talk about:
- Components and devices that make up an aircraft’s electrical system.
- How does a typical aircraft electrical system operates under different conditions.
NB: I will be using a typical passenger aircraft such as the Airbus A320 as a reference for most of my explanations. The workings, components, and magnitude of power supply of other aircraft electrical systems may slightly differ.
Components of Aircraft Electrical Systems
Some of the main components that make up aircraft electrical systems are:
- Integrated drive generators (IDGs): there is one attached to each engine, so for a two-engine aircraft, there are two IDGs. They are engine-driven generators which are called IDGs because the Constant Speed Drive (CSD) and the generator are integrated. The High Pressure (HP) rotor blades of their corresponding engines drive the IDGs through an accessory gearbox. The CSD is a hydromechanical unit that is responsible for producing a constant speed supply for the generator from the engine which produces different frequencies/speeds. The generators are typically multi-stage and have their own oil cooling systems.
The two IDGs supply 90 KVA of 3 phase 115/200 V 400 Hz AC power to their respective AC buses which then go on to supply other components within the electrical system. The generators operate at 12,000 RPM.
- Auxiliary Power Unit (APU) Generator: it is driven by a self-contained APU which is a single-shaft gas turbine that produces mechanical power for driving a generator. The gas turbine also produces bleed air for starting the engines for the IDGs and supplies to pneumatic systems. The APU drives its generator, which is very identical to the IDG generators, at a constant rate of 24, 000 RPM. It has a gearbox that contains oil for cooling and lubricating the generator, just as in the IDGs. The APU generator produces the same AC output as both IDGs and can replace either or both at any time.
- External power: This is the ground power connection near the nose wheel. Just like the APU and IDG, it supplies 90 KVA of 3phase 115/ 200 V AC power to the system. On the ground, when just ground services are needed, external power can supply a few electrical buses without being connected to the entire aircraft electrical network. The external power employs the use of the Ground Power Control Unit (GPCU) and a central unit which is connected to the fault display unit for in-flight maintenance purposes.
- The Ram Air Turbine (RAT): This is a turbine and essentially operates like a windmill. When the RAT switch is operated in the cockpit, the belly fairing of the aircraft opens and lets down the RAT. The wind in the atmosphere causes the RAT to spin and the mechanical energy is used to supply one of the plane’s hydraulic systems.
- Transformer rectifiers: they convert the 115/200 V 400 Hz AC power from the AC electrical buses into 200 A 28 V DC supply which then goes to the DC buses. There is also the Essential TR (ESS TR) which converts the AC power supply from the emergency generator into a 28 V DC supply for the DC essential bus. The TR’s output voltage when it is not connected to any load is 30.2 V while it is 27.5 V when supplying loads with a current of 200 A.
Transformer rectifiers control their contactors with internal logic (digital system). Its contractors close when there is minimum current flowing through it and when the TR’s temperature rises above 171 degrees Celsius. Overheating of the TR is prevented by its ventilation by air from the aircraft ventilation system.
- Batteries: in an emergency, when all other power sources fail, the batteries are the last resort. The A320 aircraft has two main Ni-Cad batteries with a nominal capacity of 23 Ah 28 V. The batteries contain 20 Ni-Cad cells in a stainless-steel case. The batteries are permanently connected to two hot battery buses. Each battery has an associated Battery charge limiter (BCL) which monitors battery charging and controls the battery contactor when coupling and uncoupling. The minimum battery voltage required is about 26 V, so when the battery voltage falls below that, the BCL enables the DC battery to use a power supply from the DC Buses to charge the batteries up to 28 V, typically for a duration of fewer than 20 minutes.
The batteries can power the electrical system (essential components) for up to 30 minutes in flight. To prevent overheating of the batteries, they are ventilated by two ducts that employ the use of differential pressure between the cabin and the atmosphere. Other common battery types used in aircraft are lead-acid batteries and lithium-ion batteries.
- Static inverter: this is a component that converts the DC power supply from the battery into 1 KVA of a single phase 115 V 400 Hz AC power. When the aircraft speed is greater than 50 knots, the inverter automatically kicks in if only the batteries are supplying electricity even if the battery push buttons are off. When the speed is less than 50 knots and the push buttons are on, it kicks in.
- Electrical bus bars: these are metallic strips of bars used to connect different loads within the electrical system together. They carry substantial amounts of current over relatively short distances. This eases the distribution of electricity in the electrical network. The various buses within a typical system are the AC Buses, AC essential buses, DC buses, Hot buses, DC essential buses, and the DC battery bus.
The AC buses are directly connected to their corresponding IDGs, or APU and External Power in abnormal operation. They are the first point of contact with the generators and go on to supply the rest of the electrical network with the 115/200 V 400 Hz AC power.
The AC essential bus is connected to the static inverter which supplies 115 V 400 Hz AC power for the AC loads in the case of emergency. It is also connected to an ACC ESS SHED which performs the same function as the DC ESS SHED, but for AC loads.
DC buses make use of the 28 V DC supply from the transformer rectifiers. The DC essential bus makes use of the 28 V DC supply from the ESS TR for the DC loads in the case of an emergency. It is also connected to the DC essential shed bus which is used to supply power to some non-essential DC loads and can be shed in an emergency, in order to reduce the load on the batteries and allow them to be used for longer.
The DC battery bus is supplied 28 V DC power by either of the DC buses and can be used to charge the batteries. In an emergency, the battery bus gets its 28V DC supply from the batteries and then supplies electricity to the rest of the emergency network (essential components only). The hot buses are always connected to the batteries.
Each battery bus has its own Bus power control unit (BPCU) to control the distribution of electricity to its various loads. The General Control Unit (GCU) and BPCU work hand in hand to control the electrical power, track faults and correct them and report any of these faults to the cockpit.
- Circuit breakers: the circuit breakers (C/Bs) work as a fuse within the circuit, however unlike fuses, they do not burn off and permanently open the circuit. Their primary function is to prevent excess power from flowing into a short circuit, to prevent overheating, however, they also have another use in aircraft electrical networks. When faults happen to any of the components, the C/B kicks in by opening and therefore, disconnecting them from the rest of the electrical network. If the faults are fixed, the C/B can reconnect it to the circuit.
In the A320 electrical network, for example, there are two types of C/Bs: monitored (green) and non-monitored (black and yellow). The green circuit breakers are for the essential components of the electrical system and trigger warnings using ECAM (cockpit display) signals.
When they are out for more than 1 minute, a warning shows on the ECAM for the pilot to see and identify the defective C/B. For the black, the Wingtip brake (WTB) C/B has red caps to prevent them from being reset. The yellow C/Bs can only be reset on battery power.
- Contactors: these are either relays or solenoids which act as switches and connect or disconnect the components within the circuit by closing or opening. Some contractors are also used to tie components from one side of the electrical system to the other side in the case that any of the connecting components fail.
- Overhead panel: it contains push-button switches for the different power sources and components. When power sources fail, the pilot presses the corresponding push button switch on the overhead panel to disconnect it. The Commercial push button switch allows shedding of the galley, cabin, and commercial-related loads.
- Cockpit displays show information and warning signals when necessary for the pilots to monitor and take action.
- Various computers process commands for various parts of the electrical network.
In aviation, safety is of utmost importance. The availability of multiple sources of electrical power contributes to that safety. In the case of failure of any single source, the redundancy provided by other sources ensures there is no total failure of the electrical network. This generally improves the reliability of the electrical system. The availability of multiple generators to power the entire electrical system also provides a more efficient means of power distribution within the network.
The batteries act as shock absorbers for the electrical system by relieving the generators during the brief high drain of some systems. For example, the electric motors that power the landing gear in many airplanes can draw more current than the generators produce when they extend or retract, and the batteries supply those additional amperes. If the batteries weren’t there, bus voltage could fall so low that avionics would drop off line.
The APU is useful for many functions in the aircraft aside from power supply. It easily provides bleed air for starting the engines before take-off. During the flight, it supplies bleed air to the air conditioning systems, thereby reducing drag, increasing fuel efficiency, and eliminating the need for an external high-pressure air unit.
This supply of bleed air makes it useful in the pneumatic system. The aircraft does not need to have a different supply unit for the pneumatic and air conditioning systems because of the presence of the APU.
Operation of Aircraft Electrical System
A standard airliner aircraft system typically comprises a three-phase (3 ∅) 115/200 V 400 Hz constant frequency AC system as well as a 28 V DC system. Under normal conditions, the AC sources are the primary supply to the system and within the system, this AC supply will then be converted to DC for the operation of some of the load within the electrical system.
The main AC sources are the engine-driven generators otherwise known as IDGs. There is also an APU generator, External Power, and Emergency Generator powered by the Ram Air Turbine (RAT). These sources all supply 115V 400 Hz constant frequency AC electricity.
In the case of an emergency, when all AC power supplies have failed, the electrical system is powered by batteries that supply 28 V DC power. The system uses a split configuration in which each electricity source supplies its own network.
Now using the above A320 electrical network as a reference:
Normal Operation
Under normal conditions in flight, the IDGs, GEN 1 and 2 supply the electrical network. GEN 1 and 2 supply their corresponding AC buses 3∅ 115 V 400 Hz AC power. From AC Bus 1, the same AC power is supplied to the AC essential bus and then the AC essential shed bus.
The 3∅ 115 V 400 Hz AC power also goes directly to TR 1 so that TR1 first steps down the AC power to single-phase 28 V AC then rectifies it to 28 V DC so it can go on to DC bus 1 and then the DC battery bus whose contactor opens to charge the battery when it is lower than 28 V.
From AC bus 2, the 115 V 3∅ 400 Hz AC power is supplied to the AC GND/FLT Bus (if the contactor is open) and TR 2. The same process of stepping down and rectifying to 28 V DC happens at TR2 so that is the supply that goes to DC bus 2 and then the DC battery bus. From the DC battery bus, the 28 V DC can be supplied to the DC Essential bus and the DC Essential shed bus.
Before Take-off:
The first thing to do when in the aircraft is to ensure the batteries are fully charged. On the ground, External Power is connected to charge up the batteries to 28 V if it is not already fully charged. The External Power supplies both AC Buses 1 and 2 with 3∅ 115/200 V 400 Hz AC power and AC buses 1 and 2 deliver that power to the rest of the network.
Before getting to DC buses 1 and 2, the AC power is converted by the transformer rectifiers, TR 1 and 2, into 28 V DC of up to 200 A. The battery is then charged from the DC BUS 1 and 2 via the DC battery bus.
The two sides of the electrical network are usually powered by two different sources, but in this case, the entire electrical network is supplied by External power. For this to happen, the two sides of the network have to be connected together using the bus tie contactors. They close automatically in order to allow both AC Buses 1 and 2 to be supplied power by the same source.
If only ground services are required before the flight, the External Power directly connects to the GND/FLT buses which are not part of the main electrical network. The power is supplied directly to the AC GND/FLT bus which is connected to TR 2. TR 2 steps down and then converts the 90 KVA 3∅ 115/ 200 V AC supply from the External Power into 28 V DC power for the DC GND/FLT bus. This configuration is activated by the flight crew using the MAINT BUS switch.
The APU is then connected. It still shows a 0 % function when the APU button is turned on because external power has priority over the APU, so external power has to be turned off on the overhead panel before the APU can kick in. External power is then disconnected. The purpose of starting the APU first is that it provides bleed air for the HP rotor blades of the 2 engines to spin, enabling them to drive the generators.
To take over from the APU, the IDGs must be connected one by one so that the system is not left without power supply at any point. The bus tie contactor opens so that two sources can power their own respective networks. GEN 1 is first started by the APU, leaving the APU GEN to power the right side of system then GEN 2 is started and takes over from APU. The fault lights of both engines go off on overhead panels once turned on. The APU is then switched off when the IDGs are running.
After landing:
The APU starts when the plane begins taxiing in (after landing). The APU generator is started and available to take over from IDGs once they are disengaged. GEN 1 and 2 are put off so that APU automatically takes over.
Then external power (EXT PWR) is plugged in and shown as “AVAIL” light on the overhead panel. Then the EXT PWR automatically takes over and APU is turned off. EXT PWR is also turned off so the ECAM page goes off. Lastly, the batteries are turned off using the overhead panel.
Abnormal Operation
An IDG can fail when there is low oil pressure or overheating. If any of those happen, a warning signal is sent to those in the cockpit. If one IDG fails, the first option is to make the other IDG supply for both sides of the electrical network.
In this case, the galley load from the AC bus is automatically shed to reduce the load on the functioning IDG. The bus tie contactor closes in order to allow one source power both sides of the electrical network.Another option is to replace the failed IDG with the APU generator if it is available. In this case, the galley load will not have to be shed.
If one TR fails due to overheating or very little current flow, the other TR automatically stands in for the failed one. The ESS TR supplies the DC ESS bus with DC power. If it is TR 1 that fails, DC BUS 1 is supplied by the DC BAT BUS which is supplied by DC BUS 2 from GEN 2. If it is TR 2 that fails, DC BUS 2 is supplied by the DC BUS 1 instead via the DC BAT BUS.
If the static inverter fails, it sends a permanent ground signal to one of the Battery Charge Limiters (BCLs). The failure can occur as a result of overheating, over-voltage or under-voltage of the input and output.
Emergency Operation
When the speed of the aircraft is above 100 knots and AC Buses 1 and 2 are lost, the RAT solenoid 1 is energized by the Hot battery bus 1, causing the RAT to extend automatically. The Constant Speed Motor/ Generator Control (CSMG) Unit activates the CMSG solenoid control valve with power supply from battery 2. If the nose landing gear of the aircraft is in the up and locked position, then the emergency generator starts running.
The emergency generator supplies the AC ESS BUS with 5 KVA single-phase 400 HZ 115 V AC power supply and DC ESS BUS with 28 V DC via the ESS TR.
If the emergency generator fails, the AC ESS bus is supplied from Battery 1 with single phase 1 KVA 115 V 400 Hz AC power via the static inverter. Battery 2 supplies the DC ESS bus directly with 28 V DC.
Before flight, the emergency generators must be tested to ensure that they work. When a switch on the overhead panel is turned on, the emergency generator is powered by the relevant hydraulic system as long as its electric pump is in operation. The AC ESS BUS and the DC ESS BUS are connected to the generator automatically.
Conclusion
From this post, it is clear that aircraft electrical systems comprise a very sophisticated network in which multiple sources of power ensure the safety of the aircraft in case of any single failure.
The electrical network supplies different loads which require both AC and DC power and in the event that AC sources are in use, components within the system must convert the AC power to DC power for the DC loads to be supplied. The electrical network has measures which can be taken in the case of any abnormal operations to ensure the network is still operational.
References
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- A320dp.com. (2019). Electrical System | AC Power | Section 3.7.1. [online] Available at: http://www.a320dp.com/A320_DP/electrical/sys-3.7.1.html
- A320dp.com. (2019). Electrical System | Batteries | Section 3.7.2. [online] Available at: http://www.a320dp.com/A320_DP/electrical/sys-3.7.2.html
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