Implementing Interconnected Generation in Future Civil Aircraft
Power protection equipment enables detection and interruption of transients and arc faults occurring at a single point in an aircraft’s electrical network.
In addition to providing thrust, the engines on conventional civil jet airliners generate power for onboard systems and ancillary loads in the form of pneumatic, hydraulic, and electrical power. There is a move to begin replacing mechanically, hydraulically, and pneumatically powered aircraft systems with electrical equivalents, towards the ultimate goal of achieving the All-Electric Aircraft (AEA).
Until recently, a number of aircraft loads were powered pneumatically using air bled from the high-pressure compressor stages in the thrust engines. On conventional aircraft, this hot, high-pressure air is used to heat the leading edges of the wing and engine nacelle to prevent potentially dangerous ice buildup, and is also cooled and expanded to provide pressurized cabin air. This is a convenient method of supplying such loads. Compressor bleeding, however, carries a significant efficiency impact, and pneumatic systems are prone to leaks and difficult to maintain.
The Boeing 787 represents the state-of-the-art in more electric civil aircraft, and is the first civil airliner to replace most of the pneumatic systems with electric equivalents. As well as progressively replacing ancillary loads with electrical equivalents, the industry is turning towards electrical power for flight control as well.
Recent developments in fast-acting, solid-state protection equipment, together with work in the field of engine efficiency and fuel savings, provide the platform for a move towards interconnected generation. Reducing the isolation of the electrical system will necessitate more advanced and faster-acting protection strategies. By increasing the degree of interconnection, a greater proportion of the aircraft’s systems will be exposed to transients and faults occurring at a single point in the network. This increased effect will need to be mitigated by protection equipment that can detect and clear faults in the network before bus conditions become in breach of the power quality standards.
Several advancements in the field of electrical network protection equipment have been realized that may enable such protection coverage to be attained. In particular, Solid State Power Controllers (SSPCs) and Fault Isolation Devices (FIDs) offer advanced functionality that may facilitate the safe interconnection of generation. Such solid-state devices offer very fast (25 – 50 μs) operation with improved reliability and the possibility of more intelligent control. They provide a more predictable and consistent operation over a longer life because they have no moving parts; therefore, they do not suffer as much from issues with wear and tear. Unlike conventional circuit breakers, they are capable of detecting and interrupting arc faults, and can operate according to a number of profiles.
A salient requirement in an initial implementation of interconnected generation would be the provision of a minimum of two channels. This approach would be required to comply with current standards that require separate isolated supplies for critical systems such as flight control and pilots’ instruments. It would also create a stepwise path to achieving a fully interconnected system while still offering potential fuel savings and operability improvements through the inter con nection of generators connected to multiple shafts on the same engine.
Sources that are interconnected must be protected from becoming impaired or damaged by transients and faults caused by the failure of other sources. This is an explicit requirement of existing standards, but is also a sensible consideration to ensure continuity of supply and compliance with the reliability requirements. It is anticipated that in addition to use of faster-acting protection equipment, a suitably advanced protection strategy will make use of distributed local systems with authority over smaller portions of the overall network, together with careful network design with consideration to fault propagation times.
Ideally, a dynamically reconfigurable network would be realized that is capable of adapting to current load conditions and component health. This may mean that the eventual embodiment of this technology will be “dynamically interconnected,” with the network topology able to adapt to existing conditions.
It is possible for the essential electrical bus to be supplied from any of the isolated generator buses on current aircraft. A more interconnected system would necessarily reduce the available supply redundancy for the essential bus, and this would have to be taken into account when designing the interconnected architecture. It is expected that the topology of the essential system itself (with attached loads and emergency supplies) would remain unchanged, but that some appropriate interface be made with the interconnected system bus/buses.
Interconnected generation could provide the platform for a number of fuel efficiency and engine operability improvements. It could also provide a more reliable, dynamic network that is more able to meet the increasing electrical power demands of modern aircraft.
This work was done by Gordon Mackenzie- Leigh, Patrick Norman, Stuart Galloway, and Graeme Burt of the University of Strathclyde; and Eddie Orr of Rolls-Royce PLC. The full technical paper on this technology is available for purchase through SAE International at http://papers.sae.org/2013-01-2125 .