This project aimed to develop a clear methodology for the management of partial discharge in wound components (such as machines, brakes, actuators, solenoids) and connecting components (connectors and cables) used in aerospace systems in low pressure environments. Partial discharge is difficult to detect but can have a significant impact on the lifetime of electrical systems. As such elimination of partial discharge is preferable and while this can be assessed in new insulation systems, no guidelines exist to help select test voltage requirements when ageing of insulation over the lifetime of the component is accounted for. Partial discharge elimination is not, however, always possible owing to the impact that increased insulation thicknesses can have on the volume of a component. As such partial discharge may have to be managed in a component subject to significant environmental stress throughout its lifetime.
The project delivered clear guidance on the design of components and the selection of insulation materials in a way that ensures the risk of partial discharge is minimised at the design stage. A significant test programme then supported the development of guidelines for the qualification of wound / connecting components and their test during production.
Electrical systems used in aircraft have previously been used at reasonably low voltages which have not been associated with a significant risk of electrical discharge. A move towards More Electric Aircraft (MEA) has increased the load requirements and quantity of power electronics on-board aircraft. These increased power requirements have led to an increase in system voltage, placing the components at increased risk of electrical discharges occurring. Through this push to increase performance, the requirements to ensure they operate safely and reliably remain the same. This project focuses on the means to eliminate, control and evaluate partial discharge in power electronic components.
The use of any high voltage system on an aircraft introduces a risk of damage from electrical discharge. Partial discharges are small discharges that do not completely bridge a gap between two electrodes and can occur in a range of locations, such as within voids in machine and actuator winding encapsulation systems, around sharp edged electrodes and on the surfaces of insulating material across which an electric field is present. Partial discharge is difficult to detect, yet the presence of partial discharge can reduce the service life of a system by degrading insulation.
Repetitive discharge events can cause irreversible mechanical and chemical deterioration of the insulating material. Damage can be caused by the energy dissipated by high energy electrons or ions, ultraviolet light from the discharges and ozone attacking the insulation. The chemical transformation of the dielectric can also tend to increase the electrical conductivity of any surrounding dielectric material and can increase the electrical stress in the remaining unaffected insulation, leading to an acceleration of the breakdown process.
The ideal insulation system design would see partial discharge eliminated and to some extent the above issues can be managed through careful design of converter and selection of insulation materials. However, the low air density of a low pressure / high temperature environment experienced by many machine insulation systems is still likely to make elimination of partial discharge an option that can only be achieved through significant compromise in power density.
The alternative to eliminating partial discharge is the evaluation of the insulation system to ensure it remains resilient to partial discharge over the duration of its life, i.e. partial discharge can exist without compromising operational performance. The manner in which to evaluate the ability of an aerospace insulation system to fulfil this role is, however, not clearly defined by standards which are designed to cover machines operating in a standard industrial environment.
The project sought to improve the process of developing electrical systems operating in an aerospace environment by:
- Providing clear design guidelines for the selection of insulation systems for use in wound components and connecting components for a range of environmental conditions.
- Demonstrating the key mechanisms associated with the ageing of wound components and connecting components to support best practice in the selection of insulation systems and the development of appropriate test methodologies for use during equipment qualification.
- Delivering recommendations for test procedures required to support equipment qualification and assessment during series production. These recommendations, where appropriate, will be based on existing IEC guidance in this area but will be modified to reflect the challenges associated with deployment in an aerospace system.
The main outputs of the project were a series of design guidelines and an accompanying design tool based on the knowledge acquired through extensive testing during the project. The guidelines consider a number of subjects, including, the applicability of existing standards, and recommending modifications to such standards where applicable, recommended test practices when testing at simulated altitude and recommendations on insulation system design.
The design tool was used to determine insulation specifications for a number of different applications and operating conditions, or, is used to determine the safe operating voltage of a specific insulation system. Other outputs of the project include a recommended process for the qualification of electrical insulation systems in wound and connecting components and a recommendation on product acceptance testing for the manufacture of wound and connecting components.