Community Research and Development Information Service - CORDIS

FP7

PDMAN Report Summary

Project ID: 338528
Funded under: FP7-JTI
Country: United Kingdom

Final Report Summary - PDMAN (Partial Discharge Management In Compact Insulation Systems)

Executive Summary:
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 are 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 is 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.

Project Context and Objectives:
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. However, given the increase in system voltage needed for the increasing loads on the more electric aircraft, these are now at increased risk of electrical discharge, while the pressure to ensure they operate safely and reliably remains.
This project focuses on the means to eliminate, control and evaluate partial discharge in power electronic components. As detailed in the call document, power electronic components are taken to be both wound components (including motors, actuators/brakes and solenoids) and the accompanying connecting components (i.e. the cable harnesses and connectors that are their means of connection to power electronic converters).
The use of any high voltage system on an aircraft introduces a risk of damage from electrical discharge. This risk is exacerbated by low pressure, humid and variable temperature environments. Electrical discharge can be subdivided into three categories; disruptive discharge, partial discharge and tracking. The risk from all of these must be considered in the design of any insulation system. As aerospace power systems move to higher voltages, these forms of discharge must be considered in their design. At present, there is little guidance in this area and SAE have recognised this through the development of an Aerospace Information Report ‘High Voltage Design Guidelines for Aerospace Systems’.
The consideration of partial discharge is a critical issue for insulation systems and particularly those used in low pressure environments where partial discharge inception voltages are usually lower than those observed at atmospheric pressure. 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 tends 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 elimination of partial discharge from an electrical system is usually dependent on the management of the maximum electric field through the use of increased insulation thickness at higher voltages and increasingly careful control of electrode geometries.
Wound components generally employ a number of different insulation materials to deliver reliability in service. Taking a typical electrical machine used at the voltage levels typical of a more-electric aircraft as an example, a thin insulation layer in the order of 50μm would be placed on the winding wire while slot liners and phase separators with thicknesses in the order of 200μm would provide extra phase to earth and phase to phase capability respectively. However, these thin insulation layers are not enough to prevent partial discharge at relatively modest values of voltage. IEC 60034-18-41 subdivides electrical machines into two categories; Type 1 machines which typically operate below 700 Vrms and in which partial discharge can be eliminated and Type 2 machines typically operating above 700 Vrms and in which partial discharge cannot be eliminated but must be managed. It is important to note that the voltage limit of 700 Vrms is not likely to be applicable in the low pressure environment of many aerospace systems.
Partial discharge could be eliminated from any wound component but this would usually require an increase in insulation size sufficient to result in a major reduction in the percentage of slot area (or similar) filled with copper (often referred to as the packing factor). Elimination of partial discharge would therefore be at the expense of a reduction in power density leading to a solution that was overly large / heavy. The ability to eliminate partial discharge from the insulation system of a wound component is further complicated by a number of factors which include:
• Converter fed systems can see an increase in peak voltage magnitude (possibly up to 2x) on the leading edge of a voltage transition leading to the insulation system being stressed with a higher voltage than would otherwise be expected if the DC bus voltage was considered
• A fast rise time on the leading edge of a voltage transition will cause a non-uniform voltage distribution across the winding with the first two turns potentially being subject to the full voltage being applied across the winding. This imposes a significant voltage stress across an extremely small thickness of insulation commonly leading to an insulation failure of the first turns of a winding
• The insulation systems (both the turn insulation and encapsulant) are vulnerable to space charge accumulation which means the electric field observed in the insulation system is not just related to the instantaneous voltage but also to the voltage that has previously stressed the system. For example, a system fed from a +/-270 Vdc bus would need to be designed to consider a minimum voltage stress equivalent to 540 V.
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.
While the wound components used in an aerospace system are of major concern in terms of partial discharge, it is usually relatively straightforward to eliminate partial discharge from the connecting components. As with a wound component, partial discharge elimination is usually a matter of selecting appropriate insulation thicknesses. In the case of connecting components, space is not as significant a driver with insulation thicknesses partly being based on mechanical robustness. A number of standards (both MIL and SAE) exist that would guide a system designer to appropriate component selection but it is still recognised that some questions remain, namely the impact of squarewave voltages (either unipolar or bipolar in nature), the impact of space charge and the manner in which these components deteriorate over the lifetime of the system.
This project intends to progress the state of the art in aerospace power electrical component design by delivering the following outputs / innovations:
• Providing clear design guidelines for the selection of insulation systems for use in wound components and connecting components for a range of environmental conditions along with illustrative guidance on how the choice influences the power density of the component
• 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.

Project Results:
The main achievements of the project are outlined below:
Design Guidelines: The main output of the project are a series of design guidelines based upon the observations made whilst performing work packages 2 and 3. Work package 2 worked to benchmark the PD performance of a number of types of sample with varying insulation specifications. Tests were performed on wound and connecting components using a variety of voltage waveforms and atmospheric conditions. Work package 3 involved subjecting the same samples to a variety of ageing mechanisms in order to better appreciate how each mechanism could degrade the insulation and the effect this would have on the partial discharge performance.
The knowledge acquired from these two work packages led to the creation of a series of design guidelines for insulation systems for aerospace applications. They make recommendations on the design and qualification process for insulation systems used in aerospace applications. One key recommendation is on the applicability of IEC 60034-18-41 to aerospace components. From observations made within the laboratory it is being recommended that several modifications are made to the qualification tests outlined within the standard. These modifications are aimed at making the standard more applicable for the types of machines used in aircraft and the conditions in which they operate.
Design Tool: Another significant output from the project is the creation of a design tool to aid design decisions for future aircraft systems. The design tool allows engineers to modify the properties of an insulation system and monitor the effect these changes would have on the partial discharge performance of the system. This enables more informed decisions on the insulation system requirements for wound and connecting components at an early stage of design.
The design tool has the capability of determining:
• Safe clearances between uninsulated electrodes
• Partial Discharge Inception Voltage (PDIV) for wound and connecting components
• Minimum insulation thickness to prevent PD
Each of the above models account for operating pressure and temperature. This allows for the comparison of insulation performance at ground level and cruising altitude. The large amount of testing has provided the project with a vast database of test results from an array of test samples with varying insulation specifications. This data has assisted both the development and validation of the design tool.
Recommendations on qualification and production acceptance testing: Based on the experience gained in work packages 2 and 3, procedures have been developed for the qualification testing of both wound and connecting components. These procedures recommend a series of ageing simulations and functional tests to stress the insulation system in a manner that represents the stress that would be expected during a component’s life cycle. Ageing and functional tests are based on existing standards where applicable, with modifications made based on the design guidelines developed during the project.
A procedure for the production acceptance of components has also been developed. This document details a process to ensure that the insulation systems in manufactured components confirm to the same specification as the components used in the qualifying process. Both of these procedures exist for both wound and connecting components.
Other key outcomes of the project include the discovery of difficulties performing partial discharge tests according to IEC 61934. At atmospheric pressure the test results were found to be inconsistent. At low pressure the results were found to be incorrect. This is of concern as the tests were performed as per the standard, which could be used to inform qualification criteria for commercial systems. All of the above outputs of the project address this issue.

Potential Impact:
The project has several potential commercial impacts for the industrial partner. The substantive evidence collected during the testing phase has led to the development of a bespoke version of the qualification procedure of IEC 60034-18-41. This should lead to a reduction in the demands of the insulation system for electrical machines, resulting in an improvement in power density or reduce the need to verify the long term partial discharge performance.
The production of a design tool should also reduce the development time of new systems for the industrial partner by allowing for the comparison of a number of insulation materials without the need for testing. This should lead to reduced research and development costs.
A significant impact that this project may have on both the industrial partner and the wider aerospace community is the potential for an SAE Aerospace Recommended Practice (ARP) regarding the design and qualification of aerospace components that are robust to partial discharge. ARPs are design guidelines that are recognised by industry and support the development processes that support qualification of aircraft systems. There is also potential for an ARP to update of existing standards. Work is expected to commence on this shortly following agreement by SAE.
The findings of the project have been disseminated to the wider aerospace and electrical insulation communities. The challenges associated with testing wound components for partial discharge in an aerospace environment were presented at the IEEE Electrical Insulation Conference in Seattle, Washington, in June 2015 and at a special seminar session at Rolls Royce, UK in February 2015.

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Liz Fay, (EU Funding and Development Manager)
Tel.: +441612757114
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Record Number: 189674 / Last updated on: 2016-10-07