SAE AIR 6127-2017 Managing Higher Voltages in Aerospace Electrical Systems.pdf

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1、_ SAE Technical Standards Board Rules provide that: “This report is published by SAE to advance the state of technical and engineering sciences. The use of this report is entirely voluntary, and its applicability and suitability for any particular use, including any patent infringement arising there

2、from, is the sole responsibility of the user.” SAE reviews each technical report at least every five years at which time it may be revised, reaffirmed, stabilized, or cancelled. SAE invites your written comments and suggestions. Copyright 2017 SAE International All rights reserved. No part of this p

3、ublication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE. TO PLACE A DOCUMENT ORDER: Tel: 877-606-7323 (inside USA and Canada) Tel: +1 724-776-497

4、0 (outside USA) Fax: 724-776-0790 Email: CustomerServicesae.org SAE WEB ADDRESS: http:/www.sae.org SAE values your input. To provide feedback on this Technical Report, please visit http:/standards.sae.org/AIR6127 AEROSPACE INFORMATION REPORT AIR6127 Issued 2017-04 Managing Higher Voltages in Aerospa

5、ce Electrical Systems RATIONALE This SAE Aerospace Information Report (AIR) considers ways of managing high voltages in aerospace vehicles. This document provides a basis for identifying high voltage design risks, defines areas of concern as a function of environment, potential risk mitigation metho

6、ds and test and evaluation techniques. TABLE OF CONTENTS 1. SCOPE . 3 2. REFERENCES . 3 2.1 Applicable Documents . 3 2.2 Definitions . 6 2.3 Glossary . 6 3. ELECTRICAL DISCHARGES IN AEROSPACE HIGH VOLTAGE SYSTEMS 7 4. OVERVIEW OF ELECTRICAL DISCHARGE TYPES . 8 4.1 Paschens Law . 8 4.2 Disruptive Dis

7、charges 10 4.3 Partial Discharges 10 4.4 Tracking . 12 5. CONTROLLING ELECTRICAL DISCHARGES . 13 5.1 Conductor Spacings in Air 14 5.2 Selection of Solid Insulation Thickness 16 5.3 Partial Discharge In/Around Solid Insulation 17 5.4 Machine Design 20 5.5 Creepage Distance Specification . 23 6. TESTI

8、NG AND VALIDATION OF DESIGN 24 6.1 Overvoltage Testing/Insulation Resistance Testing . 25 6.2 Partial Discharge Testing . 25 6.3 Impulse Testing in an Aerospace Environment 28 6.4 Test Methods to Determine Creepage Distances 29 7. SUMMARY AND RECOMMENDATIONS 30 8. NOTES . 30 8.1 Revision Indicator .

9、 30 SAE INTERNATIONAL AIR6127 Page 2 of 41 APPENDIX A RECOMMENDED GUIDELINES FOR PARTIAL DISCHARGE MEASUREMENTS AT SUB-ATMOSPHERIC PRESSURES A1 31 FIGURE 1 PASCHENS CURVE FOR AIR, FOR A UNIFORM ELECTRIC FIELD (Y-AXIS - BREAKDOWN VOLTAGE VOLTS-PEAK, X-AXIS - PRESSURE-DISTANCE PA-M) 13 9 FIGURE 2 BREA

10、KDOWN VOLTAGE OF AIR ACCORDING TO PASCHENS LAW AT A RANGE OF PRESSURES (Y-AXIS - BREAKDOWN VOLTAGE VOLTS-PEAK) 13 10 FIGURE 3 ILLUSTRATION OF SYSTEMS THAT COULD BE VULNERABLE TO PARTIAL DISCHARGE 11 FIGURE 4 PARTIAL DISCHARGE BETWEEN AN AEROSPACE CABLE AND A GROUNDED ELECTRODE 12 FIGURE 5 BASIC PROC

11、ESS OF TRACKING ACROSS AN INSULATING SURFACE CONTAMINATED WITH AN AQUEOUS LAYER 13 FIGURE 6 CLEARANCE DISTANCES ACCORDING TO IEC 60664 AND IPC221A (THE LATTER BEING CONVERTED TO PD PRODUCTS USING AN ALTITUDE OF 50000 FT) . 15 FIGURE 7 BREAKDOWN VOLTAGES MEASURED ON PTFE INSULATED AEROSPACE CABLING 3

12、0. 17 FIGURE 8 VARIATION OF THE SAFE OPERATING VOLTAGE “SOV” WITH INCREASING INSULATION THICKNESS. COMPARISON OF TWO MATERIALS WITH DIFFERENT RELATIVE PERMITTIVITY AND THE EFFECT OF INCREASING PRESSURE 18 FIGURE 9 ELECTRIC FIELD VALUES FOR SAFE/DISCHARGING VOIDS AT VARYING PD PRODUCTS . 19 FIGURE 10

13、 TIME TO FAILURE OF COMMERCIAL TWISTED MACHINE WIRE SAMPLES AT VARYING PRESSURE (POLYIMIDE INSULATION) 38 . 21 FIGURE 11 IMPACT OF THERMAL (LEFT PLOT) AND ELECTRICAL AGEING (RIGHT PLOT) ON PARTIAL DISCHARGE EXTINCTION VOLTAGE OF MACHINE COILS 35 22 FIGURE 12 REQUIRED CREEPAGE DISTANCES ACCORDING TO

14、IEC 60664 AND IPC 2221B 23 FIGURE 13 STANDARD PARTIAL DISCHARGE TEST CIRCUIT . 25 FIGURE 14 COMPARISON OF POWER FREQUENCY (LEFT) AND IMPULSE TEST (RIGHT) RESULTS FOR TWISTED WIRES AT 1000 MBAR 45 . 26 FIGURE 15 TYPICAL AC PARTIAL DISCHARGE TEST RESULT 27 FIGURE 16 TYPICAL DC PARTIAL DISCHARGE TEST R

15、ESULT 27 FIGURE 17 IMPULSE TEST CIRCUIT USING ANTENNA (IEC 61934 40) . 28 FIGURE 18 EFFECT OF PRESSURE ON PD VOLTAGES FOR TWISTED WIRES 28 FIGURE 19 TURN-TURN (A), PHASE-PHASE (B) AND PHASE-GROUND (C) PD VOLTAGES FOR STATOR (STR) AND MOTORETTE (MTR) SAMPLES AT 1000 AND 116 MBAR 45. 29 SAE INTERNATIO

16、NAL AIR6127 Page 3 of 41 1. SCOPE This SAE Aerospace Information Report (AIR) considers the issue of proper design guidance for high voltage electrical systems used in aerospace applications. This document is focused on electrical discharge mechanisms including partial discharge and does not address

17、 personnel safety. Key areas of concern when using high voltage in aerospace applications are power conversion devices, electrical machines, connectors and cabling/wiring. The interaction between components and subsystems will be discussed. The AIR is intended for application to high voltage systems

18、 used in aerospace vehicles operating to a maximum altitude of 30000 m (approximately 100000 feet), and maximum operating voltages of below 1500 VRMS (AC)/1500 V peak (DC). These upper voltage limits have been incorporated because this report focuses on extending the operating voltage of non-propuls

19、ive electrical systems beyond that of existing aerospace systems. It is noted that electrical systems for electrical propulsion may consider operating voltages beyond 1500 VAC/DC. 2. REFERENCES 2.1 Applicable Documents The following publications form a part of this document to the extent specified h

20、erein. The latest issue of SAE publications shall apply. The applicable issue of other publications shall be the issue in effect on the date of the purchase order. In the event of conflict between the text of this document and references cited herein, the text of this document takes precedence. Noth

21、ing in this document, however, supersedes applicable laws and regulations unless a specific exemption has been obtained. 1 Weimer, J.A.: “Past, Present both at the primary power system level as well as within aircraft sub-systems (e.g., flight actuation, invertors, window heating, etc.). The increas

22、es in the operating voltage of aircraft have been gradual. Aircraft electrical systems operated at 14.25 VDC in 1936, rose to 28 VDC in 1946 1 which was slowly phased out in the 1950s for larger aircaft but kept as the standard for low voltage sections of aircraft electrical systems. In the 1930s, d

23、evelopment of single and three phase alternating current systems begain with the Boeing XB-15 having single phase 120 VAC-800 Hz power, and the Douglas XB-19 having the first 3-phase 120/207 VAC-400 Hz system. In the late 1940s the transition to a 115/200 VAC-400 Hz standard began with the British “

24、Brabazon” civil aircraft and the American B-36 “Peacemaker” military aircraft. Today, 115/200 VAC-400 Hz systems are used in the majority of civil aircraft. 270 VDC was selected by the military to provide further weight savings in the 1980s 2. The current aircraft in operation with a more electric a

25、rchitecture include the Boeing 787 which has 500 kVA of electrical generation capability on each engine 3. To support the generation levels without a significant weight penalty, the Boeing Company has moved from a 115 VAC-400Hz system to a combined 230/400 VAC-360-800Hz and 270 VDC (540 VDC) systems

26、 3. Airbus is also using high voltages for distribution on the A350 platform. Although various military aircraft have employed “double voltage” (230/400 VAC) primary power systems, these two civilian aircraft represent a significant change in the industry with potenally thousands of aircraft to be p

27、roduced over the next few decades. While these voltages are relatively low in terms of those found on land-based power systems, moving to higher voltage thresholds introduces an increased likelihood of electrical discharge 4, particularly the risk of continuous partial discharge that does not cause

28、instant failure but reduces the life of insulation systems over time. While it is reasonably straightforward to design an insulation system that will prevent discharge, the need to minimize weight and the constrained volumes that are available in aircraft mean that there can be a conflict between ac

29、hieving maximum reliability and the lowest possible weight and volume. The design of an insulation system is further complicated given insulation systems may be designed for mechanical robustness well beyond the HV needs to support installation or other needs. An optimal design must be achieved in w

30、idely varying environmental conditions. Parts of the aircraft system that are in an unpressurized zone will see a low pressure for most of the flight and temperatures reaching average lows of around -56 C 5 with many modern systems now being designed for -65 C. However, localized heat sources can le

31、ad to systems having to operate at very high temperatures up to several hundred degrees. Any systems located in the engine (including the starter generators that are being considered for future aircraft) can be subject to high pressure (HP) compressor delivery air which can be at around 400 C. Engin

32、e accesories, such as generators, can have ambient temperatures exceeding 180 C, and power terminations can exceed 240 C. In any pressurized location, flight critical equipment must be designed to ensure that failure does not take place should the aircraft suffer a loss of cabin pressure. This means

33、 that equipment has to be designed to function over a wide range of pressure and temperature. There is a clear need to guarantee the reliability of any equipment operating at high voltage. Consideration must therefore be given to issues such as the insulating material lifetime. Insulation lifetime c

34、an be reduced by a number of factors including exposure to high temperatures, thermal cycling, vibration, chemical attack and electrical discharges. In addition to the bulk dielectric life issues, surface dielectric deterioration and effects must be considered as well. Primarily for maintenance and

35、corrosion prevention reasons, civilian aircraft electrical power terminations are not coated. Therefore, an understanding of the material, voltage stresses and contamination exposure, nearly always reconstituted in aqueous form, over the life of the aircraft is very important. SAE INTERNATIONAL AIR6

36、127 Page 8 of 41 Past work has identified a number of the concerns of operating electrical systems at higher voltages in an aerospace environment. Bilodeau, Dunbar and Sarjeant discussed the increased demand for higher voltage usage in space power systems owing to the need to reduce the weight that

37、would result from the use of lower voltage systems and the necessary test regimes 6. Many papers examine some of the basic issues relating to the operation/testing of higher voltage systems in a low pressure environment. Dunbar 7 stressed a wire with high voltage to determine the effect of high freq

38、uency at high altitudes. His results indicate a reduction in the partial discharge inception voltage of around 15% at frequencies above 40 kHz for an altitude of 33000 feet. Dunbar states in another paper 8 that partial discharge can cause significant numbers of failures in high voltage systems. Bro

39、ckschmidt 9 also discusses the problems associated with the operation of higher voltage systems in a low pressure environment and states that the voltage required to sustain partial discharge can be lower than that required to initiate it. Karady et al. 10 examined the corona inception voltage of si

40、mple electrode geometries including ones that involved thin layers of electrical insulation. The authors show that for certain experimental arrangements, corona inception can take place at voltages lower than Paschens minimum. Hammoud and Stavnes 11 conducted breakdown tests on different types of ae

41、rospace cable observing a reduction in the breakdown voltage of 20% occurred when using a testing frequency of 400 Hz at 200 C. A study was conducted on polypropylene cable by 12 at different frequencies, ranging from 50 to 400 Hz. The results of these tests correlated with 11 in that breakdown volt

42、ages of such insulation could fall as a function of frequency. A key document that while dated provides excellent information across many of the pertinent issues was produced by Dunbar 12. This describes the operating environment of an aircraft, the types of electrical discharge that could take plac

43、e and provides guidelines and precautions to be taken into consideration in the insulation design of electrical and electronic equipment. It also provides guidelines for monitoring and testing. More recent literature includes an overview of the key damage mechanisms in higher voltage aerospace syste

44、ms along with an examination of the optimum voltage level of cabling systems. This document seeks to provide the reader with a thorough overview of the risks that are involved in using high voltages in an aerospace environment and the methods that can be used to mitigate the risk both in terms of th

45、e selection of appropriate insulation systems and through the use of electrical test. Totally mitigating the risk of electrical discharge occuring is, however, unlikely to be achievable given the impact that other failures may have. For example, pressure seals may reduce the pressure below the value

46、 for which clearances have been designed, or component failures could emit clouds of ionised gas/deposit conductive pollution onto surfaces. This document will use the term “high voltage” to define any voltage above that which can cause partial or disruptive discharges in air, typically about 300 V

47、peak. This will include systems with nominal operation at less than 300 V, but in which abnormal conditions can elevate the voltage above this level. Other organizations have specific legal definition, such as 600 V per the National Electric Code (NEC) or 1000 V by various IEEE standards. Other stan

48、dards adopt specific limits such as BS EN 60071-1:2006:2010 that is applicable to terrestrial three-phase AC systems above 1 kV rms. This voltage level is known to be where high voltage effects such as electrical discharge become apparent in an electrical system. However, the standard is not intende

49、d for use in aerospace applications. 4. OVERVIEW OF ELECTRICAL DISCHARGE TYPES In the context of aerospace systems, electrical discharges can be generally subdivided into three types; disruptive discharge, partial discharge and tracking. For discharges that take place wholly or partly within the air, an understanding of Paschens law is particularly useful. This law descr