Safety and failure analysis of electrical powertrain for fully electric vehicles and the development of a prognostic health monitoring system A R Ruddle*, A Galarza†, B Sedano †, I Unanue‡, I Ibarra* and L Low* *MIRA Limited, UK (e-mail: alastair.ruddle@mira.co.uk), †CEIT, Spain, ‡Jema, Spain Keywords: electrical powertrain; fully electric vehicle; hazard analysis; safety. Abstract As it is not practicable to address the entire vehicle in the HEMIS project, this analysis focuses on those elements of the system that are important for the PHMS and the electrical powertrain components that this system aims to monitor. One of the main aims of the EC project HEMIS is to design a prognostic health monitoring system for electrical powertrain components in order to enhance the safety and maintainability of electric vehicles. This paper outlines some of the preliminary work carried out in the areas of safety and failure analysis, based on a generic architecture that aims to represent common features of a wide range of electric vehicles. Some assumptions concerning the nature of the vehicle were required in order to describe the vehicle architecture at a level that is suitable for analysis. Firstly, it is assumed that the target application for the HEMIS PHMS is probably a nearfuture, high-end passenger vehicle. Previous EU research projects (e.g. [1]–[3]) indicate that vehicles of this type can be expected to be equipped with the following: 1 Introduction x Progress towards mass production of hybrid and electric vehicles presents vehicle manufacturers with new challenges due to the relative immaturity of the new technologies that are involved. The most notable of these is the electrical powertrain, comprising the electric traction machine and its associated power electronics controller. The defining feature of fully electric vehicles (FEVs) is that they are wholly reliant on the electrical powertrain to provide traction. One of the main aims of the EC project HEMIS is therefore to design a Prognostic Health Monitoring System (PHMS) for the electric powertrain in order to enhance the safety and maintainability of FEVs. In order to achieve this, a generic electric vehicle architecture has been defined and analysed in order to investigate relevant safety and reliability issues and hence derive requirements for the PHMS. x x an in-vehicle data network, partitioned between a number of “functional domains”; car-car/infrastructure communications capabilities; Advanced Driver Assistance Systems (ADAS). Although the physical and electrical architectures of alternative powertrain vehicles vary widely, the focus of the HEMIS project is fully electric vehicles (FEVs) as defined in the context of the European Green Cars Initiative [4], which includes: x x x 2 Generic electric vehicle architecture electrically-propelled vehicles that provide significant driving range on purely battery-based power; including vehicles with range extenders; including small light-weight passenger and light duty vehicles. The FEV concept therefore includes series hybrid architectures and vehicles equipped with other energy sources such as fuel cells, as well as the purely battery powered. Thus, the initial assumptions that have been made concerning the electrical powertrain for the purposes of HEMIS are that: An underlying architecture is required as an input to the RAMS (Reliability, Availability, Maintainability and Safety) analysis tasks in the HEMIS project. As the target market for the HEMIS PHMS is a broad class of electric vehicles, rather than any specific vehicle, the architecture that is defined has to be generic, representing the common features of electric vehicles that are relevant to the HEMIS PHMS. The generic electric vehicle architecture that is proposed must therefore reflect a balance between the desire to make the analysis generic (and therefore high level) whilst also considering sufficient detail to make the RAMS analysis practicable. It is anticipated that the architecture may need to be further developed in the course of the RAMS analysis activities. x x x traction power is provided only via electrical machines, and not mechanically from any on-board source such as an internal combustion engine (ICE); the electrical machine may be operated as a traction motor, or as a generator under braking conditions; the vehicle contains at least one such machine, but possibly more (e.g. one in each wheel, or one for each axle). 1 Authorized licensed use limited to: KIT Library. Downloaded on March 03,2023 at 04:25:38 UTC from IEEE Xplore. Restrictions apply. x The assumed network architecture, based on [3], is illustrated in Figure 1, while Figure 2 provides a functional view of the generic FEV architecture and its external interfaces. electrical energy storage is provided by a high voltage traction battery, as this is the most commonly used solution; It is assumed that energy may be obtained from: Vehicle Backbone Network x x x the electricity grid (by conductive or inductive charging – note that the latter may be achieved by wireless power transfer, during which the vehicle may active but temporarily stationary above a source coil embedded in the road, or possibly even while in motion as in [5]); energy recovery during regenerative braking; possibly from an on-board energy source (which could be an ICE or turbine coupled to a generator, or a fuel cell system generating electricity). Powertrain Controller Charging Power Interface Charging Data Interface Chassis & Safety Controller Battery Management System Driver Interfaces ACC System Powertrain Thermal Management Powertrain Sensors (e.g. PNRD, driver pedals, speed etc.) On-board Energy Source Powertrain Domain Network Lighting Control Passive Safety Airbag LV Source Bus Mobile Device Telephone Infotainment Domain Network Seat Control Body Electronics Domain Network Diagnostic Systems Infotainment Interfaces Powertrain Demand Driving Demand Diagnostic Interface Nearby Vehicles ACC Radar Antenna LV DC Power Bus ACC System Steering Chassis Sensors CHASSIS AND SAFETY INFOTAINMENT Pedal and Direction Sensors BODY ELECTRONICS Battery Management System Navigation Figure 1: HEMIS generic FEV architecture: network view. COMMUNICATIONS Vehicle Backbone Network Auxiliary LV Battery HV DC Source Bus Display/ Video Chassis & Safety Domain Network Functional Domains Charging System USB Audio Climate Control Steering Sensors (steer angle) Chassis Sensors (e.g. yaw rate, lateral/longitudinal acceleration etc.) PHMS Analysis and Warning Door Modules Instrument Panel Energy System Infotainment Controller Instrument Panel Environmental Sensors Passengers Communications Antennas Body Electronics Controller Braking System Driver External Networks DSRC Bluetooth Inverter Controller In addition, it is assumed that the vehicle is equipped with a HEMIS PHMS that is focused on the electrical transmission components (i.e. the electrical machine(s) and their associated power electronics and control systems). Vehicle Charging Station Communications GPS/Galileo UMTS Unit Diagnostic Interface HEMIS PHMS Steering Sensors EM Field Monitor Braking System Analysis & Warning POWERTRAIN Powertrain Controller HV Traction Battery DC/DC Converter Thermal Bus Transmission Sensors Powertrain Domain Network Powertrain Thermal Management Thermal Bus Wheels Control Mechanical Thermal High voltage (HV) Low voltage (LV) Signals/Data Gas On-board Energy Source HV AC Power Bus Electrical Machine Electrical Transmission Vehicle HV DC Power Bus Air Intake Exhaust Transmission Mechanical Transmission Radiator Environment Atmosphere Figure 2: HEMIS generic FEV architecture: functional view. 2 Authorized licensed use limited to: KIT Library. Downloaded on March 03,2023 at 04:25:38 UTC from IEEE Xplore. Restrictions apply. R o a d required as an input to the hill-hold function that is implemented by the Powertrain Controller and Electrical Transmission. 3 Main vehicle functions In Figures 1–2 it is assumed that the vehicle systems are distributed amongst five functional domains, described as Powertrain, Chassis and Safety, Body Electronics, Infotainment and Communications. Each domain has its own network providing intra-domain communications, and all domains are connected to a Vehicle Backbone Network that provides inter-domain communications. Each domain has a domain controller that acts as a gateway to the Vehicle Backbone Network and may also take control of domain system functions as a back-up if the associated systems fail. 4 Vehicle hazard analysis A preliminary hazard analysis (PHA) is carried out by reviewing the high level functions of a system together with its operating environment. In this way it is possible to identify the hazards that may result when the system mission is not fulfilled. As the PHA is intended to be systematic and repeatable the use of guidewords is encouraged. The PHA distinguishes between system hazards and failures, and the system under analysis is to be considered without any safeguards or mitigations. Furthermore, implementation details are not relevant for this type of study. Although the domain of primary interest in HEMIS is the Powertrain Domain, some systems of the Chassis and Safety Domain may also have significant interactions with systems in the Powertrain Domain. The Body Electronics Domain is also of interest, primarily in terms of its role in providing information and warnings to the driver. The Infotainment and Communication domains, however, are less significant for the operation of the PHMS. The generic architecture outlined in section 2 formed the basis of the HEMIS PHA. The focus of this analysis was to identify hazards associated with acceleration and deceleration (i.e. unavailable, un-demanded, excessive, insufficient and reversed), as well as those affecting vehicle handling and stopping distance. The high level functions outlined in section 3 were assessed in order to identify functional failures that could result in potential hazards. The functional domains that were analysed included the Powertrain Domain and the Chassis and Safety Domain, with the Electrical Transmission and Energy Systems being the primary interests. The high level functions of the systems were specified, based on the architectural description. In the Powertrain Domain, the Energy System provides facilities for obtaining, generating and storing energy, as well as supplying energy to the Electrical Transmission. The Powertrain Thermal Management system ensures that the temperatures of key powertrain sub-systems including the HV Traction Battery, DC/DC Converter, On-Board Energy Source, Control, and Electrical Machine are controlled for optimum performance. Driver demand for the powertrain is delivered through the Pedal and Direction Sensors, which monitor the accelerator pedal and PNRD (Park, Neutral, Reverse and Drive) selection lever. The hazard identification was carried out in two parts: the first part identified hazards related to functional failures of the system, while the second part identified non-functional hazards that are inherent in the novel technologies assumed in the vehicle. The latter included high voltage traction batteries, high voltage electrical power networks and on-board energy generation such as by fuel cells. The potential hazards associated with these include fire, explosion, and exposure to hazardous substances, and exposure to high voltages. The basic function of the electrical powertrain (the Electrical Transmission of Figure 2) is to supply torque to the Wheels via the Mechanical Transmission. More sophisticated functions of the electrical powertrain, such as idle creep, hillhold and torque vectoring, are assumed to be implemented by the Electrical Transmission under the control of the Powertrain Controller. A regenerative braking function is also considered, but is assumed to be Category A as defined in [6], essentially providing energy harvesting when the accelerator pedal is released if the rotor of the machine is still rotating. The objective of the PHA is to translate system hazards into design constraints, or functional safety requirements. Once the hazards were identified, each was assessed in terms of their potential consequences (“severity”), probability of exposure to the hazard (“exposure”) and opportunities for the driver to influence the outcome (“controllability”), using qualitative classifications described in ISO 26262 [10]. A risk graph was then used in order to establish and classify the associated risks in terms of the Automotive Integrity Levels (ASILs) of ISO 26262 [10]. The primary source of braking is the Braking System of the Chassis and Safety Domain. This system is also assumed to provide a number of enhanced braking functions, including anti-lock braking (mandatory since 2007), brake assist (mandatory since 2009 [7]) of Category A as described in [8], electronic stability control (mandatory since 2011 [9]) and an electric parking brake. Other systems of the Chassis and Safety domain that interact with the electrical powertrain include an Adaptive Cruise Control (ACC) system and the Steering Sensors and Chassis Sensors. The ACC system has a speed control function and the Chassis Sensors include parameters such as longitudinal acceleration, which is In addition, fault tree analysis (FTA) and failure mode and effects analysis (FMEA) were also used to identify the specific systems and functions that may lead to the potential hazards. The FMEA and FTA methods are complementary because the focused, deductive nature of FTA may identify failures that might be missed by the broader, inductive FMEA approach. Conversely, the broad coverage provided by FMEA may identify relevant failures that are outside the scope of the more narrowly focused FTA evaluations. 3 Authorized licensed use limited to: KIT Library. Downloaded on March 03,2023 at 04:25:38 UTC from IEEE Xplore. Restrictions apply. relationships between all of the potential causes, from which one is more readily able to identify the root causes of the problem. The FMEA approach provided a mechanism for identifying and prioritizing those failure modes that would require corrective action in order to ensure that functional safety targets are satisfied. 5 Electrical Transmission architecture The architecture of the Electrical Transmission was further refined, as outlined in Figure 3, in order to facilitate more detailed analysis of the components that are intended to be monitored by the PHMS. 6 Electrical Transmission faults Powertrain Domain Network LV DC Power Bus Inverter Controller Voltage Regulator Thermal Bus ENERGY SYSTEM Gate Drivers HV DC Power Bus Microcontroller Inverter Instrumentation DC Bus Link Capacitor Inverter Gates Inverter Control Specific information regarding faults in automotive traction machines and associated power electronics converters is not readily available as these technologies are still in relative infancy. However, some information is available concerning similar types of equipment used in other applications. ELECTRICAL TRANSMISSION Network Interface HEMIS PHMS HV AC Power Bus Machine Instrumentation Traction Machine Faults in the connections of electrical machines are reported to be very unusual at voltages below 1 kV RMS [13], but more common at higher voltages due to increased dielectric stresses and forces on conductors. Traction battery voltages reported for hybrid and electric vehicles range from 120 V [14] up to 650 V [15], suggesting that connection faults are perhaps unlikely in traction machines currently used in automotive applications. MECHANICAL TRANSMISSION Electrical Machine Figure 3: Details of Electrical Transmission architecture. Based on surveys of current vehicles on the market (e.g. [11]), as well as discussions with vehicle and machine manufacturers, three types of traction machine were considered. These included the squirrel cage induction machine, permanent magnet synchronous machine, and switched reluctance machine. However, bearing related faults are widely reported as the most common cause of failure in electrical machines, with stator related faults and other rotor related faults providing the next most significant fault categories. A breakdown of these categories into more specific component faults is given in Table 10, which is derived from the surveys reported in [16] and [17]. The results from both of these surveys are very similar, with 41% of faults bearing related, 35–36% stator related, and 9–10% rotor related. The major components of the three types of machine are essentially the same (rotor, stator, bearings, windings etc.). The main difference between them is in the implementation of the rotor magnetic field source. In the squirrel cage induction machine the magnetic field of the rotor is provided by a steel cage, which comprises rotor bars and rotor end rings. For the permanent magnetic machine the rotor magnetic field sources are permanent magnets, typically rareearth materials. In the switched reluctance machine the rotor magnetic field is provided by salient rotor poles formed from soft-magnetic material projecting from the rotor core. Thus, there are some minor differences in the failure modes, but overall the failure behaviours are very similar. The largest single contributor to machine failures [16]–[17] relates to stator ground insulation faults, at 22–23%. However, these figures are dominated by larger, higher voltage machines with higher vibration and dielectric stress levels, which may not reflect the characteristics of automotive traction motors. It is also noted in [13] that bearings in large machines are generally more reliable than those of smaller machines. A survey of small (<75 kW), low-voltage machines (generally squirrel cage IM) indicates that bearing faults accounted for 95% of the machine failures, with stator and rotor faults at only 2% and 1%, respectively [18]. Similarly, the nature of the power electronics and controller used to drive these machines is fundamentally the same. Thus, the failure behaviours are again very similar for the different circuit topologies that are required to drive the different types of machine. In [19], however, it is suggested that electrical problems may be much more common in induction motors used for automotive traction applications than in similar machines used for industrial applications due to more frequent rapid temperature rises. These temperature changes may accelerate insulation degradation and give rise to mechanical stresses that could cause cracks to form at the junctions between rotor bars and end rings in squirrel cage induction machines. Thus, in automotive traction applications, bearing faults may not be the overwhelming source of machine failures that the results of [18] would appear to suggest, and insulation and rotor related faults may also be significant contributors to electrical machine failures. Potential functional failure mechanisms of the generic electrical powertrain were also analysed using FMEA and FTA, using functions defined based on the architecture illustrated in Fig. 3. Furthermore, Ishikawa diagrams [12] were used to develop an overview of how electrical powertrain faults contribute to the vehicle-level functional safety hazards, providing a structured representation of all causes that could contribute to produce the undesirable effect. This approach provides a graphical representation of the 4 Authorized licensed use limited to: KIT Library. Downloaded on March 03,2023 at 04:25:38 UTC from IEEE Xplore. Restrictions apply. Stator current signature monitoring may also provide information on machine vibration characteristics without requiring additional sensors (which may be more expensive and less robust) or the need for access to the machine. Moreover, the stator current is often already monitored for other applications, such as protecting the machine against destructive fault currents, as well as monitoring the performance of inverters. From a survey based on 200 products from 80 companies [20], failures in the converter were reported to be due to faults in capacitors (30%), PCBs (26%), semiconductors (21%) and solder (13%). The results of another industry-based survey [21] reports faults in semiconductors (40%), capacitors (26%) and gate drivers (24%). These observations suggest that power semiconductors and DC link capacitors are likely to account for a significant proportion (perhaps 50–60%) of possible inverter failures. Faults associated with vibration that could potentially be detected through their impact on stator current signature include damaged bearings, broken rotor bars and air-gap eccentricity [22]–[23]. For automotive applications, however, more sophisticated signal processing techniques may be needed to overcome the wide variations in operating conditions that result during driving. Techniques based on short-time Fourier Transform and Wavelet Transform methods have been shown to be suitable for varying load conditions [23]. 7 Possible PHMS inputs The HEMIS PHMS is intended to monitor sensors associated with the Inverter and Electrical Machine, as well as other vehicle parameters that may be of relevance to their performance, in order to assess the condition of these key components of the electrical powertrain. This information would be used to identify faults and degradation, as well as to predict the remaining useful life. The driver could then be alerted to potential problems and maintenance needs, thus enhancing reliability, availability, maintainability and safety. Methods for detecting IGBT failures in a PWM voltage source inverter drive for an induction machine are described in [24], based on monitoring the stator current vector. These approaches are reported to allow the defective semiconductor to be identified, while data clustering techniques permit a robust evaluation that is independent of rotor speed. The latter is of particular interest for FEV applications because of the variable speed and load conditions. A review of physical characteristics used to monitor the symptoms of common induction machine faults, based on the reviews reported in [22] and [23], is summarized in Table 1 below. Winding short circuit Insulation X X X X Vibration X X X X Temperature X X X Partial discharge X X Gaseous emission X X Air-gap torque X Power X Magnetic flux X Acoustic emission X X Stator Core Rotor Bars X Rotor Core Air-gap eccentricity Current Fault Indicators Rotor Shaft Bearing and seals Fault Types X X X X X Failures because of ageing in drive capacitors are mostly monitored in terms of the capacitor ESR (equivalent series resistance) from Fourier Transform analyses of current or voltage signals [25]. In [26] it is concluded that the capacitor ripple voltage and ripple current are good indicators of ageing when loads are relatively constant: if not, the ratio of ripple voltage to ripple current is used. If ripple monitoring is not available, an alternative technique based on system modelling is proposed to estimate the ripple voltage from the converter input current. A real-time condition monitoring alternative is also proposed in [26], in which the ESR and capacitance values are estimated using a low-cost analogue circuit. 7 Conclusions A generic electric vehicle architecture has been proposed and used as a basis for safety analysis (using the methods of ISO 26262 as well as FMEA and FTA) in order to investigate requirements for a prognostic health monitoring system (PHMS) to monitor the electrical powertrain components. The failure mechanisms associated with the electrical machine and its associated power electronics have also been analysed in more detail, also using FMEA and FTA methods as well as Ishikawa diagrams. X Table 1: Possible indicators for faults in induction motors. Faults associated with bearings, insulation and rotor components are considered to be the most likely causes of automotive traction machine failures (see section 6). Consequently, the results shown in Table 1 suggest that current, vibration and temperature may be useful indicators for the HEMIS PHMS. Opportunities for condition monitoring of electrical powertrain components have also been briefly reviewed. For automotive applications, however, more sophisticated signal processing techniques may be needed to overcome the wide variations in operating conditions that result during driving. 5 Authorized licensed use limited to: KIT Library. Downloaded on March 03,2023 at 04:25:38 UTC from IEEE Xplore. Restrictions apply. Thus, combining results from a number of different sensors, analysed using a number of different processing techniques, is expected to improve the scope and reliability of condition monitoring for electrical powertrain components, as well as enhancing the associated prognosis capabilities. [13] P. Tavner, L. Ran, J. Penman and H. Sedding, “Condition Monitoring of Rotating Electrical Machines”, Institution of Engineering and Technology, London, 2008, ISBN 978-0-86341-739-9. [14] L. Chen, J. Wang, P. Lombard, P. Lazari and V. Leconte, “High efficiency motor design for electric vehicles”, Proc. 2012 Flux Conf., Rome, Italy, October 2012. [15] G. Schmid, R. Überbacher and P. Göth, “ELF and LF magnetic field exposure in hybrid- and electric cars”, Proc. Bio-electromagnetics Conf. 2009, Davos, Switzerland, June 2009, Paper 9–3. [16] P. O’Donnell, “Report of Large Motor Reliability Survey of Industry and Commerical Installations, Part I”, IEEE Trans. Ind. Apps., IA-21(4), July/August 1983, pp. 853–864. [17] O.V. Thorsen and M. Dalva, “A survey of faults on induction motors in offshore oil industry, petrochemical industry, gas terminals and oil refineries”, IEEE Trans. Ind. Apps., 31(5), September/October 1995, pp. 1186– 1196. [18] P.J. 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Industrial Technology ICIT 2006, Mumbai, India, December 2006, pp. 3061–3066. [23] N. Mehala, “Condition monitoring and fault diagnosis of induction motor using motor current signature analysis”, PhD Thesis, National Institute of Technology, Kurukshetra, India, 2010. [24] S. Chafei, F. Zidani, R. Nsit-Said and M.S. Boucherit, “Fault detection and diagnosis on a PWM inverter by different techniques”, J. Elect. Sys., 4(2), 2008, pp. 235– 247. [25] H. Ma and L. Wang, “Fault diagnosis and failure prediction of aluminum electrolytic capacitors in power electronic converters”, Proc. 31st IEEE Ann. Conf. Industrial Electronics Society (IECON 2005), Raleigh, USA, November 2005. [26] A.M. Imam, “Condition monitoring of electrolytic capacitors for power electronics applications”, PhD Thesis, Georgia Institute of Technology, May 2007. Future work will therefore include selection of the most appropriate parameters for monitoring by the HEMIS PHMS and the development of suitable analysis algorithms for eventual implementation and demonstration in a prototype. Acknowledgements The research leading to these results has received funding from the European Community’s Framework Programme (FP7/2007-2013) under grant agreement nº 314609. The authors are grateful for the support and contributions from other members of the HEMIS project consortium, from CEIT (Spain), IDIADA (Spain), Jema (Spain), MIRA (UK), Politecnico di Milano (Italy), VTT (Finland) and York EMC Services (UK). Further information can be found on the project website (www.hemis-eu.org). References [1] VEESA project, “Vehicle e-Safety Architecture”, EU project IST-2001-37598, Deliverable D0: Final Report, 29th February 2004. [2] M. Hiller et al., “General architecture framework”, EASIS EU Project Deliverable D0.2.4, 13th August 2004. [3] E. Kelling et al., “Specification and evaluation of esecurity relevant use cases”, EVITA EU Project Deliverable D2.1, Version 1.2, 30th September 2009. [4] M. Boukerche, “ICT for Fully Electric Vehicles 4 th call for proposals Objective GC-ICT-2013.6.7 Electromobility”, PPP European Green Cars Iinitiative Infoday, Brussels, 9th July 2012. [5] S. Ahn and J. Kim, “Magnetic field design for high efficient and low EMF wireless power transfer in on-line electric vehicle”, Proc. 5th European Conf. Antennas and Propagation, Rome, April 2011, pp. 3979–3982. [6] UNECE Regulation No. 13-H, “Uniform provisions concerning the approval of passenger cars with regard to braking”, Revision 1 – Amendment 2, 11/11/2009. [7] Commission Regulation (EC) No 78/2009, O. J. EU, 4/2/2009, L35, pp. 1–31. [8] Commission Regulation (EC) No. 631/2009, O. J. EU, 25/7/2009, L 195, pp. 1–60. [9] Commission Regulation (EC) No. 661/2009, O. J. EU, 31/7/2009, L 200, pp. 1–24. 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