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Hybrid & Electric Vehicle High Voltage Isolation Fault Systems

Understanding the vehicle on-board diagnostic system and off-board testing methods for locating faults.
Tuesday, December 30, 2014 - 09:00
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High Voltage (HV) systems that are resident on hybrid, electric and fuel cell powered vehicles provide many advantages, relative to improved fuel economy and enhanced flexibility of propulsion modes to acquire the improvement of fuel economy. The HV systems are mounted to the vehicle body/chassis and there must be a minimum electrical isolation (resistance) maintained between the HV system and the body/chassis to ensure a safe vehicle during operation or repair.  Because the automotive industry has adopted that any voltage greater than 60 volts as the threshold for HV and federal government regulations require the OEMs to monitor the chassis for HV leakage to the chassis, the OEMs must provide safety systems and sensing for any vehicle with systems that are operating at voltages greater than 60 volts. 

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The HV systems offer new testing and diagnostic challenges to the industry that need to be understood by the automotive technician to ensure any Loss of Isolation (LOI) problems in the HV system can be identified and repaired.  The LOI on electric powered vehicles is one of the more common failure modes, irrespective of manufacturer, and it is critical that technicians are aware of the systems that sense these failures.  This article will explore how the hybrid system monitors the body/chassis to determine if the isolation resistance has been lowered/reduced, between any HV system component and chassis, compromising the isolation and causing a HV LOI.  This article will also cover systems that are embedded in the vehicle systems software to test the HV system and manual testing methods that can be used by a technician to test the HV system to determine where the fault is/has occurred so it can be repaired. 

For most hybrid and electric vehicles, the HV component family typically includes the battery pack, power inverter, electric-machines (MGUs), dc-dc converters, and in most cases an electric air conditioning compressor.  Other HV systems, such as electric heating systems (e.g., PTC heaters), also would be considered part of the HV component family. This includes the control system for each of the family of components (Figure 1)

As hybrid electric vehicles (HEV) and its derivatives continue to populate the market, HV LOI failures have become a common failure mode for technicians to analyze, diagnose and repair.  There are currently more than 70 different HEV models in the market today that will need service; many of them are no longer under warranty.  The 70 models referenced do not include battery electric (BEV) or plug-in electric (PHEV) vehicles.  The LOI failures and how the HV systems controllers detect these failures is not well understood in the automotive repair industry.  Therefore, this article will dissect the HV related controls and diagnostic systems to help simplify how the HV systems operate and detect LOI that will help assist technicians in understanding how these failures occur, and how to test the HV component systems to locate the source of the LOI. 

What Is Isolation and an Isolation Barrier?
For the purposes of this article, Isolation will be defined as an electrical resistance barrier that exists between the HV components and the vehicle chassis.  Although HV components use a lower voltage (12 volt) controller to control HV functions, there must be an isolation barrier maintained between the HV system components (including the 12 volt controller) and the chassis to maintain a high level of safety while operating or repairing the vehicle.  The HV system is considered to be a floating system.  Specifically, the HV components fasten to the chassis but, the HV components do not use the chassis for ground. 

However, the 12 volt controllers do use vehicle chassis for ground.  The HV battery pack or power inverter serve as the power and grounding points for the HV system (depending on mode of operation), not the vehicle chassis.  Therefore, with respect to the vehicle chassis, the HV components are electrically connected in parallel.  These components are fastened/mounted to the chassis but, electrically float on the chassis because, they do not use the chassis for grounding.  Therefore, an electrical resistance barrier between HV current and the chassis must be maintained to ensure safety for anyone interfacing with it.

 To simplify and understand the circuit, using Ohm’s or Kirchhoff’s Laws for parallel circuits can be applied to determine how much HV electrical current would be injected onto the chassis from the HV system.  Figures 2 and 3 illustrate a simplified HV parallel circuit and the second diagram illustrates the HV component isolation resistances in parallel when fastened to the vehicle chassis.  

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