Hydrogen Fuel Cell Vehicle Fuel System Integrity Research: Electrical Isolation Test Procedure Development and Verification
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Hydrogen Fuel Cell Vehicle Fuel System Integrity Research: Electrical Isolation Test Procedure Development and Verification

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    The Federal Motor Vehicle Safety Standards (FMVSS) establish minimum levels for vehicle safety, and manufacturers of motor vehicle and equipment items must comply with these standards. The National Highway Traffic Safety Administration (NHTSA) contracted Battelle to develop a procedure for testing electrical isolation on hydrogen fuel cell vehicles (HFCVs) when crash testing without hydrogen. FMVSS 305, Electric Powered Vehicles: Electrolyte Spillage and Electrical Shock Protection, requires electrical isolation as the basis for defining electrical safety and for establishing a criterion for the prevention of electrical shock. Enhancement is required to address electrical safety of hydrogen vehicles, particularly when testing without hydrogen onboard the vehicle. The current electrical isolation test procedure described in FMVSS TP-305-01, Laboratory Test Procedure for Electric Powered Vehicles: Electrolyte Spillage and Electrical Shock Protection, uses an active onboard high-voltage power system to determine the electrical safety of the vehicle. FMVSS TP-305-01 is written specifically to require full engagement of the test vehicle’s propulsion system before the crash test. Due to safety concerns, however, some automobile manufacturers and international regulatory bodies are proposing to conduct crash testing of compressed hydrogen vehicles using an inert gas substitute, such as helium. Unlike electric and hybrid electric vehicles, the absence of hydrogen in a hydrogen-fueled vehicle renders the fuel cell inactive, prohibiting the generation of high voltage and preventing the propulsion system from being fully engaged. Consequently, anTech Report method for verifying electrical isolation between the high-voltage source(s) and potential human contact points is needed for hydrogen-fueled vehicles if the propulsion system is not engaged fully. Task Order 4 developed and verified an alternative electrical isolation test procedure for HFCVs with an inactive fuel cell using a megohmmeter or “Megger™,” a piece of test equipment that supplies high voltage and measures the resulting leakage current to the vehicles’ chassis as necessary to verify the isolation. A systematic approach was used to develop the procedure and was initiated with a test requirements review. The review examined the latest standards and literature from industry and regulatory bodies to solidify the requirements for the final test procedure. In addition to current documentation, Battelle received technical insight and guidance from General Motors Corporation (GMC) and Ford Motor Company. The information was compiled and evaluated on the applicability to electrical safety of postcrash HFCV with an inert gas. The test procedure was written to test electrical isolation of an inactive high-voltage source using a megohmmeter. The test instrumentation was selected based on current HFCV power systems and the proposed acceptance criteria. A market survey to find test instrumentation was performed, and the Quad Tech 1855 was selected as a suitable instrument for this effort. After the proposed acceptance criteria and instrumentation were selected, the detailed procedure steps were outlined. During this phase of the program, the procedure was refined after several trips to the original equipment manufacturers (OEMs). During the OEM visits, the draft procedures were evaluated on several HFCVs for possible test point locations, test sequencing, and safety precautions. These dry runs were essential in order to finalize the test procedure and instrumentation. The final dry-run or procedure verification was performed on a hydrogen fueled Ford Focus and a Chevrolet Equinox at their respective development sites in Michigan. The successful procedure verification was performed to refine further the procedure and to confirm that the detailed steps and instrumentation can accurately test electrical isolation on an inactive fuel cell. Since the HFCVs were not subjected to an actual crash test, a resistor was inserted between the fuel cell power terminals and the chassis to simulate an electrical isolation failure. Several limitations on the capabilities of the megohmmeter were identified during the development of the procedure. The megohmmeter is an acceptable tool for measuring isolation in accordance with the test procedure; however, the QuadTech 1885 and Tech Report commercial available megohmmeters have unique restrictions. The Quadtech 1855 cannot measure isolation accurately between a power source and the chassis if the chassis has a conductive connection to earth ground. Therefore, Tech Report types of electrical safety test equipment were evaluated and tested during Task Order 4. The successful completion of Task Order 4 yielded a verified electrical isolation test procedure utilizing a megohmmeter for testing a HFCV with an inert gas and inactive fuel cell. In addition to this procedure, some additional research activities evaluated the effects of temperature and age on fuel cell coolant and the potential for environmental conditions to alter isolation measurements.
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