Improved Accuracy in Calculating of Isolation Resistance of xEV High-Voltage Systems

The integrity of isolation resistance of the xEV high-voltage subsystems is critically important to ensure the safety of vehicle occupants. The term xEV collectively refers to the hybrid electrical vehicle (HEV), plug-in HEV (PHEV), battery electric vehicle (BEV), fuel cell electric vehicle (FCEV), and range extended electric vehicles (REEV) etc. As described in the Federal Motor Vehicle Safety Standards (FMVSS) 305, the isolation resistance is indirectly determined based on the measured voltages. The existing formula used for calculating the isolation resistance does not take into account the finite input-resistance of the voltage measurement equipment. The assumption of infinite input resistance of the voltage measurement equipment leads to significant error, which will be quantified in this paper.

The proposed method in this paper includes the input-resistance and results in accurate calculation of isolation resistance. Both real-word data and simulation results will be included to verify the accuracy and the effectiveness of the proposed method. The contribution of this paper would establish the technical basis for future improvement of the regulations and industry standards, which will benefit the automotive industry with more improved high-voltage safety without adding unnecessary cost to the designs.