The present invention relates in general to electrified vehicles using a high voltage bus, and, more specifically, to accurate estimation of the effective isolation resistance present between each high-power bus and a chassis ground.
Electrified vehicles such as for electric vehicles and hybrid electric vehicles typically utilize a high voltage power bus driven by a DC power source which may include storage and/or conversion devices such as a multi-cell battery pack or a fuel cell. The presence of high-voltage buses creates a need for monitoring of the electrical isolation of each bus with respect to the electrically conductive components of the vehicle chassis (ground).
Any leakage resistance present between a DC bus and chassis ground must be sufficiently large. In typical leakage resistance detection systems, there is an assumption that leakage resistance will be between one of the positive or negative DC buses and chassis ground. A typical leakage detector circuit operates by periodically connecting one bus at a time to chassis ground through a current-limiting resistance, and using the resulting current flow to calculate the leakage resistance between the opposite bus and ground. The battery voltage divided by the calculated leakage resistance characterizes the electrical isolation.
The invention is based, in part, on a determination that conventional leakage resistance detection systems based on supplying a current from one bus through a known resistance to the leakage resistance between the ground and the other bus may ignore a potential balanced component of leakage resistance from both buses to ground that can sometimes result in the mischaracterization of the electrical isolation because of a potential discrepancy in the derived leakage resistance values. More specifically, a resistance may exist between the positive bus and chassis ground as well as a resistance of equal value between the negative bus and chassis ground. These resistances, both being equal in value, are hereafter referred to as symmetrical or balanced leakage resistance. A resistance on one bus to chassis without a matching value on the other bus to chassis is hereafter referred to as non-symmetrical or unbalanced leakage resistance. The additional current flow through the balanced leakage resistance may cause the prior art detection system to overestimate the composite balanced and unbalanced resistance which exists between one bus and chassis ground. Estimating this latter composite resistance is desirable in order to more accurately determine the isolation.
One typical source of a balanced leakage resistance would be a hydrogen fuel cell vehicle, wherein a deionizer intended to remove ions from water being used as a coolant fails to maintain proper deionization. As ions build up, the conductance of the cooling water increases and the electrical isolation between both positive and negative fuel cell electrodes and ground is reduced. Another possible source of a balanced leakage resistance includes a symmetrical breakdown in cable insulation.