Method and apparatus to monitor electric isolation of a high-voltage direct current electrical circuit

Method for monitoring a high-voltage direct-current electrical circuit including an electric energy storage device electrically connected via a positive DC electric power bus and a negative DC electric power bus to an electric machine, the high-voltage direct-current electrical circuit electrically isolated from a chassis ground, the method comprising:

• periodically sampling electrical potentials associated with the positive DC electric power bus and the negative DC electric power bus and a corresponding load current to the electric machine;
• calculating a ground isolation index corresponding to each of the periodically sampled electrical potentials;
• calculating trend elements using a plurality of successive ones of the ground isolation indexes;
• characterizing a trend corresponding to the trend elements;
• comparing the characterized trend with an expected trend that is based upon the load current to the electric machine;
• and
• detecting a potential fault associated with electric isolation of the high-voltage direct-current electrical circuit when the characterized trend deviates from the expected trend.

Method for monitoring a high-voltage direct-current electrical circuit including an electric energy storage device electrically connected via a positive DC electric power bus and a negative DC electric power bus to an electric machine, the high-voltage direct-current electrical circuit electrically isolated from a chassis ground, the method comprising:

• monitoring the positive DC electric power bus and the negative DC electric power bus and an electric load associated with the electric machine;
• periodically sampling electrical signals associated with the positive DC electric power bus and the negative DC electric power bus and calculating respective ground isolation indexes comprising
• sampling a first electric potential between the positive DC electric power bus and the negative DC electric power bus,
• sampling a second electric potential between the negative DC electric power bus and the chassis ground, and
• calculating a respective ground isolation index comprising a ratio of the first and second electric potentials;
• calculating trend elements using three successively calculated ground isolation indexes including a first trend element comprising an arithmetic difference between a first and a second of the three successively calculated ground isolation indexes and a second trend element comprising an arithmetic difference between the second and a third of the successively calculated ground isolation indexes.
• characterizing a trend corresponding to the trend elements;
• comparing the characterized trend with an expected trend, the expected trend associated with the monitored electric load associated with the electric machine;
• and
• detecting a potential fault associated with electric isolation of the high-voltage direct-current electrical circuit when the characterized trend deviates from the expected trend.