Ground fault detection apparatus

A ground fault detection apparatus connected to a high-voltage battery and configured to detect lowering of insulation resistance of a system provided with the high-voltage battery, the ground fault detection apparatus comprising:

• (a) a detection capacitor which operates as a flying capacitor;
• (b) a positive-electrode-side termination resistor connecting a positive-electrode side of the high-voltage battery and a ground;
• (c) a negative-electrode-side termination resistor connecting a negative-electrode side of the high-voltage battery and the ground;
• (d) a positive-electrode-side bypass resistor having one end connected to the ground;
• (e) a negative-electrode-side bypass resistor having one end connected to the ground;
• (f) a bypass twin relay configured to:
• in a first state, make no connection between the positive-electrode side of the high-voltage battery and a predetermined positive-electrode-side connection point, and make no connection between the negative-electrode side of the high-voltage battery and a predetermined negative-electrode-side connection point;
• and
• in a second state, connect between the positive-electrode side of the high-voltage battery and the predetermined positive-electrode-side connection point, and connect between the negative-electrode side of the high-voltage battery and the predetermined negative-electrode-side connection point;
• (g) a positive-electrode-side twin relay configured to:
• in a first state, connect a first end of the detection capacitor to the positive-electrode side of the high-voltage battery, while making no connection between the predetermined positive-electrode-side connection point and the positive-electrode-side bypass resistor;
• and
• in a second state, connect the first end of the detection capacitor to the ground, and connect the predetermined positive-electrode-side connection point and the positive-electrode-side bypass resistor;
• (h) a negative-electrode-side twin relay configured to:
• in a first state, connect a second end of the detection capacitor to the negative-electrode side of the high-voltage battery, while making no connection between the predetermined negative-electrode-side connection point and the negative-electrode-side bypass resistor;
• and
• in a second state, connect the second end of the detection capacitor to the ground, and connect the predetermined negative-electrode-side connection point and the negative-electrode-side bypass resistor;
• and
• (i) a control unit,
• wherein the control unit is configured to:
• compare a first charge voltage (Vp) of the detection capacitor with a second charge voltage (Vn) of the detection capacitor, the first charge voltage (Vp) being obtained when the detection capacitor is fully charged under a condition where (i) the positive-electrode-side twin relay is in the first state, (ii) the negative-electrode-side twin relay is in the second state, and (iii) the bypass twin relay is in the first state, and the second charge voltage (Vn) being obtained when the detection capacitor is fully charged under a condition where (i) the positive-electrode-side twin relay is in the second state, (ii) the negative-electrode-side twin relay is in the first state, and (iii) the bypass twin relay is in the first state;
• and
• determine that the insulation resistance on the positive-electrode side is lowered when the first charge voltage (Vp) is smaller and when degree of the smallness of the first charge voltage (Vp) is greater than a predetermined reference, or determine that the insulation resistance on the negative-electrode side is lowered when the second charge voltage (Vn) is smaller and when degree of the smallness of the second charge voltage (Vn) is greater than a predetermined reference.