Insulation resistance detection method and circuit for ungrounded DC power supply system

An insulation resistance detection circuit for an ungrounded DC power supply system, comprising

• a power supply system including a positive bus, a negative bus and at least one ground terminal, and connected with at least one load;
• a first ground capacitor, wherein one terminal of the first ground capacitor is connected with the positive bus, and another terminal of the first ground capacitor is connected with the ground terminal;
• a second ground capacitor, wherein one terminal of the second ground capacitor is connected with the negative bus, and another terminal of the second ground capacitor is connected with the ground terminal;
• a power conversion unit, wherein an input terminal of the power conversion unit is connected with the positive bus and the negative bus, and an output terminal of the power conversion unit is connected with the positive bus and the ground terminal;
• a control/processing unit connected with the power conversion unit;
• two voltage detection units respectively connected with the first ground capacitor, the second ground capacitor ground, the ground terminal and the control/processing unit, wherein one of the voltage detection units measures a voltage of a positive bus of the power supply system with respect to a ground terminal (PB-G voltage), and another one of the voltage detection units measures a voltage of a negative bus of the power supply system with respect to the ground terminal (NB-G voltage);
• at least one current detection unit connected with the control/processing unit and the load of the power supply system;
• wherein the control/processing unit divides the PB-G voltage by the NB-G voltage to obtain a ratio, determines an absolute value of the ratio, and determines whether the absolute value of the ratio exceeds a preset range, the control/processing unit controlling the power conversion unit to inject a DC (Direct Current) current into a circuit between the positive bus and the ground terminal to boost the PB-G voltage to a rated value if the absolute value of the ratio is below the preset range;
• or controlling the power conversion unit to inject a DC current into a circuit between the negative bus and the ground terminal to boost the NB-G voltage to have an absolute value equal to the rated value if the absolute value of the ratio is above the preset range.