Detecting ground faults on non-isolated DC systems
A power conversion topology and control method intended for use with a DC bus is disclosed, providing a means to prevent the flow of ground fault current while detecting a fault condition and optionally continuing to operate the DC bus. In one control configuration, a DC-DC power stage with an active current control loop drives differential current to zero, and a separate voltage control loop maintains the desired differential bus voltage. Absolute bus voltage with respect to ground is monitored in order to detect fault conditions. In another configuration, the absolute voltage of the bus with respect to ground is controlled and differential bus current is monitored to detect ground fault conditions.
1. A method of detecting ground fault currents comprising:
providing a DC bus where ground fault currents are to be detected, the rails of the bus being tied to ground through a high impedance;
providing a power converter capable of independently driving each rail of the bus from a source;
driving the rails independently to achieve zero differential current; and
detecting imbalance in the voltage of the rails.
2. The method of claim 1 where the voltage of the source exceeds the voltage of the bus.
3. The method of claim 1 where the voltage of the source is at least two times the voltage of the bus.
4. A method of detecting ground fault currents in a DC bus having rails tied to ground through a high impedance, the method comprising:
independently driving each rail of the bus from a source;
detecting a differential current between the rails; and
driving the rails independently to achieve zero differential current.
5. A DC-DC power stage comprising:
an active current control loop that drives differential current on a two rail DC bus to zero; and
a separate voltage control loop that maintains a specified differential bus voltage.
6. The DC-DC power stage of claim 5 and further comprising an absolute bus voltage with respect to ground monitor to detect fault conditions.
7. The DC-DC power stage of claim 5 and further comprising:
a controller to control the absolute voltage of the differential bus with respect to ground; and
a current monitor configured to monitor differential bus current to detect ground fault conditions.
8. A circuit comprising:
an AC stage having a plurality of half bridges driven independently by pulse-width-modulated signals to couple to an AC source of power;
a DC-DC stage having a plurality of half bridges driven independently by pulse-width-modulated signals, the AC and DC-DC stages coupled via an internal DC link capacitor coupled between internal DC rails;
a DC output stage having a positive rail and a negative rail coupled to ground via high-impedance grounding resistors, the rails coupled to respective half bridges of the DC-DC stage to receive positive and negative DC voltages respectively; and
a differential sensor coupled to the DC output stage positive and negative rails to measure a difference in current or a difference in voltage responsive to a ground fault.
9. An AC to DC power converter comprising:
an AC stage having multiple half-bridges disposed between shared internal rails to couple to an corresponding multiple legs of an AC supply;
an internal DC stage joined to the AC stage by an internal DC link capacitor coupled between the shared internal rails, the internal DC stage having two half-bridges coupled between the shared internal rails;
an output DC stage having output rails coupled to respective half-bridges of the internal DC stage via an output filter;
a current sensor coupled to detect a difference in current between the output rails; and
a voltage detector coupled to detect a voltage across each output rail with respect to ground.
10. The power converter of claim 9 and further comprising a controller to operate the AC stage to maintain the detected voltage at a selected differential value.
11. The power converter of claim 10 wherein the controller controls a duty cycle of each of the two half-bridges of the internal DC stage to control the voltages on the output rails.
12. The power converter of claim 11 wherein the controller drives the differential rail current to zero by driving one of the rails to zero to compensate for a ground fault in one of the rails.
13. The power converter of claim 12 wherein the differential rail voltage is maintained at the selected differential value.
14. The power converter of claim 11 wherein the controller allows the rail voltages to flow away from ground provided the differential current is within safe bounds.
15. The power converter of claim 11 wherein the voltage of each rail of the internal rails exceeds the differential voltage of the output rails to allow the output rails to provide a full differential voltage swing regardless of which output rail is ground faulted.