Detecting shorted diodes

1. A method for detecting a shorted diode in a bridge rectifier having two series-connected diodes for each phase of the bridge rectifier, the method comprising:

determining a phase-to-phase voltage (V ab , V bc , V ca ) between two phase inputs of the bridge rectifier;

providing a phase input between the two series-connected diodes of each respective phase;

determining whether the phase-to-phase voltage is zero for more than a commutation time of the bridge rectifier; and

indicating a shorted diode fault when the phase-to-phase voltage is zero for more than the commutation time of the bridge rectifier.

2. The method of claim 1 , wherein the phase-to-phase voltage is determined by:

measuring a first phase voltage at a first phase input of the bridge rectifier;

measuring a second phase voltage of a second phase input of the bridge rectifier; and

calculating the phase-to-phase voltage as a difference between the first phase voltage and the second phase voltage.

3. The method of claim 1 , comprising:

determining that the phase-to-phase voltage is zero when the phase-to-phase voltage is smaller than a predefined voltage threshold value.

4. The method of claim 3 , wherein the predefined voltage threshold value is smaller than about 10% of a maximal phase voltage at a phase input of the bridge rectifier.

5. The method of claim 1 , comprising:

determining that the phase-to-phase voltage is zero for more than a commutation time of the bridge rectifier by determining that a time, during which the phase-to-phase voltage is zero, is longer than a predefined time threshold value.

6. The method of claim 5 , wherein the predefined time threshold value is smaller than about 20% of a period of an AC voltage at a phase input of the bridge rectifier.

7. The method of claim 1 , wherein the bridge rectifier comprises:

at least three phase inputs; and

at least three phase-to-phase voltages are determined between the at least three phase inputs of the rectifier.

8. The method of claim 7 , comprising:

determining whether a circuit breaker connected to the phase inputs is open; and

only indicating a shorted diode, when the circuit breaker is closed.

9. The method of claim 8 , comprising:

determining that the circuit breaker is open by determining that the three phase-to-phase voltages are smaller than a predefined threshold voltage.

10. The method of claim 1 , comprising:

determining whether a circuit breaker connected to the phase inputs is open; and

only indicating a shorted diode, when the circuit breaker is closed.

11. A controller unit for a bridge rectifier, wherein the controller unit stores a computer program in a non-transitory medium for causing the controller unit to execute a method for detecting a shorted diode in a bridge rectifier having two series-connected diodes for each phase of the bridge rectifier, the method comprising:

determining a phase-to-phase voltage (V ab , V bc , V ca ) between two phase inputs of the bridge rectifier;

providing a phase input between the two series-connected diodes of each respective phase;

determining whether the phase-to-phase voltage is zero for more than a commutation time of the bridge rectifier; and

indicating a shorted diode fault when the phase-to-phase voltage is zero for more than the commutation time of the bridge rectifier.

12. The controller unit of claim 11 , wherein the phase-to-phase voltage is determined by:

measuring a first phase voltage at a first phase input of the bridge rectifier;

measuring a second phase voltage of a second phase input of the bridge rectifier; and

calculating the phase-to-phase voltage as a difference between the first phase voltage and the second phase voltage.

13. The controller unit of claim 11 , wherein the controller is programmed for determining that the phase-to-phase voltage is zero when the phase-to-phase voltage is smaller than a predefined voltage threshold value.

14. The controller unit of claim 11 , comprising:

a FPGA (Field Programmable Gate Array) for executing at least a portion of the method.

15. The controller unit of claim 14 , wherein the method is executed by a FPGA.

16. A bridge rectifier system, comprising:

a bridge rectifier with two series connected diodes for each phase providing a phase input between them, wherein the series connected diodes for each phase are connected in parallel to an DC output of the bridge rectifier; and

a controller unit, which stores a computer program in a non-transitory medium for causing the controller unit to execute a method for detecting a shorted diode in a bridge rectifier having two series-connected diodes for each phase of the bridge rectifier, the method including:

determining a phase-to-phase voltage (V ab , V bc , V ca ) between two phase inputs of the bridge rectifier;

providing a phase input between the two series-connected diodes of the respective phase;

determining whether the phase-to-phase voltage is zero for more than a commutation time of the bridge rectifier; and

indicating a shorted diode fault when the phase-to-phase voltage is zero for more than the commutation time of the bridge rectifier.

17. The bridge rectifier system of claim 16 , comprising:

a transformer interconnecting the phase inputs of the bridge rectifier with a voltage source; and

wherein the controller unit is configured with a computer program to measure a phase voltage at a phase input of the bridge rectifier between the transformer and the phase input.

18. The bridge rectifier system of claim 16 , comprising:

a circuit breaker connected for disconnecting a voltage source from the bridge rectifier.

19. The bridge rectifier system of claim 16 , wherein the diodes are medium voltage diodes.

20. The bridge rectifier system of claim 16 , wherein the controller is programmed for determining phase-to-phase voltage by:

measuring a first phase voltage at a first phase input of the bridge rectifier;

measuring a second phase voltage of a second phase input of the bridge rectifier; and

calculating the phase-to-phase voltage as a difference between the first phase voltage and the second phase voltage.