Method and system for detecting a failed rectifier in an AC/DC converter

1. A method for detecting a failed rectifier in an AC/DC converter for converting three-phase AC power to DC power and having its AC side connected to an AC source for power supply and its DC side connected to a DC load, the AC/DC converter including a rectifier circuit comprising at least three legs, each of the legs connected to a corresponding AC phase and including at least one rectifier, the method comprises

measuring instantaneous AC currents of each of the phases,

estimating an instantaneous DC current based on the measured instantaneous AC currents during one period of the power supply frequency,

characterized in that the method further comprises

calculating a ripple value of the estimated instantaneous DC current,

determining if the calculated ripple value exceeds a pre-defined threshold value,

initiating an alarm upon the calculated ripple value exceeding the pre-defined threshold value,

upon the calculated ripple value exceeding the pre-defined threshold value, for each of the legs, calculating a rectifier status value based on a maximum instantaneous AC phase current and a minimum instantaneous AC phase current during the same one period, and

determining, for each of the legs, if the rectifier in the leg has a non-conducting fault based on the calculated rectifier status value.

2. The method according to claim 1 , further comprising,

upon the calculated ripple value exceeding the pre-defined threshold value and a non-conducting rectifier not detected, for each of the legs, calculating a DC component in AC current based on measured instantaneous AC value of the phase, and

determining and identifying, for each of the legs, if the rectifier in the leg is short-circuited based on the calculated DC component.

3. The method according to claim 1 , further comprising calculating a maximum DC current and a minimum DC current during the same one period and calculating the ripple value in percent of the estimated DC current based on the calculated maximum instantaneous and minimum instantaneous DC currents.

4. The method according to claim 3 , further comprising calculating an average DC current based on the measured instantaneous AC currents during the same one period and calculating the ripple value in percent of the estimated DC current based on the calculated maximum instantaneous and minimum instantaneous DC currents and the calculated average DC current.

5. A device for detecting a failed rectifier in an AC/DC converter for converting three-phase AC power to DC power and having its AC side connected to a three-phase AC source and its DC side connected to a DC load, the AC/DC converter including a rectifier circuit comprising at least three legs, each of the legs connected to a corresponding AC phase and including at least one rectifier, the device including a measuring unit for measuring and sampling AC current for each of the phases and a calculating unit configured to estimate an instantaneous DC current based on the measured instantaneous AC currents during one period of the power supply frequency, characterized in that the calculating unit is further configured to,

calculate a ripple value of the estimated instantaneous DC current,

determine if the calculated ripple value exceeds a pre-defined threshold value,

initiate an alarm upon the calculated ripple value exceeding the pre-defined threshold value,

upon the calculated ripple value exceeding the pre-defined threshold value, for each of the legs, calculate a rectifier status value based on a maximum instantaneous AC phase current and a minimum instantaneous AC phase current during the same one period, and

determine, for each of the legs, if the rectifier in the leg has a non-conducting fault based on the calculated rectifier status value.

6. The device according to claim 5 , wherein the calculating unit is further configured to,

upon the calculated ripple value exceeding the pre-defined threshold value and a non-conducting rectifier not detected, for each of the legs, calculate a DC component in the AC current based on measured instantaneous AC value of the phase, and

determine and identify, for each of the legs, if the rectifier is short-circuited based on the calculated DC component.

7. The device according to claim 6 , wherein the calculating unit is further configured to calculate a maximum DC current and a minimum DC current during the same one period and calculate the ripple value in percent of the estimated DC current based on the calculated maximum and minimum instantaneous DC currents.

8. The device according to claim 7 , wherein the calculating unit is further configured to calculate an average DC current based on the measured instantaneous AC currents during the same one period and calculating the ripple value in percent of the estimated DC current based on the calculated maximum instantaneous and minimum instantaneous DC currents and the calculated average DC current.

9. An excitation system for exciting a synchronous rotating electrical machine, wherein the synchronous rotating electrical machine has a rotor, the excitation system comprising,

a transformer having a primary winding and a secondary winding and,

an exciter for exciting the rotor of the synchronous rotating electrical machine and comprising an AC/DC converter including a rectifier circuit comprising at least three legs, wherein each of the legs has an AC input connected to the secondary winding and an DC output connected to the rotor and, each of the legs includes at least one rectifier,

wherein the excitation system further comprises a device according to claim 5 for detecting a failed rectifier in the rectifier circuit.

10. An excitation system according to claim 9 , wherein the measuring unit of the device is further configured to measure instantaneous AC current at either the primary winding or the secondary winding.

11. An excitation system according to claim 9 , comprising a second transformer having a primary winding and a secondary winding, wherein each of three additional legs of the rectifier circuit has an AC input connected to the secondary winding of the second transformer and an DC output connected to the rotor, and each of the additional three legs comprises at least one rectifier, and the measuring unit is further configured to measure instantaneous AC current flowing through the winding of the second transformer at the same side as the first transformer.

12. An excitation system according to claim 9 , wherein the transformer further comprises a tertiary winding and each of three additional legs of the rectifier circuit has an AC input connected to the tertiary winding and the DC output connected to the rotor and the measuring unit is further configured to measure instantaneous AC current at the tertiary winding in case that the measuring unit has configured to measure the secondary winding.