Method and device for enhancing the reliability of generator ground fault detection on a rotating electrical machine

1. A method for detecting a ground fault in a rotating electrical machine having a system frequency by injection, wherein the machine includes a winding and the method comprises:

injecting a test signal at a predefined frequency to the winding, where the predefined frequency is different from the system frequency,

measuring an electrical quantity of a response signal in the winding resulting from the injected test signal,

detecting a ground fault based on the measured value of the electrical quantity,

continuously determining the frequency of the response signal, and

discarding a present measured value corresponding to the determined frequency of the response signal when the determined frequency of the response signal deviates from the predefined frequency of the test signal by threshold value.

2. The method according to claim 1 , wherein the threshold value is in a range of 5-100 mHz.

3. The method according to claim 1 , wherein the measured electrical quantity is any one of current or voltage.

4. The method according to claim 1 , the method further comprising estimating values of a second electrical quantity based on the measured values of first electrical quantity and detecting the ground fault based on the estimated values of the second electrical quantity.

5. A device for detecting a ground fault in a rotating electrical machine having a winding and operating at a system frequency, the device comprising:

a signal injection unit configured to inject a test signal at a predefined frequency to the winding of the machine, where the predefined frequency is different from the system frequency,

a measurement unit configured to measure an electrical quantity of a response signal in the winding resulting from the injected test signal, and

a fault detecting unit configured to detect a ground fault based on the measured value of the electrical quantity, said fault detecting unit configured to:

continuously determine the frequency of the response signal, and

discard a present measured value corresponding to the determined frequency when the determined frequency deviates from a predefined frequency of the test signal by a threshold value.

6. The device according to claim 5 , wherein the threshold value is in a range of 5-100 mHz.

7. The device according to claim 5 , wherein the measured electrical quantity is any one of current or voltage.

8. The device according to claim 5 , the fault detecting unit is further configured to estimate values of a second electrical quantity based on the measured values of first electrical quantity and detect the ground fault based on the estimated values of the second electrical quantity.

9. A method for detecting a ground fault in a rotating electrical machine having a winding and operating at a system frequency, the method comprising the steps of:

injecting a test signal at a predefined frequency into the winding, where the predefined frequency is different from the system frequency,

measuring an electrical quantity of a response signal in the winding resulting from the injected test signal,

detecting a ground fault based on the measured value of the electrical quantity and the determined frequency,

determining the frequency of the response signal over a period of time, and

suspending the ground fault detection during a time interval when a present measured value of the determined frequency of the response signal deviates from the predefined frequency of the test signal by a threshold value;

wherein the time interval is determined by a ratio of the threshold and a nominal frequency of the rotating electrical machine multiplied by a maximum acceleration time or a maximum retardation time.

10. The method according to claim 9 , wherein the measured electrical quantity is selected from the group consisting of: voltage, current, impedance and combinations thereof.

11. The method according to claim 9 , wherein the threshold value is in a range of 5-100 mHz.

12. The method according to claim 9 further comprising the steps of:

estimating values of a second electrical quantity based on the measured values of first electrical quantity, and

detecting the ground fault based on the estimated values of the second electrical quantity.

13. The method according to claim 12 , wherein the first electrical quantity is either voltage or current, and the second electrical quantity is impedance.

14. A method for detecting a ground fault in a rotating electrical machine having a winding and operating at a system frequency, the method comprising the steps of:

injecting a test signal at a predefined frequency into the winding, where the predefined frequency is different from the system frequency,

measuring an electrical quantity of a response signal in the winding resulting from the injected test signal,

detecting a ground fault based on the measured value of the electrical quantity and the determined frequency,

determining the frequency of the response signal over a period of time, and

suspending the ground fault detection during a time interval when a present measured value of the determined frequency of the response signal deviates from the predefined frequency of the test signal by a threshold value,

wherein the function of detecting the ground fault is blocked only when at least two consecutive present measured values deviate from the predefined frequency by the threshold value.