Method for adaptation of ground fault detection

1. A method for adapting a ground fault detection to a change of an operating condition of an electrical machine, performed by a protective device connected to the electrical machine, wherein the machine includes a winding, the electrical machine being in a first machine operating condition, the method comprising the steps of:

defining a first reference value for measured values of an electrical quantity,

continuously measuring the electrical quantity in the winding and detecting a ground fault based on the measured values of the electrical quantity and the first reference value,

receiving a signal corresponding to a change in the operating condition of the electrical machine,

detecting the change of the operating condition of the electrical machine from the first machine operating condition to a second machine operating condition based on the received signal, the second machine operating condition being different than the first machine operating condition, and

defining a second reference value for the measured values of the electrical quantity, the second reference value being different from the first reference value, when the change of the machine operating condition is detected, the second reference value corresponding to the detected second machine operating condition,

wherein the first machine operating condition comprises one of: acceleration, retardation, full speed or full load and the second machine operating condition comprises one of: acceleration, retardation, full speed or full load.

2. The method according to claim 1 , wherein the received signal is an internal signal used in the ground fault detection and the method further comprises analyzing the received internal signal and detecting the change of the machine operating condition based on the analyzed signal.

3. The method according to claim 1 , wherein the received signal is a signal external to the ground fault detection and the method further comprises detecting the change of the machine operating condition based on the external signal.

4. The method according to claim 3 , wherein the external signal is any one of breaker status, machine load or excitation and in form of analog or logic.

5. The method according to claim 4 , wherein the method further comprises analyzing an analog external signal and detecting the change of the machine operating condition based on the analyzed external signal.

6. The method according to claim 2 , wherein the change of the machine operating condition is detected by any one of the properties of the analyzed signal including: Root-Mean-Square amplitude, denoted as RMS, dominating frequency or dominating amplitude.

7. The method according to claim 1 , wherein the ground fault detection comprises injecting a test signal at a predefined frequency to the winding of the machine and the method comprises receiving measured values of an electrical quantity, wherein the measured values being measured at the test signal injection point, and the method further comprises continuously analyzing the measured signal and detecting the change of machine operating condition based on the analyzed signal.

8. The method according to claim 1 , wherein the ground fault detection comprises injecting a test signal at a predefined frequency to the winding of the machine and the method comprises receiving an external signal and detecting the change of the machine operating condition based on the received external signal.

9. The method according to claim 1 , further comprising the steps of:

receiving a second signal corresponding to a change in the operating condition of the electrical machine,

detecting the change of the operating condition of the electrical machine from the second machine operating condition to a third machine operating condition based on the received signal, the third machine operating condition being different than the first and second machine operating conditions, and

defining a third reference value for the measured values of the electrical quantity, the third reference value being different from the first and second reference values, the third reference value corresponding to the detected third machine operating condition,

wherein the third machine operating condition comprises one of: standstill, acceleration, retardation, full speed or full load.

10. A system for adapting a ground fault detection to a change of an operating condition of an electrical machine, the system comprising:

a protective device connected to the electrical machine having a winding, the machine being in a first machine operating condition; and

a measurement device coupled to said protective device, said measurement device measuring values of an electrical quantity of the electrical machine;

said protective device defining a first reference value based on the measured values of the electrical quantity;

said measurement device continuously measuring the electrical quantity in the winding while the electrical machine is in the first machine operating condition, and said protective device detecting a ground fault based on the measured values of the electrical quantity and the first reference value;

said protective device receiving a signal corresponding to a change in the operating condition of the electrical machine from the first machine operating condition to a second machine operating condition;

said protective device defining a second reference value based on the second machine operating condition;

said measurement device continuously measuring the electrical quantity in the winding while the electrical machine is in the second machine operating condition, and said protective device detecting a ground fault based on the measured values of the electrical quantity and the second reference value;

wherein the first machine operating condition comprises one of: acceleration, retardation, full speed or full load and the second machine operating condition comprises one of: acceleration, retardation, full speed or full load.

11. The system according to claim 10 , wherein the received signal is any of: breaker status, machine load or excitation.

12. The system according to claim 10 , wherein the change of the machine operating condition is detected by any one of the properties of the received signal including: Root-Mean-Square amplitude, denoted as RMS, dominating frequency or dominating amplitude.

13. The system according to claim 10 , wherein the machine operating condition is in any one of: standstill, acceleration, retardation, full speed or full load.

14. The system according to claim 10 , wherein the ground fault detection comprises injecting a test signal at a predefined frequency into the winding of the machine.

15. The system according to claim 10 , wherein,

said protective device receiving a second signal corresponding to a change in the operating condition of the electrical machine from the second machine operating condition to a third machine operating condition;

said protective device defining a third reference value based on the third machine operating condition;

said measurement device continuously measuring the electrical quantity in the winding while the electrical machine is in the third machine operating condition, and said protective device detecting a ground fault based on the measured values of the electrical quantity and the third reference value;

wherein the third machine operating condition comprises one of: standstill, acceleration, retardation, full speed or full load.