Method for identifying the fault by current differential protection and device thereof

1. A method for operating a current differential relay, wherein said method comprises:

measuring full component currents of two terminals of a two-terminal line system and calculating corresponding fault component current vectors;

obtaining an operate value by calculating a first difference between an absolute value of a sum of said fault component current vectors and a first preset value;

obtaining a restrain value by multiplying a second difference with a control factor, in which said control factor includes a phase angle of fault component currents, and in which said second difference is calculated between a maximum of absolute values of said fault component currents and a second preset value or between an absolute value of the difference of said fault component currents and a second preset value;

identifying a fault as an external one or internal one by comparing said operate value with said restrain value; and

tripping the current differential relay in response to identifying the fault as internal.

2. The method according to claim 1 , wherein said control factor is a first positive value for an external fault and second positive value or negative value for an internal fault, wherein the first positive value is greater than the second positive value.

3. The method according to claim 1 , wherein if the operate value is smaller than the restrain value, a fault is identified as an external fault or no fault occurs in the system; and if the operate value is larger than the restrain value, a fault is identified as an internal fault.

4. The method according to claim 1 , further comprising:

converting a multi-terminal system into a virtual two-terminal system; and

employing the virtual two-terminal system as the two-terminal line system.

5. The method according to claim 4 , further comprising:

measuring the full component currents of all terminals of the multi-terminal line system and calculating corresponding fault component current vectors;

selecting a maximum from the calculated fault component currents;

calculating a sum of all the other calculated fault component current except said maximum; and

deeming said maximum and said sum as two terminals' currents of said virtual two-terminal system.

6. The method according to claim 1 , further comprising:

directly using the full component current vectors corresponding to said fault component current vectors for identifying the fault.

7. A device for current differential protection, wherein said device comprises:

a measurement module configured to measure full component currents of two terminals of a two-terminal line system;

a calculating module configured to calculate corresponding fault component current vectors; obtain an operate value by calculating a first difference between an absolute value of a sum of said fault component current vectors and a first preset value; obtain a restrain value by multiplying a second difference with a control factor, in which said control factor relates to a phase angle of fault component currents, and in which said second difference is calculated between a maximum of absolute values of said fault component currents and a second preset value or between an absolute value of the difference of said fault component currents and the second preset value;

an identifying module configured to identify a fault as an external one or internal one by comparing said operate value with said restrain value; and

a relay structured to trip in response to the identifying module identifying the fault as internal.

8. The device according to claim 7 , wherein said control factor has a first positive value for an external fault and second positive value or negative value for an internal fault, wherein the first positive value is greater than the second positive value.

9. The device according to claim 7 , wherein said identifying module identifies a fault as an external fault or no fault in the system if the operate value is smaller than the restrain value; and as an internal fault if the operate value is greater than the restrain value.

10. The device according to claim 7 , wherein said device is applied to a multi-terminal system by converting said multi-terminal system into a virtual two-terminal system.

11. The device according to claim 10 , wherein

said measurement module is further configured to measure full component currents of all terminals of a multi-terminal line system and calculate corresponding fault component current vectors;

said calculating module is further configured to select a maximum from the calculated fault component currents, and calculate a sum of all the other calculated fault component current except said maximum; and

a converting module is configured to deem said maximum and said sum as two terminals' currents of said virtual two-terminal system.

12. The device according to claim 7 , wherein said device directly uses the full component current vectors corresponding to said fault component current vectors for identifying the fault.

13. The device according to claim 11 , wherein said device directly uses the full component current vectors corresponding to said fault component current vectors for identifying the fault.

14. The method according to claim 5 , further comprising:

directly using the full component current vectors corresponding to said fault component current vectors for identifying the fault.