Method and control device for current differential protection

1. A methods of current differential protection performed in a control device, the control device having a first operate-restrain characteristic with a differential characteristic pick-up setting I D , the method comprising:

determining currents of all terminals of a protected object,

determining a differential current based on the determined currents,

determining direct current, DC, components, I DC in the respective determined currents,

detecting a fault, and determining whether it is an internal or external fault, and

adjusting, for a detected external fault, the operate-restrain characteristic by setting an adjusted differential characteristic pick-up setting I D _ adj to be equal to the sum of the differential characteristic pick-up setting I D and the determined DC components, I DC , providing an adjusted operate-restrain characteristic.

2. The method as claimed in claim 1 , including issuing a trip signal for opening one or more circuit breakers when the determined differential current is within an operate area of the adjusted operate-restrain characteristic.

3. The method as claimed in claim 1 , including, for a detected internal fault, issuing a trip signal for opening one or more circuit breakers when the determined differential current is within an operate area of the first operate-restrain characteristic.

4. The method as claimed in claim 1 , wherein the object includes a transformer.

5. The method as claimed in claim 1 , wherein the detecting the fault includes using a negative sequence current method, a zero sequence current method or an incremental current method.

6. A non-transitory computer readable medium storing a computer program for a control device for current differential protection, the computer program comprising a computer program code, which, when executed on at least one processor on the control device causes the control device to perform a method including:

determining currents of all terminals of a protected object,

determining a differential current based on the determined currents,

determining direct current, DC, components, I DC in the respective determined currents,

detecting a fault and determining whether it is an internal or external fault, and

adjusting, for a detected external fault, an operate-restrain characteristic having a differential characteristic pickup setting I D by setting an adjusted differential characteristic pick-up setting I DC _ adj to be equal to the sum of the differential characteristic pick-up setting I D and the determined DC components, I DC , providing an adjusted operate-restrain characteristic.

7. A control device arranged to protect an object of an electrical power system by current differential protection, the control device having a first operate-restrain characteristic with a differential characteristic pick-up setting I D , the control device being configured to:

determine currents of all terminals of a protected object,

determine a differential current based on the determined currents,

determine direct current, DC, components I DC in the respective determined currents,

detect a fault and determine whether it is an internal or external fault, and

adjust, for a detected external fault, the operate-restrain characteristic by setting an adjusted differential characteristic pick-up setting I D _ adj to be equal to the sum of the differential characteristic pick-up setting I D and the determined DC components I DC , providing an adjusted operate-restrain characteristic.

8. The control device as claimed in claim 7 , configured to issue a trip signal for opening one or more circuit breakers when the determined differential current is within an operate area of the adjusted operate-restrain characteristic.

9. The control device as claimed in claim 7 , configured to, for a detected internal fault, issue a trip signal for opening one or more circuit breakers when the determined differential current is within an operate area of the first operate-restrain characteristics.

10. The control device as claimed in claim 7 , wherein the object includes a transformer.

11. The control device as claimed in claim 7 , configured to detect the fault by using a negative sequence current method, a zero sequence current method or an incremental current method.

12. The method as claimed in claim 2 , including, for a detected internal fault, issuing a trip signal for opening one or more circuit breakers when the determined differential current is within an operate area of the first operate-restrain characteristic.

13. The method as claimed in claim 2 , wherein the object includes a transformer.

14. The method as claimed in claim 2 , wherein the detecting the fault includes using a negative sequence current method, a zero sequence current method or an incremental current method.

15. The control device as claimed in claim 8 , configured to, for a detected internal fault, issue a trip signal for opening one or more circuit breakers when the determined differential current is within an operate area of the first operate-restrain characteristic.

16. The control device as claimed in claim 8 , wherein the object includes a transformer.

17. The control device as claimed in claim 8 , configured to detect the fault by using a negative sequence current method, a zero sequence current method, or an incremental current method.