Differential protection method, system and device

1. A differential protection method within a power network for determining type of fault occurring within said power network, said power network comprising a protected object having two or more ends, wherein a current differential protection device is arranged at each end, characterized by the steps of:

obtaining, in the current differential protection device at a first end of said protected object, measured values from the current differential protection device at a second end of said protected object,

comparing, in the current differential protection device at said first end, changes in measured values taken at said first end with changes in said measured values obtained from the current differential protection device at said second end,

determining, upon said step of comparing changes in measured values showing differing results, type of fault occurring within said power network.

2. The method as claimed in claim 1 , wherein said step of determining type of fault comprises determining occurrence of a primary fault if said step of comparing changes in measurement values shows changes at two or more ends.

3. The method as claimed in claim 2 , comprising the further step of tripping a circuit breaker, in response to said step of comparing showing a change in measured values at two or more ends.

4. The method as claimed in claim 1 , wherein said step of determining type of fault comprises determining occurrence of a secondary fault if said step of comparing changes in measured values shows changes at only one end.

5. The method as claimed in claim 4 , comprising the further step of blocking a tripping command upon said step of determining showing a change in measured values at only one end.

6. The method as claimed in claim 1 , comprising the further step of repeating said steps of obtaining and comparing changes in measured values, if said step of comparing shows no changes in measured values.

7. The method as claimed in claim 1 , wherein said changes in measured values comprises comparing current changes at said two or more ends.

8. The method as claimed in claim 1 , wherein said step of obtaining measured values comprises obtaining numerical values over a fibre-optical connection.

9. The method as claimed in claim 1 , wherein said step of comparing changes in measured valued comprises comparing, in the current differential protection device at said first end, changes in synchronized time tagged measured values taken at the first end with changes in said measured values obtained from the current differential protection device at said second end, which measured values are time tagged measured values synchronized to the same clock as the measured values at said first end.

10. The method as claimed in claim 1 , wherein said current differential protection devices are connected to said protected object by means of a respective current transformer.

11. The method as claimed in claim 1 , wherein said protected device comprises a transmission line.

12. A differential protection system within a power network comprising a protected object having two or more ends, wherein a current differential protection device is arranged at each end, characterized by:

means, in said current differential protection devices, for measuring at each respective end a change of measured values,

means, in said current differential protection device, for receiving from the other current differential protection device, measured values,

means, in each of said current differential protection device, for comparing changes in measured values taken at said first end to changes in said measured values obtained from said second end, and

means, in each of said current differential protection device for determining type of fault occurring within said power network.

13. The differential protection system as claimed in claim 12 , wherein said current differential protection devices are connected to said protected object by means of a respective current transformer.

14. A current differential protection device for a power network comprising a protected object having two or more ends, characterized by:

means for measuring a change of measured values,

means for obtaining measured values from other end(s) of said protected object,

means for comparing changes in measured values taken at said first end to changes in said measured values obtained from said second end, and

means for determining type of fault occurring within said power network.

15. The current differential protection device as claimed in claim 14 , comprising means for implementing a differential protection method within said power network for determining type of fault occurring within said power network, said power network comprising a protected object having two or more ends, wherein a current differential protection device is arranged at each end, characterized by the steps of obtaining, in the current differential protection device at a first end of said protected object, measured values from the current differential protection device at a second end of said protected object; comparing, in the current differential protection device at said first end, changes in measured values taken at said first end with changes in said measured values obtained from the current differential protection device at said second end; and determining, upon said step of comparing changes in measured values showing differing results, type of fault occurring within said power network.