Distance protection relay and method

1. A protective relay for an electrical power distribution system comprising one or more power lines, said protective relay comprising:

a first and a second current transformer arranged to sense a respective current flowing into a power line,

a voltage sensing device arranged to sense a voltage of said power line,

a first device for determining a direction of a fault within the system based on a current signal from said first current transformer and a voltage signal from said voltage sensing means,

a second device for determining the direction of a fault within the system based on a current signal from said second current transformer and a voltage signal from said voltage sensing means,

a third device for determining the direction of a fault within the system based on a sum of the current signals from said first and second current transformers and a voltage signal from said voltage sensing means, and

controller means for determining whether a fault is internal or external based on the fault directions determined by said first, second and third devices.

2. The protective relay according to claim 1 , wherein if one or more of said first, second and third devices indicates an external fault, then the fault is determined to be an external fault.

3. The protective relay according to claim 1 , further comprising:

means for setting a current level, wherein said first and second devices are arranged to be disregarded if the current signal in the respective device is below said set current level.

4. The protective relay according to claim 3 , wherein said current level is set below a current level at which there is a risk of saturation of said current transformers.

5. The protective relay according to claim 3 , wherein said current level is set to be higher than the maximum load current utilized.

6. The protective relay according to claim 1 , wherein said first, second and third devices comprise directional elements.

7. The protective relay according to claim 1 , further comprising:

a plurality of circuit breakers.

8. The protective relay according to claim 7 , further comprising:

means for tripping a circuit breaker in response to a determined fault.

9. The protective relay according to claim 1 , wherein said protective relay is utilized in a breaker-and-a half system.

10. An electrical power distribution system, comprising:

one or more protective relays comprising a first and a second current transformer arranged to sense a respective current flowing into a power line, a voltage sensing device arranged to sense a voltage of said power line, a first device for determining a direction of a fault within the system based on a current signal from said first current transformer and a voltage signal from said voltage sensing means, a second device for determining the direction of a fault within the system based on a current signal from said second current transformer and a voltage signal from said voltage sensing means, a third device for determining the direction of a fault within the system based on a sum of the current signals from said first and second current transformers and a voltage signal from said voltage sensing means, and controller means for determining whether a fault is internal or external based on the fault directions determined by said first, second and third devices.

11. A method in a protective relay of an electrical power distribution system comprising one or more power lines, said protective relay comprising a first and a second current transformer arranged to sense a respective current flowing into a power line, and a voltage sensing means arranged to sense the voltage of said power line, the method comprising:

determining, in a first device, a direction of a fault within the system based on a current signal from said first current transformer and a voltage signal from said voltage sensing means,

determining, in a second device, the direction of a fault within the system based on a current signal from said second current transformer and a voltage signal from said voltage sensing means,

determining, in a third device, the direction of a fault within the system based on a sum of current signals from said first and second current transformers and a voltage signal from said voltage sensing means, and

determining, in controller means, whether a fault is internal or external based on the fault directions determined by said first, second and/or third devices.

12. The method as according to claim 11 , wherein if one or more of said first, second and third devices indicates an external fault, then the fault is determined to be an external fault.

13. The method as according to claim 11 , further comprising:

setting a current level, wherein said first and second devices are disconnected if the current signal in the respective device is below said set current level.

14. The method as according to claim 13 , wherein said current level is set below a current level at which there is a risk of saturation of one of said current transformers.

15. The method as according to claim 13 , wherein said current level is set to be higher than the maximum load current.

16. The method as according to claim 11 , wherein said first, second and third devices comprise directional elements.

17. The method as according to claim 11 , wherein said protective relay further comprises a number of circuit breakers.

18. The method as according to claim 17 , wherein said protective relay trips a circuit breaker in response to a determined fault.