Protective relay for power systems having fault distance measurement filter logic

1. A system for improving the performance of a distance-type protective relay for power systems, wherein the relay includes a calculation circuit responsive to voltage and current values from the power line to produce a quantity analogous to the distance between the relay and a fault on the power line, wherein the quantity is applied to a distance element for comparison of said quantity with a setting reach value for a selected zone of protection, the system comprising:

a filter circuit responsive to said quantity for filtering said quantity before the quantity is applied to the distance element, resulting in noise attenuation of the quantity; and

a control circuit for controlling the application of the filtered quantity to the distance element such that the filtered quantity is applied only when said quantity is above a preselected first threshold value and below a preselected second threshold value.

2. The system of claim 1 , wherein the preselected first threshold value is a selected percentage of the setting reach value.

3. The system of claim 2 , wherein the selected percentage is 100% minus an error of the system plus 5% for the relay.

4. The system of claim 2 , wherein the selected percentage is approximately 92%.

5. The system of claim 1 , further comprising a circuit for pre-charging the filter to the preselected second threshold value when said quantity decreases to the preselected second threshold value from said high value, in response to a fault.

6. The system of claim 1 , wherein the preselected second threshold value is approximately four times the setting reach value.

7. A method for selecting between one of an unfiltered m value and a filtered m value provided to a distance element of a protective relay providing protection for a transmission line of a power system, the protective relay including a calculation circuit adapted to provide the unfiltered m value indicative of a distance between the protective relay and a fault, and a filter adapted to filter the unfiltered m value to form the filtered m value, the method comprising:

comparing the unfiltered m value to a first percentage of a zone reach value to form a first binary output, the first percentage of the zone reach value greater than the zone reach value;

comparing the unfiltered m value to a second percentage of the zone reach value to form a second binary output, the second percentage of the zone reach value less than the zone reach value.

providing the unfiltered m value to the distance element when the first binary output comprises a low binary value or when the second binary output comprises a high binary value; and

providing the filtered m value to the distance element when the first binary output comprises a high binary value and the second binary output comprises a low binary value.

8. The method of claim 7 , wherein the first binary output has a binary high value when the first percentage of the zone reach value is greater than the unfiltered m value, and has a binary low value when the first percentage of the zone reach value is less than the unfiltered m value, and wherein the second binary output has a binary high value when the second percentage of the zone reach value is greater than the unfiltered m value, and has a binary low value when the second percentage of the zone reach value is less than the unfiltered m value.

9. The method of claim 7 , wherein the filter is charged immediately after the unfiltered m value is equal to or less than the first percentage of the zone reach value, the unfiltered m value equaling the first percentage of the preselected setting indicating an occurrence of a fault in the transmission line.

10. The method of claim 7 , wherein the filter operation is defined by ms k−1 .