Control of operation of protection relay

1. A method of controlling a protection relay, comprising;

inputting an input characteristic quantity; and

determining a triggering condition for the protection relay based on a value of a calculation equation, wherein for an overfunction measure the value increases when the input characteristic quantity exceeds a first threshold value and decreases when the input characteristic quantity undershoots a second threshold value, wherein for an underfunction measure the value of the calculation equation increases when the input characteristic quantity undershoots a first threshold value and decreases when the input characteristic quantity exceeds a second threshold value, wherein the calculation equation includes a first measure and a second measure, and wherein triggering occurs when the first measure is greater than the second measure.

2. A method according to claim 1 , wherein the calculation equation is arranged into a form of a division, where the first measure is arranged as a nominator of the division and the second measure is arranged as a denominator of the division.

3. A method according to claim 1 , wherein the second threshold value is obtained by reducing a hysteresis setting from the first threshold value for an overfunction measure, and by adding a hysteresis setting to the first threshold value for an underfunction measure, a region between the first threshold value and the second threshold value defining a hysteresis region in which calculation of the calculation equation is stopped.

4. A method according to claim 1 , wherein the calculation equation includes at least one cumulatively increasing sum term which is increased upon a predetermined condition, and a predetermined maximum value is defined for the at least one sum term, and wherein the sum term is divided by a divisor if the sum term reaches the maximum value.

5. A method according to claim 4 , wherein sum term values are indexed in a look-up table at pre-defined intervals as a function of the characteristic measure, and upon reaching a cumulative sum maximum value, the indexed look-up-table sum term is divided by the same divisor as the cumulative sum itself.

6. A method according to claim 5 , wherein the pre-determined maximum value differs between different sum terms.

7. A method according to claim 5 , wherein the intervals between the look-up table indices vary stepwise, such that more indices are located in an abrupt part of a curve representing the sum term values as a function of the indices.

8. A method according to claim 1 , comprising:

monitoring at least one variable in the calculation equation, and comparing the value of each of the at least one variables to a predetermined threshold value of the variable, and reducing each term in the calculation equation similarly, if a variable reaches its threshold value.

9. A protection relay, comprising;

means for inputting an input characteristic quantity; and

means for determining a triggering condition for the protection relay based on a value of a calculation equation, wherein for an overfunction measure the value is configured to increase when the input characteristic quantity exceeds a first threshold value and to decrease when the input characteristic quantity undershoots a second threshold value, wherein for an underfunction measure the calculation equation is configured to increase when the input characteristic quantity undershoots a first threshold value and to decrease when the input characteristic quantity exceeds a second threshold value, wherein the calculation equation includes a first measure and a second measure, and wherein triggering occurs when the first measure is greater than the second measure.

10. A protection relay according to claim 9 , wherein the calculation equation is arranged into a form of a division, where the first measure is arranged into a nominator of the division and the second measure is arranged into a denominator of the division.

11. A protection relay according to claim 9 , wherein the second threshold value is obtained by reducing a hysteresis setting from the first threshold value for an overfunction measure, and by adding a hysteresis setting to the first threshold value for an underfunction measure, a region between the first threshold value and the second threshold value defining a hysteresis region in which calculation of the calculation equation is stopped.

12. A protection relay according to claim 9 , wherein the calculation equation includes at least one cumulatively increasing sum term, which is increased upon a predetermined condition, and a predetermined maximum value is defined for the at least one sum term, and wherein the sum term is divided by a divisor if the sum term reaches the maximum value.

13. A protection relay according to claim 12 , wherein sum term values are indexed in a look-up table at pre-defined intervals as a function of the characteristic measure, and upon reaching a cumulative sum maximum value the indexed look-up-table sum term is divided by the same divisor as the cumulative sum itself.

14. A protection relay according to claim 12 , wherein the predetermined maximum values differ between different sum terms.

15. A protection relay according to claim 12 , wherein the intervals between the look-up table indices vary stepwise, such that more indices are located in an abrupt part of a curve representing the sum term values as a function of the indices.

16. A protection relay according to claim 9 , wherein at least one variable is monitored in the calculation equation, and the value of each of the at least one variables is monitored to a predetermined threshold value of the variable, and each term in the calculation equation is similarly reduced if a variable reaches its threshold value.