A method for controlling an operation of a distance relay for protection of a transmission line having a minimized reactance effect, said distance relay being adapted to, if a single phase to earth fault or a phase to phase short fault occurs on said transmission line, calculate an apparent impedance, detect an apparent fault location as a result of the calculation and generate a trip signal if the detected apparent fault location belongs to a protection zone, said method comprising the steps of:
- a) receiving a voltage/current signal flowing through said transmission line;
- b) filtering the received signal;
- c) obtaining a power frequency component from the filtered signal;
- d) comparing a level of the obtained power frequency component with a first predetermined threshold value to determine whether it is greater than said first predetermined threshold value, so as to determine whether overcurrent flows through said transmission line;
- e) if the overcurrent is determined to flow through said transmission line at said step d), comparing the amount of zero-sequence current of said filtered signal with a second predetermined threshold value to determine whether it is greater than said second predetermined threshold value, so as to determine which one of said earth fault and short fault has occurred on said transmission line;
- f) if the amount of said zero-sequence current of said filtered signal is determined to be greater than said second predetermined threshold value at said step e), determining that said earth fault has occurred on said transmission line and calculating a first apparent resistance (Rapp) and a first apparent reactance (Xapp) of a first apparent impedance;
- g) obtaining a first apparent fault location on the basis of said first apparent reactance;
- h) calculating a zero-sequence current distribution factor (CDFSa0) at said first apparent fault location;
- i) calculating a first phase angle (α) on the basis of the calculated zero-sequence current distribution factor;
- j) obtaining a first corrected impedance by correcting said first apparent impedance on the basis of said first phase angle;
- k) determining whether a difference between said first corrected impedance and a previous first apparent impedance is smaller than a third predetermined threshold value and returning to said step g) if the difference between said first corrected impedance and said previous first apparent impedance is determined not to be smaller than said third predetermined threshold value;
- l) if the amount of said zero-sequence current of said filtered signal is determined not to be greater than said second predetermined threshold value at said step e), determining that said short fault has occurred on said transmission line and calculating a second apparent resistance (Rapp) and a second apparent reactance (Xapp) of a second apparent impedance;
- m) obtaining a second apparent fault location on the basis of said second apparent reactance;
- n) calculating a positive-sequence current distribution factor (CDFSa1) at said second apparent fault location;
- o) calculating a second phase angle (α) on the basis of the calculated positive-sequence current distribution factor;
- p) obtaining a second corrected impedance by correcting said second apparent impedance on the basic of said second phase angle;
- q) determining whether a difference between said second corrected impedance and a previous second apparent impedance is smaller than a fourth predetermined threshold value and returning to said step m) if the difference between said second corrected impedance and said previous second apparent impedance is determined not to be smaller than said fourth predetermined threshold value;
- r) determining whether said first or second corrected impedance belongs to said protection zone, if said difference between said first corrected impedance and said previous first apparent impedance is determined to be smaller than said third predetermined threshold value at said step k) or if said difference between said second corrected impedance and said previous second apparent impedance is determined to be smaller than said fourth predetermined threshold value at said step q);
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- s) returning to said step b) if it is determined at said step r) that said first or second corrected impedance does not belong to said protection zone, and generating said trip signal if it is determined at said step r) that said first or second corrected impedance belongs to said protection zone.