a) Calculate the apparent impedance seen by a distance relay at terminal X for a

Question

a) Calculate the apparent impedance seen by a distance relay at terminal X for a three terminal line with the following impedance and fault current values. The fault is at Terminal Y. State any assumptions you make. b) Will a Zone 2 distance relay at terminal X set with a 10 Ohm secondary reach detect this fault within the protected line? Z_XT = j 1.2 Ohm secondary Z_YT = j 3.8 Ohm secondary Z_WT = j 2.0 Ohm secondary I_X = 8.5 A secondary I_W = 17.0 A secondary Assume Z_base = 115 kV^2/100 MVA = 132.25 Ohm

Explanation / Answer

Actual impedance = ZXT + ZYT =1.2 +3.8 =5 ohms

Apparent impedance is E/I = (8.5 x1.2) +(25.5x3.8)/17

=6.3 ohms

Hence apparent impedance is greater than actual impedance

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