Question

Q6. In the power system of Fig. 4, the transmission lines are protected by directional impedance relays. The relay polar characteristics are of the MHO type (a circle passing through the origin of coordinate system). The relays are set for two-zone operation. For Zone 1, the reach is 80% of the closest line length and the trip time is 0.25 s. For Zone 2, the reach is 120% of the closest line length, and the trip time is 0.5 s. The line impedance in each section is (0.05 + j0.15) $\Omega$/km. The transmission line lengths between buses are given in Fig. 4. 6.a Calculate the impedance setting of relay Q12, in terms of the actual line impedance, for Zone [3 marks] 1 and Zone 2. 6.b Draw the corresponding polar characteristics of relay Q12 for Zone 1. Indicate the scale of the coordinates. Show the impedance setting point and the point corresponding to a fault at a distance of 12 km from Q12. Mark the region in which the relay trips with a delay of 0.25 s [6 marks] 6.c Plot the relationship between the trip time of Q12 and Q23, and the distance of the fault from Q12 in km. Indicate the scale of the coordinate axes. [7 marks] 6.d In your exam book, draw the boundaries of protective zones in the power system shown in Fig.4. Make sure that you show without ambivalence which components of the power system are included in each boundary. [4 marks] Total Marks for Question 6 = 20 marks 1 2 3 Q1 Q12 Q21 Q23 Q32 Q2 60 km 60 km Figure 4. Power system with impedance relay protection

Q6.
In the power system of Fig. 4, the transmission lines are protected by directional impedance relays.
The relay polar characteristics are of the MHO type (a circle passing through the origin of coordinate
system). The relays are set for two-zone operation. For Zone 1, the reach is 80% of the closest line
length and the trip time is 0.25 s. For Zone 2, the reach is 120% of the closest line length, and the trip
time is 0.5 s. The line impedance in each section is (0.05 + j0.15) $\Omega$/km. The transmission line
lengths between buses are given in Fig. 4.
6.a Calculate the impedance setting of relay Q12, in terms of the actual line impedance, for Zone
[3 marks]
1 and Zone 2.
6.b Draw the corresponding polar characteristics of relay Q12 for Zone 1. Indicate the scale of the
coordinates. Show the impedance setting point and the point corresponding to a fault at a
distance of 12 km from Q12. Mark the region in which the relay trips with a delay of 0.25 s [6
marks]
6.c Plot the relationship between the trip time of Q12 and Q23, and the distance of the fault from
Q12 in km. Indicate the scale of the coordinate axes.
[7 marks]
6.d In your exam book, draw the boundaries of protective zones in the power system shown in
Fig.4. Make sure that you show without ambivalence which components of the power system
are included in each boundary.
[4 marks]
Total Marks for Question 6 = 20 marks
1
2
3
Q1
Q12
Q21
Q23
Q32
Q2
60 km
60 km
Figure 4. Power system with impedance relay protection

Added by Yolanda A.

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