Unit III Practice Questions

Q1. Draw the positive, negative and zero sequence impedance networks for the power system given in the figure. Choose a base of 50 MVA, 220kV in the 100 ohms transmission line and mark all impedances in pu. The ratings of the generators and transformers are: Generator 1: 25 MVA, 11kV, X1=20%; X2=20%; Generator 2: 25 MVA, 11kV, X1=20%; X2=20%; Three-phase transformer (each): 20MVA, 11 Y/220 Y kV, X=15%. The zero sequence reactance of each machine is 8%. The neutral reactance is 5% on individual machine ratings.

Q2. Draw the positive, negative and zero sequence impedance networks for the power system given in the figure. Choose a base of 50 MVA, 220kV in the 50Ω transmission lines and mark all impedances in pu. The ratings of the generators and transformers are:

Generator 1: 25 MVA, 11kV, X1=X2=20%, Xg0 = 10%Generator 2: 25 MVA, 11kV, X1=X2=15%, Xg0 = 8%Three-phase transformer (each): 20MVA, 11 Y/220 Y kV, X=15% The zero sequence reactance of each machine is 8%. Assume that the zero sequence reactance of the lines is 250% of their positive sequence reactance.

Q3. Draw the positive, negative and zero sequence impedance networks for the power system given in the figure. Choose a base of 100 MVA, 200kV in the 100 ohms transmission line and mark all impedances in pu. The ratings of the generators, transformers are:

G1: 20 MVA, 11kV, X1=20%; X2=10%; G2: 50 MVA, 33kV, X1=20%; X2=15%; YY Transformer: 50MVA, 200/11KV, X= 10%; Y∆ Transformer: 45MVA, 200/33KV, X = 15%. The zero sequence reactance of each machine is 5%. The neutral reactance is 3% on individual machine ratings.

Q4. Draw all the sequence networks for the power system shown in the figure below: Choose a base of 50 MVA, 220KV in the 50 ohm transmission lines and mark all the reactances in pu. The ratings of the generators and the transformers are:

Generator 1 : 25MVA, 11KV, X1=X2=15%, Xg0 = 5%

Generator 2 : 25MVA, 11KV, X1=X2=10%, Xg0 = 8% 3 phase transformers(each) = 20 MVA, 11 /220 KV, X = 15% The negative sequence reactance of each synchronous machine is equal to its positive seq. reactance. The zero sequence reactance of each machine is 8%. The neutral reactance is 5%.

Q5. Draw the positive, negative and zero sequence impedance networks for the power system given in the figure. Choose a base of 150 MVA, 220kV in the 60 ohms transmission line and mark all impedances in pu. The ratings of the generators, transformers are: G1: 50 MVA, 13kV, X1=25%; X2=15%; G2: 40 MVA, 25kV, X1=15%; X2=10%; YY Transformer: 45MVA, 220/15KV, X= 15%; Y∆ Transformer: 50MVA, 230/35KV, X = 10%.

The zero sequence reactance of each machine is 4%. The neutral reactance is 5% on individual machine ratings.