Electrical Power Systems

Power System Components and Per Unit System 123
ig. 5.33: (a) Transmission line with series capacitor, (b) Voltage phasor diagram
with series capacitor.
4. The shunt capacitor improves the power factor of the load whereas the series capacitor has
little effect on the power factor.
5. or long transmission lines where the total reactance is high, series capacitors are effective
for improvement of stability.
EXERCISE
5.1. Three generators are rated as follows: G1 : 100 MVA, 33 kV, x g1 = 0.10 pu. G2 : 150 MVA, 32 kV,
x g2 = 0.08 pu G3 : 110 MVA, 30 kV, x g3 = 0.12 pu. Determine the reactance of the generator
corresponding to base values of 200 MVA and 35 kV.
Ans. x g1 = 0.177 pu, x g2 = 0.089 pu, x g3 = 0.16 pu
5.2. Determine the transformer tap ratios when the receiving end voltage is equal to the sending end
voltage. The high voltage line operates at 230 kV and transmits 80 MW at 0.8 pf and the impedance
of the line is (40 + j150)W. Ans. tS = 1.14, tR = 0.87
5.3. Draw an impedance diagram for the electric power system as shown in ig. 5.34, and all impedances
in per-unit on a 100 MVA base. Select 20 kV base voltage for generator. The three-phase power and
line ratings are given below.
G1 : 90 MVA, 20 kV, xg1 = 0.09 pu
G2 : 90 MVA, 18 kV, xg2 = 0.09 pu
T1 : 80 MVA, 20/200 kV, xt1 = 0.16 pu
T2 : 80 MVA, 200/20 kV, xt2 = 0.20 pu
Line : 200 kV, xline = 120 W
Load : 200 kV, S = (48 + j64)MVA.
ig. 5.34: Single line diagram for problem 5.3.