Electrical Power Systems

Power System Components and Per Unit System 119
Example 5.11: Determine the transformer tap ratios when the receiving end voltage is equal to
the sending end voltage, the high voltage line operates at 220 kV and transmit 80 MW at 0.8 pf
an the impedance of the line is (40 + j140)W. Assume tS tR = 1.0.
Solution: Using eqn. (5.43), we have
t S =
L
NM
HG
| V2|
PR + QX
1 -
| V | | V|| V|
1 1 2
|V 2| = |V 1| = 220
3
| V2|
= 1.0
| V |
1
I
KJ
O
QP
-1
1/ 2
= 127.017 KV,
P = 1
× 80 × 0.8 = 21.33 MW
3
Q = 1
× 80 × 0.6 = 16 MVAr.
3
R = 40 W, X = 140 W
L
NM
2133 40 16 140
\ tS = 1 1
127 017 2
. ´ + ´
× -
( . )
\ t S = 1.11, \ t R = 1
t S
HG
I KJ
O
QP
1 1/2
-
= 0.90 Ans.
Example 5.12: A 132 kV line is fed through 33/132 kV transformer from a constant 33 kV
supply as shown in ig. 5.28. The total impedance of the line and transformers at 132 kV is (10
+ j25)W. If the load on the system is 100 MW at 0.65 pf long, calculate the setting of the tap
changer required to maintain the voltage of the load bus at 33 kV.
Solution: Using eqn. (5.43), we have
ig. 5.28: Single line diagram of example 5.12.
t S =
L
NM
HG
| V2|
PR + QX
1 -
| V | | V|| V|
1 1 2
I
KJ
O
QP
-1
1/ 2
P = 100
3 × 0.65 = 21.66 MW |V2| = |V1|= 132
3
Q = 100
3
× 0.76 25.33 MVAr.
= 76.2 KV