Electrical Power Systems

120 Electrical Power Systems
L
NMHG
2166 10 25 33 25
\ tS = 1
76 2 2
. ´ + . ´
-
( . )
\ t S = 1.08, \ t R = 0.92 Ans.
I KJ
O
QP
1 1/2
-
Example 5.13: A 12.66 kV line is fed through an 11/132 kV transformer from a constant 11 kV
supply. At the load end of the line the voltage is reduced by another transformer of nominal ratio
132/11 kV. The total impedance of the line and transformers at 132 kV is (25 + j66)W. Both
transformers are equipped with tap-changing facilities which are arranged so that the product
of the two off-nominal settings is unity. If the load on the system is 100 MW at 0.9 pf lagging,
calculate the settings of the tap-changers required to maintain the voltage of the load busbar at
11 kV.
Solution: As the voltage drop is to be completely componsated,
Using eqn. (5.43)
|V 1 | = |V 2 | = 132
t S =
\ t S =
L
NM
HG
3
| V2|
PR + QX
1 -
| V | | V|| V|
1 1 2
KV = 76.2 KV
I
KJ
O
QP
-1
1/ 2
P = 1
× 100 × 0.9 = 30 MW
3
Q = 1
× 100 × 0.435 = 14.53 MVAr., R = 25 W, X = 66 W
3
L
NM
HG
30 ´ 25 + 14.53 ´ 66
1× 1 –
\ t S = 1.19; t R = 0.84.
(76.2) 2
I
KJ
O
QP
-1 12 /
There settings are large for the normal range of tap-changing transformers. It would be
necessary, in this system, to inject vars at the load end of the line to maintain the voltage at the
required value.
5.5.2 Booster Transformers or Regulating Transformers
Regulating transformers also known as booster transformers. These are used to change the
voltage magnitude and phase angle at an intermediate point in a line rather than at the ends
as with tap-changing transformers or the system may not warrant the expense of tap-changing.
A. Voltage Magnitude Control
igure 5.29 shows the connection of a booster transformer for phase a. The secondary of the
exciting transformer is tapped and the voltage obtained from it is applied to the primary of the
series transformer. The corresponding secondary voltage of the series transformer is added to
the input voltage. Therefore, the output voltage is