Electrical Power Systems

Base impedance for the load is
( )
ZB, load = 11
W = 1.21 W
100
2
Power System Components and Per Unit System 115
(. 1532 + j 11495 . )
\ ZL (pu) = pu = (1.266 + j 0.95) pu
121 .
The per-unit equivalent circuit diagram is shown in ig. 5.22.
ig. 5.22: Per-unit impedance diagram of example 5.8.
5.5 METHODS O VOLTAGE CONTROL
The methods for voltage control are the use of (i) Tap changing transformer, (ii) Regulating
transformers or Boosters, (iii) Shunt capacitors, (iv) Series capacitors, (v) ACTS devices.
In this book, first four category will be described.
5.5.1 Tap Changing Transformer
The main purpose of all power transformer and distribution transformers is to transform
electric energy from one voltage level to another. Practically all power and many distribution
transformers have taps for changing the turns ratio. Voltage magnitude is altered by changing
the tap setting and affects the distribution of VARS and may be used to control the flow of
reactive power. There are two types of tap changing transformers.
(i) Off-load tap changing transformers.
(ii) Tap changing under load (TCUL) transformers.
The off-load tap changing transformer requires the disconnection of the transformer when
the tap setting is to be changed. igure 5.23 gives the connection of off-load tap changing
transformer. A typical off-load tap changing transformer might have four taps in addition to the
nominal setting.
Tap changing under load is used when changes in turn ratio may be frequent. Basically, a
TCUL transformer is a transformer with the ability to change taps while power is connected.
igure 5.24 gives the diagram of on-load tap changing transformer. In the position shown in
ig. 5.24, the voltage is maximum. To change the voltage, following operations are required: (i)
open A1, (ii) move selector switch P1 to the next contact, (iii) Close A1, (iv) open A2, (v) move
selector switch P2 to the next contact, (vi) close A2. These operations are required for one
change in tap position. Step-down units usually have TCUL in the low voltage winding and
de-energized taps in the high voltage winding.