Electrical Power Systems

Power System Components and Per Unit System 121
Van¢ = Van + DVan ...(5.44)
This type of booster transformer is called an in-phase booster because the voltage are in
phase. Van¢ can be adjusted by changing the taps of excitation transformer. The polarity of the
voltage across the series transformer can be made reversed by changing the switch position
from S1 to S2, such that Van¢ is less than Van. ig. 5.29: Booster transformer for voltage magnitude control.
B. Phase Angle Control
Booster transformers are also used to control the voltage phase angle. A typical arrangement
for phase ‘a’ of a booster transformer for phase angle control is shown in ig. 5.30.
ig. 5.30: Booster transformer for voltage phase angle control.
Phase shifting is used to control active power flow. rom ig. 5.30, we can write,
Van¢ = Van + DVbc ...(5.45)
igure 5.31 gives the phasor diagram showing phase
shifting effect of phase a.
Now let us assume.
× 3 - 90°
DVbc = a.Vbc = a Van
\ DVbc = - ja 3 Van
...(5.46)
Using eqn. (5.45) and (5.46). We have
Van¢ = ( 1-ja 3)
Van ...(5.47)
ig. 5.31: Voltage phasor diagram.