The Three-Phase Tap-Changing Transformer (Two-Windings) block is used to model an On Load Tap Changer Transformer (OLTC) that regulates positive-sequence voltage at B2 25-kV bus. The reference voltage is set at 1.0 pu.

Observe OLTC reaction for the following 3 events:

1.1 pu over-voltage in 120 kV network at t= 2 sec.

0.95 pu under-voltage in 120 kV network at t= 10 sec.

0.2 sec single-phase fault at B2 bus at t= 20 sec. OLTC does not react because under-voltage duration is lower than specified Delay (0.4 sec)

Note: In order to reduce simulation time, the tap selection time (usually a value between 3 sec and 10 sec) has been shortened to 0.5 sec.