Q$4)$ The three$-$phase bidirectional AC controller shown in Fig. $(3)$ supplies a wye$-$connected resistive load of $R=20$ and the line$-$to$-$line input voltage is $400$ V $($ rms $).50$ Hz $.$

a$)$ Draw output voltage firi. z angles : $=75.$ Explain the operation of each thyristor.

b$)$ Determine the output voltage for $a=30$ and $120.$

c$)$ Determine the output power Po for $=30$ and $120.$ At what angle minimum power is transferred from source to load?