In this problem there is a full wave rectifier (with capacitor filter) on each of the three phases.
Operation:
Assume steady state condition and negligible diode voltage drop, like the answer says. Pick a diode pair (top left and bottom right for positive half cycle). Because a capacitor is present at the full wave rectifier output, the diodes are reverse biased until the magnitude of the input voltage (Vin) exceeds the voltage on the capacitor (Vc). When Vin > Vc the diodes are forward biased and current flows. When Vin < Vc the diodes are reverse biased and the cap starts discharging through R.
The question is on neutral current which is on the unrectified side. Neutral current will flow in one direction for half-cycle (part of half-cycle actually) and will reverse for the other half. Also, the neutral is shared among the phases and will carry current for all 3 phases (at different times though).
Answer D is wrong because there'll be some current flow to charge the capacitor.
Answers B and C are wrong ‘cause neutral current flows in both directions.
Problem 514:
Refer to the second figure that shows the switching sequence:
When you go through the switching sequence and look at the L-L voltage graph, you can see that the voltage "magnitude" is either VDC or zero. So the maximum voltage applied L-L is 679 volts.
