DC Fault Characteristics of a Voltage Source Converter
Q: the grid-side current feeding stage takes the source AC into consideration. How is this possible?


A:  the condition is a grid fault (0V sc.to gnd) with the voltage source now appearing as a current sink to the 400V charged microgrid capacitor.


  Q2: Also, the first and second stage is described as a natural response and the third stage is described as a forced response. What exactly is forced here? 


A:  the Inductor charges up the current which becomes the new forcing Function and must be protected by opening some  IGBT (s) to allow the free-wheel diodes to clamp the natural impulse Voltage. 

When the diodes conduct, this current absorbed is “forced back onto the grid” for the RC time duration while the grid is in fault to ground with some natural LC damped ringing.  


it is the same method used for regenerative braking in e-cars except instead of a motor-generator on DC,you have a storage inductor with flyback current or back EMF voltage clamped thru diode current rectifiers to a shorted grid fault.