According to this paper, the three stages in the DC Fault characteristic of a VSC are
- Capacitor-Discharge State (Natural Response)
- Diode Freewheeling Stage (When source AC gets disconnected and \$V_C = 0\$; Natural Response)
- Grid Side Current Feeding Stage (Forced Response)
The diode freewheeling stage comes into picture when the source of the AC power is disconnected and the capacitor is completely discharged. But then, the grid-side current feeding stage takes the source AC into consideration. How is this possible?
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?
The referenced content is in Section II, Subsection B under the title "VSC DC Fault Characteristics".