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According to this paper, the three stages in the DC Fault characteristic of a VSC are

  1. Capacitor-Discharge State (Natural Response)
  2. Diode Freewheeling Stage (When source AC gets disconnected and \$V_C = 0\$; Natural Response)
  3. 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".

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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.
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The fault characteristics can be divided into three stages as described because "the VSC...interfaces to the ac side through an inductor (Lac)." In addition, there is inductive impedance on the DC side in series with the fault. That impedance controls the timing of the response such that it can be analyzed in three stages.

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  • \$\begingroup\$ I got that. My question is what makes the first two a natural response and the third stage, a forced response? And, how did the source AC which was disconnected in the second stage get connected in the third? \$\endgroup\$
    – Niteesh Shanbog
    Commented Jul 15, 2019 at 11:57
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    \$\begingroup\$ I assume that the AC source is not physically disconnected in the first stage, but delayed by the source impedance. It may be ultimately disconnected physically, but the protective device can not act quickly enough to disconnect it until the third stage. \$\endgroup\$
    – user80875
    Commented Jul 15, 2019 at 12:05
  • \$\begingroup\$ That makes sense. \$\endgroup\$
    – Niteesh Shanbog
    Commented Jul 15, 2019 at 12:50

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