In the case of DC circuits, since the supply voltage does not go through a zero value, some external source is required to commutate the device. This process with the thyristor inverter is known as the ______ commutation process.

Question

Question

This question was previously asked in

BDL MT Electrical 17 April 2022 Official Paper

Answer 1

The correct answer is option 3):(Forced)

Concept:

Turning OFF SCR:

To turn OFF the conducting SCR, the anode or forward current of SCR must be reduced to zero or below the level of holding current, and then a sufficient reverse voltage must be applied across the SCR to regain its forward blocking state.
The reverse voltage which causes to commutate of the SCR is called the commutation voltage

Natural Commutation:

In natural commutation, the source of commutation voltage is the supply source itself.
If the SCR is connected to an AC supply, at every end of the positive half cycle the anode current goes through the natural current zero, and also immediately a reverse voltage is applied across the SCR.
Thereby reducing the current to zero and allowing time to recover thyristor junctions
These are the conditions to turn OFF the SCR.

Forced Commutation:

In the case of DC circuits, there is no natural current zero to turn OFF the SCR. In such circuits, the forward current must be forced to zero with an external circuit to commutate the SCR hence named forced commutation.
In the case of DC circuits, since the supply voltage does not go through a zero value, some external source is required to commutate the device. This process with the thyristor inverter is known as the forced commutation