Two types of commutation process

1) Natural commutation

2) Forced commutation

Generally there are two methods of improving Commutation :-

1.Resistance Commutation

2.Emf or Voltage Commutation

The Commutation can also be improve by using interpoles and compensating windings.

it is a type of forced commutation technique of thyristor

the process of turning off of a thyrisror is known as commutation

Voltage commutation: This is possible in ac circuits...The thyristor gets turned off by reverse bias (negative half cycle) voltage applied across it. This is also called line commutation.

Current Commutation: This is possible in dc circuits ...This type of commutation makes use of auxiliary circuit which contains inductor,capacitor,diode and even a thyristor(auxiliary). This is called Forced commutation.