Load Commutation Technique of Thyristor

Commutation is defined as the process of turning-OFF a thyristor. Several commutation techniques have been developed with the sole objectives of reducing their turn-OFF time. Use resonant LC or under-damped RLC circuits to force the current and/or voltage of a thyristor to zero to turn-OFF the device.

1. Load commutation
2. Resonant pulse commutation
3. Complementary commutation
4. Impulse commutation
5. External pulse commutation
6. Line commutation

Load commutation used in series inverter and also called as resonant commutation, self commutation or Class-A commutation.

• For low value of load R, L and C are connected in series with R.

• For high value of load R, C is connected across load R.

• The essential requirement for both the circuits is that the overall circuit must be under-damped.
• Current i first rises to maximum value and then begins to fall.
• When current decays to zero and tends to reverse, thyristor T is turned-OFF on its own at instant a.
• Possible in dc circuits and not in ac circuits.

Reference

Dr. P. S. Bimbhra, "Power Electronics", Khanna Publishers, Fifth Edition, 2013