Forced commutation of thyristor

Forced commutation of a thyristor refers to externally turning it OFF using a commutation circuit

Types of Forced Commutation

Forced commutation circuits are categorized into different classes. The most common ones are:

Class A (Resonant Commutation)

• A capacitor and inductor are used in a resonant LC circuit.
• The thyristor conducts until the LC circuit reverses the voltage across it, forcing it to turn off.

Class B (Self-Commutation)

• A capacitor is charged during the conduction of the main thyristor.
• Upon triggering an auxiliary thyristor, the capacitor discharges in the reverse direction across the main thyristor, turning it off.

Class C (Complementary Commutation)

• Uses two thyristors.
• When one thyristor is ON, it helps commutate the other OFF through a pre-charged capacitor.

Class D (Auxiliary Commutation)

• Uses an auxiliary thyristor and a charged capacitor.
• When the aux thyristor is triggered, the capacitor applies a reverse voltage across the main thyristor.

Class E (External Pulse Commutation)

• An external pulse source forces the current to zero using a pulse transformer or similar method.

Example: Class B Commutation Circuit

Components:

• Main Thyristor (T1)
• Auxiliary Thyristor (T2)
• Commutating Capacitor (C)
• Load (R or RL)
• DC Supply (V)

Working:

1. T1 is turned ON and conducts load current.
2. C charges while T1 is ON.
3. T2 is triggered → C discharges through T2 in reverse direction across T1.
4. T1 is forced OFF as reverse voltage is applied.

Simulink Implementation

In MATLAB Simulink, you can simulate forced commutation using:

• SCR (Thyristor) block (from Simscape > Electrical > Specialized Power Systems > Power Electronics)
• Commutating Capacitor and Inductor
• Pulse Generators to control triggering of thyristors
• Scopes to observe voltage and current waveforms