Class D Commutation of Thyristor | Impulse commutation | Auxiliary Voltage Commutation

The auxiliary voltage commutation is also called as “Impulse commutation”, or class D commutation. The circuit diagram for class D type commutation is as shown in Fig. 1(a). SCR₁ is the main load carrying SCR whereas SCR₂ is an auxiliary SCR which is turned on to turn off the main SCR. Auxiliary SCR is a low-power device.  The auxiliary SCR, i.e. SCR₂ is turned on first in order to charge the commutating capacitor C with the polarities as shown in Fig. 1(b). As soon as C is charged, SCR₂ will turn off due to a lack of current. Thus SCR₂ gets commutated naturally. This positive voltage will then be held on the commutating capacitor, as there is no discharge path for it.

(a) Circuit diagram for auxiliary voltage commutation.

(b) Equivalent circuit before mode I

Fig. 1: Auxiliary Voltage Commutation

Operation of the circuit:

Mode I (t₀ to t₁):

Fig. 2: Equivalent circuit for mode I

The main thyristor SCR₁ is triggered at an instant t₀. As shown in Fig. 2 there are two different currents flowing in the circuit, the load current I_R and the discharge current of the capacitor I_C which is sinusoidal in nature. The current through the main thyristor is the sum of these two currents.

I_SCR1 = I_R + I_C                       …(1)

The positive voltage on the capacitor decreases first to zero and then the capacitor is charged with a reverse polarity (See Fig. 3). When voltage on the capacitor reverses its polarity completely, the discharge current I_C goes to zero at t = t₁.

Fig. 3: Voltage and current waveforms for auxiliary voltage commutation (class D commutation)

Mode II (t₁ to t₂):

Fig. 4: Equivalent circuit for mode II

The negative voltage on the capacitor is held constant as the “hold off” diode D is now reverse biased. SCR₁ continues to conduct the load current. See Fig. 4. At the instant t₂ the auxiliary SCR₂ is triggered in order to turn off the main thyristor SCR₁ (See Fig. 3).

Mode III (t₂ to t₃) :

Fig. 5: Equivalent circuit for mode III

At instant t₂, the auxiliary thyristor SCR₂ is turned on. This connects the negative capacitor voltage (Fig. 5) across the conducting main thyristor SCR₁ via SCR₂. The main thyristor SCR₁ is turned off due to reverse voltage applied across it.

Thus auxiliary commutation is a “voltage commutation”. Now the capacitor starts charging through SCR₂ and R as shown in Fig. 4. The load current I_R is equal to the capacitor charging current I_C. The voltage on the capacitor increases towards zero first and then towards the positive maxima (See Fig. 3).

At the end of this mode, the voltage across the capacitor is positive, SCR₂ is turned off at “t₃” as the current through it goes to zero.

The voltage across SCR₁ will be nothing else but V_C as the capacitor is connected across SCR₁ via SCR₂ throughout this mode. The capacitor current and the load current go to zero at instant t₃.

Mode IV (t₃ to t₄) :

In this mode, all the devices will remain in their off state. The positive voltage on the commutating capacitor will be held constant as there is no discharge path for the capacitor.

At instant t₄ the main SCR is turned on and the cycle repeats itself. The voltage and current waveforms for auxiliary voltage commutation are shown in Fig. 3.