In continuous gating, the correct answer is option 'B' which states that the SCR is heated up.

Explanation:

Continuous gating is a technique used in power electronics to control the firing angle of a thyristor (such as a silicon-controlled rectifier or SCR). This technique allows for the continuous control of the power delivered to a load.

When a thyristor is gated continuously, it means that the gate signal is applied continuously throughout the conduction period of the thyristor. This is in contrast to the conventional single pulse gating, where the gate signal is applied only once at the beginning of each half-cycle.

Now, let's discuss each option and understand why option 'B' is the correct answer.

a) Overlap angle is very high:
The overlap angle refers to the period of time during which both the anode current of the conducting thyristor and the forward voltage across the thyristor are non-zero. In continuous gating, the overlap angle is usually small to ensure effective commutation. A high overlap angle would lead to inadequate commutation, but it is not directly related to the heating of the SCR.

b) SCR is heated up:
When the SCR is continuously gated, it remains in the conducting state for a longer duration compared to single pulse gating. This prolonged conduction leads to a higher dissipation of power within the SCR, resulting in increased heating. The SCR's temperature rises due to this increased heat dissipation, which can affect its performance and reliability.

c) Size of the pulse transformer is small:
The size of the pulse transformer is not directly related to continuous gating. The pulse transformer is used to provide the necessary gate signal to the SCR, and its size depends on the required voltage and current ratings of the gate signal, not the gating technique itself.

d) Commutation cannot be achieved effectively:
Continuous gating does not inherently prevent effective commutation. Commutation refers to the process of turning off the thyristor after it has conducted for a certain period. Although continuous gating may require more careful consideration of commutation circuits and techniques, it does not imply that effective commutation cannot be achieved.

In conclusion, the correct answer is option 'B' because continuous gating leads to increased heating of the SCR due to prolonged conduction, which can affect its performance and reliability.