What exactly are you controlling?
I am primarily intrested in controlling the current, thus my main aim in this project is to keep the current through the heater constant.

Even if you keep the current constant, but at a lower voltage, the power has still dropped!
Right Andy but In my application things are not that way. As I have already said I am using two SCR's connected back to back to control the voltage across my heater. Thus whenever the mains voltage drops I will sense the magnitude of drop and accordingly I will adjust my SCR firing angle so that there wont be any change in the voltage across the heater. Thus the heater will always see a constant voltage across it. Now the current going to the heater by ohms law I=V/R.
- R is a contant [ Resistance of heating coil ].
- V is kept constant by SCR firing angles.
Thus "I" through the heater will also remain more or less constant.
Power won't drop. Sine Power = I*I*R As discussed above I and R are not changing so Power will remain almost constant.

My main aim here is to keep the current through heater constant and the best way of doing this would have been directly measure the current going into the system with the help of a Current Transformer. And then adjust the firing angles if this current tends to rise or fall thus keeping a constant current. But this approach is rather expensive. Thus I am looking at a cheapest way i.e. Monitor the voltage and adjust the firing angles.

Regards,
Prahlad Purohit