think the first step in solving the problem is to determine the response rates of the various signals. Firstly the current sensor - what sort of response delay does this have?

For Current sensing I am using a Current transformer -> Precission rectifier built around TL084-> ADC0809 And thus it is pretty fast response time in microseconds. I am planning to read atleat 16 samples every half cycle and then calculate RMS current from these 16 samples. In the next design I desire to measure Secondary current i.e. current in the secondary winding of welding transformer In that case I will be using Rogowski coil -> Integrator built around a suitable op-Amp -> PCF8591 ADC Even in this response time will be equally fast.

As for the thyristor control - we can only change this on a per half cycle basis (preferably on a full cycle as we don't want to introduce too much assymetry into the transformer).

You are right assymetrical magnetisation in transformer can lead to troubles.

Is the thyristor timing fine enough to give you the control you require? If you have only control in 10% steps the best output you could expect is +/-10%!

My existing program routines can fire the thyristor with an accuracy of approx. 2 degrees. But the firing range is kept in between 45 and 90 degrees. The minimum firing angle is 45 since the load is inductive.

Thanks & Regards,
Prahlad Purohit