What happens when both ac and dc supply is combined?

Regardless of AC or DC nature of multiple voltage sources, one just applies the superposition theorem for unrelated voltages.

you need to review your nonsinusiodal topics.. i think the answer is there...

This is a very complex situation and anything may happen. It depends on

a) what are the various circuit elements

b) How do they react to the changed situation.

For DC Inductor is a short circuit, capacitor- Open circuit

For AC - Inductor is high impedance, capacitor is low impedance

For magnetic core components you have the added complexity of origin shifting, and hence the magnetic field intensity reaching saturation or beyound

Several other circuit components may create non-linearity due to DC.

If there is a transformer - DC shorts + nonlinearity in AC (as in above)

etc.

You have to analyse every circuit element with respect to the situation and find the result.

yes i agree, but his statement or the question he gave is too vague to clearly define the certain scenarios that might happen..

hope he will clear out on what particular aspect he is trying to point out.

Thank you for giving your valuable reply

Not much. The technique is widely used to provide traction and signalling segregation in modern railway installations.

It depends on the ratio of AC to DC.