How to do Norton equivalent circuit? Why part of the circuit can be ignored?
If you're finding the Norton equivalent looking into the "a" and "b" terminals, the 30V ideal voltage source keeps the voltage at its positive node at a constant 30V, regardless of anything in parallel with it.  So any components to the left of the ideal source can't affect the output across the a-b terminals.  You can write the mesh equations or simplify the circuit in any way you like, but you will come up with the same result regardless.

Remember, an IDEAL source has zero source impedance and regardless of what you put across it will force the voltage to its given value.