D - Part in PID doesn't impact steady state error? Why does the D-part in a PID controller not do any impact on the steady state error. I mean if tries to resist changes, should it not then make it stay at wanted configuration?
A PID controller generates a control input 

$$u(t) = u_\text{P}(t)+u_\text{I}(t)+u_\text{D}(t).$$

The contributions of each of the terms can be written explicitly by using the error $e$.

$$=k_\text{P}e(t)+k_\text{I}\int_{\tau=t_0}^{t}e(\tau)~d\tau + k_\text{D}\dfrac{d}{dt}e(t).$$

Now, we assume that our system is in a state with constant error $e(t) = \text{const}$. What will happen to the different components? It is clear that the derivative term $k_\text{D}\dot{e}(t)$ will vanish. Only the proportional and the integral terms will provide a non-zero term for the total control input. The proportional term will not increase its action even if we have a remaining constant error. The integral term will accumulate errors and increase its action. Hence, only the integral term will increase its action for a steady state error. Note, that the integral term might lead to a phenomenon called integrator windup if the control input is saturated.
If the steady state error is nonzero but settles down then the derivative is zero hence no correction. That's why there is always, though small, integral action added in the controller. Such that when it builds up, the system is forced to return to the designated reference.