when doing small signal (AC) analysis.. only alternating (that change with time) voltages are considered. since DC voltage source is not varying (fixed value) it is considered zero and is short circuited to ground.
similarly if its a DC current source it is open circuited..
While doing small signal analysis (AC) we are bothered about the small changes in the circuit(remember there is no change in your DC supply which is large compared to your i/p and hence called large signal analysis.) so we make the DC sources effect on the circuit zero by shorting voltage sources and open circuiting current sources.....
it is basically due to superposition theorem.... in AC analysis they omit the DC supplies once they have found the circuit is properly biased in DC analysis....
while applying superposition we replace the sources with their equivalent impedance and it is open circuit of current source and short circuit for voltage source...
there is no AC component in the supply.
We find the PSRR of the circuit for some AC harmonics in the power supply.
When we do AC analysis we only concider AC sources