when the turbine is started or stopped, the cylinder and the rotor will be thermal expansion, cold shrink. Due to differences in the quality of the cylinder and the rotor, the heating conditions are not the same, expansion and contraction of the rotor faster than the cylinder, the cylinder rotor along the axial expansion of the differential, known as differential expansion. When differential expansion is positive, indicating that axial expansion of the rotor is greater than the amount of expansion of cylinder, when differential expansion is negative, indicating that the rotor  axial expansion amount is less than cylinder expansion value.

When the turbine start, the rotor heat faster, are generally positive; when the turbine downtime or load rejection, differential expansion prone to negative.