What may be the effect if we run turbine in low load?

1) Low power output

2) clearance between rotor and stator in the generator.

At low load the steam to the turbine is throttled, this causes a drop in steam temp and consequently it effects the turbine shaft expansion, so like you pointed out there will be a narrowing of the gap between the rotor (turbine blades?) and nozzles (stator?), The turbine manufactures will have limits on positive and negative expansion. If you are refering to the alternator? then I think the H20 cooling kepts it stable, an electrician may help you there. If it is the alternator maybe you should post it in Electrical Engineering?