After generators are in parallel operation, the increasing speed of the active load depends on the prime mover, the load of the steam turbine shall be increased /decreased according to the situations of the steam turbine and the boiler gradually. 

If the load is suddenly increased, firstly the coil heats, but the heating of conductor or insulator needs a period of time, the allowable temperature cannot be reached immediately; The heating makes expansion, the expansion ability of different materials is different. In the design of the generator, the heating is calculated. If the generator is suddenly loaded without exceeding the rated load by 50% - 70%, there will not a big problem, otherwise the residual deformation of the rotor conductors will occur.