محدودیت تولید توان راکتیو پس فاز به ظرفیت ولت آمپری ژنراتور بر می گردد اما محدودیت توان راکتیو پیش فاز به پدیده تخریب حرارتی هسته مطابق توضیحات زیر بر می گردد.
A cylindrical-rotor machine can experience thermal damage at the ends of the stator core when operated at reduced field current. This limitation forms the leading Var boundary of the generator capability curve. A loss of field represents the extreme in field current reduction. A loss of field from full load can result in leading Var loading in excess of the generator’s MVA rating. Typically, the generator manufacturer’s capability curve limits leading Var intake to about 40 to 60% of the generator rating. The leading Var limitation has been shown to vary inversely with terminal voltage. Although the reduction in terminal voltage that accompanies a LOF markedly increases the Var capability, the increase is insufficient to accommodate a potential 2.0 pu leading Var inflow. The limit is circular on the P–Q plane with the following characteristics:
The voltage-dependent leading Var capability of the sample system generator was estimated using this approach. Values for K1 and K2 were chosen to approximately reproduce the leading Var limit defined by the generator manufacturer’s capability curve at et = 1.0. The resulting voltage variant limits are plotted in Figure below.
Excessive end-core heating would result in bluing of metallic end-core structure, charring of stator winding insulation and failure of the insulation medium between laminations.