Impact of under-excitation limit control on power system dynamicperformance

Several control functions, in addition to terminal voltage control, are performed by the excitation systems of the synchronous machines in a power plant. The under-excitation limit (UEL) is one of these. In this paper, the authors deal with the effect of such control on the dynamic performance of power systems. Certain simulation results properly characterize UEL action which, depending on the nature and severity of the contingency, can be relatively fast. Their objective is to encourage representation of the under-excitation limit control function in synchronous generator AVR modeling for power system dynamic studies. Essentially, the control acts by precluding excessively binder-excited conditions. In practice, it is applied to prevent both operation beyond the steady-state stability limit and the large magnetic losses produced by leakage fluxes in stator winding end-of-turns during machine operation with very small field circuit excitation