The action of excitation system protective limiters and var regulation are often not taken into account in planning studies, and actual units may be quite different than simulations. This may lead to incorrect predictions of actual stability limitations, or even voltage collapse phenomena, The design of the excitation system protective limiters such as Over Excitation Limiters (OEL), Under Excitation Limiters (UEL) and Volts/Hertz (V/Hz) is explained, and models presented, For system events where these controls and limiters may become active, modeling in transient stability studies is important to correctly predict volt/var performance. Specific models applicable to the GE EX2000 digital-based control systems are shown for reference, although the general concepts discussed here are applicable to any other excitation equipment, An important aspect of the new digital-based excitation controls is that they allow for more intelligent limiter functions and protective features not convenient with older equipment, Test procedures and results are shown to illustrate the limiter performance.