Excitation systems are used to increase power system stability and power transfer capability. IEEE 421.4, a revision to IEEE 421.4-1990, provides vital information to those who are preparing a specification for the purchase of an excitation system. The standard includes descriptions of functions typical of excitation systems, along with the various types of excitation systems and information for sizing the excitation system. It covers: exciter ration consideration, type and power sources; excitation system performance; and control considerations and equipment tests. It also includes and excitation questionnaire.