IEEE 421.1:2021

Revision Standard - Active.
Defined in this standard are elements and commonly used components in excitation systems. Also contained are definitions for excitation systems applied to synchronous machines. An included annex contains one-line block diagrams of some typical excitation systems. These are presented to illustrate the defined terminology referenced in this standard and clarify the understanding of the excitation control system.

This standard defines elements and commonly used components in excitation control systems and contains definitions for excitation systems as applied to synchronous machines. These definitions should be useful in the following areas:
- Writing excitation systems specifications
- Evaluating excitation system performance
- Specifying methods for excitation system tests
- Preparing related excitation system standards
- Serving as an educational means for those becoming acquainted with excitation systems
- Modeling excitation systems
This standard provides a common base for terminology in common usage related to excitation control systems. The definitions given in this standard assist in describing terms specifically related to excitation systems. With the advent of new excitation systems and changes in common terminology and usage there are revisions required to keep this standard relevant.