Stability
The ability to maintain equilibrium during normal and abnormal conditions.
Last Updated: June 22, 2025
Definition by NERC
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The ability of an electric system to maintain a state of equilibrium during normal and abnormal conditions or disturbances.
Definition in an IEEE Article by a Joint Task Force of IEEE and CIGRE
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Stability of a power system refers to the continuance of intact operation following a disturbance. It depends on the operating condition and the nature of the physical disturbance.
Revised Definition in an IEEE Article by a Task Force
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Stability is the ability of an electric power system, for a given initial operating condition, to regain a state of operating equilibrium after being subjected to a physical disturbance, with most system variables bounded so that practically the entire system remains intact.
Elaboration in an IEEE Article by a Joint Task Force of IEEE and CIGRE
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Relationships between Reliability, Security, and Stability
- Reliability is the overall objective in power system design and operation. To be reliable, the power system must be secure most of the time. To be secure, the system must be stable but must also be secure against other contingencies that would not be classified as stability problems e.g., damage to equipment such as an explosive failure of a cable, fall of transmission towers due to ice loading or sabotage. As well, a system may be stable following a contingency, yet insecure due to post-fault system conditions resulting in equipment overloads or voltage violations.
- System security may be further distinguished from stability in terms of the resulting consequences. For example, two systems may both be stable with equal stability margins, but one may be relatively more secure because the consequences of instability are less severe.
- Security and stability are time-varying attributes which can be judged by studying the performance of the power system under a particular set of conditions. Reliability, on the other hand, is a function of the time-average performance of the power system it can only be judged by consideration of the system’s behavior over an appreciable period of time.