Synchronism

Synchronism is the condition in which synchronous machines in an interconnected power system operate at a common electrical frequency while maintaining bounded relative rotor-angle differences. Machines do not need to have identical rotor angles, but their angles must remain coordinated so power can be exchanged in a controlled way.

Loss of synchronism occurs when those relative angles separate without bound, often after a severe disturbance or under weakly damped stressed transfers. Once that happens, power oscillations can become violent enough to threaten shafts, generators, and the wider network.

Key Aspects of Synchronism:

• Common Frequency Condition: Generators in synchronism share the same electrical frequency across the interconnected system. This is a basic requirement for stable parallel operation of synchronous machines.
• Angle Difference Matters: Stable operation allows nonzero angle differences because those differences are what transfer active power through the network. The key requirement is that the differences remain bounded rather than diverging after a disturbance.
• Relation to Stability: Transient stability and small-signal stability are both fundamentally concerned with preserving synchronism. One focuses on large disturbances, while the other focuses on the damping of small oscillations.
• Loss of Synchronism Consequences: Pole slipping can cause severe current and torque oscillations, damage to rotating equipment, and wide-area disturbance propagation. Protection schemes may intentionally separate areas to contain the event.
• Monitoring and Control: Engineers monitor synchronism through rotor-angle studies, synchrophasor measurements, transfer limits, and out-of-step protection logic. Strong network connections and well-tuned controls help maintain it.