Definition
The angular difference (δ) between the voltage phasors at two electrically connected buses, which directly controls the flow of real power between them. In a simplified lossless two-bus system, real power flow is P = (V₁ × V₂ / X) × sin(δ), where X is the line reactance. As the power angle increases toward 90°, small perturbations can cause synchronous generators to fall out of step with the rest of the grid — the phenomenon known as loss of synchronism or transient instability. Maximum power transfer occurs at 90°, defining the steady-state stability limit.
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Topic Deck
Electrical Engineering Fundamentals
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Source
FERC Pro Forma OATT / LGIP