Q5Power System 2

The power angle (delta), the phase angle difference between a synchronous machine's internal EMF and the system/receiving-end voltage, directly governs the real electrical power transferred through the relation P=(EV/X)sin(delta) for a lossless line of reactance X. Since this power-angle relationship is sinusoidal, power transfer increases with delta only up to delta=90 degrees (where P reaches its maximum, Pmax=EV/X), beyond which further increase in delta actually reduces transferable power; stable equilibrium operation is only possible on the ascending portion of the curve (delta less than 90 degrees), since only there does an increase in delta correspond to an increase in restoring (synchronizing) power that opposes any further angular deviation, while on the descending portion beyond 90 degrees any small perturbation cannot be corrected and synchronism is lost. This is why the maximum power transfer angle and the available margin below 90 degrees are central considerations in both steady-state and transient stability assessment of a power system.