Defining constraint thresholds by angles in a stability constrained corridor with high wind

Conventionally, transient stability limits are defined in terms of the active power transfer through a cut-set of the transmission lines connecting the participating centres of inertia. However, the industry is experiencing two changes that suggest a change of practice in defining the limits. Firstly, increasing penetration of renewable and distributed generation, often connecting at nodes between the centres of inertia, increases the volatility of power transfers. This generation does not contribute to inertia, and breaks the direct relationship between power and angle that underlies the conventional expression of a transient limit in MW. The definition in MW is often very conservative. Secondly, the ability to measure angle differences is a relatively recent capability, and affords the opportunity to include angle difference in the definition of power system limits. This paper describes studies to investigate the potential benefit of defining a stability limit in terms of angle difference. The study is motivated by the British grid, where there is a transient stability limit in the corridor between Scotland and England, with 1.5GW of wind generation connected between the centres of inertia. The studies indicate that a definition of the limit by angle difference can enable 10–12% uplift in the corridor transfer during high wind conditions when the capacity is most valuable.