Quantifying Implicit Overload Mandates in Phase Jump Requirements for Grid Forming Inverters
Grid codes increasingly require grid-forming (GFM) inverters to demonstrate prescribed active-power response to phase-angle jumps at the point of interconnection (POI). This paper shows that such requirements embed an implicit current-overload mandate whose severity depends on the test parameters but is nowhere made explicit in the specifications. First, an analytic expression for the instantaneous power is derived at an arbitrary measurement point, establishing that a momentary power excursion in the non-opposing direction is an inevitable physical consequence of the phase jump itself, independent of control action. Second, the phase-jump recovery is formulated as a constrained optimal control problem with the characteristic GFM objective of minimizing terminal voltage deviation from the pre-disturbance value while subject to a hard current limit. As the plant dynamics are linear and the constraints are convex, the solution constitutes a controller-architecture-independent physical bound on the achievable power-recovery trajectory. Sweeping the current limit, the phase-jump acceptance criterion is converted into an equivalent minimum overload ratio, making the implicit hardware mandate quantitative. The bound is validated against three WECC generic GFM inverter models (REGFM_A1, B1, C1) in electromagnetic transient simulations, confirming both validity and tightness of the bound. Recommendations are offered for interpreting compliance test results and for structuring test specifications to distinguish physical hardware limitations from control deficiencies.