Architecture-Driven Observer–Energy Event Discrimination in Grid-Connected Virtual Synchronous Generators

Grid disconnection in converter-dominated systems is difficult to distinguish from faults and load changes with comparable or larger transients. This paper investigates an observer–energy event-discrimination framework built on a role-separated architecture for a grid-connected virtual synchronous generator (VSG): the VSG keeps the voltage-forming role, so its power balance is available internally, while a phase-locked loop (PLL) acts only as an external, communication-free observer at the point of common coupling. Three signatures are examined: PLL-frequency deviation, VSG–PLL phase deviation, and the power-balance signature. Phase deviation settles at a nonzero value after a permanent grid-connected load step, and is excluded from the decision rule. In steady state, the swing equation and damping approximately relate the two thresholds (0.10 p.u. ≈ 0.1006 Hz); both must be exceeded for 300 ms. For a 30 kW local load, simulated disconnection is detected 0.553 s after the grid opens (threshold crossing plus dwell), and the fault and grid-connected load step raise no false alarm; no speed advantage is claimed. The deficit-side non-detection-zone edge lies near 26.6 kW under reference conditions and varies with reactive-power mismatch, load model, and power reference. Offline one-at-a-time variations of parameters, grid strength, X/R, and voltage noise preserved 30 kW detection without false alarms.