A diagnostic of voltage infeasibility at low net load and the role of inverter-based grid support

At low net load a cost-based unit commitment keeps few synchronous units online, as in the hour of the 2025 Iberian blackout. A violation of the upper voltage limits in a security check is then usually read as an excess of reactive power to be absorbed. This paper examines that reading with two tools. The first is an operability test that poses voltage operability under single generation outages as the minimum-violation problem of a preventive security-constrained optimal power flow. The second is a diagnostic procedure that, with a few more solutions of the same problem, finds which limit carries the violation and whether the state lacks reactive power or has too much. In three test systems and in hours scheduled by a unit commitment, almost every failed state lacked reactive power. The diagnostic exposed how an upper-limit violation can come from a lack: an area without nearby units must be held high to keep a radial load bus within limits, and the end of a lightly loaded cable rises above the ceiling. Capacitors at the radial bus reduced the violation, and reactors at the cable end increased it. With plants at fixed or unity power factor most failures occurred before any outage, and few did with plant voltage control. Batteries that keep charging after their plant trips removed about a quarter of the states operable in voltage control in the largest system. An upper-limit violation is therefore not by itself evidence of excess reactive power, and the diagnostic tells the two situations apart.