Analysis of voltage bounds in grid-connected inverters with automatic reactive power control under persistent load variations

The rapid growth of variable electricity generation in low-voltage electricity grids poses new challenges to the operation of power systems. Distributed non-dispatchable electricity from renewable sources, such as photovoltaic solar energy, can potentially impact power quality, particularly voltage regulation. In response, inverter manufacturers and recent standards have introduced automatic inverter response functions, such as the commonly known as Volt-Watt and Volt-VAR functions. These functions aim to mitigate under- and over-voltage events by gracefully curtailing injected active power, or injecting/absorbing reactive power when the voltage at the point of coupling exceeds the predefined regulation limits. While the utility of these new inverter functionalities has been established in various studies, the analysis of their stability properties, particularly in the context of persistent changes in load, remains scarce. This paper contributes a framework for nonlinear analysis of voltage stability and boundedness in distribution networks with variable loads. The framework is applied to the case of a single grid-connected inverter to derive sufficient conditions for steady-state voltage boundedness as a function of the load variation levels in the inverter bus. Simulation results illustrate the accuracy of these bounds for varying load conditions.