Voltage stability in a grid-connected inverter with automatic Volt-Watt and Volt-VAR functions

The growing uptake of solar power in low-voltage grids across the world has drawn attention to potential overvoltage issues. In an effort to mitigate overvoltage, automatic Volt-Watt and Volt-VAR inverter response functions have been introduced in recent standards. The utility of these functions has been established in a number of recent studies. However, relatively little analysis exists on the stability of grid-connected inverters where decentralised operation of these response functions can potentially trigger undesirable interactions. This paper presents a rigorous stability analysis of a grid-connected inverter under simultaneous operation of automatic Volt-Watt and Volt-VAR response functions. Conditions for the existence of an equilibrium voltage are established together with tests that characterise its stability in terms of the inverter and line parameters. The analysis reveals a little-known stability vulnerability arising when both Volt-Watt and Volt-VAR functions are in operation if Watt output takes precedence over the provision of VAR support, a generally recommended setting. To circumvent this vulnerability, the proposed stability tests allow parameter selection for guaranteed stability margins. A safe configuration alternative is also identified to avoid instability at the expense of real power output. Numerical methods to compute equilibrium voltages are presented and used in two illustrative numerical studies.