Optimal Control of Grid-Interfacing Inverters With Current Magnitude Limits

TL;DR

This paper develops a Lyapunov-based control method for grid-interfacing inverters that explicitly considers current magnitude limits, ensuring stability and improving performance over traditional saturation handling techniques.

Contribution

It introduces a nonlinear control framework with a Lyapunov stability condition for inverters with current limits, and learns a linear-feedback controller via offline sampling and SDP optimization.

Findings

The proposed method guarantees stability under current saturation constraints.

The learned controller outperforms existing control strategies.

The approach effectively handles nonlinear current limits in inverter control.

Abstract

Grid-interfacing inverters act as the interface between renewable resources and the electric grid, and have the potential to offer fast and programmable controls compared to synchronous generators. With this flexibility there has been significant research efforts into determining the best way to control these inverters. Inverters are limited in their maximum current output in order to protect semiconductor devices, presenting a nonlinear constraint that needs to be accounted for in their control algorithms. Existing approaches either simply saturate a controller that is designed for unconstrained systems, or assume small perturbations and linearize a saturated system. These approaches can lead to stability issues or limiting the control actions to be too conservative. In this paper, we directly focus on a nonlinear system that explicitly accounts for the saturation of the current magnitude. We use a Lyapunov stability approach to determine a stability condition for the system, guaranteeing that a class of controllers would be stabilizing if they satisfy a simple SDP condition. With this condition we fit a linear-feedback controller by sampling the output (offline) model predictive control problems. This learned controller has improved performances with existing designs.