Adaptive Grid Impedance Shaping Approach Applied for Grid-Forming Power Converters

Abstract

This paper proposes an adaptive grid impedance shaping approach, implemented through a variable structure virtual impedance control strategy (VS-VIS), to improve the power flow control of grid-forming power converters (GF-PC). In this strategy, an equivalent grid impedance estimator (EGIE) evaluates the feeder X/R ratio to determine the required virtual impedance to reach the grid shaping objective. Besides, the virtual impedance definition also considers the converter power rating to ensure the system's feasibility. A virtual negative resistance series interconnected with a positive virtual reactance implements the virtual impedance. Moreover, the required virtual impedance is only updated when the X/R ratio extrapolates deviation limits determined by allowed grid voltages boundaries specified by the standards. Besides, the virtual reactance is split into two parts that use the conventional and sliding mode (SMC) approaches. The first implements the main part of required virtual reactance, while the second provides a fine-tuning action matching the X/R control objectives and compensating grid impedance uncertainties. This grid-shaping method provides a smooth compensation increasing the GF-PC resilience under grid impedance variation. These features constitute a novelty compared to the shaping methods presented in the literature. Experimental tests obtained from a GF-PC laboratory setup demonstrate the effectiveness and robustness of the proposed grid impedance shaping method.