New grid-forming control keeps power stable during unbalanced faults

A new paper from Finnish researchers tackles a longstanding problem in power electronics: keeping inverter-based resources online during unbalanced grid faults. The team—Juho Määttä, Marko Hinkkanen, Tuure Nurminen, Orcun Karaca, Rayane Mourouvin, Jarno Kukkola, and Lennart Harnefors—proposes a grid-forming control method that regulates positive-sequence active power while actively suppressing the negative-sequence converter voltage. This lets the converter maintain a constant voltage magnitude even when grid conditions are severely asymmetric, as long as physical limits allow.

The key innovation is a disturbance observer that performs synchronization and provides integral and resonant action, eliminating the need to measure AC-side voltages directly. Only converter current is measured, which simplifies hardware and reduces cost. The observer estimates both positive- and negative-sequence grid voltages, feeding them into the control loop. The design also integrates a current-limitation scheme that works for both balanced and unbalanced faults, preventing overcurrent damage during transients. Experimental validation on a 12.5-kVA converter demonstrated stable operation through severe fault conditions. The paper includes comprehensive stability analysis and tuning guidelines, making it practical for engineers implementing grid-forming controls in renewable energy systems, microgrids, and energy storage.