This paper presents a new control method that helps grid-forming converters behave in a predictable and stable way. Grid-forming converters (GFMs) are devices used when lots of wind and solar power connect to the grid. They are often made to act like a slowly changing voltage source behind a tunable resistive–inductive impedance. The authors design a virtual admittance (VA) inner-loop controller that makes the converter’s terminal behavior follow a desired admittance. Admittance is simply the ease with which current flows; it is the inverse of impedance.

The researchers build the VA using internal model control (IMC). IMC is a design idea that embeds a simple model of the converter’s inner dynamics into the controller so the controller can compensate for those dynamics. The proposed VA explicitly accounts for two things that matter in real converters: the inner current-control loop and the voltage feedforward path. By including these effects in the controller design, the terminal admittance of the converter more closely matches the intended target over the frequency range where outer control loops do not dominate.

At a high level, this closer match matters because it affects small-signal stability. The paper shows that with some existing VA designs, a small-signal instability can appear in the harmonic frequency range if the voltage feedforward uses a low-pass filter with a high cut-off frequency. In plain terms, that means that some common filter choices can allow unwanted oscillations at higher frequencies. The IMC-based VA avoids this problem: the authors report that their design keeps the converter stable regardless of the feedforward filter cut-off frequency, at least in the situations they studied.

To support these claims, the authors tested the method on a laboratory prototype made of three-phase voltage-source converters. This experimental validation shows the approach is practical and not only theoretical. Improving the small-signal stability and making the terminal admittance predictable are useful for large-scale integration of renewables, because they help converters provide reliable grid support and follow setpoints precisely.