Negative Resistance Caused by Intra-Loop Coupling in Virtual-Admittance-Based Grid-Forming Control
For power electronics engineers, this reveals a previously unrecognized instability mechanism and offers a practical damping solution without requiring grid impedance information.
The paper identifies a harmonic instability mechanism in virtual-admittance-based grid-forming control caused by intra-loop coupling, which induces negative resistance independent of control delay. A passivity-oriented damping method is proposed and validated experimentally.
This paper addresses the harmonic instability problem of the virtual-admittance (VA)-based grid-forming control. It is revealed that the intra-loop coupling among the VA control, the inner-loop current control, and the voltage feedforward control results in an \(s^2\)-term in the equivalent output impedance of the inverter, which induces a negative-resistance property in the harmonic range. It is worth highlighting that this negative resistance is independent of the control delay. Consequently, this harmonic instability mechanism is fundamentally different from the extensively investigated cases in the literature, which are induced by the digital control delay of inverters. Then, a simple passivity-oriented damping control is proposed to mitigate the negative resistance arising from the intra-loop coupling. The method fully retains the well-established current controller and voltage feedforward, and does not require grid impedance information. Finally, experimental tests verify the theoretical findings and the effectiveness of the damping method.