Inertia Provision and Overload Protection of Grid-Forming Doubly Fed Induction Machines

New technical requirements for grid-forming (GFM) capabilities include providing power system inertia to limit the rate of change of frequency (ROCOF). Doubly fed induction machines (DFIMs) are widely used in wind turbines and can provide inertia, e.g., by extracting kinetic energy reserves. However, research on GFM DFIMs remains limited compared to the extensive research on GFM inverters. This article examines the inertia provision capability of GFM DFIMs using virtual synchronous machine (VSM) control with inner rotor current control (RCC) loop. Because the GFM DFIM inertial response to a ROCOF event depends on the initial operating point, we introduce operational constraints to satisfy German grid code requirements for inertia provision. Minimum power reserve and grid synchronization speed constraints result in minimum excitation or rotor current amplitude conditions. A 5MW DFIM example demonstrates the feasible operating range, including the active and reactive power range of the grid-side inverter (GSI). Combining input-output (I/O) linearization-based RCC with nonlinear DFIM modeling ensures that the rotor-side inverter (RSI) current limit is not exceeded. Considering severe ROCOF events, we propose a novel overload protection strategy that limits the DFIM power, while prioritizing inertia provision over primary frequency (droop) control. Furthermore, the proposed control approach ensures synchronization stability by limiting the load angle. Experiments verify the proposed approach using a small-scale DFIM test bench.