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 by extracting kinetic energy. However, research on GFM DFIMs remains limited compared with that on GFM inverters. This article investigates the inertia provision capability of GFM DFIMs using virtual synchronous machine (VSM) control with an inner rotor current control (RCC) loop. Because the inertial response of a GFM DFIM 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 a minimum allowable excitation, i.e., rotor current amplitude. A 5 MW DFIM example demonstrates the feasible operating ranges, including the active and reactive power capability of the back-to-back inverter configuration. Integrating a nonlinear DFIM model into the RCC through input-output (IO) linearization ensures that the rotor-side inverter (RSI) current limit is not exceeded. To address severe ROCOF events, we propose a novel overload protection strategy that limits DFIM power while prioritizing inertia provision over primary frequency (droop) control. Furthermore, the proposed control approach ensures synchronization stability by limiting the load angle. Experimental results obtained using a small-scale DFIM test bench verify the proposed approach.
