Synchronous mode operation of DFIG based wind turbines for improvement of power system inertia

• Fixed dc rotor voltages at 1 p.u. rotor speed shifts DFIGs to synchronous mode.
• Electromagnetic principles and control methods behind the operation are explained.
• Conversion of a DFIG to a synchronous generator is demonstrated experimentally.
• Contribution to power system inertia validated through simulations.

Inertia emulation methods exist to compensate for the reduced inertial support provided by doubly fed induction generator (DFIG) based wind turbines. Instead of emulating inertia, this paper proposes to temporarily convert DFIGs to synchronous generators, enabling supply of real inertia to the system. In order to achieve this, the voltage supplied to the DFIG rotor needs to be made independent of the grid frequency. Feeding the rotor with a fixed dc voltage while it is rotating at synchronous speed enables the DFIG to operate in synchronism with the grid and couple the inertia of its rotating mass to the power system. The rotor side converter of a DFIG can be controlled to function as the dc voltage source, allowing convenient switching between the two operation modes according to system requirements.