Control Scheme for DC Voltage Regulation of Stand-Alone DFIG-DC System

A scheme for the dc output voltage control of a stand-alone DFIG-dc system is proposed. Any load variation on the dc link will primarily affect the active component of stator current. The change in reactive component of a stator current is minimum, which will also manifest in a slight variation in the stator voltage and frequency. The control philosophy is designed such that the active rotor current is decided by an active stator current, while the dc-link voltage is created and maintained by a flux loop with a reactive rotor current. Stator voltage/flux ratio is nearly constant, since the flux is decided by the voltage loop. This will indirectly maintain stator frequency around a constant reference frequency. The main advantages of this scheme are direct control of the load current, load independent dc voltage regulation, indirect stator frequency regulation, and stator voltages sensorless. Control scheme is verified experimentally and demonstrated with results for two cases: with and without the use of step-down transformer between stator and rectifier.