A LOW COST OPERATIONAL METHOD OF DOUBLY FED INDUCTION GENERATORS (DFIG) WITHOUT USE OF POWER ELECTRONIC DEVICES TO ACHIEVE HIGH POWER DENSITY, HIGH TORQUE AND HIGH EFFICIENCY

摘要

The invention relates to a low cost operational method of doubly fed induction generators( DFIG) without use of power electronic devices to achieve high power density, high torque and high efficiency, comprising the steps of providing a DFIG machine having three phase windings in the stator and rotor, the stator winding and the rotor windings being respectively fed from mains and a low frequency source via a set of slip rings matching the slip-frequency of the machine; providing an auxiliary slip-ring type induction machine coupled to the DFIG machine; a phase shifting transformer (PST) connected to the stator of the auxiliary machine and receiving power from the mains at a voltage and frequency identical to that supplied to the DFIG, wherein the PST comprises two windings in each phase which are connected in a zig-zag configuration to effect a phase change, and a plurality of tapings on the winding to control the phase angle, wherein the magnitude of the voltage fed to the stator of the auxiliary induction motor is controlled by a step-up transformer having tapings on its winding, wherein said controlling of phase angle and magnitude of the voltage fed to the auxiliary induction motor is effected by an on board tap changer in combination with a programmable logic controller (PLC), and wherein a slip frequency voltage depending on said phase angle, voltage magnitude, and speed is induced in the rotor windings of the auxiliary induction motor which in turn is fed to the rotor windings of the DFIG machine to allow corresponding change in the speed and power factor of the DFIG machine.