A control scheme for improving the efficiency of DFIG at low wind speeds with fractional rated converters

• Extending the low speed range of operation of a Doubly Fed Induction Generator (DFIG).
• When the rotor speed is between 70% and 130% of the synchronous speed, the machine is operated in the normal DFIG mode.
• When the rotor speed falls below 70%, it is operated in ‘Stator Short Circuited (SSC)’ mode.
• An algorithm to minimize the total copper loss is proposed for SSC operation for obtaining maximum efficiency at low speeds.

This study proposes a scheme for extending the low speed range of operation of a Doubly Fed Induction Generator (DFIG) without down grading the efficiency. Also, only fractional rated converters are employed. The technique involves two operational modes for the generator. When the rotor speed is between 70% and 130% of the synchronous speed, the machine is operated in the normal Doubly Fed Induction Generator (DFIG) mode and when the rotor speed falls below 70%, it is operated in Stator Short Circuited (SSC) mode. The switch-over from the DFIG mode to the SSC mode is carried out at a threshold speed to maintain the efficiency of generator with the same fractional rated converters. The computer simulations on a typical DFIG (250 kW) in Matlab/Simulink environment illustrate that the range of efficiency improvement is from zero to 15%. Further, the experimental results on a 2.3 kW DFIG set up are also illustrated to demonstrate the efficacy of the scheme.