Pole-Changing Dual-Speed Hybrid-Excited Synchronous Generator for Grid-Strengthening Slip-Synchronous Wind Turbine Applications

In this paper, a new pole-changing dual-speed hybrid-excited synchronous generator (DS-HESG) is proposed to be used within the slip-synchronous wind turbine system (SS-WTS). The DS-HESG is excited with a combination of field windings and permanent magnets (PMs), where the direction of field current in the field windings alters, allowing the number of poles on the rotor to vary. This allows the DS- HESG to operate at two distinct shaft speeds while being directly synchronous grid-connected. This was performed in pursuit of enhanced grid-strengthening capabilities through a synchronous direct grid connection, where an increase in energy harvesting is acquired compared to fixed-speed wind turbines. This paper shows that the DS-HESG possesses adequate reactive power compensation capabilities and stability under terminal voltage variations. Further, the DS- HESG was designed to operate with little field assistance in the lower power (higher pole) configuration, aiding in increased efficiency. This was coupled with a low synchronous reactance in the higher pole configuration, ensuring high short-circuit current is provided, ensuring adequate ancillary support capabilities under fault conditions. However, it has been found that the leakage flux from the PMs to the field windings requires special attention in the design of the rotor because it negatively affects the optimum design and performance of the DS-HESG.