Overview of different wind generator systems and their comparisons 2-4~2-7 陳昱希

Outline Variable speed direct-drive concept with a full- scale power converter Variable speed single-stage geared concept with a full-scale power converter Variable speed multiple-stage geared concept with a full-scale power converter

Variable speed direct-drive concept with a full-scale power converter The full-scale power converter can perform smooth grid connection over the entire speed range. However, it has a higher cost and a higher power loss in the power electronics, since all the generated power has to pass through the power converter Types of direct-drive generators used in the market can be classified into the electrically excited synchronous generator (EESG) and the PMSG.

Variable speed direct-drive concept with a full-scale power converter Direct-drive types V.S. geared-drive Lower generator rotor speed Higher torque Larger size Simplified drive train Omitting the gearbox. Higher overall efficiency High reliability The most important difference between geared drive wind turbines and direct-drive types is the generator rotor speed. The direct-drive generator rotates at a low speed, because the generator rotor is directly connected on the hub of the turbine rotor

Electrically excited synchronous generator (EESG) The amplitude and frequency of the voltage can be fully controlled by the power electronic at the generator side. The EESG has the opportunities of controlling the flux for a minimized loss in different power ranges, because the excitation current can be controlled by means of the power converter in the rotor side.

PM synchronous generator The advantages of PM machines over EESG Higher efficiency No additional power supply for the magnet field excitation. Disadvantages High cost of PM material Difficulties to handle in manufacture

PM synchronous generator Based on the direction of flux penetration. Radial-flux PM machines Axial-flux PM machines Transversal-flux PM machines

Radial-flux PM machines The PMs of radial-flux machines are radically oriented. Two types of RFPM machines, the slotted surface mounted PM machine and the slotted flux concentrating PM machine

Axial-flux PM machines The axial-flux PM (AFPM) machine is a machine producing magnetic flux in the axial direction. The slotless single stator double rotor is a typical structure of slotless AFPM machines.

Axial-flux PM machines AFPM V.S RFPM Advantages Simple winding Short axial length Higher torque/volume ratio. Disadvantages Lower torque/mass ratio , Difficulty to maintain air gap in large diameter Difficulty in production of stator core

Transversal-flux PM machines The path of the magnetic flux is perpendicular to the direction of the rotor rotation. TFPM machines have some advantages, such as higher force density, considerably low copper losses and simple winding. TFPM machines have some disadvantages, such as the electromagnetic structure is much more complicated , low power factor.

Variable speed single-stage geared concept with a full-scale power converter In this scheme, a variable speed pitch control wind turbine is connected to a single-stage planetary gearbox that increases the speed by a factor of roughly10 and a low-speed permanent-magnet generator. It has the advantages of a higher speed than the direct-drive concept and a lower mechanical component than the multiple-stage gearbox concept.

Variable speed multiple-stage geared concept with a full-scale power converter A PMSG system with a multiple gearbox is used in order to reduce the generator’s volume and improve the generator efficiency in variable speed wind turbine concepts with a full-scale power converter.

PMSG system compared with the DFIG system Advantages The generator has a better efficiency. The generator can be brushless. Disadvantages Larger More expensive converter The losses in the converter are higher

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