Variable speed wind turbine drive and control system

All article:
A variable speed wind turbine utilizes a hydraulic transmission including a hydraulic pump coupled to a rotor and a hydraulic motor coupled to an induction generator to produce variable frequency power that is converted to grid frequency power by electronic controls.

The present invention relates to a wind driven electric power production system, and more particularly to a variable speed wind turbine drive and control system which utilizes a hydraulic pump coupled to the rotor to drive a hydraulic motor coupled to an induction generator producing AC power and utilizing electronic controls to produce a grid frequency power generation at variable rotor speeds.

The majority of wind turbines utilize a mechanical speed increasing gearbox which is prone to failure resulting in extended down time and high maintenance costs. It is possible to remove the gearbox from the wind turbine system by utilizing a hydrostatic transmission.

These prior art wind turbines have generally been limited to fixed speed generators in order to enable the system to be attached to a power grid. This has been accomplished using blade control systems to compensate for changes in wind speed, pressure control systems to control changes in hydraulic flow, as well as variable displacement motors and pumps to compensate for pressure changes in the hydrostatic systems.

The use of power electronics to convert a variable frequency output of a variable speed AC generator into useable AC power is well known. U.S. Pat. No. 5,083,039 deals with this type of control used on variable speed wind turbines. It would therefore be an advantage to provide a variable speed wind turbine that utilizes a hydrostatic transmission that does not require the use of a gearbox.

The proposed system 110 is designed as a means of converting kinetic energy from the wind into electrical power suitable to be transferred over the electrical grid. Referring now to FIG. 1, a rotor 1 is shown mounted on a nacelle of a wind tower.

A hydraulic pump positioned in the nacelle is coupled to the rotor. In one embodiment of the invention, the pump is directly driven by the shaft of the rotor 1 and the pump 3 rotates at the same speed as the rotor 1. A first hydraulic fluid line 31 connects the hydraulic pump output to a hydraulic motor 11 which may be positioned on the ground or in the nacelle.

A second hydraulic fluid line 32 connects the hydraulic motor 11 to the fluid input of the hydraulic pump 3 in the nacelle to complete the fluid circuit. The flow going through the motor 11 turns the motor output shaft, which is directly coupled to an electric generator 12. The generator 12, coupled to the rotation of the rotor 1 through the hydraulic transmission, produces variable frequency AC power.

A generator drive inverter 27 converts the variable frequency AC power to DC power which is transferred into the Active Front End (AFE) 28. The AFE 28 is synchronized with the frequency of the grid, so it can convert the DC power into grid frequency AC power. The AFE 28 synthesizes the sinusoidal input voltage by Pulse-Width-Modulation (PWM). A tuned L-C-L filter 38 reduces potential PWM power line harmonics to negligible levels.

The hydraulic circuit for this system 110 is shown in FIG. 2 and the system is discussed in additional detail. The rotor 1 is placed in an air stream to turn a shaft. The shaft is shown directly coupled to the drive shaft of a hydraulic pump 3. A high pressure hydraulic line 31 connects the outlet of the pump to the inlet of the motor 11, and a low pressure return line 33 connects the outlet of the motor 11 to the inlet of the pump 3.

Flow is generated through this hydraulic loop as the pump shaft is driven by the rotor 1. The flow going through the motor 11 turns the motor output shaft, which is directly coupled to an electric generator 12. The pump 3 has a much larger displacement than the motor 11, so the hydraulic transmission acts as a speed increaser. This allows the use of a relatively small generator 12 even though the rotor input speed is slow.

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