The present invention relates in general to wind energy plants, and, more particularly, to storm protection of wind energy plants.
Changing the pitch, or angle, of air foil type propellers for speed regulation has long been in use for preventing overspeeding in normal winds and light storms. However, most of such governing devices, if they change the pitch sufficiently to prevent overspeeding in winds about 50 to 60 miles per hour, when the blade angle to the wind is increased beyond a certain point in order to prevent any further increase in speed, the outer portion of the propeller begins to act as a high speed centrifugal "paddle" fan. Back pressures of 100 miles per hour or more are thus created against the back portions of the propeller and this "back pressure" in turn allows tremendous pressure buildup against the front side of the revolving propellers, often breaking them as well as subjecting the entire plant and tower to severe storm damage. Winds exceeding 100 miles per hour are encountered at the tops of towers, and successful wind energy systems must be automatically controlled when such storms strike.
The inventors of the hereinafter disclosed invention also have patents relating to wind energy plants having the propeller axis offset from the vertical. Such patents include U.S. Pat. No. 4,068,131. Furthermore, the present inventors also have patents wherein the mast of a wind electric plant is offset from the center of the gear assembly, see, e.g., U.S. Pat. No. 4,088,420.
While the devices disclosed in the just-mentioned patents afford protection to the wind energy plant in storms wherein wind velocities do not exceed about 40 miles per hour, and are thus adequate under most conditions, there are storms in which wind velocites exceed 40 miles per hour, especially at the heights of the propellers of the wind energy power plants. As used herein, the term "high winds", or the like, will refer to wind velocities with respect to the wind energy plant of about 40 miles per hour and above. Even though such storms are rare in many areas, even the slimmest possibility of such a storm should be accounted for.
A drawback to those known wind energy power plants is that protection against high winds, that is, winds in excess of 40 miles per hour, is either not possible at all, or can be accomplished only at considerable expense. Accordingly, all known wind energy power plants either are not protected against high winds, or are protected inadequately, or are extremely expensive.
During storms, ice may form on the plant propeller blades creating a severe imbalance. The rotational velocities of the blades in a typical wind power plant are such that blade imbalance can set up severe vibrations in the plant. These vibrations can be serious enough to damage or even destroy a plant unless the plant is shut down. Known plants have no means of automatically shutting down the blades in a manner which, itself, does not present a possibility of damaging the plant. A shutdown which is too rapid may cause severe damage to the plant. As ice formation on propeller blades is a possibility in storms in many areas, and in some storms in all areas, complete storm protection for wind energy plants should account for this contingency.
Yet a further drawback to such plants is the uncontrolled manner in which manual shutdown can be effected. If the manual shutdown is not controlled properly, it is possible for an operator, either through inexperience or panic, or the like, to shut down the plant too rapidly in a storm condition, thereby causing the plant to be severely damaged, if not destroyed, due to the wind pressure placed on the blades as they slow down. Thus, complete storm protection for a wind energy plant should provide control over manual shutdown so that operation also is protected.
The inventors of the presently disclosed device also have a patent on an automatic storm protection system which overcomes the above-discussed drawbacks. The just-mentioned patent is U.S. Pat. No. 4,297,075, and the disclosure of such patent is incorporated herein by reference thereto.
It is also known that wind energy plants that use bevel gears to transfer power from a horizontal propeller driven shaft to a vertical drive shaft have a problem of the torque action rotating the horizontal shaft gear around the vertical shaft gear when power is applied to the horizontal shaft by the propeller.
In the device disclosed in U.S. Pat. No. 4,088,420, the disclosure of which is incorporated herein by reference thereto, the horizontal propeller hub shaft is offset several inches to the right side (viewed facing upwind, e.g., the starboard side) of the vertical drive shaft to create a lateral swing pressure against the gear case, that is, opposite the gear torque of the vertical shaft. Thus, the two opposing forces act to offset each other and the propellers of a wind electric plant which incorporate hypoid gear drives as disclosed in the just-referenced patent face directly forward into the wind when operating. U.S. Pat. No. 4,297,075 discloses the offset propeller hub principle in a wind electric plant to fold the propellers around to the right side in wind velocities above 30 miles per hour through a tail vane frame hinge system that operates against a tension spring. As used herein, the term "fold" implies yaw action of the propeller. The yaw action, or swing action, can only operate (yaw to the starboard side) in excess winds when the winding velocity (and hence wind pressure) exceeds that value required to deliver the predetermined amount of power to the vertical shaft (a wind speed of approximately 25 miles per hour). The folding action prevents excessive torque from being applied by the propeller to the vertical drive shaft in high winds or storms. The design disclosed in U.S. Pat. No. 4,297,075 only permits folding of the power head to the right (for high winds), and does not accommodate folding to the left, or port. Thus, the propellers in the U.S. Pat. No. 4,297,075 design face forward in full power position when the brake is applied.
When applying a brake to the vertical drive shaft to stop the propellers of a wind electric plant, the added brake torque (in addition to the power torque) keeps the propellers facing forward toward the wind, and when propeller speed is reduced, by the brake, an automatic variable blade pitch governor alters the blade pitch angle into full power position. This occurs in high winds, and makes it impossible to stop the plant. High winds can thus damage the brake and apply too much pressure to the propellers, bending them into the tower. For purposes of this disclosure, the term "high winds" implies wind velocities of 40 miles per hour or more, as stated hereinabove.
While the propellers in the U.S. Pat. No. 4,297,075 device will fold to the right under normal operation to prevent excessive wind pressure in storms, these propellers cannot be stopped because the added brake torque applied to the vertical drive shaft pulls the propellers back into an orientation facing the storm (even if those propellers are folded to the right), and when the propellers are slowed down by the brake, the governor action automatically changes the propellers from high storm pitch to a flat full power position even though the wind may be greater than 40 miles per hour. Storm pressure can thereby damage the plant.