Patent Publication Number: US-2012038155-A1

Title: Step up converter for a wind power plant

Description:
This invention relates generally to the field of wind turbines for power generation, and more particularly, but not by way of limitation, to a system for controlling wind power plants in case of a power outage. 
     Among the available renewable energy sources, wind power is considered to have acquired the prime position in terms of use, along with solar power. Hence, the use of wind power plants is steadily on rise. With this the need to improve the features to effectively control the wind turbine, especially during power outages, is also gaining more and more attention. 
     Most of the wind power plants today use adjustable rotor blades to maximize the efficiency. While controlling the rotational speed of the rotor blades the pitch of the rotor blades are changed with respect to the direction of the wind, changing their orientation in such a way that the wind can engage the rotor blades in the most aerodynamically favorable way. Accordingly, altering the orientation of the rotor blades is also a method for decreasing the rotational speed of the rotor blades should the rotation be too fast. The pitch of the rotor blades is changed by an electrical motor coupled to the rotor blades. In order to safely control the wind power plant even during a power outage, due to failure of a power grid or due to lightning strike, a back-up power source is required to drive the electrical motor. 
     A solution previously in use for this problem was using hydraulic accumulators in which a non-compressible hydraulic fluid is retained under pressure from an external source. The main function of these hydraulic accumulators is to store hydraulic energy and make the energy available again to the system, i.e. the wind power plant in this case during the power outages. However, the hydraulic accumulators are quite big and bulky and hence add heavily to the maintenance costs as well as cover a lot of space. Hence, with the ever increasing size of the wind power plants the hydraulic accumulators are not a very suitable option. 
     Another solution in use is based on an arrangement of accumulating energy in a set of electrical energy storage devices, such as batteries and capacitors, connected in a series connection. The arrangement ensures that even in case of a power outage the power required to drive the electrical motor will be available from the set of electrical energy storage devices, which are connected to a DC-to-AC converter, converting DC from the set of electrical energy storage devices to AC, before reaching the electrical motor. However, the electrical energy storage devices are expensive, and with the larger wind power plants being built lager number of electrical energy storage devices are required to produce enough voltage to drive the electrical motor. Further, the electrical energy storage devices are sensitive to temperature fluctuations and are also unfavorable from an environmental aspect. These all again add up to the maintenance costs and the space requirement which is unwanted, especially in case of off-shore wind power plants. Accordingly there is a need for an improved back-up power source arrangement for the wind power plants. 
     It is an object of the invention to eliminate or at least minimize the above problems/disadvantages, which is achieved by means of the apparatus, according to the claims. 
     An advantage with the present invention is that less number of electrical energy storage devices is required to produce the voltage to drive an electrical motor coupled to the rotor blades of a wind power plant. 
     Another advantage with the present invention is that less expenditure on installation and subsequent maintenance of the electrical energy storage devices is required. 
     Yet another advantage with the present invention is that the space requirement for the electrical energy storage devices is less. 
     Another advantage with the present invention is that due to less number of electrical energy storage devices the related impacts on environment are less. 
     These and other objects are attained in accordance with the present invention wherein there is provided, a system for controlling a wind power plant, having at least one rotor blade, in case of a power outage. The system includes a plurality of electrical energy storage devices, wherein the plurality of electrical energy storage devices are connected in series connection, a step-up DC-to-DC converter, wherein the step-up DC-to-DC converter amplifies the voltage supplied by the plurality of electrical energy storage devices, and a motor coupled to the rotor blade for controlling the pitch angle of the rotor blade of the wind power plant. Preferably, but not necessarily, the motor is an AC motor, in which case the system also comprises a DC-to-AC converter connected in series between the DC-to-DC converter and the AC motor and the AC from the DC-to-AC converter is supplied to the AC motor. 
     It is understood that an AC motor according to the inventive system may be controlled by components other than a DC-to-AC converter, and that such a system will still provide the desired effect of an amplified voltage to the plurality of electrical energy storage devices. 
    
    
     
       Various objects of the present invention together with additional features contributing thereto and advantages accruing there from, will be apparent from the following description of a preferred embodiment of the invention which is shown in the accompanying drawings with like reference numerals indicating like components throughout, wherein: 
         FIG. 1  is a block diagram of a back-up power system for an electrical motor coupled to the blades of a wind power plant, according to the prior art, and 
         FIG. 2  is a block diagram of a back-up power system for an electrical motor coupled to the blades of the wind power plant, according to a preferred embodiment of the present invention. 
     
    
    
     Various embodiments are described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. It may be evident, however, that such embodiments may be practiced without these specific details. 
       FIG. 1  is a block diagram of a back-up power system  100  for an electrical motor  102  coupled to the blades of the wind power plant, according to the prior art. The back-up power system  100  includes a plurality of electrical energy storage devices  104   a,    104   b,    104   c,    104   d  and a DC-to-AC converter  103 , wherein the DC-to-AC converter  103  converts DC provided by the plurality of electrical energy storage devices  104   a,    104   b,    104   c,    104   d  to AC before providing it to the electrical motor  102  in the case of a power outage due to failure of a power grid or lightning. The plurality of electrical energy storage devices  104   a,    104   b,    104   c,    104   d  can be charged using power from a power grid or using a part of the power generated by the wind power plant. 
       FIG. 2  is a block diagram of a back-up power system  200  for the electrical motor  102  coupled to the rotor blades of the wind power plant, according to a preferred embodiment of the present invention. The back-up power system  200  includes a plurality of electrical energy storage devices  204   a,    204   b,  a step-up DC-to-DC converter  201 , wherein the step-up DC-to-DC converter  201  amplifies the voltage supplied by the plurality of electrical energy storage devices  204   a,    204   b,  and a DC-to-AC converter  103 , wherein the DC-to-AC converter  103  converts the amplified DC voltage provided by the plurality of electrical energy storage devices  204   a,    204   b  to AC voltage before providing it to the electrical motor  102  in the case of a power outage due to failure of a power grid or lightning. In an embodiment, the electrical energy storage devices  204   a,    204   b  are capacitors. In another embodiment, the electrical energy storage devices  204   a,    204   b  are batteries. 
     During normal operation of the wind power plant the electrical motor  102  receives power directly from a power grid for controlling the pitch angle of the rotor blades. In the meantime, the plurality of electrical energy storage devices  204   a,    204   b  in the back-up power system  200  get charged using power from a power grid or using a part of the power generated by the wind power plant. On occurrence of a power outage the electrical motor  102  stops receiving power from the power grid, however, at the same time the electrical motor  102  needs to be operated to control the pitch angle of the rotor blades to slow down the rotational speed of the rotor blades and bring the wind power plant to a halt. This is necessary for safety and maintenance of the wind power plant until the time supply of power from the power grid is resumed, or the turbine has been stopped to a safe mode. In this case, the plurality of electrical energy storage devices  204   a,    204   b  provides power to the electrical motor  102  to control the rotor blades. However, the electrical motor  102  preferably being an AC motor, the DC voltage provided by the plurality of electrical energy storage devices  204   a,    204   b  is, first amplified using the step-up DC-to-DC converter  201  and then passed through the DC-to-AC converter  103  to convert the amplified DC voltage into AC voltage. 
     The advantages of using the step-up DC-to-DC converter  201  can be detailed with the help of  FIG. 2 . At point A, as can be seen from the graph of Voltage Output from the plurality of electrical energy storage devices  204   a,    204   b  versus time, the output voltage decreases with passes of time as the electrical charge stored in the plurality of electrical energy storage devices  204   a,    204   b  is spent. In turn, in absence of the step-up DC-to-DC converter  201 , the output voltage provided by the DC-to-AC converter  103  to the electrical motor  102 , as can be seen from the graph at point C, would gradually diminish over time and the wind power plant may not be stopped in a proper manner due to insufficient supply of power to control the electrical motor  102 . Hence, one way to solve this problem could be using a large number of electrical energy storage devices to provide power to the electrical motor  102  for sufficient time to bring the wind power plant to a halt. Another way could be using the step-up DC-to-DC converter  201  which can, as can be seen from the graph at point B of Voltage Output from the step-up DC-to-DC converter  201  versus time, amplify the output voltage from the plurality of electrical energy storage devices  204   a,    204   b  to provide a steady amplified voltage over time, as depicted by a straight line in the graph. It is noteworthy that due to using the step-up DC-to-DC converter  201  the number of electrical energy storage devices in the back-up power system  200 , required to provide sufficient power to operate the electrical motor  102 , decreases in comparison to the number of electrical energy storage devices required in the back-up power system  100 . 
     Accordingly, the inventive back-up power system  200  described herein provides various advantages over the prior art. An advantage offered is that due to presence of the step-up DC-to-DC converter  201  less number of electrical energy storage devices is required to produce the required voltage to drive the electrical motor  102 . Further, due to requirement of a smaller number of electrical energy storage devices less expenditure on installation and subsequent maintenance of the electrical energy storage devices is required. Moreover, the smaller number of electrical energy storage devices also ensure requirement of less space, further adding up to the savings and minimizing the related impacts on the environment.