DC LOCAL GRID FOR WIND FARM

Abstract: The invention relates to a wind farm comprising two or more windmills, each provided with a generator by means of which energy can be generated. The energy supplied by the generators is output to a general a.c. grid which predominantly is at a standard voltage. The electrical power supplied by the windmills is injected into the a.c. grid via a d.c. grid to which a plurality of windmills are linked, and a converter. According to the invention, the d.c. voltage in the d.c. interconnecting grid is set as a function of the electrical power to be generated by a generator.

Description: DC LOCAL GRIDFOR WIND FARM The invention relates to a method according to the preamble of claim 1. A method of this type is disclosed by US 5225712. In the case of the known method, the level of the d.c. voltages is constant and independent of the speed of rotation of the generator or the wind speed. The efficiency of the generator at the various speeds of rotation is affected, inter alia, by the voltage output by the generator, which means that for a single setting of the level of the direct current the efficiency of the generator is unnecessarily low either at high speeds of rotation and wind speed or at low speeds of rotation and wind speed, unless the generator is fitted with special and expensive provisions to improve efficiency. To overcome the abovementioned drawback, the method according to the invention is implemented according to the characterizing feature of claim 1. As a result, the d.c. voltage is adjusted according to circumstances, as a result of which the generator of the windmill is able to operate at a higher efficiency over the entire working range or can be of simpler design. Furthermore, the invention relates to an apparatus according to the preamble of claim 4. An apparatus of this type is disclosed by the abovementioned document US 5225712 and has the abovementioned drawbacks. To avoid the abovementioned drawbacks, the apparatus according to the invention is of a design according to the characterizing features of claim 4. As a result of the voltage level in the d.c. interconnecting grid being varied and possibly being tailored to the power to be supplied by the generator, control of the windmills is greatly simplified, since owing to the reduction in the d.c. voltage it is possible for the generator likewise to supply a lower voltage and thus to supply its power at a lower speed of rotation with higher efficiency. According to a further refinement, the apparatus is of a design according to claim 5. As a result of the voltage level of the d.c. interconnecting grid being adjusted, inter alia, on the basis of the wind speed, the generator is able to provide maximum efficiency at all wind speeds. According to a further refinement, the apparatus is of a design in accordance with claim 6. This allows the voltage set to be made directly dependent on the wind speed experienced by the windmills and the effects thereof on the speed of rotation of the generator (s) . This is an advantage particularly if the various windmills are spaced far apart and/or local conditions vary rapidly. According to a further refinement, the apparatus is of a design according to claim 7. This allows the windmill to be provided with energy by using the same cable via which generated energy is supplied, resulting in cost savings. According to a further refinement, the apparatus is of a design according to claim 8. This provides a simple option for carrying out maintenance on the windmill and to start the mill. According to a further refinement, the apparatus is of a design according to claim 9. This makes it possible, even in the event of a loss of voltage in the d.c. interconnecting grid, to stop the windmill safely and without damage. According to a further refinement, the apparatus is of a design according to claim 10. This makes it possible for the windmills to be provided with energy if all the mills are at a standstill, for example in the event of a lull. The invention is explained hereinafter with reference to a specific embodiment with the aid of a few figures in which Figure 1 depicts a schematic top view of a wind farm and Figure 2 shows a schematic depiction of the various components. Corresponding components have been assigned, as far as possible, the same reference numeral. Figure 1 shows the top view of a wind farm comprising eight windmills 1, each provided with vanes 2 whose rotary shaft points towards the prevailing wind direction W. The windmills are set in water S and are connected to one another and to a control building 4 via a d.c. interconnecting grid 3. The control building 4 is located on dry land L. From the control building 4 a cable runs to an interconnection point 5 and thence to a public A.C. grid (not shown in any detail). Situated on top of the control building 4 is a wind direction- and wind speed-measuring instrument 6. Figure 2 schematically depicts the various components. Incorporated in the schematically depicted windmill 1 is an a.c. generator 8, which may be provided with a step-up transformer 8a and a rectifier 9. The generator 8 is driven by a rotary shaft 7 to which the vanes 2 are attached (but are not shown in any detail) . Also fitted can be a revolution counter for measuring the speed of rotation of the rotary shaft 7 and thus of the generator 8. In addition to the components shown here, the windmill 1 inter alia also incorporates a capstan drive of the vanes 2, as well as a controller for the generator 8, if this is required for the particular generator type. The windmill 1 is further provided with means for adjusting the pitch angle of the vanes 2, in order to effect, where necessary, a lower efficiency of the vanes 2 at high wind speeds, thus avoiding an excessive speed of rotation of the windmill 1. The generator 8 can be a field-controlled a.c. generator or an a.c. generator equipped with permanent magnets. If field control is used, a separate controller is present for this purpose in the windmill 1. The windmill 1 further includes known means for signalling that the windmill 1 in question is in use, said signal being relayed to a controller 10a via a link 11. In another embodiment, the generator 8 can be designed as a d.c. generator including the provisions necessary for this purpose. In that case, a separate rectifier 9 is unnecessary although there are then additional provisions to prevent the undesirable situation where the vanes 2 would be driven by the generator 8. Positioned in the control building 4 is converter 10 for converting the energy supplied to the d.c. interconnecting grid 3 into constant-voltage alternating current which, via the interconnecting point 5, is fed into the public a.c. grid. The converter 10 is connected to the d.c. interconnecting grid 3, to which the rectifiers 9 of the various windmills 1 are linked. The length of the cables of the d.c. interconnecting grid 3 can, without any difficulties, be from a few kilometres to tens of kilometres, so that the windmills 1 can be situated at as favourable a location as possible in the water S, and the control building 4 can be on land L. The length of the cables in the d.c. interconnecting grid 3 does not affect the potential to transport energy, in contrast to the situation with an a.c. interconnecting grid. The controller 10a is fitted on the converter 10 to control the converter 10. By means of the controller 10a, the converter 10 is driven in such a way that the voltage level in the d.c. interconnecting grid 3 is maintained at an adjustable constant value. The controller 10a is connected to the windmills via the link 11, and to one or more wind direction- and wind speed-measuring instruments 6 which are located near the windmills 1. The wind direction- and wind speed-measuring instruments 6 are connected to the controller 10a via a cable 12 along which the wind data are carried. Optionally, these instruments may also be located on a few, for example three, windmills 1. In the controller 10a it is established what the voltage level in the d.c. interconnecting grid 3 must be. This voltage level may depend on the wind speed which has been measured by means of the anemometers 6. The voltage level may also depend on the rotary shaft 7 speed of rotation which, as measured by the revolution counter, gives an indication of the wind speed around a windmill 1 in question. Preferably, the measured values of wind speed and/or speed of rotation are averaged for the various measurements for the windmills 1 as linked to the d.c. interconnecting grid. As a result of a particular d.c. voltage prevailing in the d.c. interconnecting grid 3, all the generators 8 linked via a d.c. interconnecting grid 3 produce the same d.c. voltage and therefore operate in the same operating range of the generator 8. Since the windmills 1 are located in each others vicinity and will encounter more or less the same wind speed, they will all be able to operate at their most efficient operating point, which allows the highest efficiency to be achieved in a simple manner at any wind speed. In those situations where the wind speed is high the rotary shafts 7 of the windmills 1 will rotate at the maximum permissible speed of rotation of the vanes 2. This speed is determined, inter alia, by the aerodynamic requirements and permissible load for which the vanes 2 have been designed. At the maximum speed of rotation, the vanes 2 will also provide maximum power, and the generator 8 will likewise have to be able to output its maximum power. This is what determines the highest level of the d.c. voltage in the d.c. interconnecting grid 3. If the wind rises further, the speed of rotation of the vanes 2 must not or virtually not increase, and the power to be output by the generator 8 will only be capable of being increased by an increase in the current intensity. The d.c. voltage in the d.c. interconnecting grid will not increase. In that situation, the vanes 2, if the wind rises further, will have to work at lower efficiency, which is achieved by the known (but now shown) vane adjustment. The generator 8 is of such design that its efficiency is highest at maximum power and speed of rotation. To ensure that in situations with less wind the efficiency of the generator 8 is nevertheless as high as possible, it is important to lower the voltage at which the generator 8 has to operate, which is effected by reducing the voltage level of the d.c. interconnecting grid 3. This is done in the converter 10 which is controlled for this purpose by the controller 10a on the basis of the information from the anemometers 6 or the revolution counter. To achieve effective control of this voltage it may be of interest, moreover, to know how many on-stream windmills 1 are connected to the d.c. interconnecting grid 3. Via a link 11, each windmill 1 is connected to the controller 10a, for example to indicate whether the windmill 1 is supplying current and, for example, to indicate the speed of rotation of the rotary shaft 7. The link 11 is shown in Figure 2 as a separate link used for transmitting information regarding the operation of the windmills 1 to the control building 4. The link 11 can also be used for transmitting control data to the windmills 1, such as wind direction data which are calculated on the basis of the data from the wind direction measuring instruments 6. This information is used to point the windmills 1 in the wind direction , avoiding an arrangement where each windmill 1 is fitted with failure-prone wind direction measuring instruments. The cable for transmitting the information can be designed as an optical cable, there being a facility in the windmills 1 which stops the windmill when the link is broken. According to another embodiment, the link 11 can be implemented as a signal present on the d.c. interconnecting grid 3, the d.c. interconnecting grid 3 acting as a network and each windmill 1 extracting from the network the signal intended for said windmill. According to one embodiment, separate facilities may be provided to ensure that the windmills 1 are always positioned correctly with respect to the wind direction W, i.e. when they are operational they are pointing in the wind direction and when they are turned off they are at right angles to the wind direction W, so that the vanes 2 capture as little wind as possible. To this end, each mill is provided with the capstan drive (not shown) , which is supplied, for example, from the d.c. interconnecting grid 3, possibly via a converter, and which is driven via the link 11. The above-described embodiment can be implemented using windmills 1 each having a maximum capacity of, for example, 0.5 - 1.0 megawatts. The d.c. interconnecting grid 3 will have a voltage, at that capacity, of 25,000 volts d.c. In the mill, the a.c. generator 8 then generates, via the step-up transformer 8a, a three-phase a.c. voltage of about 20,000 volts, the rotary shaft 7 of the generator turning at 18-30 revolutions per minute. As well as in accordance with the above- described specific embodiment, the various components can be of some other, known design. As well as the previously described a.c. generator 8, for example, a d.c. generator can be used instead, which is likewise provided with a controller by means of which the behaviour of the generator is tailored to the voltage level in the d.c. interconnecting grid 3. In the previously described specific embodiment it was described how the energy generated by the generator 8, which energy may, for example, amount to many hundreds of kilowatts, is transported to the interconnection point 5. Present in the windmill 1 is not only energy-generating equipment such as the generator 8 , but also energy-consuming equipment such as a drive for adjusting the vanes 2, and a capstan drive for turning the rotary shaft 7 of the vanes 2 into the wind direction . These drives together do not consume more than, for example, from a few up to tens of kilowatts. The controller of the windmill 1 and the equipment present in it likewise require energy. To provide this energy, the windmill 1 includes an a.c. grid to which the various drives and equipment can be linked. This a.c. grid can be fed in various ways. During normal operation of the windmill 1, the a.c. grid can be fed from one or more windings in the generator 8, energy at the same time being fed to one or more storage batteries for the situation where the generator 8 is at a standstill. In this arrangement, the storage batteries are sufficiently large for it to be possible to use them, after the generator 8 has come to a standstill, to turn the rotary shaft 7 of the vanes 2 by means of the capstan drive in the direction of the wind, so that the mill will start to turn again. Optionally, there is also enough energy for setting the vanes at the correct pitch angle and for emergency lighting while work is being carried out on the windmill 1. Instead of electrical storage batteries it is also possible to use hydraulic accumulators in conjunction with a hydraulic drive. A second way of feeding the a.c. grid in the windmill 1 is to position a converter between the d.c. interconnecting grid 3 and the a.c. grid, by means of which the d.c. voltage is converted into a.c. voltage. In such an arrangement, the said converter must be suitable for converting a d.c. voltage of many thousands of volts into three-phase a.c. voltages of, for example, 380 volts at an output of about ten kilowatts. Thus a.c. voltage will always be present in the windmill 1 as long as there is a voltage in the d.c. interconnecting grid 3. A third way of feeding the a.c. grid into the windmill 1 is to use an engine-generator unit which is switched on if required. A fourth way is to connect the a.c. grid to the interconnection point 5 via cables and possibly transformers. The above describes each windmill 1 having its own a.c. grid. Alternatively, however, the a.c. grid of two or more windmills 1 can be connected to one another to form a local a.c. grid which can optionally be connected, in a previously described manner, to the interconnection point 5. This local a.c. grid can, if required, be provided with energy in any of the ways described above for the a.c. grid in the windmill 1.

Claims: Claims 1. Method for generating electric power by means of a windmill (1) provided with a generator (8) possibly equipped with a rectifier (9) for feeding direct current to a d.c. interconnecting grid (3), the electrical power of two or more windmills 1) being conducted via the d.c. interconnecting grid (3) to a converter (10) and in the converter (10) being converted into alternating current and output to an a.c. grid (5) characterized in that the d.c. voltage in the d.c. interconnecting grid (3) is set, for example by the converter (10) , as a function of the power to be supplied by a generator (8) . 2. Method according to Claim 1, characterized in that the d.c. voltage is set on the basis of the wind speed. 3. Method according to Claim 2 , characterized in that the d.c. voltage is set on the basis of the speed of rotation of at least one of the windmills. 4. Apparatus for generating electrical power, comprising two or more windmills (1) each provided with a generator (8) , which may include a rectifier (9) for supplying direct current, a d.c. interconnecting grid (3) for conducting the electrical power generated by the windmills to an a.c. grid (5) via a converter (10) , characterized in that means (10, 10a) are provided for altering the voltage set in the d.c. interconnecting grid (3) . 5. Apparatus according to claim 4, characterized in that the means (10, 10a) for altering the voltage set are connected to sensors (6) for detecting wind speed. 6. Apparatus according to claim 4, characterized in that the means (10, 10a) for altering the voltage set are connected to sensors (6) for detecting the speed of rotation of the generator of one, or the average of the speeds of rotation of a number of, windmill (s) (1) . 7. Apparatus according to any one of claims 4-6, characterized in that a windmill (1) is provided with means for converting electrical power present in the d.c. interconnecting grid (3) into a form which can be used by equipment in the windmill (1) , for example into a.c. voltage. 8. Apparatus according to any one of claims 4-7, characterized in that a windmill (1) is provided with means for temporarily generating energy in a form which can be used by equipment in the windmill (1) , for example in the form of a.c. voltage. 9. Apparatus according to any one of claims 4-8, characterized in that a windmill (1) is provided with means for intermittently storing energy for the purpose of stopping the windmill safely. 10. Apparatus according to any one of claims 4-9, characterized in that conversion means are provided for feeding the electrical energy present in the a.c. grid (5) to the d.c. interconnecting grid (3) .

Inventors: LAGERWEY HENDRIK LAMBERTUS, LAGERWEY, HENDRIK, LAMBERTUS
Applicants: LAGERWEY HENDRIK LAMBERTUS, LAGERWEY WINDTURBINE BV, LAGERWEY, HENDRIK, LAMBERTUS
Application year: 1999
Main classification: F03D9
IPC codes: F03D9, H02J1, H02P9