WO2008039121A1

WO2008039121A1 - A power plant

Info

Publication number: WO2008039121A1
Application number: PCT/SE2006/001110
Authority: WO
Inventors: Lars Gertmar; Bengt FRANKÉN
Original assignee: Abb Research Ltd.
Priority date: 2006-09-29
Filing date: 2006-09-29
Publication date: 2008-04-03

Abstract

A wind power plant has a plurality of wind power stations (1) distributed over an area (3), in which each station comprises a wind turbine (2) including a generator and a first step-up transformer (4). The power stations are interconnected in groups (10). Each group is connected to a separate second step-up transformer (7) located in the region of the group, and each said second step-up transformer is connected to a main cable (8) in common to all the stations for transfer of the total power generated by the plant to an electric power transmission network at a considerable distance to said area.

Description

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Applicant: ABB Research Ltd.

A POWER PLANT

TECHNICAL FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a wind power plant having a plurality of wind power stations distributed over an area, each said station comprising a wind turbine generator including a generator and a first step-up transformer for raising the voltage of the electric power generated by the wind turbine, said sta- tions being by means of cables connected to a further second step-up transformer connected to a main cable in common to all stations for transfer of the total power generated by the plant to an electric power transmission network at a considerable distance to said area. "Considerable distance" is typically > 3 km, often > 10 km.

Said plurality of wind power stations or so-called wind mills may be of any number and is often in the order of 100 arranged in a so-called wind park or wind farm formed thereby. The invention is particularly directed to an offshore wind power plant having said stations distributed in the sea and said power transmission network on land, and this application will hereinafter be discussed. However, the power stations may just as well be arranged on land, such as in a desert and be connected through said main cable to a network outside the desert.

The invention is not restricted to any particular ranges with respect to power or voltage levels.

The generator associated with each said wind turbine may be of any type, such as synchronous as well as asynchronous type. A known wind power plant of this type is very schematically illustrated in appended Fig 1. It is shown how a plurality of wind power stations V comprising a wind turbine 2' are distributed in the sea 3. The generator of each wind turbine is connected to a first step-up transformer 4' for raising the voltage of the electric power generated by the wind turbine generator. The voltage may by said first step-up transformers 4' typically be raised from 1 kV to 6, 12, 24 or 36 kV. The first step-up transformer 4' may be arranged in the nacelle, be padmounted or arranged in the column. The transformers of each individual power station Y are connected to a cable of a large medium-voltage cable system 5', which connects all the captured wind power into a common point in the form of an offshore platform 6' for a large said second step-up transformer 7' adapted to transform the electric power from the entire wind power plant to a higher voltage, often in the region of 150 kV, then transferred through a main cable 8' to an electric power transmission network on land 9'. A switch gear and a converter station not shown, such as for converting the alternating voltage into direct voltage, may be arranged on land for receiving said power and transmitting the power through a High Voltage Direct Current (HVDC) power transmission line connected to the converter station. However, HVAC transmission is also conceivable.

There is an ongoing attempt to improve different aspects of a wind power plant of the type shown in Fig 1. Such aspects are the availability and redundancy of parts of the wind power plant, the capacitance of the cable systems of the plant, transient over-voltages at switching, costs for phase compensation equipments, weight of transformers and so on.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a wind power plant of the type defined in the introduction, which addresses at least some of the problems of such wind power plants already known and mentioned above.

This object is according to the invention obtained by providing such a wind power plant in which said power stations are interconnected in groups, each said group is connected to a separate said second step-up transformer located in the region of the group, and each said second step-up transformer is connected to said main cable in common to all the stations.

A number of advantages are achieved by such a layout of a wind power plant with respect to a wind power plant of the type shown in Fig 1. The main advantage is that said second step-up transformers may be much smaller than a second step-up trans- former handling all the power generated by the wind power plant, so that a said very costly transformer platform with crane- vessels associated therewith may be omitted. These second step-up transformers may instead without any major investments be located in the region of the group in question, such as in connection with a wind power station belonging to the group. Furthermore, the cable lengths interconnecting the power stations of said group will be much shorter than said medium voltage cable system connecting to the second step-up transformer on the platform in known such wind power plants, which means reduced problems with high capacitance. Another advantage is that the ratio of the first step-up transformers may be lower reducing the weight of these transformers, so that they may conveniently be located in the nacelle or wind turbine house of the power station in question. Another advantage is that the ground- ing system of the wind power plant may be divided into more separate areas, which will improve the detection possibility of relay protection and thus the selectivity. It will also be easier to insert phase compensating reactors at the lower voltage level in the cable interconnecting the power stations belonging to the same group than in said medium-voltage cable system in known wind power plants. It is pointed out that "a main cable in common" does in this context also cover the case of a number of main cable parts extending in parallel for the transfer of the total power generated by the plant when this is needed.

According to an embodiment of the invention each said second step-up transformer is arranged in one of the power stations belonging to the respective group. A minimum of costs will by this be involved for integrating the second step-up transformers in the wind power plant, and the cost saving with respect to an extra transformer platform will be huge. According to another embodiment of the invention said second step-up transformer is arranged on the column structure supporting said wind turbine with generator or on the fundament supporting said column structure by being so-called padmounted. These are convenient ways of arranging said second step-up transformers for suitable connection of all the first step-up transformers belonging to the group in question to this second step-up transformer.

According to another embodiment of the invention said first step-up transformers of the power stations belonging to the same group are connected to a cable in common, which in its turn connects them all to said second step-up transformer of the group. Such a cable will have a comparatively low capacitance, and phase compensating shunt reactors may easily be inserted, especially because of the voltage of said cable which may be even lower than the voltage from the first step-up transformers in known wind power plants thanks to the short length of said interconnecting cable.

According to another embodiment of the invention said wind power plant comprises > 3, > 5, > 8 or > 10 said groups. By reducing the number of wind power stations in a said group that much with respect to the total number of wind power stations in the wind power plant the second step-up transformers may be given a considerably reduced size with respect to one transformer used for all the power stations.

According to another embodiment of the invention each said group has less than 10, 2-8 or 4-6 said power stations. These are suitable numbers of wind power stations in a said group for obtaining a suitable size of said second step-up transformers and keeping the length of the cable interconnecting the power stations of the same group suitably short.

According to another embodiment of the invention the power plant has > 15, > 30 or > 50 and < 200 said wind power stations. These are suitable numbers of wind power stations in a wind power plant for which it may be interesting to interconnect the power stations in groups in accordance with the invention.

According to another embodiment of the invention each said second step-up transformer is adapted to transform the voltage to a level of 60 kV-300 kV or 120 kV-200 kV for said transfer to land. These are suitable levels of the voltage for the main cable for a transfer of the electric power to a converter station and/or switch gear located at said considerable distance, such as on land. Second step-up transformers with this ratio may be suitably arranged at a low cost in a region of a said group thanks to the reduced number of power stations connected thereto.

According to another embodiment of the invention each said first step-up transformer is adapted to transform the voltage generated by said generator to a voltage of 5-12 kV. As already stated above the structure of the wind power plant according to the invention makes it possible to have such a low ratio of said first step-up transformers making it easier to locate them in the nacelle or wind turbine house of the respective wind power station. This low ratio also makes it easier to obtain phase compensa- tion along the cable interconnecting the power stations within the same group. The invention also relates to a High Voltage Direct Current (HVDC) transmission system, which comprises a wind power plant according to the invention connected to an onshore con- verter station comprising a Voltage Source Converter (VSC) and an HVDC power transmission line connected thereto. HVDC and wind power is and advantageous combination, especially for keeping transfer losses at a low level.

Further advantages as well as advantageous features of the invention appear from the following description.

BRIEF DESCRIPTION OF THE DRAWING

With reference to the appended drawings, below follows a specific description of an embodiment of the invention cited as an example.

In the drawings:

Fig 1 is a very schematic view of a wind power plant according to the prior art,

Fig 2 is a view corresponding to Fig 1 of a wind power plant according to an embodiment of the present invention, and

Fig 3 is a schematic view illustrating a possible way of arranging the first and second step-up transformer in a power station of a wind power plant according to the present invention. DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

A wind power plant according to an embodiment of the invention is very schematically illustrated in Fig 2. This may for instance have a plurality of wind power stations 1 distributed in the sea 3 according to a 24x6 matrix pattern with a distance of say 300 metres between each short and 600 metres between each long row. Other configurations are of course possible. The rated power of the plant is typically 50 MW-500 MW, although other sizes are possible. Each wind power station comprises a wind turbine 2 including a generator and a first step-up transformer 4 adapted to raise the voltage of the electric power generated by the wind turbine generator from 1 kV to 6 kV or 12 kV. The power stations 1 are interconnected in groups 10 of four power stations by connecting the step-up transformers of each station to a cable 1 1 in common, which in its turn connects them all to a second step-up transformer 7 adapted to transform the voltage from 6 kV to for instance 66 kV or 132 kV.

The second step-up transformer connects each group to a cable 8 in common to all the stations for transfer of the total power generated by the plant to an electric power transmission network on land. It is illustrated how the cable 8 has for this sake a plu- rality of branches, one for each long row of said matrix.

Fig 3 illustrates schematically the construction of a wind power station 1 of the power plant according to Fig 2. It is shown how the first step-up transformer 4 may advantageously be arranged in the nacelle 12 of the wind turbine, and how the second step- up transformer 7 is arranged on the fundament or pad 13, which supports the column structure 14 supporting the wind turbine. The second step-up transformer 7 of each group may in this way be arranged in one of the wind power stations belonging to the group, and the first step-up transformers of all the wind power stations belonging to the group will then be connected to the low voltage side of this second step-up transformer 7 in common.

An arrangement of the wind power stations in groups in this way means that second step-up transformers possible to arrange in wind power stations may be used, so that the costs for a transformer platform and of crane-vessels, etc may be avoided. Other advantages have been listed above in the discussion of embodiments of the invention.

The invention is of course not in any way restricted to the embodiment thereof described above, but many possibilities to modifications thereof would be apparent to a person with ordinary skill in the art without departing from the basic idea of the invention as defined in the appended claims.

As already mentioned, the number of the wind power stations in each group may be different than shown in the figures, such as preferably any number between 3 and 9, but also higher num- bers are possible.

Although the present invention enables the omission of a transformer platform, a platform for a switch gear may be necessary or suitable when the wind power stations are located at a long distance to land.

It is pointed out that the wind power stations need not be arranged in groups, but they may be substantially evenly distributed over a wind park area, but they may also be arranged and not only interconnected in groups.

Claims

1. A wind power plant having a plurality of wind power stations (1 ) distributed over an area, each said station comprising a wind turbine (2) generator including a generator and a first step-up transformer (4) for raising the voltage of the electric power generated by the wind turbine generator, said stations being by means of cables (1 1 ) connected to a further second step-up transformer (7) connected to a main cable (8) in common to all stations for transfer of the total power generated by the plant to an electric power transmission network at a considerable distance to said area, characterized in that said power stations (1 ) are interconnected in groups (10), that each said group is connected to a separate said second step-up transformer (7) located in the region of the group, and that each said second step-up transformer is connected to said main cable (8) in common to all the stations.

2. A wind power plant according to claim 1 , characterized in that each said second step-up transformer (7) is arranged in one of the power stations (1 ) belonging to the respective group (10).

3. A wind power plant according to claim 2, characterized in that said second step-up transformer (7) is arranged on the column structure (14) supporting said wind turbine with generator or on the fundament (13) supporting said column structure.

4. A wind power plant according to any of the preceding claims, characterized in that said first step-up transformers (4) of the power stations (1 ) belonging to the same group (10) are connected to a cable (1 1 ) in common, which in its turn con- nects them all to said second step-up transformer (7) of the group.

5. A wind power plant according to any of the preceding claims, characterized in that it comprises > 3, > 5, > 8 or > 10 said groups (10).

6. A wind power plant according to any of the preceding claims, characterized in that each said group (10) has less than 10, 2-8 or 4-6 said power stations (1 ).

7. A wind power plant according to any of the preceding claims, characterized in that the power plant has > 15, > 30 or > 50 and < 200 said wind power stations (1 ).

8. A wind power plant according to any of the preceding claims, characterized in that each said second step-up transformer

(7) is adapted to transform the voltage to a level of 60 kV- 300 kV or 120 kV-200 kV for said transfer to land.

9. A wind power plant according to any of the preceding claims, characterized in that each said first step-up transformer (4) is adapted to transform the voltage generated by said generator to a voltage of 5-12 kV.

10. A wind power plant according to any of the preceding claims, characterized in that each said first step-up transformer (4) is arranged in the nacelle (12) or wind turbine house of the respective wind power station (1 ).

1 1. A wind power plant according to any of claims 1 -10, charac- terized in that it is an offshore wind power plant and said stations (1 ) are distributed in the sea, and that said network is located on land.

12. A High Voltage Direct Current (HVDC) transmission system, characterized in that it comprises a wind power plant according to any of claims 1 -10 connected to an onshore con- verter station comprising a Voltage Source Converter (VSC) and an HVDC power transmission line connected thereto.