Patent Description:
In one relatively widespread approach, AC power from the wind turbine is transferred via multiple conductors - one for each phase - suspended from the nacelle and extending through the tower for connection (at a level near the tower base) to a transmission line. Conductors are usually made of copper on account of its high conductivity. In AC transmission, each conductor may be provided as a separate cable, or multiple conductors may be arranged in a single cable.

Alternatively, a DC cable can be used to transfer the wind turbine output power to a transmission line. In this approach, the wind turbine is equipped with a first power converter (in the nacelle) for converting the generator AC output to DC, a DC transmission cable extending from nacelle to tower base, and a second power converter (e.g. at or near the tower base) for converting the DC voltage to the desired export form (AC or DC). The first power converter can transform the generator AC power to a suitable DC voltage level for transmission in the tower cable to the second power converter. The advantage of implementing DC transmission through the tower is that more power can be transferred with the same cross-sectional conductor area, compared to AC transmission due to impedances, skin effects and electrical insulation requirements.

The DC transmission cable or "tower DC cable" is the link between the first power converter and the second power converter. Any DC transmission cable requires two conductors to complete the electrical circuit between both ends, i.e. between source and load. In the usual type of DC transmission cable, a positive conductor and a negative conductor are arranged in a co-axial configuration with a dielectric insulation layer separating the two conductors. Alternatively, a DC transmission link may have one cable for the outward current and a separate cable for the return current. A relevant prior art background document is for example <CIT>.

However, it is desirable to minimize losses in the tower cable. This can be done by transmitting at a higher DC voltage, resulting in reduced copper losses. However, a power converter with high DC output is generally more costly. Alternatively or in addition, losses can be reduced by increasing the available conductor area in the DC tower cable, since the resistance of an electrical conductor is inversely proportional to its cross-sectional area. However, DC cables with thicker conductors require more copper and are more expensive than small cross-sectional area conductors.

It is therefore an object of the invention to provide an improved way of transmitting DC power through a wind turbine This object is achieved by the wind turbine of claim <NUM>; and by the method of claim <NUM> of transmitting output power of a wind turbine generator to a transmission link.

The wind turbine in the context of the invention shall be understood to comprise a generator that is driven by an aerodynamic rotor. Components of the wind turbine may be assumed to be protected by a nacelle that is mounted on top of a tower. The various regions of the wind turbine (rotor blades, hub, nacelle, tower etc.) may be assumed to each comprise an earthing connection to electrical ground, for example earthing connections that fulfil the relevant requirements. The tower shall be understood to be mounted on a support structure such as a concrete foundation, a transition piece, a monopile, a jacket foundation etc., and it may be assumed that any tower earth connection leads via the support structure to electrical earth.

The wind turbine further comprises a first power converter arranged in the nacelle and configured to convert the generator output power to a DC voltage, and a second power converter arranged at the base of the tower and configured to convert the wind turbine DC power to a form suitable for a transmission link to which the wind turbine is connected.

According to the invention, the wind turbine also comprises a DC arrangement in which a DC transmission link or cable is connected between the first power converter and the second power converter. This DC cable extends from the top of the tower to the tower base and may be referred to in the following as the "DC tower cable". The DC tower cable can comprise a single conductor, or it can comprise two electrically isolated conductors (e.g. a co-axial cable with an inner conductor separated from an enclosing outer conductor by a dielectric layer).

The inventive wind turbine is characterized in that a first current path is provided between the power converters through the DC tower cable by connecting a first terminal of the first power converter to the DC tower cable at its upper end, and a first terminal of the second power converter is connected to the DC tower cable at its lower end. Furthermore, a second current path is provided between the power converters through the electrically grounded metal of the tower by electrically connecting the second terminal of the first power converter to earth, and by electrically connecting the second terminal of the second power converter to earth.

It shall be understood that the first and second terminals of the first power converter are the terminal pair at the DC output side. Similarly, it shall be understood that the first and second terminals of the second power converter are the terminal pair at the DC input side.

A wind turbine tower generally has a high metal content in order to meet structural requirements. For example, a tower can be made primarily of stacked steel sections. The invention is based on the insight that the metal of the tower can provide a path for DC current in a DC circuit, and the inventors have realised that it is not necessary for the DC tower cable to transport current in both outward and return directions, but instead the entire DC tower cable (e.g. both conductors of a co-axial cable) can be used to transport current in one direction only, and that the grounded tower metal can carry DC current in the other direction. With the inventive approach to the DC link between nacelle and tower base, it is still safe to touch any earthed tower metal, since the potential difference between the tower metal and earth is zero.

An advantage of the invention is that one part of the current loop between the power converters can be provided by the DC tower cable, and the other part can be provided by the tower, thereby completing the electric circuit. For example, the DC tower cable can be used to carry the output current (positive with respect to ground), and the grounded tower metal is used to carry the return current (at zero potential). The magnitude of the return current is essentially the same as the magnitude of the output current. Alternatively, the DC tower cable can be used to carry the return current (negative with respect to ground), and the grounded tower metal is used to carry the output current (at zero potential). In this way, the total area of the conductor(s) of the DC tower cable is exploited to transport current in only one direction. A significant advantage of this novel approach is that cable losses of the DC tower cable can be halved by doubling the effective cable cross-sectional area, compared to the conventional approach in which a DC tower cable has one conductor for the output current and another conductor for the return current.

According to the invention, the method of transferring output power of a wind turbine generator through the wind turbine tower comprises the steps of providing a first power converter to convert the generator output power to a DC output voltage; providing a second power converter to receive a DC input voltage; and arranging a DC transmission link to extend essentially vertically through the wind turbine tower between the power converters. The DC transmission link may comprise a cable enclosing a single conductor. Alternatively, the DC transmission link may be a coaxial cable, for example an already installed cable, so that both conductors of the coaxial cable are used to carry current in one direction. The inventive method is characterized by the steps of connecting a first terminal of the first power converter to a first terminal of the second power converter through the DC tower cable, and connecting the second terminals of the power converters to the wind turbine's ground connection so that the tower metal provides one current "leg" of the DC circuit.

The inventive method can be used to upgrade an already existing wind turbine that is equipped with two such power converters and a DC tower cable with two conductors. The converter terminals can be connected as above, so that both conductors of the DC tower cable now collectively provide one current path, and the tower metal (via the earthing connector) now provides the other current path.

A wind turbine can be installed on land or offshore. Such a wind turbine may be part of a wind park, or can be a stand-alone wind energy plant. In the following, without restricting the invention in any way, it may be assumed that the wind turbine generator is constructed to generate AC power, for example a variable-speed wind turbine with a doubly-fed induction generator.

The power converters can be realised in any suitable manner, for example the generator-side rectifier may be realised as a voltage source converter (VSC). Depending on the transmission line leading from the wind turbine, the second power converter can be any suitable DC-AC inverter, as will be known to the skilled person.

In an embodiment of the invention, an electrolyzer unit is connected to the second power converter. The electrolyzer unit is configured to generate hydrogen and oxygen from water, wherein the electrolyzer unit is powered by the generator of the wind turbine.

Advantageously, the second power converter is configured as a DC-DC converter. Thus, the second power converter is advantageously configured such that it converts the DC input power received from the first power converter into a direct current with a voltage level that is optimum for the electrolyzer unit.

The first power converter can be configured to convert the generator output AC power to a low, medium or high DC voltage. As used in the context of the invention and in view of the capabilities of wind energy plants being deployed at present, "low", "medium" and "high" DC voltage levels shall be understood to be voltage levels in the order of <NUM> kV, <NUM> kV and <NUM> kV respectively. These values are merely of an exemplary nature and, as the skilled person is aware, such quantities are relative and may be re-interpreted according to technological developments.

In the following, a power converter may be referred to simply as a "converter". The expressions "first power converter", "first converter", "upper converter" and "generator-side converter" may be used interchangeably to refer to the module at the top of the tower (usually installed in the nacelle) that performs AC-DC conversion; similarly the expressions "second power converter", "second converter", "lower converter" and "grid-side converter" may be used interchangeably to refer to the module at the base of the tower that performs DC-AC conversion.

The expression "conductive tower path" refers to the path that would be taken by electrical current through the conducting metal of the tower. In the case of a tower made of metal, for example from stacked steel sections, the "conductive tower path" can be the entire tower structure. In the case of a tower that comprises a concrete structure, the "conductive tower path" refers to the electrically conductive reinforcing elements embedded in the concrete structure, e.g. a mesh of reinforcing bars ("rebar"). The terms "ground connection", "earthing connection", "tower ground" and "tower earth" may all be used interchangeably in the following.

In a preferred embodiment of the invention, the DC output current from the first converter flows through the DC tower cable. In such an embodiment of the invention, the path of the return current is through the conductive tower metal from the negative output terminal of the second converter to the negative input terminal of the first converter.

In an alternative embodiment, the DC output current from the first converter flows through the conductive tower path, and the return current flows through the DC tower cable.

A wind turbine of the type mentioned above, i.e. a VSIG used in an offshore wind park, may have a rated power output in the order of several MW. In a preferred embodiment of the invention, the first power converter is configured to output a DC voltage of at least <NUM> kV. The conductors of the DC cable are preferably realised to collectively carry an output current of at least <NUM> kA.

The earthing connection of a wind turbine tower can be realised in the usual manner, as will be known to the skilled person, for example by ensuring equipotential bonding to all metal parts. Similarly, the second terminal of the generator-side converter can be connected to the nacelle earthing system in any suitable manner; and the second terminal of the grid-side converter can be connected to a converter housing earth in any suitable manner. For example, the second terminal of the generator-side converter can be connected to an earthing conductor of the nacelle, since any such earthing conductor is also electrically connected to the tower earthing system as part of the wind turbine lightning protection system (LPS).

There are several economic benefits to the installation described above: the DC tower cable can be lighter and cheaper, since only one conductor is necessary for a single current path; a more economical generator-side converter can be deployed since the ability of the DC tower cable to transport a higher current means that the output voltage of the generator-side converter can be lower than would be required in a comparable prior art wind turbine. A wind park can comprise any number of wind turbines equipped as described above, for example one hundred or more, so that the savings and benefits are even more relevant.

<FIG> is a simplified schematic showing the principle underlying the invention. The diagram shows a wind turbine <NUM> with a generator <NUM> and a first converter <NUM> at the level of the nacelle <NUM>, which is mounted on top of a tower <NUM>. An aerodynamic rotor (not shown) turns a rotor shaft of the generator <NUM>. The AC power thus generated is converted to DC power by the generator-side converter <NUM>. A DC cable <NUM> extends from the first converter <NUM> to a second converter <NUM> at the base of the tower <NUM>.

The tower <NUM> in this exemplary embodiment is made primarily of metal, for example from one or more essentially cylindrical steel tower sections. The tower <NUM> is grounded in the usual manner, for example by a robust metal connection <NUM> between the tower and electrical ground G. For example, a metal tower of an offshore wind turbine is bolted or welded to a grounded steel monopile or jacket foundation. The tower's earth connection <NUM> leads to ground as indicated by the standard earth symbol, and may be realised by appropriate assemblies at one or more suitable location(s) in the tower structure. The nacelle <NUM> is also grounded by connecting any metal components to a suitable grounding network <NUM> as will be known to the skilled person. Similarly, an earthing connector <NUM> connects the second power converter <NUM> and its housing <NUM> to earth as will be known to the skilled person.

The first converter <NUM> converts the generator AC power to a DC voltage V<NUM> at a low, medium or high level depending on the converter's configuration. The first converter <NUM> has a positive output terminal <NUM> (labelled "+") and a negative input terminal <NUM> (labelled "-").

The second converter <NUM> converts the incoming DC voltage V<NUM> to an output voltage, which may be DC or AC, depending on the type of export link <NUM> connecting that wind turbine <NUM> to a point of common connection. The export link <NUM> may be a HVDC transmission line ("HVDC link"), for example, connecting a wind farm to a substation such as a transmission substation, a distributor substation, etc. Equally, the export link can transport AC power. The second converter <NUM> has a positive input terminal <NUM> (labelled "+") and a negative output terminal <NUM> (labelled "-").

A DC tower cable <NUM> links the converters <NUM>, <NUM>. In this exemplary embodiment, the DC tower cable <NUM> comprises a single conductor <NUM>. One end of the conductor <NUM> is connected to the positive output terminal <NUM> of the first converter <NUM> and its other end is connected to the positive input terminal <NUM> of the second converter <NUM>, and this single conductor <NUM> transports the output DC current Iout. To complete the electrical circuit, the negative input terminal of the first converter <NUM> is connected to ground (through the nacelle ground connection <NUM>) and the negative output terminal <NUM> of the second power converter <NUM> is connected to ground (through the housing ground connection <NUM>), and because all grounding connectors <NUM>, <NUM>, <NUM> are connected, the tower metal provides a path for the return current Ireturn.

It shall be understood that the various earthing connectors <NUM>, <NUM>, <NUM> are usually physically independent, but are essentially linked by virtue of their connection to electrical ground. This concept is indicated by the dotted line for the return current path Ireturn.

<FIG> is a simplified schematic showing an alternative embodiment of the invention. In this case, the DC tower cable <NUM> is a co-axial cable with two conductors <NUM>, <NUM> separated by an insulating dielectric. In this exemplary embodiment of the invention, each conductor <NUM>, <NUM> of the DC cable <NUM> is connected at one end to the positive output terminal <NUM> of the first converter <NUM> and at the other end to the positive input terminal <NUM> of the second converter <NUM>. In this way, the total cross-sectional area of both conductors <NUM>, <NUM> is available to the output DC current Iout. To complete the electrical circuit, the negative input terminal of the first converter <NUM> and the negative output terminal <NUM> of the second power converter <NUM> are connected to tower ground <NUM>, so that a path is available for the return current Ireturn.

In this embodiment, the total combined area of both conductors <NUM>, <NUM> is used to carry the output DC current Iout. This approach to implementing a co-axial cable significantly increases the efficiency of the DC connection. Compared to a prior art approach, in which only one conductor is available to carry the output DC current Iout, the cable losses are effectively halved.

As described above, both conductors <NUM>, <NUM> are also connected at the other end of the DC cable <NUM> to the second converter <NUM>. The other terminal <NUM> of the DC output stage of the first power converter <NUM> and the other terminal <NUM> of the DC input stage of the second power converter <NUM> are connected as described in <FIG> to provide a path through the tower ground connector for the return current, thereby completing the DC circuit.

<FIG> shows an alternative realisation of the invention. In this exemplary embodiment, the output current Iout is negative with respect to ground. The DC tower cable conductor <NUM> is connected at one end to the negative output terminal <NUM> of the second converter <NUM> and at the other end to the negative input terminal <NUM> of the first converter <NUM>. In this way, the conductor <NUM> carries the negative return current Ireturn. The positive output terminal <NUM> of the first converter <NUM> and the positive input terminal <NUM> of the second power converter <NUM> are grounded as described above, so that tower metal provides a path for the output current Iout.

<FIG> shows a prior art system <NUM>. Here also, electrical power from the generator <NUM> is transferred by means of a first converter <NUM> and a coaxial DC tower cable <NUM> to a second converter <NUM> at the base of the tower <NUM>. In the known configuration, one DC cable conductor <NUM> is connected between the positive output terminal <NUM> of the first converter <NUM> and the positive input terminal <NUM> of the second converter <NUM>; the other DC cable conductor <NUM> is connected between the negative input terminal <NUM> of the first converter <NUM> and the negative output terminal <NUM> of the second converter <NUM>. As explained above, the cross-sectional area of a single conductor <NUM>, <NUM> places a constraint on the magnitude of the current that it can transport, in this case the cross-sectional area of a conductor is a physical constraint that limits the DC output current Iout that can be output by the first power converter <NUM>. In order to be able to export more power, it is necessary to provide a generator-side converter <NUM> capable of higher voltage output and/or to provide a thicker conductor <NUM>, <NUM> in the DC cable <NUM>. However, both solutions add significantly to the overall expense. In the case of a wind farm with tens or even hundreds of wind turbines, the added expense may be prohibitive.

Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

Claim 1:
A wind turbine (<NUM>) comprising
- a generator (<NUM>) driven by an aerodynamic rotor;
- a first power converter (<NUM>) arranged in a nacelle (<NUM>) supported by an electrically grounded tower (<NUM>);
- a second power converter (<NUM>) arranged at the base of the tower (<NUM>);
- a DC transmission link (<NUM>) extending through the wind turbine tower (<NUM>) and connected between the first power converter (<NUM>) and the second power converter (<NUM>);
characterized by
- a first current path between the power converters (<NUM>, <NUM>) through the DC transmission link (<NUM>); and
- a second current path between the power converters (<NUM>, <NUM>) through electrically grounded metal of the tower (<NUM>) .