ELECTRIC CIRCUIT AND WIND TURBINE

Abstract

An electric circuit providing a ground connection includes at least two brushes and a contact member, wherein each brush and the contact member form a sliding contact, wherein the contact member is connected to a ground potential by at least one of the sliding contacts, wherein at least two of the sliding contacts are connected to a current source adapted to provide a current through the sliding contacts.

Claims

1. An electric circuit providing a ground connection, comprising at least two brushes and a contact member, wherein each brush and the contact member form a sliding contact, wherein the contact member is connected to a ground potential by at least one of the sliding contacts, wherein at least two of the sliding contacts are connected to a current source adapted to provide a current through the sliding contacts.

2. The electric circuit according to claim 1, wherein the current source is adapted to provide a voltage at each brush that is equal to or larger than a breakdown voltage of an isolation film arising at the sliding contact in a state without current.

3. The electric circuit according to claim 1, wherein the electric circuit comprises a measuring means for measuring a contact impedance of at least one of the sliding contacts, wherein the current source adapted to provide a current in dependence of a measured contact impedance.

4. The electric circuit according to claim 1, wherein the at least two brushes are electrically connected by the contact member and at least one capacitor.

5. The electric circuit according to claim 1, wherein the brushes consist at least partly of carbon and/or copper, wherein the current source is adapted to provide a current density in at least one of the brushes of 1 A/cm.sup.2-50 A/cm.sup.2, in particular of 5 A/cm.sup.2-20 A/cm.sup.2.

6. The electric circuit according to claim 1, wherein the electric circuit comprises a surge protection device.

7. The electric circuit according to claim 1, wherein the ground connection comprises at least one further brush not connected to the current source, wherein at least one of the brushes of the sliding contacts connected to the current source and the at least one further brush are movable relatively to the contact member at least partly on a mutual path.

8. The wind turbine comprising the electric circuit according to claim 1.

9. The wind turbine according to claim 8, wherein the electric circuit provides a ground connection of a rotating part of a generator of the wind turbine.

10. The wind turbine according to claim 8, wherein the electric circuit provides a ground connection of a lightning conductor of the wind turbine.

11. The wind turbine according to claim 8, wherein the contact member is a break disc of a rotatable part of the wind turbine.

12. The wind turbine according to claim 8, wherein the current source is a common mode current induced by a generator of the wind turbine.

Description

[0027] Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. The drawings, however, are only principle sketches designed solely for the purpose of illustration and do not limit the invention. The drawings show:

[0028] FIG. 1 an embodiment of a wind turbine according to the invention,

[0029] FIG. 2 a first embodiment of an electric circuit according to the invention, and

[0030] FIG. 3 a second embodiment of an electric circuit according to the invention.

[0031] In FIG. 1, a wind turbine 1 according to the invention is shown. The wind turbine 1 comprises a tower 2, a nacelle 3 and a hub 4, wherein the hub 4 bears a plurality of rotor blades 5. The hub 4 is connected to a rotatable shaft 6, which connects the hub 4 to a generator 7.

[0032] The generator 7 comprises a rotor 8 rotatable relatively to a stator 9 of the generator 7. The rotatable shaft 6 is supported on the stationary nacelle 3 by a main bearing 10 of the wind turbine 1.

[0033] The wind turbine 1 comprises an electric circuit 11, wherein the electric circuit 11 comprises two brushes 12, 13 each forming a sliding contact to a contact member 14 of the electric circuit 1. In this embodiment, the contact member 14 is a break disc of the wind turbine 1, which is attached to the rotor 8 of the generator 7 and which can be used to brake, or to stop, respectively, a rotation of the shaft 6. The rotating brake disc forms a slip ring creating a sliding contact with each of the brushes 12, 13.

[0034] The electric circuit 11 provides a grounding connection of the wind turbine 1 connecting one or more lightning conductors 15, which are arranged inside the rotor blades 5 and therefore in a rotating part of the wind turbine 1, to a down conductor 16 which is arranged inside the tower 2 of the wind turbine 1 and which is therefore stationary. The lightning conductor 15 is for instance electrically connected to the shaft 6 and to the rotor 7 of the generator, wherein a ground connection is provided by the electric circuit 11 by at least one of the sliding contacts between the brushes 12, 13 and the contact member 14 and the down conductor 16.

[0035] In normal operation of the wind turbine 1 neither the lightning conductors 15, 16 nor the sliding contacts between the brushes 12, 13 and the contact member 14 are permanently energized by a current. A current is conducted only in the rare circumstance of a lightning strike hitting the wind turbine 1. This may lead to the formation of an isolation film on the brushes 12, 13 and/or the contact member 14, which increases an electrical impedance of the connection between the brushes 12, 13 and the contact member 14, or of the sliding contacts, respectively, and thereby created a degenerated bond of the sliding contact.

[0036] The amount of electromagnetic emissions generated during the operation of the wind turbine 1 may depend, besides other effects, also on the electrical impedance of each of the sliding contacts between the brushes 12 or 13, respectively, and the contact member 14. To decrease the amount of electromagnetic emissions by reducing the contact resistance of the sliding contacts between the brushes 12, 13 and the contact member 14, the electric circuit 11 comprises a current source 17 that is connected to the sliding contacts provided by brushes 12, 13 and the contact member 14.

[0037] By the current source 17, a bias current is generated which circulates through the brushes 12, 13 and the contact member 14. The contact member 14 is at least partly electrically conductive and connects the sliding contacts of the brush 12 electrically to the sliding contact of the brush 13. As a result, a closed current loop is created which can be fed by the current from the current source 17, wherein the current flows through the sliding contacts between the brushes 12, or 13, respectively, and the contact member 14. By the current energizing the sliding connections, an existing isolation layer can be removed and/or the formation of a new isolation layer can be prevented advantageously.

[0038] In FIG. 2, a first embodiment of an electric circuit 11 is shown. In addition to the previously mentioned components, the electric circuit 11 also comprises a surge protection device 18 and a capacitor 19. By the surge protection device 18, also the sliding contact between the brush 12 and the contact member 14 is connected to a ground potential if a high voltage occurs in the electrical circuit, for instance in the event of an lightning strike into one of the lightning conductors 5. The capacitor 19 is connected in parallel to the current source 17 and is used for decoupling the brushes 12 and 13.

[0039] The current source 17 may comprise a direct current (DC) current source 20 and/or an alternating current (AC) current source 21. It is possible that the alternating current source 21 comprises a transformer 22, which allows for instance to use an alternating current created by the generator 7 as current source for the electric circuit 11. It is possible that for instance a common mode current induced by the operation of the generator 7, in particular in addition to an alternating current produced by the generator 7 for power generation, is used as current source 17 for the electric circuit 11, for instance by coupling it into the transformer 22 of the AC current source 21.

[0040] Additionally or alternatively, the current source 17 may comprise an alternating current source 21 powered by an external power source, for instance a power grid. Also the direct current source 20 may be powered from an external power source, so that the current created by the DC power source 20 is independent from the operation of the wind turbine 1.

[0041] The current source 17 provides an open loop voltage, which is above a breakdown voltage of the isolation film, which appears at the brushes 12, 13 and/or the contact member 14, when the sliding contacts are not energized by a current. This enables to remove an isolation film by providing a current with the current source 17 to the sliding contact.

[0042] The electric circuit 11 comprises a measuring means 23, for instance an ohmmeter, which measures the contact impedance, or contact resistance, respectively, of at least one of the sliding contacts between the brushes 12, 13 and contact member 14. The measuring means 23 is connected (connection not shown) to a control unit 24 of the electric circuit 11. By the control unit 24, the current source 17 can be controlled in dependence of the contact impedance of the at least one sliding contact between the brushes 12, 13 and the contact member 14. This allows for instance to reduce a current provided by the current source 17 if the contact resistance between the brushes 12, 13 and the contact member 14 is low, hence in a state, where no or only a neglectable isolating film between the brushes 12, 13 and the contact member 14 exists. Vice versa, in case that a larger impedance is determined by the measuring means 23, a larger current can be provided from the current source 17 removing an existing isolation film and/or preventing the formation of a new isolating film.

[0043] The brushes 12, 13 may consist at least partly of carbon and/or copper. In other words, the brushes 12, 13 may be manufactured from material that comprise carbon and/or copper. A contact member 14, which is a slip ring, in particular a break disc of a wind turbine 1, may consist or comprise for instance of steel or stainless steel. To prevent deformation of an isolating film in particular at the brushes 12, 13, a current can be provided by the current source 17 so that a current density in at least one of the brushes 12, 13 of 1 A/cm.sup.2-50 A/cm.sup.2, in particular of 5 A/cm.sup.2-20 A/cm.sup.2, is obtained. This prevents deformation of an isolating layer at the brushes 12, 13 and/or at the surface of the contact member 14.

[0044] In FIG. 3, a second embodiment of an electric circuit 11 is shown. The electric circuit 11 comprises two further brushes 25, 26, which each form a sliding contact with the contact member 14. The further brushes 25, 26 are also movable relatively to the contact member 14. The brush 25 runs on the contact member at least partly on a mutual path with the brush 12, and the brush 26 runs on the contact member at least partly on a mutual path with the brush 13. By the bias current provided by the current source 17, all sliding contacts between the brushes 12, 13 and the further brushes 25, 26 and the contact member 14 can kept in a state without or essentially without isolating film and therefore with reduced contact impedance.

[0045] The current source 17 may be provided correspondingly to the first embodiment. The measuring means 23 may measure the contact resistance of one or more of the sliding contacts between the brushes 12, 13 and/or the further brushes 25, 26 and the contact member 14 and control the current source 17 as described with regard to the first embodiment. It is possible that the brush 13 is also connected to a ground potential correspondingly to the first embodiment.

[0046] Although the present invention has been described in detail with reference to the preferred embodiment, the present invention is not limited by the disclosed examples from which the skilled person is able to derive other variations without departing from the scope of the invention.