Electric circuit and wind turbine

The invention relates to an electric circuit providing a ground connection, comprising at least two brushes and a con tact 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. Furthermore, the invention relates to a wind turbine.

Sliding contacts may be used to ensure an electrical contact for instance between rotating structures and stationary structures. In wind turbines, such sliding contacts may be used to contact a rotating structure of the wind turbine and connect it via the sliding contact to a stationary part of the wind turbine. Sliding contacts, which are part of a ground connection and which are not conducting a current reg ularly during operation, tend to increase their impedance due to the built-up of an insulation film at the brush and/or at the surface of a corresponding counterpart, for instance a rotating member. Due to this increased contact resistance, electromagnetic noise may be emitted from the sliding con tact, reducing for instance an electromagnetic compatibility of the electric circuit, or of a device comprising the elec tric circuit, respectively.

Patent publications US 2014/183864 Al, EP 3255276 Al,

US 2009/053062 Al, and DE 102007052525 Al show wind turbine solutions with sliding electrical contacts, for example to avoid stray currents.

It is therefore an object of the invention to provide an electric circuit with improved sliding contacts, in particu lar with sliding contacts that exhibit an improved electro magnetic compatibility. According to the invention, this problem is solved by an electric circuit as initially described, wherein at least two of the sliding contacts are connected to a current source adapted to provide a current through the sliding contacts.

By the electric circuit, a ground connection is provided, wherein the ground connection involves at least one of the sliding contacts of the electric circuit. During operation of the electric circuit, or of an apparatus comprising the elec tric circuit, respectively, the ground connection is not or at least not permanently guiding a current. The electric cir cuit may be used for instance for potential equalization and/or in a lightning conductor path.

To clean the surfaces of the brushes and/or the contact mem ber, at least two of the sliding contacts are connected to a current source, which provides a current through the sliding contacts. This allows for energizing the sliding contacts with a current provided from the current source, so that in particular the formation of an isolating film at the brushes and/or at the contact member can be prevented. The sliding currents energized by the current from the current source may exhibit advantageously clean surfaces and therefore a reduced contact impedance, which in particular improves the electro magnetic compatibility (EMC) of the electric circuit, or a device or apparatus comprising the electric circuit, respec tively. Also the reduced contact impedance is advantageous since it improves a ground connection provided by the elec tric circuit and/or provides a better small signal impedance of the sliding contacts.

Providing a bias current by the current source is in particu lar advantageous for sliding contacts that do not conduct a current, or that do not frequently conduct a current, respec tively, since an isolating film is formed preferably at slid ^¬ ing contacts not conducting a current. The isolation film may cause a degraded bond between the surface of a brush and the surface of the contact member. The contact member is at least partially electrically conduc tive, wherein by at least one of the brushes, the current provided from the current source is fed into the contact mem ber. The current source may be connected also to the ground potential, so that a closed loop involving in particular all of the sliding contacts is obtained. The current source may be connected to one or more of the brushes, wherein one or more other brushes, in particular the remainder of the brush es, connect the contact member to a ground potential. This allows to provide a current through the sliding contacts of the electric circuit to obtain and/or to maintain cleaned surfaces of each of the sliding contacts. In other words, the current source provides a bias current energizing the other wise not or at least not regularly energized sliding contacts leading to a self-cleaning of the brushes.

The contact member may be for instance a slip ring, which is rotatable relatively to the at least two brushes. A current source may be an alternating current (AC) current source or a direct current (DC) current source. It is also possible that the current source provides a superposition of an alternating current and the direct current.

Preferably, the current source is adapted to provide a volt age 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. By providing a voltage that ex ceeds the breakdown voltage of an isolation film, which may arise at the sliding contact, or at the brush and/or the con tact member, respectively, at each brush of a sliding contact connected to the current source, also energizing of the slid ing contacts is possible in a state, where already an isola tion film is existing. Furthermore, the current provided by the current source can be used to remove the existing isola tion film and to keep it clean and hence to improve the con tact impedance of the sliding contact. In a preferred embodiment of the invention, the electric cir cuit comprises a measuring means for measuring a contact im pedance of at least one of the sliding contacts, wherein the current source is adapted to provide a current in dependence of a measured contact impedance. This allows for providing a current by the current source adapted to an existing contact impedance of at least one of the sliding contacts. This al lows for adjusting a current from the current source to the actual contact impedance, so that it is advantageously possi ble to save power during operation or the electric circuit, or the current source, respectively.

The measuring means may be for instance a sensor, in particu lar an ohmmeter, which measures the electrical impedance of one or more of the sliding contacts. The electric circuit may involve a control unit that is connected to both the measur ing means and the current source, wherein a measured imped ance of at least one of the sliding contacts is transmitted to the control unit. The control unit may then control the current source to provide a current depending on the actual contact impedance.

In a preferred embodiment of the invention, the at least two brushes are electrically connected by the contact member and at least one capacitor. The capacitor may be used for in stance to decouple the brushes from a direct current and/or to further improve the electromagnetic compatibility of the electric circuit.

In a preferred embodiment of the invention, the brushes con sist at least partly of carbon and/or copper (i.e. the brush es comprise 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 ² - 50 A/cm ² , in particular of 5 A/cm ² - 20 A/cm ² . Such a current density in a brush inhibits the for mation of an isolation film, in particular at the brush and/or the contact member and therefore at the sliding con tact, respectively. In particular, the current density is provided in at least one of the brushes of the at least one sliding contact that provide the ground connection. It is possible that one or more sliding contacts, in particular a sliding contact that is connected to the current source, ex hibit a higher current density in the respective brush, in particular in order to obtain a current density of 1 A/cm ² - 50 A/cm ² , in particular of 5 A/cm ² - 20 A/cm ² , in the further sliding contacts that provide the ground connection.

Preferably, the electric circuit comprises a surge protection device. The surge protection device may be in particular con nected in parallel to the current source. This may prevent a damaging of the current source, in particular when the ground connection of the electric circuit is used in a lightning conductor used for grounding the current created by a light ning strike. Furthermore, using the surge protection device, also the at least one sliding contact connected to the cur rent source may be used for instance to ground the lightning current in the case that the surge protection device becomes conductive and closes a connection to a ground potential, in particular as a result to the lightning strike.

In a preferred embodiment of the invention, the ground con nection 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 con tact member at least partly on a mutual path. This allows for instance to provide a closed loop for the bias current by two or more brushes connected to both the contact member and the current source and to provide the ground connection, or addi tional ground connections, respectively, by the at least one sliding contact between the contact member and the at least one further brush.

A ground connection of the closed loop involving the current source may be obtained for instance using a surge protection device connecting the closed loop to the ground potential. The closed loop involving the at least two sliding contacts may also cause a reduced contact impedance of the sliding contact comprising the at least one further brush, which is not connected to the current source, but which moves rela tively to the contact member at least partly on the same path as the brushes connected to the current source. By biasing the brushes, which are connected to the current source, with the current from the current source, also the contact imped ance of the at least one sliding contact comprising the at least one further brush and the contact member can be advan tageously reduced. Since at least on the surface of the con tact member, which is in contact with the brushes and the at least one further brush, the formation of an isolation layer reducing the contact resistance may be prevented by the brushes connected to the current source. In particular, both the brushes connected to the current source and the at least one further brush may be moveable relatively to the contact member on a circular path, in particular when the contact member is a slip ring.

A wind turbine according to the invention comprises an elec tric circuit according to the invention. The electric circuit can be used with an advantage in a wind turbine, in particu lar for reducing electromagnetic noise emitted from the wind turbine. A wind turbine comprises several rotating parts, in cluding the rotor blades, a hub bearing the rotor blades and a shaft connected to both the hub and a rotor of a generator of the wind turbine. At several positions, a provision of grounding connections using sliding contacts may be required, so that an electric circuit according to the invention can be used with an advantage in the wind turbine. Advantageously, the contact impedance of these grounding connections may be reduced by the usage of the current source as previously de scribed.

The reduction of the contact impedance of the sliding con tacts of the electric circuit has the additional advantage that electromagnetic emissions created by the wind turbine can be reduced. Especially in a frequency range from 150 kHz - 30 MHz, a significant reduction of the electromagnetic emissions can be obtained by using brushes connected to a current source in the respective grounding connections of the wind turbine. This helps to attenuate electromagnetic noise created in form of radiated and conducted emissions during the operation of the wind turbine. Additionally, also an im munity to external electromagnetic energy sources is im proved.

Preferably, the electric circuit provides a ground connection of a rotating part of a generator of the wind turbine. The generator may comprise for instance a rotor as rotating part, which rotates around a stator of the generator arranged in side the rotor. Since the rotor of the generator is connected to the shaft of the wind turbine as well as to the hub, an electrical contact between the rotor of the generator and one or more stationary parts of the wind turbine is necessary for grounding these components. By providing the ground connec tion by self-cleaning sliding contacts of an electric cir cuit, which can reduce the contact impedance by energizing the sliding contacts using a current source, the contact im pedance of the sliding contacts comprising the brushes and a contact member attached to the rotating part of the generator can be kept low.

Additionally, a potential equalization between the rotating part of the generator and one or more stationary parts of the wind turbine can be improved by the reduced contact impedance of the sliding contacts. This may be in particular advanta geous with regard to a bearing, which supports a rotating part of the wind turbine on a stationary part, in particular with regard to a main bearing supporting the rotatable shaft on a stationary housing. An improved contact resistance in a grounding connection providing a potential equalization be tween one or more rotating parts and the stationary parts prevents the appearance of undesired high voltages between the rotating parts and the stationary parts. These voltages may cause high noise voltages in the bearing, in particular over an oil film in a main bearing of the wind turbine. The reduction of these noise voltages decreases the risk of a flashover through the oil film and thereby increases the bearing lifetime.

In a preferred embodiment of the invention, the electric cir cuit provides a ground connection of a lightning conductor of the wind turbine. The ground connection provided by the elec tric circuit may connect for instance a lightning conductor provided in one of the blades of the wind turbine to a sta tionary lightning conductor provided for instance inside the tower of the wind turbine. For connection of these lightning conductors, at least one connection between a rotatable part and a stationary part is required. This connection can be provided advantageously by the ground connection of the elec tric circuit.

In a preferred embodiment, the contact member is a break disc of a rotatable part of the wind turbine. The break disc may be for instance attached to a rotor of the generator of the wind turbine allowing to break the rotor of the wind turbine during operation. The break disc may consist at least partly of a conductive material (i.e. the break disc may comprise conductive material), for instance steel or stainless steel, wherein the brushes of the electric circuit each form a slid ing contact to the break disc.

In a preferred embodiment, the current source is a common mode current induced by a generator of the wind turbine. Be sides the main alternating current generated by the generator for power generation, also additional voltages or currents, in particular of a lower voltage, can be generated by the generator during operation of the wind turbine. An addition ally induced common mode current can be directed to take the path of the brushes of the sliding contact by feeding it into a closed loop comprising the sliding contact, for instance using a transformer or the like. This has the advantage that no additional current source has to be implemented in the wind turbine for providing the bias current through the brushes. However, additionally or alternatively, also a cur rent source, which is independent on the operation of the generator, can be used to facilitate in particular a control of the current source based on a measured actual contact im pedance.

The advantages and details of the electric circuit according to the invention apply correspondingly to a wind turbine ac cording to the invention and vice versa.

Other objects and features of the present invention will be come apparent from the following detailed description consid ered in conjunction with the accompanying drawings. The draw ings, however, are only principle sketches designed solely for the purpose of illustration and do not limit the inven tion. The drawings show:

Fig. 1 an embodiment of a wind turbine according to the invention,

Fig. 2 a first embodiment of an electric circuit according to the invention, and

Fig. 3 a second embodiment of an electric circuit accord ing to the invention.

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.

The generator 7 comprises a rotor 8 rotatable relatively to a stator 9 of the generator 7. The rotatable shaft 6 is sup ported on the stationary nacelle 3 by a main bearing 10 of the wind turbine 1. 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 elec tric 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 rotat ing brake disc forms a slip ring creating a sliding contact with each of the brushes 12, 13.

The electric circuit 11 provides a grounding connection of the wind turbine 1 connecting one or more lightning conduc tors 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.

In normal operation of the wind turbine 1 neither the light ning conductors 15, 16 nor the sliding contacts between the brushes 12, 13 and the contact member 14 are permanently en ergized 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.

The amount of electromagnetic emissions generated during the operation of the wind turbine 1 may depend, besides other ef fects, also on the electrical impedance of each of the slid- ing contacts between the brushes 12 or 13, respectively, and the contact member 14. To decrease the amount of electromag netic 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.

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 re sult, 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.

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 de vice 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.

The current source 17 may comprise a direct current (DC) cur rent 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 cur- rent 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 alternat ing 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.

Additionally or alternatively, the current source 17 may com prise an alternating current source 21 powered by an external power source, for instance a power grid. Also the direct cur rent source 20 may be powered from an external power source, so that the current created by the DC power source 20 is in dependent from the operation of the wind turbine 1.

The current source 17 provides an open loop voltage, which is above a breakdown voltage of the isolation film, which ap pears 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 cur rent with the current source 17 to the sliding contact.

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 mem ber 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 pro vided by the current source 17 if the contact resistance be tween 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 ex ists. Vice versa, in case that a larger impedance is deter mined by the measuring means 23, a larger current can be pro- vided from the current source 17 removing an existing isola tion film and/or preventing the formation of a new isolating film.

The brushes 12, 13 may consist at least partly of carbon and/or copper. In other words, the brushes 12, 13 may be man ufactured 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 ² - 50 A/cm ² , in particular of 5 A/cm ² - 20 A/cm ² , is ob tained. This prevents deformation of an isolating layer at the brushes 12, 13 and/or at the surface of the contact mem ber 14.

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 rela tively 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 con tacts 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.

The current source 17 may be provided correspondingly to the first embodiment. The measuring means 23 may measure the con tact resistance of one or more of the sliding contacts be tween 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 pos- sible that the brush 13 is also connected to a ground poten tial correspondingly to the first embodiment.

Although the present invention has been described in detail with reference to the preferred embodiment, the present in vention 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.