Field of the invention The invention relates to a method of transmitting a current from a first conductor to a second conductor, which conductors are in contacting relationship along a contact interface. The invention also relates to a device for transferring current from a first conductor to a second conductor via a contact interface. Background of the invention

When electrical current needs to be transferred between parts that show relative rotation, such as machine parts, wind turbines or offshore high voltage swivels, many different solutions are known, some of which allow a limited angle of rotation while others allow unlimited rotational angles. Increasing high voltages, such as several KV need to pass from one electrode to the other via the contact interface. In known slide contacts, such as available from Schleifring or Cavotec, a stack of rings or discs is contacted by one or more sliding contacts or carbon brushes per ring to provide electrical contacts. The slide contacts have several disadvantages such as wear of the contact surfaces. Wear is counteracted by the use of expensive metal alloys and reduced contact pressures between the slide contacts and the rings. In some signal slip ring units a paste is used for offering conductivity and lubrication, typical components including grease and a high content of silver or copper particles, such as for instance Highly Conductive Grease, marketed by the firm Electrolube.

For contacts that are not in a sliding contacting relationship, such as for instance in circuit breakers, a high current needs to be passed through the contact interface which current is limited by the resistance formed by the interfaces at microscopic level not forming a even surface. Increasing the contact pressures may cause local deformation and results in a decrease of the resistance across the contact interface.

From EP 0 089 725, a rotary device for transfer of electric power is known in which in the gap between rotor and stator, mercury is used as a conductive fluid. The gap may have a width of about 15 μηι. Mercury is environmentally undesirable in view of its toxic qualities and is difficult to contain by means of seals. Also a mixture of conductive particles and a non-conductive fluid is described as an alternative for the liquid metal, such as silver particles and ethylene glycol. The known device has as a disadvantage that the metal slurry formed by the particles and liquid may over time be unevenly distributed in the gap, resulting in irregular current transfer and increased wear.

It is an object of the present invention to provide a method and a device in which the resistance at the contact interface is reduced and which provides for a stable transfer of relatively high currents from one conductor to the other via the contact interface.

Summary of the invention Hereto the method according to the invention is characterized by introducing a nonconducting liquid comprising conducting particles at the contact interface, wherein the liquid and the particles form a colloidal suspension, the particle size being between 1 and 1000 nm. The applicants have surprisingly found that by the use of a colloidal suspension of conducting particles, the resistance at the interface can be strongly reduced and currents can be increased. The liquid with particles as a colloidal suspension provides a stable and homogeneous medium allowing uniform current transmission on each position of the conductor that is stable over a large period of time. It was found that the colloidal suspension according to the invention provides an insulating behavior at gap distances between the first and second conductors above 0.5 mm but becomes highly conductive at smaller gap widths, such as gaps smaller than 0.5 mm, preferably smaller than 0.3 mm and more preferably smaller than 0.1 mm. No accumulation or increased concentration of suspended particles was found to occur over time. This stability of the colloidal suspension over time ensures that the combination of conductivity at short distances (small gap widths) and isolation at larger distances prevents either loss of conducting contact between the conductors or possible short circuits forming.

The colloidal suspension may comprise lubricating or preferably penetrating oil. Two exemplary brands of oil that can be successfully applied are high quality penetrating oil based on graphite in a spray can, marketed by Griffon under trade name EVIAL

Penetrating Oil (see www.griffon.eu) or Standard Penetrating Oil marketed by LATOL in jerry can (see www.metropa-rotterdam.nl), both containing graphite particles in a colloidal suspension.

A colloidal suspension is formed by a substance that is microscopically dispersed through the liquid, such that over time Brownian motion resists separation of the substances, and no separation of the substances occurs. The colloidal particles generally range in size from 1 nm to 1000 nm.

The colloidal suspension according to the invention does not provide a conducting medium for gaps between the conductors that are larger than 0.5 mm but shows a highly increased conductivity when the conductors approach each other to form a gap with a width of 0.1mm and smaller. It is believed that the oil will transport the conducting particles into interspaces at the uneven areas of the contact interface and will bridge exiting gaps at the interface for establishing an improved conductive path. Hereby it was possible to transfer high currents at a voltage loss of less than 50 mV. It was for instance measured that by adding graphite oil between two copper contacts at 12 V DC and a current of 40 A, the voltage loss was by 2/3 from 33mV to 1 lmV.

With the term "contacting relationship" as used herein it is meant that the conductors may be in stationary or dynamic contact, such as rolling contact or sliding contact and may touch at the contact interface at pressures ranging from 0 to relatively high contact pressures or are separated at the contact interface by a distance of no more than 0.5 mm, preferably no more than 0.1mm. In case a gap is formed between the conductors, this gap is bridged by a film of fluid containing the conducting particles. It is noted that in WO 2013/087487 the use of a lubricant in combination with a microstructure on an electrical plug type connector is known for reducing the frictional force required to engage the plug and socket. The lubricant may have for example, oil, grease, a paste or a solid-state lubricant such as graphite, CNT, Graphene MoS2 and admixtures thereof. No colloidal suspension of small-size conducting particles in a nonconducting liquid is disclosed however.

In a stationary embodiment, the conductors are interconnected by a connector device compressing the conductors together at the contact interface. It was found that two interconnected conductors, such as copper terminals that are connected by a bolted clamping, after time lose their contact pressure due to the oxidation and the effects of current transfer at the contact interface resulting in a widening gap between the conductors and a loosening of the conductors. This causes a need to regularly check and refasten the conductors on a frequent basis. By the use of the colloidal suspension at the interface between the conductors, it was found that loosening of the conductors does no longer occur and regular inspecting and refastening of the conductors is obviated.

Suitable connector devices according to the invention comprise bolt and nut connections, resilient clamping connectors or plug and socket connections, bayonet connections, threaded connections and the like.

In case the conductors move relative to one another along a contact interface plane, the oil will also help in lubricating the contact plane and prevents corrosion of the electrodes. In addition, the graphite particles will assist in reducing the friction coefficient of both conductors.

The non conducting liquid may be formed by liquids, and are preferably oils either based on mineral oil distilled from crude oil (nafta) or based on synthetic oil.

Preferably the liquid comprises an oil which facilitates suspension of the particles. These particles may comprise any kind of conductive particles either based on metals or carbon, and have preferably a size smaller than 1000 nm. A particularly advantageous form of particles are graphite particles. These particles are especially advantageous in case of movement at the contact interface as they exhibit both a conducting and a lubricating effect.

The conductors can be made of metal, such a metal typically used in slip ring applications for instance phosphor bronze, carbon or other conductive materials.

In a further embodiment, at least one of the conductors comprises a replaceable conducting material at the contact interface, comprising carbon, such as full carbon brushes or partly carbon and metal containing brushes. The carbon containing replaceable parts, or "(carbon brushes" provide a conducting path and can be used in combination with the colloidal suspension as an additional path for current transfer or to serve as a backup in case the colloidal suspension is no longer present between the conductors. It was found that the combination of the replaceable conductors and the colloidal suspension according to the present invention results in increased lifetime of the replaceable conductors by reduction of wear and that formation of a slurry of carbon wear particles and non-conducting fluid did not occur. Hence there is no risk of short circuiting and homogeneous and stable current transfer is warranted at high currents. For instance for metal graphite brushes on phosphor bronze rings the resistance was found to drop by 33% and currents could be increased by 200%.

The kinematic viscosity of the oil is preferably low in order to allow penetration between the conductors at the contact interface and may be between 2 and 10 mm ² /s. In one embodiment, the conductors move relative to one another along a contact interface plane in a sliding manner, wherein a pressure at the contact interface is below 0.75 N/mm ² .

In a further embodiment, the conductors move relative to one another along a contact interface plane in a rolling manner, wherein a pressure at the contact interface is between 100-150 N/mm ² . At these increased pressures, the uneven metal surfaces of the electrodes (for instance made of or containing copper) may be pressed together beyond the yield pressure of the softest metal such that improved electrical contact is achieved. Furthermore, the conductors that are pressed together along the contact interface plane are very stable and resistant to vibrations.

The conductors may first be moved relative to one another along a contact interface plane in a rolling manner without a current passing from one to the other. Hereby the contact interface plane is smoothened by rolling and the need for high pressure forces is obviated. Thereafter, the conductors can be moved relative to one another along the contact interface plane in a rolling manner at pressures of between 25% and 35% of the yield strength of the softest metal of the contacting metal conductors. The lower pressures at the contact interface may for copper lie between 30-70 N/mm ² .

In an embodiment, a current density at the interface plane is between 50 and 75 A/mm ² . By use of the insulating oil containing conducting particles, a high current can be passed through a small contact surface at relatively low resistance (in the voltage range of 20-50 mV), which is favorable for power transfer.

In case low currents are transferred, for instance in the case of signal transfer, the resistance can be very low (in the range of 1-5 mV). Simultaneously, the signal noise is reduced to very low values such as 0.1-0.5 mV for copper, allowing high data transfer rates. This is important for instance when transmitting high resolution camera images from a rotating support to a cable, the invention allowing establishing a 1 GB/s

Ethernet connection via the moving contacts.

A device for transferring current from a first conductor to a second conductor via a contact interface may comprise a reservoir containing a non-conducting liquid with conducting particles therein, the contact interface being in fluid communication with the reservoir.

The contact interface may placed within the reservoir to be submerged in the oil, or the reservoir may be connected to a dispenser for dispensing non conducting liquid with conducting particles therein, onto the contact interface. This can be in continuous or intermittent flow when the oil remains in place at the contact interface. The colloidal graphite oil has as an advantage that it avoids short circuits as the graphite particles do not stick together and the oil remains non-conductive.

Brief description of the drawings

Some embodiments of a device according to the invention will be explained in detail with reference to the accompanying drawings. In the drawings:

Fig. 1 shows a schematic view of moving electrodes submerged in a non-conducting oil containing conducting particles suspended therein,

Fig. 2 shows a schematic view of an alternative configuration of moving electrodes connected to a dispenser device for supplying non-conducting oil with conducting particles to the contact interface,

Fig. 3 shows a schematic view of a circuit breaker arrangement with oil supplied to the contact interface.,

Fig. 4 shows a stationary bolted connection with a colloidal suspension at the interface,

Figs. 5 and 6 show embodiments of rotary conductors in combination with carbon brushes, and

Fig. 7 shows a graph of the conductivity of the colloidal suspension according to the invention vs. the gap width between the conductor for conductors with and without a colloidal suspension according to the invention.

Detailed description of the invention Fig. 1 shows a set-up in which a copper tube 1 rolls on a flat copper strip 2. Current is supplied from power supply 8 and flows via motor 7, through the contact 3, the tube 1 and through strip 2 to contact 4. Weights are placed on the tube 1 such that the pressure at the contact interface was about 150 N/mm ² . In the reservoir 5 penetrating oil containing graphite was placed, such that the contact interface 6 was submerged below the oil level. At currents of 60 A/mm ² and at a voltage of 12V, the voltage loss that was measured across the tube 1 and the strip 2 was 10-20 mV.

Fig. 2 shows a stationary ring-shaped outer electrode 11 and a rotating inner electrode 10, rolling along the contact interface 16. Insulating oil containing graphite particles is stored in container 17'. Via a dispenser 12 comprising a pump 13, a duct 14 and an outflow device 15, a film of oil containing graphite particles is applied onto the contact interface 16, hence strongly reducing the resistance. Current is supplied by contact 17 to the inner electrode and flows via the inner electrode 10 to the outer electrode 11 and outer contact terminal 18.

Fig. 3 shows a stationary application of the oil in a circuit breaker 22 comprising a first electrode 20 and a second electrode 21, that can be separated by withdrawing electrode 20 away from contact interface 23. A reservoir 24 comprising oil with conducting particles therein surrounds the contact interface 23.

Fig. 4 shows a first and second conductors 31, 32 that are clamped together via a bolt 34 and nut 33. A double internal and external seal 35 such as an O-ring or a metal seal, is provided between the conductors to contain the colloidal suspension 36 within the gap d, which may have a width of 0.1 mm or less.

Fig. 5 shows a inner conductor 40 and a rotary outer conductor 41 comprising four replaceable carbon brushes 42, 42' in combination with the colloidal suspension forming a layer around the inner conductor 40. The suspension layer can be created by rotating in a part submerged condition (not shown in figure) or by supplying the suspension by means of spraying, dripping, a contacting wick or any other suitable means. Fig. 6 shows the carbon brushes 42,42'being placed on a rotary inner conductor 40 and a stationary outer conductor 41 in combination with the colloidal suspension layer 43. Fig. 7 shows a graph of the gap resistance in Ohm vs. the gap width in mm, with and without the colloidal suspension according to the invention. The measurements were carried out for two copper contacts, one having a flat surface and the other a half circular rod diameter of 15mm. It was found that in dry conditions, the resistance increases abruptly at a gap width of about 0.03 mm and that using graphite containing oil as a colloidal suspension as obtained from the firm Metropa in The Netherlands, a low resistance is maintained at larger gap widths (up to 0.04 mm). After 0.04 mm also for the colloidal suspension of graphite particles in oil, a stark increase in resistance was observed. Surprisingly, the high conductivity due to the colloidal suspension is present in a relatively narrow gaps.