WO2023075604A1

WO2023075604A1 - Improved low-emi electric cable and electric circuit comprising such cable

Info

Publication number: WO2023075604A1
Application number: PCT/NO2022/050234
Authority: WO
Inventors: Erlend Frisvold
Original assignee: Ezone Green Energy As
Priority date: 2021-10-26
Filing date: 2022-10-18
Publication date: 2023-05-04
Also published as: TW202324453A; EP4174881A1

Abstract

The invention relates to an electric cable (300-1, 300-1a, 300-1b, 300-2) for coupling an electric power source (100e) with an electric load (300). The electric cable (300-1, 300-1a, 300-1b, 300-2) extends from an input end (E1) to an output end (E2) and comprises at least two sub-cables (310, 320, 330). Each sub-cable (310, 320, 330) comprises at least two electrically-conductive wires (L11..L33) laid out in a twisted or braided fashion. A first wire (L11, L21, L31) of said wires (L11..L33) is configured for carrying an electric signal (L1, L2, L3) from the input end (E1) to the output end (E2) of the sub-cable (310, 320, 330). A second wire (L12, L22, L32) of said wires (L11.L33) is configured for forming a current return path (IRC, IRC1, IRC2, IRC3) for the electric signal (L1, L2, L3) of the first wire (L11, L21, L31), wherein the second wire (L12, L22, L32) is electrically connected with a ground terminal (318) at the output end (E2) through an electrical connection (319). The invention provides for an electric cable that is much less susceptible to EMI enabling to feed much larger currents through the cable without causing undesired effects.

Description

IMPROVED LOW-EMI ELECTRIC CABLE AND ELECTRIC CIRCUIT COMPRISING

SUCH CABLE

FIELD OF THE INVENTION

The invention relates to an electric cable for coupling an electric power source with an electric load. The current invention serves to further improve the EMI-properties of electric cables that are used as power cables.

BACKGROUND OF THE INVENTION

Isolation transformers block transmission of the DC components in signals from one circuit to the other, while allowing AC components in signals to pass. Transformers that have a ratio of 1 to 1 between the primary and secondary windings are often used to protect secondary circuits and individuals from electrical shocks between energized conductors and earth ground. Suitably designed isolation transformers block interference caused by ground loops. Isolation transformers with electrostatic shields are used for power supplies for sensitive equipment such as computers, medical devices, or laboratory instruments.

Faraday cages are typically used for blocking electrical fields. An external electrical field causes the electric charges within conducting material (which the cage comprises) to be distributed such that they cancel the field's effect in the interior of the cage. This phenomenon is used to protect sensitive electronic equipment within the cage from external radio frequency interference (RFI). Faraday cages are also used to enclose devices that produce RFI themselves, such as radio transmitters. The Faraday cage then prevents the radio waves from interfering with other nearby equipment outside the respective cage. In the case of varying electromagnetic fields, it applies that the faster the variations are (i.e. , the higher the frequencies), the better the material resists magnetic field penetration. In such case the shielding also depends on the electrical conductivity, the magnetic properties of the electrical ly-conductive materials used in the cages, as well as their thicknesses.

The problem with the above-mentioned known isolation transformers is that they still suffer from a lot of electric magnetic interference (EMI) when used in accordance with the international standards for connecting isolation transformers. The noise levels can even be an order of magnitude higher than the prescribed maximum allowable levels. Thus, there is a clear need for a further improvement of isolation transformers. The most relevant international standard is “2011 NEC” which refers to the UL, CSA and NEMA standards (NEMA ST-20).

The current inventor earlier proposed in WO2019/013642 a low-EMI transformer comprising: i) a Faraday cage comprising a magnetic core and at least one primary coil and at least one secondary coil; ii) input terminals connected to the at least one primary coil via input wires; iii) output terminals connected to the at least one secondary coil via output wires, iv) and an input ground terminal for connecting to the Faraday cage and an output ground terminal connected to the Faraday cage for further connection to a further circuit to be connected to the isolation transformer. The isolation transformer in WO2019/013642 further comprises: v) a clean ground input terminal for receiving an external clean ground; vi ) a clean ground output terminal for connecting to a further clean ground input terminal of the further circuit, and vii) a physical electrical node placed at a location within the Faraday cage where the magnetic flux and electric field are the lowest, preferably close to zero. The clean ground input terminal is electrically fed into the isolation transformer and connected to the physical electrical node through a first electric connection. Furthermore, the physical electrical node is further electrically connected to a clean ground output terminal through a second electric connection.

An important feature of the transformer in WO2019/013642 is that the transformer is provided with a separate (extra) input terminal for receiving a clean ground and a separate (extra) output terminal for supplying a clean ground to the further circuit, whereas in the earlier prior art solutions all grounds are connected to each other, i.e. , there is no separate low-EMI ground. The (normal) input ground terminal is connected to the Faraday cage, which may be further connected to other Faraday cages of other circuitry, which as such is also the case for the earlier prior art solutions. The clean ground input terminal is fed to the physical electrical node, from which it is further fed towards the clean ground output terminal. The inventor discovered that the placement of this physical electrical node is very critical, i.e., that it must be placed where there is the least magnetic flux and the lowest electric field. Furthermore, the ideal position of the physical electrical node is also dependent on the load of the transformer in that the load determines the internally created electric and magnetic fields. Furthermore, the clean ground output terminal is, in operational use, fed to a further clean ground input of the further circuit. The first electric connection and the second electric connection are preferably placed such that EMI generation is minimized in these connections, for example by using shielded wires and by making the wires run parallel with other signal carrying conductors. In addition, the first and second electric connections must have a low-impedance, not only at low frequencies, but also at high frequencies. By taking these technical measures the transformer in WO 2019/013642 provides for a transformer where EMI that is generated in the further circuit will be fed back to the transformer through the low-impedance clean ground connection instead of through the high-impedance ground connections which creates a lot of noise in the supply voltage of the further circuit, but also in the circuitry and components connected to the further circuit. The consequence of the combination of the above-mentioned features is an isolation transformer that is much less susceptible to EMI than the isolation transformers as known from the earlier prior art.

In order to maintain low EMI-levels throughout the circuit, WO2019/013642 proposes a special cable for coupling the transformer with its load. This cable is adapted in that it also feeds the clean ground towards the load besides the conventional ground. The adapted cable comprises a multi-core shielded cable and aims at reducing induced noise (EMI) by minimizing or cancelling electric and magnetic fields to which cables and wires in the isolation transformer are exposed. The multi-core shielded cable effectively comprises two cables a first core and a second core combined into one cable sleeve. The cable sleeve may comprise oil-resistant PVC for example. The first core comprises of a double twistedpair wire connecting the clean ground output terminal of the transformer with the clean ground input terminal of the load. The second core comprises the output wires each carrying a respective output phase/signal. Both the first core and the second core comprise a shield that is connected to ground (PE).

However, a drawback of the cable disclosed in WO2019/013642 is that the inventor discovered it is still susceptible to EMI. The return current does not always appear to fully flow through the twisted-pair wires of the first core.

This problem led to the inventor further developing the low EMI cable in order to reduce EMI. SUMMARY OF THE INVENTION

The invention has for its object to remedy or to reduce at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art.

The object is achieved through features, which are specified in the description below and in the claims that follow.

The invention is defined by the independent patent claims. The dependent claims define advantageous embodiments of the invention.

In a first aspect the invention relates to an electric cable for coupling an electric power source with an electric load, the electric cable extending from an input end to an output end. The electric cable comprises at least two sub-cables, each sub-cable comprising at least two electrical ly-conductive wires laid out in a twisted or braided fashion. A first wire of said wires is configured for carrying an electric signal from the input end to the output end of the sub-cable. Furthermore, a second wire of said wires is configured for forming a current return path for the electric signal of the first wire. Also, the second wire is electrically connected with a ground terminal at the output end through an electrical connection.

The effects of the electric cable in accordance with the invention are as follows.

A first important feature of the invention is that instead of a single conductor per phase line (one for L1 , one for L2 and one for L3 in case of a three-phase system) a sub-cable having at least two electrically-conductive wires is used. The first wire is used for carrying the electric signal (L1 , L2 or L3) from the input end to the output end. Then at the output end there is provided the ground terminal, which allows for feeding the return current back into the same sub-cable through the second wire via the electrical connection. The first wire and second wire are laid out in a twisted (when there are two wires) or braided fashion (when there are three wires). The consequence of this is that the magnetic fields that may be built up in the cable are strongly reduced. First of all, the amount of magnetic field that is caught is lower, because of the lower area of the enclosed loop defined between signal wire (the first wire) and the current return path (second wire). Second, the magnetic field that is caught is averaged out, because of the twisting (or braiding in some embodiments) of the wires in the sub-cable. In contrast with the cable of the current invention, in WO2019/013642 the enclosed loop defined between the signal wires and the current return path, which is laid out in a separate cable, is much larger. The inventor discovered that EMI may still be built up in the cable of WO2019/013642. In the cable of the invention, the amount of EMI is much lower.

An interesting consequence of the cable of the invention is that the return current wire is in the current invention provided per phase and not as a shared conductor between all phases. The inventor discovered that in case of a shared conductor the individual return currents as triggered by the electric signals (phase signals) would start to “fight” for place in the return current wire, resulting in more EMI, a higher resistance, and higher temperatures, which leads to a lower voltage. The conventional way of solving this problem in the prior art is to increase the diameter of the return current wire to reduce the resistance, which makes the cable heavier and more costly. In the invention this “fighting” between return currents no longer happens, which reduces the EMI, and thereby the temperature and thereby the wires can be designed with less diameter.

Another interesting feature of the invention is that the second wire may in fact be designed with a smaller diameter than the first wire. Despite the fact that this increases the resistance of the second wire, the inventor has found out by experiment that the return current still flows through the second wire. The amount of required conductor material (i.e. , copper) is thereby strongly reduced. In three-phase power supply systems the individual (per phase) return currents are all 120 degrees offset to their “neighbours” and therefore no longer need to “fight” as hard with each other. In addition, as operation frequency increase, the current density decreases from the surface of a conductor to the centre of the conductor. The spatial distribution of the current density in a particular conductor is additionally altered (similar to the skin effect) due to currents in adjacent current-carrying conductors the density will be less due the 120 degrees offset in phase. The parasitic capacitance of the cable is solely dependent on the geometry of the arrangement and the dielectric permittivity of the medium between the conductors will be charged up with the 120 degree offset in phase between the signal and return currents.

The invention provides for an electric cable that is much less susceptible to EMI enabling to feed much larger currents through the cable without causing undesired effects, such as overheating, melting, induced EMI, reduced maximum allowable current, and other problems.

In order to facilitate understanding of the invention one or more expressions, used throughout this specification, are further defined hereinafter. Whenever the wording “sub-cable” is used, this is to be interpreted as a self-contained (multi-conductor) cable as part of a larger cable. In the current invention this sub-cable serves as replacement for a single phase line, wherein multiple conductors are contained in a cable sleeve, all being held together by filling material, such as moulding material, i.e. , the filling material fills the spaces between the conductors. The individual conductors in the sub-cable may have their own (electrically insulating) sleeves also. An alternative word for “sub-cable” may also be “electrical phase cable” or “cable for carrying an electrical phase (signal)”.

Whenever the wording “electric cable” is used, this is to be interpreted as a cable for car- rying/conducting two or three phase signals from the electric power source to the electric load. Alternative words for “electric cable” may be main cable, (main) power cable, electric power cable, and the like. The electric cable may be a self-contained cable comprising a main cable sleeve embodying multiple sub-cables, all being held together with filling material, such as moulding material, i.e., the filling material filling the spaces between the subcables.

Wherever the wording “coil” is used, this is to be interpreted to be a winding (at least one) of a conductor formed such that an inductance is formed.

Wherever the wording “electrically-conductive loop” is used, this is to be interpreted to be a winding (at least one) of a conductor formed such that an inductance is formed.

Whenever the wording “Faraday cage” is used, this is to be interpreted as an enclosure used to block electromagnetic fields. A Faraday shield may be formed by a continuous covering of electrically-conductive material or in the case of a Faraday cage, by a mesh of such materials. Faraday cages are named after the English scientist Michael Faraday, who invented them in 1836.

In an embodiment of the electric cable in accordance with the invention a distance between neighbouring twisting points or braiding points is chosen between 5cm and 80cm. Preferably, the distance is chosen between 10cm and 60cm. By experiments it was found that this distance (expressed in mm) is preferably set between 10 to 50 times the diameter, to around 30 times the cross-sectional area (expressed in mm2) of the wires. A typical distance in a prototype cable is around 15cm for a cable with 6mm2 copper wires. A typical distance in a prototype cable is around 40cm for a cable with 16mm2 copper wires.

In an embodiment of the electric cable in accordance with the invention each sub-cable comprises at least three electrically-conductive wires in a braided fashion, wherein a third wire of said wires is configured for conducting a further electric signal from the input end to the output end of the sub-cable. Using two wires, namely the first wire and the third wire, for carrying electric signals from the input end to the output end provides for some significant advantages. By laying out the wires in a braided fashion a better nulling effect of the magnetic field may be achieved.

In an embodiment of the electric cable in accordance with the invention the first wire and the third wire are provided as a twisted-pair of wires, while the second wire is fed back and forth through openings formed by the twisted pair of wires. Such twisted/braided wires has so far not been reported before and experiments have shown that the nulling effect of the magnetic field is greatly improved. What is exactly meant with the braided fashion of laying out the wires is further explained with reference to the drawings.

An embodiment of the electric cable in accordance with the invention further comprises a ground sub-cable comprising two electrically-conductive wires formed as a twisted-pair for conducting a ground current between the electric power source and the electric load. Adding a ground-sub-cable makes the electric cable even more suitable for being used as power cable between the electric power source and electric load. No separate ground cables are required in this embodiment.

In an embodiment of the electric cable in accordance with the invention the at least two sub-cables comprises two sub-cables that are laid-out as a twisted-pair of sub-cables with a predefined twisting distance between neighbouring twisting points along the length of the electric cable. This embodiment is associated with an electric cable for a one-phase power system, i.e. , a system using only two phase lines (L1 and L2). When only two subcables are used for carrying two phase signals, changing the order or placement of the sub-cables automatically implies twisting of the sub-cables using the predefined twisting distance.

In an embodiment of the electric cable in accordance with the invention the predefined twisting distance is chosen between 20cm and 200cm. Preferably the predefined twisting distance is chosen between 30cm and 120cm, and even more preferably between 40cm and 90cm. It must be noted that the predefined twisting distance of the sub-cables is also dependent on the thickness (and thereby stiffness) of the sub-cables. The thicker the subcables, the larger the twisting distance that is feasible.

In an embodiment of the electric cable in accordance with the invention the at least two sub-cables comprise three sub-cables. This embodiment concerns a cable suitable for feeding (at least) three phase lines from the electric power source to the electric load. This embodiment is associated with an electric cable for a three-phase power system, i.e., a system using only three phase lines (L1 , L2 and L3). When three sub-cables are used for carrying three phase signals, changing the order or placement of the sub-cables enables braiding of the sub-cables using the predefined twisting distance.

In an embodiment of the electric cable in accordance with the invention the three sub-cables are braided with a predefined braiding distance between neighbouring braiding points along the length of the electric cable. Preferably, the predefined braiding distance is chosen between 20cm and 200cm. Preferably, the predefined braiding distance is chosen between 30cm and 120cm, and even more preferably between 40cm and 90cm. It must be noted that the predefined braiding distance of the sub-cables is also dependent on the thickness (and thereby stiffness) of the sub-cables. The thicker the sub-cables, the larger the braiding distance that is feasible.

In an embodiment of the electric cable in accordance with the invention the ground terminals of all sub-cables at the output end are electrically connected together. The advantage of connecting the ground terminals together is that, if one or two of said individual ground terminals would not be properly connected with the electric load, a remaining connection would still enable the return current to flow through the individual current return paths of the sub-cables (second wires). One way of connecting said ground terminals together is to use a copper wire mesh that is a known part of the EMC brush. The respective ground terminals can be conveniently placed in electric contact with such copper wire mesh, which on its turn may be electrically connected with the metallic outside of a round screw connector, such as a hull-connector.

In an embodiment of the electric cable in accordance with the invention each second wire is further provided at the output end with a respective auxiliary ground connector. Further to the previously discussed embodiment, this document provides for auxiliary ground connectors, which is advantageous in case the electric load does not have a metallic housing to which the metallic connector is connected, i.e., some electric loads have non-metallic, non-conductive housings. If that is the case, there is a need to connect the ground terminals on the output using the auxiliary ground connectors. This aspect is further illustrated with reference to the figures.

In an embodiment of the electric cable in accordance with the invention the first wire and third wire of each sub-cable are provided with a shared connector at the input end and the output end. This embodiment constitutes the alternating-current (AC) variant of the invention, that is that each sub-cable is used for carrying one electrical signal (phase signal). By connecting the respective first wire and third wire electrically together at the input end and the output end effectively doubles the cross-sectional area of the input wire and reduces the resistance by a factor 2. An interesting note is that the cross-sectional area of the second wire can be lower than the cross-sectional area of the first and third wire individually without reducing the effect of the current return path significantly. Experiments have shown that the return current will still flow through the second wire (from the output end towards the input end).

In an embodiment of the electric cable in accordance with the invention first wire and third wire of each sub-cable are provided with individual connectors at the input end and the output end. This embodiment constitutes the direct-current (DC) variant of the invention. This embodiment exploits the fact that there are in fact two signal wires running from the input end to the output end. By providing the first wire and third wire with individual connectors the sub-cable has become suitable for carrying a DC-voltage signal between the first wire and the third wire. Also, it is now possible to electrically connect the respective first wires of each sub-cable together (at both ends) as well as the third wires of each subcable (at both ends), which increases the cross-sectional area with a factor three compared to the situation where the respective wires are not connected together. In other words, the electrical cable in accordance with the invention enables a very interesting bonus application, namely the DC-application.

An embodiment of the electric cable in accordance with the invention further comprises a communication cable extending from the input end to the output end. Example of such communication cable are a UTP cable, an ethernet cable, a telephone cable, a subsea umbilical, and the like. This embodiment constitutes a major breakthrough in the technical field of power cables and communication. When using conventional power cables, it is not possible to run a communication cable close to and in parallel with the power cable. The EMI created by the cable would totally ruin the signal on the communication cable. One would typically keep a safe distance between the power cable and the communication cable, for instance at least half a meter. In contrast, in the invention it is possible to add the communication cable to the electric cable as an integrated part, simply adding it as a subcable to the electric cable and moulding it together with the other sub-cables. This leads to an enormous advantage when routing power and communication cables through buildings and structures. BRIEF INTRODUCTION OF THE DRAWINGS

In the following is described examples of embodiments illustrated in the accompanying drawings, wherein:

Fig. 1 shows a known low EMI-transformer as known from the prior art;

Fig. 2 illustrates a known low-EMI electric cable to be used with the low-EMI transformer of Fig. 1 ;

Fig. 3 shows a cross-sectional view of the cable of Fig. 2;

Fig. 4 illustrates a problem with the known low-EMI transformer and low-EMI cable of Figs. 1 and 2 respectively;

Fig. 5 schematically illustrates a first embodiment of an improved low-EMI electric cable in accordance with the invention;

Fig. 6 shows a cross-sectional view of the electric cable of Fig. 5;

Figs. 7a-7b some aspects and variants of the electric cable of Fig. 5;

Fig. 8 shows yet a further aspect of the electric cable of Fig. 5;

Fig. 9a illustrates how respective sub-cables in the electric cable may be braided in accordance with a further embodiment of the invention;

Fig. 9b shows in a 3D-view how such braiding may be done;

Fig. 10 illustrates how the problem of Fig. 4 is reduced by the electric cable of the invention;

Fig. 11 shows a cross-sectional view of a sub-cable in accordance with a further embodiment of the invention;

Fig. 12a illustrates the sub-cable in Fig. 11, wherein respective wires are braided;

Fig. 12b shows in a top view how such braiding may be done;

Fig. 12c shows in a sideview how such braiding may be done; Fig. 13 shows a further embodiment of an electric cable in accordance with the invention, which further comprises a communication cable, and

Fig. 14 schematically illustrates some further aspects of the electric cable of Fig.

13.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Various illustrative embodiments of the present subject matter are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers’ specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming. Nevertheless, it would be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

The present subject matter will now be described with reference to the attached figures. Various systems, structures and devices are schematically depicted in the drawings for purposes of explanation only and so as not to obscure the present disclosure with details that are well known to those skilled in the art. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the present disclosure. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase.

When the demands of transformers are higher, typically an isolation transformer is used. Isolation transformers block transmission of the DC-component in signals from one circuit to the other, while allowing AC-components in signals to pass. Transformers that have a ratio of 1 -to-1 between the primary and secondary windings are often used to protect secondary circuits and individuals from electrical shocks between energized conductors and earth ground. A known way of tackling noise caused by EMI is to build expensive and complex filters to subdue the noise actively.

It was realized in WO 2019/013642 that the problem is in fact worsened by the way isolation transformers are built and used. It was realized that the problem is often caused by the fact that all ground terminals are simply connected together without people realizing that such connection worsens the amount of ground loops induced in the systems. In other words, the grounding in the traditional way of building and using isolation transformers is hardly effective, i.e. , more problems are created than there are solved.

The first improvement in WO 2019/013642 concerns the design of the isolation transformer. As a first step the isolation transformer of the invention is provided with a separate electrical ground node provided inside the Faraday cage at a position where the magnetic flux and electric field are substantially zero. The main idea by this separate ground node is to keep it as clean as possible, but also to keep the impedance to this separate ground node as low as possible. In case it would be placed at a location where there is significant magnetic and/or electric field, the separate electrical ground node would catch unwanted signals again (act as an antenna).

Fig. 1 shows a known low EMI-transformer 100e as known from the prior art, namely WO 2019/013642. The transformer 100e is a three-phase isolation transformer (the three phases are conventionally called L1 , L2 and L3) having three input terminals Ti1 , Ti2, Ti3 that are fed via respective input wires i1 , i2, i3 via a first isolated junction box 180 to respective primary coils 120-1 , 120-2, 120-3 that are connected in a star network in this embodiment. The secondary coils 130-1 , 130-2, 130-3 are connected to respective output terminals To1 , To2, To3 via respective output wires o1 , o2, o3 via a second isolated junction box 181. The secondary coils 130-1 , 130-2, 130-3 are also connected in a star network in this embodiment. It must be noted however that also other types of networks may be used, such as the delta network, either on the input side, on the output side or on both sides of the transformer. This is all dependent on the type of electric grid to which the transformer is coupled and on the type of electric grid that needs to be generated by the transformer.

Furthermore, there is the earlier-mentioned Faraday cage 150 as illustrated, which is connected to the input ground terminal GT1 (and thus to ground PE). The Faraday cage 150 is also connected to the electrostatic shields 140-1 , 140-2 and further to the ground output terminal GT2 to be connected to further circuits. So far, all mentioned parts in Fig. 1 are conventional for isolation transformers.

What renders the isolation transformer 100e of Fig. 1 special is that there is provided a physical electrical node 175 inside a further Faraday cage 170 (defining the earlier discussed no-field (or low-field) zone NFZ) within the Faraday cage 150 that is defined by a Faraday shield 171 as illustrated. The physical electrical node 175 is connected to a clean ground input terminal 179 via a first electric connection 185 (for instance a double isolated cable, which is typically used before the earth-leakage circuit breaker in an electric system of a house-hold). The physical electrical node 175 is further connected to a clean ground output terminal 199 via a second electric connection 195. The second electric connection 195 in this embodiment constitutes a twisted-pair shielded cable comprising two wires 196 that are intertwined as illustrated. Each of said wires 196 is connected to the physical electrical node 175 and fed to the clean ground output terminal 199 as illustrated. In Fig. 1 the second electric connection 195 is drawn as running parallel with and in between said electrostatic shields 140-1 , 140-2, but that is not essential. In fact, the second electric connection 195 may alternatively be fed out of the isolation transformer 100e parallel to said output wires o1 , o2, 03 for instance.

Fig. 1 further illustrates a sensor and controller circuit 190 (CPU) that is configured for measuring noise on said inputs and outputs as illustrated by the arrows and eventually controlling the position of said physical electrical node 175 to minimize the electric field and magnetic fields experienced by this node for reducing/minimizing the noise. In the embodiment of Fig. 1 the position of said physical electrical node 175 is controllable as illustrated by said arrows.

When the transformer 100e of Fig. 1 is properly investigated it can be easily observed that this transformer is not a conventional transformer because on the outside it has two different type of ground terminals, namely the conventional ground terminals GT 1 , GT2 for being connected with the conventional ground potential PE, and the special clean ground terminals 179, 199, which serve to be connected with a separate ground potential, which also is being referred to as ISPE in WO 2019/013642. In other words, this low-EMI transformer does not follow the standard for connection isolation transformers and requires a new standard as was properly explained in the same document. This is as such not a problem but may prevent rapid commercialisation of the transformer. Fig. 2 illustrates a known low-EMI electric cable 300 to be used with the low-EMI transformer 100e of Fig. 1. Fig. 3 shows a cross-sectional view of the cable 300 of Fig. 2. As already discussed with reference to Fig. 1 the transformer aims at reducing induced noise (EMI) by minimizing or cancelling electric and magnetic fields to which cables and wires in the isolation transformer are exposed. Figs. 2 and 3 show a special cable that was earlier developed by the inventor to further improve the performance of the isolation transformer. The multi-core shielded cable 300 effectively comprises two cables (a first core 311 and a second core 321) combined into one cable sleeve 301 as Figs. 2 and 3 illustrate. The cable sleeve 301 may comprise oil-resistance PVC for example. The first core 311 is in fact the earlier-discussed second electric connection 195. The second core 321 comprises the output wires o1 , o2, o3 each carrying a respective output phase/signal L1 , L2, L3 as discussed above view of Fig. 1. Both the first core 311 as well as the second core 321 comprise a shield that may be connected to ground (PE).

In the twisted-core shielded cable 300 all output signals are intertwined within the shielded cable for reducing EMI. The effect of using the twisted-core shielded cable 300 is that EMI that is generated inside the isolation transformer is reduced. The twisted-pair shielded cable for the clean ground and the twisted-core shielded cable for the output signals are, at least over a certain length, combined into the multi-core shielded cable 300 comprising the shields of said shielded cables with their twisted wires inside of them. The advantage of combining said cables is that it becomes much easier to ensure that said wires are running parallel.

Fig. 4 illustrates a problem with the known low-EMI transformer 100e and low-EMI cable 300 of Figs. 1 and 2, respectively. The low-EMI transformer 100e is typically connected to a power grid 900 as illustrated, wherein the power grid 900 provides three phases L1 , L2, L3 as illustrated. The power grid 900 comprises also an earthing 999 (also referred to as ground spear or earth rod), which is preferably dedicated to the actual building where the transformer 100e is provided. Such measure avoids unnecessary noise on the ground line caused by electric activity in the neighbourhood.

The transformer 100e is on its turn coupled with an electric load 400 via the low-EMI cable 300 of Fig. 2 as illustrated. The electric load 400 may be a motor, a pump, or the like. Fig. 4 illustrates how a signal current through the first phase line L1 causes a return current IRC through the clean ground line 311 back to the respective clean ground node 175 of the transformer 100e. However, as can be seen in Fig. 4 the enclosed loop of the signal current and the return current actually covers a significant area. It is exactly this area, which may cause increased EMI, i.e., the supposedly low-EMI cable 300 adds to the EMI problem, which was targeted to be reduced in the first place. The illustrated becomes even worse when all three phase lines L1 , L2, L3 carry their respective signal currents, each signal current causing a return current, wherein all return currents are supposed to return through the same clean ground line 311. As discussed earlier in this specification, this causes increased EMI, as the return currents start to “fight” for a place in the ground line 311.

Fig. 5 schematically illustrates a first embodiment of an improved low-EMI electric cable 300-1 in accordance with the invention. The electric cable 300-1 comprises a cable sleeve 301 comprising four sub-cables 310, 320, 330, 340. Three of the sub-cables 310, 320, 330 are designed for each carrying one of the phase signals L1 , L2, L3, respectively. The special feature of the low-EMI electric cable 300-1 of the invention is that each of the phase signals L1 , L2, L3 have their dedicated sub-cable 310, 320, 330, wherein each subcable 310, 320, 330 comprises three respective electrically-conductive wires. The first sub-cable 310 comprises a first wire L11 , a second wire L12 and a third wire L13. The second sub-cable 320 comprises a first wire L21 , a second wire L22 and a third wire L23. The third sub-cable 330 comprises a first wire L31 , a second wire L32 and a third wire L33. The third wires L13, L23, L33 are optional as will be discussed in view of Figs. 13-14. However, the illustrated embodiment with three wires is advantageous. The respective first wires L11 , L21 , L31 and the respective third wires L13, L23, L33 are both supposed to carry an electrical signal (either the same AC-signal or a different DC-signal), whereas the respective second wires L12, L22, L32 serve as current return paths for these signal wires L11 , L13, L21 , L23, L31 , L33. The respective first and third wires are provided in a twisted-pair fashion, while the respective second wires L12, L22, L32 are fed back and forth through the holes formed by the twisted-pair. The second wire L12 of the first subcable carries a first return current IRC1 as illustrated. The second wire L22 of the second sub-cable carries a second return current IRC2 as illustrated. The second wire L23 of the third sub-cable carries a third return current IRC3 as illustrated.

The first sub-cable 310 comprises a non-conductive inner layer 312 to achieve double-insulation together with the outer layer. Similarly, the second sub-cable 320 comprises an electrically-conductive armouring/sheathing 322 and the third sub-cable 330 comprises electrically-conductive armouring/sheathing 332. The fourth sub-cable 340 serves for carrying ground. This may be the conventional ground PE, but also a clean ground ISPE as proposed in WO 2019/013642. Fig. 6 shows a cross-sectional view of the electric cable of Fig. 5. This figure shows some further aspects of the electric cable 300-1. First of all, it shows how the fourth sub-cable 340 comprises two wires PE1 , PE2, which each are configured for carrying ground PE/ISPE. Furthermore, the drawings shows an (optional) fifth sub-cable 350, which may be a communication cable for example, such as a UTP-cable, an ethernet cable, a telephone cable, a subsea umbilical, and the like. The communication cable 350 is preferably placed in the centre of the cable, but it could be placed in other locations of the cable as well, such as near the outside.

Figs. 7a and 7b some aspects and variants of the electric cable of Fig. 5. Fig 7a shows a first variant of the electric cable 300-1 a. This first variant is to be used in AC-mode. Fig. 7b shows a second variant of the electric cable 300-1 b. This second variant is to be used in DC-mode. Both figures mainly show parts of the first sub-cable. The first variant of the electric cable 300-1 a shows how the first wire L11 and the third wire L13 are electrically connected together at an input end E1 of the cable, and each carry the same electrical signal, here the first phase signal L1 . At the output end E2 the first wire L11 and the third wire L13 are also connected together by terminating them both in the same shared connector CL1 . The first wire L11 and the third wire L13 are clearly laid out in a twisted-pair fashion forming crossings X13 (twisting points) of said wires and between the crossings (twisting points) there are openings 013 as illustrated. Fig. 7a further shows how the second wire L12 runs in between the first wire L11 and the third wire L13 along the length of the cable 300-1 a. Around the openings 013 a maximum heart-heart distance D13 is set such that it actually forms an opening. Typically, the heart-heart distance D13 may be set to about twice the diameter of the respective first wire L11 and third wire L13. This diameter includes the electrically insulating sleeve of said wires. With the heart-heart distance D13 set in this way the second wire L12 can be conveniently fed up and down (back and forth) through the openings 013. As already explained the second wire L12, which acts as current return path may be designed with a small diameter than the first and third wire L11 , L13, which save a lot of material and thereby costs. Fig. 7a also shows the predefined twisting distance LT as referred to in the claims. This twisting distance LT may be chosen between 5 and 80cm, and preferably between 10cm and 60cm.

Fig. 7a further shows how at the output end E2 of the electric cable 300-1 a there are provided an outer nipple 314 and an inner nipple 316. These parts are quite common in electric cables and only serve to keep the cable properly together mechanically. A much more important electrical component is a ground terminal 318 at the output end E2 as illustrated. The ground terminal 318 serves for connecting ground of the electric load (not shown) with the current return path formed by the second wire L12. In order to make this electrical connection there are two options, which may be combined.

A first connection may be established by providing, for example, a ring- or cylinder-shaped metal conductor 318 around the second wire L12 and connecting these electrically through an electrical connection 319 as illustrated. When all sub-cables have such ring- or cylinder shaped metal conductor 318 (for illustration purposes one extra conductor 318 has been schematically drawn) then these conductors 318 may be electrically connected together with each other through a component such as a metallic (copper) brush 317, which is a component known from the EMC-brushes. This metallic (copper) brush 317 may then conveniently be in electrical contact with or enclose said conductors 318 and further connect them with a metallic outer housing of a cable connector (not shown). Such cable connector is often screwed with the housing on a mating part of the electrical connector on the electric load.

A further electrical connection may be established by feeding the second wire L12 further through the conductor 318 establishing the further electrical connection 319-2 as illustrated. This further electrical connection 319-2 together with an auxiliary connector CR1 in which it is terminated serves as an extra safety measure. Especially when a housing of the electric load (not shown) is not electrically conductive, i.e., when it is made in plastic, the earlier discussed cable connector will not automatically establish a connection with ground when screw on the mating part. Then, the further electrical connection 319-2 may provide for an alternative connection with ground, which then should be fed in the electric load to some ground pin/connector (not shown) to be connected with the auxiliary connector CR1 of each sub-cable.

Fig. 7b will be further discussed in as far as it differs from Fig. 7a. At the input end E1 the first wire L11 and the third wire L13 are not electrically connected. Instead, at the input end E1 the first wire L11 may be coupled with the first phase signal L1 and the third wire L13 with the second phase signal L2. In the DC-mode the respective first wires L11 of all sub-cables may be connected together at both the input end E1 and the output end E2. The same applies for the respective third wires L13 and the respective second wires L12. Furthermore, at the output E2 each of the first and third wires L11 , L13 have an individual connector CL1, CL2 as illustrated.

In both Figs. 7a and 7b the second wire L12 may be coupled to a star point or neutral phase signal N of the electric power source (not shown) as illustrated. Alternatively, it may be coupled to ground PE (which is often connected to the neutral phase signal N)

Fig. 8 shows yet a further aspect of the electric cable of Fig. 5. The figure only shows one of the sub-cables and in particular a cross-sectional view through the connector 318. The first wire L11 and the third wire L13 are visible on the outside of the cable, whereas the second wire L12 is located in between the first wire L11 and the third wire L13. In order to be able to connect the second wire L12 electrical connections 319 are made as illustrated. In this example these electrical connections 319 are formed by two electrically conductive wires, but this may virtually be any number of wires.

Fig. 9a illustrates how the respective sub-cables 310, 320, 330 in the electric cable 300-1 may be braided in accordance with a further embodiment of the invention. It must be noted that braiding can only be done using three sub-cables. In case there are only two sub-cables for the phase signals (single phase system) then the only thing what is possible is twisting. When there are three sub-cables braiding is possible. The braiding in Fig. 9a only includes the sub-cables (schematically presented as lines) for the three phase signals L1 , L2, L3. The figures shows the braiding points B123 and their distance LB from each other, which is preferably chosen between 20cm and 200cm, and preferably between 30cm and 120cm, and even more preferably between 40cm and 90cm. It must be noted that the thicker the sub-cables, the stiffer they will be and the longer the braiding distance will need to be.

Fig. 9b shows in a 3D-view how such braiding may be done. Braiding is in fact known from, amongst others, the field of hair styling. But for the sake of completeness Fig. 9b is added such that it is unambiguously clear what the wording “braiding” in the claims is supposed to mean. The ground cable 340 has been left out for facilitating understanding of the drawing. It is the sub-cables 310, 320, 330 for the phase signals L1 , L2, L3 that are to be included in the braiding. There is no need to include the ground sub-cable 340 (for PE, ISPE) in the braiding. Experiments have shown that this has no significant effect. The ground sub-cable may be wound around the others or just kept straight. There was no significant effect detected of this on the EMI-levels. Fig. 9b also illustrates the communication cable 350, which is not braided with the other sub-cables 310, 320, 330 in the current embodiment.

Fig. 9b shows how the sub-cables 310, 320, 330 are braided (change order) every predefined braiding distance LB. The figure also illustrates how the cable sleeve 301 may be provided with an electrically-conductive sleeve 302 made of a conductive metal, for example. This conductive sleeve 302 may serve for both increasing the mechanical strength of the cable 300-1 as well as feeding/carrying the ground potential PE along the cable 300-1. The impact of braiding the sub-cables is mainly seen in the DC behaviour of the cable, because it nulls out potential DC-components of the EMI. There is no impact of the braiding on the performance from an AC perspective.

An important note is that, in embodiments having only two sub-cables for the phase signals L1 , L2 the braiding turn effectively in a twisting of the sub-cables. The predefined braiding distance then effectively becomes a predefined twisting distance. Furthermore, the braiding points effectively become twisting points.

Fig. 10 illustrates how the problem of Fig. 4 is reduced by the electric cable 300-1 of the invention. Similar to Fig. 4 a low-EMI (isolation) transformer 100e is coupled to an electric load 400 via the low-EMI electric cable 300-1 in accordance with the invention, wherein the low-EMI electric cable 300-1 comprises three sub-cables for the phase signals L1 , L2, L3 (phase lines). For the second sub-cable 320 the return current IRC2 through its second wire L22 is illustrated. The load current first flows from the transformer 100e through the respective first and third conductive wire L21 , L23 then through the load 400 and then back to cable 300-1, where it enters the second conductive wire L22 and flows extremely close to the original signal wires L21, L23 back to the transformer. It must be noted that twisting of the electrically-conductive wires and braiding of the sub-cables is not shown in this figure in order to facilitate understanding of the figure.

The situation is the same for the first sub-cable 310 and the third sub-cable 330 with their respective conductive wires L11, L12, L13, L31 , L32, L33. The fourth sub-cable 340 for the ground connections PE, ISPE is not further discussed.

Fig. 11 shows a cross-sectional view of a sub-cable 320 in accordance with a further embodiment of the invention. Fig. 12a illustrates the sub-cable 320 in Fig. 11 , wherein respective wires L21, L22, L23 are braided, that is the first wire L21 and the third wire L23 are laid out in a twisted-pair fashion, while the second wire L22 is fed back and forth the openings. The embodiment in Fig. 12 shows a DC-cable, wherein the first wire L21 and the third wire L23 are not coupled together at the input and output (not shown), but the same applies for an AC-cable, wherein these wires are coupled together at the input and output. Fig. 12b and 12c show in a 3D-view how such braiding may be done. The figure shows how the first wire L21 and the third wire L23 are laid out in a twisted-pair formation, leaving openings 013 through which the second wire L22 is laced back-and-forth (up-and- down as referred to in the claims).

Fig. 13 shows a further embodiment of an electric cable 300-2 in accordance with the invention, which further comprises a communication cable 350. The first main difference between this embodiment and the embodiment of Figs. 5-7b is that the electric cable 300-2 only comprises two sub-cables 310, 320 for the phase signals L1 , L2. In other words, this cable constitutes a single-phase cable (meaning there are only two phase signals L1 , L2, wherein the voltage to be carried is taken as the difference between these phase signals). A second main difference is that the sub-cables 310, 320 each only comprise two electrical ly-conductive wires L11 , L12, L21 , L22. From an EMI point of view the performance of this cable 300-2 is less. However, the same trick is applied in that the current return path for each phase signal L1 , L2 is formed by the respective second wire L12, L22. Having two wires per sub-cable only has the consequence that it is not possible to apply the wires in a braided fashion. Twisted pair is the only feasible option. Beside the two sub-cables 310, 320 for the phase signals there is also provided the same sub-cable 340 for the ground connection. This sub-cable 340 also has two wires PE1 , PE2 as in Figs. 5-7b. Furthermore, the electric cable of Fig. 13 also has the option of adding a communication cable 350 as discussed in view of Figs. 5-7b.

Fig. 14 schematically illustrates some further aspects of the electric cable 300-2 of Fig. 13. For the same of simplicity, the respective connectors of the wires L11 , L12, L21 , L22 have been left out. On the left side, similar to in Figs. 5-7b the first wires L11 , L21 are coupled with the phase signals L1 , L2 respectively, and the second wires L21 , L22 are coupled with the neutral phase signal N (or ground PE, ISPE). At the output end the phase signals L1 , L2 are drawn and the return-current connectors R1 , R2 for connecting the return current back into the cable are schematically shown. In the embodiment of Fig. 18 the respective metal conductors 318 and the electrical connections 319 are also drawn. The figure show how the respective wires are laid out in a twisted-pair fashion defining twisting points X12 between the respective first wires L11 , L21 and the respective second wires L12, L22.

The inventors built several prototypes of the electric cable in accordance with the invention. A first prototype cable concerns a three-phase cable having 6mm2 copper wires, wherein the cables are of the Radox 4GKW-AX type. In this prototype the twisting distance was optimized towards 15cm. The recommended maximum twisting distance was determined to be 21cm. Experiments have shown that the current return wire (the second wire) may not be centralized properly anymore when the twisting distance becomes too large. This recommended maximum twisting distance is dependent on the cable diameter also. The same prototype has a braiding distance of the sub-cables of 51cm, whereas the recommended maximum braiding distance was determined to be 83cm. Experiments have shown that the parasitic capacitive load as seen by the electric source (i.e. transformer) is no longer properly controlled when the braiding distance becomes too large. The recommended maximum braiding distance is dependent on the cable diameter also. A second prototype cable concerns a three-phase cable having 16mm2 copper wires, wherein the cables are of the Radox 4-GKW-AX type. In this prototype the twisting distance was optimized towards 39cm. The recommended maximum twisting distance was determined to be 55cm. The same prototype has a braiding distance of the sub-cables of 79cm, whereas the maximum braiding distance was determined to be 111cm.

Many variations, optimizations and adjustments are possible, during further development of this product.

The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. For example, the method steps set forth above may be performed in a different order. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope of the invention. Accordingly, the protection sought herein is as set forth in the claims below.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware.

Claims

C l a i m s

1. Electric cable (300-1 , 300-1 a, 300-1 b, 300-2) for coupling an electric power source (100e) with an electric load (300), the electric cable (300-1 , 300-1a, 300-1b, 300-2) extending from an input end (E1) to an output end (E2), the electric cable (300-1 , 300-1a, 300-1 b, 300-2) comprising at least two sub-cables (310, 320, 330), each sub-cable (310, 320, 330) comprising at least two electrically-conductive wires (L11 ..L33) laid out in a twisted or braided fashion, wherein a first wire (L11 , L21 , L31) of said wires (L11..L33) is configured for carrying an electric signal (L1 , L2, L3) from the input end (E1) to the output end (E2) of the sub-cable (310, 320, 330), and wherein a second wire (L12, L22, L32) of said wires (L11.L33) is configured for forming a current return path (IRC, IRC1 , IRC2, IRC3) for the electric signal (L1 , L2, L3) of the first wire (L11 , L21 , L31), wherein the second wire (L12, L22, L32) is electrically connected with a ground terminal (318) at the output end (E2) through an electrical connection (319).

2. The electric cable (300-1 , 300-1 a, 300-1 b, 300-2) according to claim 1 , wherein a distance (LT) between neighbouring twisting points (X13) or braiding points (B123) is chosen between 5cm and 80cm.

3. The electric cable (300-1 , 300-1 a, 300-1 b) according to claim 1 or 2, wherein each sub-cable (310, 320, 330) comprises at least three electrically-conductive wires (L11 ..L33) in a braided fashion, wherein a third wire (L13, L23, L33) of said wires

(L11..L33) is configured for conducting a further electric signal (L1 , L2, L3) from the input end (E1) to the output end (E2) of the sub-cable (310, 320, 330).

4. The electric cable (300-1 , 300-1 a, 300-1 b) according to claim 3, wherein the first wire (L11 , L21 , L31) and the third wire (L13, L23, L33) are provided as a twisted-pair of wires, while the second wire (L12, L22, L32) is fed back and forth through openings formed by the twisted pair of wires.

5. The electric cable (300-1 , 300-1 a, 300-1 b) according to any one of the preceding claims, further comprising a ground sub-cable (340) comprising two electrically-conductive wires (PE1 , PE2) formed as a twisted-pair for conducting a ground current between the electric power source (100e) and the electric load (400).

6. The electric cable (300-1 , 300-1 a, 300-1 b, 300-2) according to any one of the preceding claims, wherein the at least two sub-cables comprises two sub-cables (310, 320) that are laid-out as a twisted-pair of sub-cables (310, 320) with a predefined twisting distance (LB) between neighbouring twisting points (B123) along the length of the electric cable (300-1, 300-1 a, 300-1 b, 300-2).

7. The electric cable (300-1, 300-1 a, 300-1 b) according to claim 6, wherein the predefined twisting distance (LB) is chosen between 20cm and 200cm.

8. The electric cable (300-1, 300-1 a, 300-1 b) according to any one of claims 1 to 5, wherein the at least two sub-cables comprise three sub-cables (310, 320, 330).

9. The electric cable (300-1, 300-1 a, 300-1 b) according to claim 8, wherein the three sub-cables (310, 320, 330) are braided with a predefined braiding distance (LB) between neighbouring braiding points (B123) along the length of the electric cable (300-1 , 300-1 a).

10. The electric cable (300-1, 300-1 a, 300-1 b) according to claim 9, wherein the predefined braiding distance (LB) is chosen between 20cm and 200cm.

11. The electric cable (300-1 , 300-1 a, 300-1 b) according to any one of the preceding claims, wherein the ground terminals (318) of all sub-cables (310, 320, 330) at the output end (E2) are electrically connected together.

12. The electric cable (300-1, 300-1 a, 300-1 b) according to any one of the preceding claims, wherein each second wire (L12, L22, L32) is further provided at the output end (E2) with a respective auxiliary ground connector (319-2, CR1).

13. The electric cable (300-1 , 300-1 a, 300-1 b) according to any one of claim 2 and claims 3 to 11 in as far as directly or indirectly dependent on claim 2, wherein the first wire (L11, L21 , L31) and third wire (L13, L23, L33) of each sub-cable (310, 320, 330) are provided with a shared connector (CL1) at the input end (E1) and the output end (E2).

14. The electric cable (300-1, 300-1 a, 300-1 b) according to any one of claim 2 and claims 3 to 11 in as far as directly or indirectly independent on claim 2, wherein first wire (L11 , L21 , L31) and third wire (L13, L23, L33) of each sub-cable (310, 320, 330) are provided with individual connectors (CL1, CL2) at the input end (E1) and the output end (E2).

15. The electric cable (300-1, 300-1 a, 300-1 b) according to any one of the preceding claims, further comprising a communication cable (350) extending from the input end (E1) to the output end (E2).