Device for injecting a current

Abstract

The invention relates to a device (100) for injecting a current into an inner conductor (13) of a cable (10). The cable (10) comprises a shield (15) arranged around the inner conductor (13). The device (100) comprises an AC voltage device (110) designed to apply an AC voltage to the shield (15), and a converter device (120) designed to convert a frequency of an alternating current induced in the inner conductor (13) by the shield and thus to inject the current.

Description

[0001] Device for imprinting a current

[0002] The present invention relates to a device and a method for inducing a current into a cable with an inner conductor and shielding, and in particular to a concept for inducing current into shielded cables.

[0003] BACKGROUND

[0004] For testing medium-voltage cables and especially high-voltage cables, which are of central importance in the context of the energy transition, the energizing of the cable during the application of high voltage is a significant problem.

[0005] Fig. 6 schematically shows conventional devices for inducing a current Ide into a cable io. The cable io is at a DC potential Ude. Six different conventional devices are shown, each inducing the current Ide into the cable io according to a different principle. In an upper row, (1) shows a capacitive current induction with a phase converter 20, power transformers 30, and a rectifier 40; (2) a hydraulic current induction with a motor M, a pump 50, and a generator G; and (3) an inductive DC current induction. In a lower row, (4) shows an induction via an insulated shaft 60 and an electric motor M; (5) shows an induction based on isolation transformers 70; and (6) shows an induction via a fuel system.

[0006] The devices shown here each have a rectifier 40 with connections for the cable 10, through which the current Ide is impressed into the cable 10. Almost exclusively, an inductive imprinting of an alternating current into the cable 10 under test is used, which, due to the inductance, results in a very high reactive power requirement and thus necessitates very large transformers 30. Electrical insulation of those components of the devices that are at the DC voltage potential Ude is also a complex undertaking.

[0007] From the German patent application DE io 2016 105 897 Ai, it is further known that power can be transmitted capacitively to a DC source. While this technique is particularly suitable for long cable lengths, the capacitance of the setup increases significantly, which is a disadvantage for surge voltage tests. Furthermore, large and expensive coupling capacitors and a correspondingly higher charging current are required for AC tests.

[0008] Therefore, there is a need for further and simpler solutions for imprinting a current into a cable at high potential.

[0009] BRIEF DESCRIPTION OF THE INVENTION

[0010] A contribution to solving the aforementioned problems is achieved by a device for inducing a current according to claim 1 and a method for inducing a current according to claim 8. The dependent claims relate to advantageous embodiments of the subject matter of the independent claims.

[0011] The present invention relates to a device for inducing a current in the inner conductor of a cable or a similar current carrier. The cable comprises the inner conductor and a shield arranged around the inner conductor. The cable itself is not part of the device. The device includes an alternating voltage device configured to apply an alternating voltage to the shield, and a conversion device configured to convert the frequency of an alternating current induced in the inner conductor and thus induce the current. The alternating current is induced in the inner conductor by the alternating voltage applied across the shield.

[0012] The term "imprinting a current" can refer in particular to the application or injection of an electric current. The device can be designed as a test fixture or test equipment for the cable. The cable can be the cable to be tested or inspected. Both the inner conductor and the shielding of the cable are electrically conductive or each comprise one or more conductive materials. The cable can be a coaxial cable.

[0013] A DC potential can be present on the cable, more precisely on the inner conductor. This can be the case, in particular, when the cable is to be tested or inspected. Advantageously, the DC potential is then present along the entire length of the inner conductor; the cable, or rather the inner conductor, is thus at the DC potential as a constant electrical potential relative to a reference potential, which can be, in particular, earth and / or a ground potential of the device. The DC potential can be relatively high relative to the reference potential. The cable itself, or a sheath between the inner conductor and the shield, acts as an insulator; fundamentally, only the insulating capacity of the cable itself limits the magnitude of the DC potential. In exemplary embodiments, the DC potential can be, for example, 50, 100, or 150 kV, or in a range of 50 to 500 kV above the reference potential.

[0014] The AC voltage device is designed to provide the alternating voltage. It may have suitable connections, such as terminals or plugs, to apply the alternating voltage to the shield. The AC voltage device may be designed to generate the alternating voltage itself; it may therefore include a generator. Alternatively or additionally, the alternating voltage may be designed to be connected to an electrical power supply device, to be energized by a direct current or another alternating current from the electrical power supply device, and to provide the alternating voltage based on this. In exemplary embodiments, the cable, and in particular the shield, each has two ends, and one end of the shield is connected to a terminal, so that both ends of the shield are then provided with terminals for the alternating voltage.

[0015] In some embodiments, the alternating voltage can be transferred to the shield via electrically conductive connections, i.e., by direct galvanic contact. The cable can, in particular, have an insulating sheath with one or more layers between the inner conductor and the shield. Therefore, no current flows directly between the shield and the inner conductor. The alternating voltage on the shield induces a current in the inner conductor.

[0016] The conversion device is designed to convert the frequency of an alternating current induced in the inner conductor by the shield, based on power provided by the AC voltage device and initially transferred to the shield and from there to the inner conductor, thus imprinting the current. The conversion device is therefore designed to convert the frequency of an alternating current induced in the inner conductor by the shield. In exemplary embodiments, the conversion device is specifically designed to rectify the induced alternating current. The conversion device can be electrically connected directly to the inner conductor of the cable. The conversion device can be spatially separated from the AC voltage device.

[0017] One technical effect of the device is based on the understanding that a cable with an inner conductor and (conductive) shielding is capable of inducing an alternating current in the inner conductor based on an alternating voltage. This is based on electrical induction. The device thus utilizes the fact that the cable itself acts like a (coreless) transformer. The alternating voltage applied to the shielding causes the alternating current in the inner conductor, or an induced alternating voltage that drops along a certain length of the inner conductor. This alternating current can be a component of the total current flowing in the inner conductor. The frequency of the alternating current or the induced alternating voltage corresponds to the frequency of the alternating voltage provided by the AC voltage generator.

[0018] Accordingly, in exemplary embodiments, the device can also consist solely of the AC voltage device, which is designed only to provide and apply an AC voltage to the shield, and the conversion device, which is designed only to convert the frequency of an AC current induced in the inner conductor by the shield and thus impress the current. Compared to many conventional devices, this device can dispense with a transformer for inductively impressing the AC current into the inner conductor. In particular, the device does not need to include a transformer core around which the cable would have to be wound or coiled.

[0019] The conversion device can comprise a transformer, an AC / AC converter, and / or an AC / DC converter to convert the induced alternating current or AC component in the inner conductor. The impressed current can include time-varying components (in particular an AC component) and a DC component. In advantageous embodiments, the impressed current is entirely or substantially a DC current. In exemplary embodiments, the conversion device comprises a rectifier or AC / DC converter configured to smooth and / or regulate the current in the inner conductor.

[0020] Optionally, the AC power supply unit has a connection for an AC supply voltage; the electrical power supply unit described above then serves as the source of the AC supply voltage. The power supply unit can, for example, be or comprise a standard low-voltage network. The AC supply voltage can thus have a frequency of 50 Hz, be single-phase, three-phase, or have a different number of phases. A nominal RMS value of the AC supply voltage can be, for example, 0 V, 220 V, or 230 V. The power supply unit can also be or comprise a standard medium- or high-voltage network. In other embodiments, the AC supply voltage can also already have a high frequency.

[0021] Alternatively or additionally, the AC voltage device includes a transformer to provide the AC voltage. The transformer can be configured to transform the supply AC voltage to the AC voltage applied to the shield, and / or to transform a current associated with the supply AC voltage to a current associated with the AC voltage. This transformer can be significantly smaller than transformers in conventional devices because it does not need to directly induce a current in the cable under test as a conductor loop, but merely serves to provide the AC voltage applied to the shield.Accordingly, although the transformer may be designed for a high frequency, in exemplary embodiments a transformation to voltages of only a few hundred volts is sufficient.

[0022] Alternatively or additionally, the AC voltage device includes an AC / AC converter to provide the AC voltage. The converter can have a DC link, be another type of indirect converter, or be a direct converter.

[0023] Alternatively or additionally, the AC voltage device includes a compensation component designed to compensate for stray inductance in a supply line. The compensation component can generally be designed to provide reactive power compensation, i.e., to reduce displacement reactive power that would otherwise be generated in a section of the AC voltage device when the AC voltage is applied to the shield.

[0024] Optionally, the AC voltage generator is configured to provide AC voltage at least at one frequency in the range of 1 kHz to 10 MHz. Higher frequencies allow for easier decoupling of the AC current in the inner conductor. In exemplary embodiments, frequencies are low enough that the corresponding wavelengths of the AC current are longer than the length of the cable. This can also reduce or minimize radiation.

[0025] Optionally, the conversion device includes a coupling component designed to extract at least a portion of the induced alternating current. In exemplary embodiments, the coupling component is thus designed to extract a high-frequency voltage from the DC circuit or from the total current flowing through the cable. The term "coupling" can be understood here as separating or diverting; the coupling component is therefore designed to separate the induced alternating current from the already impressed current.

[0026] Optionally, the coupling component includes a blocking circuit. A blocking circuit can be understood as an arrangement of electrical and / or electronic components designed to block or at least reduce the propagation of current and / or voltage of a specific frequency or frequency range. The blocking circuit thus exhibits a particularly high electrical resistance for current of the blocked frequency or frequency range. The blocking circuit can, for example, consist of a capacitor and an inductor connected in parallel. Similarly, the blocking circuit can comprise a capacitor and an inductor. Advantageously, the blocking circuit is designed to block a specific frequency of the induced alternating current, or a range of frequencies that the induced alternating current also exhibits.The coupling component can be designed based on the blocking circuit to couple the induced alternating current from a current already impressed in the inner conductor.

[0027] Optionally, the conversion device includes a rectifying component configured to rectify the induced alternating current. The rectifying component can comprise one or more rectifiers. A rectifier can be an arrangement of electrical and / or electronic components configured to convert alternating current into direct current (or, if applicable, vice versa). Each rectifier can be an uncontrolled rectifier, specifically comprising diodes. The rectifying component can be configured to switch rectifiers on and off incrementally, thus gradually increasing the capacitance of the rectifying component. Switching on rectifiers can occur, for example, depending on the magnitude of the induced alternating current and / or the magnitude of the already impressed current.In this way, currents occurring through individual rectifiers can be limited, thus protecting components such as diodes. In some embodiments, at least one of the rectifiers can also be a controllable rectifier, i.e., it can include control electronics.

[0028] Optionally, the rectifying component comprises at least two transformers, each with a first coil and a second coil, where the at least two first coils are connected in series and the at least two second coils are connected in parallel. Advantageously, the series of first coils forms part of a first line through which an alternating current coupled out by the coupling component, or the alternating current component induced in the inner conductor, is passed. The second coils, on the other hand, can be arranged in parallel in a second line, through which the already impressed current is passed. The poles or ends of each of the second coils can each be coupled to an anode of a diode. This circuit offers a very simple and cost-effective way to implement the rectifying component to smooth the induced alternating voltage, or to rectify the alternating current or its component and convert it into the current to be impressed.

[0029] Exemplary embodiments also relate to a method for inducing a current in the inner conductor of a cable or a similar current carrier. The cable or current carrier comprises an inner conductor and a shield arranged around the inner conductor. The method includes applying an alternating voltage to the shield and converting the frequency of an alternating current induced in the inner conductor by the shield in order to induce the current. The method can be carried out, in particular, with a device as described above.

[0030] Important aspects of the presented device and method can be described as follows.

[0031] Induction of current into the inner conductor of shielded cable loops, particularly during testing of high-voltage cables operating at high voltage potential, is a significant problem, especially over long cable lengths. Conventional methods include inductive injection through transformers, mechanical methods (insulated motor shaft, hydraulic pump, generator), and capacitive coupling.

[0032] In the device and method presented here, the cable under test itself is used as a coreless coaxial transformer for inductive current transfer, acting as an isolating transformer. An alternating current applied to the shield is inductively transferred to the inner conductor. The inner conductor then carries an induced alternating current or component in addition to the already impressed current. This induced alternating current is converted to transfer further power into the desired current, thus impressing the current. In exemplary embodiments, this conversion includes extracting and rectifying the induced alternating current.

[0033] In exemplary embodiments of the presented device or method, only the cable under test is subjected to the high-voltage level or DC voltage. No additional insulating components are required. The method can be applied in the MHz range even for very short cable lengths and also works for lengths in the kilometer range, whereby the frequency of the AC voltage or AC current component is selected to be high enough to efficiently transmit power and thus induce current in the inner conductor. On the other hand, the frequency is advantageously kept low enough that the wavelength is greater than the cable length. In this way, problems with standing waves in the cable can be avoided.

[0034] Some advantages of the presented device and method can be described as follows.

[0035] Besides significant cost savings due to the minimal effort required, theoretically any length of cable can be tested, since the cable itself acts as a coaxial transformer, thus eliminating large stray inductances. This significantly simplifies the testing of the high-voltage cables that are essential for the energy transition, thereby considerably reducing costs as well as material and energy consumption.

[0036] BRIEF DESCRIPTION OF THE FIGURES

[0037] The embodiments of the present invention are better understood with reference to the following detailed description and the accompanying drawings of the different embodiments, which, however, should not be understood as limiting the disclosure to the specific embodiments, but merely serve for explanation and understanding.

[0038] Fig. 1 shows a device for imprinting a current into an inner conductor of a cable according to the present invention.

[0039] Fig. 2 shows further details of an embodiment of the device.

[0040] Fig. 3 illustrates aspects of an exemplary embodiment of the device.

[0041] Fig. 4 illustrates further aspects of the exemplary embodiment of the device.

[0042] Fig. 5 shows steps of a method for impressing a current into an inner conductor of a cable according to the present invention.

[0043] Fig. 6 shows conventional devices for impressing a current into an inner conductor of a cable.

[0044] DETAILED DESCRIPTION

[0045] Fig. 1 shows a device 100 for inducing a current in an inner conductor 13 of a cable 10, or a similar current carrier. The cable 10 comprises, in addition to the inner conductor 13, a shield 15 arranged around the inner conductor 13. A DC voltage potential is applied to the cable 10, in exemplary embodiments particularly to the inner conductor 13. In exemplary embodiments, the DC voltage potential can be several kV above a ground or another reference potential of the device 100. The device 100 comprises an AC voltage generator 110 configured to apply an AC voltage to the shield 15. The device 100 further comprises a converter 120 configured to convert the frequency of an AC current induced in the inner conductor 13 and thus induce the current.

[0046] The device 100 utilizes the structure of the cable 10 with inner conductor 13 and shield 15 to induce the current by using the cable 10 itself as a coreless transformer for the alternating voltage. The power for the current is transferred by the alternating voltage to the shield 15 and from there by induction to the inner conductor 13. The conversion device 120 is designed to transfer the power transferred to the inner conductor 13 to the current to be induced, or to generate the current to be induced based on the power transferred to the inner conductor 13.

[0047] The device 100 allows for the simple application of current to the shielded cable 10, which can be of any length. The DC voltage potential can be, in particular, a high or medium voltage. The applied current can be a direct current. The device 100 is therefore particularly suitable for testing medium-voltage cables and especially high-voltage cables where the current is to be applied to the inner conductor 13 while the DC voltage is applied to it. Such cables 10 to be tested are frequently designed as shielded cables 10 with an inner conductor 13 and shielding 15. The cable 10 can have a length ranging from a few decimeters to one or even several kilometers.

[0048] The frequency of the alternating current can be in the range of kHz to MHz and may be, for example, 1 kHz, 100 kHz, 1 MHz, or 10 MHz. During the application of the current, the shield 15 and the inner conductor 13 are subjected to a high-frequency current component, which, however, in exemplary embodiments is less than 1% of the applied current. In exemplary embodiments, a high-frequency voltage load occurs only along the cable 10 and is less than 10 V / m. It can be advantageous to keep the frequency of the alternating voltage applied to the shield 15 by the AC voltage device 110 so low that the wavelength is significantly greater than the length of the cable 10. However, a high frequency of this alternating voltage can be advantageous in order to more easily separate an alternating current component from the already applied current in the inner conductor 13 (by means of the conversion device 120).

[0049] Overall, the device 100 is designed to transmit power via cable 10 or the device under test itself. This minimizes the number of components required, which can result in significant cost savings and enable very simple on-site testing.

[0050] Fig. 2 shows an embodiment of the device 100 with further details. The shield 15 can be connected to ground via an earthing point 80 or a point connection. The AC voltage device 110 has a connection 114 to a high-frequency AC supply voltage. The AC voltage device 110 also includes a high-frequency transformer 116 to provide the AC voltage for the shield 15. The RMS value of the AC supply voltage can be, for example, 200 V, and the transformer 116 can be configured to provide the AC voltage with a lower RMS value. The AC voltage device 110 includes a compensation component 118, in this case a capacitor, which is arranged in a line leading to connections 112 on the shield 15 to compensate for stray inductance.Lower values ​​of the alternating voltage have proven advantageous in exemplary embodiments.

[0051] On the secondary side of the device 100, the converter 120 comprises an output coupling component designed to extract at least a portion of the induced alternating current. The output coupling component includes a blocking circuit 123 formed by a parallel connection of an inductor 1231 and a capacitor 1232. The inductor 1231 and the capacitor 1232 are configured such that the blocking circuit 123 exhibits a high electrical resistance for current components with a frequency equal to or within a frequency range encompassing a frequency equal to the alternating voltage. For other frequencies, however, the electrical resistance formed by the blocking circuit 123 is low. In particular, the applied current can advantageously pass through the blocking circuit 123 almost unimpeded.A first conductor 121, running parallel to the blocking circuit 128, is connected to the inner conductor 13 via a capacitor 129 (also part of the output coupling component). The capacitor 129 is designed to decouple current components in the first conductor 121 from the already impressed current. The alternating current or alternating current component induced in the inner conductor 13 therefore flows predominantly in the first conductor 121, while the already impressed direct current flows predominantly in a second conductor 122, which is separate from the first conductor 121.

[0052] On the secondary side of the device 100, the high-frequency voltage is thus coupled out of the DC circuit by the parallel resonant circuit 123. The inductor 1231 is advantageously designed to carry the impressed current, which is essentially a DC current. It has been found that the AC voltage advantageously has a high frequency, since the inductor 1231 can then be designed as a relatively small conductor loop with a large cross-section.

[0053] The converter 120 comprises a rectifying component 125, which here includes several center-tap rectifiers 126, 126' or center-tap rectifier switching arrangements. Each rectifier 126, 126' comprises a high-frequency transformer 128 with two coils 1281, 1282, which can have a winding ratio of 1:1. Each transformer 128 initially comprises a first (primary) coil 1281. The first coils 1281 of all transformers 128 are connected in series and are connected on one side to the first line 121, in which the induced alternating current predominantly flows. On another side, the series of first coils 1281 is connected to the inner conductor 13 via the capacitor 129. Furthermore, in each transformer, a second (secondary) coil 1282 is electrically connected to the inner conductor 13 via its center point. The ends of the second coil 1282 are each connected to an anode of a diode 127.The diodes 127 are designed and arranged to rectify the current through the second coil 1282. The second coils 1282, or rather the diodes 127 of all rectifiers 126, 126', are connected in parallel to the second line 122. The rectification component 125 thus transfers power from the induced alternating current to the impressed current. The impressed direct current is therefore generated via one or more transformers 128 in rectifiers 126, 126', which are decoupled from the DC voltage resulting from the voltage drop by means of the capacitor 129.

[0054] Fig. 3 illustrates an embodiment of the device 100 in a specific application. A medium-voltage cable of type Nexans NA2XS(F)2Y 1X70 / 16 with a length of 15 m is used as cable 10.

[0055] In this embodiment, the AC device comprises, in particular, a converter 111 configured to convert a 50 Hz AC current into a high-frequency AC current with a frequency in the range between 10 kHz and 10 MHz. The coupling component comprises a parallel resonant circuit 123 with an inductor 1231 or a conductor loop and a capacitor 1232, as well as a further capacitor 129 for decoupling the AC current induced in the inner conductor 13 from the impressed DC current. The device 100 comprises a rectifier component 125 with 50 rectifiers 126, 126' connected in parallel, each with a transformer 128. Each transformer 128 has a primary coil 1281 with one primary turn and a secondary coil 1282 with a 2x1 secondary turn.Current through the secondary coil 1282 is rectified by a dual Schottky diode (instead of the two parallel diodes 127). This design is particularly simple because the primary conductor, or first line 121, can be implemented as a single conductor that runs through all transformers 128.

[0056] Based on the rectifying component, a current transformation ratio of 1:50 is achieved. This results in a current ripple (the ratio of the AC component to the impressed DC component in the inner conductor 13) of only 2%. The rectifying component is designed to supply the impressed current with a current intensity of at least 1 kA. The cable 10 has a rated current of 230 A. Its operating frequency is 200 kHz.

[0057] Fig. 4 shows, for the embodiment according to Fig. 3, the rectifying component 125 with the 50 individual rectifiers 126, 126' and the capacitor 129 for decoupling the AC component from the DC component in the inner conductor 13 of the cable 10.

[0058] Fig. 5 shows steps of a method for inducing a current in an inner conductor 13 of a cable 10. The cable 10 comprises, in addition to the inner conductor 13, a shield 15 arranged around the inner conductor 13. A DC voltage potential is applied to the cable 10. The method includes applying an AC voltage S110 to the shield 15. Furthermore, the method includes converting the frequency S120 of an AC current induced on the inner conductor 13 in order to induce the current. In particular, this eliminates the need for a separate transformer to induce the current in the inner conductor 13.

[0059] The method is advantageously carried out using a device as described above. In particular, the method can include decoupling the alternating current or the alternating voltage induced in the inner conductor. Furthermore, the method advantageously includes rectifying the decoupled alternating current or the decoupled induced alternating voltage.

[0060] The features of the invention disclosed in the description, claims and figures may be essential for the realization of the invention, either individually or in any combination.

[0061] Reference symbol list io cable

[0062] 13 inner conductors

[0063] 15 Shielding

[0064] 20 phase converters

[0065] 30 Power transformer

[0066] 40 rectifiers

[0067] 50 Pumping equipment

[0068] 60 wave

[0069] 70 Isolation transformer

[0070] 80 Grounding

[0071] 100 Device

[0072] 110 AC voltage device

[0073] 111 converters

[0074] 112 connection

[0075] 114 Connection to AC power supply

[0076] 116 High-frequency transformer

[0077] 118 Compensation component

[0078] 120 changing facilities

[0079] 121 first (primary) line

[0080] 122 second (secondary) line

[0081] 123 restricted area

[0082] 1231 Inductance

[0083] 1232 capacity

[0084] 125 Rectifier component

[0085] 126 rectifiers

[0086] 127 Diode

[0087] 128 Transformer

[0088] 1281 first (primary) coil

[0089] 1282 second (secondary coil)

[0090] 129 Capacitor

[0091] S110, S120 Steps of a procedure

Claims

REQUIREMENTS 1. Device (100) for inducing a current in an inner conductor (13) of a cable (10), wherein the cable (10) comprises a shield (15) arranged around the inner conductor (13), comprising: an alternating voltage device (110) configured to apply an alternating voltage to the shield (15); and a conversion device (120) configured to convert a frequency of an alternating current induced in the inner conductor (13) and thus induce the current.

2. The device (100) according to one of the preceding claims, wherein the AC voltage device (110) comprises at least one of the following: - a connection (114) to an AC power supply, - a transformer (116) to provide the alternating voltage based on that, - an inverter (111) to provide the alternating voltage based on that, - a compensation component (118) designed to compensate for a stray inductance.

3. The device (100) according to one of the preceding claims, wherein the AC voltage device (110) is configured to provide the AC voltage with at least one frequency in a range from 1 kHz to 10 MHz.

4. The device (100) according to one of the preceding claims, wherein the conversion device (120) comprises a coupling component configured to coupling out at least a portion of the induced alternating current. 5- The device (100) according to claim 4, wherein the decoupling component comprises a blocking circuit (123).

6. The device (100) according to any one of the preceding claims, wherein the conversion device (120) comprises a rectifying component (125).

7. The device (100) according to claim 6, wherein the rectifying component (125) comprises at least two transformers (128), each with a first coil (1281) and a second coil (1282), wherein the at least two first coils (1281) are connected in series and the at least two second coils (1282) are connected in parallel.

8. A method for inducing a current in an inner conductor (13) of a Cable (10), wherein the cable (10) comprises an inner conductor (13) and a shield (15) arranged around the inner conductor (13), with: Applying an alternating voltage to the shield (15); and Converting the frequency of an alternating current induced on the inner conductor (13) in order to impress the current.

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