Mobile testing device and method for testing a test object, in particular a high-voltage or medium-voltage cable

Abstract

The invention relates to a mobile testing device (1) for testing a test object (3), in particular a high-voltage or medium-voltage cable, by means of a very low-frequency (VLF) high-voltage source (5) providing a VLF test voltage, which mobile testing device comprises a high-voltage connection (7A), a grounding connection (7C), a noise-suppressing filter (21), a high-voltage (HV) coupling capacitor (23), a voltage pulse generator (25) for generating a voltage pulse with respect to the device ground potential, and a data acquisition unit (9A). For a partial discharge test of the test object (3), a low-voltage side (23B) of the HV coupling capacitor (23) can be electrically connected to a signal input (19A) of the data acquisition unit (9A) via an impedance circuit (27) in order to detect a partial discharge generated in the test object (3) using the VLF test voltage. In order to locate a position of a fault in the test object (3), a pulse output (25A) of the voltage pulse generator (25) can be electrically connected to the low-voltage side (23B) of the HV coupling capacitor (23) via the impedance circuit (27) in order to couple a voltage pulse into the test object (3), and the low-voltage side (23B) of the HV coupling capacitor (23) can be electrically connected to the signal input (19A) of the data acquisition unit (9A) via the impedance circuit (27) in order to detect a voltage pulse reflected at the fault position.

Description

[0001] Mobile test device and method for testing a test object, in particular a high-voltage or medium-voltage cable

[0002] [1] The present invention relates to a (particularly multimodal) mobile test device for testing a test object, in particular a high- or medium-voltage cable, in particular for testing insulation in coaxial cables for power distribution in electrical supply networks using very low frequency (VLF) test methods (such as partial discharge and fault location test methods). Furthermore, the invention relates to a method for testing a test object, in particular a high- or medium-voltage cable, using a mobile test device, for example using partial discharge and fault location test methods.

[0003] [2] The testing of high-voltage or medium-voltage cables (herein also generally referred to as the test object, for example cables laid underground or running through water as part of local / regional energy networks) includes the testing for the purpose of determining possibly existing defects or pre-damage to a high-voltage or medium-voltage cable (hereinafter generally referred to as the fault location). Fault locations can, for example, form in the insulation of a high-voltage cable. Common defects include insulation short circuits or insulation defects in the cable insulation, which ignite during operation or during testing at high voltages and represent a short circuit for a short time in which the energy stored in the cable is discharged in a current pulse.A fault location (e.g., a void in the insulation) can also be a defect in the cable insulation that does not (yet) cause a voltage breakdown in the voltage range being tested. Such fault locations can be, for example, so-called water trees or electrical trees.

[0004] [3] With regard to the presence of pre-damage, mobile VLF test devices enable testing of a test object using a test voltage, for example, in the range of 14 kV peak to 120 kV peak (generally not limited), which is generated with a high-precision voltage curve at a frequency in the range of 0.01 Hz to 1 Hz, the so-called "Very Low Frequency" (VLF), and is applied to the test object as a power cable on the conductor opposite protective earth. Circuit arrangements for generating such test voltages are disclosed, for example, in DE 10 2012 024 560 B3 by the applicant or DE 195 13 441 A1.

[0005] [4] VLF-based test methods are known and defined, for example, in IEEE 400.2 (IEEE Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF) (less than 1 Hz)). To diagnose the presence of pre-damage (e.g., electrical tree fault), a so-called partial discharge measurement can be carried out. If a VLF test voltage is applied, the pre-damage can lead to a signal that can be recorded using a coupling capacitor (measuring capacitance / impedance) and fast data acquisition (oscilloscope). The measurement of partial discharges (partial discharge diagnosis) is the subject of IEC60270 (DIN EN 60270 VDE 0434:2016-11 High-voltage test techniques, Partial discharge measurements). Furthermore, to diagnose the presence of pre-damage, a loss factor can be determined using a so-called tangent-delta measurement.In this case, the current through a fault location is measured - provided the test object is connected to protective earth - i.e. without the test object having to be disconnected from the protective earth.

[0006] [5] In addition to determining the presence of a fault, it is necessary to locate the position of the fault in order to eliminate the fault. Methods for fault location are known and are defined, for example, in IEEE 1234 ("IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems"). State-of-the-art methods can, for example, be based on the detection of a fault location, i.e., on the determination that a fault exists in a power cable. The fault location can, for example, have been detected due to a voltage breakdown between the conductor and the shield during operation or during a scheduled test of, for example, a high-voltage cable.

[0007] [6] To locate the fault, a test voltage up to a maximum value in the range of, for example, 8 kV to 40 kV can be applied to the high-voltage cable using a high-voltage generator. The voltage is increased until a breakdown occurs again on the high-voltage cable. The "breakdown point", i.e. the position of the fault along the cable, can be found, for example, using a voltage pulse generator. During breakdown, a voltage pulse can be injected into the cable using the voltage pulse generator. An echometer (time-domain reflectometer TDR) measures the propagation time of the artificially injected pulse to the fault location and back, generally the propagation time of a voltage pulse reflected at the fault location (also referred to as an "echo"). See, for example, IEEE 1234-2019, Chapter 7.2.4 or Cigre Technical Brochure 773.For such fault location under applied high voltage, the echometer can be connected to the test object via a coupling capacitor as an isolating impedance.

[0008] [7] Furthermore, setups for partial discharge measurement are known, for example, from the aforementioned IEC 60270-2013 (e.g., Chapter 11.4) and DE 3 737 373 C2. [8] Other fault location methods can be based on the fact that a current pulse triggered by the breakdown causes an oscillation (transient wave) in the test object, whereby the distance to the fault location is determined by the frequency of the oscillation. See, for example, IEEE 1234-2019, Chapter 7.2.9 "Impulse current method (also surge pulse reflection)" and Chapter 7.2.11 "Decay method" as well as CIGRE Bl 773 September 2019, Chapter 2.3.5 "Decay method and differential decay method" or Chapter 2.3.6.1 "Impulse Current Method". A new method for fault location related to the tangent-delta measurement is further disclosed in the German patent application DE 102023106958.6 (filed by the applicant on March 20, 2023).

[0009] [9] Depending on the type of fault, a differentiated approach is required to determine the type of fault and its position within the test object. Especially for cable lengths of several kilometers, it is advantageous to locate the fault location along the test object as quickly as possible.

[0010]

[0010] Within the scope of testing technology, measurement methods and test devices are developed and deployed that cover as many different aspects of fault locations and test objects as possible. For example, it is known to combine VLF test voltage generation and a tangent-delta measurement diagnosis in one test device. However, to avoid interference, test devices for partial discharge measurement are usually connected downstream of a VLF generator as a standalone (e.g., mobile) add-on device.

[0011]

[0011] The inventors have recognized the disadvantages that arise from a change in the measurement setup or an addition to a measurement setup when testing, for example, a laid high-voltage or medium-voltage cable, such as additional time expenditure, additional measuring devices and associated costs.

[0012]

[0012] One aspect of this disclosure is based on the object of providing a device and a method for versatile, flexible cable testing. Another aspect of this disclosure is based on the object of providing a compact and cost-effective design of a testing device for testing a high- or medium-voltage cable, which enables safe, simple, and cost-effective testing and locating methods.

[0013]

[0013] At least one of these objects is achieved by a mobile testing device for testing a test object according to claim 1 and by a method for testing a test object according to claim 14. Further developments are specified in the subclaims.

[0014]

[0014] In one aspect, a mobile test device for testing a test object, in particular a high-voltage or medium-voltage cable, by means of a VLF high-voltage source providing a very low frequency (VLF) test voltage, comprises a high-voltage terminal for connecting the VLF high-voltage source, a test object terminal for connecting the test object, and an earth terminal for providing a device earth potential in the mobile test device by connecting the earth terminal to earth potential.The mobile test device further comprises a noise-suppressing filter whose high-voltage input is electrically connected to the high-voltage terminal and whose high-voltage output is electrically connected to the test object terminal, a high-voltage (HV) coupling capacitor, with a high-voltage side of the HV coupling capacitor electrically connected to the high-voltage output of the noise-suppressing filter, a voltage pulse generator for generating a voltage pulse with respect to the device ground potential, and a data acquisition unit with a signal input. For a partial discharge test of the test object, a low-voltage side of the HV coupling capacitor is electrically connectable via an impedance circuit to the signal input of the data acquisition unit for detecting a partial discharge generated in the test object with the VLF test voltage.Furthermore, in order to locate a position of a fault in the test object, a pulse output of the voltage pulse generator can be electrically connected via the impedance circuit to the low-voltage side of the HV coupling capacitor for coupling a voltage pulse into the test object, and the low-voltage side of the HV coupling capacitor can be electrically connected via the impedance circuit to the signal input of the data acquisition unit for detecting a voltage pulse reflected at the fault location.

[0015]

[0015] In a further aspect, a method for testing a test object, in particular a high-voltage or medium-voltage cable, in particular using a mobile testing device, comprises the following steps:

[0016] - Connecting a VLF high voltage source to a high voltage terminal of the test device,

[0017] - Connecting a conductor of the test object to a test object terminal of the test instrument, whereby a shield of the test object and an earth terminal of the test instrument are connected to protective earth,

[0018] - Generating a very low frequency (VLF) test voltage with the VLF high voltage source and coupling the VLF test voltage into the test object via the test device,

[0019] - by means of an impedance circuit, deactivating an electrical connection between a voltage pulse generator of the test device and a signal input of a data acquisition unit of the test device and activating an electrical connection between the voltage pulse generator and an HV coupling capacitor,

[0020] - Generating a voltage pulse with the voltage pulse generator of the test device and coupling the voltage pulse into the test object via the impedance circuit and the HV coupling capacitor,

[0021] - by means of the impedance circuit, activating an electrical connection between the HV coupling capacitor, the impedance circuit and the signal input of the data acquisition unit of the test device, and effecting an electrical decoupling or a high-impedance connection between the impedance circuit and the voltage pulse generator,

[0022] - using the data acquisition unit to acquire a healthy signal resulting from a voltage pulse reflected at a cable end of the test object and / or a location signal resulting from a voltage pulse reflected at a fault location, and

[0023] - Processing the location signal using the healthy signal in an evaluation electronics provided in the data acquisition unit or connected externally to obtain a position of the fault location in the test object.

[0024]

[0016] In some embodiments of the test device, the impedance circuit may comprise at least one semiconductor-based switchable impedance, which is particularly designed as a fast-switching impedance, for example, switchable in the time range of less than 100 ns, less than 50 ns, or less than 10 ns. The at least one switchable impedance may be arranged between the HV coupling capacitor and the signal input and / or between the HV coupling capacitor and the voltage pulse generator. For example, the impedance circuit may comprise an electronic switch, in particular a semiconductor switch, which is designed to switch between low- and high-impedance states (generally between a first and a second state) within a few nanoseconds, in particular in the time range of less than 100 ns, less than 50 ns, or less than 10 ns.

[0025]

[0017] Furthermore, for the partial discharge test, the low-impedance (first) state can be configured with a resistance in the range of 0.1 Ω to 100 Ω between the HV coupling capacitor and the signal input, and the high-impedance (second) state can be configured with a resistance in the range of 1 MΩ to 100 MΩ between the voltage pulse generator and the HV coupling capacitor. Additionally or alternatively, for the location test, the high-impedance (second) state can be configured with a resistance in the range of 1 MΩ to 100 MΩ between the HV coupling capacitor and the signal input, and the low-impedance (first) state can be configured with a resistance in the range of 0.1 Ω to 100 Ω between the voltage pulse generator and the HV coupling capacitor.

[0026]

[0018] In some embodiments of the test device, the data acquisition unit may have a further signal input and the test device may further have a voltage divider that electrically connects the high-voltage output of the noise-suppressing filter to the further signal input in order to provide the data acquisition unit in particular with a phase information signal and / or the test voltage, for example for the partial discharge test or for a tangent-delta measurement.

[0027]

[0019] In some embodiments, the test device may further comprise a control unit. The control unit comprises, in particular, a computing unit, a memory, and optionally a display, and is connected to the voltage pulse generator and the data acquisition unit for transmitting and receiving control signals. Furthermore, the control unit is configured to control the impedance circuit such that, upon the transmission of a voltage pulse to the low-voltage side of the HV coupling capacitor, the impedance circuit assumes a circuit state in which the signal input of the data acquisition unit is highly impedantly decoupled, in particular electrically isolated, from the low-voltage side of the HV coupling capacitor.

[0028]

[0020] In some embodiments of the test device, an adjustable current measuring device, e.g., a measuring impedance, can be provided between the noise-suppressing filter and the test object connection. This adjustable current measuring device is connected to the control unit and configured to detect a test current supplied to the test object during a VLF test and to supply it to the control unit as a current measurement signal. In particular, the control unit is configured to output a trigger signal to the voltage pulse generator, depending on the current measurement signal, for the voltage pulse generator to emit a voltage pulse for performing fault location. The trigger signal is output, in particular, when the current measurement signal indicates a breakdown of the VLF voltage through an insulation of the test object.

[0029]

[0021] In some embodiments of the test device, the control unit can be configured to control the impedance circuit—particularly after detecting a breakdown or a breakdown during a VLF partial discharge measurement or a breakdown current during a VLF tangent-delta measurement or as part of a "healthy image" detection—to deactivate and activate the coupling of the low-voltage side of the HV coupling capacitor to the signal input. Additionally or alternatively, the control unit can be configured to detect a breakdown during a VLF measurement.

[0030]

[0022] In some embodiments of the test device, the noise suppressing filter may be designed as a high-frequency filter, the HV coupling capacitor may have a capacitance in the range of 0.5 nF to 25 nF and / or the voltage pulse generator may be designed to generate and deliver voltage pulses in the range of 1 V to 1 kV voltage peak value.

[0031]

[0023] In some embodiments of the test device, the ground terminal for providing a device ground potential may be electrically connected to the noise suppressing filter, to the voltage pulse generator and / or to the data acquisition unit via a filter capacitor.

[0032]

[0024] In some embodiments, the test device can further be configured to use the provided VLF test voltage to generate both a diagnostic signal for an insulation test of the test object via a partial discharge measurement and / or via a VLF phase rotation measurement and a location signal for locating the position of the fault location of the test object using the voltage pulse, and / or to output the VLF test voltage provided by the VLF high-voltage source connected to the high-voltage connection for a partial discharge measurement and / or tangent-delta measurement at the test object connection and to generate, for internal and / or external evaluation, a diagnostic signal for an insulation test of the test object via a partial discharge measurement and / or via a VLF phase rotation measurement or a location signal for fault location pre-location via a propagation time measurement of an echo signal of a voltage pulse.

[0033]

[0025] In some embodiments of the test device, an evaluation electronics integrated in the data acquisition unit and / or connectable to the data acquisition unit can be configured to determine from the diagnostic signal a partial discharge and / or partial discharges (if present) associated with the test object and / or a loss factor of the test object and to determine from the locating signal a distance along the test object to a fault location causing a breakdown.

[0034]

[0026] In some embodiments of the test device, the evaluation electronics may comprise at least one analog and / or digital signal processing unit and / or a processor and / or a buffer memory.

[0035]

[0027] In some developments of the method, a breakdown can be triggered with the VLF test voltage to transfer the fault location into a detectable state, and the steps of deactivating the electrical connection, generating the voltage pulse and activating the electrical connection to detect the locating signal can be carried out during the breakdown, so that the locating signal is based on a reflection of the voltage pulse at a position of the breakdown in the test object and the obtained position of the fault location corresponds to the position of the breakdown in the test object.

[0036]

[0028] In some developments of the method, a trigger signal can be triggered upon detection of a breakdown and the steps of deactivating the electrical connection, generating the voltage pulse and activating the electrical connection can be triggered by the trigger signal.

[0037]

[0029] In some developments of the method, a trigger signal can be triggered after detecting a current pulse in the test object and the steps of deactivating the electrical connection, generating the voltage pulse and activating the electrical connection after detection can be triggered by the trigger signal, wherein in particular the current pulse can be evaluated.

[0038]

[0030] In some developments of the method, the steps of deactivating the electrical connection, generating the voltage pulse, and activating the electrical connection can be triggered continuously and repeatedly in order to detect healthy signals and location signals while testing the device under test with a VLF test voltage. Alternatively or additionally, the steps of deactivating the electrical connection, generating the voltage pulse, and activating the electrical connection can be triggered specifically within the framework of a voltage pulse causing a voltage breakdown.

[0039]

[0031] In some developments of the method, the steps of deactivating the electrical connection, generating the voltage pulse and activating the electrical connection for detecting the healthy signal can be carried out at a value of the VLF test voltage at which no breakdown has yet been triggered in the test object, so that the healthy signal is based on a reflection of the voltage pulse at one end of the test object and in particular indicates the current state of the test object by means of a pulse signal.Furthermore, alternatively or in addition to detecting a basic healthy signal, the steps of deactivating the electrical connection, generating the voltage pulse, and activating the electrical connection for detecting the healthy signal can be carried out before applying a VLF test voltage, so that the basic healthy signal is based on a reflection of the voltage pulse at one end of the voltage-free test object and, in particular, indicates the current state of the test object using a pulse signal.

[0032] In some developments of the method, the deactivation and activation of the electrical connection by means of the impedance circuit can be carried out by switching an electronic switch, in particular a semiconductor switch, which is designed, in particular, to switch between low- and high-impedance states within a few nanoseconds, in particular in the time range of less than 100 ns or less than 50 ns or less than 10 ns.

[0040]

[0033] The aspects of the multimodal test device disclosed herein relate in particular to a joint implementation of the operating modes of a partial discharge test and a fault location in a mobile test device connected downstream of a VLF generator, wherein the fault location is to be enabled in particular in the event of a voltage breakdown within the framework of a VLF test procedure.

[0041]

[0034] The concepts described herein may, among others, have the following advantages over the prior art or avoid corresponding disadvantages of the prior art:

[0042]

[0035] In particular, the advantages of providing multiple functionalities, such as partial discharge measurement and fault location, as well as optional tangent-delta measurement, in a mobile test device were recognized. Early diagnosis with and without forced breakdown can delay the need to replace a defective cable and, in particular, save time if testing and diagnosis can be performed simultaneously with one test device.

[0043]

[0036] Since partial discharge measurement, fault location, and tangent-delta measurement are performed using different, independent workflows, two or even three separate devices are typically required for the various test procedures in the prior art. With the cleverly arranged connection and wiring in the test device proposed herein, it is possible according to the invention to provide a voltage pulse generator within a partial discharge measurement setup at a low-voltage base point of a coupling capacitor. This allows a voltage pulse to be injected into the test object for fault location and the echo to be detected using the coupling capacitor used for the partial discharge measurement. Since the coupling capacitor already serves to insulate the data acquisition unit from the high voltage, this requirement for the measurement is also met for the fault location measurement.In the inventive approach, only one capacitor is required for partial discharge measurement and fault location. Furthermore, the inventive setup allows for fault location to be performed directly—without modifying the measurement setup. In other words, the use and connection of an additional device specifically designed for fault location, which includes its own (expensive) high-voltage decoupling via a coupling capacitor, is eliminated.

[0044]

[0037] When combining state-of-the-art test devices, a tangent-delta measuring device arranged internally in the VLF generator, for example, can no longer be used due to the circuit arrangement of several devices with a "downstream" partial discharge measurement (i.e., arranged between the VLF generator including the tangent-delta measuring device and the test object). This is because the circuitry required for the partial discharge measurement can falsify the tangent-delta measurement, in this case the measured current for the phase angle measurement. By using a tangent-delta measuring device arranged internally downstream in a mobile test device for partial discharge measurement, this disadvantage can be avoided, and the tangent-delta measuring device can be implemented integrated together with the partial discharge measurement and the fault location measurement in one device.

[0045]

[0038] In general, fault location can also be avoided by means of an additional external fault location device according to the prior art.

[0046]

[0039] Concepts are disclosed herein that allow aspects of the prior art to be improved, at least in part. In particular, further features and their usefulness will become apparent from the following description of embodiments with reference to the figures. The figures show:

[0047] Fig. 1 is a schematic representation of a test setup for using an exemplary test device for testing a high or medium voltage cable according to the inventive concept,

[0048] Fig. 2 is a schematic representation of a structure of a first exemplary

[0049] Test device for testing a high or medium voltage cable,

[0050] Fig. 3 is a schematic diagram of the test device structure with exemplary illustrations of the impedance circuit,

[0051] Fig. 4 is a schematic representation of a test device structure with a further exemplary illustration of the impedance circuit and a control concept,

[0052] Fig. 5 is a schematic representation of a structure of another exemplary test device with an additional tangent-delta measuring unit, Fig. 6 is a schematic flow diagram for explaining an exemplary

[0053] Procedure for testing a candidate,

[0054] Fig. 7 shows a representation of a health image recorded for a test subject and

[0055] Fig. 8 a superimposed representation of the healthy image with a

[0056] Flaw image captured for the test specimen with detectable flaw.

[0057]

[0040] In general, the aim of VLF testing technology is to detect a fault in a test object (e.g., an insulation defect in a high- or medium-voltage cable) and to determine its position along the cable. A fault that has not yet caused a breakdown can be detected and localized in the test object, for example, using a partial discharge measurement method. For example, in the tangent-delta measurement method, the detection of a fault (in particular, a "water tree") can also be detected before a breakdown occurs. If a fault triggers a voltage breakdown, the position of the breakdown in the test object can be localized, for example, using a fault location (pre-)location method that uses the injection of a voltage pulse into the test object and the detection of a portion of the voltage pulse reflected at the fault location (the so-called echo).

[0058]

[0041] The concepts for fault location described herein are particularly unique in that they combine partial discharge measurement and fault location, which makes it possible to directly determine the distance to the fault location in the event of a breakdown during the test.

[0059]

[0042] Fig. 1 shows a schematic representation of a test setup with a (mobile / portable) test device 1 for testing a test object 3, e.g., a high- or medium-voltage cable such as a coaxial cable, wherein the test device 1 can be operated in multiple operating modes for multiple test methods. The test device 1 is connected to a VLF high-voltage source 5. A circuit arrangement of the test device 1 is explained in various embodiments in conjunction with Figures 2-5. Use of the test device 1 for fault location is described by way of example in conjunction with Figures 6-8.

[0060]

[0043] The VLF high-voltage source 5 is designed to generate a suitable (VLF) test voltage. For example, a high-voltage generator 5A of the VLF high-voltage source 5 is electrically connected to a high-voltage terminal 7A of the mobile test device 1. The VLF high-voltage source 5 is supplied with energy (power terminal 5B), for example, via the power grid (generally a supply voltage source). The high-voltage generator 5A generates, for example, by means of transformers and downstream cascade multipliers, an HV test voltage, which is provided at the high-voltage terminal 7A with a variable amplitude for the VLF test and ensures a preferably sinusoidal HV voltage curve at the test object 3. For further details on the generation and control of the test voltage, please refer to the aforementioned publications DE 10 2012 024 560 B3 and DE 195 13 441 Al.

[0061]

[0044] The test device 1 further comprises a circuit arrangement 8 (see also Figures 2-5) and a data acquisition unit 9A. As shown in Figure 1, the data acquisition unit 9A can also be connected to an externally implemented evaluation electronics unit 9B (indicated in Figure 1, for example, as a laptop) for evaluating the measurement (for example, electrically isolated, optical, or wireless). Alternatively, the evaluation electronics unit 9B can be implemented as part of the test device 1 or partially integrated into the data acquisition unit 9A.

[0062]

[0045] Furthermore, the test device 1 comprises a control unit 15 (see also Figures 2-5) for controlling one or more components of the circuit arrangement 8.

[0063]

[0046] The data acquisition unit 9A, the evaluation electronics 9B, and the control unit 15 can be configured for multiple operating modes for performing a partial discharge measurement, a fault location, and optionally a tangent-delta measurement. The underlying hardware (usually one or more computing units) comprises, for example, digital processor systems with microprocessor circuits having data inputs and control outputs, which operate according to computer-readable instructions stored on a computer-readable medium.Typically, the data acquisition unit 9A, the evaluation electronics 9B and the control unit 15 provide high computing power for real-time analysis of the continuously acquired and evaluated data sets and include long-term (non-volatile) memory for storing the program instructions as well as very fast short-term (volatile) memory for storing acquired data (measurement data, control data) and evaluation results during (or resulting from) the data acquisition and data processing described herein.

[0064]

[0047] The data acquisition unit 9A, the evaluation electronics 9B, and the control unit 15 can, for example, comprise electronic components such as signal processing with operational amplifiers, at least one integrator, a sample and hold element, and at least one analog-to-digital converter for digitization for further processing in a processor having at least one memory for storing sampling data (measurement data, control data).

[0048] The evaluation electronics 9B can comprise an operating display (e.g., laptop display) for displaying the acquired test data and at least one operating element (e.g., laptop keyboard) for setting a measurement parameter. The operating display and the operating element form, for example, a user interface for controlling the test device 1 and for evaluating the acquired measurement data.

[0065]

[0049] As shown in Figure 1, a conductor 3A of the test object 3 is connected to a test object terminal 7B of the test device 1 via a (HV) connection cable 11. As further shown in Figure 1, the test device 1, in particular a ground terminal 7C of the test device 1, a shield 3B of the test object 3, and the VLF high-voltage source 5 are connected to protective earth 13, whereby the power terminal 5B can also provide a protective earth. Between the conductor 3A of the test object 3 and the shield 3B lies an insulation 3C to be tested for fault locations.

[0066]

[0050] Figure 2 shows an exemplary embodiment of the circuit arrangement 8 in a housing 17 of the test device 1. The control unit 15 comprises a control output 15A for controlling components of the circuit arrangement 8. Furthermore, the data acquisition unit 9A comprises, for example, signal inputs 19A, 19B for acquiring measurement data (in particular for different operating modes) and a control input 19C, via which the data acquisition unit 9A is connected to the control output 15A (in particular for setting one or more operating mode-specific parameters) (dashed control connection in Figure 2).

[0067]

[0051] The circuit arrangement comprises a noise-suppressing filter 21, whose high-voltage input 21A is electrically connected to the high-voltage terminal 7A of the test device 1 and whose high-voltage output 21B is electrically connected to the test object terminal 7B. The noise-suppressing filter 21 is, for example, a high-pass filter, which is connected, in particular, via a filter capacitor 21C to a ground terminal 7C of the circuit arrangement. The circuit arrangement further comprises a high-voltage (HV) coupling capacitor 23 with a capacitance in the range of, for example, 0.5 nF - 25 nF. A high-voltage side 23A of the HV coupling capacitor 23 is electrically connected to the high-voltage output 21B of the noise-suppressing filter 21.

[0068]

[0052] The circuit arrangement comprises the ground terminal 7C for providing a device ground potential in the mobile test device 1 by connecting the ground terminal 7C to protective earth 13.

[0069]

[0053] The circuit arrangement further comprises the voltage pulse generator 25 for generating a voltage pulse. As shown schematically in Figure 1, the voltage pulse generator 25 is connected to the ground terminal 7C and thus to the protective earth 13, so that the voltage pulse can be generated with respect to the device's ground potential (protective earth).

[0070]

[0054] According to the embodiment shown in Figure 2, both a pulse output 25A of the voltage pulse generator 25 and the (first) signal input 19A of the data acquisition unit 9A are connected via an impedance circuit 27 to a low-voltage side 23B of the HV coupling capacitor 23 to provide different operating modes.

[0071]

[0055] For a partial discharge test of the test object 3, the impedance circuit 27 connects the low-voltage side 23B of the HV coupling capacitor 23 to the first signal input 19A of the data acquisition unit 9A. This, in particular low-resistance, electrical connection is designed such that a partial discharge generated in the test object 3 with the VLF test voltage can be detected as a partial discharge measurement signal.

[0072]

[0056] To locate a position of a fault in the test object 3, the impedance circuit 27 connects the pulse output 25A to the low-voltage side 23B of the HV coupling capacitor 23. At the same time, a high-impedance state is provided between the signal input 19A and the low-voltage side 23B to protect the data acquisition unit. The electrical connection, particularly a low-resistance connection, between the pulse output 25A and the low-voltage side 23B is designed such that a voltage pulse from the voltage pulse generator 25 can be coupled into the test object 3 via the coupling capacitor 23. To temporally control the delivery of the voltage pulse to the test object 3 (e.g., during a voltage breakdown), the voltage pulse generator 25 can be connected via a control input 25B to the control output 15A for receiving control signals from the control unit 15.Furthermore, when locating a fault location, the impedance circuit 27 can electrically connect the low-voltage side 23B to the first signal input 19A for detecting a voltage pulse (echo pulse) reflected at the fault location, particularly a low-impedance one. To prevent the reflected voltage pulse from being influenced, a high-impedance state can be created between the voltage pulse generator 25 and the low-voltage side 23B of the HV coupling capacitor 23 simultaneously during the detection of the echo pulse.

[0073]

[0057] To measure the voltage across the test object 3, a voltage divider 31 can optionally be provided in the test device 1 between the high-voltage output 21B and the (second) signal input 19B, for example in the form of high-voltage resistors (e.g. with resistance values ​​in the range from 100 MΩ to 1000 MΩ), which electrically connect the high-voltage output 21B of the noise-suppressing filter 21 to the (second) signal input 19B and / or the control unit 15. With the aid of the voltage divider 31, the phase information can be derived, for example, to determine the zero crossing or the maximum of the VLF test voltage, and a current proportional to the test voltage can be acquired by the data acquisition unit 9A. The phase information signal can be used, for example, for the partial discharge test (phase angle in ° - see IEC60270) or for tangent-delta measurement.

[0074]

[0058] Furthermore, a current detection element 33 (e.g., an adjustable impedance or a resistor as a current shunt) can optionally be provided between the high-voltage output 21B and the test object connection 7B, in particular for a tangent-delta diagnostic measurement in the test device 1, with which the current through the test object can be detected. See, for example, DE 10 2012 024 560 B3 regarding a particularly advantageous arrangement for a highly accurate tangent-delta diagnostic measurement. The test current is detected, for example, in a frequency range up to typically 5 kHz (not more than 50 kHz) and serves to determine the power loss (resistive leakage current) via the phase shift resulting from the test current.

[0075]

[0059] As explained below in connection with Figure 3, the current measurement with the current detection element 33 can also be used in the context of fault location.

[0076]

[0060] In the embodiment illustrated in Figure 3, the circuit arrangement comprises, in addition to the components described in connection with Figure 2 (for clarity, the corresponding reference numerals have been omitted), a control connection 35 (shown as a dashed line) between the current detection element 33 and the control unit 15.

[0077]

[0061] For example, based on the measured current, the time of a breakdown in the test object 3 can be detected during a VLF test, wherein the breakdown is triggered by the applied VLF high voltage. Such a breakdown extends over a period of, for example, 1 us - 1 ms, so that if the breakdown is detected in time, a voltage pulse, which may, for example, have a pulse length of a few nanoseconds to a few microseconds, can be coupled into the test object 3 to locate the breakdown. The breakdown causes an impedance change at the location of the fault, so that an inverted reflection of the voltage pulse (with formation of the echo) occurs at the fault.

[0078]

[0062] For example, the control unit 15 can be configured to detect the time of a breakdown from the current measurement value obtained with the current detection element 33 and to accordingly control the voltage pulse generator 25 to emit a voltage pulse. For coupling the voltage pulse and detecting the echo, the impedance circuit 27 is controlled according to the sequence of the locating process; see in particular the description in connection with Figure 6.

[0079]

[0063] On the one hand, the impedance circuit 27 is thus designed to protect the data acquisition unit 9A from the voltage pulses of the voltage pulse generator 25. On the other hand, the impedance circuit 27 is designed to adapt the impedance to the various components, in particular the HV coupling capacitor 23 and the voltage pulse generator 25, for undisturbed propagation of a measurement signal (partial discharge measurement signals and echo pulse signals) to the data acquisition unit 9A. Fast-switching switches (switching cycle of, for example, less than 100 ns) are preferably used for the switching. For example, the impedance circuit 27 can use at least one switchable semiconductor-based impedance.

[0080]

[0064] The impedance circuit 27 may comprise an analog switch, in particular a semiconductor switch, which is designed in particular for switching between low- and high-impedance states, for example within a few nanoseconds, in particular in the time range of less than 100 ns or less than 50 ns or less than 10 ns.

[0081]

[0065] For the partial discharge test, the low-impedance state can be formed with a resistance in the range of 1 Q (or smaller, for example 0.5 Q or 0.1 Q) - 100 Q between the HV coupling capacitor 23 and the signal input 19A and the high-impedance state between the voltage pulse generator 25 and the HV coupling capacitor 23 with a resistance in the range of 1 MQ - 100 MQ.

[0082]

[0066] For the location, at the time of generation of the voltage pulse, the high impedance state can be formed with a resistance in the range of 1MQ - 100 MQ between the HV coupling capacitor 23 and the signal input 19A and the low impedance state between the voltage pulse generator 25 and the HV coupling capacitor 23 with a resistance in the range of 1 Q (or smaller, for example 0.5 Q or 0.1 Q) - 100 Q.

[0083]

[0067] In Figure 3, the impedance circuit 27 is schematically illustrated by two switches 41A, 41B. The switch 41A is arranged between the data acquisition unit 9A and the HV coupling capacitor 23 as well as the voltage pulse generator 25 (or more precisely the switch 41B); the switch 41B is arranged between the voltage pulse generator 25 and the HV coupling capacitor 23 as well as the data acquisition unit 9A (or more precisely the switch 41A). The switches 41A, 41B are designed such that they can be switched very quickly in order to separate the connected components with high impedance. For example, the switches 41A, 41B are switchable impedances based on semiconductors, e.g. B. designed as two controllable solid-state relays 43A, 43B (see schematically indicated MOSFETS in the enlarged section 45 of Figure 3).

[0084]

[0068] As shown in the enlarged section 45, the control unit 15 can control an optical fast control unit 47 (for example by means of LED (Light Emitting Diode)) for activating / deactivating the solid-state relays 43A, 43B, so that the solid-state relays 43A, 43B cause a high impedance between the data acquisition unit 9A and the HV coupling capacitor 23 (as well as the voltage pulse generator 25) for coupling the voltage pulse into the HV coupling capacitor 23 and a low impedance between the data acquisition unit 9A and the HV coupling capacitor 23 for data acquisition. In this way, it can be avoided that the data acquisition unit 9A is damaged when the voltage pulse is coupled in, for example a 1 kV test pulse with a pulse duration in the range of 10 ns to 10 ps, ​​and that the measurement signal is corrupted due to the connected voltage pulse generator.

[0085]

[0069] In the embodiment of the impedance circuit 27 illustrated in Figure 4, it comprises (only) one switch 4 IC, which is arranged between the voltage pulse generator 25 and the HV coupling capacitor 23 as well as the data acquisition unit 9A. A high-ohm resistor 49 is additionally arranged between the data acquisition unit 9A and the HV coupling capacitor 23 as well as the voltage pulse generator 25. The high-ohm resistor 49 is selected such that it adapts the impedance in the circuit arrangement, in particular in the impedance circuit 27, for efficient coupling of the voltage pulse into the HV coupling capacitor 23 (for example, in the range of 1 MΩ to 1000 MΩ).

[0086]

[0070] In the embodiment of the test device 1 illustrated in Figure 5, a tangent-delta measuring unit 51 is provided—in addition to, for example, Figure 2. The tangent-delta measuring unit 51 is electrically connected to the current sensing element 33 and to the low-voltage side of the voltage divider 31. The tangent-delta measuring unit 51 receives the test current from the current sensing element 33. The tangent-delta measuring unit 51 receives the phase information and the value of the test voltage from the voltage divider 31. Based on these input variables, the tangent-delta measuring unit 51 is configured for tangent-delta diagnosis.

[0087]

[0071] Figure 6 illustrates, in a flowchart, the process of testing insulation in, for example, a coaxial cable for current / energy distribution in electrical supply networks. Generally, for testing and diagnostic measurements, information on the insulation is obtained using partial discharge measurements and, optionally, phase rotation measurements (tangent-delta measurements) during or before a breakdown of the cable insulation. In particular, in the case of a breakdown, the position of the fault location is evaluated using echometry / time-of-fault measurements. The flowchart encompasses both the acquisition of a "healthy image" of the test object (healthy image measurement, i.e., echo signal curve without the presence of a breakdown - "no-arc" measurement; see Figure 7) and the acquisition of a "fault location image" of the test object (fault location image measurement, i.e., echo signal curve in the presence of a breakdown or already in the presence of impedance changes along the insulation - "in-arc" measurement; see Figure 8).

[0088]

[0072] Referring to Figures 1 and 6, to test the device under test 3, the VLF high-voltage source 5 is connected to the high-voltage terminal 7A of the test device 1 (step 101 - "connect VLF source"). Furthermore, the conductor 3A of the device under test 3 (accessible at one end of the device under test 3) is connected to the device under test terminal 7B of the test device 1. The shield 3B of the device under test 3 and a ground terminal 7C of the test device 1 are connected to protective ground 19. With the VLF high-voltage source 5, a very low frequency (VLF) test voltage can be generated and coupled into the device under test 3 via the test device 1 (step 103 - "connect device under test and apply voltage").

[0089]

[0073] In the following, a (high-voltage-free) healthy image measurement 104 carried out as an example at the beginning of a VLF diagnosis is described, wherein in particular no, in particular no high VLF test voltage, optionally only a low basic voltage, is applied.

[0090]

[0074] In step 105 ("deactivate DAQ mode / activate impulse mode"), an electrical connection between a voltage pulse generator 25 of the test device 1 and the signal input 19A of the data acquisition unit 9A of the test device 1 is avoided / deactivated with the impedance circuit 27 (assuming a high-impedance state; this can already be provided by the implementation of the impedance circuit 27 - see, for example, Figure 4). Furthermore, an electrical connection between the voltage pulse generator 25 and the HV coupling capacitor 23 of the test device 1 is activated (assuming a low-impedance state by providing a resistance in the range of 0.1 Ω (or 1 Ω, 5 Ω, or 10 Ω) to, for example, 100 Ω).

[0091]

[0075] In step 107 ("inject impulse"), a voltage pulse is generated by the voltage pulse generator 25 and coupled into the test object 3 via the impedance circuit 27 and the HV coupling capacitor 23.

[0092]

[0076] In step 109 ("activate DAQ mode / deactivate impulse mode"), the impedance circuit 27 is used to provide / activate an electrical connection between the HV coupling capacitor 23, the impedance circuit 27 and the signal input 19A of the data acquisition unit 9A, and to effect electrical decoupling or a high-impedance connection between the impedance circuit 27 and the voltage pulse generator 25.

[0093]

[0077] The data acquisition unit 9A acquires a healthy signal, which is based on a voltage pulse (echo) reflected at a (remote) cable end of the test object 3 (step 111 - "record transient image (no-arc)"). This can be regarded as a basic healthy signal, in which the test object 3 is (still) voltage-free.

[0094]

[0078] Figure 7 shows, in an exemplary healthy image 61, the healthy signal 63, which is plotted as a voltage value U (in V) recorded by the data acquisition unit 9A over a transit time t (in ps). Starting at t=0, a residual signal 65 of positive voltage associated with the voltage pulse can be seen, as well as an echo signal 67 (also with positive voltage) at t=13. Echo signal 67 corresponds to the voltage pulse reflected at the (far) cable end of the test object 3. The transit time of 13 ps determines the distance between the two cable ends depending on the specific cable parameters.

[0095]

[0079] Referring again to Figure 6, the healthy image can be stored in a memory 112 for later comparison with a fault location measurement (step 113 - "disturb no-arc data stream for later comparison").

[0096]

[0080] Using the VLF test voltage, a VLF diagnosis of the device under test 3, for example a partial discharge test and / or a tangent delta test, can now be performed (step 115 - "perform VLF diagnostic (partial discharge PD and / or tandelta TD)").

[0097]

[0081] The data obtained from the healthy signal on the test subject 3 can be included in the VLF diagnosis (step 117 - "examine / compare with transient record; compare PD and / or TD data (levels)").

[0098]

[0082] Triggered by results of the partial discharge test and / or the Tanges-Delta test or continuously repeated at the same time as the VLF diagnosis, the location measurement 118 described below can be carried out, with the VLF test voltage now being applied to the test object 3.

[0099]

[0083] During the location measurement 118, a location signal is detected, which is based on a voltage pulse (echo) reflected at a fault location. The fault location forms during the VLF diagnosis due to the applied high voltage over a period of time, for example, over a few microseconds of a breakdown ("in-arc" measurement), such that an injected voltage pulse can interact with the fault location.

[0084] The location measurement 118 comprises steps 105 to 109 of the healthy image measurement 104, which are referred to below as steps 105' to 109' in the context of location.

[0100]

[0085] That is, in step 105' ("deactivate DAQ mode / activate impulse mode"), the electrical connection between the voltage pulse generator 25 of the test device 1 and the signal input 19A is avoided / deactivated with the impedance circuit 27 (assuming the high-impedance state). Furthermore, an electrical connection between the voltage pulse generator 25 and the HV coupling capacitor 23 is activated (assuming the low-impedance state).

[0101]

[0086] In step 107' ("inject impulse"), a voltage pulse is generated by the voltage pulse generator 25 and coupled into the test object 3 via the impedance circuit 27 and the HV coupling capacitor 23.

[0102]

[0087] In step 109' ("activate DAQ mode / deactivate impulse mode"), an electrical connection between the HV coupling capacitor 23, the impedance circuit 27 and the signal input 19A of the data acquisition unit 9A is provided / activated with the impedance circuit 27, and an electrical decoupling or a high-impedance connection between the impedance circuit 27 and the voltage pulse generator 25 is effected.

[0103]

[0088] Upon formation of a detectable fault location, the data acquisition unit 9A now acquires a location signal which is attributable to a voltage pulse reflected at a fault location (step 119 - "record transient image (no-arc or in-arc)"). To convert (create) the fault location into a detectable state according to the concepts disclosed herein, a breakdown can be triggered with the VLF test voltage. Accordingly, for locating, steps 105' to 109', i.e., the steps of deactivating the electrical connection, generating the voltage pulse, and activating the electrical connection for acquiring the location signal during the breakdown, can be performed. Accordingly, the location signal is attributable to a reflection of the voltage pulse at a breakdown position in the test object 3, wherein the acquired position of the fault location corresponds to the breakdown position in the test object 3.

[0104]

[0089] Figure 8 shows a measurement image 71 - superimposed with the healthy signal 63 - an exemplary location signal 73, which is again plotted as a voltage value U (in V) recorded by the data acquisition unit 9A over a transit time t (in ps). Starting at t=0, one can again see a residual signal 75 of positive voltage associated with the voltage pulse, as well as an echo signal 77 at t=9.5 (albeit with a negative voltage due to the presence of a breakdown). The echo signal 77 corresponds to the voltage pulse reflected at the fault location - here, for example, at the breakdown induced by the VLF test voltage. The transit time of 9.5 ps determines the distance from the cable end, at which the pulse is coupled, to the fault location, depending on the specific cable parameters.

[0105]

[0090] To improve the evaluation, a plurality of location signals 73 can be detected, particularly during a breakdown. For example, a trigger signal based on a breakdown detection, for example, can trigger a sequence of several tens of location measurements 118.

[0106]

[0091] Referring again to Figure 6, the measurement image 71 can be stored in the memory 112 for later analysis (step 121 - "store arc data stream for later comparison").

[0107]

[0092] In step 123 ("analyze image(s)"), the location signal 73 is optionally analyzed with the aid of the healthy signal 63 in the evaluation electronics 9B provided in the data acquisition unit 9A or externally connected to obtain a position of the fault location in the test object 3.

[0108]

[0093] The data for the measurement and a detected fault location, in particular the position of the fault location in the test object (for example a distance value of the fault location to one of the ends of the test object), is summarized in a measurement report in step 125 ("generate report on or more fault locations") and output on a display of the evaluation electronics 9B and, for example, additionally stored in the memory 112.

[0109]

[0094] After completion of the (VLF) test procedure, the device under test 3 (step 127 - "disconnect device under test") and subsequently the VLF high-voltage source 5 are disconnected from the test device 1 (step 129 - "disconnect VLF source").

[0110]

[0095] The location measurement 118 can be triggered, for example, by the control unit 15 detecting a trigger signal. Thus, steps 105' to 109', i.e., the steps of deactivating the electrical connection, generating the voltage pulse, and activating the electrical connection, can be triggered, for example, specifically within the context of a voltage pulse causing a voltage breakdown. For example, detecting a breakdown can trigger the trigger signal, with steps 105' to 109', i.e., the steps of deactivating the electrical connection, generating the voltage pulse, and activating the electrical connection, being triggered by the trigger signal, in particular by the control unit 15.Alternatively, the trigger signal can be triggered after detecting a current pulse, for example, with the current detection element 33, in the test object 3, wherein steps 105' to 109' are triggered after detection by the trigger signal, in particular initiated by the control unit 15.

[0096] For example, the current pulse detected by the current detection element 33 can additionally be evaluated for a tangent-delta measurement with the tangent-delta measuring unit 51.

[0111]

[0097] As already mentioned, steps 105' to 109', i.e., the steps of deactivating the electrical connection, generating the voltage pulse, and activating the electrical connection, can be continuously and repeatedly triggered to detect healthy signals and—e.g., upon the occurrence of a breakdown—location signals while testing the device under test 3 with a VLF test voltage. This is illustrated in Figure 6 by arrow 131.

[0112]

[0098] Regarding the generation of healthy images during the VLF measurement process, steps 105' to 109', i.e., the steps of deactivating the electrical connection, generating the voltage pulse, and activating the electrical connection, can be performed to detect healthy signals at a value of the VLF test voltage at which no breakdown has yet been triggered in the test object 3. Accordingly, the healthy signal is based on a reflection of the voltage pulse at one end of the test object 3 and, in particular, indicates the current state of the test object based on a pulse signal.

[0113]

[0099] As already described, steps 105' and 109', i.e. the steps of deactivating and activating the electrical connection by means of the impedance circuit 27, can be carried out by switching an electronic switch (analogue switch), for example the semiconductor switch 43A, 43B. The analog switch can be designed in particular to switch between low- and high-impedance states within a few nanoseconds, in particular in the time range of less than 100 ns or less than 50 ns or less than 10 ns.

[0114]

[0100] The features of the methods and devices used in the present description, which have been referred to as "unit", "apparatus" or the like, may be implemented, for example, as discrete physical units, as conceptual functional units, e.g., as software code (as part of an evaluation program) stored in a storage unit (memory), routines from a microprocessor, and / or within a hybrid hardware / firmware structure within the scope of the skill of the art. Furthermore, two or more "units", etc., may be integrated together in a single physical circuit structure (e.g., an integrated unit or structure). For example, a processor may be controlled by programming code (stored instructions), where the programming code is capable of performing the respective functions when executed by a processor, such as a microprocessor.

[0101] The features specifically mentioned in the claims can thus be implemented as software, hardware, and / or a combination of hardware and software. Specific details of the individual units are described in the description (and in particular in the exemplary sections). This provides a person skilled in the art with sufficient information to implement the corresponding structures in hardware circuits or software code. As an example, the "control unit" disclosed herein can be embodied in the structure of a central processing unit (CPU) configured with instructions for performing the operations for controlling the various components. Furthermore, as an example, the "evaluation unit" disclosed herein can be embodied in the structure of a central processing unit (CPU) configured with instructions for performing the operations for evaluating the acquired signals.Such CPUs may comprise one or more microprocessors in conjunction with one or more memory elements. A memory element may store one or more microprocessor-readable instructions (programs) that, when executed by the microprocessor, generate, for example, control signals.

[0115]

[0102] It is explicitly emphasized that all features disclosed in the description and / or the claims are to be considered separate and independent of each other for the purpose of the original disclosure as well as for the purpose of limiting the claimed invention, regardless of the combinations of features in the embodiments and / or the claims. It is explicitly stated that all range specifications or specifications of groups of units disclose every possible intermediate value or subset of units for the purpose of the original disclosure as well as for the purpose of limiting the claimed invention, in particular also as a limit of a range specification.

Claims

Patent claims 1. Mobile test device (1) for testing a test object (3), in particular a high-voltage or medium-voltage cable, by means of a VLF high-voltage source (5) providing a very low frequency (VLF) test voltage, comprising: a high-voltage terminal (7A) for connecting the VLF high-voltage source (5), a test object terminal (7B) for connecting the test object (3), an earth terminal (7C) for providing a device earth potential in the mobile test device (1) by connecting the earth terminal (7C) to earth potential, a noise-suppressing filter (21), the high-voltage input (21A) of which is electrically connected to the high-voltage terminal (7A) and the high-voltage output (21B) of which is electrically connected to the test object terminal (7B), a high-voltage (HV) coupling capacitor (23), wherein a high-voltage side (23A) of the HV coupling capacitor (23) is electrically connected to the high-voltage output (21B) of the noise-suppressing filter (21),a voltage pulse generator (25) for generating a voltage pulse with respect to the device earth potential and a data acquisition unit (9A) with a signal input (19A), wherein for a partial discharge test of the test object (3), a low-voltage side (23B) of the HV coupling capacitor (23) is electrically connectable via an impedance circuit (27) to the signal input (19A) of the data acquisition unit (9A) for detecting a partial discharge generated in the test object (3) with the VLF test voltage,and for locating a position of a fault in the test object (3), a pulse output (25 A) of the voltage pulse generator (25) is electrically connectable via the impedance circuit (27) to the low-voltage side (23B) of the HV coupling capacitor (23) for coupling a voltage pulse into the test object (3), and the low-voltage side (23B) of the HV coupling capacitor (23) is electrically connectable via the impedance circuit (27) to the signal input (19A) of the data acquisition unit (9A) for detecting a voltage pulse reflected at the fault.

2. Mobile testing device (1) according to claim 1, wherein the impedance circuit (27) comprises at least one switchable impedance (41A, 41B) based on a semiconductor, which is designed in particular as a fast-switchable impedance, for example switchable in the time range of less than 100 ns or less than 50 ns or less than 10 ns, and wherein the at least one switchable impedance (41A, 41B) is arranged between the HV coupling capacitor (23) and the signal input (19A) and / or between the HV coupling capacitor (23) and the voltage pulse generator (25).

3. Mobile testing device (1) according to claim 1 or 2, wherein the impedance circuit (27) comprises an electronic switch, in particular a semiconductor switch, which is designed to switch between low- and high-impedance states within a few nanoseconds, in particular in the time range of less than 100 ns or less than 50 ns or less than 10 ns.

4. Mobile testing device (1) according to claim 3, wherein for the partial discharge test the low-impedance state is formed with a resistance in the range of 0.1 Ω to 100 Ω between the HV coupling capacitor (23) and the signal input (19A) and the high-impedance state is formed between the voltage pulse generator (25) and the HV coupling capacitor (23) with a resistance in the range of 1 MΩ to 100 MΩ, and / or for the location the high-impedance state is formed with a resistance in the range of 1 MQ to 100 MQ between the HV coupling capacitor (23) and the signal input (19A) and the low-impedance state between the voltage pulse generator (25) and the HV coupling capacitor (23) is formed with a resistance in the range of 0.1 Q to 100 Q.

5. Mobile test device (1) according to one of the preceding claims, wherein the data acquisition unit (9A) has a further signal input (19B) and the test device (1) further comprises a voltage divider (31) which electrically connects the high-voltage output (21B) of the noise-suppressing filter (21) to the further signal input (19B) in order to provide the data acquisition unit (9A) in particular with a phase information signal and / or the test voltage, for example for the partial discharge test or for a tangent-delta measurement.

6. Mobile testing device (1) according to one of the preceding claims, further comprising a control unit (15), in particular comprising a computing unit, a memory and optionally a display, wherein the control unit (15) is connected to the voltage pulse generator (25) and the data acquisition unit (9A) for emitting and receiving control signals and is set up to control the impedance circuit (27) in such a way that when a voltage pulse is emitted to the low-voltage side (23B) of the HV coupling capacitor (23), the impedance circuit (27) assumes a circuit state in which the signal input (19A) of the data acquisition unit (9A) is highly impedantly decoupled, in particular electrically separated, from the low-voltage side (23B) of the HV coupling capacitor (23).

7. Mobile test device (1) according to one of the preceding claims, wherein an adjustable current measuring device (33), e.g. a measuring impedance, is provided between the noise-suppressing filter (21) and the test object connection (7B), which is connected to the control unit (15) and is configured to detect a test current supplied to the test object (3) during a VLF test and to supply it as a current measurement signal to the control unit (15), and the control unit (15) is configured to - depending on the current measurement signal, to output a trigger signal to the voltage pulse generator (25) for outputting a voltage pulse from the voltage pulse generator (25) for carrying out fault location, wherein the trigger signal is output in particular when the current measurement signal indicates a breakdown of the VLF voltage through an insulation (3C) of the test object (3).

8. Mobile testing device (1) according to one of the preceding claims, wherein the control unit (15) is designed to control the impedance circuit (27), in particular after detecting - a carbon copy or - a breakdown during a VLF partial discharge measurement or - a breakdown current during a VLF tangent-delta measurement or - within the scope of a "health image" detection, for deactivating and activating the coupling of the low-voltage side (23B) of the HV coupling capacitor (23) to the signal input (19A) and / or wherein the control unit (15) is designed to detect a breakdown within the scope of a VLF measurement.

9. Mobile testing device (1) according to one of the preceding claims, wherein - the noise-suppressing filter (21) is designed as a high-frequency filter and / or - the HV coupling capacitor (23) has a capacitance in the range of 0.5 nF to 25 nF, and / or - the voltage pulse generator (25) is designed to generate and emit voltage pulses in the range from 1 V to 1 kV voltage peak value.

10. Mobile testing device (1) according to one of the preceding claims, wherein the ground terminal (7C) is for providing a device ground potential - via a filter capacitor (21C) with the noise suppressing filter (21) and / or - with the voltage pulse generator (25) and / or - is electrically connected to the data acquisition unit (9A).

11. Mobile testing device (1) according to one of the preceding claims, further configured to - to generate with the provided VLF test voltage both a diagnostic signal for an insulation test of the test object (3) via a partial discharge measurement and / or via a VLF phase rotation measurement and a locating signal for locating the position of the fault location of the test object (3) by means of the voltage pulse, and / or - to output the VLF test voltage provided by the VLF high-voltage source (5) connected to the high-voltage connection (7A) for a partial discharge measurement and / or Tanges-Delta measurement at the test object connection (7B) and to generate a diagnostic signal for an insulation test of the test object (3) via a partial discharge measurement and / or via a VLF phase rotation measurement or a location signal for fault pre-location via a propagation time measurement of an echo signal of a voltage pulse for internal and / or external evaluation.

12. Mobile testing device (1) according to claim 11, wherein an evaluation electronics (9B) integrated in the data acquisition unit (9A) and / or connectable to the data acquisition unit is configured to to determine from the diagnostic signal a partial discharge and / or partial discharges (if present) assigned to the test object (3) and / or a loss factor of the test object (3) and to determine from the locating signal a distance along the test object (3) to a fault location causing a breakdown.

13. Mobile testing device (1) according to claim 12, wherein the evaluation electronics (9B) - at least one analog and / or digital signal processing unit and / or - comprises a processor and / or a buffer memory.

14. Method for testing a test object (3), in particular a high-voltage or medium-voltage cable (3), in particular using a mobile testing device (1) according to one of the preceding claims, comprising the steps: - Connecting a VLF high-voltage source (5) to a high-voltage terminal (7A) of the test device (1), - connecting a conductor (3 A) of the test object (3) to a test object terminal (7B) of the test device (1), wherein a shield (3B) of the test object (3) and an earth terminal (7C) of the test device (1) are connected to protective earth (19), - generating a very low frequency (VLF) test voltage with the VLF high voltage source (5) and coupling the VLF test voltage into the test object (3) via the test device (1), - by means of an impedance circuit (27) deactivating an electrical connection between a voltage pulse generator (25) of the test device (1) and a signal input (19A) of a data acquisition unit (9A) of the test device (1) and activating an electrical connection between the voltage pulse generator (25) and an HV coupling capacitor (23), - generating a voltage pulse with the voltage pulse generator (25) of the test device (1) and coupling the voltage pulse via the impedance circuit (27) and the HV coupling capacitor (23) into the test object (3), - by means of the impedance circuit (27) activating an electrical connection between the HV coupling capacitor (23), the impedance circuit (27) and the signal input (19A) of the data acquisition unit (9A) of the test device (1), and causing a electrical decoupling or a high-impedance connection between the impedance circuit (27) and the voltage pulse generator (25), - with the data acquisition unit (9A) detecting a healthy signal which is attributable to a voltage pulse reflected at a cable end of the test object (3) and / or a locating signal which is attributable to a voltage pulse reflected at a fault location, and - Processing the location signal with the aid of the healthy signal in an evaluation electronics unit (9B) provided in the data acquisition unit (9A) or connected externally in order to obtain a position of the fault location in the test object (3).

15. The method according to claim 14, wherein a breakdown is triggered with the VLF test voltage to transfer the fault location into a detectable state and the steps of deactivating the electrical connection, generating the voltage pulse and activating the electrical connection to detect the locating signal are carried out during the breakdown, so that the locating signal is based on a reflection of the voltage pulse at a position of the breakdown in the test object (3) and the obtained position of the fault location corresponds to the position of the breakdown in the test object (3).

16. The method according to claim 14 or 15, wherein a trigger signal is triggered upon detection of a breakdown and the steps of deactivating the electrical connection, generating the voltage pulse and activating the electrical connection are triggered by the trigger signal.

17. The method according to claim 14 or 15, wherein a trigger signal is triggered after detection of a current pulse in the test object (3) and the steps of deactivating the electrical connection, generating the voltage pulse and activating the electrical connection are triggered after detection by the trigger signal, and wherein in particular the current pulse is evaluated.

18. The method according to claim 14 or 15, wherein the steps of deactivating the electrical connection, generating the voltage pulse and activating the electrical connection are continuously and repeatedly triggered to detect healthy signals and location signals concomitantly with the testing of the device under test (3) with a VLF test voltage, and / or wherein the steps of deactivating the electrical connection, generating the voltage pulse and activating the electrical connection are specifically triggered within the framework of a voltage pulse causing a voltage breakdown.

19. The method according to any one of claims 14 to 18, wherein the steps of deactivating the electrical connection, generating the voltage pulse, and activating the electrical connection for detecting the healthy signal are performed at a value of the VLF test voltage at which no breakdown has yet been triggered in the test object (3), so that the healthy signal is based on a reflection of the voltage pulse at one end of the test object (3) and, in particular, indicates the current state of the test object using a pulse signal, or wherein, for detecting a basic healthy signal, the steps of deactivating the electrical connection, generating the voltage pulse, and activating the electrical connection for detecting the healthy signal are performed before applying a VLF test voltage,so that the basic health signal is based on a reflection of the voltage pulse at one end of the voltage-free test object (3) and in particular indicates the current state of the test object (3) by means of a pulse signal.

20. Method according to one of claims 14 to 19, wherein the deactivation and activation of the electrical connection by means of the impedance circuit (27) is carried out by switching an electronic switch, in particular a semiconductor switch (43A, 43B), which is designed in particular for switching between low- and high-impedance states within a few nanoseconds, in particular in the time range of less than 100 ns or less than 50 ns or less than 10 ns.

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