Test device and method for testing a high-voltage or medium-voltage cable and method for testing a transformer arranged in a test device

Abstract

A testing device (1) for testing a test specimen (3), e.g. B. a high- or medium-voltage cable, comprises a power converter (31), a transformer (41, 43, 61) for transforming an RF signal into a high-voltage signal, wherein the transformer (41, 43, 61) comprises a primary winding (63), a protective earth shield (71) for shielding the primary winding (63), wherein the protective earth shield (71) is capacitively coupled to the primary winding (63) and electrically connected to the protective earth potential (19), a secondary winding (65) and an internal earth shield (73) for shielding the secondary winding (65), wherein the internal earth shield (73) is capacitively coupled to the secondary winding (65) and electrically connected to the internal earth collection point (55).The test device (1) further comprises a rectifier circuit (45) electrically connected to the high-voltage output (41A, 43A) for outputting a rectified high-voltage signal and a high-voltage circuit arrangement (35) for generating the VLF test voltage based on the rectified high-voltage signal.

Description

[0001] The present invention relates to a test device for testing a test object (e.g., a high- or medium-voltage cable), in particular for testing insulation in, for example, coaxial cables for power / energy distribution in electrical supply networks using VLF testing methods. Furthermore, the invention relates to a method for testing a transformer in such a test device.

[0002] The testing of high- or medium-voltage cables, for example cables laid underground or running through water as part of regional energy networks, includes testing to determine any existing defects or pre-damage that may, for example, develop in the insulation of a high-voltage cable. Example defects also include defects in the cable insulation such as water trees or electrical trees that do not yet cause a breakdown or that cause a breakdown during the VLF test. High- or medium-voltage cables (also referred to herein as Device Under Test (DUT)) are generally cables that are designed to distribute energy / power using voltages in the high- and medium-voltage range starting at approximately 1 kV up to several hundred kV (and more), e.g. in power grids for electrical energy supply. The distribution of energy / power can, for example,over distances of a few hundred meters to a few tens of kilometers (with up to over 1 GW power and up to 500 kV voltage).

[0003] To determine the presence of pre-damage, mobile VLF test devices enable testing of a device under test using a test voltage, e.g., in the range of 20 kV peak to 120 kV peak (generally not limited), which is generated with a highly precise sinusoidal voltage waveform at a frequency in the range of 0.01 Hz to 1 Hz, the so-called Very Low Frequency (VLF). This is applied to the device under test as a power cable on the conductor opposite the protective earth potential (PE). VLF-based test methods are well known and defined, for example, in IEEE 400.2. Essential for VLF-based test methods is the most interference-free measurement of a test current that follows the voltage waveform of the generated kV test voltage.

[0004] For voltage generation, VLF test devices include special VLF high-voltage sources (also referred to as VLF high-voltage sources). A VLF high-voltage source can preferably comprise two high-voltage sources (cascade-connected transformers) and an output amplifier (in the form of current sources), which together generate a very low-noise sinusoidal output voltage (the "test voltage"). Circuit arrangements for generating such test voltages are disclosed, for example, in the applicant's DE 10 2012 024 560 B3 or DE 195 13 441 A1.

[0005] To diagnose the presence of pre-damage (e.g., Water Tree), a loss factor can be determined in a so-called tangent-delta measurement. For a tangent-delta measurement, the current through the DUT containing the defect can be measured despite the device under test being connected to protective earth, i.e., without the device under test having to be disconnected from protective earth. For example, a current sensing element (e.g., impedance as a current shunt) for a tangent-delta measurement can be provided internally in a VLF test generator. This can be used to measure / detect the current and phase through the device under test, in particular between protective earth potential and internal earth potential, at a so-called internal earth collection point, also referred to herein as the current collection point. In other words, the current collection point in the VLF test generator is selected such that the current flows from the test object back to the high-voltage cascade through the current collection point.DE 10 2012 024 560 B3 discloses an advantageous arrangement for a high-precision diagnostic measurement, in which a high-precision test current can be generated and measured using a central clock and a sinusoidal test voltage envelope. The test current is acquired, for example, with a resistance in the range of 0.01 kΩ to 1 kΩ at a sampling rate in the range of 5 kHz to 100 kHz and is used to determine the power loss (resistive leakage current) via the phase shift, for example, using Fourier transformation (e.g., Discrete Fourier Transformation (DFT)).

[0006] As also disclosed in DE 10 2012 024 560 B3, increased test voltage quality leads to improved tangent-delta measurability. However, as the power of a VLF test generator increases, so do the noise at the current collection point, especially when a non-isolated DC link generator is used to convert an input AC voltage into an RF signal referenced to a floating reference potential. In the latter case, the necessary potential isolation is typically achieved during the high voltage generation process using transformers (HV transformers) for the cascade circuit(s). (For details, see the following description.)

[0007] For a device for testing high-voltage equipment, reference is also made to DE 10 2009 023 713 A1, which proposes, among other things, shifting the electrical isolation to the input side of the static frequency converter to suppress asymmetrical interference. Furthermore, US 2022 / 0 037 080 A1 discloses a shielding structure for transformer structures.

[0008] One aspect of this disclosure is based on the object of specifying a device and a method for a tangent-delta measurement in which disturbances on the test current used for the measurement by the device under test (in particular a disturbance current due to harmonics of the mains voltage and / or a leakage current of a transformer) are reduced.

[0009] A further 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-voltage or medium-voltage cable, which enables safe, simple and cost-effective testing methods to be carried out.

[0010] At least one of these objects is achieved by a testing device for testing a test object, in particular a high-voltage or medium-voltage cable, according to claim 1 and by a method according to claim 12 or a method according to claim 14. Further developments are specified in the subclaims.

[0011] In one aspect, a test device for testing a device under test, e.g., a high- or medium-voltage cable, using a test method employing a very low-frequency (VLF) test voltage comprises a power converter. The power converter comprises an AC input for receiving an AC input voltage referenced to a protective earth potential, a low-voltage circuit arrangement comprising a rectifier circuit and at least one switched-mode power supply configured to generate an RF signal referenced to a floating reference potential from the AC input voltage, and at least one RF output pair for outputting the RF signal and the floating reference potential. The test device further comprises a transformer for transforming the RF signal into a high-voltage signal.The transformer comprises a primary winding whose winding ends are electrically connected to the RF output pair, a protective earth shield for shielding the primary winding, wherein the protective earth shield is capacitively coupled to the primary winding and electrically connected to the protective earth potential, a secondary winding whose first winding end is electrically connected to an internal earth collector point and whose second winding end is electrically connected to a high-voltage output for outputting the high-voltage signal, and an internal earth shield for shielding the secondary winding, wherein the internal earth shield is capacitively coupled to the secondary winding and electrically connected to the internal earth collector point.The test device further comprises a rectifier circuit electrically connected to the high-voltage output for outputting a rectified high-voltage signal, and a high-voltage circuit arrangement for generating the VLF test voltage based on the rectified high-voltage signal.

[0012] In a further aspect, a method for testing a test object, e.g., a high-voltage or medium-voltage cable, using such a test device comprises the following steps: - Generating a test voltage with the test device, - causing a measuring current with the test voltage when a high- or medium-voltage cable is connected in a connecting conductor that electrically connects an internal earth connection point of the test device to a protective earth connection of the test device, so that the connecting conductor is traversed by the measuring current during the test of the device under test (e.g. the high- or medium-voltage cable), which current develops between the internal earth connection point and a shield of the device under test (e.g. the high- or medium-voltage cable) that is also at protective earth potential, - generating a low-frequency diagnostic signal associated with the measuring current for a VLF phase rotation measurement with a low-frequency signal tap on the connecting conductor, and - Evaluation of the low-frequency diagnostic signal in an evaluation electronics with regard to a fault in the high- or medium-voltage cable.

[0013] In a further aspect, a method for testing a transformer arranged in such a test device, wherein an internal earth shield of the secondary winding of the transformer is electrically connected to an internal earth collector point of the test device via a leakage current measuring impedance, comprises the following steps: - Generating a test voltage with the test device, - Measuring a leakage current with the leakage current measuring impedance and - Evaluation of the leakage current in the evaluation electronics of the test device with regard to insulation damage of the transformer.

[0014] In some embodiments of the test device, an intermediate circuit for generating a DC voltage with respect to the floating reference potential can be formed in the low-voltage circuit arrangement, wherein the DC voltage is subject to harmonics depending on the received input AC voltage and the protective earth shield capacitively coupled to the primary winding is designed to divert an interference current caused by the harmonics to the protective earth potential.

[0015] In some embodiments of the test device, the internal ground shield capacitively coupled to the secondary winding may be configured to divert an RF high-voltage leakage current generated in the transformer to the internal ground collection point.

[0016] In some embodiments of the test device, the transformer may further comprise a transformer core formed as part of the internal ground shield capacitively coupled to the secondary winding and electrically connected to the internal ground collection point.

[0017] In some embodiments of the test device, the transformer may further comprise a winding body, in particular one having a U-shaped cross section, on which the primary winding is arranged embedded in the protective earth shield, an insulating foil, the internal earth shield and the secondary winding, and wherein the winding body may be arranged in particular between sections of a core of the transformer and / or wherein the primary winding may be delimited radially on both sides by the protective earth shield and / or the secondary winding may be delimited radially on both sides by the internal earth shield.

[0018] In some embodiments of the test device, the power converter and the transformer may form a high voltage source and the high voltage circuit arrangement may include at least one semiconductor switch cascade and at least one amplifier associated with the transformer.

[0019] In some embodiments of the test device, the low-voltage circuitry may comprise two switching power supplies configured to output a positive and a negative RF signal at corresponding RF output pairs. Furthermore, the test device may comprise two transformers, each comprising a protective earth shield and an internal earth shield, for transforming the positive and negative RF signals into positive and negative high-voltage signals, and two rectifier circuits electrically connected to high-voltage outputs of the transformers for outputting a rectified positive and a rectified negative high-voltage signal. In particular, the high-voltage circuitry may be configured to generate the VLF test voltage based on the positive rectified high-voltage signal and the negative rectified high-voltage signal.

[0020] In some embodiments of the test device, the high-voltage circuit arrangement may comprise a test object connection for connecting a high-voltage side output of the high-voltage circuit arrangement to a conductor of the test object (e.g., the high-voltage or medium-voltage cable), in particular via a measuring connection cable.

[0021] In some embodiments of the test device, the test device can further comprise a protective earth terminal, a connecting conductor that electrically connects the internal earth collection point to the protective earth terminal, so that during testing of the device under test (e.g., the high- or medium-voltage cable), the connecting conductor is flowed through by a measuring current that forms between the internal earth collection point and a shield of the device under test (e.g., the high- or medium-voltage cable) that is also at protective earth potential, a low-frequency signal tap at the current collection point, at which a low-frequency diagnostic signal is generated based on the measuring current, and evaluation electronics that are connected to the low-frequency signal tap for receiving the low-frequency diagnostic signal and are designed for a VLF phase rotation measurement.

[0022] In some embodiments of the test device, the low-frequency signal tap can be configured to detect an instantaneous value of a current in the connecting conductor and / or comprise an impedance in the connecting conductor, in particular a parallel circuit comprising a resistor and a capacitor. Additionally or alternatively, the evaluation electronics can be configured to determine a loss factor associated with the high- or medium-voltage cable from the diagnostic signal. In particular, it can be designed to detect low frequencies of the diagnostic signal in the range from 0.01 Hz to 1 Hz with a sampling rate with frequencies in the range of, in particular, 5 kHz to 100 kHz. Additionally or alternatively, the evaluation electronics can comprise at least one analog and / or digital signal processing unit and / or a processor and / or a buffer memory.

[0023] In some embodiments, the test device may comprise a signal tap, in particular a leakage current measuring impedance or a leakage current measuring resistor, which electrically connects the internal earth shield of the secondary winding of the transformer to the internal earth collection point and is provided for detecting a structural fault of an insulation of the transformer.

[0024] In some embodiments of the method for testing a device under test (e.g., a high- or medium-voltage cable), a disturbance current caused by harmonics of a received input AC voltage can be diverted to the protective earth potential via a protective earth shield. Additionally or alternatively, a leakage current generated in the transformer can be diverted to the internal earth collection point via an internal earth shield of the transformer.

[0025] The concepts described herein may, among others, have the following advantages over the prior art or avoid corresponding disadvantages of the prior art:

[0026] One source of interference – particularly in a tangent-delta measurement – is the AC input voltage used (e.g., mains input voltage) and its multiplication (double or multiple) in the amplification path. For example, currents can be traced back to an (input bridge) rectifier in the power converter (where the rectifier is designed, for example, as a "power factor corrector" or "power factor correction filter"), even if the rectifier itself is coupled to protective ground via capacitors (EMC / EMV). Typically, the power converter uses an intermediate circuit to generate an RF signal from the AC input voltage with reference to a floating reference potential, also known as "floating power ground" (PGND). Interference resulting from the non-galvanically isolated DC link voltage can be capacitively coupled to the transformation and thus transferred to the current measurement.As a result, in non-isolated setups, frequency components that are due to the input AC voltage can propagate into the measurement signal.

[0027] Furthermore, when using a current collection point for an integrated tangent-delta measurement, leakage currents may occur within the transformers due to the high voltage, which must be considered as possible disturbances during the measurement.

[0028] The inventors recognized that using a filter with a corresponding bandwidth to remove such interference signals can also clip the useful signal and lead to a phase shift, which would also affect the tangent-delta measurement. Furthermore, they recognized that a filter cannot compensate for frequency fluctuations (e.g., caused by generator operation and / or 50 / 60 Hz grids) at the grid input, or only to a limited extent.

[0029] In contrast, the concept proposed by the inventors of equipping transformers with a "double" shield does not alter the useful signal. Rather, as explained below, frequency components attributable to the input AC voltage can be diverted to protective earth, and capacitively coupled leakage currents from the transformer can be diverted directly to internal earth.

[0030] Specifically, the inventors discovered that by connecting a shield of the primary winding of a transformer to protective earth, also referred to herein as a protective earth shield, interference currents can be avoided through the device under test or bypassed during the tangent-delta measurement. In other words, interference currents attributable to the input AC voltage are returned to the source by inserting a shield of the primary winding connected to protective earth. This protective earth shield directs the interference currents attributable to the input AC voltage back to the source.

[0031] The "double" shield configuration also includes a shield on the transformer's secondary winding, also referred to herein as the internal ground shield. This shield is electrically connected to the internal ground collection point, allowing leakage currents on the transformer's secondary side to bypass the measurement. In other words, leakage currents are coupled directly to the current collection point.

[0032] By introducing the protective earth shield and the internal earth shield, a sensitive tangent-delta measurement can be enabled. In general, the concepts proposed herein can further improve the accuracy of integrated diagnostic measurements (such as a tangent-delta measurement). They can also improve external diagnostic measurements (such as a partial discharge measurement).

[0033] The advantages of the concepts proposed herein are particularly evident with large test currents (load capacities), i.e., at higher powers and applied voltages. In particular, VLF test generators in power classes from approximately 500 W output power and test voltages from, for example, 45 kV can be used for the "low harmonics" measurement method described above (see also DE 10 2012 024 560 B3) thanks to the concepts proposed herein. (For powers in the range from, for example, 1 kW to 4 kW, even greater interference can be avoided if such high power is required.)

[0034] Disclosed herein are concepts 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: Fig. 1 a schematic representation of an exemplary test device for testing a high or medium voltage cable according to the inventive concept, Fig. 2 a schematic circuit diagram of an exemplary structure of a circuit arrangement in a test device, for example according to Fig. 1, Fig. 3 a flowchart of an example of a test procedure for a high or medium voltage cable, Fig. 4 a schematic representation of an exemplary structure of a transformer for use, for example, in a device as in Fig. 2 shown circuit arrangement in a test device for testing a high or medium voltage cable, Fig. 5 and Fig. 6 diagrams of a VLF test voltage, generated without a “double” shield configuration ( Fig. 5) and creates with “double” screen configuration ( Fig. 6), to illustrate the interference reduction and Fig. 7 and Fig. 8 diagrams of frequency spectra of generated VLF test voltages, generated without “double” shielding configuration ( Fig. 7) and creates with “double” screen configuration ( Fig. 8), to illustrate the interference reduction.

[0035] The invention particularly relates to the implementation of a tangent-delta measurement within a non-isolated VLF test device while avoiding AC interference signals and transformer leakage currents, whereby a test of a device under test / high- or medium-voltage cable can be carried out with high resolution and high sensitivity within the framework of the tangent-delta diagnosis.

[0036] The inventors propose a concept of double shielding of (high-performance) transformers used in a test device for testing a high-voltage or medium-voltage cable, which can be used instead of, for example, a complete potential separation that is complex in terms of circuitry, subject to power loss and expensive to implement - be it on the input side, in the DC link, between H-bridges and HV transformers or in the high-voltage path.

[0037] According to the invention, interference signals originating from the input AC voltage, such as line ripple, are fed back to protective earth potential, while simultaneously diverting a VLF leakage current to an internal earth potential. In this way, an interference signal coupled via the PGND is bypassed by the test object and is not included in the current measurement. The interference signal can be fed back by a shield (shield winding) surrounding the primary winding – in this case, a shield that runs all the way around but does not form a short-circuit winding. For example, the generation of a 100 Hz ripple on the measurement signal for tangent-delta diagnostics with a 50 Hz input AC voltage can be avoided.

[0038] However, leakage currents in the transformers used for high-voltage generation can affect the measurement signal for tangent-delta diagnostics. An internal ground shield, introduced according to the invention and shielding the secondary winding, can divert such leakage currents to the current collection point in a controlled manner. In a simple structural arrangement, a transformer core (e.g., a ferrite core) can be used as part of the internal ground shield and electrically connected to the internal ground potential.

[0039] Furthermore, the internal earth shield of the inventive double-shielded structure can be used for testing transformers. For example, a second measuring shunt (measuring impedance / measuring resistance between the shield of the secondary coil shield and the internal earth collector point) can be used to detect a leakage current in the transformer caused, for example, by faulty insulation in the transformer. Thus, using the double shielding, the functionality of the VLF test device's transformers can be monitored during operation.

[0040] Fig. Figure 1 shows a schematic representation of a portable test device 1 for testing a test object 3, e.g., a high- or medium-voltage cable, such as a coaxial cable, shown schematically here, according to the inventive concept. The test device 1 comprises a circuit arrangement 5, essentially a VLF test voltage generation unit 7 for generating a suitable (VLF) test voltage, and (measurement and) evaluation electronics 9 coupled to the circuit arrangement 5 and integrated into the test device. The evaluation electronics 9 are configured, for example, for a tangent-delta measurement (tangent-delta measurement unit 11) and optionally for transformer monitoring (transformer monitoring unit 13).The hardware (processing unit) underlying the evaluation electronics 9 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. The evaluation electronics 9 typically includes high computing power for real-time analysis of the continuously acquired and evaluated data sets, as well as long-term (non-volatile) memory for storing the program instructions and very fast short-term (volatile) memory for storing acquired data and evaluation results during (or resulting from) the data acquisition and data processing of low- and / or high-frequency signals.

[0041] The test device 1 is supplied with energy (power connection 15), for example, via the mains supply as source 14 (generally a supply voltage source for delivering an input AC voltage to the test device 1, e.g., a 50 Hz / 60 Hz power network or a regulated generator with adjustable frequencies in the range of, e.g., 40 Hz to 65 Hz). For the test, a conductor 3A of the test object 3 is connected to the test device 1 via a (HV) connection cable 17 (with exemplary lengths of 5 m to 15 m). A shield 3B of the test object 3 and the circuit arrangement 5 are connected to a protective earth potential 19. The power connection 15 can also comprise a line connected to protective earth (see, e.g., Fig. 2). Between the conductor 3A of the test object 3 and the shield 3B there is an insulation 3C of the test object 3 to be tested.

[0042] The circuit arrangement 5 is arranged in a housing 21 of the test device 1 and can 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). The circuit arrangement 5 can further be connected to, or (partially) integrated into, a controller 23 of the test device 1 provided in the test device 1 or entirely or partially outside the test device 1. The controller 23 and the high-voltage sources contained in the circuit arrangement 5 jointly generate the test voltage to be applied to the test object 3 by, for example, providing and controlling the power for the current sources required to regulate the test voltage via transformers and downstream cascade multipliers, see Fig. 2 for an example circuit arrangement.

[0043] For example, on a top side 21A of the housing 21, an operating display 25A (display) for displaying the acquired test data and at least one operating element 25B for setting a measurement parameter are shown schematically in Fig. 1. The operating display 25A and the operating element 25B, for example, form a user interface of the controller 23.

[0044] Fig. 2 shows an exemplary circuit diagram for the components of the circuit arrangement 5 provided in the housing 21 of the Fig. 1. The test object 3 is connected to a test object connection 27A, provided for example on the housing 21, via the connecting cable 17 and is further electrically connected to the protective earth potential 19 at the shield. To supply the circuit arrangement 5 with an input AC voltage, the test device 1 is connected to the supply voltage source 14 (see Fig. 1). The mains connection 27B includes, for example, a live, a neutral, and a grounded connection. Furthermore, Fig. 2 optionally a protective earth connection 27C, for example provided separately on the housing 21, for sufficient earthing of the housing 21 and for coupling the protective earth with low impedance to the DUT (e.g. for a tangent-delta measurement).

[0045] The circuit arrangement comprises a power converter 31, two transformers with cascade circuits (in Fig. 2 summarized in box 33) as high-voltage sources and a high-voltage circuit arrangement 35.

[0046] The power converter 31 is supplied with power via the mains connection 27B and includes a rectifier circuit 37 (AC / DC converter) that generates a DC voltage (of controlled amplitude) from the input AC voltage. The power converter 31 further includes two switching power supplies 39 (DC / AC converters) that are inversely electrically connected to DC voltage outputs 37A, 37B of the rectifier circuit 37. The rectifier circuit 37 and the switching power supplies 39 represent an example of a low-voltage circuit arrangement that, based on the input AC voltage, generates two inverse RF signals for the two high-voltage sources 33 at two pairs of RF outputs 39A, 39B of the switching power supplies 39.

[0047] The outlined, non-potentially isolated design of the power converter 31 particularly applies to VLF test generators for higher power / high voltages, which use a power factor corrector (PFC) on the mains input side to rectify an AC voltage provided by the mains (or another source such as mobile generators) and to provide a DC voltage for further amplification. The output voltage for the power range 1 kW to 4 kW is, for example, 400 V. Due to the lack of potential isolation, a voltage reference point of the output DC voltage is not at protective earth potential, but represents a PGND, whose potential, compared to protective earth potential, results from the rectifier circuit at half the input voltage. Due to the technically induced capacitive coupling, the PGND can produce a ripple of twice ormultiple input frequency, which - if not addressed - can lead to corresponding effects on the measurement.

[0048] The high-voltage sources 33 are configured to provide a positive (+) or negative (-) high voltage of variable amplitude at their respective outputs 33A, 33B, for example, by operating a modulation at a multiple of the mains frequency. The high-voltage sources 33 each comprise a (high-voltage) transformer 41, 43 for transforming the RF signal into a high-voltage signal present at a respective high-voltage output 41A, 43A of the respective transformer 41, 43. For example, amplifications from, for example, 400 V (at the RF output pair) to, for example, 13 kV (at the high-voltage output 41A, 43A) are possible. The high-voltage sources 33 further each comprise a rectifier circuit 45 electrically connected to the high-voltage output 41A, 43A for outputting a rectified high-voltage signal at the outputs 33A, 33B of the high-voltage sources 33.

[0049] The high-voltage circuit arrangement 35 is provided between the outputs 33A, 33B of the high-voltage sources 33 and the test object 3. The high-voltage circuit arrangement 35 is designed to form the VLF test voltage based on the rectified high-voltage signal. The high-voltage circuit arrangement 35 is acted upon by a control unit 47 for the defined charging and discharging of the test object 3, which represents a certain capacitive load. The control unit 47 is designed to ensure a preferably sinusoidal voltage curve at the test object 3. In the exemplary embodiment shown in Fig. In the embodiment shown in Figure 2, the high-voltage switch arrangement 35 comprises, for example, semiconductor switch cascades 49 and amplifiers 51, on which the control unit 47 acts.

[0050] Furthermore, the switching power supplies 39 of the low-voltage circuit arrangement are controlled by a controller 53 using a clock signal generator T, so that the two switching power supplies 39 in combination with the high-voltage sources 33 can each provide a test voltage that is synchronized by means of the clock signal generator T, can be defined in terms of curve shape and amplitude, is advantageously edge-free and in particular sinusoidal, and in particular is not influenced by the control unit 47.

[0051] For further details on the generation and control of the test voltage as well as other design alternatives for the circuit arrangement, reference is made to the aforementioned DE 10 2012 024 560 B3 and DE 195 13 441 A1.

[0052] Essential to the VLF concept of tangent-delta measurement is that on the high-voltage side of the transformers 41, 43, a low-voltage ground input, referred to herein as internal ground collection point 55, and the high-voltage circuit arrangement 33 includes a high-voltage output, here the test object terminal 27A. The high-voltage output is electrically connected to conductor 3A for testing (see Fig. 1) of the test object 3. The low-voltage ground input represents an internal ground potential during operation and is electrically connected to the protective earth terminal 27C, whereby a measurement signal for the tangent-delta diagnosis can be acquired at this electrical connection. For testing using tangent-delta diagnosis, the shield 3B (see Fig. 1) of the test object 3 is connected to protective earth, so that the shield 3B and the VLF test device 1, in particular the test voltage generation unit 7, are at a common protective earth potential 19. In the event of a defect in the test object 3, if the protective earth terminal 27C is connected to the same earth potential (protective earth potential 19), an electrical circuit can form through which a measuring current flow can develop and which extends from the high-voltage circuit arrangement 35 via the test object 3, in particular a defect, and through the protective earth terminal 27C via the internal earth collection point 55 back to the high-voltage side of the transformers 41, 43.

[0053] As in Fig. 2 shows the measurement current at a connecting conductor 57, which electrically connects the low-voltage ground input (internal ground connection point 55) to the protective ground terminal 27C, accessible for measurement within the test device 1 and particularly advantageously in a low-voltage environment. The connecting conductor 57 can be used, for example, for a low-frequency signal tap 59, for example via an impedance, to generate a diagnostic signal. The diagnostic signal can be used as part of analog and / or digital signal processing in the tangent-delta measuring unit 11 to determine the phase and thus the power loss.

[0054] The measuring process is exemplified in connection with Fig. 3 summarized. According to a method for testing a device under test (e.g. the high-voltage or medium-voltage cable), a test voltage is generated using the test device 1 (step 101). With the high-voltage or medium-voltage cable connected, the test voltage causes a measuring current in the connecting conductor 57 (step 103). Since the connecting conductor 57 electrically connects the internal earth collecting point 55 to the protective earth connection 27C, a measuring current flows through the connecting conductor 57 during the testing of the device under test (e.g. the high-voltage or medium-voltage cable). Due to the applied test voltage, this current forms between the internal earth collecting point 55 and the shield 3B of the device under test / high-voltage or medium-voltage cable, which is also at protective earth potential 19. With the low-frequency signal tap at the current collecting point, a low-frequency diagnostic signal assigned to the measuring current is generated, for example for B. a VLF phase rotation measurement is generated (step 105).An evaluation of the low-frequency diagnostic signal in an evaluation electronics, for example the tangent-delta measuring unit 11, checks whether there is a fault in the high-voltage or medium-voltage cable (step 107).

[0055] In contrast to low-power VLF test generators, which can usually be implemented with purchased potential-isolated power supplies, VLF test generators for higher power / high voltages usually do not implement potential isolation of the input voltage to the DC link, since potential-isolated power supplies are large and cost-intensive.

[0056] The previously discussed lack of potential isolation in VLF test generators for higher power / high voltages requires the presence of the PGND in the intermediate circuit, as a connection to protective earth potential is not possible due to the input-side rectifier. Due to the PGND reference, an output and smoothed voltage signal may contain an AC voltage component with a frequency twice the frequency of the input AC voltage (e.g., mains frequency) and corresponding harmonics (multiples of the mains frequency) relative to the protective earth potential. This means that, for technical reasons, harmonic currents are not suppressed in non-isolated test generator designs.

[0057] In common circuit arrangements of VLF test generators for higher power / high voltages, (high-voltage) transformers are also used in the voltage generation path, see transformers 41, 43 in Fig. 2. Each transformer provides an (inductive) primary-secondary coupling to amplify the voltages to, for example, +10 kV or -10 kV. Without countermeasures, twice or multiples of the input frequency (based on the incoming AC output voltage) can be applied to the measuring current through the device under test via the capacitive coupling of the transformers and thus influence the diagnostic signal via the current measurement (current shunt). At high voltages, interference signals occur with—relative to the useful signal—up to four times the amplitude of the useful signal. Referring to Fig. 2, these interference currents (mains-related interference) have their origin in the AC / DC converter of the power converter 31.

[0058] Since such interference currents have similar frequencies to the measurement currents recorded in the tangent-delta measurement, they can influence the measurement result. Common (AC) power sources - such as a 50 Hz / 60 Hz power supply network or a regulated generator with frequencies in the range of 40 Hz to 65 Hz - can lead to doubled frequencies in the range of 80 Hz to 130 Hz and the associated integer multiples, for example up to the tenth harmonic, as interference currents. In other words, the resulting frequencies are in the range of, for example, 0.8 kHz to 1.3 kHz, for example 1 kHz or 1.2 kHz for the tenth harmonic. Signal sampling in the tangent-delta measurement, for example, uses a sampling rate in the range of, for example, B. 5 kHz to 100 kHz to capture a signal bandwidth of 5 kHz to 50 kHz, so that such source-related interference signals can also be captured.In other words, the frequency range of a network-related interference signal lies within the bandwidth of the tangent-delta measurement.

[0059] As explained, for the concepts described here, the low-voltage side of the transformers is not potential-decoupled from the voltage source, so that in principle a ripple (harmonics of the input AC voltage) can develop on the high-voltage side as a disturbance current that interferes with the measurement.

[0060] In VLF test generators for higher power / high voltages, leakage currents can also occur on the high-voltage side of the transformers due to the high voltages. This is because leakage currents can be inherent in high-voltage transformers at differential voltages greater than, for example, 1 kV. Furthermore, such leakage currents are variable, particularly temperature- and load-dependent, and thus represent a dynamic error. Their influence on, for example, a tangent-delta measurement is therefore unpredictable.

[0061] The inventors have recognized that these disturbances are not only caused by the power drawn, but that the power class (and thus the topology) of the power electronics can also lead to the disturbances.

[0062] Both mains-related interference currents and high-voltage leakage currents can, in principle, affect the tangent-delta measurement (the diagnostic signal).

[0063] Within the framework of the double-shield concept, the inventors have now implemented a concept to prevent such interference. The adverse effects of interference and / or leakage currents can be reduced, preferably avoided, by specifically dissipating the interference and leakage currents through specially configured tuning within the transformer.

[0064] Fig. Figure 2 shows, on the one hand, an exemplary implementation for diverting network-related interference currents and, on the other hand, two exemplary implementations for diverting leakage currents. An exemplary configuration of a transformer 61 for use according to the invention in a test device is shown in Fig. 4, where the specific structure is exemplified by the transformer 41 in Fig. 2. Comparable structural components were found in the Fig. 2 and Fig. 4 are provided with the same reference numerals.

[0065] Generally, each of the transformers 41, 43, 61 (generally each transformer) comprises at least one coil pair comprising a primary winding 63 and a secondary winding 65. Winding ends of the primary winding 63 are connected to the RF output pair (in Fig. 2 the RF outputs 39A, 39B) are electrically connected and are accordingly, for example, at a maximum of 400 V. A first winding end of the secondary winding 65 is connected to the internal earth potential (in Fig. 2 the internal earth collector point 55) and a second winding end of the secondary winding 65 is connected to the high voltage output (in Fig. 2 high-voltage outputs 41A, 43A) for outputting the high-voltage signal (with voltages of, for example, more than 3 kV, e.g. up to 20 kV) are electrically connected.

[0066] The windings are wound, for example, in trapezoidal arrangements with multiple layers as primary or secondary coils, one after the other on an annular winding body 67 (made of plastic, for example) and encapsulated, for example, with casting resin. The winding body 67 serves, among other things, for mechanical stabilization during production / winding of the coils. The winding body 67 also separates and insulates the primary coil from the core. Fig. Figure 4 shows the winding body 67 with a U-shaped cross-section. Distances between the coils are, for example, a few millimeters for voltage differences of several tens of kV.

[0067] In the example of Fig. 4, the arrangement of primary winding 63 and secondary winding 65 is radially delimited by a transformer core 69. The transformer core 69 facilitates the generation of several kilovolts of secondary voltage. The transformer core 69 is designed, for example, as a ferrite core with two half-shells.

[0068] An insulating film 70, for example a polyethylene terephthalate film or a film consisting of polycondensed aromatic dianhydride and aromatic diamine, is arranged between the primary winding 63 and the secondary winding 65 for electrical insulation. Particularly in an inventive implementation of the double-shield concept, the insulating film 70 serves as low-voltage insulation.

[0069] The double-shield concept uses, on the one hand, a protective earth shield 71. The protective earth shield 71 is designed to shield the primary winding 63 and encloses it circumferentially, but in particular without forming a short-circuit winding. The protective earth shield 71 is capacitively coupled to the primary winding 63 and electrically connected to the protective earth potential 19. The protective earth shield 71 is used to divert interference current caused by harmonics to the protective earth potential 19.

[0070] The double shield concept also uses an internal ground shield 73. The internal ground shield 73 is designed to shield the secondary winding. For this purpose, the internal ground shield 73 is capacitively coupled to the secondary winding 65 and electrically connected to the internal ground collector point 55. The internal ground shield 73 is used to absorb leakage currents, in this case, for example, an RF high-voltage leakage current generated in the transformer, and to transfer them to the internal ground potential in the structure of the Fig. 2 to the internal earth collection point 55.

[0071] The protective earth shield 71 and / or the internal earth shield 73 are each formed, for example, as a circumferential copper strip (generally made of a highly conductive material), wherein in particular no short-circuit winding is formed.

[0072] In general, a compact design is more efficient, so the transformer filling ratio can be kept high. Fig. 4 shown and in Fig. In the embodiment schematically indicated for transformer 41 in Figure 2, the internal ground shield 73 is partially realized by the transformer core 69. This makes it possible to keep the structure of the HV transformers compact while maintaining the insulation distances. Since the primary winding 63 is enclosed by the protective ground shield 71, there is no "power-ground coupling" from the primary side to the core. Furthermore, there is usually only a voltage difference of a few volts (e.g., 5 V) between the protective ground shield 71 and the internal ground shield 73, so the insulation requirements for the insulating film 70 are reduced.

[0073] Alternatively, as for transformer 43 in Fig. 2, an internal earth shield completely surrounding the secondary coil (circumferential, but not forming a short-circuit winding) can be provided. The introduction of such a shield winding can ensure the maintenance of an appropriate insulation distance as well as a voltage-resistant feedthrough of the high-voltage outputs 41A, 43A (see Fig. 2) and therefore require a larger spatial structure.

[0074] Based on the double-shield concept, information regarding the quality of a transformer insulation can be obtained during operation of the test device 1, in particular during a tangent-delta measurement. In particular, a structural fault in the transformer insulation can be detected in this way. For this purpose, a signal tap, for example, in Fig. 2, a leakage current measuring impedance 75 may be provided, which electrically connects the internal earth shield 73 of the secondary winding 65 of the transformer to the internal earth collector point 55. Using, for example, the leakage current measuring impedance 75, a leakage current in the transformer can be continuously detected and evaluated. The evaluation includes, for example, a plausibility check or a temporal development of the detected leakage currents.

[0075] Fig. Figure 3 illustrates, in addition to the test method of a test object, a method for testing a transformer arranged in a test device, in which an internal earth shield of a secondary winding of the transformer is electrically connected to an internal earth collector point of the test device via a signal tap (e.g., a leakage current measuring resistor or a capacitive coupling). For the test, a test voltage is generated with the test device (step 101). For example, a leakage current is measured with the leakage current measuring resistor (step 111). The leakage current is evaluated in the evaluation electronics of the test device for insulation damage in the transformer (step 113).

[0076] The Fig. Figures 5 to 8 illustrate the effects of using a double-shield concept according to the invention. If transformers with a double-shield structure are used in a test device, interference currents can be largely suppressed or returned to the source. This significantly improves the quality of the measurement, particularly for high voltages. To illustrate that the effects addressed by the double shield, as described above, can manifest as interference in the measurement current and diagnostic signal, the current was additionally analyzed with an oscilloscope in a measurement at the current shunt (measurement resistor / signal 59), each for a transformer without and with a double-shield structure.

[0077] The Fig. 5 and Fig. Figure 6 shows the time course of the applied voltage 81 and the corresponding high-frequency time course of the measured currents 83, recorded with an oscilloscope. Without the double shielding concept, interference currents of up to 40 µA can be detected. When the double shielding concept is used for the transformer, the interference currents are significantly reduced to just a few µA.

[0078] In the in the Fig. 7 and Fig. In the spectra 85 and 86 shown in Figure 8, the dominant frequency 87 is the 0.1 Hz test frequency, which is used for VLF testing and diagnosis. The second and third highest frequencies 88, 89 in spectrum 85 are in Fig. 7 (ie without using the double shielding concept) at 50 Hz and 100 Hz (fundamental and harmonic) respectively and represent the interference caused by the mains voltage at 50 Hz and coupled to the measuring current. Due to the double shielding of the primary and secondary windings of the transformer, Fig.9 these frequencies are no longer in spectrum 86 when the double-screen concept is used.

[0079] As can be seen, the double shielding concept can also reduce interference frequencies 91 and 93 (see spectrum 85), which can be generated, for example, by other network consumers.

[0080] The features of the methods and devices referred to as "unit," "apparatus," or similar in this description 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 into 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.

[0081] The features specifically recited in the claims can thus be embodied 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 "evaluation unit" disclosed herein can be embodied in the structure of a central processing unit (CPU) configured with instructions for performing the operations for deriving fundamental wave information. The CPU can comprise one or more microprocessors in conjunction with one or more memory elements.A memory element can store one or more microprocessor-readable instructions (programs) that, when executed by the microprocessor, perform, for example, the Fourier or wavelet transformation. Furthermore, the measurement and evaluation unit 9 and the controller can comprise various units that interact with each other to perform the desired actions, such as receiving, accessing, and / or transmitting data sets, identifying maxima, etc.

[0082] 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 feature combinations 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 subgroup 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

[1] Test device (1) for testing a test object (3), in particular a high-voltage or medium-voltage cable, by means of a test method using a very low frequency (VLF) test voltage, comprising: a power converter (31), wherein the power converter (31) comprises: - an AC input for receiving an AC input voltage with reference to a protective earth potential (19), - a low-voltage circuit arrangement comprising a rectifier circuit (37) and at least one switching power supply (39) which is designed to generate an RF signal with reference to a floating reference potential from the input AC voltage, and - at least one RF output pair (39A, 39B) for outputting the RF signal and the floating reference potential, a transformer (41, 43, 61) for transforming the RF signal into a high-voltage signal, the transformer (41, 43, 61) comprising: - a primary winding (63) whose winding ends are electrically connected to the RF output pair, - a protective earth shield (71) for shielding the primary winding (63), wherein the protective earth shield (71) is capacitively coupled to the primary winding (63) and electrically connected to the protective earth potential (19), - a secondary winding (65), the first winding end of which is electrically connected to an internal earth collector point (55) and the second winding end of which is electrically connected to a high-voltage output (41A, 43A) for outputting the high-voltage signal, and - an internal earth shield (73) for shielding the secondary winding (65), wherein the internal earth shield (73) is capacitively coupled to the secondary winding (65) and electrically connected to the internal earth collection point (55), a rectifier circuit (45) electrically connected to the high-voltage output (41A, 43A) for outputting a rectified high-voltage signal, and a high-voltage circuit arrangement (35) for generating the VLF test voltage based on the rectified high-voltage signal. [2] Test device (1) according to claim 1, wherein an intermediate circuit for generating a DC voltage with respect to the floating reference potential is formed in the low-voltage circuit arrangement, wherein the DC voltage is subject to harmonics as a function of the received input AC voltage and the protective earth shield (71) capacitively coupled to the primary winding (63) is designed to divert an interference current caused by the harmonics to the protective earth potential (19). [3] Test device (1) according to claim 1 or 2, wherein the internal earth shield (73) capacitively coupled to the secondary winding (65) is designed to divert an RF high-voltage leakage current generated in the transformer (41, 43, 61) to the internal earth collection point (55). [4] Test device (1) according to one of the preceding claims, wherein the transformer (41, 43, 61) further comprises a transformer core (69) formed as part of the internal earth shield (73) capacitively coupled to the secondary winding (65) and electrically connected to the internal earth collection point (55). [5] Test device (1) according to one of the preceding claims, wherein the transformer (41, 43, 61) further comprises a winding body (67), in particular one with a U-shaped cross section, on which a primary winding (63) embedded in the protective earth shield (71), an insulating film (70), the internal earth shield (73) and the secondary winding (65) are arranged, and wherein the winding body (67) is arranged in particular between sections of a core of the transformer (41, 43, 61) and / or wherein the primary winding (63) is delimited radially on both sides by the protective earth shield (71) and / or the secondary winding (65) is delimited radially on both sides by the internal earth shield (73). [6] Test device (1) according to one of the preceding claims, wherein the transformer (41, 43, 61) and the rectifier circuit (45) form a high-voltage source (33) and the high-voltage circuit arrangement (35) comprises at least one semiconductor switch cascade (49) and at least one amplifier (51) which are assigned to the transformer (41, 43, 61). [7] Test device (1) according to one of the preceding claims, wherein the low-voltage circuit arrangement comprises two switching power supplies (39) which are designed such that a positive and a negative RF signal are output at corresponding RF output pairs (39A, 39B), the test device (1) comprises two transformers (41, 43, 61), each comprising a protective earth shield (71) and an internal earth shield (73), for transforming the positive and negative RF signals into a positive and a negative high-voltage signal, and two rectifier circuits (45) electrically connected to high-voltage outputs of the transformers (41, 43, 61) for outputting a rectified positive and a rectified negative high-voltage signal, and wherein the high-voltage circuit arrangement is configured to generate the VLF test voltage based on the positive rectified high-voltage signal and the negative rectified high-voltage signal. [8] Test device (1) according to one of the preceding claims, wherein the high-voltage circuit arrangement comprises a test object connection (27A) for connecting a high-voltage side output of the high-voltage circuit arrangement to a conductor (3A) of the test object (3), in particular via a measuring connection cable (17). [9] Test device (1) according to one of the preceding claims, further comprising: a protective earth connection (27C), a connecting conductor (57) which electrically connects the internal earth connection point (55) to the protective earth connection (27C), so that the connecting conductor (57) is traversed by a measuring current during the testing of the test object (3), which current is formed between the internal earth connection point (55) and a shield (3B) of the test object (3) which is also at protective earth potential (19), a low-frequency signal tap (59) at the current collection point, at which a low-frequency diagnostic signal is generated based on the measuring current, and an evaluation electronics unit (9) which is connected to the low-frequency signal tap (59) for receiving the low-frequency diagnostic signal and is designed for a VLF phase rotation measurement. [10] Test device (1) according to claim 9, wherein the low-frequency signal tap (59) is designed to detect an instantaneous value of a current in the connecting conductor (57), and / or comprises an impedance in the connecting conductor (57), in particular a parallel circuit comprising a resistor and a capacitor, and / or the evaluation electronics (9) are designed to determine a loss factor assigned to the test object (3) from the diagnostic signal, and are designed in particular to detect low frequencies of the diagnostic signal in the range from 0.01 Hz to 1 Hz with a sampling rate with frequencies in the range of in particular 5 kHz to 100 kHz, and / or the evaluation electronics (9) comprises at least one analog and / or digital signal processing unit and / or a processor and / or a buffer memory. [11] Test device (1) according to one of the preceding claims, further comprising: a signal tap, in particular a leakage current measuring impedance (75) or a leakage current measuring resistor, which electrically connects the internal earth shield (73) of the secondary winding (65) of the transformer (41, 43, 61) to the internal earth collection point (55) and is provided for detecting a structural fault in an insulation of the transformer (41, 43, 61). [12] Method for testing a test object (3), in particular a high-voltage or medium-voltage cable, using a test device (1) according to one of claims 1 to 11, comprising the steps: Generating (step 101) a test voltage with the test device (1), Effecting (step 103) a measuring current with the test voltage with the test object (3) connected in a connecting conductor (57) which electrically connects an internal earth collecting point (55) of the test device (1) to a protective earth terminal (27C) of the test device (1), so that the connecting conductor (57) is traversed by the measuring current during the testing of the high-voltage or medium-voltage cable, which current forms between the internal earth collecting point (55) and a shield (3B) of the test object (3) which is also at protective earth potential (19), Generating (step 105) a low-frequency diagnostic signal associated with the measuring current (83) for a VLF phase rotation measurement with a low-frequency signal tap (59) on the connecting conductor (57), and Evaluating (step 107) the low-frequency diagnostic signal in an evaluation electronics (9) with regard to a fault in the high-voltage or medium-voltage cable. [13] The method of claim 12, wherein an interference current caused by harmonics of a received input alternating voltage is diverted via a protective earth shield (71) to the protective earth potential (19) and / or a leakage current generated in the transformer (41, 43, 61) is diverted via an internal earth shield (73) of the transformer (41, 43, 61) to the internal earth collection point (55). [14] Method for testing a transformer (41, 43, 61) arranged in a test device (1) according to one of claims 1 to 11, wherein an internal earth shield (73) of the secondary winding (65) of the transformer (41, 43, 61) is electrically connected to an internal earth collector point (55) of the test device (1) via a leakage current measuring impedance (75), comprising the steps of: Generating (step 101) a test voltage with the test device (1), Measuring (step 111) a leakage current with the leakage current measuring impedance (75), and Evaluating (step 113) the leakage current in an evaluation electronics (9) of the test device (1) with regard to insulation damage of the transformer (41, 43, 61).

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