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

Abstract

A test apparatus (1) for testing a test object (3) (e.g., a high-voltage or medium-voltage cable) includes: a power converter (31); a transformer (41, 43, 61) for converting an HF signal into a high-voltage signal, wherein the transformer (41, 43, 61) includes: a primary winding (63); a protective ground shield (71) for shielding the primary winding (63), wherein the protective ground shield (71) is capacitively coupled to the primary winding (63) and electrically connected to a protective ground potential (19); a secondary winding (65); and an internal ground shield (73) for shielding the secondary winding (65), wherein the internal ground shield (73) is capacitively coupled to the secondary winding (65) and electrically connected to an internal ground collection point (55). The test apparatus (1) also includes: a rectifier circuit (45) electrically connected to a high-voltage output (41A, 43A) for outputting a rectified high-voltage signal; and a high-voltage circuit arrangement (35) for generating a VLF test voltage based on the rectified high-voltage signal.

Description

Technical Field

[0001] This invention relates to a testing apparatus for testing test objects (e.g., high-voltage or medium-voltage cables), particularly for testing the insulation layer in coaxial cables, for example, used to distribute current / energy in a power supply network, using the VLF testing method. The invention also relates to a method for testing transformers in such testing apparatus. Background Technology

[0002] Testing of high-voltage or medium-voltage cables (e.g., cables laid underground or across water in the context of regional energy networks) includes tests conducted to identify potential defects or pre-damage that may form in, for example, the insulation of high-voltage cables. Exemplary defects further include defects in the cable insulation, such as water trees or electrical trees, which may not yet result in breakdown or may break down during VLF testing. High-voltage or medium-voltage cables (also referred to herein as test objects or devices under test (DUTs)) are typically designed to distribute energy / power using voltages in high-voltage and medium-voltage ranges, starting from about 1 kV and extending to several hundred kV (or higher), for example, in power networks used for power supply. Energy / power distribution can, for example, be carried out over distances ranging from several hundred meters to tens of kilometers (at power capacities exceeding 1 GW and voltages up to 500 kV).

[0003] Regarding the presence of pre-damage, mobile VLF testing equipment allows for testing of the test object using a test voltage, for example, a test voltage in the range of 20 kVpeak to 120 kVpeak (typically unlimited), generated using a high-precision sinusoidal voltage distribution at a frequency in the range of 0.01 Hz to 1 Hz (i.e., the so-called Very Low Frequency (VLF)), and applied to the conductor of the test object, which is an energy cable, relative to the protective earth potential (PE). VLF-based testing methods are known and are defined, for example, in IEEE 400.2. Crucially for VLF-based testing methods is that the measurement of the test current is as undisturbed as possible, wherein the test current follows the voltage distribution of the generated kV test voltage.

[0004] For voltage generation, VLF test equipment includes a special VLF high-voltage source (also referred to as one or more VLF high-voltage sources). The VLF high-voltage source may preferably include two high-voltage sources (transformers with cascaded circuitry) and an output amplifier (in the form of a current source), which together produce a highly undisturbed sinusoidal output voltage (“test voltage”). Circuit arrangements for generating such test voltages have been disclosed, for example, in the applicant’s DE 10 2012 024 560 B3 or DE 195 13441 A1.

[0005] To diagnose the presence of pre-damage (e.g., water tree), the loss factor can be determined in a so-called loss tangent measurement. For loss tangent measurements—even though the test object is connected to protective ground—current through a defective DUT can be detected, meaning the test object does not need to be disconnected from protective ground. For example, a current sensing element (e.g., as the impedance of a current shunt) for loss tangent measurements can be located inside a VLF test generator. Using this current sensing element, the current and phase through the test object can be measured / detected at a so-called internal ground collection point (also referred to herein as the current collection point), particularly between the protective ground potential and the internal ground potential. In other words, the current collection point is selected in the VLF test generator such that current flows from the test object through the current collection point back to the high-voltage cascade. For example, DE 10 2012 024 560 B3 discloses an advantageous arrangement for high-accuracy diagnostic measurements, in which a high-accuracy test current can be generated and measured by means of a central clock signal and a sinusoidal envelope of the test voltage. For example, the test current is detected using a resistor 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 phase shift, for example by Fourier transform (e.g., Discrete Fourier Transform (DFT)).

[0006] As also disclosed in DE 10 2012 024 560 B3, the improvement in test voltage quality leads to improved measurability of the loss tangent. However, with increased power of the VLF test generator, interference signals at the current collection point also increase, especially when non-potentially isolated intermediate circuitry is used to convert the input AC voltage into an HF signal relative to a floating reference potential. In the latter case, necessary potential isolation is typically provided during the generation of high voltage for (multiple) cascaded circuits using a transformer (HV transformer). (See also the description below for more information on this.) Summary of the Invention

[0007] One aspect of this disclosure is based on the objective of providing an apparatus and method for measuring loss tangent, wherein interferences to the test current (used for measurement) passing through the test object (in particular, parasitic currents caused by harmonics of the mains voltage and / or leakage current of the transformer) are reduced.

[0008] Another aspect of this disclosure is based on the objective of providing a compact and cost-effective setup for testing high-voltage or medium-voltage cables, which enables safe, simple, and cost-effective testing methods to be performed.

[0009] At least one of these objectives is achieved by the testing equipment for testing test objects (particularly high-voltage or medium-voltage cables) according to claim 1, and by the method according to claim 12 or the method according to claim 14. Further variations are described in detail in the dependent claims.

[0010] In one aspect, a test apparatus for testing a test object (e.g., a high-voltage or medium-voltage cable) using a test method employing a very low frequency (VLF) test voltage includes a power converter. The power converter includes: an AC voltage input for receiving an input AC voltage relative to a protective ground potential; a low-voltage circuit arrangement including a rectifier circuit and at least one switching power supply, wherein the low-voltage circuit arrangement is configured to generate an HF signal relative to a floating reference potential from the input AC voltage; and at least one HF output pair for outputting the HF signal and the floating reference potential. The test apparatus also includes a transformer for converting the HF signal into a high-voltage signal. The transformer includes: a primary winding, the ends of which are electrically connected to the HF output pair; a protective ground shield for shielding the primary winding, wherein the protective ground shield is capacitively coupled to the primary winding and electrically connected to the protective ground potential; a secondary winding, the first winding end of which is electrically connected to an internal ground collection point, and the second winding end of which is electrically connected to the high-voltage output for outputting a high-voltage signal; and an internal ground shield for shielding the secondary winding, wherein the internal ground shield is capacitively coupled to the secondary winding and electrically connected to the internal ground collection point. The test equipment also includes: 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 a VLF test voltage based on the rectified high-voltage signal.

[0011] On the other hand, a method for testing a test object (such as a high-voltage or medium-voltage cable) using such testing equipment includes the following steps:

[0012] -Use testing equipment to generate test voltage.

[0013] - In the connecting conductor, when a high-voltage or medium-voltage cable is connected, a test voltage induces a measuring current. This connecting conductor electrically connects the internal grounding collection point of the test equipment to the protective grounding connection of the test equipment. This ensures that when the object under test (e.g., a high-voltage or medium-voltage cable) is tested, a test current flows through the connecting conductor. This measuring current is formed between the internal grounding collection point and the shielding layer of the object under test (e.g., a high-voltage or medium-voltage cable), which is also at the protective grounding potential.

[0014] - A low-frequency diagnostic signal is generated using a low-frequency signal tap on the connecting conductor and assigned to the measurement current for use in VLF phase rotation measurements, and

[0015] - Evaluate low-frequency diagnostic signals in the assessment of faults in high-voltage or medium-voltage cables in electronic devices.

[0016] On the other hand, a method for testing a transformer arranged in such a test apparatus, wherein the internal grounding shield of the transformer's secondary winding is electrically connected to the internal grounding collection point of the test apparatus via a leakage current measurement impedance, the method comprising the following steps:

[0017] -Use testing equipment to generate test voltage.

[0018] - Measure leakage current using leakage current measurement impedance, and

[0019] - Evaluate leakage current in the context of transformer insulation damage in the evaluation electronics of the test equipment.

[0020] In some embodiments of the test equipment, an intermediate circuit for generating a DC voltage relative to a floating reference potential may be formed in a low-voltage circuit arrangement, wherein the DC voltage is affected by harmonics depending on the received input AC voltage, and a protective ground shield capacitively coupled to the primary winding is configured to discharge parasitic currents caused by harmonics to the protective ground potential.

[0021] In some embodiments of the test equipment, the internal grounding shield layer capacitively coupled to the secondary winding can be configured to discharge the HF high-voltage leakage current generated in the transformer to the internal grounding collection point.

[0022] In some embodiments of the test equipment, the transformer may also include a transformer core configured as a portion of an internal grounded shielding layer of the secondary winding and electrically connected to an internal grounded collection point.

[0023] In some embodiments of the test equipment, the transformer may further include: a winding body, particularly having a U-shaped cross-section, with a primary winding arranged on the winding body and embedded in a protective grounding shield; an insulating film; an internal grounding shield; and a secondary winding; wherein the winding body may be particularly arranged between sections of the transformer core, and / or wherein the primary winding may be radially defined on both sides by the protective grounding shield, and / or the secondary winding may be radially defined on both sides by the internal grounding shield.

[0024] In some embodiments of the test equipment, the power converter and transformer can form a high-voltage source, and the high-voltage circuit arrangement can include at least one cascaded semiconductor switch assigned to the transformer and at least one amplifier.

[0025] In some embodiments of the test apparatus, the low-voltage circuitry may include two switching power supplies configured to output a positive HF signal and a negative HF signal at corresponding HF output pairs. Furthermore, the test apparatus may include: two transformers, each including a protective ground shield and an internal ground shield, for converting the positive and negative HF signals into positive and negative high-voltage signals; and two rectifier circuits electrically connected to the high-voltage outputs of the transformers for outputting rectified positive and negative high-voltage signals. Specifically, the high-voltage circuitry may be configured to generate a VLF test voltage based on the rectified positive and negative high-voltage signals.

[0026] In some embodiments of the test equipment, the high-voltage circuit arrangement may include a test object connection for connecting the high-voltage side output of the high-voltage circuit arrangement to a conductor of the test object (e.g., a high-voltage or medium-voltage cable), particularly via a measurement connection cable.

[0027] In some embodiments of the test apparatus, the test apparatus may further include: a protective grounding connection; a connecting conductor that electrically connects an internal grounding collection point to the protective grounding connection such that when the test object (e.g., a high-voltage or medium-voltage cable) is tested, a measurement current flows through the connecting conductor, the measurement current being formed between the internal grounding collection point and the shielding layer of the test object (e.g., a high-voltage or medium-voltage cable), the shielding layer also being at the protective grounding potential; a low-frequency signal tap on the current collection point, at which a low-frequency diagnostic signal is generated based on the measurement current; and evaluation electronics connected to the low-frequency signal tap to receive the low-frequency diagnostic signal and configured for VLF phase rotation measurement.

[0028] In some embodiments of the test equipment, low-frequency signal taps may be configured to detect instantaneous values ​​of current intensity in the connecting conductors, and / or include impedances in the connecting conductors, particularly parallel circuits of resistors and capacitors. Additionally or alternatively, the evaluation electronics may be configured to determine the loss factor assigned to high-voltage or medium-voltage cables based on diagnostic signals. Specifically, it may be designed to detect low-frequency diagnostic signals in the range of 0.01 Hz to 1 Hz at sampling rates, particularly in the range of 5 kHz to 100 kHz. Additionally or alternatively, the evaluation electronics may include at least one analog and / or digital signal processing unit and / or processor and / or buffer memory.

[0029] In some embodiments, the test equipment may include signal taps, particularly leakage current measuring impedances or leakage current measuring resistors, which electrically connect the internal grounding shield of the transformer's secondary winding to an internal grounding collection point and are provided for detecting structural faults in the transformer's insulation.

[0030] In some embodiments of the method for testing a test object (e.g., a high-voltage or medium-voltage cable), parasitic currents caused by harmonics of the received input AC voltage can be discharged to the protective ground potential via a protective grounding shield. Additionally or alternatively, leakage currents generated in a transformer can be discharged to an internal grounding collection point via the transformer's internal grounding shield.

[0031] Compared with existing technologies, the concepts described herein can have the following advantages or avoid the corresponding disadvantages of existing technologies:

[0032] One source of interference—especially in the case of loss tangent measurements—is the input AC voltage (e.g., mains input voltage) used in the amplification path and its multiplication (double or more). For example, current can be attributed to the rectifier (input bridge) in the power converter (where the rectifier is embodied as, for example, a "power factor corrector" or "power factor correction filter"), even though the rectifier itself is coupled to protective ground via a capacitor (EMC / EMC). Power converters typically use intermediate circuitry to generate an HF signal relative to a floating reference potential from the input AC voltage, where the HF signal is also referred to as "floating power ground" (PGND). Interference generated by the voltage of the non-potentially isolated intermediate circuitry can capacitively couple to the converter and thus propagate to the current measurement. Therefore, for non-potentially isolated structures, frequency components attributable to the input AC voltage can propagate to the measurement signal.

[0033] Furthermore, when the current collection point is used for integrated loss tangent measurement, leakage current may occur in the transformer due to the high voltage, which will be considered as a possible interference variable in the measurement.

[0034] The inventors have recognized that using filters with corresponding bandwidth to remove such interference signals may also prune the useful signal and cause a phase shift, thereby affecting the loss tangent measurement. Furthermore, it has been recognized that filters cannot compensate for, or can only compensate for to a limited extent, fluctuations in frequency at the network input (e.g., caused by generator operation and / or 50 / 60Hz networks).

[0035] In contrast, the inventors' concept of equipping the transformer with a "double" shielding layer does not alter the useful signal. Specifically, as explained below, this is because the frequency components of the input AC voltage can be discharged to the protective ground, and the capacitively coupled leakage current of the transformer can be directly discharged to the internal ground.

[0036] Specifically, the inventors have recognized that when the shielding layer of the primary winding of a transformer (also referred to herein as the protective grounding shielding layer) is connected to a protective ground, parasitic currents can be avoided by the test object or can be bypassed for loss tangent measurement. In other words, by introducing a shielding layer of the primary winding at a protective ground, parasitic currents attributable to the input AC voltage are returned to the source. This protective grounding shielding layer returns parasitic currents attributable to the input AC voltage to the source.

[0037] The "double" shielding configuration also includes a shielding layer on the transformer's secondary winding, referred to herein as an internal grounding shield. This shielding layer is electrically connected to the internal grounding collection point, allowing leakage current on the transformer's secondary side to bypass measurement. In other words, the leakage current is directly coupled to the current collection point.

[0038] By introducing a protective grounding shield and an internal grounding shield, sensitive loss tangent measurement can be achieved. Typically, the concepts proposed in this paper can further improve the accuracy of comprehensive diagnostic measurements, such as loss tangent measurement. Furthermore, they can also improve external diagnostic measurements, such as partial discharge measurement.

[0039] The advantages of the concept presented in this paper are particularly evident in the case of large test currents (load capacitance), i.e., at higher power and higher applied voltages. Specifically, due to the concept presented in this paper for the aforementioned "low harmonic" measurement method, a VLF test generator with a power rating starting from approximately 500 W output power and a test voltage starting from, for example, 45 kV can be used (see also DE 10 2012 024 560 B3). (For example, in the case of a power range of 1 kW to 4 kW, if such high power is required, even greater effects can be avoided.) Attached Figure Description

[0040] The concepts disclosed herein allow for at least partial improvements to various aspects of the prior art. Specifically, further features and their usefulness are derived from the following description of embodiments with reference to the accompanying drawings. In the drawings:

[0041] Figure 1 A schematic diagram of an exemplary test apparatus for testing high-voltage or medium-voltage cables, based on a concept conceived according to the present invention, is shown.

[0042] Figure 2 The test equipment is shown (e.g., according to...) Figure 1 A schematic circuit diagram illustrating an exemplary configuration of the circuit layout in ).

[0043] Figure 3 A flowchart illustrating an exemplary sequence for testing high-voltage or medium-voltage cables is shown;

[0044] Figure 4This illustrates a test setup used for testing high-voltage or medium-voltage cables, for example, in... Figure 2 A schematic diagram of an exemplary configuration of the transformer used in the circuit layout shown;

[0045] Figure 5 and Figure 6 A graph showing the VLF test voltage generated without a "double" shielding layer configuration is presented. Figure 5 ) and a graph showing the VLF test voltage generated with a "double" shielding configuration ( Figure 6 ), used to illustrate the reduction of interference; and

[0046] Figure 7 and Figure 8 The spectrum of the VLF test voltage generated without a "double" shielding layer configuration is shown. Figure 7 ) and the spectrum of the VLF test voltage generated in the case of a "double" shielding configuration ( Figure 8 ), used to illustrate the reduction of interference. Detailed Implementation

[0047] Specifically, this invention relates to achieving loss tangent measurement in the context of a non-potentially isolated structure of a VLF test equipment, while avoiding AC interference signals and transformer leakage current, thereby enabling testing of the test object / high-voltage or medium-voltage cable in a high-resolution and high-sensitivity manner in the context of loss tangent diagnosis.

[0048] The inventors here propose a concept of double shielding for (high-power) transformers used in test equipment for testing high-voltage or medium-voltage cables, which can be used to replace, for example, complete potential isolation that is circuitically complex, has power losses, and is expensive to implement—whether on the input side, in the DC intermediate circuit, between the H-bridge and HV transformers, or in the high-voltage path.

[0049] According to the invention, the interference signal attributable to the input AC voltage (e.g., network ripple) is fed back to the protective ground potential, while simultaneously discharging the VLF leakage current to the internal ground potential. In this way, the interference signal coupled via PGND bypasses the test object and is not added to the current measurement. The feedback of the interference signal can be achieved through a shielding layer (shielded winding) surrounding the primary winding—here, a circumferential shielding layer, but without forming a short-circuit winding. For example, a 100 Hz ripple, for example, at a 50 Hz input AC voltage, can be avoided on the measurement signal used for loss tangent diagnosis.

[0050] However, leakage current in transformers used for high-voltage generation may be present on the measurement signal used for loss tangent diagnosis. The internal grounded shielding layer of the shielded secondary winding introduced according to the invention can discharge such leakage current to the current collection point in a controlled manner. In a simple configuration, the transformer's (e.g., ferrite) core can be used as part of the internal grounded shielding layer and correspondingly electrically connected to the internal ground potential.

[0051] Furthermore, the internal grounding shield provided by the double shielding layer according to the present invention can be used for transformer testing. For example, leakage current in the transformer, caused, for example, by an insulation fault in the transformer, can be detected via a second measuring shunt (a measuring impedance / resistance between the shielding layer of the secondary coil shielding layer and the internal grounding collection point). Therefore, using the double shielding layer, the transformer function of the VLF test equipment can be monitored during operation.

[0052] Figure 1 A schematic diagram of a portable testing device 1 for testing a test object 3 (e.g., a high-voltage or medium-voltage cable, such as a coaxial cable, schematically shown) according to the present invention is shown. The testing device 1 includes a circuit arrangement 5, which is 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 testing device. The evaluation electronics 9 is configured, for example, for loss tangent measurement (loss tangent measurement unit 11) and optionally for transformer monitoring (transformer monitoring unit 13). The hardware (computing unit) on which the evaluation electronics 9 is based has, for example, a digital processor system with microprocessor circuitry having data inputs and control outputs, which operates according to computer-readable instructions stored on a computer-readable medium. Evaluation electronics 9 typically include high computing power for real-time analysis of continuously detected and evaluated datasets, long-term (non-volatile) memory for storing program instructions, and extremely fast short-term (volatile) memory for storing data acquired (or generated) during data collection and data processing of low-frequency and / or high-frequency signals.

[0053] Test device 1 is powered via, for example, a power network acting as source 14 (typically a supply voltage source for outputting the input AC voltage to test device 1, such as a 50 Hz / 60 Hz power network or a regulated generator with an adjustable frequency in the range of, for example, 40 Hz to 65 Hz) (power connection 15). For testing, conductor 3A of test object 3 is connected to test device 1 via (HV) connection cable 17 (with an exemplary length of 5 m to 15 m). The shielding layer 3B and circuit arrangement 5 of test object 3 are connected to protective ground potential 19. Power connection 15 may also include lines at protective ground (see, for example...). Figure 2 The insulation layer 3C of the test object 3 is located between the conductor 3A and the shielding layer 3B of the test object 3.

[0054] Circuit arrangement 5 is arranged within housing 21 of test equipment 1 and may include electronic components such as signal processing with operational amplifiers, at least one integrator, sample-and-hold elements, and at least one analog-to-digital converter for digitization for further processing in a processor having at least one memory for storing sampled data (measurement data). Circuit arrangement 5 may also be connected to control device 23 of test equipment 1, which may be located within test equipment 1, wholly or partially external to test equipment 1, or may be (partially) integrated into control device 23. Control device 23, together with a high-voltage source included in circuit arrangement 5, generates a test voltage to be applied to test object 3, wherein control device 23 provides and controls the power of a current source required to regulate the test voltage, for example via a transformer and downstream cascaded multiplier. See [reference needed]. Figure 2 An exemplary circuit arrangement.

[0055] exist Figure 1 In the example, an operation display 25A (display) for displaying acquired test data and at least one operating element 25B for setting measurement parameters are schematically shown on the upper side 21A of the housing 21. The operation display 25A and the operating element 25B form a user interface for, for example, a control device 23.

[0056] Figure 2 It shows the use of Figure 1 The circuit diagram shown is an exemplary circuit diagram of the components of the circuit arrangement 5 of the test device 1, which are housed in the housing 21. The test object 3 is connected to the test object connection 27A (e.g., located on the housing 21) via a connecting cable 17, and is also electrically connected to the protective ground potential 19 on the shielding layer. To supply input AC voltage to the circuit arrangement 5, the test device 1 is connected to a power voltage source 14 (see power connection 27B, e.g., located on the housing 21) at the power connection 27B. Figure 1 Power connections 27B include, for example, voltage-bearing connections, neutral connections, and grounding connections. Figure 2 Also optionally shown is a protective grounding connection 27C, which is provided separately, for example, on housing 21, for adequate grounding of housing 21 and for coupling the protective grounding to the DUT with low impedance (e.g., for loss tangent measurement).

[0057] The circuit layout includes a power converter 31 and two transformers in a cascaded circuit serving as a high-voltage source (in... Figure 2 The middle assembly is in frame 33), and the high-voltage circuit is arranged in 35.

[0058] Power converter 31 is powered via power connection 27B and includes rectifier circuit 37 (AC / DC converter) which generates a DC voltage (with regulated amplitude) from the input AC voltage. Power converter 31 also includes two switching power supplies 39 (DC / AC converters) electrically connected in reverse to the DC voltage outputs 37A, 37B of rectifier circuit 37. Rectifier circuit 37 and switching power supplies 39 here represent an example of a low-voltage circuit arrangement that generates two inverse HF signals for two high-voltage sources 33 at the two pairs of HF outputs 39A, 39B of switching power supplies 39, starting from the input AC voltage.

[0059] The non-potentially isolated design of the depicted power converter 31 specifically relates to a VLF test generator for higher power / higher voltage applications, which uses a power factor corrector (PFC) on the mains input side to rectify the AC voltage supplied by the mains (or another source such as a mobile generator) and provide a DC voltage for further amplification. The output voltage for a power range of 1 kW to 4 kW is, for example, 400 V. Due to the lack of potential isolation, the voltage reference point for the output DC voltage is not at the protective earth potential, but rather represents PGND, whose potential relative to the protective earth potential is generated by the rectifier circuitry and is half the value of the input voltage. Due to technically induced capacitive coupling, PGND can have ripple two or more times that of the input frequency, and this ripple—unresolved—can have a corresponding impact on measurements.

[0060] High-voltage sources 33 are configured to provide variable-amplitude positive (+) or negative (-) high voltages at their respective outputs 33A, 33B, wherein, for example, modulation is performed at multiples of the mains frequency. Each high-voltage source 33 includes (high-voltage) transformers 41, 43 for converting HF signals into high-voltage signals present at the high-voltage outputs 41A, 43A of the respective transformers 41, 43. For example, there is an amplification from, for example, 400 V (at the HF output pair) to, for example, 13 kV (at the high-voltage outputs 41A, 43A). Each high-voltage source 33 also includes a rectifier circuit 45 electrically connected to the high-voltage outputs 41A, 43A for outputting rectified high-voltage signals at the outputs 33A, 33B of the high-voltage source 33.

[0061] A high-voltage circuit arrangement 35 is disposed between the outputs 33A and 33B of the high-voltage source 33 and the test object 3. The high-voltage circuit arrangement 35 is configured to shape the VLF test voltage based on the rectified high-voltage signal. The high-voltage circuit arrangement 35 is operated by means of a control unit 47 for defined charging and discharging of the test object 3, which represents a specific capacitive load. The control unit 47 is configured to ensure a preferably sinusoidal voltage distribution at the test object 3. Figure 2 In the embodiment shown by way of example, the high-voltage switch arrangement 35 includes, for example, a semiconductor switch cascade 49 and an amplifier 51, to which the control unit 47 operates.

[0062] Furthermore, the switching power supply 39 of the low-voltage circuit arrangement is driven by the controller 53 via a clock signal generator T, so that the two switching power supplies 39 combined with the high-voltage source 33 can each provide a test voltage, which is synchronized using the clock signal generator T, and can be predefined in a defined manner in terms of curve shape and amplitude, advantageously edgeless and especially sinusoidal, and particularly unaffected by the control unit 47.

[0063] For further details on the generation and regulation of the test voltage, as well as alternative embodiments of the circuit layout, see, for example, DE 10 2012 024 560 B3 and DE 195 13 441 A1 above.

[0064] For the VLF design of loss tangent measurement, it is important that the low-voltage side ground input (referred to herein as internal ground collection point 55) and the high-voltage circuit arrangement 33 include a high-voltage side output (here, test object connection 27A, located on the high-voltage side of transformers 41 and 43). For testing purposes, the high-voltage side output is electrically connected to conductor 3A of test object 3 (see...). Figure 1 During operation, the low-voltage side ground input represents the internal ground potential and is electrically connected to the protective ground connection 27C, where a measurement signal for loss tangent diagnostics can be detected. For testing via loss tangent diagnostics, the shielding layer 3B of test object 3 (see...) Figure 1 The protective grounding connection 27C is also connected to the protective ground, so that the shielding layer 3B and the VLF test equipment 1 (especially the test voltage generating unit 7) are at the same protective grounding potential 19. In the event of a defect in the test object 3, when the protective grounding connection 27C is connected to the same grounding potential (protective grounding potential 19), a circuit can be formed through which a measurement current can be established, and this circuit extends from the high-voltage circuit arrangement 35 through the test object 3 (especially the defect), and returns to the high-voltage side of the transformers 41 and 43 through the internal grounding collection point 55 via the protective grounding connection 27C.

[0065] like Figure 2 As shown, the measurement current at the connecting conductor 57, which electrically connects the low-voltage side ground input (internal ground collection point 55) to the protective ground connection 27C, is available within the test equipment 1 for measurement, and is particularly advantageous in low-voltage environments. The connecting conductor 57 can, for example, be used for a low-frequency signal tap 59, e.g., via impedance, to generate a diagnostic signal. This diagnostic signal can be used in the context of analog and / or digital signal processing in the loss tangent measurement unit 11 to determine the phase and thus the power loss.

[0066] Combination Figure 3 The measurement process is summarized exemplarily. A test voltage is generated using test equipment 1 according to a method for testing a test object (e.g., a high-voltage or medium-voltage cable) (step 101). When the high-voltage or medium-voltage cable is connected, a measurement current is induced in the connecting conductor 57 using the test voltage (step 103). Since the connecting conductor 57 electrically connects the internal ground collection point 55 to the protective ground connection 27C, a measurement current flows through the connecting conductor 57 when the test object (e.g., a high-voltage or medium-voltage cable) is tested. This measurement current is formed by the test voltage applied between the internal ground collection point 55 and the shielding layer 3B of the test object / high-voltage or medium-voltage cable, which is also at the protective ground potential 19. A low-frequency diagnostic signal is generated using a low-frequency signal tap at the current collection point, assigned to the measurement current, for example, for VLF phase rotation measurement (step 105). Evaluation of the low-frequency diagnostic signal in an evaluation device (e.g., loss tangent measurement unit 11) checks for faults in the high-voltage or medium-voltage cable (step 107).

[0067] Compared to low-power VLF test generators that can typically be implemented using commercially available potential-isolated power supply units, potential isolation from the input voltage to the DC intermediate circuit is usually not implemented in VLF test generators used for higher power / higher voltage, because potential-isolated power supply units are large and expensive.

[0068] The lack of potential isolation in VLF test generators for higher power / higher voltage, as explained above, results in the presence of PGND in the intermediate circuitry because a connection to the protective earth potential is impossible due to the rectifier on the input side. Because of the PGND reference, the output and smoothed voltage signals can have an AC voltage component with a frequency twice that of the input AC voltage (e.g., the mains frequency) and corresponding harmonics relative to the protective earth potential (multiples of the mains frequency). In other words, harmonic currents are not suppressed in the non-potential isolation configuration of the test generator due to technical limitations.

[0069] In the standard circuit layout for VLF test generators used for higher power / higher voltage applications, a (high-voltage) transformer is also used in the voltage generation path; see [link to relevant documentation]. Figure 2 Transformers 41 and 43 are included. Each transformer provides (inductive) primary-secondary coupling to amplify the voltage to, for example, +10 kV or -10 kV. Without countermeasures, two or more times the input frequency (originating from the incoming output AC voltage) can now be applied to the measuring current passing through the test object via the transformer's capacitive coupling, and thus affect the diagnostic signal via current sensing (current shunt). At high voltages, interference signals appear—relative to the useful signal—for example, up to four times the amplitude of the useful signal. (Reference) Figure 2 These parasitic currents (mains induced interference) originate from the AC / DC converter of power converter 31.

[0070] Because the frequencies of these parasitic currents are similar to the measured currents detected in the context of loss tangent measurements, they can affect the measurement results. Conventional (AC) power sources, such as 50 Hz / 60 Hz power supply networks or regulated generators with frequencies in the range of 40 Hz to 65 Hz, can result in double frequencies in the range of 80 Hz to 130 Hz, as well as associated integer multiples (e.g., up to the tenth harmonic) as parasitic currents. In other words, the resulting frequencies are in the range of, for example, 0.8 kHz to 1.3 kHz, or, for example, 1 kHz or 1.2 kHz for the tenth harmonic. Signal sampling in loss tangent measurements uses, for example, a sampling rate in the range of 5 kHz to 100 kHz to detect a signal bandwidth of 5 kHz to 50 kHz, making it possible to detect such source-induced interference signals as well. In other words, the frequency range of the mains-induced interference signal is within the bandwidth of the loss tangent measurement.

[0071] As explained, for the concept described in this paper, the low-voltage side of the transformer is not decoupled from the voltage source potential, so in principle, the ripple of the interference measurement (harmonics of the input AC voltage) may form a parasitic current on the high-voltage side.

[0072] Furthermore, in VLF test generators used for higher power / higher voltage applications, leakage current may occur on the high-voltage side of the transformer due to the high voltage. The background is that leakage current may inherently exist in high-voltage transformers at differential voltages greater than, for example, 1 kV. Moreover, this leakage current is variable, particularly related to temperature and load, and therefore represents a dynamic error variable. Consequently, their effects, such as on loss tangent measurements, are unpredictable.

[0073] The inventors have recognized that these interferences are caused not only by the requested power, but also by the power level (and therefore the topology) of the power electronics.

[0074] In principle, both the parasitic current induced by the mains power and the leakage current on the high-voltage side can affect the loss tangent measurement (diagnostic signal).

[0075] In the context of the double-shielded concept, the inventors have now realized a concept for avoiding such effects. According to the invention, the adverse effects of parasitic currents and / or leakage currents can be reduced, and preferably avoided, because the parasitic currents and leakage currents are discharged in a targeted manner through specially configured adjustments within the transformer.

[0076] Figure 2 On the one hand, an exemplary implementation for discharging parasitic current induced by mains power is shown; on the other hand, two exemplary implementations for discharging leakage current are shown. Figure 4 The diagram depicts an exemplary configuration of a transformer 61 used in a test apparatus according to the present invention, wherein the specific configuration exemplary corresponds to... Figure 2 Transformer 41 in the middle. Figure 2 and Figure 4 In the figures, similar structural components are provided with the same reference numerals.

[0077] Typically, each of transformers 41, 43, and 61 (usually each transformer) includes at least one coil pair comprising a primary winding 63 and a secondary winding 65. The winding ends of the primary winding 63 are electrically connected to the HF output pair (in... Figure 2 In the middle, HF outputs 39A and 39B) and are correspondingly at a maximum value of, for example, 400 V. The first winding end of the secondary winding 65 is electrically connected to the internal ground potential (in Figure 2 In the middle, connected to the internal ground collection point 55), and the second winding end of the secondary winding 65 is electrically connected to the high voltage output (in Figure 2 In the middle, connected to the high voltage output 41A, 43A) for outputting high voltage signals (with voltages, for example, greater than 3 kV, for example, up to 20 kV).

[0078] The windings, for example, are arranged in a trapezoidal shape with multiple layers, successively wound as primary or secondary coils onto a toroidal winding body 67 (e.g., made of plastic), and are cast, for example, with a casting resin. The winding body 67 is particularly used for mechanical stability during coil manufacturing / winding. The winding body 67 also separates and insulates the primary coil from the core. Figure 4 In the diagram, winding body 67 is shown with a U-shaped cross-section. At a voltage difference of tens of kilovolts (10 kV), the distance between the coils is, for example, a few millimeters.

[0079] exist Figure 4In the example, the arrangement of the primary winding 63 and the secondary winding 65 is radially defined by the transformer core 69. The transformer core 69 simplifies the generation of a secondary voltage of several kilovolts. The transformer core 69 is, for example, configured as a ferrite core with two half-shells.

[0080] An insulating film 70 (e.g., a polyethylene terephthalate film or a film composed of condensed aromatic dianhydrides and aromatic diamines) is disposed between the primary winding 63 and the secondary winding 65 for electrical insulation. Specifically, in an implementation of the double-shielded layer concept according to the invention, the insulating film 70 is used for low-voltage insulation.

[0081] The double-shielding design utilizes a protective grounding shield 71. The protective grounding shield 71 is configured to shield the primary winding 63 and circumferentially surround it, but specifically does not form a short-circuit winding. The protective grounding shield 71 is capacitively coupled to the primary winding 63 and electrically connected to a protective ground potential 19. The protective grounding shield 71 serves to discharge parasitic currents caused by harmonics to the protective ground potential 19.

[0082] On the other hand, the double-shielded design utilizes an internal grounded shielding layer 73. The internal grounded shielding layer 73 is configured to shield the secondary winding. For this purpose, the internal grounded shielding layer 73 is capacitively coupled to the secondary winding 65 and electrically connected to the internal grounding collection point 55. The internal grounded shielding layer 73 is used to receive leakage current, such as the HF high-voltage leakage current generated in the transformer, and discharge it to the internal grounding potential. Figure 2 In the setup, discharge to the internal ground collection point 55.

[0083] The protective grounding shield 71 and / or the internal grounding shield 73 are each configured, for example, as circumferential copper strips (typically made of highly conductive materials), wherein in particular, short-circuit windings are not formed.

[0084] Typically, a compact setup is more efficient, allowing for a higher transformer packing density. Figure 4 As shown and Figure 2 In the embodiment schematically indicated for transformer 41, the internal grounding shield 73 is partially implemented by the transformer core 69. Therefore, the HV transformer can be kept compact while maintaining the insulation path. Since the primary winding 63 is surrounded by the protective grounding shield 71, there is no "power-to-ground coupling" from the primary side to the core. Furthermore, the voltage difference between the protective grounding shield 71 and the internal grounding shield 73 is typically only a few volts (e.g., 5 V), thereby reducing the insulation requirements on the insulating film 70.

[0085] Alternatively, such as Figure 2As shown in transformer 43, an internal grounded shielding layer can be provided that completely surrounds the secondary coil (circumferentially grounded, but not forming a short-circuit winding). The introduction of a shielding winding configured in this way may require maintaining corresponding insulation distances and high-voltage outputs 41A, 43A (see [reference]). Figure 2 The pressure-resistant bushings require a larger space for installation.

[0086] Based on the double-shielded design, information about the quality of a given transformer insulation layer can be continuously obtained during the operation of test equipment 1, particularly during loss tangent measurement. Specifically, structural faults in the transformer insulation layer can be detected in this manner. For this purpose, signal taps, such as... Figure 2 The leakage current measuring impedance 75 electrically connects the internal grounding shield 73 of the transformer's secondary winding 65 to the internal grounding collection point 55. Using, for example, the leakage current measuring impedance 75, leakage current in the transformer can be continuously detected and evaluated. This evaluation includes, for example, a plausibility test or time evolution of the detected leakage current.

[0087] In addition to the test methods of the test object Figure 3 A method for testing a transformer arranged in a test apparatus is also shown, wherein the internal grounded shield of the transformer's secondary winding is electrically connected to the internal grounding collection point of the test apparatus via signal taps (e.g., leakage current measuring resistors or capacitive coupling). A test voltage for testing is generated using the test apparatus (step 101). For example, leakage current is measured using a leakage current measuring resistor (step 111). The leakage current is evaluated in terms of insulation damage to the transformer in the evaluation electronics of the test apparatus (step 113).

[0088] Figures 5 to 8 The effects of using the double-shielded design according to the invention are shown. If a transformer with a double-shielded structure is used in the test equipment, parasitic currents can be largely suppressed or returned to their source. In this way, the quality of measurements can be significantly improved, especially for high voltages. To illustrate the effects addressed by the double shielding as described above, interference in the measured current and diagnostic signal is shown, additionally analyzed using an oscilloscope during measurements at the current shunt (measuring resistor / signal 59), for both transformers with and without a double-shielded structure.

[0089] Figure 5 and Figure 6The process of the applied voltage 81 changing over time, as detected by an oscilloscope, and the associated high-frequency process of the detected measured current 83 changing over time are shown. Without using a double-shielded design, parasitic currents as high as 40 μA are observed. When the transformer uses a double-shielded design, the parasitic current is significantly reduced to a few μA.

[0090] exist Figure 7 and Figure 8 In the spectrums 85 and 86 shown, the 0.1 Hz test frequency used for VLF testing and diagnostics can be considered the dominant frequency 87. The second high frequency 88 and the third high frequency 89 in spectrum 85... Figure 7 The frequencies (i.e., without using a double-shielded design) are at 50 Hz or 100 Hz (fundamental and harmonic frequencies), and represent interference caused by the mains voltage at 50 Hz and coupled to the measuring current. When using a double-shielded design, these frequencies are no longer shown in spectrum 86 in Figure 9 due to the double shielding of the transformer's primary and secondary windings.

[0091] Furthermore, it can be seen that the double-shielded design can also additionally reduce parasitic frequencies 91 and 93 (see spectrum 85), which may be generated, for example, by other mains loads.

[0092] The features of the methods and apparatus used in this description, referred to as “units,” “devices,” etc., can, in the knowledge of those skilled in the art, be implemented, for example, as discrete physical units, as conceptual functional units, such as software code stored in a memory unit (memory) (in the context of an evaluation program), as routines of a microprocessor, and / or within a hybrid hardware / firmware architecture. Furthermore, two or more “units,” etc., can be integrated together in a single physical circuit structure (e.g., an integrated unit or structure). For example, a processor can be controlled by programming code (stored instructions), wherein the programming code is capable of performing corresponding functions when executed by a processor (such as, for example, a microprocessor).

[0093] Therefore, the features mentioned, particularly those mentioned in the claims, can be configured as software, hardware, and / or a combination of hardware and software. Specific details of each unit are described herein (and particularly in the exemplary section). This provides sufficient information for those skilled in the art to implement the corresponding structure in hardware circuitry or software code. For example, the “evaluation unit” disclosed herein can be embodied in the structure of a central processing unit (CPU) configured with instructions for performing operations that derive basic oscillation information. The CPU may include one or more microprocessors connected to one or more memory elements. The memory elements may store one or more microprocessor-readable instructions (programs) that, when executed by the microprocessor, perform, for example, Fourier or wavelet transforms. Furthermore, the measurement and evaluation unit 9 and the control device may include various units that interact with each other to perform desired actions, such as receiving, accessing, and / or transmitting datasets, identifying maximum values, etc.

[0094] It is explicitly emphasized that, for the purposes of the original disclosure and for the purpose of defining the claimed invention, all features disclosed in this description and / or claims are considered to be individual and independent of each other, and are unrelated to the combination of features in the embodiments and / or claims. It is explicitly stated that, for the purposes of the original disclosure and for the purpose of defining the claimed invention, and particularly as a limitation of scope indications, all scope indications or unit group indications disclose any possible intermediate values ​​or unit subgroups.

Claims

1. A test apparatus (1) for testing a test object (3), particularly a high-voltage or medium-voltage cable, using a test method employing a very low frequency (VLF) test voltage, comprising: Power converter (31), wherein the power converter (31) includes: - AC voltage input, for receiving an input AC voltage relative to the protective grounding potential (19), - A low-voltage circuit arrangement, including a rectifier circuit (37) and at least one switching power supply (39), wherein the low-voltage circuit arrangement is configured to generate an HF signal relative to a floating reference potential from the input AC voltage, and - At least one HF output pair (39A, 39B) for outputting the HF signal and the floating reference potential; Transformers (41, 43, 61) are used to convert the HF signal into a high-voltage signal, wherein the transformers (41, 43, 61) include: - Primary winding (63), the winding ends of which are electrically connected to the HF output pair. - A protective grounding shield (71) for shielding the primary winding (63), wherein the protective grounding shield (71) is capacitively coupled to the primary winding (63) and electrically connected to the protective grounding potential (19). - Secondary winding (65), the first winding end of which is electrically connected to an internal ground collection 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 ground shield (73) for shielding the secondary winding (65), wherein the internal ground shield (73) is capacitively coupled to the secondary winding (65) and electrically connected to the internal ground collection point (55). The rectifier circuit (45), electrically connected to the high-voltage outputs (41A, 43A), is used to output rectified high-voltage signals; and A high-voltage circuit arrangement (35) is used to generate the VLF test voltage based on the rectified high-voltage signal.

2. The test apparatus (1) according to claim 1, wherein an intermediate circuit for generating a DC voltage relative to the floating reference potential is formed in the low-voltage circuit arrangement, wherein the DC voltage is affected by harmonics depending on the received input AC voltage, and the protective ground shield (71) capacitively coupled to the primary winding (63) is configured to discharge parasitic currents caused by the harmonics to the protective ground potential (19).

3. The test apparatus (1) according to claim 1 or 2, wherein the internal grounding shield (73) capacitively coupled to the secondary winding (65) is configured to discharge the HF high-voltage leakage current generated in the transformer (41, 43, 61) to the internal grounding collection point (55).

4. The test apparatus (1) according to any one of the preceding claims, wherein the transformer (41, 43, 61) further comprises a transformer core (69) configured to be capacitively coupled to a portion of the internal grounding shield (73) of the secondary winding (65) and electrically connected to the internal grounding collection point (55).

5. The test apparatus (1) according to any one of the preceding claims, wherein The transformer (41, 43, 61) further comprises: The winding body (67), in particular having a U-shaped cross-section, has a primary winding (63) arranged on the winding body and embedded in the protective grounding shield (71); Insulating film (70); the internal grounding shield (73); and the secondary winding (65); and wherein the winding body (67) is specifically arranged between sections of the core of the transformer (41, 43, 61), and / or The primary winding (63) is radially defined on both sides by the protective grounding shield (71), and / or the secondary winding (65) is radially defined on both sides by the internal grounding shield (73).

6. The test apparatus (1) according to any 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) includes at least one cascaded semiconductor switch (49) and at least one amplifier (51) assigned to the transformer (41, 43, 61).

7. The test apparatus (1) according to any one of the preceding claims, wherein The low-voltage circuit arrangement includes two switching power supplies (39) configured to output a positive HF signal and a negative HF signal at corresponding HF output pairs (39A, 39B); The test equipment (1) includes: Two transformers (41, 43, 61), each transformer including a protective grounding shield (71) and an internal grounding shield (73), for converting the positive HF signal and the negative HF signal into a positive high voltage signal and a negative high voltage signal; and two rectifier circuits (45), electrically connected to the high voltage output of the transformers (41, 43, 61), for outputting rectified positive high voltage signals and rectified negative high voltage signals; as well as The high-voltage circuit arrangement is configured to generate the VLF test voltage based on the rectified positive high-voltage signal and the rectified negative high-voltage signal.

8. The test apparatus (1) according to any one of the preceding claims, wherein the high voltage circuit arrangement includes a test object connection (27A) for connecting the high voltage side output of the high voltage circuit arrangement to a conductor (3A) of the test object (3), particularly via a measurement connection cable (17).

9. The test apparatus (1) according to any one of the preceding claims further includes: Protective grounding connection (27C); A connecting conductor (57) electrically connects the internal ground collection point (55) to the protective ground connection (27C) such that when the test object (3) is tested, a measuring current flows through the connecting conductor (57), the measuring current being formed between the internal ground collection point (55) and the shielding layer (3B) of the test object (3), the shielding layer also being at the protective ground potential (19). A low-frequency signal tap (59) located at the current collection point generates a low-frequency diagnostic signal based on the measured current at the low-frequency signal tap; as well as Evaluation electronics (9) are connected to the low-frequency signal tap (59) to receive the low-frequency diagnostic signal and are configured for VLF phase rotation measurement.

10. The test apparatus (1) according to claim 9, wherein The low-frequency signal tap (59) is configured to detect the instantaneous value of the current intensity in the connecting conductor (57), and / or include the impedance in the connecting conductor (57), particularly a parallel circuit of resistors and capacitors; and / or The evaluation electronics (9) are configured to determine the loss factor assigned to the test object (3) based on the diagnostic signal, and are specifically configured to detect the low frequency of the diagnostic signal in the range of 0.01 Hz to 1 Hz at a sampling rate in the range of 5 kHz to 100 kHz, in particular; and / or The evaluation electronics (9) includes at least one analog and / or digital signal processing unit and / or processor and / or buffer memory.

11. The test apparatus (1) according to any one of the preceding claims further includes: Signal taps, particularly leakage current measuring impedance (75) or leakage current measuring resistors, electrically connect the internal grounding shield (73) of the secondary winding (65) of the transformer (41, 43, 61) to the internal grounding collection point (55) and are provided for detecting structural faults in the insulation of the transformer (41, 43, 61).

12. A method for testing a test object (3), particularly a high-voltage or medium-voltage cable, using the test equipment (1) according to any one of claims 1 to 11, comprising the following steps: The test voltage (step 101) is generated using the test equipment (1); When the test object (3) is connected in the connecting conductor (57), the test voltage causes (step 103) a measurement current. The connecting conductor electrically connects the internal grounding collection point (55) of the test equipment (1) to the protective grounding connection (27C) of the test equipment (1), so that when the high voltage or medium voltage cable is tested, the measurement current flows through the connecting conductor (57). The measurement current is formed between the internal grounding collection point (55) and the shielding layer (3B) of the test object (3), which is also at the protective grounding potential (19). A low-frequency diagnostic signal is generated (step 105) by using the low-frequency signal tap (59) on the connecting conductor (57) and assigned to the measuring current (83) for VLF phase rotation measurement. as well as In evaluating the low-frequency diagnostic signal in terms of the fault of the high-voltage or medium-voltage cable in the evaluation of electronic devices (9) (step 107).

13. The method of claim 12, wherein Parasitic currents caused by harmonics of the received input AC voltage are discharged to the protective grounding potential (19) via the protective grounding shield (71); and / or The leakage current generated in the transformer (41, 43, 61) is discharged to the internal grounding collection point (55) via the internal grounding shield (73) of the transformer (41, 43, 61).

14. A method for testing a transformer (41, 43, 61) arranged in a test apparatus (1) according to any one of claims 1 to 11, wherein the internal grounding shield (73) of the secondary winding (65) of the transformer (41, 43, 61) is electrically connected to an internal grounding collection point (55) of the test apparatus (1) via a leakage current measuring impedance (75), the method comprising the steps of: The test voltage (step 101) is generated using the test equipment (1); The leakage current is measured (step 111) using the leakage current measuring impedance (75); as well as The leakage current is evaluated (step 113) in the evaluation electronics (9) of the test equipment (1) in terms of insulation damage of the transformer (41, 43, 61).

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