Procedure and device for testing a voltage transformer

Abstract

The present invention relates to a device (50) for testing a voltage converter (10, 20), comprising a frequency response analyzer (60) and an impedance converter (70). The frequency response analyzer (60) is configured to measure an electrical transfer function over a predefined frequency range. The frequency response analyzer (60) has a test signal output (61) for generating a test signal for the voltage converter (10, 20), a reference signal input (64) for receiving a reference signal applied to the voltage converter (10, 20) for testing purposes, and a response signal input (67) with a predefined input impedance (68) for receiving a response signal from the voltage converter (10, 20).The impedance converter (70) comprises an impedance converter input (71), which has a variable input impedance (72) that can be adjusted to a voltage converter impedance (10, 20), and an impedance converter output (75), which is coupled to the response signal input (67) and has an output impedance (74) that matches the input impedance (68) of the response signal input (67).

Description

AREA OF INVENTION

[0001] The present invention relates to a device for testing a voltage transformer, for example an inductive voltage transformer or a low power voltage transformer (LPVT), and a corresponding method for testing a voltage transformer. BACKGROUND

[0002] The shift in the topology of power distribution and transmission networks towards decentralization of electricity generation is significantly increasing the number of electronic components. So-called "green" energy (from wind farms, solar parks, and other alternative energy sources) is increasing considerably. This generated electrical energy is often fed into the distribution and transmission networks using semiconductor technologies. Energy from these sources is frequently dependent on environmental changes, and therefore, any change in the time of day or weather directly impacts a number of switching operations necessary for maintaining grid stability. Furthermore, the increasing number of loads based on electronically controlled technologies, such as power electronics or variable frequency drives, also affects the grid.The aforementioned influences can lead to an increased occurrence of transient voltage pulses, harmonics, subharmonics, or offset voltages with DC voltages up to several kHz. Such phenomena can only be detected or monitored using high-voltage measurements with correspondingly high accuracy, especially in the DC range up to a frequency of several kHz.

[0003] In electrical power engineering, voltage transformers can be used as instrument transformers for measuring alternating voltages. The function of a voltage transformer is to proportionally convert a high voltage to a lower voltage value. This lower voltage, for example, around 100 V, is then transmitted to voltmeters, energy meters, and similar devices, for purposes such as measurement or protection. A voltage transformer can be implemented as an inductive (so-called conventional) voltage transformer or as a low-current voltage transformer (so-called LPVT, Low Power Voltage Transformer, or LPIT, Low Power Instrument Transformer).

[0004] Low-voltage voltage converters (LPVTs) can take various forms. Besides resistive and capacitive voltage dividers (undamped and damped), resistive-capacitive voltage dividers in a wide variety of configurations are also found in the field. Resistive-capacitive voltage converters are formed from two voltage dividers connected in parallel, one of which is a capacitive voltage divider and the other a resistive voltage divider. Both the capacitive and resistive voltage dividers typically consist of at least two elements connected in series. The parallel connection of these two voltage dividers is also known as an RC divider. One end of the RC divider is connected to the high voltage being measured, and the other end is connected to ground. A lower voltage, proportional to the high voltage being measured, is present at a tap between the RC dividers and can be applied to a voltmeter.Faults in the resistive-capacitive voltage converter can be caused by defects in the resistive-capacitive voltage divider. There are various reasons for defects in the capacitors of the capacitive voltage divider, such as moisture ingress into the insulation.

[0005] Inductive voltage converters are fundamentally constructed like transformers. They consist of a primary winding, which is electrically connected to the high voltage being measured, and a secondary winding, which is galvanically isolated but, for safety reasons, generally grounded on one side to the connected devices. Faults in an inductive voltage converter can occur, for example, due to defects in the winding insulation, displacement of winding turns, or defects in the iron core that magnetically couples the primary and secondary windings.

[0006] Conventional voltage converter technologies are predominantly used worldwide, however the number of LPVTs is increasing significantly due to their greater suitability for power quality measurement.

[0007] Voltage transformers (both conventional and low-voltage voltage transformers) exhibit a pronounced frequency-dependent transmission characteristic due to their internal design. Applications for detecting the aforementioned phenomena, and thus for monitoring power quality, require information about this frequency-dependent transmission behavior. Corresponding measurement procedures and evaluation methods for determining and assessing the transmission characteristics, and therefore the suitability of LPVTs and conventional voltage transformers for power quality measurement, are defined. Since these measurement procedures require extensive electrical equipment, they are primarily performed at the manufacturer's site or, with considerable effort, on-site. Typically, a reference setup capable of measuring frequencies up to 9 kHz is selected.To determine the frequency response of conventional voltage converters, a so-called two-frequency method is used, which achieves a pre-linearization of the core by means of a 50 Hz fundamental frequency. The high-frequency components are modulated onto this fundamental frequency.

[0008] D. Gopp, "Influencing factors in the model-based testing of capacitive voltage transformers - What are the most influential parameters of the model-based testing of capacitive voltage transformers and how do these affect the translation and phase error?", Master's thesis, University of Applied Sciences Vorarlberg, August 1, 2019, discloses a device according to the preamble of claim 1.

[0009] German patent DE 10 2018 200869 A1 discloses a method and a device for testing an inductive charging system. US patent 2015 / 288277 A1 discloses a method for testing the frequency response of a switching converter. SUMMARY OF THE INVENTION

[0010] There is a need for improved methods for testing voltage transformers, both conventional voltage transformers and LPVTs, especially with methods and devices that can be easily applied on site.

[0011] According to the present invention, a device for testing a voltage transformer and a method for testing a voltage transformer are provided, as defined in the independent claims. The dependent claims define embodiments of the invention.

[0012] An inventive device for testing a voltage converter comprises a frequency response analyzer and an impedance converter. The frequency response analyzer is configured to measure an electrical transfer function over a predetermined frequency range. The frequency response analyzer has a test signal output, a reference signal input, and a response signal input.

[0013] The frequency response analyzer can output a test signal for the voltage converter under test at its test signal output. This test signal can, for example, be a voltage signal with a specified voltage and variable frequency. The frequency can be varied, for example, in a range from 1 Hz to 30 MHz, particularly in a range from 20 Hz to 2 MHz. The voltage can be in a range of several volts, for example, in the range of 5–300 V. The voltage can be, for example, 10 V. The voltage can be an alternating voltage, for example, 10 V peak-to-peak. The test signal output can include a connection for a coaxial cable, with the test signal being output on the center conductor of the coaxial cable and the outer conductor of the coaxial cable being connected to ground.The inner conductor of the voltage transformer under test is connected to a terminal of the transformer, for example, the primary side, and the outer conductor is connected to the transformer's ground. This reduces or prevents interference from the environment affecting the test signal.

[0014] The frequency response analyzer can receive a reference signal via the reference signal input. For example, the reference signal input can be connected to the same terminal on the voltage converter where the test signal is applied. The reference signal input can include a coaxial cable connection, with the reference signal received via the center conductor of the coaxial cable and the outer conductor connected to ground. On the voltage converter, the center conductor is connected to the same terminal where the test signal is applied, and the outer conductor is connected to the voltage converter's ground. The test signal applied to the voltage converter can be precisely determined via the reference signal input and used as a reference signal. Based on this reference signal, the voltage converter's transfer function can be accurately determined.

[0015] The response signal input allows the frequency response analyzer to receive a response signal generated by the voltage converter under test in response to the output test signal. The response signal input has a predefined input impedance, for example, 50 ohms.

[0016] The frequency response analyzer could, for example, be a device used to test power transformers using sweep frequency response analysis (SFRA). Such a frequency response analyzer could be designed to be transported by an operator, for example, as a portable device in a case.

[0017] The impedance converter has an input and an output. The input has an adjustable input impedance. The output is coupled to the response signal input of the frequency response analyzer and has an output impedance matched to the input impedance of the response signal input. The input can be coupled to another terminal of the voltage converter, such as a terminal on the secondary side of the voltage converter. The input can include a coaxial cable terminal, with the secondary side of the voltage converter being coupled to the center conductor of the coaxial cable and the outer conductor of the coaxial cable being coupled to ground at both the voltage converter and the impedance converter.The impedance converter thus receives an output signal from the voltage converter, which the latter outputs in response to the test signal, and forwards this output signal as a response signal to the response signal input of the frequency response analyzer, whereby the impedance is adjusted accordingly.

[0018] In summary, the device is based on the SFRA method and uses, for example, an SFRA instrument as a frequency response analyzer. Both conventional and LPVT voltage converters can, by their very nature, have arbitrary impedances, which generally do not correspond to the input impedance of the response signal input of the SFRA instrument. For example, the response signal input of the SFRA instrument, i.e., the frequency response analyzer, may have a predetermined input impedance of 50 ohms, whereas conventional voltage converters can have impedances in the range of up to several hundred ohms, and LPVTs can even have impedances up to several megaohms. An output impedance at the test signal output of the frequency response analyzer can be 50 ohms, and an input impedance of the reference signal input of the frequency response analyzer can also be 50 ohms.The response signal input is the critical path when determining the frequency-dependent transfer characteristics of conventional voltage converters and LPVTs. This means that any deviation of the secondary-side impedance of the voltage converter from the input impedance of the SFRA measuring instrument leads to inaccurate determinations of the voltage converter's frequency-dependent transfer characteristics. To avoid this, an impedance converter is connected between the voltage converter and the response signal input. The output impedance of the impedance converter is matched to the input impedance of the response signal input. The input impedance of the impedance converter can be adjusted to match the output impedance of the voltage converter. For example, the input impedance of the impedance converter can be adjustable within a range of 30 ohms to 100 megaohms, preferably within a range of 50 ohms to 100 megaohms.The output impedance of the impedance converter matches the input impedance of the response signal input, resulting in impedance matching on both sides of the converter. This allows the frequency-dependent transfer characteristic of the voltage converter to be measured under optimal conditions (e.g., at the voltage converter's nominal burden).

[0019] According to one embodiment, the device, which comprises the frequency response analyzer and the impedance converter, can be designed as a mobile, portable device. In this context, mobile and portable means that the device can be carried by a single person and, for example, housed in a suitcase or bag. The device can, for example, weigh only a few kilograms, perhaps in the range of 1 to 10 kg.

[0020] According to one embodiment, the device comprises at least one battery configured to provide electrical energy for operating the frequency response analyzer and / or the impedance converter. For example, one rechargeable battery may be provided for the frequency response analyzer and another for the impedance converter. Alternatively, a single (rechargeable) battery may be provided to power both the frequency response analyzer and the impedance converter. The battery may, for example, be housed together with the frequency response analyzer and the impedance converter in the aforementioned case or bag, making the entire device, including the battery and any connecting cables, mobile and portable.This allows the device for testing voltage transformers to be used quickly and easily in a variety of locations, extending over large parts or the entire power supply network.

[0021] In another embodiment, the impedance converter includes an amplifier with adjustable gain. This allows response signals from the voltage converter under test to be adjusted and adapted to a measurement range of the frequency response analyzer. Furthermore, it is possible to test a large number of different voltage converters, which can exhibit a wide range of different transformation ratios between the primary and secondary sides.

[0022] The invention further relates to a method for testing a voltage converter. In this method, a device as described above is provided, comprising the frequency response analyzer and the impedance converter. The impedance converter output is coupled to the response signal input of the frequency response analyzer. Finally, the input impedance of the impedance converter is adjusted to match the impedance of the voltage converter under test, thus ensuring impedance matching between the voltage converter under test and the impedance converter input. This impedance matching between the voltage converter under test and the impedance converter input, as well as between the impedance converter output and the response signal input, allows for the precise determination of the voltage converter's transfer function.

[0023] According to one embodiment, the method can include calibrating the frequency response analyzer, the impedance converter, and the measuring leads used. The method comprises, for example, connecting the test signal output to the reference signal input and the impedance converter input via measuring leads connected to the test signal output, the reference signal input, and the impedance converter input, respectively. For example, the first end of a first measuring lead can be connected to the test signal output, the first end of a second measuring lead to the reference signal input, and the first end of a third measuring lead to the impedance converter input. The second ends of the three measuring leads are then connected together.If the measuring leads are coaxial, the inner conductors of the second ends of the three measuring leads are connected together, and the outer conductors of the second ends of the three measuring leads are connected together. The impedance converter output of the impedance converter is connected to the response signal input of the frequency response analyzer, as described previously.

[0024] Several test signals at different frequencies are output via the test signal output. Corresponding calibration values ​​are acquired at the reference signal input and the response signal input. It is clear that the test signal output is acquired at the response signal input via the impedance converter, i.e., via the third measuring line connected to the impedance converter input and via the coupling between the impedance converter output and the response signal input. Each of the multiple calibration values ​​is assigned to a corresponding test signal or a corresponding frequency of that test signal.

[0025] Each of the multiple calibration values ​​can, for example, include an amplitude of a voltage signal at the reference signal input, an amplitude of a voltage signal at the response signal input, a ratio between the amplitude of the voltage signal at the reference signal input and the amplitude of the voltage signal at the response signal input, and / or a phase difference between the voltage signal at the reference signal input and the voltage signal at the response signal input.

[0026] Based on the calibration values, the gain of the impedance converter's amplifier can be adjusted, for example, to compensate for voltage drops on the measuring leads during subsequent measurements on a voltage converter. Similarly, phase differences caused by the measuring leads can be taken into account during subsequent measurements on a voltage converter.

[0027] After calibration, the connections between the second ends of the measuring leads are disconnected again.

[0028] To test a voltage converter, its transfer function can be determined at various frequencies. The transfer function might, for example, represent the voltage ratio between an input voltage and an output voltage over a specified frequency range. Alternatively or additionally, the transfer function might represent the phase shift between an input voltage and an output voltage over a specified frequency range.

[0029] According to one embodiment, for example, the test signal output and the reference signal input can be connected to a first terminal of the voltage converter via appropriate measuring leads. The first terminal of the voltage converter can, for example, be a terminal on an input side, such as a primary side, of the voltage converter. Furthermore, the impedance converter input can be connected to a second terminal of the voltage converter via a measuring lead. The second terminal of the voltage converter can, for example, be a terminal on an output side, such as a secondary side, of the voltage converter. Several test signals are output via the test signal output at different frequencies and fed into the voltage converter. For example, a signal with a specific voltage can be output, the frequency of which changes over time.For example, an alternating voltage of constant amplitude can be output, the frequency of which continuously sweeps through a predefined range, such as a range from a few hertz to several megahertz, for example, a range from 20 Hz to 2 MHz. Such signals are also called sweep or chirp signals.

[0030] While the test signals are output via the test signal output, several measured values ​​are acquired at the reference signal input and the response signal input. It is clear that to acquire the measured values ​​at the response signal input, signals are received from the voltage converter via the impedance converter input, the impedance converter including amplifier, the impedance converter output, and the coupling between the impedance converter output and the response signal input. Each of the multiple measured values ​​is assigned to a corresponding test signal among the multiple test signals.Each of the multiple measured values ​​can, for example, include an amplitude of the voltage signal at the reference signal input, an amplitude of the voltage signal at the response signal input, a ratio between the amplitude of the voltage signal at the reference signal input and an amplitude of the voltage signal at the response signal input, and a phase difference between the voltage signal at the reference signal input and the voltage signal at the response signal input.

[0031] The measuring leads used, as well as the connections where the measuring leads are coupled to the voltage transformer and the device, typically exhibit an impedance that can be frequency-dependent. To determine the transfer function of the voltage transformer as accurately as possible, it is desirable to consider and compensate for the effects of these (frequency-dependent) impedances. Precise information about the corresponding impedances is sometimes unavailable or can be variable, for example, due to different geometries at the connections or different routing of the measuring leads. If calibration values ​​have been determined, as described above, these calibration values ​​can be used to correct the measured values ​​so that at least the influence of the (frequency-dependent) impedances of the measuring leads on the measured values ​​can be substantially accounted for.According to one embodiment, one of several measured values ​​is corrected using a corresponding calibration value, wherein the measured value and the corresponding calibration value are assigned to a respective test signal with the same frequency. For example, at a given frequency, a corresponding calibration value assigned to that frequency can be subtracted from a measured value assigned to that frequency.

[0032] If a correction of the measured values ​​is carried out using the calibration values, in the following embodiments the measured values ​​preferably relate to the measured values ​​corrected using the calibration values.

[0033] According to one embodiment, a voltage ratio error between an expected voltage signal and a measured voltage signal at different frequencies is determined based on multiple measured values. An expected voltage signal can be determined, for example, based on a voltage signal at the reference signal input and the voltage converter's turns ratio. For instance, a respective voltage ratio error can be determined for different frequencies based on the amplitude of the voltage signal at the response signal input and the amplitude of the voltage signal at the reference signal input, taking into account the voltage converter's turns ratio. Furthermore, a phase shift at different frequencies can be determined based on the measured values.For example, a phase shift between the voltage signal at the response signal input and the voltage signal at the reference signal input can be determined for different frequencies.

[0034] The voltage ratio error or phase shift at different frequencies can be displayed on a display device coupled to the frequency response analyzer. This display device could, for example, be a screen on a notebook, tablet PC, or smartphone connected to the frequency response analyzer.

[0035] Furthermore, characteristic values ​​of the voltage converter can be determined based on the multiple measured values. Characteristic values ​​of a voltage converter include, for example, a frequency at a voltage ratio error of 2%, a frequency at a voltage ratio error of 5%, a frequency at a voltage ratio error of 10%, a resonant frequency, and / or a voltage ratio error at a frequency of 50 Hz.

[0036] The characteristic values ​​of the voltage converter can also be displayed on a display device coupled to the frequency response analyzer and stored, for example, for long-term monitoring, such as on a notebook, tablet PC or smartphone.

[0037] The condition of a voltage transformer can be determined based on the voltage ratio error, phase shift, and characteristic values, for example, by comparing them with corresponding setpoint values ​​or values ​​recorded during commissioning, or by observing changes in these values ​​over a longer period. This allows it to be determined whether the voltage transformer is in proper working order.

[0038] The procedure described above can be carried out, for example, using the device described above. BRIEF DESCRIPTION OF THE FIGURES

[0039] The invention is explained in more detail below with reference to the drawings and preferred embodiments. In the drawings, identical reference numerals denote identical elements. Fig. 1 Figure 1 schematically shows a device for testing a voltage converter according to an embodiment of the present invention in conjunction with a conventional voltage converter to be tested. Fig. 2 schematically shows the device for testing a voltage converter of the Fig. 1 in conjunction with an LPVT under test (for example, a resistive-capacitive voltage divider). Fig. 3 shows procedure steps for testing a voltage converter according to one embodiment. DETAILED DESCRIPTION OF EXAMPLES OF EXECUTION

[0040] The present invention is now explained in more detail with reference to preferred embodiments and the figures. In the figures, identical reference numerals denote identical or similar elements. The figures are schematic representations of various embodiments of the invention. The elements depicted in the figures are not necessarily shown to scale. Rather, the various elements shown in the figures are represented in such a way that their function and purpose are understandable to a person skilled in the art.

[0041] The connections and couplings between functional units and elements depicted in the figures can be implemented as direct or indirect connections or couplings. A connection or coupling can be implemented wired or wirelessly.

[0042] The following section describes in detail the procedures and equipment used to test a voltage transformer. The condition of a conventional voltage transformer (i.e., an inductive voltage transformer) can be affected by defects in the windings, such as insulation faults or winding misalignment. The condition of an LPVT (low-voltage voltage transformer) can be affected by defects in the capacitors or the resistive components of the RC voltage divider. There are various reasons for these defects, such as moisture ingress into the insulation. Testing a voltage transformer can help prevent the mismanagement of a power transmission network due to faulty readings from the voltage transformer, or it can prevent a total failure of the voltage transformer. A total failure can endanger other equipment or people.

[0043] Fig. 1 Figure 1 schematically shows a conventional inductive voltage converter 10. The voltage converter 10 comprises a transformer 11, which is arranged in a housing 12. The transformer 11 comprises a primary winding 13 and a secondary winding 14. The primary winding 13 and the secondary winding 14 determine a turns ratio Ü of the transformer 11. One end of the primary winding 13 is connected to a terminal 15, and another end of the primary winding 13 is connected to a terminal 19, which is coupled to ground. The secondary winding 14 is connected to two terminals 17 and 18. Reference numeral 16 denotes a ground terminal of the housing 12. The output impedance of the voltage converter 10, as can be measured, for example, at terminals 17 and 18, is essentially determined by the secondary winding 14 and can, for example, be in the range of a few ohms to several hundred ohms or several kilohms.

[0044] Fig. 1 Figure 50 further shows a device 50 for testing the voltage converter 10. The device 50 comprises a frequency response analyzer 60 and an impedance converter 70. The frequency response analyzer 60 and the impedance converter 70 are in Fig. 1 While depicted as two separate units, they can be designed as one unit or at least integrated into a common housing.

[0045] The frequency response analyzer 60 comprises a signal generation device 63 with an output impedance 62, configured to output a test signal with variable frequency and a predefined voltage at a test signal output 61. The test signal can, for example, be a low-voltage signal with a voltage of, say, 10 V. The signal generation device 63 can, for example, output a sinusoidal voltage with a continuously increasing frequency, for example, in a frequency range of 10 Hz to 10 MHz, or, for example, in a range of 20 Hz to 2 MHz.

[0046] The frequency response analyzer 60 further comprises a reference signal acquisition device 66 with an input impedance 65, which is coupled to a reference signal input 64.

[0047] Furthermore, the frequency response analyzer 60 includes a measurement signal acquisition device 69 with an input impedance 68, which is coupled to a response signal input 67.

[0048] The frequency response analyzer 60 can be a device that can be used for SFRA (Sweep Frequency Response Analysis) measurements on power transformers. The output impedance 62 and the input impedances 65, 68 can each be, for example, 50 ohms.

[0049] For example, while the frequency response analyzer 60 outputs a sinusoidal test voltage with a continuously increasing frequency at the test signal output 61, the frequency response analyzer 60 can receive a reference signal at the reference signal input 64 and a response signal at the response signal input 67 and relate the response signal to the reference signal.

[0050] The frequency response analyzer 60 can, for example, be supplied with electrical energy for the operation of the frequency response analyzer 60 by a battery 90.

[0051] The device 50 further comprises the impedance converter 70. The impedance converter 70 has an impedance converter input 71 with an adjustable input impedance 72. The input impedance 72 can be set, for example, in a range from a few ohms to several megaohms. The input impedance can be adjustable, for example, in a range from 1 ohm to 10 megaohms. The impedance converter 70 further comprises an amplifier 73, for example, an operational amplifier, with adjustable gain. The gain can be adjustable in a range from 1 to several thousand, for example, up to 2000 or 10,000. An output of the amplifier 73 is connected via an output impedance 74 to an impedance converter output 75. The output impedance 74 can, for example, be equal to the input impedance 68 of the frequency response analyzer 60, i.e., for example, 50 ohms.The impedance converter 70 can, for example, be powered by a battery 91. Batteries 90 and 91 can be separate or combined. Batteries 90 and 91 can be rechargeable. Alternatively or additionally, the frequency response analyzer 60 and the impedance converter 70 can be powered by a power supply unit.

[0052] To test the voltage converter 10, the primary side of the transformer 11 is connected to the test signal output 61 and the reference signal input 64. Corresponding lines 81, 82, which are also commonly referred to as test leads, can be, for example, coaxial cables. The outer conductors of the coaxial cables 81, 82 are each connected to ground at the frequency response analyzer 60, for example, via the housing of the frequency response analyzer 60. At the voltage converter 10, the outer conductors of the coaxial cables 81, 82 are each connected to the housing ground 16. The center conductor of the coaxial cable 81 is connected to the test signal output 61 at the frequency response analyzer 60 and to terminal 15 at the voltage converter 10, which is connected to the primary winding 13 of the transformer 11.The inner conductor of the coaxial cable 82 is connected to the reference signal input 64 of the frequency response analyzer 60 and to terminal 15 of the voltage converter 10. The reference signal acquisition device 66 thus detects the test signal from the signal generation device 63 via the reference signal input 64, as it is fed into the voltage converter 10, i.e., taking into account any interference or losses due to transmission via the coaxial cable 81. It is clear that the cables 81 and 82 can be implemented in any other way, for example, as twisted pairs or as single cables that only transmit the test signal or the reference signal, respectively, but do not provide a ground connection. In this case, a suitable ground connection can be established via a separate connection between the device 50 and the voltage converter 10.

[0053] Another line 83, in particular a measuring line, for example a coaxial line, connects the secondary side of the voltage converter 10 to the impedance converter input 71. For example, at the voltage converter 10, an inner conductor of the coaxial line 83 can be connected via terminal 17 to one side of the secondary winding 14 of the transformer 11, and an outer conductor of the coaxial line 83 can be connected via terminal 18 to another side of the secondary winding 14. Terminal 18 can also be connected to ground. At the impedance converter 70, the inner conductor of the coaxial line 83 can be connected to the impedance converter input 71, and the outer conductor of the coaxial line 83 can be connected to ground, for example via a housing of the impedance converter 70.

[0054] The impedance converter output 75 is connected to the response signal input 67 via a line 84, in particular another measuring line, for example a coaxial cable. For example, at the impedance converter 70, an inner conductor of the coaxial cable 84 can be connected to the impedance converter output 75 and an outer conductor of the coaxial cable 84 to ground, for example via the housing of the impedance converter 70. At the frequency response analyzer 60, the inner conductor of the coaxial cable 84 can be connected to the response signal input 67 and the outer conductor of the coaxial cable 84 to ground, for example via the housing of the frequency response analyzer 60.

[0055] Lines 83 and 84 can be implemented in any other way, for example as twisted pairs or as single lines that only transmit the response signal from voltage converter 10 to impedance converter 70 or the impedance-matched response signal from impedance converter 70 to frequency response analyzer 60, but do not establish a ground connection. A suitable ground connection can be established via separate connections between voltage converter 10, impedance converter 70, and frequency response analyzer 60.

[0056] Fig. 2 Figure 1 schematically shows a voltage converter 20 of the type of a resistive-capacitive LPVT. The voltage converter 20 comprises a series connection of two capacitors 21 and 22, which operate as capacitive voltage dividers. The series connection is connected to terminals 15 and 19. A resistive voltage divider 23 and 24 is connected in parallel. The output impedance of the voltage converter 20, as measured at terminals 17 and 18, is therefore essentially determined by the capacitor 22 and the resistor 24. In contrast to the output impedance of the [unclear text] Fig. 1 The output impedance of the voltage converter 10 shown, which can range from a few ohms to a few kilohms, can be in the range of a few hundred kilohms to a few megaohms. The turns ratio of the resistive-capacitive voltage converter 20 is determined by both the capacitances C1 and C2 of capacitors 21 and 22, respectively, and by the resistance values ​​R1 and R2 of resistors 23 and 24, respectively. A complex transfer function k R ( jω ) = U 2 / U 1 with the complex voltage U 1 between terminals 15 and 16 and the complex voltage U The distance between terminals 17 and 18 is: k _ R jω = U _ 2 U _ 1 = Z 2 Z _ ges = R 2 R 2 + R 1 ⋅ 1 + jωC 2 R 2 1 + jωC 1 R 1

[0057] The in Fig. 2 The resistive-capacitive voltage converter 20 shown is of the same type as the one in Fig. 1 The inductive voltage converter 10 shown is connected to the device 50 using the measuring leads 81 to 83.

[0058] With reference to Fig. 3 The following describes a procedure 300 for testing a voltage transformer with the one described in the Fig. 1 and 2 The device shown in 50 will be described in detail.

[0059] In step 301, the frequency response analyzer 60 and the impedance converter 70 are positioned near the voltage converter under test. The voltage converter can, for example, be the one described in Fig. 1 the inductive voltage converter 10 shown or the one in Fig. 2 The illustrated resistive-capacitive voltage converter 20 comprises the following components. In step 302, the impedance converter output 75 of the impedance converter 70 is coupled to the response signal input 67 of the frequency response analyzer 60 via line 84. As previously described, the output impedance 74 of the impedance converter 70 at the impedance converter output 75 essentially corresponds to the input impedance 68 of the frequency response analyzer 60 at the response signal input 67.

[0060] Depending on the voltage transformer 10, 20 being tested, the input impedance 72 of the impedance transformer 70 is set in step 303. The output impedance of the voltage transformer 10, 20 can either be measured, taken from the nameplate of the voltage transformer, or determined from it, for example, from the burden specified on the nameplate of the voltage transformer.

[0061] Optionally, in steps 304-306, the device 50 can be calibrated taking into account the measuring lines 81-83. For this purpose, a calibration configuration can be set up in step 304. Line 81 is connected to the test signal output 61, line 82 to the reference signal input 64, and line 83 to the impedance converter input 71. The three free ends of lines 81, 82, and 83 are connected directly to each other. If lines 81, 82, and 83 are coaxial, the inner conductors of lines 81, 82, and 83 are connected directly to each other, and separately, the outer conductors of lines 81, 82, and 83 are connected directly to each other. In step 305, test signals are generated by the signal generation device 63 and output via the test signal output 61.The test signals can include, for example, chirp signals, i.e., a signal whose frequency changes over time. The test signals can also include sweep signals, i.e., an alternating voltage of constant amplitude whose frequency periodically and continuously sweeps through a predefined range. Finally, the test signals can include voltage signals with an amplitude in the range of a few volts, for example, 10 V.

[0062] While the test signals are output in step 305, corresponding calibration values, such as voltage signals, are acquired in step 306 at the reference signal input 64 and (via the impedance converter 50) at the test signal input 67. The transmission characteristics of, in particular, line 82, line 83, the impedance converter 70, and line 84 can be determined by analyzing the calibration values ​​and subsequently used to correct measured values ​​during testing of the voltage converter 10, 20. One calibration value can, for example, be a voltage signal at the reference signal input, and another calibration value can, for example, be a voltage signal at the response signal input. Additional calibration values ​​can be determined from the acquired calibration values.For example, an amplitude ratio between the amplitude of the voltage signal at the reference signal input and the amplitude of a voltage signal at the response signal input can be determined as an additional calibration value. Similarly, a phase difference between the voltage signal at the reference signal input and the voltage signal at the response signal input can be determined as an additional calibration value. The acquired and additionally determined calibration values ​​can be acquired or determined at different frequencies and assigned to those frequencies. For example, corresponding amplitude ratios and phase differences can be assigned to some or all of the multiple different frequencies at which the test signal was output.

[0063] At the end of the calibration, the directly connected ends of lines 81, 82 and 83 are separated from each other.

[0064] Next, in step 307, the gain of amplifier 73 of the impedance converter 70 is adjusted. When adjusting the gain, findings from the previous calibration can be taken into account. For example, the amplitude ratio at a specific frequency or an average of the amplitude ratios over a specific frequency range can be determined in order to adjust the gain of amplifier 73 so that the amplitude ratio is essentially balanced. Furthermore, the turns ratio of the voltage converter and the input sensitivity of the response signal input can be considered when adjusting the gain of amplifier 73, so that the voltage range expected at the output of the voltage converter 10 due to the test signal lies within the measuring range of the signal acquisition device 69 and utilizes it as fully as possible.

[0065] In step 308, a test configuration is set up in conjunction with the voltage transformer 10 or 20. As in Fig. 1 and Fig. 2As shown, the test signal output 61 is connected via line 81 to terminal 15 of the voltage converter 10 or 20. If line 81 also carries a ground connection, this is connected to ground 16 of the housing 12 of the voltage converter 10 or 20. The reference signal input 64 is also connected via line 82 to terminal 15 of the voltage converter 10 or 20, and if line 82 carries a ground connection, this is connected to terminal 16 (ground) of the housing 12 of the voltage converter 10 or 20. The impedance converter 71 is connected via line 83 to terminal 17 of the voltage converter 10 or 20, and if line 83 carries a ground connection, this is connected to terminal 18 of the housing 12 of the voltage converter 10 or 20. It should be noted that line 84 still connects the impedance converter output 75 to the response signal input 67 of the frequency response analyzer 60.

[0066] In step 309, test signals are generated by the signal generation device 63 and fed into the primary side of the voltage converter 10, 20 via the test signal output 61 and line 81. The test signals can include, for example, chirp signals, i.e., a signal whose frequency changes over time. The test signals can also include sweep signals, i.e., an alternating voltage of constant amplitude whose frequency periodically and continuously sweeps through a predefined range. The test signals can also include voltage signals with an amplitude in the range of a few volts, for example, 10 V. Other test signals are possible, such as signals with constant frequency and variable amplitude, pulse signals, and the like.

[0067] While the test signals are output in step 309, corresponding measured values, for example voltage signals, are acquired in step 310 at the reference signal input 64 and (via the impedance converter 50) at the test signal input 67 using the reference signal acquisition device 66 and the measurement signal acquisition device 69, respectively. The transfer characteristics of the voltage converter 10, 20 can be determined by analyzing these measured values, for example, using a processing device (e.g., a microprocessor with associated memory) of the frequency response analyzer 60 (not shown). If the previously described calibration has been performed, the acquired measured values ​​can be corrected in step 311 using the calibration values. In particular, this allows the influence of line 82, line 83, the impedance converter 70, and line 84 on the acquired measured values ​​to be corrected.

[0068] The measured values ​​can include, for example, a voltage signal at the reference signal input and a voltage signal at the response signal input. Additional values ​​can be determined from the acquired measurements. For example, an amplitude ratio between the amplitude of the voltage signal at the reference signal input and the amplitude of the voltage signal at the response signal input can be determined. Furthermore, a phase difference between the voltage signal at the reference signal input and the voltage signal at the response signal input can be determined. The acquired measurements and the additionally determined values ​​can be acquired at different frequencies or determined for different frequencies and assigned to those different frequencies.For example, corresponding amplitude ratios and phase differences can be assigned to some or all of the several different frequencies at which the test signal was output.

[0069] The amplitude ratio and / or phase difference can be corrected using corresponding values ​​from the calibration. The correction can be performed for each frequency to which the amplitude ratio or phase difference is assigned.

[0070] Based on the measured values ​​obtained in this way and additionally determined values, a transfer function of the voltage transformer can be determined. For example, in step 312, a voltage ratio error of the voltage transformer can be determined; in particular, voltage ratio errors can be determined for the different frequencies at which the test signal was fed into the voltage transformer. Furthermore, in step 312, a phase shift of the voltage transformer can be determined for the different frequencies. In step 313, the voltage ratio error and / or the phase shift can be displayed, for example, as a graph versus frequency on a display device.

[0071] The display device can be, for example, a display device of a notebook, tablet PC or smartphone connected to the device 50.

[0072] Furthermore, in step 314, characteristic values ​​of the voltage transformer 10, 20 can be calculated and displayed based on the measured values ​​and the additionally determined values. A characteristic value of the voltage transformer 10, 20 could, for example, be a frequency at a voltage ratio error of 2%. For instance, starting from a nominal frequency of 50 Hz, it can be determined up to which higher frequency the voltage ratio error is less than 2%. The frequency at which the voltage ratio error first exceeds 2% can be displayed as a corresponding characteristic value. A corresponding characteristic value can, for example, be determined starting from the nominal frequency of 50 Hz and moving towards lower frequencies. Other characteristic values ​​of the voltage transformer could, for example, be a frequency at a voltage ratio error of 5% or 10%.Another characteristic value of the voltage converter can be its resonant frequency (10, 20), for example, the frequency at which the largest output amplitude is achieved with a constant input amplitude, or at which the output signal has a phase angle of 90° to the input signal. The voltage ratio error at the nominal frequency, for example at 50 Hz, can also be determined as a further characteristic value.

[0073] In summary, the combination of the frequency response analyzer 60 and the impedance converter 70 offers the possibility of testing both conventional inductive voltage converters 10 and LPVT voltage converters 20. Furthermore, such a frequency response analyzer 60, in conjunction with the impedance converter 70, can be designed as a compact, portable device, allowing such tests to be easily performed on-site.

[0074] The described method is suitable for on-site measurements, allowing for verification of integrity and transmission behavior in the installed state (e.g., during on-site acceptance testing or routine measurements) and the monitoring and display of critical frequencies (e.g., voltage ratio errors of 2%, 5%, 10%) over time. Furthermore, this method is also suitable for manufacturers during the production process, as the device used is compact and lightweight, making it easy to integrate into the production workflow. Additionally, the voltage levels used are low, thus reducing the risk to operating personnel. The measurement itself is highly accurate, particularly due to impedance matching and, if necessary, calibration.

[0075] The impedance converter 70 not only adapts the impedance to the nominal load of the voltage converter 10, 20, but also amplifies the signal present on the secondary side of the voltage converter 10, 20. Furthermore, this measurement setup has the advantage that the connection between the impedance converter 70 and the secondary side of the voltage converter 10, 20 can be kept short to avoid reflections. A 50-ohm impedance match is achieved on the other side of the impedance converter. This measurement setup also allows for simple gain / phase calibration, e.g., a measurement setup calibration.

Claims

1. A device for testing a voltage converter, comprising: - a frequency response analyser (60) which is configured to measure an electrical transfer function over a predefined frequency range, wherein the frequency response analyser (60) comprises a test signal output (61) for outputting a test signal for the voltage converter (10, 20), a reference signal input (64) for receiving a reference signal which is applied to the voltage converter (10, 20) for the purpose of testing the voltage converter (10, 20), and a response signal input (67) having a predefined input impedance (68) for receiving a response signal from the voltage converter (10, 20), characterized by - an impedance converter (70) having an impedance converter input (71), which has a variable input impedance (72) which can be adjusted to an impedance of the voltage converter (10, 20), and an impedance converter output (75), which is coupled to the response signal input (67) and has an output impedance (74) matched to the input impedance (68) of the response signal input (67).

2. The device according to claim 1, wherein the input impedance (72) of the impedance converter input (71) can be adjusted in a range of from 30 ohms to 100 megaohms, preferably in a range of from 50 ohms to 100 megaohms.

3. The device according to claim 1 or claim 2, wherein the predefined input impedance (68) of the response signal input (67) of the frequency response analyser (60) is 50 ohms.

4. The device according to any one of the preceding claims, wherein an output impedance (62) at the test signal output (61) of the frequency response analyser (60) is 50 ohms and an input impedance (65) of the reference signal input (64) of the frequency response analyser (60) is 50 ohms.

5. The device according to any one of the preceding claims, wherein the device (50) comprises at least one battery (90, 91) which is configured to provide electrical power for the purpose of running the frequency response analyser (60) and / or the impedance converter (70).

6. The device according to any one of the preceding claims, wherein the device (50) is configured as a mobile portable device.

7. A method of testing a voltage converter, wherein the method (300) comprises: - providing (301) a device according to any one of claims 1-6, - coupling (302) the impedance converter output (75) to the response signal input (67), and - adjusting (303) the input impedance (72) of the impedance converter (70) to an impedance of the voltage converter (10, 20).

8. The method according to Claim 7, further comprising: - connecting (304) the test signal output (61) to the reference signal input (64) and the impedance converter input (71) via measuring lines (81-83) which are connected to the test signal output (61), the reference signal input (64) and the impedance converter input (71) respectively, - outputting (305) a plurality of test signals via the test signal output (61) at different frequencies, and - acquiring (306) a plurality of calibration values at the reference signal input (64) and the response signal input (67) via the impedance converter (70) and impedance converter input (71), wherein each calibration value of the plurality of calibration values is assigned to a corresponding test signal of the plurality of test signals.

9. The method according to claim 8, wherein each calibration value of the plurality of calibration values comprises at least one of the following values: an amplitude of a voltage signal at the reference signal input (64), a ratio between the amplitude of the voltage signal at the reference signal input (64) and an amplitude of a voltage signal at the response signal input (67), and a phase difference between the voltage signal at the reference signal input (64) and the voltage signal at the response signal input (67).

10. The method according to Claim 8 or Claim 9, further comprising: - adjusting an amplification (307) of an amplifier (73) of the impedance converter (70) depending on at least one of the plurality of calibration values.

11. The method according to any one of Claims 7 to 10, further comprising: - connecting (308) the test signal output (61) and the reference signal input (64) to a first terminal (15) of the voltage converter (10, 20) via measuring lines (81, 82) and connecting (308) the impedance converter input (71) to a second terminal (17) of the voltage converter (10, 20) via a measuring line (83), - outputting (309) a plurality of test signals via the test signal output (61) at different frequencies, and - acquiring (310) a plurality of measurement values at the reference signal input (64) and the response signal input (67) via the impedance converter (70) and impedance converter input (71), wherein each measurement value of the plurality of measurement values is assigned to a corresponding test signal of the plurality of test signals.

12. The method according to claim 11, wherein each measurement value of the plurality of measurement values comprises at least one of the following values: an amplitude of a voltage signal at the reference signal input (64), a ratio between the amplitude of the voltage signal at the reference signal input (64) and an amplitude of a voltage signal at the response signal input (67), and a phase difference between the voltage signal at the reference signal input (64) and the voltage signal at the response signal input (67).

13. The method according to Claim 11 or Claim 12, further comprising: - correcting (311) a measurement value of the plurality of measurement values using a calibration value, wherein the measurement value and the calibration value are assigned to a respective test signal with an identical frequency.

14. The method according to any one of Claims 11 to 13, further comprising: - determining (312) a voltage ratio error and / or a phase shift at the different frequencies based on the plurality of measurement values, and - depicting (313) the voltage ratio error and / or the phase shift at the different frequencies on a display device which is coupled to the frequency response analyser (60).

15. The method according to any one of Claims 11 to 14, further comprising: - determining (314) characteristic values of the voltage converter (10, 20) based on the plurality of measurement values, wherein the characteristic values comprise at least one value from a group comprising: - a frequency at a voltage ratio error of 1%, - a frequency at a voltage ratio error of 5%, - a frequency at a voltage ratio error of 10%, - a resonant frequency, and - a voltage ratio error at a frequency of 50 Hz.

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