Method and device for testing transformer

Abstract

The invention relates to a method (200) for testing a transformer (180). The method comprises determining (202) an edge gradient of the test signal (702) from the transformer (180) to be tested and generating (204) the test signal (702) with the determined edge gradient. The method further includes testing (206) the transformer (180) using the generated test signal (702).

Description

Field of the invention

[0001] The present invention relates to a method for testing a transformer and a corresponding configured device. In particular, the invention is suitable for testing and measuring a transformer used as a current transformer. The testing of the transformer can comprise, for example, the measurement of a magnetization curve. BACKGROUND

[0003] In the energy technology, measuring transformers, in particular current transformers, are used, which are based on transformers designed to meet the measuring requirements, which enable a potential-free measurement of large alternating currents at high voltages. In order to ensure the suitability of the current transformer, the requirements for the current transformer must be checked, which are defined, for example, in the standard EN 61869-2. These requirements relate, inter alia, to the magnetization parameters, of which, inter alia, the magnetization curve and the saturation magnetization can be relevant.

[0004] For this purpose, test devices can generate, for example, a sinusoidal signal with mains frequency, which is fed into the device under test, for example a current transformer. Thus, a voltage and a current can be generated on the secondary side of the current transformer, which also occurs in normal operation. Thus, in the case of conventional current transformers, test signals are required, for example, with a power of up to 5 KW. Due to this high output power, these test devices are very heavy, on the other hand, they can only test small power current transformers.

[0005] In addition, there are test methods in which a test signal with a reduced test frequency or test voltage is used to test the respective current transformer. However, these methods are generally very imprecise, since the varying behavior of the current transformer is not adequately taken into account when operating at a frequency deviating from the nominal frequency or the mains frequency.

[0006] Against this background, EP 1 398 644 B1 discloses a method for testing a transformer, in which, in order to measure a plurality of, in particular frequency-dependent, transformer parameters, a test signal with a frequency lower than the rated frequency or the operating frequency of the transformer is applied to the transformer. From the parameters measured according to the test signal, a simulation model is derived, which enables the behavior of the transformer at different frequencies to be simulated. By means of the simulation model or using the parameters measured using the test signal, the behavior of the transformer when operating at a frequency deviating from the frequency of the test signal, for example the behavior of the transformer when operating at the rated frequency, can be inferred. In particular, a rectangular signal with a fundamental frequency lower than the rated frequency can be used as a test signal.

[0007] The use of rectangular signals is particularly suitable for achieving a correspondingly large magnetic flux in the magnetic core. However, in this case, problems can arise in that at the steepness of the high edges of the rectangular signal, in combination with the high voltages in the sensor due to its design having a high parasitic capacitance, capacitive current peaks can occur. These currents can overload the test device or require a correspondingly larger size, thus more expensive and heavier test device. Furthermore, this can impair the accuracy of the measurement. SUMMARY

[0008] It is therefore an object of the present application to provide a method for accurately measuring the magnetization parameters of a transformer, in particular the magnetization curve of a current transformer, which can be implemented cost-effectively and is suitable for a wide range of transducers.

[0009] This object is achieved by the method for testing a transformer and the device for testing a transformer, which are defined in the independent claims. The dependent claims define advantageous embodiments of the application.

[0010] According to the application, in a method for testing a transformer, the edge steepness of a test signal is determined depending on the transformer to be tested, and a test signal having the thus determined edge steepness is generated. The transformer is tested using the generated test signal.

[0011] In other words, a test signal is used with a signal form having an edge steepness depending on the transformer to be tested. The test signal used in this signal form thus has defined harmonics, i.e. defined components of the frequency above the fundamental frequency of the test signal. In particular, it is thereby possible to achieve that the test signal in this signal form has fewer frequency components than, for example, a rectangular signal, which is usually generated by a constant voltage source connected via a bridge. Furthermore, undershoots and overshoots can be avoided or reduced by adjusting the edge steepness. The edge steepness of the test signal is not determined by the switching mechanism used in the generation of the test signal, but is adjusted depending on the transformer. For this purpose, a corresponding controllable signal source can be used. Due to the adjustable edge steepness, in particular a lower edge steepness compared to a connected constant voltage source, large capacitive current peaks can be avoided, so that on the one hand an overload of the device generating the test signal can be avoided, and on the other hand, because the current to be measured occurs within a limited range, so that it can be accurately acquired, the accuracy of the measurement can be improved. By determining and adjusting the edge steepness depending on the transformer, it is also possible to test, for example, current transformers having a high capacitance as a result of the design; in particular, for example, the magnetization curve can be accurately measured.

[0012] The test signal can be generated, for example, by driving at least one adjustable voltage converter. The voltage converter can be, for example, a direct current voltage converter (so-called DC-DC converter). In order to produce an alternating voltage with a desired edge steepness, for example, two voltage converters can be used. One of the two voltage converters produces an adjustable positive voltage to ground, and the other of the two voltage converters produces an adjustable negative voltage to ground. Both the adjustable positive voltage and the adjustable negative voltage can be adjusted by a control device to a plurality of voltage values and are supplied to the transformer to be tested. The previously determined edge steepness of the rising and falling edges can be achieved by a corresponding adjustment of the change in voltage over time. The adjustable range of the output voltage of the voltage converter can be, for example, from zero to 25 V or from zero to 50 V, for example, in steps of 1 V or 0.1 V.

[0013] Due to the adjustable positive voltage to ground and the negative voltage to ground, for example, the core of the transformer can be magnetized in both directions in order to measure the magnetization curve.

[0014] For example, the stray capacitance of the transformer can be determined in order to determine the edge steepness of the test signal. From the stray capacitance and a predetermined maximum current that can occur during the testing of the transformer, the edge steepness can be determined. The stray capacitance can be caused, for example, by connection lines, connection terminals and the structure of the transformer. These capacitances can lead to current peaks, in particular at high frequencies. By adjusting the edge steepness of the test signal, high frequencies can be avoided, for example, by reducing the edge steepness. This means that current peaks can be avoided. The load of the test device is reduced, since it has to provide a lower maximum current. As a result, the reliability of the test device can be improved and the test device can be made more compact and lighter.

[0015] The stray capacitance can be determined by measurements on the transformer to be tested. In particular, for example, a high-frequency measurement can be made on the transformer. A high-frequency measurement on the transformer means, for example, that the frequency at which the stray capacitance is measured is higher than the rated frequency, for example, a multiple of the rated frequency. The rated frequency of the transformer can be, for example, 50 or 60 Hz. The high frequency used in the measurement of the stray capacitance can be, for example, higher than 1 kHz or even higher than 100 kHz. The voltage used in this case can be correspondingly low, in particular lower than the voltage to be used when testing the transformer. For example, the transformer can be tested at + / - 25 V, while the stray capacitance is measured at + / - 5 V.

[0016] Alternatively or additionally, the stray capacitance can be input by a user. The user can have determined the value of the stray capacitance using additional measurement equipment, for example, or the stray capacitance can be stored in a database. Furthermore, the type of transformer to be tested and, if necessary, its connection configuration can be input, for example, the stray capacitance can then be retrieved from the database.

[0017] It is clear that the parasitic capacitance currently measured is advantageous in that in this case the actual state of the transformer is taken into account and the edge steepness can be optimally adjusted to the test signal. A correspondingly configured test device can for example first determine the parasitic capacitance automatically by means of a measurement and then start the actual test of the transformer, for example the measurement of the magnetization curve of the transformer, using the edge steepness determined from the parasitic capacitance. Taking the parasitic capacitance into account can improve the measurement accuracy. On the one hand, the current peaks caused by the parasitic capacitance can be reduced and determined by the selected edge steepness, so that the regular inductive current through the transformer and the capacitive current peaks can be distinguished. For example, the regular inductive current thus determined can then be used to measure the magnetization curve of the transformer very precisely.

[0018] According to one embodiment, the test signal is a periodic test signal. The edge steepness can be determined using an iterative method. In this iterative method, the edge steepness of a subsequent cycle of the test signal is determined from the maximum current measured during the use of the test signal in the previous cycle of the test transformer based on the edge steepness in one cycle of the test signal. For example, the edge steepness can be increased slightly from one cycle to the next. For example, starting from a relatively low edge steepness, the edge steepness can be increased slowly, thereby increasing the capacitive current peaks. This can continue until a desired or allowed maximum current is reached. The maximum current can correspond to, for example, the rated current of the test device or a certain percentage of the maximum current of the test device, for example 90% of the maximum current. Once the edge steepness has been determined by means of the iterative method, the edge steepness determined in this way can be used to make measurements on the transformer.

[0019] The test signal can be, for example, a periodic voltage signal. Each cycle, the voltage signal can comprise a voltage rising portion from a lower voltage value to a higher voltage value with the determined edge steepness, a higher portion with substantially constant voltage at the higher voltage value, and a voltage falling portion from the higher voltage value to the lower voltage value. It is clear that the lower voltage value is smaller than the higher voltage value. The lower voltage value can be, for example, a negative voltage to ground, for example, the higher voltage value can be a positive voltage to ground. The values of the higher voltage value and the lower voltage value can be different or the same. The voltage signal can comprise a lower portion with substantially constant voltage at the lower voltage value each cycle. The length of the lower portion and the length of the higher portion can be substantially equal. The voltage falling portion can have an edge steepness equal to the edge steepness size of the voltage rising portion.

[0020] It will be appreciated that the lower portion can also have a different length than the higher portion. Also, the voltage falling portion can have a different edge steepness in amplitude than the voltage rising portion. However, in many embodiments, the higher portion will have substantially the same length as the lower portion, and the edges of the voltage falling portion and the voltage rising portion will have substantially the same steepness in amplitude. In these cases, the voltage signal is symmetrical to ground.

[0021] The edge steepness can be constant with the rise time or the fall time, so the test signal has a trapezoidal signal course. However, the edge steepness also varies with the rise time or the fall time, for example, starting with a lower steepness in amplitude, then increasing in amplitude and reaching its maximum value in the region of the zero crossing of the voltage signal, and then decreasing again. This results in a trapezoidal signal course with "soft" transitions between the substantially constant plateau portions and the edges.

[0022] According to other embodiments, the time of the edge start of the test signal is adjusted or initiated in dependence on a comparison of the current generated by the test signal through the transformer with a threshold value. As described above, the test signal can be periodic with a substantially constant higher portion and a substantially constant lower portion. The edge of the test signal starts, for example, when the voltage falls from a substantially higher (positive) voltage value or the voltage rises from a substantially lower voltage value. In a transformer test, for example, the substantially constant voltage of the higher portion results in a (positive) current through the winding of the transformer which rises due to the inductance of the winding. The current through the winding can be measured, and when the current reaches a threshold value, the voltage fall is initiated with a previously determined edge steepness. However, as long as the voltage is still positive during the falling edge, the current through the winding continues to increase. Only after the zero crossing of the voltage, the current through the winding starts to decrease. The voltage reaches a lower (negative) voltage value and then remains substantially constant. The (positive) current further decreases, becomes zero, and then flows in the opposite direction (negative current) through the winding and increases in amplitude, while the voltage value remains substantially constant at the lower voltage value. As soon as the (negative) current through the winding reaches a threshold value in amplitude, the rising edge starts. As long as the voltage is still negative during the rising edge, the (negative) current continues to increase in amplitude. Only when the rising edge reaches the zero crossing and the subsequent positive voltage is applied to the transformer again, the amplitude of the (negative) current decreases. When the voltage has a substantially constant higher (positive) voltage value, the (negative) current first falls to zero in amplitude and then rises again in amplitude in the opposite direction (positive current) until the threshold value is reached. Then the above cycle starts again.

[0023] As mentioned above, because the edges do not have an infinite steepness, and thus the voltage remains positive (falling edge) or negative (rising edge) until the zero crossing, the current initially continues to increase in amplitude even at the beginning of the edge. In contrast, the edge steepness is relatively low in order to not contain high frequency components, thereby avoiding current peaks due to parasitic capacitances. Thus, it is unavoidable that the current further rises even after the beginning of the edge, and has to be taken into account in order not to exceed a desired maximum current. For example, in order to avoid overloading of the test device. Thus, the magnitude of the threshold of the start edge can be set to the magnitude of the maximum rated current of the test device providing the test signal, for example to a value in the range of 70% to 95% of the rated current, for example 90% of the rated current, or depending on the further increase of the inductive current which in turn depends on the set edge steepness.

[0024] For example, the magnitude of the threshold can be dynamically adjusted according to an observation of the actual current through the transformer. For example, the threshold can be determined according to the edge steepness of the test signal, the voltage swing of the test signal, and the absolute current rise rate of the current generated by the test signal through the transformer. For example, in the case of a periodic test signal, the voltage swing of the test signal is the voltage difference between the higher and lower voltage values. Assuming, for example, that the edge steepness is constant, i.e. the voltage drop per unit of time from the beginning to the end of the edge is constant, the time from the beginning of the edge to the voltage zero crossing can be determined in a simple manner. Even if the edge does not have a constant edge steepness, the time from the beginning of the edge to the voltage zero crossing can be determined since the edge shape is determined and adjusted. During this time, as mentioned above, the current through the transformer continues to increase at least due to the inductance of the transformer winding. In addition, the current through the parasitic capacitances can increase. The current rise rate of the current through the transformer can be measured by, for example, continuously measuring the current during the test of the transformer and determining the corresponding current change, i.e. the current change per unit of time. Continuous can mean, for example, that the current is continuously measured at short time intervals, for example 1 μs, and the current change is determined from the current change per microsecond. The longest period of time during which the current can continue to rise is the time from the beginning of the edge to the voltage zero crossing. For example, the last determined current rise rate is multiplied by this period of time from the beginning of the edge to the voltage zero crossing to estimate the further voltage rise that can be achieved during this time. Based on this estimate, the threshold can be set such that the current does not exceed a desired maximum, for example the maximum rated current of the test device. In other words, the threshold can be determined as the difference between the magnitude of the desired maximum current generated by the test signal through the transformer and the product of the actual current rise rate and the polarity reversal time of the test signal. The polarity reversal time depends on the voltage swing and the edge steepness of the test signal.

[0025] Due to the parasitic capacitances, an additional margin of e.g. 10% of the maximum current can be provided for the additional current. However, this margin can remain relatively small, since the edge steepness is purposefully adjusted, the additional current caused by the parasitic capacitances being relatively small compared to the corresponding current peaks caused by the edge steepness obtained by simply switching the test signal between the higher voltage value and the lower voltage value.

[0026] According to an embodiment, the test can comprise determining a magnetization curve of the transformer from the periodic voltage signal and a time course of the current generated by the transformer from the voltage signal.

[0027] According to the invention, there is also provided a device for testing a transformer, i.e. a test device. The device comprises a test signal source for generating a test signal for the transformer and a processing device coupled to the test signal source. The processing device is configured to determine an edge steepness of the test signal from the transformer to be tested, to generate the test signal with the thus determined edge steepness by means of the test signal source, and to test the transformer using the generated test signal. The device is thus suitable for carrying out the above-described method and thus also has the above-described advantages.

[0028] The above-described embodiments and features of aspects of the invention can be combined with each other arbitrarily, unless explicitly stated otherwise. In particular, the features can be used not only in the described combinations, but also in other combinations or alone.

[0029] BRIEF DESCRIPTION OF DRAWINGS

[0030] The invention will be described in detail below with reference to the attached drawings.

[0031] Figure 1 A device for testing a transformer according to an embodiment of the invention is schematically shown in connection with the transformer.

[0032] Figure 2 Method steps of a method for testing a transformer according to an embodiment of the invention are schematically shown.

[0033] Figure 3 A test signal source according to an embodiment of the invention is schematically shown.

[0034] Figure 4 A test signal source according to another embodiment of the invention is schematically shown.

[0035] Figure 5 A further test signal source according to yet another embodiment of the invention is schematically shown.

[0036] Figure 6 A voltage course and an associated current course of a test signal fed into a transformer are schematically shown.

[0037] Figure 7 A voltage course of a test signal of a feed-in transformer is schematically shown, with an edge steepness determined according to an embodiment of the application, and an associated current course.

[0038] Figure 8 Method steps for determining an edge steepness according to an embodiment of the application are schematically shown.

[0039] Figure 9 A threshold is determined from a current course for determining a time of a test signal edge start.

[0040] Detailed description of example embodiments

[0041] The above-mentioned characteristics, features and advantages of the present application as well as the manner in which they are implemented will become clearer and better understood in connection with the following description of example embodiments, in conjunction with the accompanying drawings.

[0042] The present application will be explained in more detail in the following with reference to example embodiments in the context of which the application will be explained in more detail. In the drawings, the same reference designations are used for identical or similar elements. The drawings are schematic representations of various embodiments of the application. The elements depicted in the drawings do not necessarily appear to scale. The manner in which the various elements depicted in the drawings are reproduced makes their function and general purpose understandable to the person skilled in the art. Connections and couplings between the functional units and elements depicted in the drawings can also be realized as indirect connections or couplings. The functional units can be realized as hardware, software or a combination of hardware and software.

[0043] Figure 1 A transformer 180 is shown connected with a device 100 for testing the transformer 180. The transformer 180 can be, for example, a current transformer arranged in a power engineering electrical system. The device 100 can be, for example, coupled to a secondary side of the transformer 180. The device 100 can comprise, for example, a portable test device which is transported to the transformer 180 for testing the transformer 180 and is coupled to the transformer 180 via lines 150, 152. In this context, the transformer to be tested is also referred to as a device under test (DUT).

[0044] The device 100 comprises a test signal source 102 and a processing device 104. The processing device 104 can comprise, for example, a microprocessor controller comprising a working memory, a mass storage for program code and data, and input / output devices. Furthermore, the device 100 can comprise a user interface 106 and a current measuring device 108. The user interface 106 can comprise, for example, an optional, touch-sensitive screen, a keyboard or keys, a loudspeaker, a signal light, etc. The current measuring device 108 can be provided, for example, in or on a line to the transformer 180, for example in the line 152, to measure a current flowing through the transformer 180 based on a voltage generated by the test signal source 102. The current measuring device 108 can comprise an analog / digital converter which determines the current intensity at a predetermined sampling frequency and converts it into a corresponding digital value provided to the processing device 104. The test signal source 102, the user interface 106 and the current measuring device 108 are coupled to the processing device 104 and controlled thereby and / or provide measurement values and other information, for example user inputs, to the processing device 104. The device 100 can comprise more components, for example a power supply device for providing electrical energy to the above-mentioned components, and further measuring devices for measuring current, voltage or power on the lines 150, 152. The device 100, in particular the processing device 104, can be configured to perform the methods and techniques described in the following. The device 100 can also be configured to perform further tests on other devices under test, for example high-voltage switches, electrical bushings or electrical lines.

[0045] When testing a transformer, it can be necessary to feed test signals into the transformer which do not correspond to the rated signals. For example, the frequency of these test signals can be lower or higher than the rated frequency of the transformer. Thus, it is possible to test properties of the transformer which cannot be tested at the rated frequency or which require a great deal of effort to test, for example the saturation magnetization. Possible test signals are sinusoidal signals, in particular for determining the saturation magnetization, and also rectangular signals, however, which also contain very high-frequency components. The test signals can be, for example, voltage signals. In response, a current flows, the time course of which, i.e. the current intensity as a function of time, can be acquired by the current measuring device 108. It is possible to use a model to back-calculate the corresponding rated values from the results of the test signals. The use of rectangular signals is particularly necessary in order to achieve a correspondingly large magnetic flux in the magnetic core. However, the high edge steepness of the rectangular signals in combination with structure-related parasitic capacitances leads to large capacitive current peaks. The parasitic capacitances can come, for example, from connections and connecting lines and also the structure of the transformer itself. The large current peaks can overload the test signal source 102 and / or affect the accuracy of the measurement.

[0046] Thus, the test signal source 102 generates a test signal with reduced high frequency components compared to a switching rectangular signal, i.e. a test signal with defined rising and falling edges. For example, a combination of a triangular signal and a rectangular signal, i.e. a test signal with a trapezoidal shape, can be generated instead of a simple rectangular signal. However, in order to have as long a constant voltage portion in the test signal as possible, for example in order to achieve a saturation magnetization, and in addition, in order to be able to provide high frequency components in the test signal which can be used, for example, for a model use for determining properties of the transformer, the method 200 as shown in Figure 2 is carried out under control of the processing device 104.

[0047] In the method 200, in step 202, an edge steepness of the test signal is determined depending on the transformer 180 to be tested, in step 204, a test signal with the determined edge steepness is generated by means of the test signal source 102. Then, in step 206, the transformer 180 is tested using the generated test signal. The edge steepness of the test signal is determined such that a long portion with constant voltage and high frequency components are included in the test signal, but the test signal source 102 is not overloaded by capacitive current peaks and the accuracy of the measurement is not impaired.

[0048] Technically, a test signal with adjustable edge steepness can be realized by using one or more voltage sources with adjustable output voltage. In a conventional generation of a rectangular signal, for example, two direct current (DC) voltage sources with fixed output voltage can be used. One of the direct current voltage sources has a positive output voltage to ground, for example, and the other direct current voltage source has a negative output voltage to ground. The two output voltages can be identical in terms of amplitude. For example, one changeover switch or two switches either connect the direct current voltage source to the positive output voltage or to the negative output voltage and to one end of a winding of the transformer, while the other end of the winding of the transformer is grounded. However, the edge steepness cannot be adjusted here.

[0049] A test signal with adjustable edge steepness can be generated, for example, by using and driving at least one adjustable voltage converter. The voltage converter can be, for example, a direct current voltage converter (so-called DC / DC converter). For example, under control of the processing device 104, the adjustable voltage converter can selectively adjust to one of a plurality of voltage values, for example, to a voltage between zero volts and 25 V or 50 V (with a precision of 1 V or 0.1 V). For example, the voltage converter can comprise a clocked or linear direct current amplifier (DC-DC amplifier) which can generate a positive output voltage and / or a negative output voltage. The voltage converter or amplifier is able to provide the necessary voltages and currents with the necessary dynamic properties.

[0050] Figure 3An implementation example of the test signal source 102 is schematically shown. In this example, the test signal source 102 comprises two adjustable voltage converters 302, 304 and two switches 306, 308. The switches 306, 308 can be mechanical or electronic switches controllable by the control device 104. The voltage converter 302 is configured to output, for example, an output voltage U1 which is negative with respect to a ground line 310 and which can be set in amplitude to a number of values between zero and a maximum voltage U max . For example, U max may be in the range of 10 V to 100 V. For example, U max may be 25 V. For example, the voltage converter 304 is configured to provide an output voltage U2 which is positive with respect to the ground line 310 and which can be set in amplitude to a number of values between zero and U max . The output voltages of the voltage converters 302, 304 can be adjusted by the control device 104. By means of the switches 306, 308, the output voltage from the voltage converter 302 or the output voltage from the voltage converter 304 can be selectively connected to a line 312. As shown in Figure 1 , by appropriately driving the voltage converters 302, 304 and the switches 306, 308, an alternating voltage in the range of -U max to +U max can be provided on the lines 310, 312 which can be used for testing the transformer 180. For example, a periodic trapezoidal alternating voltage can be provided. The fundamental frequency of this periodic alternating voltage may, for example, correspond to the rated frequency of the transformer 180, for example 50 Hz or 60 Hz. It can be understood that the fundamental frequency can have any other value higher or lower than the rated frequency of the transformer, for example 30 Hz or 100 Hz. Within one period, the alternating voltage can have, for example, the following voltage course: (1) from a lower voltage value (for example -U max ) to a higher voltage value (for example +U max) ; (2) a higher portion having a substantially constant voltage at a higher voltage value; (3) a voltage falling portion from the higher voltage value to a lower voltage value with a desired edge steepness; and (4) a lower portion having a substantially constant voltage at the lower voltage value. It is clear that the lower voltage value and the higher voltage value can differ in amplitude. It is also clear that the edge steepness from the lower voltage value to the higher voltage value can differ from the edge steepness from the higher voltage value to the lower voltage value. In many examples, however, the lower voltage value is equal in amplitude to the higher voltage value, and the edge steepness of the voltage rising is equal in amplitude to the edge steepness of the voltage falling. In case the voltage swing between the higher and lower voltage values is for example 50 V, the required edge steepness can be for example 50 V per 28 μβ. The period of the exemplary test signal at 50 Hz is 20 ms, so the lower portion and the higher portion are each slightly shorter than 10 ms (exactly 9.972 ms).

[0051] Figure 4 Another implementation example of the test signal source 102 is schematically shown. In this example, the test signal source comprises two adjustable voltage converters 402, 404. The adjustable voltage converter 402 is configured to provide an adjustable output voltage U1 on a line 408, which voltage is positive with respect to a connection line 406 connecting the two adjustable voltage converters 402, 404, and adjustable in amplitude to a number of values between zero and 2xU max The adjustable voltage converter 404 is configured to provide an adjustable output voltage U2 on a line 410, which voltage U2 is also positive with respect to the connection line 406, and adjustable in amplitude to a number of values between zero and U max By driving the voltage converters 402, 404 appropriately by means of the control device 104, an alternating voltage with a voltage swing of 2xU max and an adjustable edge steepness can be generated on the lines 408, 410.

[0052] Figure 5 Yet another implementation example of the test signal source 102 is schematically shown. In this example, the test signal source comprises an adjustable voltage converter 502 and a fixed voltage source 504. The adjustable voltage converter 502 is configured to provide an adjustable output voltage U1 on a line 508, which voltage U1 is positive with respect to a connection line 506 connecting the voltage converter 502 to the fixed voltage source 504, and adjustable in amplitude to a number of values between zero and 2xU max The fixed voltage source 504 is configured to provide a fixed output voltage U2 on a line 510, which voltage U2 is also positive with respect to the connection line 506, and for example has a value of U maxBy properly driving the voltage converter 502, a voltage with a capacity of 2xU can be generated on lines 508 and 510. max AC voltage with adjustable voltage swing and adjustable edge steepness.

[0053] Figure 6 and Figure 7 The signal processes 602 and 702 of the output voltage of the test signal source 102 are illustrated schematically and by way of example, which can be generated in the test signal source 102 by appropriately driving the aforementioned voltage converter. Figure 6 and Figure 7 Each of the above diagrams shows the signal processes 602 and 702 for the output voltage, and the corresponding signal processes 604 and 704 for the current flowing through the transformer 180 connected to the test signal source 102, respectively. The signal processes 604 and 704 for the current flowing through the transformer 180 can be achieved, for example, by means of a current measuring device 108 (see [link]). Figure 1 The output voltage can have a voltage swing of, for example, 50V, meaning the upper voltage level is, for example, +25V and the lower voltage level is, for example, -25V. In this example, the transformer 180 has a resistance of 220mΩ and an inductance of 0.5H. The frequency of the output voltage is essentially 50Hz. Ignoring the capacitive current peaks described below, the inductive current between signal edges, i.e., during the upper and lower signal portions with essentially constant voltage, increases in amplitude to, for example, about 250mA.

[0054] Figure 6 The signal process 602 of the output voltage shown has a very steep signal edge. Figure 6 The voltage swing depicted can be, for example, 50V, and the edge steepness can be, for example, 25V / μs or greater, such as 50V / μs. Due to parasitic capacitance (which may be generated by, for example, the wiring and terminals of a transformer and the structure of the transformer itself), a very high current 606 (so-called current peak) will appear during the signal edge, as... Figure 6 As shown in the lower section. These high currents are caused by high-frequency components at steep signal edges, which can generate high capacitive currents. The magnitude of these current peaks can be substantially higher than the magnitude of inductive currents, such as 500mA, 1000mA, or greater.

[0055] Figure 7 The signal processing 702 of the output voltage shown has a less steep signal edge. If the voltage swing is again, for example, 50V, the edge steepness can be, for example, 50V / 20μs or less, such as 50V / 28μs. Figure 7The signal process 704 of the respective current shown in the middle can be seen that at this lower edge steepness, due to the signal edge now having significantly lower amplitudes on the high frequency components, a significantly reduced current peak 706 occurs. The capacitive current peak 706 is correspondingly smaller, for example only 50 mA or 100 mA.

[0056] The current I of the capacitive current peak C Depending on the stray capacitance C of the transformer 180, as follows:

[0057]

[0058] In practice, the value of the stray capacitance typically occurs in the range of 100 pF to 30 nF, but is not limited to this range, and can thus be quite large or smaller, for example 3 μF. The edge steepness to be used is thus determined and set depending on the transformer currently being tested. In this regard, various processes are described below.

[0059] In a process for determining the edge steepness, the stray capacitance of the transformer is first determined. For this, the stray capacitance can be measured, for example, by a high frequency measurement on the transformer. The high frequency measurement can be performed at a lower voltage than the subsequent test voltage, for example, to ensure that the device 100 is not overloaded. The stray capacitance can also be provided by the user via input of the user interface 106. For example, the user can directly input the stray capacitance depending on information of the transformer, or the user can input the type of the transformer, from which the stray capacitance can then be determined from a database. The edge steepness can then be determined depending on the stray capacitance and a predetermined maximum capacitive current, which should not be exceeded during the testing of the transformer. The maximum capacitive current can be determined, for example, depending on the maximum rated current of the test signal source 102 and the maximum inductive current occurring during the measurement of the transformer, for example, as the difference between these two currents. The edge steepness du / dt can then be determined depending on equation 1 described above.

[0060] In combination Figure 8The illustrated method 800 describes another procedure for determining the edge steepness. First, in step 802, the edge steepness is set to an initial low steepness. The initial low steepness can be, for example, so low that it does not cause the test signal source 102 to overload and does not affect the measurement results even in the case of a very high parasitic capacitance of the transformer, for example 3 μF. For example, if the capacitive current is to be kept below 100 mA, the initial low edge steepness according to equation 1 can be set to, for example, 50 V / 1500 μs, so that the test signal source is not overloaded even with a parasitic capacitance of 3 μF. Using this edge steepness, a preliminary test signal is generated (step 804), and the maximum occurring current is determined in step 806. If the maximum occurring current is less than the allowed maximum current, for example less than the rated current of the test signal source 102 (step 808), the initial edge steepness can be increased, for example by a certain value or percentage, in step 810. The method continues in step 804 with the new initial edge steepness. If the allowed maximum current is reached in step 808, the edge steepness for testing the transformer 180 can be determined in step 812; for example, the edge steepness at which the allowed maximum current has not yet been reached can be used.

[0061] As mentioned above, the test signal can be a periodic signal. The frequency can be predetermined, for example by the test conditions; for example, the test signal can have the rated frequency of the transformer or a multiple of the rated frequency as the base frequency. The base frequency of the test signal can also be lower than the rated frequency of the transformer. However, particularly when measuring the magnetization curve of the transformer, it can be necessary to feed in as high a current as possible in order to generate as high a magnetic flux as possible in the transformer 180. Once this high current has been reached, the polarity of the applied voltage signal must be switched in order to generate as high a current as possible in the transformer in the opposite direction, thereby generating a high magnetic flux. This procedure can be repeated several times in succession when recording the magnetization curve. For this purpose, a comparator can be provided which compares the actual current from the current measurement device 108 with a threshold current, for example the rated current of the test signal source 102, for example using software in the processing device 104. However, due to the reduced edge steepness, the polarity reversal can be delayed, because the voltage, during the falling edge or the rising edge, takes a certain time until it reaches the zero crossing and the inductive current flow is reduced, thereby reducing the magnetic flux, so that the current can exceed the threshold current. To avoid this, the reaching of the threshold current can be predicted as follows.

[0062] Figure 9Exemplarily, a non-linear current course 902 over time is shown, which is an inductive current flowing through the windings of the transformer 180 due to the voltage applied to the transformer 180. The current course 902 is more non-linear, as they are typically saturated inductances. Thus, the inductance value is unknown and related to the current. Thus, the slope of the current course 902 is also unknown. Thus, a continuous determination of the slope of the current course 902 is performed. For example, the current can be sampled at successive points in time (e.g. at the vertical dashed lines shown in Figure 9 Fig. 1), thereby continuously determining the di / dt of the current course in real time. Due to the edge steepness from the voltage swing and the set edge steepness, a delay t d between the beginning of the edge and the zero crossing can be determined. According to the desired maximum current I thr , a corrected maximum current I thr_corr may be determined according to the actual slope di / dt of the current as follows:

[0063]

[0064] Once the measured current reaches the corrected maximum current I thr_corr , the polarity reversal process is initiated with the determined edge steepness, so that the maximum current I thr is not exceeded.

[0065] Of course, the above-described embodiments and features of aspects of the application can be combined with each other. In particular, these features can be used not only in the described combinations, but also in other combinations or individually, without leaving the scope of the application.

Claims

1. A method for testing a transformer, comprising: -Determine the edge steepness of the test signal (702) based on the transformer (180) to be tested. - Generate (204) a test signal (702) with the determined edge steepness, and - Use the generated test signal (702) to test (206) the transformer (180).

2. The method of claim 1, wherein determining the edge steepness comprises: - Determine the parasitic capacitance of the transformer (180), - The edge steepness is determined based on the parasitic capacitance and the predetermined maximum capacitive current that can occur during the testing of the transformer (180).

3. The method of claim 2, wherein determining the parasitic capacitance comprises: - The parasitic capacitance is measured by performing high-frequency measurements on the transformer (180), and / or - Obtain the input of the parasitic capacitance from the user.

4. The method according to any one of the preceding claims, wherein the test signal (702) is a periodic test signal, wherein determining the edge steepness comprises an iterative method (800), wherein the subsequent edge steepness of a subsequent period of the test signal (702) is determined based on the edge steepness in the period of the test signal, according to the maximum current measured during the testing of the transformer (180) using the test signal.

5. The method according to any one of the preceding claims, wherein the start time of the edge of the test signal (702) is based on the current generated by the test signal (702) through the transformer (180) and a threshold (I). thr_corr Adjust by comparing ( ).

6. The method of claim 5, wherein the threshold (I) thr_corr The value of the test signal (702) is less than the value of the maximum rated current of the device (100) that provides the test signal (702).

7. The method according to claim 5 or 6, wherein the test signal (702) comprises a voltage signal, and the threshold (I) thr_corr The determination is based on the edge steepness of the test signal (702), the voltage swing of the test signal (702), and the absolute current rise rate (Δi / Δt) of the current generated by the test signal (702) through the transformer (180).

8. The method according to claim 7, wherein the rate of rise (Δi / Δt) of the current generated by the test signal (702) is continuously determined during the test of the transformer (180).

9. The method of claim 8, wherein the threshold (I) thr_corr The value of the test signal is determined to be the expected maximum current (I) generated by the test signal through the transformer (180). thr The magnitude of the current rise (Δi / Δt) and the polarity reversal time (t) of the test signal (702) are related to the actual current rise rate (Δi / Δt). d The difference between the products of the test signal (702) and the polarity reversal time (t) of the test signal (702). d It depends on the voltage swing of the test signal and the edge steepness.

10. The method according to any one of the preceding claims, wherein the test signal (702) is a periodic voltage signal.

11. The method of claim 10, wherein the periodic voltage signal per cycle comprises: - A voltage rise portion from a lower voltage value to a higher voltage value with a defined edge steepness. -Having a higher portion of essentially constant voltage at higher voltage values, -The voltage drop from a higher voltage value to a lower voltage value, and - It has a lower portion of a basically constant voltage at a lower voltage value.

12. The method of claim 11, wherein the voltage drop portion has an edge steepness equal in magnitude to the edge steepness of the voltage rise portion.

13. The method according to any one of the preceding claims, wherein testing (206) the transformer (180) comprises determining the magnetization curve of the transformer (180) based on the test signal (702) and the time process (704) of the current generated by the test signal through the transformer (180).

14. The method according to any one of the preceding claims, wherein generating (204) the test signal (702) comprises driving at least one adjustable voltage converter (302, 304, 402, 404, 502).

15. An apparatus for testing transformers, comprising: - A test signal source (102) is used to generate (204) test signals (702) for the transformer (180). - A processing device (104), coupled to the test signal source (102) and configured to: The edge steepness of the test signal (702) is determined based on the transformer under test (180). A test signal (702) with a determined edge steepness is generated (204) by the test signal source (102), and The generated test signal (702) is used to test (206) the transformer (180).

16. The apparatus of claim 15, wherein the apparatus (100) is configured to perform the method (200) of any one of claims 1-14.

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