Method and apparatus for determining the status of a capacitive voltage transformer

Abstract

The invention relates to a method for determining a state of a capacitive voltage transformer (50) comprising a capacitive voltage divider with a first capacitor (C1) and a second capacitor (C2) on its primary side. The first capacitor (C1) has a high voltage connection (HV) for connecting to a high voltage (60) and the second capacitor (C2) has a ground connection (NHF). A first resonance frequency (ω e ) is determined by a plurality of first short circuit impedance measurements at different frequencies on the secondary side of the capacitive voltage transformer (50) with the high voltage connection (HV) grounded (70). A second resonance frequency (ω2) is determined by a plurality of second short circuit impedance measurements at different frequencies on the secondary side of the capacitive voltage transformer (50) with the high voltage connection (HV) open. A capacitance ratio (K e ) of the capacitive voltage divider (50) is determined based on the first resonance frequency (ω C ) and the second resonance frequency (ω2).

Description

Technical Field

[0001] This invention relates to a method for determining the state of a capacitive voltage transformer, and more particularly to a method for determining defects in the capacitive voltage divider within a capacitive voltage transformer. The invention also relates to an apparatus for determining the state of a capacitive voltage transformer in which the method is implemented. Background Technology

[0002] Voltage transformers are used in electrical engineering to measure AC voltage. The function of a voltage transformer is to proportionally convert a high voltage to a lower voltage value. This lower voltage (e.g., a value of approximately 100V) is then transmitted to voltmeters, energy meters, and similar devices, for example, for measurement or protection purposes. Voltage transformers can be implemented as inductive or capacitive voltage transformers, wherein capacitive voltage transformers, for example, can be designed for primary rated voltages (nominal values) up to or exceeding 1mV.

[0003] A capacitive voltage transformer includes a capacitive voltage divider on the high-voltage side, which typically consists of at least two capacitors connected in series. Typically, one end of the capacitors connected in series is connected to the high voltage to be measured, and the other end is grounded. At the tap between the capacitors, a lower voltage proportional to the high voltage to be measured exists, which can be fed via the transformer to, for example, a voltmeter.

[0004] Failures in capacitive voltage transformers can be caused by defects in the capacitive divider. There are various reasons for defects in the capacitors of the capacitive divider, such as moisture entering the insulation. Monitoring the development of capacitance over time, the so-called "trend," can help prevent complete failure of the capacitive voltage transformer. A complete failure can damage other devices (such as measuring devices connected to the capacitive voltage transformer) and may endanger personal safety. To track the temporal development of capacitance changes and identify faults in individual capacitors, the capacitance of the capacitors is measured, for example, through swept frequency response analysis (SFRA). These measurements are, for example, based on the principle of frequency-dependent short-circuit impedance measurement on the secondary side of the transformer. For this purpose, the series connection of the capacitors is usually disconnected from the high voltage and ground. In particular, to disconnect the series connection of the capacitors from ground, it is necessary to be able to access the corresponding connection points, which is often not the case in reality or is only possible if the installation is costly. Summary of the Invention

[0005] There is a need for improved methods to inspect and monitor the capacitors in the capacitive voltage divider of a capacitive voltage transformer.

[0006] According to the present invention, as defined in the independent claim, a method for determining the state of a capacitive voltage transformer and an apparatus for determining the state of a capacitive voltage transformer are provided. The dependent claims define embodiments of the invention.

[0007] This invention provides a method for determining the state of a capacitive voltage transformer. The capacitive voltage transformer includes a capacitive voltage divider with a first capacitor and a second capacitor on its primary side. Herein, the primary side of the capacitive voltage transformer refers to the side of the capacitive voltage transformer coupled to the high voltage to be measured. The high voltage can be, for example, several kilovolts to hundreds of thousands of volts, or even 1 mV or higher. Therefore, the secondary side of the capacitive voltage transformer refers to the side of the capacitive voltage transformer coupled to a measuring device or another device (e.g., an energy meter). The first capacitor has a high-voltage connection for connecting the first capacitor to the high voltage. The second capacitor has a ground connection for grounding the second capacitor. In this case, ground can be, for example, a reference potential of the high voltage. Both the first and second capacitors can include multiple capacitors connected in series to increase the voltage sustaining capability of the resulting capacitor. The first and second capacitors can be connected in series, for example, to form a capacitive voltage divider, such that the voltage drop of the high voltage can be divided at the connection point between the first and second capacitors. A capacitive voltage transformer can also include a transformer to which voltage is fed in at the primary side between the connection point and ground, and thus provides a current-distributed voltage on the secondary side of the transformer, proportional to the voltage tapped between the connection point and ground. The transfer factor of the transformer can be, for example, 1. The transformer can have any other transfer factor, particularly a transfer factor greater than 1, such that the voltage tapped by the voltage divider is stepped down by the transformer.

[0008] In this method, when the high-voltage connection is grounded, a first resonant frequency is determined by performing multiple first short-circuit impedance measurements at different frequencies on the secondary side of the capacitive voltage transformer. Furthermore, when the high-voltage connection is open, a second resonant frequency is determined by performing multiple second short-circuit impedance measurements at different frequencies on the secondary side of the capacitive voltage transformer. The capacitance ratio of the capacitive voltage divider is determined based on the first and second resonant frequencies. The capacitance ratio can be, for example, the ratio of the capacitance of the second capacitor to the capacitance of the first capacitor, or the ratio of the second capacitor to the cumulative capacitance of the first and second capacitors. During the determination of the first resonant frequency and during the determination of the second resonant frequency, the second capacitor can maintain its ground connection grounded.

[0009] When the capacitance of the capacitors in a capacitive voltage divider changes due to wear, for example, the capacitance ratio usually also changes. Therefore, the capacitance ratio can be seen as an indicator of the state of the capacitive voltage transformer. Since the capacitance ratio can be determined without disconnecting the ground connection of the second capacitor, the state of the capacitive voltage divider can be determined quickly and inexpensively.

[0010] In this method, the measurement current can also be determined based on the load of the capacitive voltage transformer and the voltage drop at the load. Multiple first short-circuit impedance measurements and multiple second short-circuit impedance measurements are performed using the measurement current. The load of the capacitive voltage transformer is related to the impedance of the secondary circuit of the capacitive voltage transformer, typically expressed as the apparent power it withstands under specified conditions. Therefore, short-circuit impedance measurements can be performed exclusively with the measurement current, whose amplitude is close to the nominal current, allowing for rapid and reliable establishment of the resonant frequency.

[0011] In one embodiment, the plurality of first short-circuit impedance measurements include a plurality of short-circuit impedance measurements in a first frequency grid, followed by a plurality of short-circuit impedance measurements in a second frequency grid. The first frequency grid has a larger frequency distance than the second frequency grid. Alternatively or additionally, the plurality of second short-circuit impedance measurements can include a plurality of short-circuit impedance measurements in a first frequency grid, followed by a plurality of short-circuit impedance measurements in a second frequency grid. Here again, the first frequency grid has a larger frequency distance than the second frequency grid. In other words, the resonant frequency can be roughly determined in the first scan. In the first scan, frequencies within a wide frequency range can be roughly selected so that the resonant frequency can be approximately determined by short-circuit impedance measurements. In the second scan, the resonant frequency can be determined more accurately because the second scan is performed within a frequency range that has already been determined by the first scan to include the resonant frequency. This frequency range has been determined in the first scan, and it is possible to scan in small steps in the second scan, thereby accurately determining the resonant frequency. Additional scans can be performed to determine the resonant frequency more accurately. The first and second resonant frequencies can be determined quickly and reliably through this iterative method.

[0012] In this method, the resonant inductance of the capacitive voltage transformer can also be determined based on the nominal values ​​of the first and second capacitors and the nominal frequency of the capacitive voltage transformer. The nominal values ​​of the first and second capacitors and the nominal frequency of the capacitive voltage transformer can be inferred, for example, from the nameplate of the capacitive voltage transformer. The actual value of the capacitance of the second capacitor can be determined based on the second resonant frequency and the resonant inductance. Since the high-voltage connection is disconnected during the determination of the second resonant frequency, the second resonant frequency is essentially generated solely by the capacitance of the second capacitor. Combined with the resonant inductance, the actual value of the capacitance of the second capacitor can therefore be determined from the second resonant frequency, for example, using Thomson's formula for oscillations.

[0013] In another embodiment, as described above, multiple actual values ​​of the capacitance of the second capacitor can be determined at different points in time, and the time trend of the capacitance of the second capacitor can be plotted and / or represented on a display device. The time trend can be used to infer or estimate whether the state of the capacitive voltage transformer is sufficient for additional reliable operation.

[0014] Based on the actual capacitance value and capacitance ratio of the second capacitor, the actual capacitance value of the first capacitor can be determined. At different points in time, multiple actual values ​​of the capacitance of the first capacitor can be determined, and the time trend of the capacitance of the first capacitor can be plotted and / or represented on a display device. Therefore, the time trend of the first capacitor can also be used to infer or estimate whether the state of the capacitive voltage transformer is sufficient for reliable operation.

[0015] Alternatively or additionally, multiple capacitance ratios of the capacitive voltage divider can be determined at different points in time, and the time trend of the capacitance ratios can be displayed on a display device. When the state of the capacitors in the capacitive voltage divider changes, the capacitance ratios relative to each other usually also change. Deterioration of the state of the capacitive voltage divider can be detected easily and promptly through the time process of the capacitance ratios.

[0016] In another embodiment, a first correlation of the capacitance ratio to a first resonant frequency is determined. For example, the first correlation may include the partial derivative of the capacitance ratio with respect to the first resonant frequency. This correlation is also referred to as the sensitivity of the capacitance ratio to the first resonant frequency. Multiple first correlations of the capacitance ratio to the first resonant frequency can be determined at multiple different time points, and the time trend of the first correlation can be represented on a display device.

[0017] A second correlation between the capacitance ratio and the second resonant frequency can be determined. This second correlation can, for example, include the partial derivative of the capacitance ratio with respect to the second resonant frequency. Multiple second correlations between the capacitance ratio and the second resonant frequency can be determined at multiple different time points, and the temporal trend of these second correlations can be displayed on a display device.

[0018] The time trend of the first correlation reflects the sensitivity of the total capacitance of the first and second capacitors, while the time trend of the second correlation reflects the sensitivity of the capacitance of the second capacitor. If the first and / or second sensitivities change over time, this means that the state of the first and / or second capacitors can be inferred over time by analyzing the time trends of the first or second sensitivities separately.

[0019] The present invention also provides an apparatus for determining the state of a capacitive voltage transformer. The capacitive voltage transformer includes a capacitive voltage divider having a first capacitor and a second capacitor on its primary side. The first capacitor has a high-voltage connection for connection to a high voltage. The second capacitor has a ground connection. The first and second capacitors can, for example, be connected in series to form a capacitive voltage divider. The apparatus includes a measuring device configured to perform short-circuit impedance measurements at an adjustable frequency. The apparatus also includes a control device, such as an electronic control system with a microprocessor configured to determine a first resonant frequency by performing multiple first short-circuit impedance measurements at different frequencies on the secondary side of the capacitive voltage transformer when the high-voltage connection is grounded. The control device is further configured to determine a second resonant frequency by performing multiple second short-circuit impedance measurements at different frequencies on the secondary side of the capacitive voltage transformer when the high-voltage connection is open. The control device determines the capacitance ratio of the capacitive voltage divider based on the first and second resonant frequencies.

[0020] The apparatus is suitable for performing the above-described method and its embodiments, and therefore also has the aforementioned advantages. Attached Figure Description

[0021] The invention will now be explained in more detail with reference to the accompanying drawings and preferred embodiments. The same reference numerals denote the same elements in the drawings.

[0022] Figure 1 A capacitive voltage transformer connected to a high-voltage line is schematically shown.

[0023] Figure 2 The diagram schematically illustrates the method used to determine the first resonant frequency. Figure 1 Interconnection of capacitive voltage transformers.

[0024] Figure 3 The diagram schematically illustrates the method used to determine the second resonant frequency. Figure 1 Interconnection of capacitive voltage transformers.

[0025] Figure 4 The diagram schematically illustrates the method used to determine another resonant frequency. Figure 1 Interconnection of capacitive voltage transformers.

[0026] Figure 5The diagram schematically illustrates short-circuit impedance measurements performed at different frequencies for determining a first resonant frequency or a second resonant frequency, according to one embodiment.

[0027] Figure 6 The diagram schematically illustrates the phase angle during short-circuit impedance measurements performed at different frequencies, according to one embodiment, for determining a first resonant frequency or a second resonant frequency, respectively.

[0028] Figure 7 The diagram schematically illustrates short-circuit impedance measurements at different frequencies for determining the resonant frequency of another, according to one embodiment.

[0029] Figure 8 The diagram schematically illustrates the phase angle during short-circuit impedance measurements performed at different frequencies for determining the resonant frequency of another, according to one embodiment.

[0030] Figure 9 The correlation between the capacitance comparison and the second resonant frequency according to one embodiment is illustrated schematically.

[0031] Figure 10 The correlation between the capacitance and the first resonant frequency according to one embodiment is illustrated schematically.

[0032] Figure 11 The diagram schematically illustrates the correlation of the capacitance of the second capacitor in a capacitor-to-capacitor voltage divider according to one embodiment.

[0033] Figure 12 The diagram schematically illustrates the correlation of the capacitance of the first capacitor in a capacitance-to-capacitance voltage divider according to one embodiment.

[0034] Figure 13 The method steps for determining the state of a capacitive voltage transformer according to one embodiment are shown.

[0035] Figure 14 A method for determining the state of a capacitive voltage transformer according to another embodiment is shown.

[0036] Figure 15 The method steps for determining the state of a capacitive voltage transformer according to yet another embodiment are shown. Detailed Implementation

[0037] The invention will now be explained in more detail with reference to the accompanying drawings and preferred embodiments. In the drawings, the same reference numerals denote the same or similar elements. The drawings are schematic diagrams of different embodiments of the invention. The elements shown in the drawings are not necessarily drawn to scale. Rather, those skilled in the art will understand that the different elements shown in the drawings are reproduced in a manner appropriate to their function and purpose.

[0038] The connections and couplings between the functional units and components shown in the attached diagram can also be implemented as indirect connections or couplings. Connections or couplings can be achieved via wired or wireless means.

[0039] The following describes in detail a method and apparatus for determining the condition of a capacitive voltage transformer. The condition of a capacitive voltage transformer can be adversely affected by defects in the capacitors of a capacitive voltage divider. There are various causes of capacitor defects, such as moisture entering the insulation layer. Therefore, monitoring changes in capacitance over time can help prevent complete failure of the capacitive voltage divider. Complete failure could endanger other parts of the installation or personnel. To assess changes in capacitance over time and locate defects in the capacitors of the capacitive voltage divider, it may be necessary to establish capacitance values ​​for the capacitors that are separate from each other.

[0040] Figure 1 A capacitive voltage transformer 50 is schematically shown, coupled to a high-voltage line 60 via a high-voltage connection HV. The capacitive voltage transformer 50 includes two capacitors C1 and C2 connected in series, forming a voltage divider. The first capacitor C1 is connected to the high-voltage connection HV, and the second capacitor C2 is grounded 70 via an NHF connection. A tap 52 is provided at the connection between the first capacitor C1 and the second capacitor C2, allowing a portion of the voltage from the high-voltage line 60 to be tapped during operation of the voltage transformer 50. The tapped voltage is transmitted via a compensating inductor L. comp The voltage is conducted to the primary winding of the current transformer T. A voltage without potential measurement is available at the secondary winding of the current transformer T, which is significantly lower than and proportional to the high voltage on the high-voltage line 60, and can be measured, for example, at the connection 51 on the secondary side of the current transformer T using a measuring device 80. Figure 1 In the schematic diagram, the current transformer T represents an ideal current transformer. The characteristics of a real current transformer are not ideal, and... Figure 1 The winding resistor R1 and leakage inductance L1 on the primary side, the winding resistor R2 and leakage inductance L2 on the secondary side, and the main inductance L h and core loss R fe Modeling.

[0041] As will be referenced below Figures 2 to 4The capacitances of capacitors C1 and C2 can be distinguished by different scanned frequency response analysis (SFRA) measurements. These measurements are based on the principle of frequency-dependent short-circuit impedance measurement on the secondary side of the transformer T. It should be noted that, for measurements based on… Figure 4 The measurements shown require the NHF connection to be disconnected from ground. However, this is only possible if the NHF connection is accessible and can be left ungrounded. Therefore, without regard to... Figure 4 Determining the condition of a capacitive voltage transformer under the conditions shown in the measurements may be advantageous. Based on... Figures 2 to 4 In all the measurements shown, the high-voltage connection HV of the voltage transformer is disconnected from the high-voltage line 60.

[0042] exist Figure 2 In the measurement setup shown, the high-voltage connection HV is grounded 70 via the additional line 91. The NHF connection is also grounded 70, so that capacitors C1 and C2 are connected in parallel between tap 52 and ground 70. The short-circuit impedance measurement is performed on the secondary side of the transformer T, that is, on the low-voltage side of the capacitive voltage transformer 50, using the measuring device 90 connected to the connection 51 on the secondary side of the transformer T. The short-circuit impedance measurement will be described in detail below.

[0043] exist Figure 3 In the measurement setup shown, the high-voltage connection HV is disconnected and the NHF connection is grounded 70. Then, essentially only the second capacitor C2 is connected between tap 52 and ground. The short-circuit impedance of the secondary side of the transformer T is measured using the measuring device 90.

[0044] Figure 4 A measurement setup is shown in which the high-voltage connection HV is grounded via an additional line 91, while the NHF connection is disconnected, such that essentially only the first capacitor C1 is connected between tap 52 and ground 70. The short-circuit impedance of the secondary side of the current transformer T is measured using the measuring device 90.

[0045] Figures 2 to 4 The difference in the measurement setup shown is that, during frequency-dependent short-circuit impedance measurements, different capacitive loads act on the primary side of the capacitive voltage transformer. Figure 2 In this configuration, capacitors C1 and C2 are both grounded, making the parallel connection of C1 and C2 a capacitive load. Figure 3 In the above, only capacitor C2 acts as a capacitive load, and... Figure 4 In this configuration, only capacitor C1 acts as a capacitive load.

[0046] As will be described below, even without knowing the induction transformer ratio of a capacitive voltage transformer defined by the ratio of secondary turns to primary turns, the capacitance ratio K between the capacitances of capacitor C1 and C2 is... C The frequency-dependent short-circuit impedance can be determined by two different frequency-dependent short-circuit impedance measurements on the secondary side of the capacitive voltage transformer. The frequency-dependent short-circuit impedance measurement is performed using a current signal with an amplitude substantially corresponding to or close to the nominal current of the capacitive voltage transformer. During the frequency-dependent short-circuit impedance measurement, the frequency is gradually changed, for example, gradually increasing from a low starting frequency to a high cutoff frequency. The current intensity of the current signal, i.e., the current intensity of the measured current, can be determined and adjusted using information from the nameplate of the capacitive voltage transformer. For example, the measured current can be based on the load S indicated on the nameplate. r and load U Sr The voltage drop at the load is used to determine and adjust the load. The load can be expressed as apparent power in volt-amperes (VA), which is the product of the nominal current appearing on the secondary side of the current transformer and the voltage drop associated with the load.

[0047] For example, the resonant frequency can be determined by performing multiple short-circuit impedance measurements at different frequencies. For instance, the frequencies at which the corresponding short-circuit impedance measurements are performed can be coarsely increased in the first scan, allowing for a rough definition of the resonant frequency range. In the second scan, the frequencies at which the corresponding short-circuit impedance measurements are performed can be increased in more precise steps within the range determined in the first scan, and therefore, the resonant frequency can be determined more accurately. In other words, multiple short-circuit impedance measurements can be performed in a first frequency grid in the first scan, and multiple short-circuit impedance measurements can be performed in a second frequency grid in the second scan, the second frequency grid having a smaller frequency range than the first frequency grid.

[0048] The following considerations are based on Thomson's formula for oscillations (Equation 1), which allows us to calculate the resonant frequency ω (=2πf) of an oscillating circuit using capacitance C and inductance L. In the following equations, the angular frequency is represented by ω, and the number of oscillations is represented by f. In this paper, these quantities ω and f are simply referred to as frequencies.

[0049] Equation 1:

[0050]

[0051] Able to determine Figures 2 to 4 The resonant frequency of the oscillating circuit shown is used to determine the capacitance ratio. For example... Figures 2 to 4 The three resonant frequencies of the oscillating circuit shown can be determined by measuring the corresponding frequency-dependent short-circuit impedance.

[0052] For according to Figure 2 The circuit layout can determine the first resonant frequency ω. e It depends on the sum of the capacitances of capacitors C1 and C2 according to Equation 2.

[0053] Equation 2:

[0054]

[0055] For according to Figure 3 The circuit arrangement can determine the second resonant frequency ω2, which depends on the capacitance of capacitor C2 according to Equation 3.

[0056] Equation 3:

[0057]

[0058] For according to Figure 4 The circuit arrangement can determine another resonant frequency ω1, which depends on the capacitance of capacitor C1 according to Equation 4.

[0059] Equation 4:

[0060]

[0061] The capacitance ratio K of the 50-type capacitive voltage transformer C It depends on the individual capacitors of the voltage divider and can be defined as the quotient of the sum of the capacitances of capacitors C1 and C2 and the capacitance of capacitor C1 (Equation 5); and refers to the international standard IEC 61869-5, Sec.3.1.528, Note501.

[0062] Equation 5:

[0063]

[0064] Equations 2 through 4, derived from frequency-dependent short-circuit impedance measurements, can be used in Equation 5. This leads to four additional equations (Equations 6 through 9). The resonant frequencies ω1 and ω... e The resonant frequencies ω1 and ω2 are included in Equation 6, and the resonant frequencies ω2 and ω are included in Equation 7. e Included in Equation 8, and the resonant frequencies ω1, ω2 and ω e This is included in Equation 9. As mentioned above, determining the resonant frequency ω1 can be problematic because it is not always (easily) feasible for the NHF connection to be ungrounded for this purpose. Equation 8 is independent of the measurement of the resonant frequency ω1, and therefore will be considered primarily below.

[0065] Equation 6:

[0066]

[0067] Equation 7:

[0068]

[0069] Equation 8:

[0070]

[0071] Equation 9:

[0072]

[0073] To provide the lowest possible measurement voltage proportional to the high voltage, a capacitive voltage divider consisting of capacitors C1 and C2 is used. It is possible to select a capacitor C2 with a capacitance significantly larger than that of capacitor C1, i.e., C2 >> C1. Therefore, the total capacitance C1 + C2 is determined according to... Figure 2 In the circuit layout, the capacitance is only slightly different from that of capacitor C2. Therefore, according to... Figure 2 The resonant frequency ω of the circuit layout e and according to Figure 3 The resonant frequencies ω2 of the circuit arrangements differ only slightly from each other. The inductors forming the corresponding resonant circuits with capacitors C1 and / or C2 can exhibit frequency-dependent nonlinearity. However, the inductors forming the corresponding resonant circuits with capacitors C2 or C1+C2 respectively will therefore exhibit frequency-dependent nonlinearity according to... Figure 2 Circuit layout and Figure 3 The circuit layout is basically the same. This is particularly applicable to Equation 8. According to Figure 2 and Figure 3 The operating points of the circuit arrangements are practically the same because the resonant inductance is essentially the same in both measurements. In particular, this means that the operating points of the nonlinear inductors are practically the same because the impedance values ​​are close to each other at the resonant frequencies measured (with C2 or C1+C2, respectively). Equation 8 is used to determine the capacitance ratio K due to the identical operating points. c The valid equation.

[0074] To consider the capacitances of capacitors C1 and C2 separately, the resonant inductance L, which can be derived from the nameplate, can be taken into account. The resonant inductance L can represent the compensating inductance L. comp Leakage inductances L1 and L2 and main inductance L h For example, the resonant inductance L can be determined using Thomson's formula for oscillations according to Equation 10.

[0075] Equation 10:

[0076]

[0077] For example, the resonant inductance L of a capacitive voltage transformer can be determined based on the nominal values ​​of the first capacitor C1 and the second capacitor C2, as well as the nominal frequency ω of the capacitive voltage transformer. However, it should be noted that the absolute value of the inductance, such as the resonant inductance L or the compensation inductance L... comp Furthermore, the absolute values ​​of capacitors C1 and C2 do not necessarily need to be known or readily apparent, as these absolute values ​​typically have some tolerance based on the specifications on the nameplate. The percentage difference between the two capacitors (C1 and C2) at different measurements is more relevant, meaning the time trend of the ratio of their capacitances relative to each other is more relevant. Therefore, it is sufficient to use the same inductance in equations with the same operating point. In this case, it is not necessary to know the induction ratio of the capacitive voltage transformer, i.e., the capacitance ratio K of the capacitive voltage transformer. C Both the resonant inductance L and the inductor can be determined independently of the inductance transformer ratio using the aforementioned nominal values ​​(e.g., from the nameplate).

[0078] When the phase angle between the current and voltage reaches 0°, resonance is achieved in the oscillating circuit. At 0°, the imaginary parts of the inductance and capacitance cancel each other out, leaving only the real parts. The frequency at which this condition is achieved is called the resonant frequency.

[0079] Alternatively or additionally, the resonant inductance L of the capacitive voltage transformer is important for separating C1 and C2 and can be derived from frequency-dependent short-circuit impedance measurements. For example, multiple short-circuit impedance measurements can be performed at different frequencies f within a predefined frequency range. When the high-voltage connection HV of the first capacitor C1 is grounded, as... Figure 2 As shown, frequency-dependent short-circuit impedance measurement can be performed on the secondary side of a capacitive voltage transformer. Alternatively or additionally, when the high-voltage connection HV of the first capacitor C1 is disconnected, as... Figure 3 As shown, frequency-dependent short-circuit impedance measurements can be performed on the secondary side of a capacitive voltage transformer. These measurements can be interpreted as measurements of a series resonant circuit. The measured short-circuit impedance Z... sc Determine the minimum impedance Z sc_f0 The bandwidth B and resonant frequency f0 are given by equation 11. The minimum impedance Z is... sc_f0 By using the short-circuit impedance Z at frequency f sc The first derivative is determined. Minimum impedance Z sc_f0 These points exist where the first derivative of the short-circuit impedance reaches zero or where the phase response crosses zero. This point is also called the impedance Z at the resonant frequency f0. sc_f0 This impedance Z sc_f0 Corresponding to the ohmic resistance R, the imaginary parts cancel each other out. The bandwidth B is also a result of frequency-dependent short-circuit impedance measurements. The bandwidth B is defined via the -3dB point of the current curve. The resulting upper limit frequency f... H and lower limit frequency fL The difference at -3dB is defined as bandwidth B, that is, bandwidth B at f L to f H The measured short-circuit current extends over a frequency range, where it is at most 3 dB lower than the maximum short-circuit current. At the two limiting frequencies f... L and f H The impedance at point Z is the impedance Z. sc_f0 =√2 times. The values ​​of the resonant inductance L” and resonant capacitance C” in Equation 12 refer to the secondary side of the current transformer T, and are determined by the induction transformation ratio K of the current transformer T. IVT This is transferred to the primary side, resulting in the resonant inductance L' and resonant capacitance C' on the primary side of the voltage transformer.

[0080] Equation 11:

[0081]

[0082]

[0083] Equation 12:

[0084]

[0085]

[0086] Therefore, the determined resonant inductance L' on the primary side corresponds to the resonant inductance L. To consider the capacitances of capacitors C1 and C2 respectively, the resonant inductance L can be taken into account, which is respectively based on... Figure 2 Or according to Figure 3 The interconnection represents the compensation inductance L. comp Leakage inductances L1 and L2 and main inductance L h .

[0087] When the high voltage connection HV of the first capacitor C1 is according to Figure 2 When performing frequency-dependent short-circuit impedance measurements during interconnection and grounding, the resonant capacitance C' corresponds to the accumulated capacitance of the first capacitor C1 and the second capacitor C2. Considering the corresponding resonant inductance L, the resonant frequency f0 or the resonant angular frequency w0 corresponds to the resonant frequency ω according to Equation 2. e .

[0088] according to Figure 3 When performing frequency-dependent short-circuit impedance measurements using the disconnected high-voltage connection HV of the first capacitor C1, the resonant capacitance C' corresponds to the capacitance of the second capacitor C2. The resonant frequency f0 or the resonant angular frequency w0 corresponds to the resonant frequency ω2 according to Equation 3, while also considering the corresponding resonant inductance L.

[0089] Figure 5The measurement results of short-circuit impedance measurements at different frequencies are shown. In detail, Figure 5 It shows that according to Figure 2 The impedance of the capacitive voltage transformer measured at different frequencies is 501 and according to Figure 3 The impedance 502 of the capacitive voltage transformer was measured at different frequencies during short-circuit impedance measurements. Figure 6 The corresponding phase angles measured at different frequencies during short-circuit impedance measurements are shown. Based on... Figure 2 During the short-circuit impedance measurement, the phase angle is measured at 60° on the capacitive voltage transformer, according to... Figure 3 During the short-circuit impedance measurement, the phase angle 60° was measured on the capacitive voltage transformer. The two measurements show very similar processes of impedance and phase changing as a function of frequency. Since capacitor C2 differs only slightly from the total capacitance C1+C2 and has similar resonant frequencies, it can be assumed that the same inductance leads to the following... Figure 2 and Figure 3 The resonance phenomenon in the circuit layout of the voltage transformer. In this example, according to Figure 2 The resonant frequency of the capacitive voltage transformer in the wiring is 49Hz to 50Hz, and in this example, according to Figure 3 The resonant frequency of the capacitive voltage transformer in the wiring is 52Hz to 53Hz.

[0090] As mentioned above, Equation 8 is particularly suitable for considering the capacitance ratio of a capacitor divider, because according to Figure 2 and Figure 3 The operating points of the circuit layouts are practically the same, making the operating points of the inductor's nonlinear ratio almost identical as well. Conversely, according to... Figure 4 The resonant frequencies of the wiring of the capacitive voltage transformer are located in significantly different ranges, such as Figure 7 and Figure 8 The measurement results are shown. Figure 7 It shows that according to Figure 4 The impedance 701 of the capacitive voltage transformer was measured at different frequencies using short-circuit impedance measurements, and... Figure 8 The corresponding phase angle 80° is shown when measurements are taken at different frequencies during short-circuit impedance measurements. From Figure 7 and Figure 8 It is clear from the text that, according to Figure 4 The resonant frequency of the capacitive voltage transformer in the wiring is 155Hz to 156Hz.

[0091] Therefore, as Figure 2 and Figure 3As shown, on the one hand, without disconnecting the grounding connection of the NHF connection, it is possible to measure the short-circuit impedance of the capacitive voltage transformer according to the wiring layout, and it is also possible to ensure that the capacitive voltage transformer operates at approximately the same operating point in both cases, so that the nonlinear behavior of the components of the capacitive voltage transformer, such as the inductance in the capacitive voltage transformer, can be ignored.

[0092] Next, consider at least two resonant frequencies based on different measurements, for example, according to Figure 2 and Figure 3 The diagram shows the short-circuit impedance measurement of the wiring arrangement. The resonant frequency can be derived from the short-circuit impedance measurement result, for example, through interpolation. Interpolation may have some inaccuracy. To compensate for this inaccuracy, multiple measurements can be performed around the phase zero crossing. For example, in the first measurement scan, a relatively wide frequency range can be scanned with a wide frequency step size, such as scanning the frequency range from 20Hz to 200Hz with a step size of a few hertz, or for example, with a step size of 0.5Hz to 5Hz. Thus, the approximate range within which the resonant frequency lies can be determined. The approximate range still only includes a few hertz. In the second measurement scan, this region can be scanned with a smaller frequency step size, such as scanning with a step size of 0.1Hz or 0.01Hz, in order to determine the resonant frequency more accurately.

[0093] Using Equation 8, the capacitance ratio can be determined based on the resonant frequency determined by short-circuit impedance measurement, for example, based on... Figure 2 and Figure 3 The resonant frequency ω is determined by the circuit layout. e And resonant frequency ω2:

[0094] Equation 13:

[0095]

[0096] For example, capacitance ratio K C It can be within 10.

[0097] The absolute values ​​of the capacitances of capacitors C1 and C2 can be derived, for example, from the capacitance ratio K. C To determine this, the total inductance of the capacitive voltage transformer must first be determined. This can be done using the capacitance values ​​on the voltage transformer's nameplate. For example, the capacitance values ​​of C1 and C2 can be indicated on the nameplate of the capacitive voltage transformer. Typical values ​​are, for example, C1 = 5140pF (+10%, -5%) and C2 = 50700pF (+10%, -5%). The nominal frequency of the capacitive voltage transformer can be separately expressed, for example, 50Hz. These values ​​can be used in Equation 14 below, from which the inductance of the capacitive voltage transformer can be determined at least approximately. The inductance in the example above is, for example, 180.1586H.

[0098] Equation 14:

[0099]

[0100] The actual capacitance of capacitor C2 can then be determined using Equation 15, which uses the resonant frequency ω2.

[0101] Equation 15:

[0102]

[0103] According to Equation 16, the capacitance of capacitor C2 and the capacitance ratio K are used. C To determine the capacitance of capacitor C1.

[0104] Equation 16:

[0105]

[0106] Therefore, the deviation of the capacitance of capacitors C1 and C2, determined according to the values ​​indicated on the nameplate, can be determined absolutely or relatively and displayed on the display device of the measuring device 90 to evaluate the condition of the capacitive voltage divider. For example, if the actual determined capacitance of capacitors C1 and C2 is within the tolerance range indicated on the nameplate, the capacitive voltage transformer can be assumed to be operating reliably.

[0107] Furthermore, it is possible to determine and display the capacitance-to-resonance frequency ω. e The correlation with the resonant frequency ω2 can be used to assess the state of the capacitors in the capacitive voltage divider. By taking the partial differential of Equation 8 based on the resonant frequency ω2, the capacitance ratio K can be determined according to Equation 17. C Correlation with resonant frequency ω2.

[0108] Equation 17:

[0109]

[0110] Similarly, by using the resonant frequency ω e Taking the partial derivative of Equation 8, the capacitance ratio K can be determined according to Equation 18. C For the resonant frequency ω e The correlation.

[0111] Equation 18:

[0112]

[0113] Therefore, the capacitance ratio is related to the resonant frequency ω. e The correlation or correlation of the resonant frequency ω2 involves the resonant frequency ω eThe correlation between the change in capacitance ratio and the change in resonant frequency ω2. This correlation is also known as sensitivity. High sensitivity helps to clearly detect even minute changes, thus enabling early and reliable detection of deterioration in the condition of the capacitive voltage divider. Figure 9 The capacitance ratio K is shown based on the variation of the resonant frequency ω2. C The sensitivity, where the demonstration is normalized to the measured resonant frequency, i.e., the measured resonant frequency is represented by line 901, and the sensitivity of the capacitance ratio is represented by graph 902 as a percentage of the change in resonant frequency. Figure 10 It shows the resonant frequency ω e The change in capacitance ratio K C The sensitivity of the capacitance ratio is also normalized to the measured resonant frequency, i.e., the measured resonant frequency is represented by line 1001, and the sensitivity of the capacitance ratio is represented by a curve 1002 as a percentage of the change in resonant frequency.

[0114] Based on Equation 5, the capacitance ratio K can be expressed according to Formula 19. C The correlation or sensitivity to the capacitance of capacitor C2. It can be clearly seen from Equation 19 that the capacitance is greater than K. C The sensitivity remains constant even if the capacitance of capacitor C2 changes.

[0115] Equation 19:

[0116]

[0117] Similarly, based on Equation 5, the capacitance ratio K can be expressed according to Formula 20. C The correlation or sensitivity of the capacitance of capacitor C1.

[0118] Equation 20:

[0119]

[0120] Figure 11 The capacitance ratio K is shown based on the capacitance change of capacitor C2. C The sensitivity is normalized to the measured resonant frequency, i.e., the measured resonant frequency is represented by line 1101, and the sensitivity of the capacitance ratio is represented by graph 1102 as a percentage of the change in capacitance of capacitor C2. Figure 12 Therefore, the capacitance ratio K is shown based on the change in capacitance of capacitor C2. C The sensitivity, where the demonstration is normalized to the measured resonant frequency. The measured resonant frequency is represented by line 1201, and the sensitivity of the capacitance ratio is represented by graph 1202 as a percentage of the change in capacitance of capacitor C1.

[0121] The method described above can be automatically executed by the measuring device 90. The operating modes of the measuring device 90 are described below. Figures 13 to 15 To summarize.

[0122] Figure 13 The method shown includes steps 1301 to 1312. In step 1301, data from the nameplate of the capacitive voltage transformer is input, for example, by an operator of the measuring device 90 via a user interface or using a corresponding scanner that scans the nameplate of the capacitive voltage transformer to be tested. The data from the nameplate may include, for example, the load and the voltage drop at the load at the nominal frequency, as well as the nominal values ​​of the first and second capacitors. Although the short-circuit impedance measurement described below is performed on the secondary side of the capacitive voltage transformer, it is not necessary to know the transformer's induction ratio when using this method. Using the data from the nameplate, the measurement current is calculated in step 1302, for example, using the load and the voltage drop at the load at the nominal frequency, as described above. In step 1303, the high-voltage connection HV is decoupled from the high-voltage line 60 and coupled to ground 70, such that a current exists according to... Figure 2 The wiring. In step 1304, the resonant frequency ω is determined. e For example, this can be determined by measuring multiple short-circuit impedances in a first frequency grid, followed by measuring multiple short-circuit impedances in a second frequency grid, wherein the first frequency grid has a larger frequency distance than the second frequency grid. In step 1305, the high-voltage connection HV is activated, such that there exists a frequency range according to... Figure 3 The wiring is then described. In step 1306, the resonant frequency ω2 is determined by short-circuit impedance measurement. For this purpose, multiple short-circuit impedance measurements can be performed sequentially in a first frequency grid, followed by multiple short-circuit impedance measurements in a second frequency grid, wherein the first frequency grid has a larger frequency distance than the second frequency grid. In step 1307, the capacitance ratio K is determined. C For example, it is determined according to Equation 8. In step 1308, the resonant inductance L is determined based on data from the nameplate, as described above with reference to Equations 10 and 14. In step 1309, considering the resonant inductance L, the capacitance of capacitor C2 can be determined according to Equation 15, and in step 1310, the capacitance of capacitor C1 can be determined according to Equation 16. In step 1311, the capacitance values ​​of capacitors C1 and C2 are displayed, for example, on the display device of the measuring device 90. Furthermore, in step 1312, the time trend of the capacitance values ​​of capacitors C1 and C2 can be displayed on the display device, or, for example, the time trend over a longer period of time can be plotted and evaluated in the processing device. For trend analysis, the capacitance of capacitors C1 and C2 can be captured over a longer period of time, and, for example, it can be transmitted to cloud storage.

[0123] Figure 14 The method shown includes steps 1401 to 1411. In step 1401, data from the nameplate of the capacitive voltage transformer is input. This data can be input via a user interface, for example, by an operator of the measuring device 90. Alternatively, the data can be acquired electronically from the nameplate of the capacitive voltage transformer under test, for example, via a scanner or camera. For example, the data from the nameplate can include the load and the voltage drop at the load at the nominal frequency. Although the short-circuit impedance measurement described herein is performed on the secondary side of the capacitive voltage transformer, it is not necessary to know the transformer's induction ratio when using this method. A suitable measurement current can be calculated in step 1402 using the data from the nameplate (see step 1302). In step 1403, the high-voltage connection HV is decoupled from the high-voltage line 60 and coupled to ground 70. Therefore, there is a possibility that... Figure 2 The wiring of the capacitive voltage transformer. Next, in step 1404, the resonant frequency ω is determined. e For example, as described above, in each case, two scans of short-circuit impedance measurements are used at different frequency grids. In step 1405, the high-voltage connection HV is enabled, such that there exists according to Figure 3 The wiring of the capacitive voltage transformer. In step 1406, the resonant frequency ω2 is determined, for example, by two scans of multiple short-circuit impedance measurements with different frequency grids in each case, as described above. In step 1407, the capacitance ratio K is determined, for example, according to Equation 8. C In step 1408, the correlation or sensitivity of the capacitance ratio is determined based on the resonant frequency ω2 (see also Equation 17). In step 1409, the correlation or sensitivity of the capacitance ratio is determined based on the resonant frequency ω2. e Determine the correlation or sensitivity of the capacitance ratio (see also Equation 18). In step 1410, based on the resonant frequency ω2 or ω e The sensitivity of the capacitance ratio is displayed on the display device of the measuring device 90. Furthermore, in step 1411, based on the resonant frequency ω2 or ω... e The time trend of the capacitance ratio sensitivity can be displayed or stored over time for evaluation by a processing device. For trend analysis, sensitivity can be captured over a longer period and transferred to, for example, cloud storage.

[0124] Figure 15The method shown includes steps 1501 to 1511. In step 1501, data from the nameplate of the capacitive voltage transformer is input. This data can be input via a user interface, for example, by an operator of the measuring device 90. Alternatively, the data can be acquired electronically from the nameplate of the capacitive voltage transformer under test, for example, via a scanner or camera. The data from the nameplate can include, for example, the load and the voltage drop at the load at the nominal frequency, as well as the induction ratio of the capacitive voltage transformer. In step 1502, a suitable measurement current can be calculated using the data from the nameplate (see step 1302). In step 1503, the high-voltage connection HV is decoupled from the high-voltage line 60 and coupled to ground 70. Therefore, there is a basis... Figure 2 The wiring of the capacitive voltage transformer. In step 1504, the resonant frequency ω is then determined. e and bandwidth B e (See also Equation 11), for example as described above, in each case, two scans of multiple short-circuit impedance measurements are used at different frequency grids. In step 1505, the high-voltage connection HV is enabled, such that there exists according to Figure 3 The wiring of the capacitive voltage transformer. In step 1506, the resonant frequency ω2 and bandwidth B2 (see also Equation 11) are determined, for example, as described above, using two scans of multiple short-circuit impedance measurements with different frequency grids in each case. In step 1507, for example, according to Equation 8, from the resonant frequency ω e The capacitance ratio K is determined by the resonant frequency ω2. C In step 1508, from the resonant frequency ω e The corresponding resonant inductance L is determined by the resonant frequency ω2 (see also Equation 12). The cumulative capacitance of capacitors C1 and C2, or the capacitance of capacitor C2, can be further determined according to Equation 12, in step 1508, using the corresponding resonant inductance L and the corresponding resonant frequency ω2. e Or the resonant frequency ω2 and the inductive transformer ratio. Conversely, the capacitance of capacitor C1 can be determined from this. In step 1509, the capacitance ratio K determined in step 1507 can be used... C The measurement is checked for validity by matching the capacitance ratio of capacitors C1 and C2 determined in step 1508. If the measurement is valid, the capacitance values ​​of capacitors C1 and C2 can be displayed on the display device of the measuring device 90, for example, in step 1510. Furthermore, in step 1511, the time trend of the capacitance of capacitors C1 and C2 can be displayed on the display device, or plotted and evaluated over a longer period, for example, in the processing device. For trend analysis, the capacitance of capacitors C1 and C2 can be captured and transmitted over a longer period, for example, to cloud storage.

[0125] In summary, the above method provides the possibility of determining and observing the changing trends of the two capacitors in a capacitive voltage divider to assess the state of the capacitive voltage transformer. These trends provide information about the time-varying capacitance of capacitors C1 and C2 in the capacitive voltage divider. For capacitance trend analysis, the time-varying variation is more meaningful than the absolute value of the capacitance. Therefore, it is reasonable to use the capacitance values ​​from the nameplate to determine the inductance L of the capacitive voltage transformer, since the absolute accuracy of the capacitance values ​​of capacitors C1 and C2 is not important for the trend of these two capacitors. Based on the thus obtained inductance L and the resonant frequency ω determined by short-circuit impedance measurement... e With the resonant frequency ω2, the capacitances of capacitors C1 and C2 can be calculated and used as reference values. These values ​​provide a reliable basis for condition monitoring through trends.

Claims

1. A method for determining the state of a capacitive voltage transformer (50), wherein the capacitive voltage transformer (50) includes a capacitive voltage divider having a first capacitor (C1) and a second capacitor (C2) on the primary side of the capacitive voltage transformer (50), wherein the first capacitor (C1) has a high-voltage connection (HV) for connection to a high voltage (60) and the second capacitor (C2) has a ground connection (NHF), wherein the method comprises: - When the high-voltage connection (HV) is grounded (70), the first resonant frequency (ω) is determined by multiple first short-circuit impedance measurements performed at different frequencies on the secondary side of the capacitive voltage transformer (50). e ), - When the high-voltage connection (HV) is disconnected, the second resonant frequency (ω2) is determined by multiple second short-circuit impedance measurements performed at different frequencies on the secondary side of the capacitive voltage transformer (50), and -Based on the first resonant frequency (ω) e The capacitance ratio (K) of the capacitor divider is determined by the second resonant frequency (ω2) and the second resonant frequency (ω2). C ).

2. The method according to claim 1, further comprising: - The measurement current is determined based on the load of the capacitive voltage transformer (50) and the voltage drop at the load of the capacitive voltage transformer (50), wherein the measurement current is used to perform the plurality of first short-circuit impedance measurements and the plurality of second short-circuit impedance measurements.

3. The method according to claim 1, in, The plurality of first short-circuit impedance measurements include a plurality of short-circuit impedance measurements in a first frequency grid, followed by a plurality of short-circuit impedance measurements in a second frequency grid, wherein the first frequency grid has a larger frequency distance than the second frequency grid, and / or The plurality of second short-circuit impedance measurements include a plurality of short-circuit impedance measurements in a first frequency grid, followed by a plurality of short-circuit impedance measurements in a second frequency grid, wherein the first frequency grid has a larger frequency distance than the second frequency grid.

4. The method according to claim 1, further comprising: -The resonant inductance of the capacitive voltage transformer (50) is determined based on the nominal values ​​of the first and second capacitors (C1, C2) and the nominal frequency of the capacitive voltage transformer (50). - The actual value of the capacitance of the second capacitor (C2) is determined based on the second resonant frequency (ω2) and the resonant inductance.

5. The method of claim 4, wherein the resonant inductance is determined independently of the inductive transformer ratio of the capacitive voltage transformer.

6. The method according to claim 1, further comprising: - When the high voltage connection (HV) is grounded (70), the first minimum impedance and the first bandwidth are determined by the plurality of first short-circuit impedance measurements performed at different frequencies on the secondary side of the capacitive voltage transformer (50). - When the high-voltage connection (HV) is disconnected, the second minimum impedance and the second bandwidth are determined by the plurality of second short-circuit impedance measurements performed at different frequencies on the secondary side of the capacitive voltage transformer (50), and - The actual values ​​of the capacitance of the first capacitor (C1) and the actual values ​​of the capacitance of the second capacitor (C2) are determined based on the first minimum impedance, the first bandwidth, the second minimum impedance of the second bandwidth, and the induction transformation ratio of the current transformer of the capacitive voltage transformer.

7. The method according to claim 6, further comprising: - Based on the first resonant frequency (ω) e The capacitance ratio (K) determined by the second resonant frequency (ω2) and the second resonant frequency (ω2) C The capacitance ratio is compared with that determined based on the actual values ​​of the capacitance of the first capacitor (C1) and the actual values ​​of the capacitance of the second capacitor (C2), and... - The validity of determining the actual value of the capacitance of the first capacitor (C1) and the actual value of the capacitance of the second capacitor (C2) based on the comparison.

8. The method according to claim 4, further comprising: - Determine multiple actual values ​​of the capacitance of the second capacitor (C2) at different time points, and - Display the time trend of the capacitance of the second capacitor (C2) on the display device.

9. The method according to claim 6, further comprising: - Determine multiple actual values ​​of the capacitance of the second capacitor (C2) at different time points, and - Display the time trend of the capacitance of the second capacitor (C2) on the display device.

10. The method of claim 4, further comprising: - Based on the actual value of the capacitance of the second capacitor (C2) and the capacitance ratio (K) C The actual value of the capacitance of the first capacitor (C1) is determined by this method.

11. The method of claim 6, further comprising: - Determine multiple actual values ​​of the capacitance of the first capacitor (C1) at different time points, and - Display the time trend of the capacitance of the first capacitor (C1) on the display device.

12. The method according to claim 1, further comprising: - Determine multiple capacitance ratios (K) of the capacitor divider at different time points. C ),and - The capacitance ratio (K) is represented on the display device. C (Time trend) 13. The method according to claim 1, further comprising: - Determine the capacitance ratio (K) C ) for the first resonant frequency (ω) e The first correlation (δK) C / δω e ).

14. The method of claim 13, further comprising: - Determine the capacitance ratio (K) C ) at different time points the first resonant frequency (ω) e Multiple first correlations (δK) C / δω e ),and - Represent the first correlation (δK) on the display device. C / δω e (Time trend) 15. The method according to claim 1, further comprising: - Determine the capacitance ratio (K) C The second correlation (δK) with the second resonant frequency (ω2) C / δω2).

16. The method of claim 15, further comprising: - Determine the capacitance ratio (K) C Multiple second correlations (δK) of the second resonant frequency (ω2) at different time points C / δω2), and - Represent the second correlation (δK) on the display device. C The time trend of / δω2).

17. An apparatus for determining the state of a capacitive voltage transformer (50), wherein the capacitive voltage transformer (50) includes a capacitive voltage divider having a first capacitor (C1) and a second capacitor (C2) on the primary side of the capacitive voltage transformer (50), wherein the first capacitor (C1) has a high-voltage connection (HV) for connection to a high voltage (60) and the second capacitor (C2) has a ground connection (NHF), wherein the apparatus (90) comprises: - A measuring device configured to perform short-circuit impedance measurement at an adjustable frequency, and - Control device, which is configured as When the high-voltage connection (HV) is grounded (70), the first resonant frequency (ω) is determined by performing multiple first short-circuit impedance measurements at different frequencies on the secondary side of the capacitive voltage transformer (50). e ), When the high-voltage connection (HV) is disconnected, the second resonant frequency (ω2) is determined by multiple second short-circuit impedance measurements performed at different frequencies on the secondary side of the capacitive voltage transformer (50). Based on the first resonant frequency (ω) e The capacitance ratio (K) of the capacitor divider is determined by the second resonant frequency (ω2) and the second resonant frequency (ω2). C ).

18. The apparatus of claim 17, wherein the apparatus (90) is configured to perform the method of claim 1.

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