Method for measuring the effective value of an alternating voltage and alternating voltage measuring device

Abstract

Method for measuring the effective value (Ueff) of an alternating voltage, comprising the steps of: (a) applying the alternating voltage to a Pockels cell (12) of an alternating voltmeter (10), (b) guiding an incident light beam (24) linearly polarized in an input polarization direction through the Pockels cell (12) to produce a linearly polarized outgoing light beam (26) whose output polarization direction is rotated by a deflection angle relative to an input polarization direction of the incident light beam (24), (c) guiding the outgoing light beam (26) through a polarizer (16) to produce a polarization-filtered light beam (28), (d) measuring an alternating voltage-dependent intensity of the polarization-filtered light beam (28) to obtain an electrical measurement signal (Umess), and (e) determining the alternating voltage from the electrical measurement signal (Umess),(f) wherein determining the alternating voltage comprises low-pass filtering of the electrical measurement signal (Umess) so that a temporal measurement signal average (Aac(t)) is obtained, characterized in that (g) the effective value (Ueff) is determined from the temporal measurement signal average (Aac(t)) and characterized by the steps: (h) applying a positive feedback DC voltage to the Pockels cell (12), (i) guiding an incident light beam (24) linearly polarized in the input polarization direction through the Pockels cell (12) so that a linearly polarized outgoing light beam (26) is created, the output polarization direction of which is rotated by a deflection angle to an input polarization direction of the incident light beam (24), (j) guiding the outgoing light beam (26) through the polarizer (16) so that the polarization-filtered light beam (28) is created,(k) measuring a DC voltage-related positive voltage intensity of the polarization-filtered light beam (28), (l) applying a negative DC voltage feedback to the Pockels cell (12), (m) measuring a DC voltage-related negative voltage intensity of the polarization-filtered light beam (28), and (n) determining the AC voltage from the positive voltage intensity, the negative voltage intensity, and the AC voltage-related intensity.

Description

[0001] The invention relates to a method for measuring the effective value of an alternating voltage. According to a second aspect, the invention relates to an alternating voltage measuring device according to the preamble of claim 5.

[0002] The voltage unit volt is typically represented by a series connection of Josephson junctions and a microwave frequency source. Alternating voltages must be converted back to a defined direct voltage. This is done using thermal converters in the current state of the art. The alternating voltage to be measured is connected to a resistor, which subsequently heats up. A feedback voltage is then applied to the same resistor, and the time at which the resistor dissipates the same amount of heat is determined.

[0003] For voltages above a few volts, additional resistors connected in series are used to measure the corresponding AC voltage, as the dynamic range of thermal converters is limited to approximately 10:1. The higher the AC voltage, the greater the released heat loss, which does not contribute to thermal transfer but heats up the measuring device. This is particularly problematic at very high AC voltages.

[0004] A generic method for measuring the effective value of an alternating voltage is known from the article by Hertz et al., "Optical wideband high-voltage measurement system," Review of Scientific Instruments, Vol. 58, 1987, pp. 1660–1664, AIP [online]. DOI: 10.1063 / 1.1139364. A disadvantage of such a method is that traceable calibration is complex.

[0005] US 2013 / 0 093 410 A1 describes a voltage sensor comprising an insulator with mutually insulated electrodes embedded therein. The electrodes are arranged coaxially and cylindrically and overlap axially along part of their length. They are offset from one another and influence the electrical equipotential surfaces such that a substantially homogeneous electric field exists outside the insulator and a substantially homogeneous, but higher, field exists within a measuring cavity within the insulator. A field sensor is arranged in the measuring chamber to measure the field locally. This design enables the production of compact voltage sensors for high-voltage applications. Even with such a system, traceable calibration is complex.

[0006] DE 44 36 454 A1 describes a method in which polarized measurement light, after passing through a Pockels sensor device under the influence of an alternating voltage or alternating field, is split by an analyzer into two linearly polarized light signals with different polarization planes. Each of the two light signals is photoelectrically converted into an electrical intensity signal. A temperature signal is derived from the DC components of the two intensity signals. An evaluation signal is derived from the AC components of the two intensity signals. A largely temperature-independent measurement signal is determined using the evaluation signal and the temperature signal. This allows temperature sensitivity to be significantly reduced.

[0007] The expert can obtain basic knowledge in the field of measurement technology from the book Semiconductor Circuit Technology by U. Tietze and Ch. Schenk.

[0008] The invention is based on the object of converting the effective value of an alternating voltage to a direct voltage with lower losses.

[0009] The invention solves the problem by a method having the features of claim 1.

[0010] According to a second aspect, the invention solves the problem by an AC voltage measuring device having the features of claim 5.

[0011] An advantage of the invention is that the current flowing through the Pockels cell is very small and can be neglected to a good approximation. It is therefore possible to determine the effective value with very low power dissipation, even when the alternating voltage is, for example, more than 100 volts. Another advantage is that the design of the alternating voltage measuring device according to the invention is comparatively simple.

[0012] Another advantage is that the Pockels cell can usually be selected to measure comparatively high alternating voltages. The maximum applicable alternating voltage, regardless of the design, is limited only by the dielectric strength of the Pockels cell.

[0013] In the context of the present description, the application of the alternating voltage to the Pockels cell is understood in particular to mean that the alternating voltage whose effective value is to be measured is applied to the Pockels cell.

[0014] The light beam passed through the Pockels cell is preferably a linearly polarized laser beam.

[0015] A Pockels cell is specifically understood to be an electro-optical modulator that utilizes the Pockels effect. This includes, for example, a crystal with electro-optical properties to which electrodes are applied. Alternatively, the electrode can be deposited directly onto the crystal. Such devices are used, for example, for modulating laser light or for Q-switching pulsed lasers.

[0016] Preferably, the method comprises the steps of (a) applying a positive feedback DC voltage to the Pockels cell, (b) passing an incident light beam linearly polarized in the input polarization direction through the Pockels cell to produce a linearly polarized outgoing light beam whose output polarization direction is rotated by a deflection angle relative to an input polarization direction of the incident light beam, (c) passing the outgoing light beam through the polarizer to produce the polarization-filtered light beam, (d) measuring a DC voltage-induced positive voltage intensity of the polarization-filtered light beam, (e) applying a negative feedback DC voltage to the Pockels cell, (f) measuring a DC voltage-induced negative voltage intensity of the polarization-filtered light beam, and (g) determining the AC voltage from the positive voltage intensity.the negative voltage intensity and the AC voltage intensity. In other words, a returned DC voltage is sufficient to determine the effective value of the applied AC voltage using the AC voltmeter.

[0017] It is particularly advantageous to use the formula Uac=Udc⋅Aac¯Adc is used. Aac¯ is the time average of the alternating voltage intensity, A dc describes the mean value of negative voltage intensity and positive voltage intensity, U dc is the direct voltage traced back to SI units and U ac describes the alternating voltage to be determined.

[0018] This formula is obtained in the following way. The Pockels effect rotates the plane of polarization by the angle α proportional to U: α=π⋅UUπ where U π the so-called half-wave voltage, which would be required for half a rotation of the polarization. The light intensity transmitted through the analysis polarizer can be expressed as A(α)=A2⋅(1−cos(α))=A2⋅(1−cos(π⋅UUπ)).

[0019] If the rotation of the polarization is small compared to a quarter turn, a Taylor series can be used to approximate the cosine function: cos(x)≈1−x22!+x44!−x66!+... and the intensity becomes A(α)≈A2⋅(α22!−α44!+α66!−⋯).

[0020] For sufficiently small DC voltages U, the higher order terms can be neglected and the light intensity is described by Adc=A4⋅(πUπ)2⋅U2.

[0021] Now let U(t) be a sinusoidal alternating voltage U(t)=U^⋅sin(ωt), which leads to a time-dependent rotation of the polarization α(t) α(t)=π⋅U^Uπ⋅sin(ωt)=α^⋅sin(ωt) with α̂ = π · Û / U π and a time-dependent light intensity at the detector of Aac(t)=A2⋅(1−cos(α^⋅sin(ωt))).

[0022] Now we examine the mean value of the light intensity, averaged over one period of the applied alternating voltage. The integral 12π∫02πcos(α^⋅sin(x))dx has no analytical solution, but if you apply the Taylor series from formula 3, the light intensity is Aac(t)=A^2⋅(a^22!sin2(ωt)−a^44!sin4(ωt)+a^66!sin6(ωt)...), where the individual summands can then be analytically averaged. Only the mean values of the even powers of the function sin(ωt) are required: sin2k'(ωt)=122k⋅(2k k).

[0023] The average intensity is calculated as Aac'(t)=A^2(α^22!12−α^44!38+α^66!516−⋯)=A^8(α^2−116α^4+1576α^6−⋯).

[0024] For sufficiently small voltages, the higher order terms can be neglected and the average light intensity is Aac'=A^8(πUπ)2⋅U^2.

[0025] Inserting the effective value U eff , where the amplitude is given as Û = √2·U eff expressed, yields: Aac'=A^4(πUπ)2Ueff2.

[0026] The comparison with equation (5) allows for an optical AC-DC transfer. Dividing equation 5 by equation 13 results in AacAdc=A^4(πUπ)2Ueff2A^4⋅(πUπ)2⋅U2

[0027] By transforming, formula 14 gives uac=Udc⋅Aac¯Adc

[0028] Preferably, the intensity meter is a photodiode. This photodiode is operated in current mode with a bias voltage. When the polarization-filtered light beam hits it, a photocurrent I Foto This flows through an ohmic resistance, so that a photocurrent I Foto proportional photovoltage U Foto This photovoltage U Foto is preferably passed through an RC low-pass filter of at least first order. This results in an electrical measurement signal that has a measurement voltage U mess coded or identical to it. The measuring voltage U mess can be measured with a digital voltmeter, which then outputs the measurement signal.

[0029] Preferably, the frequency of the alternating voltage is at least 1 hertz. It is advantageous if the frequency of the alternating voltage is at most 1 gigahertz, in particular at most 30 megahertz, preferably at most 1 megahertz.

[0030] Preferably, the amplitude of the alternating voltage is chosen at most and the Pockels cell is chosen so that a deflection angle Δα between a minimum deflection angle α min at a zero crossing of the alternating voltage and a maximum deflection angle α max , when the peak voltage of the alternating voltage is applied to the Pockels cell, is at most 9°, in particular at most 1.5°. Only small correction terms for the Taylor series truncated after the second-order term (Formula 11 and Formula 12) are then neglected, thus achieving a particularly small measurement uncertainty.

[0031] Preferably, the alternating voltage and the direct voltage are applied alternately to the Pockels cell with alternating polarity, and the resulting light intensities are used to determine the temporal drift of the AC voltmeter. Based on this drift, the effective value, which is determined from the temporal mean value of the measurement signal, is corrected for the drift.

[0032] Drift refers to the effect that the AC voltmeter detects slowly changing RMS values over time, even when the same AC voltage is applied. Since the DC voltage is known, the correct RMS value can be determined from the temporal drift. Drift can be caused, for example, by aging of the crystal of the Pockels cell or the polarizer.

[0033] The invention also provides an AC voltage measuring unit comprising an AC voltage measuring device according to the invention and a calibration certificate specifying the measurement uncertainty for measuring the AC voltage with the AC voltage measuring device. The measurement uncertainty refers to a measurement uncertainty with respect to a primary standard used to represent an SI unit.

[0034] According to a preferred embodiment, the AC voltmeter has a DC voltage source for outputting a feedback DC voltage, an AC voltage terminal, and a selector switch for alternatively connecting the DC voltage source or the AC voltage terminal to the Pockels cell. The feature that the DC voltage source is designed to output a feedback DC voltage is understood in particular to mean that the measurement uncertainty for the DC voltage output by the DC voltage source with respect to a primary standard for an SI unit is known. This measurement uncertainty is specified in a corresponding calibration certificate.

[0035] Preferably, the polarizer is positioned relative to the polarization plane of the light source such that, without any voltage applied to the Pockels cell, the transmitted light intensity is minimal, in particular zero. Alternatively or additionally, the polarizer is positioned relative to the polarization plane of the light source such that the transmitted light intensity differs by a maximum of 1% for both polarities of the DC voltage applied to the Pockels cell. In other words, the light intensity incident on the intensity meter changes by a maximum of 1% upon polarity reversal.

[0036] The Pockels cell is preferably a transverse Pockels cell.

[0037] The invention is explained in more detail below with reference to the accompanying drawings. Fig. 1 a schematic structure of an AC voltage measuring device according to the invention for carrying out a method according to the invention.

[0038] Fig. 1 shows an AC voltmeter 10 with a Pockels cell 12, a light source 14 in the form of a laser, a polarizer 16, and an intensity meter 18. The AC voltmeter 10 also includes a low-pass filter 20 and an evaluation unit 22.

[0039] The light source 14 emits a linearly polarized incident light beam 24, which enters the Pockels cell 12, creating an outgoing light beam 26. The outgoing light beam 26 falls on the polarizer 16, resulting in a polarization-filtered light beam 28, which is incident on the intensity meter 18. This produces an electrical measurement signal, in this case in the form of the photovoltage U Foto This results in a measurement signal for the temporal intensity mean A(t).

[0040] The evaluation unit 22 calculates the effective value U as stated above. eff the alternating voltage.

[0041] The AC voltmeter 10 also has a DC voltage source 30, which can be connected to the Pockels cell 12 by means of a schematically shown switch 32. The AC voltmeter 10 can also have a terminal 34 for applying the AC voltage. List of reference symbols 10 AC voltmeter 12 Pockels cell 14 Light source 16 Polarizer 18 intensity meters 20 low-pass filters 22 Evaluation unit 24 incident light beam 26 outgoing light beam 28 polarization-filtered light beam 30 DC voltage source 32 switches 34 connection Δα deflection angle difference α min Minimum deflection angle α max Maximum deflection angle ω angular frequency A dcAverage of negative voltage intensity and positive voltage intensity Amplitude normalized to 1 A(t) temporal intensity mean U □ Half-wave voltage I Foto Photocurrent U eff Effective value U Amplitude of the alternating voltage U ac AC voltage to be determined U dc DC voltage traced back to SI units U Foto Photovoltage U mess Measuring signal, measuring voltage

Claims

[1] Method for measuring the effective value (U eff ) an alternating voltage, with the steps: (a) applying the alternating voltage to a Pockels cell (12) of an alternating voltage measuring device (10), (b) guiding an incident light beam (24) linearly polarized in an input polarization direction through the Pockels cell (12) so as to produce a linearly polarized outgoing light beam (26) whose output polarization direction is rotated by a deflection angle relative to an input polarization direction of the incident light beam (24), (c) guiding the outgoing light beam (26) through a polarizer (16) so that a polarization-filtered light beam (28) is produced, (d) measuring an AC voltage-related intensity of the polarization-filtered light beam (28) so that an electrical measurement signal (U mess ) is received and (e) Determining the alternating voltage from the electrical measurement signal (Umess), (f) wherein determining the alternating voltage involves low-pass filtering of the electrical measurement signal (U mess ) so that a temporal measurement signal average (A ac (t)) is obtained, characterized by , that (g) the effective value (U eff ) from the temporal measurement signal average (A ac (t)) is determined and characterized by the steps: (h) applying a positive feedback DC voltage to the Pockels cell (12), (i) guiding an incident light beam (24) linearly polarized in the input polarization direction through the Pockels cell (12) so as to produce a linearly polarized outgoing light beam (26) whose output polarization direction is rotated by a deflection angle relative to an input polarization direction of the incident light beam (24), (j) guiding the outgoing light beam (26) through the polarizer (16) so that the polarization-filtered light beam (28) is produced, (k) measuring a DC voltage-induced positive voltage intensity of the polarization-filtered light beam (28), (I) Applying a negative feedback DC voltage to the Pockels cell (12), (m) measuring a DC voltage-induced negative voltage intensity of the polarization-filtered light beam (28) and (n) Determine the alternating voltage from the positive voltage intensity, the negative voltage intensity and the alternating voltage intensity. [2] Method according to claim 1, characterized by that determining the alternating voltage using the formula Uac=Udc⋅Aac¯Adc takes place, whereby Aac¯ is the time average of the alternating voltage intensity, A dcdescribes the mean value of negative voltage intensity and positive voltage intensity, U dc is the direct voltage traced back to SI units and U αc describes the alternating voltage to be determined. [3] Method according to one of the preceding claims, characterized by , that (a) the frequency of the alternating voltage is at least 1 hertz and / or at most 1 gigahertz and / or (b) an amplitude of the alternating voltage (U) is at most as large and the Pockels cell (12) is selected such that a deflection angle difference (Δα) between a minimum deflection angle (α min ) at a zero crossing of the alternating voltage and a maximum deflection angle (α max ), when the peak voltage of the alternating voltage is applied to the Pockels cell (12), not more than 9°, in particular not more than 2.5°, [4] Method according to one of the preceding claims, characterized by, that (a) alternating alternating voltage and direct voltages with alternating polarity (AC, DC+, AC, DC-, AC,...) are applied to the Pockels cell (12) and (b) a temporal drift of the alternating voltage measuring device (10) is determined from the resulting light intensities (measurement signals for the intensity mean values) and the alternating voltage determined from the temporal measurement signal mean value is corrected on the basis of the drift. [5] AC voltage measuring device (10) with (a) a Pockels cell (12), (b) a light source (14) for emitting a linearly polarized incident light beam (24) onto the Pockels cell (12) so that the incident light beam passes through the Pockels cell (12) and an outgoing light beam (26) is formed, (c) a polarizer (16) arranged so that the outgoing light beam (26) passes through the polarizer (16) so that a polarization-filtered light beam (28) is produced, (d) an intensity meter (18) for measuring an AC voltage-related intensity of the polarization-filtered light beam (28) and (e) an evaluation unit (22) which is designed to automatically determine the effective value (U eff ) from the alternating voltage intensity, characterized by (f) a DC voltage source (30) for supplying a feedback DC voltage, (g) an AC connection and (h) a changeover switch (32) for alternatively connecting the DC voltage source (30) or the AC voltage connection to the Pockels cell (12). [6] AC voltage measuring device (10) according to claim 5, characterized bythat the polarizer (16) is arranged relative to the polarization plane of the light source (14) such that (a) without voltage applied to the Pockels cell (12), the light intensity passing through is minimal, in particular becomes zero and / or (b) the transmitted light intensities for both polarities of the DC voltage applied to the Pockels cell (12) differ by a maximum of 1%. [7] AC voltage measuring device (10) according to one of claims 5 to 6, characterized by that the Pockels cell (12) is a transverse Pockels cell (12). [8] AC voltage measuring unit with an AC voltage measuring device (10) according to one of claims 5 to 7 and a calibration certificate in which the measurement uncertainty for the measurement of the AC voltage is specified.

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