Measuring an electric current

Abstract

Method for measuring an electric current (I), wherein - measurement signals (A) dependent on the current (I) are repeatedly acquired using at least two optical current transformers (5, 7), wherein the current transformers (5, 7) have different uniqueness ranges (E1 to E4) in which the measurement signal (A) acquired by the respective current transformer (5, 7) increases or decreases monotonically with increasing current (I), - for each current transformer (5, 7) a sign value is continuously determined which indicates the sign of the first derivative of the measurement signal (A) detected by the current transformer (5, 7) with respect to the current (I), - the measurement signals (A) of a current transformer (5, 7) are assigned a modified uniqueness range (E1 to E4) of the current transformer (5, 7) if the sign value determined for this current transformer (5, 7) changes and the sign value determined for at least one other current transformer (5, 7) does not change, and - the current strength of the electric current (I) is determined from the measurement signals (A) of at least one current transformer (5, 7).

Description

[0001] The invention relates to the measurement of an electrical current using an optical current transducer.

[0002] An optical current transducer is an optical measuring device for measuring an electric current in a conductor, based on the magneto-optical Faraday effect. This effect is the rotation of the polarization direction of a linearly polarized electromagnetic wave in a medium by a magnetic field parallel to the wave's propagation direction. The rotation of the polarization direction is proportional to the magnetic flux density of the magnetic field.

[0003] In an optical current transducer, linearly polarized light is transmitted through a fiber optic cable arranged near the current conductor, which exhibits the Faraday effect. The magnetic field generated by the current in the conductor causes a rotation of the polarization direction of the light. Since the magnetic flux density of the magnetic field depends on the current strength, the current strength can be measured by detecting the rotation of the polarization direction of the light. To detect the rotation of the polarization direction, the light emitted by the fiber optic cable is passed through a polarizer, for example, and the light intensity of the light transmitted by the polarizer is measured.

[0004] The measuring principle of an optical current transformer implies that the current transformer's measurement signals do not provide a unique function of the current being measured, since different current intensities result in the same measurement signal. The current values ​​form successive uniqueness ranges in which the current transformer's measurement signal increases or decreases monotonically with increasing current.Each uniqueness range in which the current transformer's measuring signal increases monotonically from a minimum to a maximum with increasing current is followed by a uniqueness range in which the current transformer's measuring signal decreases monotonically from the maximum to the minimum with increasing current, and each uniqueness range in which the current transformer's measuring signal decreases monotonically from the maximum to the minimum with increasing current is followed by a uniqueness range in which the current transformer's measuring signal increases monotonically from the minimum to the maximum with increasing current.

[0005] Document WO 98 / 05975 A1 describes that the optical detection of an electrical quantity involves a periodic dependence of the optical measurement signal on the electrical quantity being measured. To obtain a unique measurement signal, two optical, periodically dependent measurement signals are used. The measured values ​​obtained serve as a pair of values ​​for the unambiguous assignment of an absolute value of the electrical quantity.

[0006] Document EP 1 179 735 A1 describes how to measure an electrical voltage in an electro-optical voltage converter by passing polarized light of two wavelengths through the medium. At the output side, the light is passed through a polarizer, and the remaining signal is measured. To unambiguously determine the voltage even above the half-wave voltage, the measurement results of both wavelengths are compared, and the voltage value consistent with both measurements is used.

[0007] The document DE 37 11 062 C2 discloses a position measuring device for absolute angle or distance measurement.

[0008] The invention is based on the object of providing an improved method and an improved measuring device for measuring an electrical current using an optical current transformer.

[0009] The object is achieved according to the invention by a method having the features of claim 1 and by a measuring device having the features of claim 7.

[0010] Advantageous embodiments of the invention are the subject of the subclaims.

[0011] In the method according to the invention for measuring an electric current, measurement signals dependent on the current are repeatedly recorded using at least two optical current transformers, wherein the current transformers have different unambiguous ranges in which the measurement signal recorded by the respective current transformer increases or decreases monotonically as the current increases. For each current transformer, a sign value is continuously determined which indicates the sign of the first derivative of the measurement signal recorded by the current transformer with respect to the current. The measurement signals of a current transformer are assigned a changed unambiguous range of the current transformer if the sign value determined for this current transformer changes and the sign value determined for at least one other current transformer does not change. The current strength of the electric current is determined from the measurement signals of at least one current transformer.

[0012] The method according to the invention thus provides for the use of multiple optical current transformers to measure an electrical current, wherein the optical current transformers have mutually different unambiguousness ranges, within which the measurement signal detected by the respective current transformer monotonically increases or decreases as the current increases. By using multiple optical current transformers with mutually different unambiguousness ranges, the problem of the ambiguity of the measurement signal of an optical current transformer as a function of the current to be measured can be solved by using at least one additional optical current transformer to determine the respective unambiguousness range of this optical current transformer.In order to reliably detect the change between different unambiguousness ranges of optical current transformers, the invention provides for continuously determining a sign value for each of these current transformers. This sign value indicates the sign of the first derivative of the measurement signal detected by the current transformer with respect to the current. A change in the sign value of one current transformer while the sign value of another current transformer does not change reliably signals that the unambiguousness range of the current transformer has changed.

[0013] In one embodiment of the method according to the invention, no current transformer has a uniqueness range that is a multiple of the uniqueness range of another current transformer. This embodiment of the invention takes into account that a uniqueness range of one current transformer that is a multiple of the uniqueness range of another current transformer leads to current intensities of the electrical current to be measured at which the uniqueness ranges of both current transformers change simultaneously, so that at these current intensities it is not possible to use one current transformer to control the other current transformer.

[0014] In a further embodiment of the method according to the invention, the measurement signals of each current transformer are digitized measurement signals that assume values ​​from a value range assigned to the current transformer, and the entire value range is used for each uniqueness range of the current transformer. This embodiment of the invention exploits the fact that the invention enables a unique determination of the respective uniqueness range of an optical current transformer. Therefore, for each uniqueness range of a current transformer, the entire value range assigned to this current transformer can be used to record digital measurement signals. In other words, different uniqueness ranges do not need to be assigned a separate subset of the value range.This advantageously significantly reduces the memory requirements for the digital measurement signals while maintaining the same resolution of the measurement signals, or advantageously significantly increases the resolution of the measurement signals while maintaining the same storage space.

[0015] In a further embodiment of the method according to the invention, each current transformer has an optical waveguide and the optical waveguides of at least two current transformers are made of different materials, in particular of materials with different Verdet constants.

[0016] In a further embodiment of the method according to the invention, electromagnetic radiation of a wavelength specific to the current transformer is passed through each current transformer and the wavelengths of at least two current transformers differ from one another.

[0017] The two aforementioned embodiments of the invention enable the formation of optical current transformers with different unambiguousness ranges by means of optical waveguides of different materials and / or electromagnetic radiation of different wavelengths.

[0018] A measuring device according to the invention for measuring an electric current comprises - at least two optical current transformers, each of which is configured to repeatedly detect measurement signals dependent on the current, wherein the current transformers have different unambiguous ranges in which the measurement signal detected by the respective current transformer increases or decreases monotonically with increasing current, and - an evaluation unit that is set up, - to continuously determine for each current transformer a sign value which indicates the sign of the first derivative of the measurement signal detected by the current transformer with respect to the current, - to assign a changed uniqueness range of the current transformer to the measuring signals of a current transformer if the sign value determined for this current transformer changes and the sign value determined for at least one other current transformer does not change, and - to determine the intensity of the electric current from the measuring signals of at least one current transformer.

[0019] A measuring device according to the invention for measuring an electrical current enables the implementation of the method according to the invention. The advantages of such a measuring device therefore correspond to the above-mentioned advantages of the method according to the invention. The same applies to the following embodiments of a measuring device according to the invention, which correspond to the above-mentioned embodiments of the method according to the invention.

[0020] In one embodiment of the measuring device according to the invention, no current transformer has an unambiguousness range that is a multiple of an unambiguousness range of another current transformer.

[0021] In a further embodiment of the measuring device according to the invention, the measuring signals of each current transformer are digitized measuring signals which assume values ​​from a value range assigned to the current transformer, and the evaluation unit is configured to use the entire value range for each uniqueness range of the current transformer.

[0022] In a further embodiment of the measuring device according to the invention, each current transformer has an optical waveguide and the optical waveguides of at least two current transformers are made of different materials, in particular of materials with different Verdet constants.

[0023] In a further embodiment of the measuring device according to the invention, each current transformer is configured to guide electromagnetic radiation of a wavelength specific to the current transformer, and the wavelengths of at least two current transformers differ from one another.

[0024] The above-described properties, features, and advantages of this invention, as well as the manner in which they are achieved, will become clearer and more readily understood in connection with the following description of exemplary embodiments, which are explained in more detail in conjunction with the drawings. Fig. 1 is a block diagram of an embodiment of a measuring device for measuring an electric current, Fig. 2 Measurement signals of an optical current transformer depending on an electrical current, Fig. 3 a flowchart of an embodiment of a method for measuring an electrical current.

[0025] Fig. 1 ( Fig. 1) shows a block diagram of an embodiment of a measuring device 1 for measuring an electrical current I flowing in an electrical conductor 3. The measuring device 1 comprises two optical current transformers 5, 7 and an evaluation unit 9.

[0026] Each current transformer 5, 7 is configured to repeatedly acquire measurement signals A dependent on the current I.

[0027] Fig. 2 ( Fig. 2) schematically shows measurement signals A detected by a current transformer 5, 7 as a function of the electrical current I. Each current transformer 5, 7 has uniqueness ranges E1 to E4, in which the measurement signal A detected by the current transformer 5, 7 increases or decreases monotonically with increasing current I. Fig. 2 shows, by way of example, two uniqueness regions E1, E3 in which the measurement signal A increases monotonically from the value A=0 to a maximum value A=A0 as the current I increases, and two uniqueness regions E2, E4 in which the measurement signal A decreases monotonically from the maximum value A=A0 to the value A=0 as the current I increases. Each uniqueness region E1, E3 in which the measurement signal A increases monotonically with increasing current I is followed by a uniqueness region E2, E4 in which the measurement signal A decreases monotonically with increasing current I. Each uniqueness region E2, E4 in which the measurement signal A decreases monotonically with increasing current I is followed by a uniqueness region E1, E3 in which the measurement signal A increases monotonically with increasing current I.

[0028] The two current transformers 5, 7 of the measuring device 1 have different unambiguousness ranges E1 to E4, whereby no current transformer 5, 7 has an unambiguousness range E1 to E4 that is a multiple of the unambiguousness range E1 to E4 of the other current transformer 5, 7. For example, the current transformers 5, 7 each have an optical fiber, and the optical fibers of the two current transformers 5, 7 are made of different materials, in particular of materials with different Verdet constants. Alternatively or additionally, electromagnetic radiation of a wavelength specific to the current transformer 5, 7 is guided through each current transformer 5, 7, whereby the wavelengths of the two current transformers 5, 7 differ from one another.

[0029] The evaluation unit 9 is configured to continuously determine a sign value for each current transformer 5, 7, which indicates the sign of the first derivative of the measurement signal A detected by the current transformer 5, 7 with respect to the current I. Furthermore, the evaluation unit 9 is configured to assign a modified uniqueness range E1 to E4 of the current transformer 5, 7 to the measurement signals A of each current transformer 5, 7 if the sign value determined for this current transformer 5, 7 changes and the sign value determined for the other current transformer 5, 7 does not change. Furthermore, the evaluation unit 9 is configured to determine the current intensity of the electrical current I from the measurement signals A of at least one current transformer 5, 7. For example, the evaluation unit 9 is configured to determine the current intensity of the electrical current I from the measurement signals A of the current transformer 5, 7 which has the higher measurement sensitivity of the two current transformers 5, 7.Alternatively, the evaluation unit 9 is configured, for example, to determine the current intensity of the electrical current I from an average value or weighted average value of the measurement signals A of both current transformers 5, 7.

[0030] Fig. 3 ( Fig. 3) shows a flow chart of an embodiment of the method according to the invention for measuring an electric current I. The method is carried out with a Fig. 1 and Fig. 2 described measuring device 1.

[0031] In a first method step 11, a calculation rule is specified to be executed by the evaluation unit 9, with which the current intensity of the electrical current I flowing in the electrical conductor 3 is determined from the measurement signals A acquired by the current transformers 5, 7. For example, the calculation rule provides for determining the current intensity of the electrical current I from the measurement signals A of the current transformer 5, 7 which, of the two current transformers 5, 7, has the higher measurement sensitivity. The calculation rule depends on the unambiguous range E1 to E4 of the current transformer 5, 7 corresponding to the current intensity. If the measurement signals A of the current transformer 5, 7, for example, as in the Fig.2 in the uniqueness ranges E1 to E4 each depend linearly on the current I, the calculation rule in the uniqueness range E1 is of the form I = f · A with a proportionality constant f, in the uniqueness range E2 the calculation rule is of the form I = f · (2A0 - A) etc.

[0032] Alternatively, the calculation rule provides, for example, for determining the current intensity of the electrical current I from an average or weighted average of the measurement signals A of both current transformers 5, 7. The calculation rule depends accordingly on the unambiguousness ranges E1 to E4 of the current transformers 5, 7 corresponding to the current intensity.

[0033] After the first process step 11, a second process step 12 is carried out.

[0034] In the second method step 12, a measurement signal A dependent on the current I is acquired from each current transformer 5, 7. The measurement signal A of each current transformer 5, 7 is a measurement signal digitized by an analog-to-digital converter, which assumes a value from a value range that determines the resolution of the analog-to-digital converter. Different analog-to-digital converters, in particular analog-to-digital converters with different value ranges, can be used for the two current transformers 5, 7.

[0035] For example, an 8-bit analog-to-digital converter is used for a first current transformer 5, 7, whose value range thus includes 256 values, and a 16-bit analog-to-digital converter is used for the second current transformer 5, 7, whose value range thus includes 65,536 values. In this example, the measurement signals A from the second current transformer 5, 7 are preferably used to determine the current intensity of the current I, and the first current transformer 5, 7 is used to verify the plausibility of the measurement signals A from the second current transformer 5, 7.

[0036] The entire value range of the analog-to-digital converter assigned to a current transformer 5, 7 is used for all measurement signals A in the respective uniqueness range E1 to E4 of the current transformer 5, 7.

[0037] After the second process step 12, a third process step 13 is carried out.

[0038] In the third method step 13, the evaluation unit 9 determines the current intensity of the current I from the measurement signals A acquired in the second method step 12 in accordance with the respective current calculation rule.

[0039] After the third process step 13, a fourth process step 14 is carried out.

[0040] In the fourth method step 14, the evaluation unit 9 determines a sign value for each current transformer 5, 7, which indicates the sign of the first derivative of the measurement signal A detected by the current transformer 5, 7 with respect to the current I. For this purpose, the evaluation unit 9 evaluates temporally successive measurement signals A of each current transformer 5, 7.

[0041] After the fourth method step 14, a fifth method step 15 is carried out.

[0042] In the fifth method step 15, the evaluation unit 9 checks whether the sign value determined in the fourth method step 14 for a current transformer 5, 7, whose measurement signals A are used to determine the current intensity of the current I, has changed compared to the previous sign value for this current transformer 5, 7. If this is not the case, the second method step 12 is executed again after method step 15. Otherwise, a sixth method step 16 is performed.

[0043] In the sixth method step 16, the calculation rule used to determine the current intensity of the electrical current I flowing in the electrical conductor 3 from the measurement signals A acquired by the current transformers 5, 7 is changed. In this case, the current transformer 5, 7 for which a changed sign value was determined in the fifth method step 15 is assigned an unambiguousness range E1 to E4 that borders on the unambiguousness range E1 to E4 previously assigned to this current transformer 5, 7 and corresponds to the respective current intensity, and the calculation rule is adapted to the changed unambiguousness range E1 to E4 of the current transformer 5, 7.

[0044] After the sixth process step 16, the second process step 12 is carried out again.

[0045] Although the invention has been illustrated and described in detail by means of preferred embodiments, the invention is not limited by the disclosed examples and other variations can be derived therefrom by those skilled in the art without departing from the scope of the invention.

Claims

[1] Method for measuring an electric current (I), wherein - measuring signals (A) which are dependent on the current (I) are repeatedly detected by at least two optical current transformers (5, 7), the current transformers (5, 7) having different unambiguousness ranges (E1 to E4) in which the measuring signal (A) detected by the respective current transformer (5, 7) increases or decreases monotonically with increasing current (I), - for each current transformer (5, 7) a sign value is continuously determined which indicates the sign of the first derivative of the measurement signal (A) detected by the current transformer (5, 7) with respect to the current (I), - a changed uniqueness range (E1 to E4) of the current transformer (5, 7) is assigned to the measurement signals (A) of a current transformer (5, 7) if the sign value determined for this current transformer (5, 7) changes and the sign value determined for at least one other current transformer (5, 7) does not change, and - the current intensity of the electric current (I) is determined from the measuring signals (A) of at least one current transformer (5, 7). [2] Method according to claim 1, wherein no current transformer (5, 7) has a uniqueness range (E1 to E4) which is a multiple of a uniqueness range (E1 to E4) of another current transformer (5, 7). [3] Method according to claim 1 or 2, wherein the measurement signals (A) of each current transformer (5, 7) are digitized measurement signals which assume values ​​from a value range assigned to the current transformer (5, 7), and wherein the entire value range is used for each uniqueness range (E1 to E4) of the current transformer (5, 7). [4] Method according to one of the preceding claims, wherein each current transformer (5, 7) has an optical waveguide and the optical waveguides of at least two current transformers (5, 7) are made of different materials. [5] Method according to claim 4, wherein the optical waveguides of at least two current transformers (5, 7) are made of materials with different Verdet constants. [6] Method according to one of the preceding claims, wherein electromagnetic radiation of a wavelength specific for the current transformer (5, 7) is passed through each current transformer (5, 7) and the wavelengths of at least two current transformers (5, 7) differ from one another. [7] Measuring device (1) for measuring an electric current (I), comprising - at least two optical current transformers (5, 7), each of which is configured to repeatedly detect measurement signals (A) dependent on the current (I), wherein the current transformers (5, 7) have different unambiguousness ranges (E1 to E4) in which the measurement signal (A) detected by the respective current transformer (5, 7) increases or decreases monotonically with increasing current (I), and - an evaluation unit (9) which is set up, - to continuously determine for each current transformer (5, 7) a sign value which indicates the sign of the first derivative of the measurement signal (A) detected by the current transformer (5, 7) with respect to the current (I), - to assign a changed uniqueness range (E1 to E4) of the current transformer (5, 7) to the measurement signals (A) of a current transformer (5, 7) if the sign value determined for this current transformer (5, 7) changes and the sign value determined for at least one other current transformer (5, 7) does not change, and - to determine the current intensity of the electric current (I) from the measuring signals (A) of at least one current transformer (5, 7). [8] Measuring device (1) according to claim 7, wherein no current transformer (5, 7) has an unambiguousness range (E1 to E4) which is a multiple of an unambiguousness range (E1 to E4) of another current transformer (5, 7). [9] Measuring device (1) according to claim 7 or 8, wherein the measurement signals (A) of each current transformer (5, 7) are digitized measurement signals which assume values ​​from a value range assigned to the current transformer (5, 7), and the evaluation unit (9) is configured to use the entire value range for each uniqueness range (E1 to E4) of the current transformer (5, 7). [10] Measuring device (1) according to one of claims 7 to 9, wherein each current transformer (5, 7) has an optical waveguide and the optical waveguides of at least two current transformers (5, 7) are made of different materials. [11] Measuring device (1) according to claim 10, wherein the optical waveguides of at least two current transformers (5, 7) are made of materials with different Verdet constants. [12] Measuring device (1) according to one of claims 7 to 11, wherein each current transformer (5, 7) is arranged to guide electromagnetic radiation of a wavelength specific to the current transformer (5, 7) and the wavelengths of at least two current transformers (5, 7) differ from one another.

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