Measuring current using two optical current transformers

Abstract

The invention relates to a method for measuring an electric current (I). In the method, a measurement signal (A) dependent on the current (I) is detected a plurality of times by each of at least two optical current transformers (5, 7), the current transformers (5, 7) having different clear ranges (E1 to E4) within which the measurement signal (A) detected by the respective current transformer (5, 7) monotonically increases or monotonically decreases as the current (I) increases. A sign value is continuously determined for each current transformer (5, 7), said sign value indicating a sign of a first derivative of the measurement signal (A) detected by the current transformer (5, 7) with respect to the current (I). If the sign value determined by one current transformer (5, 7) has changed and the sign value determined by at least one other current transformer (5, 7) has not changed, the clear range (E1 to E4) of the modification of the current transformer (5, 7) is associated with the measurement signal (A) from the current transformer (5, 7). The amperage of the current (I) is determined on the basis of the measurement signal (A) of the at least one current transformer (5, 7).

Description

Field of the Invention

[0001] The present invention relates to the measurement of current using an optical current transformer. Background Art

[0002] Here, an optical current transformer is understood as an optical measuring device for measuring the current in an electrical conductor based on the magneto-optical Faraday effect. This effect is understood as the rotation of the polarization direction of a linearly polarized electromagnetic wave in a medium due to a magnetic field parallel to the propagation direction of the wave. In this case, the rotation of the polarization direction is proportional to the magnetic flux density of the magnetic field.

[0003] In the case of an optical current transformer, linearly polarized light is sent through an optical fiber arranged near the current transformer, which exhibits the Faraday effect. The magnetic field generated by the current in the electrical conductor causes the polarization direction of the light to rotate. Since the magnetic flux density of the magnetic field depends on the amperage of the current, the amperage 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 optical fiber is, for example, guided through a polarizer, and the light intensity of the light transmitted by the polarizer is detected.

[0004] The measurement principle of an optical current transformer means that the measurement signal of the current transformer does not provide any explicit function of the amperage of the current to be measured, since different amperages result in the same measurement signal. The amperage values form a continuous and distinct range, in which the measurement signal of the current transformer monotonically increases or monotonically decreases with increasing amperage in each case. In this case, after each distinct range in which the measurement signal of the current transformer monotonically increases from a minimum value to a maximum value with increasing amperage, there is a distinct range in which the measurement signal of the current transformer monotonically decreases from a maximum value to a minimum value with increasing amperage, and after each distinct range in which the measurement signal of the current transformer monotonically decreases from a maximum value to a minimum value with increasing amperage, there is a distinct range in which the measurement signal of the current transformer monotonically increases from a minimum value to a maximum value with increasing amperage. Summary of the Invention

[0005] The object of the present invention is to describe an improved method and an improved measuring device for measuring current using an optical current transformer.

[0006] According to the present invention, this object is achieved by a method having the features of claim 1 and a measuring device having the features of claim 7.

[0007] The dependent claims relate to advantageous embodiments of the present invention.

[0008] In a method for measuring current according to the present invention, measurement signals dependent on the current are repeatedly detected by means of at least two optical current transformers, wherein the current transformers have different distinct ranges within which the measurement signals detected by the respective current transformers increase or decrease monotonically as the current increases. A sign value is continuously determined for each current transformer, which sign value indicates the sign of the first derivative of the measurement signal detected by the current transformer with respect to the current I. If the sign value determined for one current transformer changes and the sign values determined for at least one other current transformer do not change, the modified distinct range of the current transformer is associated with the measurement signal from that current transformer. The amperage of the current is determined based on the measurement signals of at least one current transformer.

[0009] Thus, the method according to the present invention provides for measuring current using a plurality of optical current transformers, wherein the optical current transformers have different distinct ranges within which the measurement signals detected by the respective current transformers increase or decrease monotonically as the current increases. Using a plurality of optical current transformers with different distinct ranges makes it possible to solve the ambiguity problem of the measurement signals of the optical current transformers as a function of the current to be measured, since at least one additional optical current transformer is used to determine the respective distinct ranges of the optical current transformers. In order to reliably determine the change between different distinct ranges, the present invention provides for continuously determining a sign value for each of the current transformers, which sign value indicates the sign of the first derivative of the measurement signal detected by the current transformer. A change in the sign value of this current transformer reliably indicates that the distinct range of this current transformer has changed when the sign value of another current transformer has not changed.

[0010] In one embodiment of the method according to the present invention, no current transformer has a distinct range that is a multiple of the distinct range of another current transformer. This embodiment of the present invention takes into account the fact that the distinct range of one current transformer being a multiple of the distinct range of another current transformer results in the amperage of the current to be measured at which the distinct ranges of the two current transformers change simultaneously, such that it is not possible to use one current transformer to control the other current transformer at these amperages.

[0011] In another embodiment of the method according to the invention, the measurement signal of each current transformer is a digitized measurement signal which takes values from the value range assigned to the current transformer, and the entire value range is used for each distinct range of the current transformer. This embodiment of the invention makes use of the fact that the invention can clearly determine the corresponding distinct ranges of the optical current transformer. Thus, for each distinct range of the current transformer, the entire value range assigned to the current transformer can be used to record the digital measurement signal. In other words, in each case, different distinct ranges do not need to be associated with separate subsets of the value range. Thus, with the same resolution of the measurement signal, the storage requirements for the digital measurement signal are advantageously significantly reduced, or with the same storage space, the resolution of the measurement signal is advantageously significantly increased.

[0012] In another embodiment of the method according to the invention, each current transformer comprises an optical waveguide, and the optical waveguides of at least two current transformers are made of different materials, in particular materials having different Verdet constants.

[0013] In another embodiment of the method according to the invention, electromagnetic radiation of a specific wavelength for the current transformer is conducted through each current transformer, and the wavelengths of at least two current transformers are different from each other.

[0014] The above two embodiments of the invention allow optical current transformers with different distinct ranges to be configured by using optical waveguides of different materials and / or electromagnetic radiation of different wavelengths.

[0015] A measuring device for measuring current according to the invention comprises:

[0016] - at least two optical current transformers, each optical current transformer being configured to repeatedly detect a measurement signal depending on the current, wherein the current transformers have different distinct ranges within which the measurement signal detected by the respective current transformer increases or decreases monotonically as the current increases, and

[0017] - an evaluation unit which is configured to:

[0018] - continuously determine the sign value of each current transformer, which sign value indicates the sign of the first derivative of the measurement signal detected by the current transformer with respect to the current;

[0019] - if the sign value determined for this current transformer changes and the sign values determined for at least one other current transformer do not change, assign the changed distinct range of the current transformer to the measurement signal of the current transformer; and

[0020] - determine the amperage of the current based on the measurement signals of at least one current transformer.

[0021] The measuring device for measuring current according to the present invention enables the implementation of the method according to the present invention. Therefore, the advantages of such a measuring device correspond to the above-mentioned advantages of the method according to the present invention. A similar situation also applies to the following embodiments of the measuring device according to the present invention, which correspond to the above-mentioned embodiments of the method according to the present invention.

[0022] In one embodiment of the measuring device according to the present invention, the clear range of one current transformer without a current transformer is a multiple of the clear range of another current transformer.

[0023] In another embodiment of the measuring device according to the present invention, the measurement signal of each current transformer is a digitized measurement signal that takes values from the value range assigned to the current transformer, and the evaluation unit is configured to use the entire value range for each clear range of the current transformer.

[0024] In another embodiment of the measuring device according to the present invention, each current transformer includes an optical waveguide, and the optical waveguides of at least two current transformers are made of different materials, in particular materials having different Verdet constants.

[0025] In another embodiment of the measuring device according to the present invention, each current transformer is configured to conduct electromagnetic radiation of a specific wavelength of the current transformer, and the wavelengths of at least two current transformers are different from each other. Description of the Drawings

[0026] In connection with the following description of the embodiments (which are explained in more detail with reference to the accompanying drawings), the above-mentioned attributes, features, and advantages of the present invention and the manner of achieving them will become clearer and easier to understand, where:

[0027] Figure 1 is a block diagram of an embodiment of a measuring device for measuring current;

[0028] Figure 2 shows the measurement signal of an optical current transformer depending on the current;

[0029] Figure 3 is a flowchart of an embodiment of a method for measuring current. Detailed Description

[0030] Figure 1 is a block diagram of an embodiment of a measuring device 1 for measuring the current I flowing in a conductor 3. The measuring device 1 includes two optical current transformers 5, 7 and an evaluation unit 9.

[0031] Each current transformer 5, 7 is configured to repeatedly detect a measurement signal A depending on the current I.

[0032] Figure 2 Schematically shows the measurement signal A detected by current transformers 5, 7 according to the current I. Each current transformer 5, 7 has a distinct range from E1 to E4, where the measurement signal A detected by the current transformers 5, 7 increases or decreases monotonically as the current I increases. Figure 2 By way of example, two distinct ranges E1, E3 are shown, in which the measurement signal A increases monotonically from the value A = 0 to the maximum value A = A as the current I increases 0 ; and two distinct ranges E2, E4, in which the measurement signal A decreases monotonically from the maximum value A = A 0 to the value A = 0 as the current I increases. Each distinct range E1, E3, in which the measurement signal A increases monotonically as the current I increases, and the distinct range E1, E3 is adjacent to the distinct ranges E2, E4, in which the measurement signal A decreases monotonically as the current I increases. Within the distinct range E2, E4, the measurement signal A decreases monotonically as the current I increases, and the distinct range E2, E4 is adjacent to the distinct ranges E1, E3, in which the measurement signal A increases monotonically as the current I increases.

[0033] The two current transformers 5, 7 of the measuring device 1 have different distinct ranges E1 to E4, and none of the current transformers 5, 7 has a distinct range E1 to E4 that is a multiple of the distinct range E1 to E4 of the other current transformer 5, 7. For example, the current transformers 5, 7 each include an optical waveguide, and the optical waveguides of the two current transformers 5, 7 are made of different materials, in particular materials with different Verdet constants. Alternatively or additionally, electromagnetic radiation of a specific wavelength for the current transformers 5, 7 is conducted through each current transformer 5, 7, and the wavelengths of the two current transformers 5, 7 are different from each other.

[0034] The evaluation unit 9 is configured to continuously determine the sign value of each current transformer 5, 7, which sign value indicates the sign of the first derivative of the measurement signal A detected by the current transformers 5, 7 with respect to the current I. In addition, the evaluation unit 9 is configured to assign the modified clear ranges E1 to E4 of the current transformers 5, 7 to the measurement signal A of each current transformer 5, 7 if the sign value determined for the current transformer 5, 7 changes and the sign value determined for another current transformer 5, 7 does not change. In addition, the evaluation unit 9 is configured to determine the amperage of the current I based on the measurement signal A of at least one current transformer 5, 7. For example, the evaluation unit 9 is configured to determine the amperage of the current I from the measurement signal A of the current transformer 5, 7 having the higher measurement sensitivity of the two current transformers 5, 7. Alternatively, the evaluation unit 9 is configured, for example, to determine the amperage of the current I from the average value or weighted average value of the measurement signals A of the two current transformers 5, 7.

[0035] Figure 3 The flowchart of an embodiment of a method for measuring the current I according to the present invention is shown. This method uses the reference Figure 1 and Figure 2 measurement device 1 described to perform.

[0036] In the first method step 11, a calculation rule is specified that is executed by the evaluation unit 9, by means of which the amperage of the current I flowing in the electrical conductor 3 is determined based on the measurement signal A detected by the current transformers 5, 7. For example, the calculation rule stipulates that the amperage of the current I is determined based on the measurement signal A of the current transformer 5, 7 with the higher measurement sensitivity of the two current transformers 5, 7. In this case, the calculation rule depends on the clear ranges E1 to E4 of the current transformers 5, 7 corresponding to the amperage. If the measurement signal A of the current transformers 5, 7 (for example, in the Figure 2 example shown) is linearly related to the current I in each of the clear ranges E1 to E4, then the calculation rule in the clear range E1 is of the form I = f·A with a proportionality constant f, and in the clear range E2, the calculation rule is of the form I = f·(2A 0 – A), and so on.

[0037] Alternatively, the calculation rule stipulates that the amperage of the current I is determined, for example, based on the average value or weighted average value of the measurement signals A of the two current transformers 5, 7. In this case, the calculation rule correspondingly depends on the clear ranges E1 to E4 of the current transformers 5, 7 corresponding to the amperage.

[0038] After the first method step 11, a second method step 12 is performed.

[0039] In a second method step 12, measurement signals A that depend on the current I are detected by each of the current transformers 5, 7. The measurement signal A of each current transformer 5, 7 is a measurement signal digitized by an analog-to-digital converter, which takes values from a value range that determines the resolution of the analog-to-digital converter. In this case, two current transformers 5, 7 can use different analog-to-digital converters, in particular analog-to-digital converters with different value ranges from one another.

[0040] For example, for the first current transformers 5, 7, an 8-bit analog-to-digital converter is used, the value range of which thus has 256 values, while for the second current transformers 5, 7, a 16-bit analog-to-digital converter is used, the value range of which thus has 65,536 values. In this example, preferably, the measurement signal A of the second current transformer 5, 7 is used to determine the amperage of the current I, and the first current transformers 5, 7 are used to check the plausibility of the measurement signal A of the second current transformer 5, 7.

[0041] In this case, the entire value range of the analog-to-digital converter associated with one current transformer 5, 7 is used for all measurement signals A in the corresponding distinct ranges E1 to E4 of the current transformer 5, 7.

[0042] After the second method step 12, a third method step 13 is carried out.

[0043] In the third method step 13, the evaluation unit 9 determines the amperage of the current I according to the current calculation rule in each case based on the measurement signal A detected in the second method step 12.

[0044] After the third method step 13, a fourth method step 14 is carried out.

[0045] In the fourth method step 14, the evaluation unit 9 determines the sign value of each current transformer 5, 7, which sign value indicates 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 the measurement signal A of each current transformer 5, 7 continuously in time.

[0046] After the fourth method step 14, a fifth method step 15 is carried out.

[0047] In the fifth method step 15, the evaluation unit 9 checks whether the sign value determined for the current transformers 5, 7 in the fourth method step 14 has changed with respect to the previous sign value of the current transformers 5, 7, the measurement signal A of which is used to determine the amperage of the current I. If this is not the case, then after the method step 15, the second method step 12 is carried out again. Otherwise, a sixth method step 16 is carried out.

[0048] In the sixth method step 16, the calculation rule is changed by which the amperage of the current I flowing in the electrical conductor 13 is determined based on the measurement signal A detected by the current transformers 5, 7. In this case, in the fifth method step 15, the current transformers 5, 7 in which a change in the sign value has been determined are assigned a distinct range E1 to E4 which is adjacent to the distinct range E1 to E4 previously assigned to the current transformers 5, 7 and which corresponds in each case to the amperage of the current, and the calculation rule is adjusted according to the change in the distinct range E1 to E4 of the current transformers 5, 7.

[0049] After the sixth method step 16, the second method step 12 is carried out again.

[0050] Although the invention has been shown and described in more detail based on preferred embodiments, the invention is not limited to the disclosed examples and other variations can be derived therefrom by a person skilled in the art without departing from the scope of protection of the invention.

Claims

1. A method for measuring a current (I), wherein, - measuring signals (A) depending on the current (I) are repeatedly detected by means of at least two optical current transformers (5, 7), wherein the current transformers (5, 7) have different distinct ranges (E1 to E4), within which the measuring signals (A) detected by the respective current transformers (5, 7) increase or decrease monotonically as the current (I) increases; - continuously determining a sign value for each current transformer (5, 7), the sign value indicating the sign of the first derivative of the measuring signal (A) detected by the current transformer (5, 7) with respect to the current (I); - if the sign value determined for the current transformer (5, 7) changes, while the sign values determined for at least one other current transformer (5, 7) do not change, then the modified distinct range (E1 to E4) of the current transformer (5, 7) is associated with the measuring signal (A) from the current transformer (5, 7); and - determining the amperage of the current (I) based on the measuring signals (A) of at least one current transformer (5, 7).

2. The method according to claim 1, wherein, no distinct range (E1 to E4) of a current transformer (5, 7) is a multiple of the distinct range (E1 to E4) of another current transformer (5, 7).

3. The method according to claim 1 or 2, wherein, the measuring signal (A) of each current transformer (5, 7) is a digitized measuring signal, which takes values from a value range assigned to the current transformer (5, 7), and wherein the entire value range is used for each distinct range (E1 to E4) of the current transformer (5, 7).

4. The method according to any one of the preceding claims, wherein, each current transformer (5, 7) includes an optical waveguide, and the optical waveguides of at least two current transformers (5, 7) are made of different materials.

5. The method according to claim 4, wherein, the optical waveguides of at least two current transformers (5, 7) are made of materials having different Verdet constants.

6. The method according to any one of the preceding claims, wherein, electromagnetic radiation of a specific wavelength for the current transformer (5, 7) is conducted through each current transformer (5, 7), and the wavelengths of at least two current transformers (5, 7) are different from each other.

7. A measuring device (1) for measuring a current (I), comprising: - at least two optical current transformers (5, 7), each optical current transformer being configured to repeatedly detect a measuring signal (A) depending on the current (I), wherein the current transformers (5, 7) have different distinct ranges (E1 to E4), within which the measuring signals (A) detected by the respective current transformers (5, 7) increase or decrease monotonically as the current (I) increases, and - an evaluation unit (9), the evaluation unit (9) being configured to: - Continuously determine the sign value of each current transformer (5, 7), the sign value indicating the sign of the first derivative of the measurement signal (A) detected by the current transformer (5, 7) with respect to the current (I); - If the sign value determined for the current transformer (5, 7) changes while the sign value determined for at least one other current transformer (5, 7) does not change, then assign the changed clear range (E1 to E4) of the current transformer (5, 7) to the measurement signal (A) of the current transformer (5, 7); and - Determine the amperage of the current (I) based on the measurement signal (A) of at least one current transformer (5, 7).

8. The measuring device (1) according to claim 7, wherein, no clear range (E1 to E4) of a current transformer (5, 7) is a multiple of the clear range (E1 to E4) of another current transformer (5, 7).

9. The measuring device (1) according to claim 7 or 8, wherein, the measurement signal (A) of each current transformer (5, 7) is a digitized measurement signal that takes 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 clear range (E1 to E4) of the current transformer (5, 7).

10. The measuring device (1) according to any one of claims 7 to 9, wherein, each current transformer (5, 7) includes an optical waveguide, and the optical waveguides of at least two current transformers (5, 7) are made of different materials.

11. The measuring device (1) according to claim 10, wherein, the optical waveguides of at least two current transformers (5, 7) are made of materials having different Verdet constants.

12. The measuring device (1) according to any one of claims 7 to 11, wherein, each current transformer (5, 7) is configured to conduct electromagnetic radiation of a specific wavelength of the current transformer (5, 7), and the wavelengths of at least two current transformers (5, 7) are different from each other.

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