CA2238971A1 - Process and device for measuring a quantity, in particular an electric current, with a high measurement resolution - Google Patents
Process and device for measuring a quantity, in particular an electric current, with a high measurement resolution Download PDFInfo
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- CA2238971A1 CA2238971A1 CA002238971A CA2238971A CA2238971A1 CA 2238971 A1 CA2238971 A1 CA 2238971A1 CA 002238971 A CA002238971 A CA 002238971A CA 2238971 A CA2238971 A CA 2238971A CA 2238971 A1 CA2238971 A1 CA 2238971A1
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R15/00—Details of measuring arrangements of the types provided for in groups G01R17/00 - G01R29/00, G01R33/00 - G01R33/26 or G01R35/00
- G01R15/14—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks
- G01R15/24—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks using light-modulating devices
- G01R15/247—Details of the circuitry or construction of devices covered by G01R15/241 - G01R15/246
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R15/00—Details of measuring arrangements of the types provided for in groups G01R17/00 - G01R29/00, G01R33/00 - G01R33/26 or G01R35/00
- G01R15/14—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks
- G01R15/24—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks using light-modulating devices
- G01R15/245—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks using light-modulating devices using magneto-optical modulators, e.g. based on the Faraday or Cotton-Mouton effect
- G01R15/246—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks using light-modulating devices using magneto-optical modulators, e.g. based on the Faraday or Cotton-Mouton effect based on the Faraday, i.e. linear magneto-optic, effect
Abstract
In order to measure a measured value (I) in a predetermined measurement range (MR), a first measurement signal (M1), which is an unequivocal function of the measured quantity (I) in the measurement range (MR), and a second measurement signal (M2), which is a periodic and equivocal function of the measurement value (I) in the predetermined measurement range (I), are generated. A third unequivocal measurement signal (M3) in the measurement range (MR) which has at least the measurement resolution of the second measurement signal (M2) is derived from the two measurement signals (M1, M2).
Description
CA 02238971 1998-0~-28 GR 95 P 3844 IN ~ILE~ THIS A~L~DL~
TRANSLATI~N
Description Method and arrangement for measuring a measured variable, in particular an electric current, with a high measuring resolution The invention relates to a method and a device for measuring a measured variable.
Optical measuring arrangements are known for measuring an electric current in a conductor which are based on the magnetooptic Faraday effect, and t~ f~fs~rs) are also designated as magnetooptic current transformers~
In a magnetooptic current transformer, linearly polarized measuring light is transmitted through a Faraday element which is arranged in the vicinity of the conductor and consists of an optically transparent material exhibiting the Faraday effect. The magnetic field generated by the current effects a rotation of the polarization plane of the measuring light by a rotationalrangle p which is proportional to the path integral ~ the magnetic field along the path covered by the measuring light. The proportionality constant is called the Verdet constant V.
The Verdet constant V depends generally on the material and the temperature of the Faraday element and on the wavelength of the measuring light used. In general, the Faraday element surrounds the conductor, with the result that the measuring light circulates the conductor at least once in a virtually closed path. The rotational angle p is essentially directly proportional in this case to the amplitude I of the current to be measured, in accordance with the relationship p = N ~ V ~ I (1), N being the number of the circulations of the measuring light around the conductor. The Faraday rotational angle p is determined polarimetrically Dy polarization analysis of the measuring light which has passed through the Faraday element, in order to obtain a measuring signal for the electric current. Single-channel polariz-ation evaluation and dual-channel polarization evaluation CA 02238971 1998-0~-28 are known for the purpose of polarization analysis.
In the case of single-channel polarization evaluation, after traversing the Faraday element the measuring light is fed to a polarizer as analyser, and transmltted the measuring light p~3cd by the polarizer is converted by a photoelectric transducer into an electric signal as measuring signal S. This measuring signal S corresponds to the light intensity of the light component, projected onto the polarization axis (transmission axis) of the polarizer, of the measuring light and, neglecting dis-turbing influences such as temperature changes and vibrations, has the general form S = So/2 ~ (1 + sin(2p + ~
= So/2 ~ (1 + sin(2-N-V-I + ~)) (2).
Here, S0 is the constant ~;mllm amplitude of the measur-ing signal S, which corresponds to the case in which the polarization plane of the measuring light is parallel to the polarization axis of the polarizer (m~;mllm trans-mitted light intensity). ~ is a constant offset angle for a current zero (I = 0 A) and depends on the polarizer angle between the polarization plane of the measuring light ~or ~ unc~lng into the Faraday element and the polarization axis of the analyser. If this polarizer angle is equal to 45~, ~ = O (IEEE Transactions on Power Delivery, Vol. 7, No. 2, April 1992, pages 848 to 852).
In the case of a dual-channel polarization evalu-ation, after traversing the Faraday element the measuring light is separated by an analyser into two linearly polarized light components L1 and L2 having polarization planes directed perpendicular to one another. Polarizing beam splitters such as, for example, a Wollaston prism or else a simple beam splitter with two downstream polarizers whose polarization axes are rotated by ~/2 or 90~ relative to one another are known as analysers. The two light components L1 and L2 are converted in each cas~
by an assigned photoelectric transducer into an electric intensity signal T1 or T2, respectively, which is propor-tional to the light intensity of the respective light component L1 or L2, respectively. A measuring signal CA 02238971 1998-0~-28 T = (T1 - T2)/(Tl + T2) (3) which corresponds to the quotient of a difference and the sum of the two intensity signals T1 and T2 (WO 95/10046) is formed from these two electric signals. Neglecting disturbing influences, this measuring signal T is equal to T = sin(2p + ~) = sin(2-N-V-I + ~) (4), ~ being an offset angle for I = 0 A which depends on the angle between the ~olarization plane of the measuring at incidence lnto light fo l~ hi ~ in~ the Faraday element and a prime optical eigenaxis of the analyser.
Both in the case of single-channel and in the case of dual-channel polarization analysis, the measuring signal S in accordance with equation (1) or T in accord-ance with equation (4) is thus a periodic, sinusoidalfunction of twice the rotational angle 2p with the period ~. It therefore holds that S(p + n ~) = S(p) and T(p + n ~) = T(p) with the whole number n. The periodicity of the measuring signals S and T follows from the fact that polarization planes, rotated by an integral multiple of ~ or 180~
relative to one another, of the measuring light cannot be distinguished from one another polarimetrically.
That is why, although in accordance with equation (1) the Faraday measuring angle p is a linear and thus unique function of the current I, the measuring signals S and T of a polarimetric magnetooptic current transformer are, by contrast, unique functions of the measuring angle p only over a ~;mum angular range of ~/2 (or 90~) for the measuring angle p. It is therefore possible to use the known polarimetric magnetooptic current transformers to measure uniquely only those electric currents which lie in a current measuring range (current measuring interval) MR of interval length IMRI
and corresponding to the said m~; mllm angular range of ~/2 (or 90~) for the measuring angle p. In accordance with equation (1), the current measuring range MR has a m;~r; mllm Of IMRI = ~/(2-N-V) (5).
CA 0223897l l998-0~-28 It may be seen from equation (5) that the magnitude IMRI
of the current measuring range MR of a magnetooptic current transformer can be set by the choice of materials with different Verdet constants V for the Faraday element and/or by the number N of circulations of the measuring light around the conductor. A larger current measuring range is obtained if the product N V in the denominator is set to be smaller. However, such a choice of a larger current measuring range MR inescapably entails a reduced measuring resolution MA of the current transformer for a prescribed display resolution. The measuring resolu-tion MA is defined here and below as the absolute value ¦MS ¦ of the measuring sensitivity MS of the current transformer. The measuring sensitivity MS corresponds to the slope of the characteristic of the magnetooptic current transformer at a working point, and in the case of single-channel evaluation in accordance with equa-tion (2) is equal to MS = dS/dI = S0 ~ N ~ V ~ cos(2 N V I + ~) (6) and in the case of dual-channel evaluation in accordance with equation (4) is equal to MS = dT/dI = 2 ~ N ~ V ~ cos(2 ~N-V-I + ~) (7).
It is evident at once from equations (6) and (7) that a reduction in the product N-V leads in the case of both evaluation methods to a reduction in the measuring resolution MA = IMSI.
EP-B - 0 088 419 discloses a magnetooptic current transformer in which there are arranged parallel to one another about a common conductor two Faraday glass rings which consist of Faraday materials with different Verdet constants, and thus have current measuring ranges which are inherently different for each. Each Faraday glass ring is respectively assigned a transmitting unit for transmitting linearly polarized measuring light into the glass _ing, and a dual-channel evaluation unit for calculating a respective measuring signal for the respec-tive Faraday rotational angle. The two measuring signals of the two evaluation units are fed to an OR gate which determines a maximum signal from the two measuring CA 02238971 1998-0~-28 signals. This m~; mllm signal is used to switch over between the measuring ranges of the two glass rings.
Different measuring ranges of the two glass rings can also be achieved in the case of identical glass material for both glass rings by using measuring light of differ-ent wavelengths. In this case, the wavelength dependency of the Faraday rotation is exploited.
"International Conference of Large Hig~ Voltage Electric Systems", CIGRE, Paris, 28.8-3.9.1988, 0 Conference Proceedings, T, Pref. Subj. 1, Vol. 34, Issue 15, pages 1 to 10 discloses a fibre-optic measuring arrangement with a first magnetooptic current transformer for measuring nominal currents and with a second magneto-optic current transformer for measuring overcurrents. The first current transformer for measuring nominal currents contains an optical monomode fibre which surrounds the conductor in the form of a measuring w;n~;ng with N
turns. Linearly polarized light traverses the measuring winding, is retroreflected from a mirror into the fibre and traverses the measuring winding in the opposite direction a second time (reflection type). In this case, the Faraday rotational angle is doubled, while the undesirable temperature-dependent effects of circular birefringence of the fibre material cancel each other out precisely. After traversing the measuring winding twice, the light is subjected to dual-channel polarization evaluation. The second magnetooptic current transformer, which is provided for protective purposes, likewise comprises a monomode fibre which surrounds the conductor in the form of a measuring winding with one measuring turn. By contrast with the first current transformer, which is provided for measuring purposes, the second current transformer is of the transmission type, that is to say the linearly polarized measuring light is sub-jected to polarizatior. analysis after traversing themeasuring winding only once.
"SENSOR 93 Kongre~band IV Bll.l, pages 137 to 144" discloses a magnetooptic current transformer for protective purposes for measuring alternating currents, CA 02238971 1998-0~-28 in which after traversing a Faraday optical fibre linearly polarized light is split into two component light signals and each of these component light signals is fed to an analyser. The eigenaxes (polarization axes) of the two analysers are directed towards one another at an angle of 45~ or 58~. The light intensities passed by the analysers are firstly normalized by dividing by their dc components, which are obtained by peak value rectification. Subsequently, a product of the normalized signals is formed and this product is then differentiated. The Faraday rotational angle is obtained directly by integration. A signal is thereby obtained which is proportional to the current and is not therefore subject to any limitations on the measuring range.
However, this method is comparatively expensive.
EP-B-0 208 593 discloses a magnetooptic current transformer in which, after traversing a Faraday optical fibre surrounding a conductor, linearly polarized measur-ing light is split by a beam splitter into two component light signals and each of these component light signals is fed to an analyser. The eigenaxes of the two analysers are directed at an angle of 0~ or 45~, respectively, to the launching polarization of the measuring light. The result is a first, sinusoidal signal at the output of one analyser and a second, cosinusoidal signal at the output of the other analyser. These two signals are each ambigu-ous, oscillating functions of the current in the conduc-tor which are phase-shifted relative to one another by an angle of 90~. A unique measuring signal is now composed of these two ambiguous signals by comparing the sign and the absolute values of the measured values of the first, sinusoidal signal and of the second, cosinusoidal signal.
As soon as the absolute values of sine and cosine are equal, that is to say in the case of an integral multiple of 45~, a switchover is made, as a func~ion of the sign of sine and cosine, from a unique branch of the first, sinusoidal signal into a unique branch of the ~econd, cosinusoidal signal, or vice versa. This method is an incremental method, and so in the event of a failure of CA 02238971 1998-0~-28 the electronic system of the current transformer it is necessary for the working point at zero current to be reset again.
It is the object of the invention to specify a method and an arrangement for measuring a measured variable from a prescribed measuring range and, in particular, for measuring an electric current in a conductor from a prescribed current measuring range in which a high measuring resolution is achieved.
This object is achieved in accordance with the invention by means of the features of Claim 1 and Claim 8, respectively.
The method for measuring a measured variable from a prescribed measuring range comprises the following method steps:
a) derivation for the measured variable of a first measuring signal which is a unique function of the measured variable in the prescribed measuring range, b) derivation for the measured variable of a second measuring signal, which is an essentially periodic function of the measured variable with a period which is shorter than twice the interval length of the measuring range, and c) derivation for the measured variable, from the two said measuring signals of a third measuring signal which, in the prescribed measuring range, is a unique function of the measured variable and has at least the same measuring resolution as the second measuring signal.
The arrangement for measuring a measured variable from a prescribed measuring range contains a) a first measuring device for generating a first measuring signal which, in the prescribed measuring range, is a unique function of the measured variable, b) a second measuring device for generating a second measuring signal which is an essentially periodic function of the measured variable with a period which is shorter than twice the interval length of CA 02238971 1998-0~-28 the measuring range, and c) a signal processing-unit which is connected to the first measuring device and the second measuring device and derives from the first measuring signal and the second measuring signal a third measuring signal which is a unique function of the measured variable in the prescribed measuring range and has at least the same measuring resolution as the second measuring signal.
The third measuring signal combines in itself the advantage of the larger measuring range of the first measuring signal and the advantage of the higher measur-ing resolution of the second measuring signal, and therefore renders it possible to measure the measured variable uniquely in the prescribed measuring range with a high measuring resolution. Conversely, the third measuring signal has a large measuring range for a prescribed measuring resolution. The method and the arrangement are not incremental. In the event of a malfunction or a failure of the current transformer, the latter can be taken back immediately into operation without prior calibration. The functional reliability is thus always ensured.
Particular refinements and developments of the method and of the arrangement in accordance with the invention follow from the respectively dependent claims.
As a consequence, in one embodiment the first measuring signal is also a periodic function of the measured variable.
In a preferred embodiment, the measuring range is subdivided with the aid of the first measuring signal into uniqueness ranges over which the second measuring signal is a unique function of the measured variable, and the third measuring signal is composed of the branches of the second measuring signal over these uniqueness ranges.
The third measuring signal can be derived from the first measuring signal and the second measuring signal with the aid of a previously determined table of values, or else by computation.
CA 02238971 1998-0~-28 The measuring method and the measuring arrange-ment are preferably used-to measure an electric current in a conductor from a prescribed current measuring range.
For this purpose, at least two Faraday elements surround-ing the conductor are provided. A first linearly polar-ized light signal is sent through a first of the two Faraday elements at least once, and a first evaluation unit derives the first measuring signal for the current from a rotation of the polarization plane of this first light signal after traversal of the first Faraday element. A second linearly polarized light signal is sent at least through a second of the two Faraday elements at least once, and a second evaluation unit derives the second measuring signal for the current from a rotation of the polarization plane of this second light signal after traversal of at least the second Faraday element.
In a special embodiment of the measuring method, the second linearly polarized light signal traverses both the first Faraday element and the second Faraday element at least once in each case. The second measuring device of the measuring arrangement then also contains the first Faraday element of the first measuring device, and the two Faraday elements are connected optically in series via optical connecting means. In this embodiment, one light source suffices for transmitting linearly polarized measuring light, since the measuring light is split by the optical connecting means into two light components of which one light component is provided as first light signal and the other light component is provided as second light signal. In addition, a compact design of the arrangement is possible, since the two Faraday elements can be arranged next to one another in a space-saving fashion.
Reference is made, for the purpose of explaining the inv ntion, to the drawing, in which Figure 1 shows a design principle of an arrangement for measuring a measured variable from a prescribed measuring range, Figure 2 shows a diagram of three different measuring CA 02238971 1998-0~-28 signals for the measured variable, Figure 3 shows an arrangement for measuring an electric current from a prescribed current measuring range, having two separate Faraday measuring devices, Figure 4 shows an arrangement for measuring an electric current from a prescribed current measuring range, having two Faraday elements which are connected optically in series and operated in transmission mode in each case, Figure 5 shows an arithmetic unit for determ; n; ng the second measuring signal, Figure 6 shows an arrangement for measuring an electric current from a prescribed current measuring range, having a first Faraday element operated in transmission mode, and a series circuit composed of this first Faraday element and a second Faraday element operated in reflection mode, and Figure 7 shows an arrangement for measuring an electric current from a prescribed current measuring range, having a first Faraday element operated in reflection mode, and a series circuit, operated in transmission mode, composed of this first Faraday element and a second Faraday element, all the figures being diagrammatic illustrations. Mutual-ly corresponding parts are provided with the same refer-ence symbols.
Figure 1 shows a measuring arrangement for measuring a measured variable I in a prescribed measuring range MR. The measuring arrangement contains a $irst measuring device 5 and a second measuring device 6, as well as a signal processing unit 12. The measured vari-able I is present in ~ch case at an input 5A of the first measuring device 5 and at an input 6B of the second measuring device 6.
The first measuring device 5 converts the measured variable I into a first measuring signal M1 which is a unique function of the measured variable I at least over the prescribed measuring range MR. It there-fore holds that M1 (Ia) ~ M1 (Ib) (8) for two arbitrary, mutually differing values Ia and I~ of the measured variable from the prescribed measuring range MR. The first measuring device 5 thus has a unique characteristic in the measuring range MR. The first measuring signal M1 is fed from an output 5B of the first measuring device 5 to a first input 12A of the signal processing unit 12.
The second measuring device 6 generates from the measured variable I a second measuring signal M2 which, at least over the prescribed measuring range MR, is a periodic function of the measured variable I with a period P2. It therefore holds that:
M2 (I) = M2 (I + n ~ P2) where n is a whole number, and the characteristic of the second measuring device 6 is periodic, at least over the measuring range MR. The period P2 of the second measuring signal M2 is shorter than twice the interval length 2 IMR¦ of the measuring range MR, that is to say P2 ~ 2 ¦MRI (10).
Because of the condition (lO), the second measuring signal M2 is, by contrast with the first measuring sig-nal M1, not a unique function of the measured variable I
over the measuring range MR.
In the measuring range MR, possibly with the exception of individual subranges, the measuring resolution MA(M2) = IdM2/dI I (11) of the second measuring signal M2 and of the second measuring device 6 is higher than the measuring resolution MA(M1) = ldM1/dII (12) of the first measuring signal Ml and of the associated first measuring device 5. The second measuring signal M2 is transmitted from an output 6B of the second measuring device 6 to a second input 12B of the signal processing -CA 02238971 1998-0~-28 unit 12.
The signal processing unit 12 now forms from the two measuring signals M1 and M2 present at its inputs 12A
and 12B a third measuring signal M3 which, over the entire measuring range MR, is a unique function, on the one hand, and has at least the measuring resolution MA(M2) of the second measuring signal M2, on the other hand. This third measuring signal M3 can be tapped at an output 12C of the signal processing unit 12.
Figure 2 shows in a diagram an exemplary embodi-ment of the three measuring signals M1, M2 and M3 derived in a measuring arrangement in accordance with Figure 1.
A prescribed measuring range for the measured variable I
plotted on the abscissa is denoted by MR and corresponds to a preferably closed interval [IA' IB] ~f values of the measured variable I between a first interval limit IA and a second interval limit IB. This measuring range MR thus has the length IMRI = I IB ~ IA I -The first measuring signal M1 is a periodic and preferably sinusoidal function of the measured variable Iand oscillates between a maximum value Max~Ml) and a m;n;mllm value Min(M1) with the period P1. The first measuring signal M1 traverses the central value Cen(M1) =
0.5 ~ (Max(M1) + Min(M1)) between the two extreme values Max(M1) and Min(M1). The first measuring signal M1 is a unique function of the measured variable I over the measuring range MR, that is to say it satisfies the condition (8). In the exemplary embodiment represented, the measuring range MR lies inside a half period Pl/2 of the first measuring signal M1 between the m;n;mllm value Min(M1) and maximum value Max(M1) of said first measuring signal M1, in which range a sinusoidal function is known to be unique, and the central value Cen(M1) of the first measuring signal M1 corresponds to the value of the first measuring signal M1 at the midpoint IM = ~ . 5 ~ (IB + IA) ~f the measuring range MR, that iR to say Cen(M1) = M1(IM) . The first measuring signal Ml increases in the measuring range MR with increaRing measured variable I with a positive slope in a strictly CA 02238971 1998-0~-28 monotonic fashion, but can also be a strictly monotonically decreasing-function with a negative slope.
At least over the measuring range MR, the second measuring signal M2 is a periodic and, preferably, sinusoidal function of the measured variable I, and oscillates between a m~X;mum value Max(M2) and a m;n;mum value Min(M2) with the period P2. Three values of the measured variable I for which the second measuring sig-nal M2 assumes its m;n;mum value Min(M2) are denoted by Io~ I2 and I4, whereas three values for which the second measuring signal M2 assumes its m~X;m~m value Max(M2) are denoted by I1, I3 and I5. It holds that Io ' I1 ~ I2 ~ I3 c I4 ~ I5. The second measuring signal M2 traverses a central value Cen(M2) = 0.5 ~ (Max(M2) + Min(M2)) between two extreme values Max(M2) and Min(M2). In the exemplary embodiment represented, the measuring range MR comprises two periods 2-P2 ~f the second measuring signal M2, the second measuring signal M2 assuming its mean value Cen(M2) in each case at the end points IA and IB as well as at the midpoint IM of the measuring range MR, that is to say it holds that Cen(M~ = M~(IA) = M~ IB) = Mz~IM).
The second measuring signal M2 represented thus fulfils the previously mentioned conditions (9) and (10).
For the purpose of greater clarity, the first measuring signal M1 is represented on a larger scale than the second measuring signal M2, and thus has a flatter course in general in comparison with the second measuring signal M2, and is, moreover, illustrated offset with respect to the second measuring signal M2. The value ranges [Min(M1),Max(M1)] of the first measuring signal M1 and [Min(M2),Max(M2)] of the second measuring signal M2 can also overlap. In particular, the two central values Cen(M1) and Cen(M2) can be equal, preferably Cen(M1) =
Cen(M2) = 0 in appropriate measurement units. The measur-ing resolution MA(M1) of the first measuring signal Ml isplainly lower over the largest part of the measuring range MR than the measuring resolution MA(M2) of the second measuring signal M2. An exception is formed only by small ranges around the values of In of the measured CA 02238971 1998-0~-28 variable I with n = O, 1, 2, 3, 4 or 5, in which the second measuring signal-M2 assumes its extreme values Max(M2) and Min(M2), respectively, and in which the measuring resolution of the second measuring signal M2 is therefore zero, that is to say MA(M2(In)) = O. The m~x;ml-m measuring resolution MA(M1(IM)) of the first measuring signal M1 at the midpoint IM of the measuring range MR iS plainly lower than the maximum measuring resolution MA(M2)(IM) ) of the second measuring signal M2, which in the exemplary embodiment shown is assumed five times over the measuring range MR, and in particular at the midpoint IM of the measuring range MR.
The third measuring signal M3 is now derived in the following way from the first measuring signal M1 and the second measuring signal M2. The measuring range MR
can be subdivided into individual uniqueness ranges over which the second measuring signal M2 is a unique function of the measured variable I. These uniqueness ranges (quadrants) are separated from one another by lines in the diagram represented and correspond to the intervals [IA~I1]~ [I1,I2], [I2~I3], [I3,I4] and [I4,IB] between two neighbouring extreme values of the second measuring signal M2. The first uniqueness range [IA' I1] and the last uniqueness range [I4, IB] are only P2/4 long in this case, while the inner uniqueness ranges [I1,I2], [I2,I3] and [I3,I4], lying therebetween, have the m~;mllm length P2/2 in each case. It is, however, impossible to determine simply from the measured value M2(I) alone, which is delivered by the second measuring signal M2, in which of the uniqueness ranges the current measured value I lies.
This ambiguity of the second measuring signal M2 is now resolved with the aid of the first measuring signal M1.
The individual branches of the second measuring signal M2 in the uniqueness ranges are combined to form the new measur_ng signal M3 by transformations (operations) such as translations and, as the case may be, reflections.
These operations differ in general for each uniqueness range. The first step is now to use the current value M1(I) of the first measuring signal M1 to determine the CA 02238971 1998-0~-28 uniqueness range in which the current value of the measured variable I lies, after which the operation assigned to this uniqueness range is used to determine the measured value M3(I) of the third measuring signal M3 for the current value of the measuring variable I. For the exemplary embodiment represented, the following operations, in particular, are suitable for deriving the third measuring signal M3 from the second measuring signal M2 with the aid of the first measuring signal Ml;
10M3(I): = M2(I) for M1(IA) S M1(I) ~ M1(I1) (13a) M3(I): = 2-A2 ~ M2(I) for M1(I1) ' M1(I) ~ M1(I2) (13b)M3(I): = M2(I) + 4 A2 for M1(I2) ' M1(I) ~ M1(I3) (13c) M3(I): = 6-A2 ~ M2(I) for M1(I3) ' M1(I) ~ M1(I4) (13d) M3(I): = M2(I) + 8 A2 for M1(I4) ' M1(I) ' M1(IB) (13e), A2 being the prescribed maximum amplitude of the second measuring signal M2, with A2 = 0-5 ~ (Max(M2) - Min(M2)) (14).
Because of the uniqueness of the first measuring signal M1, the conditions, on which the operations (13a) to (13e) are based, placed on the value M1(I) of the first measuring signal M1 correspond precisely to the uniqueness ranges [IA,I1] to [I4,IB] of the second mea~ur-ing signal M2.
A further embodiment (not represented) for deriving the third measuring signal M3 is given by the following operations in which, by contrast with the exemplary embodiment represented, the second measuring signal M2 is not reflected in the uniqueness ranges:
M3(I): =M2(I) for M1(IA) ' M1(I) ~ M1(I1) (15a) 3 ) 2(I) + 2 A2 for M1(I1) c M1(I) ~ M1(I2) (15b) 3 ) 2(I) + 4 A2 for M1(I2) c M1(I) ~ M1(I3) (15c) M (I): =M2(I) + 6-A2 for M1(I3) ' M1(I) ~ M1(I4) M3(I): =M2(I) + 8 A2 for M1(I4) ' M1(I) ' M1(IB) (15e)-In this embodiment in accordance with the oper-ations (15a) to (15e), by contrast with the exemplary CA 02238971 1998-0~-28 embodiment shown in accordance with the operations (13a) to (13e), the individual-branches of the resultant third measuring signal M3 are not continuously juxtaposed.
The operations (13a) to (13e) or (15a) to (15e) can be carried out, in particular, with the aid of a table of values (look-up table) determined experimentally in advance or by calculation. The table of values assigns the value pair of the measured values Ml(I) and M2(I) of the first measuring signal M1 and the second measuring signal M2, respectively, a value M3(I) of the third measuring signal M3 as a unique measure of the measured variable I. The signal processing unit 12 then preferably contains a memory for storing the values of the table of values which have been determined in advance, an analog-to-digital converter for converting the current values of the first measuring signal Ml and of the second measuring signal M2 into a digital value in each case, and a digi-tal signal processor or microprocessor for comparing these measured digital values with the values stored in the table of values, and for assigning the value M3(I), which is likewise digital.
The said operations (13a) to (13e) or (15a) to (15e) can, however, also be carried out computationally.
The measured value M1(I) of the first measuring signal M
is then compared to the values, determined in advance, of the first measuring signal M1 at the interval limits In of the uniqueness ranges, and the second measuring signal M2 is subjected to the associated arithmetic operation as a function of this comparison. In this embodiment, the signal processing unit 12 contains, for example, a corresponding number of comparator circuits for comparing the current value M1(I) of the first measuring signal M
with the values, determined in advance, of the first measuring signal M1 at the interval limits In of the uniqueness ranges, and an analog ari_hmetic unit with analog components such as subtracters, adders and inverters, or an analog-to-digital converter and a downstream digital arithmetic unit for carrying out the arithmetic operations. As an alternative to this, the CA 02238971 1998-0~-28 signal processing unit 12 can also contain an analog-to-digital converter for converting the current values of the first measuring signal M1 and of the second measuring signal M2 into in each case one digital value, and a downstream digital signal processor or microprocessor which compares the digital value of the first measuring signal M1 with the stored digital values Ml(In) at the interval limits In and then carries out the associated digital arithmetic operations.
The current value of the measured variable I can now be determined uniquely from the third measuring signal M3 obtained, by applying the inverse function M3~
of the third measuring signal M3 to the value M3(I~
determined for the third measuring signal M3, because it holds that M3~1 (M3(I)) = I.
In the case of the represented sinusoidal second measuring signal M2 with the amplitude A2 and the period P2, the measured value I for the measured variable is yielded, for example, as:
I = Ios(Ml) + I(M2) (16), the measured value I being composed of an offset measured value IoS(Ml) which depends on the first measuring signal M1 and is constant in each case for a uniqueness range, and a component I(M2) which is a continuous function of the second measuring signal M2.
The offset measured value IoS(Ml), respectively constant in the uniqueness ranges of the second measuring signal M2, of the measured value I is given in the exemp-lary embodiment represented and in accordance with the operations (13a) to (13e), and is also given in the exemplary embodiment in accordance with the operations (15a) to (15e), by the equations IOS (M1) = IA for M1(IA) C M1 (I) ~ M1(I1) (17a) IOS (M1) = IA + ~ ~ 5 P2 for M1(I1) c M1(I) c M1(I2) (17b) IOS (M1) = IA + P2 for M1(I2) c M1(I) c Ml(I3) (17c) IoS(Ml) = IA + 1 ~ 5 P2 for M1(I3) c M1(I) ~ M1(I4) (17d) CA 02238971 1998-0~-28 IOS (M1) = IA + 2 P2 for M1(I4) c M1(I) 5 M1 (IB) (17e).
The continuously variable component I(M2) in the uniqueness ranges of the second measuring signal M2 is I(M2) = (P2/2~) arcsintM2/A2) (18a) for M1(IA) C M1 (I) ~ M1(I1) or M1(I2) c M1(I) ~ M1(I3) or M1 (I4) 5 M1 (I) 5 M1 (IB) and I(M2) = (P2/2~) arcsin(_ M2/A2) (18b) for M1(I1) 5 M1(I) c M1(I2) or M1 (I3) 5 M1 (I) ~ M1 (I4) -In an advantageous embodiment, the current value of the measured variable I can thus also be determined directly from the first measuring signal M1 and the second measuring signal M2 by the signal processing unit 12, for example again with the aid of a table of values, or else by computation with the aid of a pro-cessor. In this case, the third measuring signal M3 is equal to the identical function over the measuring 20 range MR, that is to say M3(I) = I- (19) Since the methods described are not incremental methods, the functional reliability is always given.
For the purpose of a higher measuring resolution, in a particular embodiment the first measuring signal Ml can also be used as the third measuring signal M3 in ranges around the points I~ at which the measuring resol-ution MA(M2) of the second measuring signal M2 vanishes.
For this purpose, an additional comparison of the measur-ing resolutions MA(M1) of the first measuring signal M1and MA(M2) of the second measuring signal M2 can be carried out, and the first measuring signal M1 can be used as the third measuring signal M3 as long as it holds that MA(M2) ~ MA(M1). It is preferable to determine in advance the ranges in which this condition holds. When the condition is no longer fulfilled, the third measuring CA 02238971 1998-0~-28 - signal M3 is derived again as previously described.
The second measuring signal M2 can also be another periodic function of the measured variable I, for example a linear sweep function (saw-tooth function). The first measuring signal M1 can also be a linear function o~ the measured variable I or can, in the exemplary embodiment of Figure 2, be approximated by a linear function at the midpoint IM ( linear interpolation).
The previously described embodiments of the measuring arrangement and of the measuring method in accordance with the invention are preferably used to measure an electric current I as measured variable with the aid of the Faraday effect. As already described, in the case of a polarimetric magnetooptic current trans-former it is generally the case that sinusoidal orcosinusoidal measuring signals are generated in accord-ance with one of the equations (2) or (4), which cor-respond to the exemplary embodiment shown in Figure 2.
An exemplary embodiment of such an arrangement for measuring an electric current I in a conductor 2 is represented in Figure 3. TWO Faraday measuring devices 5 and 6 are assigned to the conductor 2. Each Faraday measuring device 5 and 6 respectively has a Faraday element 3 and 4 exhibiting the magnetooptic Faraday effect, a light source 9 and 12, respectively, for transmitting a linearly polarized light signal L1 and L2, respectively, into the Faraday element 3 and 4, respect-ively, and an evaluation unit 7 and 8, respectively, for evaluating a polarization rotation (Faraday rotation) of the linearly polarized light signal L1 and L2~
respectively, after at least one traverse of the Faraday element 3 and 4, respectively, as a consequence of the magnetic field generated in the conductor 2 by an electric current I. The two Faraday elements 3 and 4 prefer_bly surround the conductor 2, with the result that light running through the Faraday element 3 or 4 circulates the conductor 2 along a virtually closed light path at least once.
In one embodiment, the first Faraday element 3 CA 0223897l 1998-0~-28 - and/or the second Faraday element 4 can be formed using solid bodies exhibiting -the Faraday effect, preferably made from glass. It is also possible to provide one or else several bodies for each Faraday element 3 or 4. A
single coherent body is then designed as a polygonal annular body in general. Such solid bodies, in particular bodles annular ~A~, are mechanically robust and exhibit virtually no circular birefringence. In another embodi-ment, the first Faraday element 3 and/or the second Faraday element 4 are formed by one or in each case one monomode optical fibre which preferably surrounds the conductor 2 in the form of a measuring winding with at least one turn.
The polarization of the first light signal L1 after traversing the first Faraday element 3 is evaluated by the first evaluation unit 7 of the first measuring device 5 for a first measuring signal M1 for the electric current I, while the polarization of the second light signal L2 is evaluated by the second evaluation unit 8 of the second measuring device 6 for a second measuring signal L2 for the electric current I. In this case, single-channel or dual-channel polarization evaluation can respectively be used for each of the two evaluation units 7 and 8, thus resulting, in particular, in sinusoidal measuring signals M1 and M2 in accordance with the relationships (2) or (4). Again, in order to correct falsifications of the measuring signals it is possible in principle to provide all the additional measures which are known for suppressing or compensating linear and/or circular birefringence, for example as a consequence of temperature changes or vibrations, in the Faraday elements 3 and 4 and the optical transmission links.
As in the embodiment in accordance with Figure 1, the two measuring signals M1 and M2 are fed to the signal processing unit 12 ail further processed for a third measuring signal M3 for the electric current I in the conductor 2. The measuring sensitivity of the first Faraday element 3 is lower than the measuring sensitivity of the second Faraday element 4 and is selected to be so CA 0223897l l998-0~-28 high that the first measuring signal M1 in a prescribed current measuring range MR is a unique measure of the current I. The measuring sensitivity of the second Faraday element 4 is, by contrast, selected such that the period P2 of the second measuring signal M2 is shorter than twice the length of the current measuring range MR.
The second measuring signal M2 is thus not a unique function of the current I in the current measuring range MR. In order to set different measuring sensitiv-ities of the two Faraday elements 3 and 4 it is possible,for example, to use materials with different Verdet constants for the two Faraday elements 3 and 4, respect-ively, or else to use Faraday elements 3 and 4 with different light path lengths along the magnetic field generated by the current I.
The prescribed current measuring range MR prefer-ably reaches from an overcurrent measuring range, which typically lies in the range between 10 kA and at least 100 kA, as far as a nominal current measuring range which usually lies below the overcurrent measuring range by a factor of about 10 to 30. The nominal current measuring range is preferably covered in this case by a uniqueness range of the second measuring signal M2 which is half a period P2 long. The third measuring signal M3, which is derived using the method and the arrangement in accord-ance with the invention, permits measurement both of nominal currents for measurement or meter applications and of overcurrents for protective purposes with the same measuring resolution.
Apart from the embodiment, described with the aid of Figure 3, with two separate measuring devices 5 and 6, embodiments are also possible in which the two Faraday elements 3 and 4 are connected optically in series via optical connecting means, and a first linearly polarized light signal L1 traverses only ti~ first Faraday element 3 and a second linearly polarized light signal L2 traverses the first Faraday element 3 and the second Faraday element 4. Such embodiments are disclosed in the older German Patent Applications P 44 29 909.5 and CA 02238971 1998-0~-28 P 44 31 615.1, which were not published prior to the date of filing of the present application and whose content is also incorporated into the present application. Figures 4 to 6 show exemplary embodiments with Faraday elements 3 and 4 connected in this way in series.
In the embodiment in accordance with Figure 4, the first Faraday element 3 and the second Faraday element 4 are connected optically in series via a three-port coupler 18 as optical connecting means. An optical fibre which surrounds the conductor 2 in the form of a measuring winding is respectively provided both for the first Faraday element 3 and for the second Faraday element 4. It is preferable to provide annealed optical fibres which are distinguished by low linear birefring-ence and virtually negligible circular birefringence. Inaddition to the selection of different materials for the two Faraday elements 3 and 4, it is also possible, in this embodiment, to vary the number of the turns of the measuring windings in order to match the current measur-ing ranges. The measuring winding of the second Faradayelement 4 in this case preferably has more turns than the winding of the first Faraday element 3. A transmitting unit 10 generates linearly polarized measuring light L
which is preferably launched into the first Faraday element 3 via an optical fibre 50 which maintains polar-ization. A laser diode, or else a light source (for example LED) with a downstream polarizer can, for example, be provided as transmitting unit 10. A low-birefringence optical fibre (LoBi fibre) can be provided as the optical fibre 50 which maintains polarization. A
splice 53 preferably joins the optical fibre 50 to the measuring winding of the first Faraday element 3. The three-port coupler 18 is designed in such a way that a first portion of the measuring light L, which is launched into the first Faraday element 3 by the transmitting u~ t 10 and runs through the first Faraday element 3, is launched as first light signal Ll into the first evaluation unit 7, and a second portion of the measuring light L running through the first Faraday element 3 is CA 0223897l l998-0~-28 launched as second light signal L2 into the second Faraday element 4. A beam-splitting, semi-transparent mirror which is arranged at an angle of generally 45~ to the beam direction of the incident measuring light L, for example a beam-splitting splice, or else a Y-coupler, in particular a fibre coupler, can be provided as three-port coupler 18. A first port 18A of the three-port coupler 18 is optically connected to the first Faraday element 3, a second port 18B is optically connected to the first evaluation unit 7, and a third port 18C is optically connected to the second Faraday element 4. An optical fibre 70 which maintains polarization is preferably provided for transmitting the first light signal L1 from the three-port coupler 18 to the first evaluation unit 7.
The second light signal L2 traverses the second Faraday element 4 only once and is then directly launched into the second evaluation unit 8, preferably likewise via an optical fibre 60 which maintains polarization, and can be connected via a splice 64 to the measuring winding of the second Faraday element 4 and/or can be designed as a LoBi fibre. The second Faraday element 4 is thus operated in transmission mode with respect to the second portion L2 of the measuring light L.
The first evaluation unit 7 evaluates the Faraday rotation of the polarization plane of the first light signal L1 for a first measuring signal M1 which can be tapped at an output of the first evaluation unit 7. The first Faraday element 3 is thus operated in transmission mode. The second evaluation unit 8 evaluates the Faraday rotation of the polarization plane of the second light signal L2 for a second measuring signal M2, which can be tapped at an output of the second evaluation unit 8. The entire Faraday rotation of the polarization plane of the second light signal L2 is composed in this case of a first Faraday rotational angle component, produced in the first Faraday element 3, and a second Faraday rotational angle component, effected in the second Faraday element 4. If the Faraday rotation was performed in the same direction in the two Faraday elements 3 and 4, in CA 02238971 1998-0~-28 accordance with an identical sensor circulation of the transmitted light in the first Faraday element 3 and second Faraday element 4 relative to the direction of flow in the conductor 2, then the entire rotational angle is equal to the sum of the two individual rotational angles. In the case of opposite directions of rotation, the resultant rotational angle corresponds, by contrast, to the difference between the two individual rotational angles.
The advantageous embodiments, shown in Figure 4, of the two evaluation units 7 and 8 in each case carry out dual-channel polarization analysis of the associated light signal L1 and L2, respectively. Means 72 which àre optically coupled to the port 18B of the three-port coupler 18 via the optical fibre 70 which maintains polarization, for example a polarizing beam splitter, preferably a Wollaston prism, or else a beam splitter and two optically downstream analysers which cross at a prescribed angle, are provided in the first evaluation unit 7 for the purpose of splitting the first light signal L1 into two linearly polarized component light signals L11 and L12 with different polarization planes. It is possible, in particular, to provide as the optical fibre 70 which maintains polarization an optical fibre with a high linear birefringence (HiBi fibre), whose eigenaxes of the linear birefringence are adjusted to the eigenaxes of the Wollaston prism 72. Furthermore, the evaluation unit 7 contains photoelectric transducers 74 and 75 for converting the component light signals L11 and L12 into an electric signal R11 and R12, respectively, in each case as a measure of the intensity of the respective component light signal L11 and L12, respectively, and electronic means 76 for deriving the first measuring signal M1 from these two electric signals R11 and Rl2. As first measuring signal M1, the electronic means 76 preferably determine a quotient signal M1 = (Rll-Rl2)/Rll+Rl2) (20) from the difference R11-R12 and the sum R11+R12 of the two electric signals R11 and R12. This quotient signal is CA 0223897l l998-0~-28 largely freed from intensity fluctuations in the trans-mitting unit 10 or in the transmission links for the measuring light L and the first light signal L1, and it holds in general that:
Ml = sin(2 Ns Vs a + ~) (21) with the Faraday rotational angle a of the first light signal L1, the number NS of the turns of the first Faraday element 3, and the Verdet constant Vs ~f the material of the first Faraday element 3, as well as the constant offset angle ~. NS is typically between 1 and 3.
The component light signals L11 and L12 can be transmitted to the transducers 74 and 75 in a free-beam arrangement, or else via optical fibres. The outputs of the photo-electric transducers 74 and 75 are respectively connected electrically to an input of the electronic means 76.
The second evaluation unit 8 is constructed in a similar way to the first evaluation unit 7. Means 82, for example a Wollaston prism, which are optically connected to the second Faraday element 4 via an optical fibre 60 which maintains polarization are provided for the purpose of splitting into two linearly polarized component light signals L2l and L22 with different polarization planes the linearly polarized light component L2 transmitted through the second Faraday element 4. The optical fibre 60 is connected to the fibre of the second Faraday element 4 via a splice 64. As the optical fibre 60 which maintains polarization, it is possible, in particular, to provide an optical fibre with a high linear birefringence (HiBi fibre) whose eigenaxes of the linear birefringence are adjusted to the eigenaxes of the Wollaston prism 82.
Furthermore, the second evaluation unit contains photo-electric transducers 84 and 85 for converting these component light signals L2l and L22 into in each case an electric signal T21 and T22, respectively, as a measure of the intensity of he respective component light signal L21 and L22, respectively, and electronic means 86 for deriving the second measuring signal M2 from the two electric signals T21 and T22. As second measuring signal M2, a quotient signal CA 02238971 1998-0~-28 M2 = (T21-T22)/(T21+T22) (22) of normalized intensity-is preferably derived by the electronic means 86 from the difference T2l-T22 and the sum T21+T22 of the two electric signals T21 and T22. This second measuring signal M2 depends on the total Faraday rotational angle ~ of the second light signal L2 in the following way:
M2 = sin(2-(Ns-vs+NM-vM)-~B + ~) (23) with the number of turns NM~ the Verdet constant VM of the second Faraday element 4, and a constant offset angle ~. The number of turns NM of the second Faraday element 4 is generally selected to be between 10 and 50.
In a preferred embodiment, the electronic means 76 and/or 86 contain an analog-to-digital converter for digitizing the two signals R11 and R12 of the trans-ducers 74 and 75, or T21 and T22 of the transducers 84 and 85 and a downstream digital arithmetic unit for calculating the first measuring signal M1 or the second measuring signal M2.
In another embodiment, represented in Figure 5, the electronic means 86 contain analog arithmetic com-ponents. Quicker signal processing can be achieved thereby. Provision is made of a subtracter 31 and an adder 32 at whose inputs the two electric signals T
and T22 of the transducers 84 and 85 (not represented) are present in each case. The outputs of the subtracter 31 and of the adder 32 are connected in each case to an input of a divider 3 3. As second measuring signal M2, the divider 33 forms the quotient signal (T21-T22)/(T21+T22) from the difference signal T21-T22 at the output of the subtracter 31 and the sum signal T21+T22 at the output of the adder 3 2. A corresponding circuit can also be provided for the electronic means 76 of the first evaluation unit 7.
Figure 6 shows a further ~mbodiment of the measuring arrangement. The first Faraday element 3 and the second Faraday element 4 are connected optically in series via an optical four-port coupler 19 with four ports 19A to l9D as optical connecting means. The second evaluation unit 8 is optically connected to the port l9D
of the four-port coupler 19. Assigned to the second Faraday element 4 are optically reflecting means 40, for example a mirror, which retroreflect the second light signal L2 into the second Faraday element 4 after a single traverse of the second Faraday element 4. The retroreflected second light signal L2' traverses the second Faraday element 4 in the reverse direction a second time and is then fed to the second evaluation unit 8 via the four-port coupler 19. The port l9B con-nected to the second Faraday element 4, and the port l9D
connected to the second evaluation unit 8, of the four-port coupler 19 are optically coupled to one another for the purpose. Likewise optically coupled to one another are the port l9A, connected to the first Faraday element 3, and the port l9C, connected to the first evaluation unit 7, of the four-port coupler 19. A beam splitter, for example, with a semitransparent mirror arranged obliquely relative to the light propagation direction, or a fibre-optic coupler can be provided as the four-port coupler 19. In this embodiment in accordance with Figure 6, the second Faraday element 4 is operated in reflection mode. The second light signal L2' evaluated by the second evaluation unit 8 has a Faraday rotation of its polarization plane which is composed of the Faraday rotation in the case of a traverse of the first Faraday element 3 and twice the Faraday rotation in the case of a traverse of the second Faraday element 4.
In the embodiment in accordance with Figure 7, the first Faraday element 3 is operated in a reflection arrangement. An optical coupler 13 connects the first Faraday element 3 both to the transmitting unit 10 for transmitting linearly polarized measuring light L and to the first evaluation unit 7. This coupler 13 can be a Y-fibre coupler, or else a beam splitter formed by means 3fa semitransparent mirror. The optical connecting means 15 for optically coupling the two Faraday elements 3 and 4 are now designed in such a way that a portion of the measuring light L running through the first Faraday CA 02238971 1998-0~-28 element 3 is retroreflected as first light signal Ll into the first Faraday element 3, and another portion is passed as second light signal L2 and launched into the second Faraday element 4. For this purpose, the optical connecting means 15 preferably contain a semitransparent mirror 35 which is arranged essentially perpendicular to the beam direction of the incident measuring light L.
However, it is also possible to provide a beam splitter with a semitransparent mirror directed at an angle, for example 45~, to the beam direction of the incident measuring light L, and a further mirror, arranged in the beam path of the component measuring light reflected at this semitransparent mirror, for the purpose of retro-reflecting this component measuring light to the semi-transparent mirror. The first light signal Lretroreflected by the optical connecting means 15 tra-verses the first Faraday element 3 in the rever~e direc-tion a second time and is fed via the optical coupler 13 to the first evaluation unit 7. Because of the non-reciprocity of the Faraday effect, the first lightsignal L1 experiences a Faraday rotation which is twice as large as in the case of only a single traverse of the Faraday element 3. By contrast, effects of a possible circular birefringence in the first Faraday element 3 cancel one another out because of their reciprocal prop-erty. The evaluation unit 7 derives the first measuring signal M1 from the first light signal L1. By contrast, after the first Faraday element 3, the second light signal L2 passed by the optical connecting means 15 also traverses the second Faraday element 4, and is fed to the second evaluation unit 8 after traversing the second Faraday element 4. The second light signal L2 arriving at the evaluation unit 8 has a polarization plane which has been rotated both in the first Faraday element 3 about a first Faraday rotational angle and in the second Faraday element 4 about a second Faraday rotational angle. The resultant total rotational angle of the second light signal L2 is evaluated in the evaluation unit 8 for a second measuring signal M2.
CA 02238971 1998-0~-28 In the embodiment represented, a fibre spool surrounding the conductor 2 and made from an optical monomode fibre is respectively provided both for the first Faraday element 3 and for the second Faraday element 4. The optical connecting means 15 are then formed with the aid of a semitransparent coating which serves as a mirror 35 and is applied by sputtering or chemical deposition to the fibre end, preferably of the first Faraday element 3 and with the aid of mechanical connecting means for connecting the fibre end provided with the mirror to the neighbouring fibre end of the other Faraday element. The mechanical connecting means can be a detachable plug-and-socket connection, or else an undetachable splice, for example a capillary tube from the Nippon Electric Glass company.
The second evaluation unit 8 is designed in a fashion similar to Figure 4. By contrast, in the embodi-ment represented, the first evaluation unit 7 contains polarizing means lOB which are connected optically between the first Faraday element 3 and the coupler 13, and a photodetector 79 which is optically coupled to the coupler 13. A light source lOA which is optically coupled to the coupler 13 is provided for transmitting light L.
The coupler 13 can be connected to the polarizing means lOB via an optical fibre 50'. In this embodiment, this fibre 50' can be a simple teleco~mnnication fibre without properties of maintaining polarization, since the polarizing means lOB linearly polarize the light L only directly before its entry into the series circuit of the two Faraday elements 3 and 4 at the input of the first Faraday element 3. Consequently, it is also possible to provide a non-polarizing, simple light source as the light source lOA. The polarizing means lOB and the light source lOA together form the transmitting unit 10 for laun~hing linearly polarized measuring light L into the first Faraday element 3. The polarizing means lOB are provided at the same time as an analyser for the first light signal Ll which is reflected by the semireflecting connecting means 15 and has its polarization plane rotated. The analyser passes only the component Ll' of the first light signal L1-which is projected onto its set polarization axis. The light component L1', passing through the analyser, of the first light signal L1 is fed via the coupler 13 to the photodetector 79 and converted there into an electric signal as first measuring sig-nal M1. This first measuring signal M1 is proportional to the light intensity of the light component L1'. It there-fore holds that M1 = K cos2(0.5 Ns Vs ~) (24) with the Faraday rotational angle a of the first light signal L1, and a proportionality factor K.
It is also possible to carry out a method for compensating temperature and/or vibration because of the high measuring resolution required for norn;~l currents.
In principle, all analog and digital evaluation methods for detecting the polarization state of linearly polarized light can be used to evaluate the Faraday rotational angle in the first evaluation unit 7 and in the second evaluation unit 8. In particular, the embodi-ments of the evaluation units 7 and 8 in accordance with Figures 4, 5 and 7 can be combined with one another at will. Preferably, the two light signals L1 and L2 are evaluated by means of dual-channel polarization analysis.
However, it is also possible in each case to provide single-channel evaluation for each light signal Ll and L2. The measuring signals Ml and M2 obtained are then of the sinusoidal configuration as in the relationship (23) or (24) and thus fulfil, in conjunction with appropriately selected measuring sensitivities NS Vs of the first Faraday element 3 and NM-VM ~f the second Faraday element 4, the preconditions for deriving the third measuring signal M3 in accordance with one of the embodiments previously described.
The optical coupling of the various optical components of the measuring arrangement is preferably supported by collimator lenses (Grin lenses) for focusing the light.
In a particular embodiment (not represented), it CA 02238971 1998-0~-28 is also possible to use a plurality of linearly polarized measuring light signals of wavelengths which differ and are generally close to one another, in conjunction with wavelength-sensitive optical connecting means 15, 18 or 19. Crosstalk between protective and measurin~
channels can thereby be avoided.
TRANSLATI~N
Description Method and arrangement for measuring a measured variable, in particular an electric current, with a high measuring resolution The invention relates to a method and a device for measuring a measured variable.
Optical measuring arrangements are known for measuring an electric current in a conductor which are based on the magnetooptic Faraday effect, and t~ f~fs~rs) are also designated as magnetooptic current transformers~
In a magnetooptic current transformer, linearly polarized measuring light is transmitted through a Faraday element which is arranged in the vicinity of the conductor and consists of an optically transparent material exhibiting the Faraday effect. The magnetic field generated by the current effects a rotation of the polarization plane of the measuring light by a rotationalrangle p which is proportional to the path integral ~ the magnetic field along the path covered by the measuring light. The proportionality constant is called the Verdet constant V.
The Verdet constant V depends generally on the material and the temperature of the Faraday element and on the wavelength of the measuring light used. In general, the Faraday element surrounds the conductor, with the result that the measuring light circulates the conductor at least once in a virtually closed path. The rotational angle p is essentially directly proportional in this case to the amplitude I of the current to be measured, in accordance with the relationship p = N ~ V ~ I (1), N being the number of the circulations of the measuring light around the conductor. The Faraday rotational angle p is determined polarimetrically Dy polarization analysis of the measuring light which has passed through the Faraday element, in order to obtain a measuring signal for the electric current. Single-channel polariz-ation evaluation and dual-channel polarization evaluation CA 02238971 1998-0~-28 are known for the purpose of polarization analysis.
In the case of single-channel polarization evaluation, after traversing the Faraday element the measuring light is fed to a polarizer as analyser, and transmltted the measuring light p~3cd by the polarizer is converted by a photoelectric transducer into an electric signal as measuring signal S. This measuring signal S corresponds to the light intensity of the light component, projected onto the polarization axis (transmission axis) of the polarizer, of the measuring light and, neglecting dis-turbing influences such as temperature changes and vibrations, has the general form S = So/2 ~ (1 + sin(2p + ~
= So/2 ~ (1 + sin(2-N-V-I + ~)) (2).
Here, S0 is the constant ~;mllm amplitude of the measur-ing signal S, which corresponds to the case in which the polarization plane of the measuring light is parallel to the polarization axis of the polarizer (m~;mllm trans-mitted light intensity). ~ is a constant offset angle for a current zero (I = 0 A) and depends on the polarizer angle between the polarization plane of the measuring light ~or ~ unc~lng into the Faraday element and the polarization axis of the analyser. If this polarizer angle is equal to 45~, ~ = O (IEEE Transactions on Power Delivery, Vol. 7, No. 2, April 1992, pages 848 to 852).
In the case of a dual-channel polarization evalu-ation, after traversing the Faraday element the measuring light is separated by an analyser into two linearly polarized light components L1 and L2 having polarization planes directed perpendicular to one another. Polarizing beam splitters such as, for example, a Wollaston prism or else a simple beam splitter with two downstream polarizers whose polarization axes are rotated by ~/2 or 90~ relative to one another are known as analysers. The two light components L1 and L2 are converted in each cas~
by an assigned photoelectric transducer into an electric intensity signal T1 or T2, respectively, which is propor-tional to the light intensity of the respective light component L1 or L2, respectively. A measuring signal CA 02238971 1998-0~-28 T = (T1 - T2)/(Tl + T2) (3) which corresponds to the quotient of a difference and the sum of the two intensity signals T1 and T2 (WO 95/10046) is formed from these two electric signals. Neglecting disturbing influences, this measuring signal T is equal to T = sin(2p + ~) = sin(2-N-V-I + ~) (4), ~ being an offset angle for I = 0 A which depends on the angle between the ~olarization plane of the measuring at incidence lnto light fo l~ hi ~ in~ the Faraday element and a prime optical eigenaxis of the analyser.
Both in the case of single-channel and in the case of dual-channel polarization analysis, the measuring signal S in accordance with equation (1) or T in accord-ance with equation (4) is thus a periodic, sinusoidalfunction of twice the rotational angle 2p with the period ~. It therefore holds that S(p + n ~) = S(p) and T(p + n ~) = T(p) with the whole number n. The periodicity of the measuring signals S and T follows from the fact that polarization planes, rotated by an integral multiple of ~ or 180~
relative to one another, of the measuring light cannot be distinguished from one another polarimetrically.
That is why, although in accordance with equation (1) the Faraday measuring angle p is a linear and thus unique function of the current I, the measuring signals S and T of a polarimetric magnetooptic current transformer are, by contrast, unique functions of the measuring angle p only over a ~;mum angular range of ~/2 (or 90~) for the measuring angle p. It is therefore possible to use the known polarimetric magnetooptic current transformers to measure uniquely only those electric currents which lie in a current measuring range (current measuring interval) MR of interval length IMRI
and corresponding to the said m~; mllm angular range of ~/2 (or 90~) for the measuring angle p. In accordance with equation (1), the current measuring range MR has a m;~r; mllm Of IMRI = ~/(2-N-V) (5).
CA 0223897l l998-0~-28 It may be seen from equation (5) that the magnitude IMRI
of the current measuring range MR of a magnetooptic current transformer can be set by the choice of materials with different Verdet constants V for the Faraday element and/or by the number N of circulations of the measuring light around the conductor. A larger current measuring range is obtained if the product N V in the denominator is set to be smaller. However, such a choice of a larger current measuring range MR inescapably entails a reduced measuring resolution MA of the current transformer for a prescribed display resolution. The measuring resolu-tion MA is defined here and below as the absolute value ¦MS ¦ of the measuring sensitivity MS of the current transformer. The measuring sensitivity MS corresponds to the slope of the characteristic of the magnetooptic current transformer at a working point, and in the case of single-channel evaluation in accordance with equa-tion (2) is equal to MS = dS/dI = S0 ~ N ~ V ~ cos(2 N V I + ~) (6) and in the case of dual-channel evaluation in accordance with equation (4) is equal to MS = dT/dI = 2 ~ N ~ V ~ cos(2 ~N-V-I + ~) (7).
It is evident at once from equations (6) and (7) that a reduction in the product N-V leads in the case of both evaluation methods to a reduction in the measuring resolution MA = IMSI.
EP-B - 0 088 419 discloses a magnetooptic current transformer in which there are arranged parallel to one another about a common conductor two Faraday glass rings which consist of Faraday materials with different Verdet constants, and thus have current measuring ranges which are inherently different for each. Each Faraday glass ring is respectively assigned a transmitting unit for transmitting linearly polarized measuring light into the glass _ing, and a dual-channel evaluation unit for calculating a respective measuring signal for the respec-tive Faraday rotational angle. The two measuring signals of the two evaluation units are fed to an OR gate which determines a maximum signal from the two measuring CA 02238971 1998-0~-28 signals. This m~; mllm signal is used to switch over between the measuring ranges of the two glass rings.
Different measuring ranges of the two glass rings can also be achieved in the case of identical glass material for both glass rings by using measuring light of differ-ent wavelengths. In this case, the wavelength dependency of the Faraday rotation is exploited.
"International Conference of Large Hig~ Voltage Electric Systems", CIGRE, Paris, 28.8-3.9.1988, 0 Conference Proceedings, T, Pref. Subj. 1, Vol. 34, Issue 15, pages 1 to 10 discloses a fibre-optic measuring arrangement with a first magnetooptic current transformer for measuring nominal currents and with a second magneto-optic current transformer for measuring overcurrents. The first current transformer for measuring nominal currents contains an optical monomode fibre which surrounds the conductor in the form of a measuring w;n~;ng with N
turns. Linearly polarized light traverses the measuring winding, is retroreflected from a mirror into the fibre and traverses the measuring winding in the opposite direction a second time (reflection type). In this case, the Faraday rotational angle is doubled, while the undesirable temperature-dependent effects of circular birefringence of the fibre material cancel each other out precisely. After traversing the measuring winding twice, the light is subjected to dual-channel polarization evaluation. The second magnetooptic current transformer, which is provided for protective purposes, likewise comprises a monomode fibre which surrounds the conductor in the form of a measuring winding with one measuring turn. By contrast with the first current transformer, which is provided for measuring purposes, the second current transformer is of the transmission type, that is to say the linearly polarized measuring light is sub-jected to polarizatior. analysis after traversing themeasuring winding only once.
"SENSOR 93 Kongre~band IV Bll.l, pages 137 to 144" discloses a magnetooptic current transformer for protective purposes for measuring alternating currents, CA 02238971 1998-0~-28 in which after traversing a Faraday optical fibre linearly polarized light is split into two component light signals and each of these component light signals is fed to an analyser. The eigenaxes (polarization axes) of the two analysers are directed towards one another at an angle of 45~ or 58~. The light intensities passed by the analysers are firstly normalized by dividing by their dc components, which are obtained by peak value rectification. Subsequently, a product of the normalized signals is formed and this product is then differentiated. The Faraday rotational angle is obtained directly by integration. A signal is thereby obtained which is proportional to the current and is not therefore subject to any limitations on the measuring range.
However, this method is comparatively expensive.
EP-B-0 208 593 discloses a magnetooptic current transformer in which, after traversing a Faraday optical fibre surrounding a conductor, linearly polarized measur-ing light is split by a beam splitter into two component light signals and each of these component light signals is fed to an analyser. The eigenaxes of the two analysers are directed at an angle of 0~ or 45~, respectively, to the launching polarization of the measuring light. The result is a first, sinusoidal signal at the output of one analyser and a second, cosinusoidal signal at the output of the other analyser. These two signals are each ambigu-ous, oscillating functions of the current in the conduc-tor which are phase-shifted relative to one another by an angle of 90~. A unique measuring signal is now composed of these two ambiguous signals by comparing the sign and the absolute values of the measured values of the first, sinusoidal signal and of the second, cosinusoidal signal.
As soon as the absolute values of sine and cosine are equal, that is to say in the case of an integral multiple of 45~, a switchover is made, as a func~ion of the sign of sine and cosine, from a unique branch of the first, sinusoidal signal into a unique branch of the ~econd, cosinusoidal signal, or vice versa. This method is an incremental method, and so in the event of a failure of CA 02238971 1998-0~-28 the electronic system of the current transformer it is necessary for the working point at zero current to be reset again.
It is the object of the invention to specify a method and an arrangement for measuring a measured variable from a prescribed measuring range and, in particular, for measuring an electric current in a conductor from a prescribed current measuring range in which a high measuring resolution is achieved.
This object is achieved in accordance with the invention by means of the features of Claim 1 and Claim 8, respectively.
The method for measuring a measured variable from a prescribed measuring range comprises the following method steps:
a) derivation for the measured variable of a first measuring signal which is a unique function of the measured variable in the prescribed measuring range, b) derivation for the measured variable of a second measuring signal, which is an essentially periodic function of the measured variable with a period which is shorter than twice the interval length of the measuring range, and c) derivation for the measured variable, from the two said measuring signals of a third measuring signal which, in the prescribed measuring range, is a unique function of the measured variable and has at least the same measuring resolution as the second measuring signal.
The arrangement for measuring a measured variable from a prescribed measuring range contains a) a first measuring device for generating a first measuring signal which, in the prescribed measuring range, is a unique function of the measured variable, b) a second measuring device for generating a second measuring signal which is an essentially periodic function of the measured variable with a period which is shorter than twice the interval length of CA 02238971 1998-0~-28 the measuring range, and c) a signal processing-unit which is connected to the first measuring device and the second measuring device and derives from the first measuring signal and the second measuring signal a third measuring signal which is a unique function of the measured variable in the prescribed measuring range and has at least the same measuring resolution as the second measuring signal.
The third measuring signal combines in itself the advantage of the larger measuring range of the first measuring signal and the advantage of the higher measur-ing resolution of the second measuring signal, and therefore renders it possible to measure the measured variable uniquely in the prescribed measuring range with a high measuring resolution. Conversely, the third measuring signal has a large measuring range for a prescribed measuring resolution. The method and the arrangement are not incremental. In the event of a malfunction or a failure of the current transformer, the latter can be taken back immediately into operation without prior calibration. The functional reliability is thus always ensured.
Particular refinements and developments of the method and of the arrangement in accordance with the invention follow from the respectively dependent claims.
As a consequence, in one embodiment the first measuring signal is also a periodic function of the measured variable.
In a preferred embodiment, the measuring range is subdivided with the aid of the first measuring signal into uniqueness ranges over which the second measuring signal is a unique function of the measured variable, and the third measuring signal is composed of the branches of the second measuring signal over these uniqueness ranges.
The third measuring signal can be derived from the first measuring signal and the second measuring signal with the aid of a previously determined table of values, or else by computation.
CA 02238971 1998-0~-28 The measuring method and the measuring arrange-ment are preferably used-to measure an electric current in a conductor from a prescribed current measuring range.
For this purpose, at least two Faraday elements surround-ing the conductor are provided. A first linearly polar-ized light signal is sent through a first of the two Faraday elements at least once, and a first evaluation unit derives the first measuring signal for the current from a rotation of the polarization plane of this first light signal after traversal of the first Faraday element. A second linearly polarized light signal is sent at least through a second of the two Faraday elements at least once, and a second evaluation unit derives the second measuring signal for the current from a rotation of the polarization plane of this second light signal after traversal of at least the second Faraday element.
In a special embodiment of the measuring method, the second linearly polarized light signal traverses both the first Faraday element and the second Faraday element at least once in each case. The second measuring device of the measuring arrangement then also contains the first Faraday element of the first measuring device, and the two Faraday elements are connected optically in series via optical connecting means. In this embodiment, one light source suffices for transmitting linearly polarized measuring light, since the measuring light is split by the optical connecting means into two light components of which one light component is provided as first light signal and the other light component is provided as second light signal. In addition, a compact design of the arrangement is possible, since the two Faraday elements can be arranged next to one another in a space-saving fashion.
Reference is made, for the purpose of explaining the inv ntion, to the drawing, in which Figure 1 shows a design principle of an arrangement for measuring a measured variable from a prescribed measuring range, Figure 2 shows a diagram of three different measuring CA 02238971 1998-0~-28 signals for the measured variable, Figure 3 shows an arrangement for measuring an electric current from a prescribed current measuring range, having two separate Faraday measuring devices, Figure 4 shows an arrangement for measuring an electric current from a prescribed current measuring range, having two Faraday elements which are connected optically in series and operated in transmission mode in each case, Figure 5 shows an arithmetic unit for determ; n; ng the second measuring signal, Figure 6 shows an arrangement for measuring an electric current from a prescribed current measuring range, having a first Faraday element operated in transmission mode, and a series circuit composed of this first Faraday element and a second Faraday element operated in reflection mode, and Figure 7 shows an arrangement for measuring an electric current from a prescribed current measuring range, having a first Faraday element operated in reflection mode, and a series circuit, operated in transmission mode, composed of this first Faraday element and a second Faraday element, all the figures being diagrammatic illustrations. Mutual-ly corresponding parts are provided with the same refer-ence symbols.
Figure 1 shows a measuring arrangement for measuring a measured variable I in a prescribed measuring range MR. The measuring arrangement contains a $irst measuring device 5 and a second measuring device 6, as well as a signal processing unit 12. The measured vari-able I is present in ~ch case at an input 5A of the first measuring device 5 and at an input 6B of the second measuring device 6.
The first measuring device 5 converts the measured variable I into a first measuring signal M1 which is a unique function of the measured variable I at least over the prescribed measuring range MR. It there-fore holds that M1 (Ia) ~ M1 (Ib) (8) for two arbitrary, mutually differing values Ia and I~ of the measured variable from the prescribed measuring range MR. The first measuring device 5 thus has a unique characteristic in the measuring range MR. The first measuring signal M1 is fed from an output 5B of the first measuring device 5 to a first input 12A of the signal processing unit 12.
The second measuring device 6 generates from the measured variable I a second measuring signal M2 which, at least over the prescribed measuring range MR, is a periodic function of the measured variable I with a period P2. It therefore holds that:
M2 (I) = M2 (I + n ~ P2) where n is a whole number, and the characteristic of the second measuring device 6 is periodic, at least over the measuring range MR. The period P2 of the second measuring signal M2 is shorter than twice the interval length 2 IMR¦ of the measuring range MR, that is to say P2 ~ 2 ¦MRI (10).
Because of the condition (lO), the second measuring signal M2 is, by contrast with the first measuring sig-nal M1, not a unique function of the measured variable I
over the measuring range MR.
In the measuring range MR, possibly with the exception of individual subranges, the measuring resolution MA(M2) = IdM2/dI I (11) of the second measuring signal M2 and of the second measuring device 6 is higher than the measuring resolution MA(M1) = ldM1/dII (12) of the first measuring signal Ml and of the associated first measuring device 5. The second measuring signal M2 is transmitted from an output 6B of the second measuring device 6 to a second input 12B of the signal processing -CA 02238971 1998-0~-28 unit 12.
The signal processing unit 12 now forms from the two measuring signals M1 and M2 present at its inputs 12A
and 12B a third measuring signal M3 which, over the entire measuring range MR, is a unique function, on the one hand, and has at least the measuring resolution MA(M2) of the second measuring signal M2, on the other hand. This third measuring signal M3 can be tapped at an output 12C of the signal processing unit 12.
Figure 2 shows in a diagram an exemplary embodi-ment of the three measuring signals M1, M2 and M3 derived in a measuring arrangement in accordance with Figure 1.
A prescribed measuring range for the measured variable I
plotted on the abscissa is denoted by MR and corresponds to a preferably closed interval [IA' IB] ~f values of the measured variable I between a first interval limit IA and a second interval limit IB. This measuring range MR thus has the length IMRI = I IB ~ IA I -The first measuring signal M1 is a periodic and preferably sinusoidal function of the measured variable Iand oscillates between a maximum value Max~Ml) and a m;n;mllm value Min(M1) with the period P1. The first measuring signal M1 traverses the central value Cen(M1) =
0.5 ~ (Max(M1) + Min(M1)) between the two extreme values Max(M1) and Min(M1). The first measuring signal M1 is a unique function of the measured variable I over the measuring range MR, that is to say it satisfies the condition (8). In the exemplary embodiment represented, the measuring range MR lies inside a half period Pl/2 of the first measuring signal M1 between the m;n;mllm value Min(M1) and maximum value Max(M1) of said first measuring signal M1, in which range a sinusoidal function is known to be unique, and the central value Cen(M1) of the first measuring signal M1 corresponds to the value of the first measuring signal M1 at the midpoint IM = ~ . 5 ~ (IB + IA) ~f the measuring range MR, that iR to say Cen(M1) = M1(IM) . The first measuring signal Ml increases in the measuring range MR with increaRing measured variable I with a positive slope in a strictly CA 02238971 1998-0~-28 monotonic fashion, but can also be a strictly monotonically decreasing-function with a negative slope.
At least over the measuring range MR, the second measuring signal M2 is a periodic and, preferably, sinusoidal function of the measured variable I, and oscillates between a m~X;mum value Max(M2) and a m;n;mum value Min(M2) with the period P2. Three values of the measured variable I for which the second measuring sig-nal M2 assumes its m;n;mum value Min(M2) are denoted by Io~ I2 and I4, whereas three values for which the second measuring signal M2 assumes its m~X;m~m value Max(M2) are denoted by I1, I3 and I5. It holds that Io ' I1 ~ I2 ~ I3 c I4 ~ I5. The second measuring signal M2 traverses a central value Cen(M2) = 0.5 ~ (Max(M2) + Min(M2)) between two extreme values Max(M2) and Min(M2). In the exemplary embodiment represented, the measuring range MR comprises two periods 2-P2 ~f the second measuring signal M2, the second measuring signal M2 assuming its mean value Cen(M2) in each case at the end points IA and IB as well as at the midpoint IM of the measuring range MR, that is to say it holds that Cen(M~ = M~(IA) = M~ IB) = Mz~IM).
The second measuring signal M2 represented thus fulfils the previously mentioned conditions (9) and (10).
For the purpose of greater clarity, the first measuring signal M1 is represented on a larger scale than the second measuring signal M2, and thus has a flatter course in general in comparison with the second measuring signal M2, and is, moreover, illustrated offset with respect to the second measuring signal M2. The value ranges [Min(M1),Max(M1)] of the first measuring signal M1 and [Min(M2),Max(M2)] of the second measuring signal M2 can also overlap. In particular, the two central values Cen(M1) and Cen(M2) can be equal, preferably Cen(M1) =
Cen(M2) = 0 in appropriate measurement units. The measur-ing resolution MA(M1) of the first measuring signal Ml isplainly lower over the largest part of the measuring range MR than the measuring resolution MA(M2) of the second measuring signal M2. An exception is formed only by small ranges around the values of In of the measured CA 02238971 1998-0~-28 variable I with n = O, 1, 2, 3, 4 or 5, in which the second measuring signal-M2 assumes its extreme values Max(M2) and Min(M2), respectively, and in which the measuring resolution of the second measuring signal M2 is therefore zero, that is to say MA(M2(In)) = O. The m~x;ml-m measuring resolution MA(M1(IM)) of the first measuring signal M1 at the midpoint IM of the measuring range MR iS plainly lower than the maximum measuring resolution MA(M2)(IM) ) of the second measuring signal M2, which in the exemplary embodiment shown is assumed five times over the measuring range MR, and in particular at the midpoint IM of the measuring range MR.
The third measuring signal M3 is now derived in the following way from the first measuring signal M1 and the second measuring signal M2. The measuring range MR
can be subdivided into individual uniqueness ranges over which the second measuring signal M2 is a unique function of the measured variable I. These uniqueness ranges (quadrants) are separated from one another by lines in the diagram represented and correspond to the intervals [IA~I1]~ [I1,I2], [I2~I3], [I3,I4] and [I4,IB] between two neighbouring extreme values of the second measuring signal M2. The first uniqueness range [IA' I1] and the last uniqueness range [I4, IB] are only P2/4 long in this case, while the inner uniqueness ranges [I1,I2], [I2,I3] and [I3,I4], lying therebetween, have the m~;mllm length P2/2 in each case. It is, however, impossible to determine simply from the measured value M2(I) alone, which is delivered by the second measuring signal M2, in which of the uniqueness ranges the current measured value I lies.
This ambiguity of the second measuring signal M2 is now resolved with the aid of the first measuring signal M1.
The individual branches of the second measuring signal M2 in the uniqueness ranges are combined to form the new measur_ng signal M3 by transformations (operations) such as translations and, as the case may be, reflections.
These operations differ in general for each uniqueness range. The first step is now to use the current value M1(I) of the first measuring signal M1 to determine the CA 02238971 1998-0~-28 uniqueness range in which the current value of the measured variable I lies, after which the operation assigned to this uniqueness range is used to determine the measured value M3(I) of the third measuring signal M3 for the current value of the measuring variable I. For the exemplary embodiment represented, the following operations, in particular, are suitable for deriving the third measuring signal M3 from the second measuring signal M2 with the aid of the first measuring signal Ml;
10M3(I): = M2(I) for M1(IA) S M1(I) ~ M1(I1) (13a) M3(I): = 2-A2 ~ M2(I) for M1(I1) ' M1(I) ~ M1(I2) (13b)M3(I): = M2(I) + 4 A2 for M1(I2) ' M1(I) ~ M1(I3) (13c) M3(I): = 6-A2 ~ M2(I) for M1(I3) ' M1(I) ~ M1(I4) (13d) M3(I): = M2(I) + 8 A2 for M1(I4) ' M1(I) ' M1(IB) (13e), A2 being the prescribed maximum amplitude of the second measuring signal M2, with A2 = 0-5 ~ (Max(M2) - Min(M2)) (14).
Because of the uniqueness of the first measuring signal M1, the conditions, on which the operations (13a) to (13e) are based, placed on the value M1(I) of the first measuring signal M1 correspond precisely to the uniqueness ranges [IA,I1] to [I4,IB] of the second mea~ur-ing signal M2.
A further embodiment (not represented) for deriving the third measuring signal M3 is given by the following operations in which, by contrast with the exemplary embodiment represented, the second measuring signal M2 is not reflected in the uniqueness ranges:
M3(I): =M2(I) for M1(IA) ' M1(I) ~ M1(I1) (15a) 3 ) 2(I) + 2 A2 for M1(I1) c M1(I) ~ M1(I2) (15b) 3 ) 2(I) + 4 A2 for M1(I2) c M1(I) ~ M1(I3) (15c) M (I): =M2(I) + 6-A2 for M1(I3) ' M1(I) ~ M1(I4) M3(I): =M2(I) + 8 A2 for M1(I4) ' M1(I) ' M1(IB) (15e)-In this embodiment in accordance with the oper-ations (15a) to (15e), by contrast with the exemplary CA 02238971 1998-0~-28 embodiment shown in accordance with the operations (13a) to (13e), the individual-branches of the resultant third measuring signal M3 are not continuously juxtaposed.
The operations (13a) to (13e) or (15a) to (15e) can be carried out, in particular, with the aid of a table of values (look-up table) determined experimentally in advance or by calculation. The table of values assigns the value pair of the measured values Ml(I) and M2(I) of the first measuring signal M1 and the second measuring signal M2, respectively, a value M3(I) of the third measuring signal M3 as a unique measure of the measured variable I. The signal processing unit 12 then preferably contains a memory for storing the values of the table of values which have been determined in advance, an analog-to-digital converter for converting the current values of the first measuring signal Ml and of the second measuring signal M2 into a digital value in each case, and a digi-tal signal processor or microprocessor for comparing these measured digital values with the values stored in the table of values, and for assigning the value M3(I), which is likewise digital.
The said operations (13a) to (13e) or (15a) to (15e) can, however, also be carried out computationally.
The measured value M1(I) of the first measuring signal M
is then compared to the values, determined in advance, of the first measuring signal M1 at the interval limits In of the uniqueness ranges, and the second measuring signal M2 is subjected to the associated arithmetic operation as a function of this comparison. In this embodiment, the signal processing unit 12 contains, for example, a corresponding number of comparator circuits for comparing the current value M1(I) of the first measuring signal M
with the values, determined in advance, of the first measuring signal M1 at the interval limits In of the uniqueness ranges, and an analog ari_hmetic unit with analog components such as subtracters, adders and inverters, or an analog-to-digital converter and a downstream digital arithmetic unit for carrying out the arithmetic operations. As an alternative to this, the CA 02238971 1998-0~-28 signal processing unit 12 can also contain an analog-to-digital converter for converting the current values of the first measuring signal M1 and of the second measuring signal M2 into in each case one digital value, and a downstream digital signal processor or microprocessor which compares the digital value of the first measuring signal M1 with the stored digital values Ml(In) at the interval limits In and then carries out the associated digital arithmetic operations.
The current value of the measured variable I can now be determined uniquely from the third measuring signal M3 obtained, by applying the inverse function M3~
of the third measuring signal M3 to the value M3(I~
determined for the third measuring signal M3, because it holds that M3~1 (M3(I)) = I.
In the case of the represented sinusoidal second measuring signal M2 with the amplitude A2 and the period P2, the measured value I for the measured variable is yielded, for example, as:
I = Ios(Ml) + I(M2) (16), the measured value I being composed of an offset measured value IoS(Ml) which depends on the first measuring signal M1 and is constant in each case for a uniqueness range, and a component I(M2) which is a continuous function of the second measuring signal M2.
The offset measured value IoS(Ml), respectively constant in the uniqueness ranges of the second measuring signal M2, of the measured value I is given in the exemp-lary embodiment represented and in accordance with the operations (13a) to (13e), and is also given in the exemplary embodiment in accordance with the operations (15a) to (15e), by the equations IOS (M1) = IA for M1(IA) C M1 (I) ~ M1(I1) (17a) IOS (M1) = IA + ~ ~ 5 P2 for M1(I1) c M1(I) c M1(I2) (17b) IOS (M1) = IA + P2 for M1(I2) c M1(I) c Ml(I3) (17c) IoS(Ml) = IA + 1 ~ 5 P2 for M1(I3) c M1(I) ~ M1(I4) (17d) CA 02238971 1998-0~-28 IOS (M1) = IA + 2 P2 for M1(I4) c M1(I) 5 M1 (IB) (17e).
The continuously variable component I(M2) in the uniqueness ranges of the second measuring signal M2 is I(M2) = (P2/2~) arcsintM2/A2) (18a) for M1(IA) C M1 (I) ~ M1(I1) or M1(I2) c M1(I) ~ M1(I3) or M1 (I4) 5 M1 (I) 5 M1 (IB) and I(M2) = (P2/2~) arcsin(_ M2/A2) (18b) for M1(I1) 5 M1(I) c M1(I2) or M1 (I3) 5 M1 (I) ~ M1 (I4) -In an advantageous embodiment, the current value of the measured variable I can thus also be determined directly from the first measuring signal M1 and the second measuring signal M2 by the signal processing unit 12, for example again with the aid of a table of values, or else by computation with the aid of a pro-cessor. In this case, the third measuring signal M3 is equal to the identical function over the measuring 20 range MR, that is to say M3(I) = I- (19) Since the methods described are not incremental methods, the functional reliability is always given.
For the purpose of a higher measuring resolution, in a particular embodiment the first measuring signal Ml can also be used as the third measuring signal M3 in ranges around the points I~ at which the measuring resol-ution MA(M2) of the second measuring signal M2 vanishes.
For this purpose, an additional comparison of the measur-ing resolutions MA(M1) of the first measuring signal M1and MA(M2) of the second measuring signal M2 can be carried out, and the first measuring signal M1 can be used as the third measuring signal M3 as long as it holds that MA(M2) ~ MA(M1). It is preferable to determine in advance the ranges in which this condition holds. When the condition is no longer fulfilled, the third measuring CA 02238971 1998-0~-28 - signal M3 is derived again as previously described.
The second measuring signal M2 can also be another periodic function of the measured variable I, for example a linear sweep function (saw-tooth function). The first measuring signal M1 can also be a linear function o~ the measured variable I or can, in the exemplary embodiment of Figure 2, be approximated by a linear function at the midpoint IM ( linear interpolation).
The previously described embodiments of the measuring arrangement and of the measuring method in accordance with the invention are preferably used to measure an electric current I as measured variable with the aid of the Faraday effect. As already described, in the case of a polarimetric magnetooptic current trans-former it is generally the case that sinusoidal orcosinusoidal measuring signals are generated in accord-ance with one of the equations (2) or (4), which cor-respond to the exemplary embodiment shown in Figure 2.
An exemplary embodiment of such an arrangement for measuring an electric current I in a conductor 2 is represented in Figure 3. TWO Faraday measuring devices 5 and 6 are assigned to the conductor 2. Each Faraday measuring device 5 and 6 respectively has a Faraday element 3 and 4 exhibiting the magnetooptic Faraday effect, a light source 9 and 12, respectively, for transmitting a linearly polarized light signal L1 and L2, respectively, into the Faraday element 3 and 4, respect-ively, and an evaluation unit 7 and 8, respectively, for evaluating a polarization rotation (Faraday rotation) of the linearly polarized light signal L1 and L2~
respectively, after at least one traverse of the Faraday element 3 and 4, respectively, as a consequence of the magnetic field generated in the conductor 2 by an electric current I. The two Faraday elements 3 and 4 prefer_bly surround the conductor 2, with the result that light running through the Faraday element 3 or 4 circulates the conductor 2 along a virtually closed light path at least once.
In one embodiment, the first Faraday element 3 CA 0223897l 1998-0~-28 - and/or the second Faraday element 4 can be formed using solid bodies exhibiting -the Faraday effect, preferably made from glass. It is also possible to provide one or else several bodies for each Faraday element 3 or 4. A
single coherent body is then designed as a polygonal annular body in general. Such solid bodies, in particular bodles annular ~A~, are mechanically robust and exhibit virtually no circular birefringence. In another embodi-ment, the first Faraday element 3 and/or the second Faraday element 4 are formed by one or in each case one monomode optical fibre which preferably surrounds the conductor 2 in the form of a measuring winding with at least one turn.
The polarization of the first light signal L1 after traversing the first Faraday element 3 is evaluated by the first evaluation unit 7 of the first measuring device 5 for a first measuring signal M1 for the electric current I, while the polarization of the second light signal L2 is evaluated by the second evaluation unit 8 of the second measuring device 6 for a second measuring signal L2 for the electric current I. In this case, single-channel or dual-channel polarization evaluation can respectively be used for each of the two evaluation units 7 and 8, thus resulting, in particular, in sinusoidal measuring signals M1 and M2 in accordance with the relationships (2) or (4). Again, in order to correct falsifications of the measuring signals it is possible in principle to provide all the additional measures which are known for suppressing or compensating linear and/or circular birefringence, for example as a consequence of temperature changes or vibrations, in the Faraday elements 3 and 4 and the optical transmission links.
As in the embodiment in accordance with Figure 1, the two measuring signals M1 and M2 are fed to the signal processing unit 12 ail further processed for a third measuring signal M3 for the electric current I in the conductor 2. The measuring sensitivity of the first Faraday element 3 is lower than the measuring sensitivity of the second Faraday element 4 and is selected to be so CA 0223897l l998-0~-28 high that the first measuring signal M1 in a prescribed current measuring range MR is a unique measure of the current I. The measuring sensitivity of the second Faraday element 4 is, by contrast, selected such that the period P2 of the second measuring signal M2 is shorter than twice the length of the current measuring range MR.
The second measuring signal M2 is thus not a unique function of the current I in the current measuring range MR. In order to set different measuring sensitiv-ities of the two Faraday elements 3 and 4 it is possible,for example, to use materials with different Verdet constants for the two Faraday elements 3 and 4, respect-ively, or else to use Faraday elements 3 and 4 with different light path lengths along the magnetic field generated by the current I.
The prescribed current measuring range MR prefer-ably reaches from an overcurrent measuring range, which typically lies in the range between 10 kA and at least 100 kA, as far as a nominal current measuring range which usually lies below the overcurrent measuring range by a factor of about 10 to 30. The nominal current measuring range is preferably covered in this case by a uniqueness range of the second measuring signal M2 which is half a period P2 long. The third measuring signal M3, which is derived using the method and the arrangement in accord-ance with the invention, permits measurement both of nominal currents for measurement or meter applications and of overcurrents for protective purposes with the same measuring resolution.
Apart from the embodiment, described with the aid of Figure 3, with two separate measuring devices 5 and 6, embodiments are also possible in which the two Faraday elements 3 and 4 are connected optically in series via optical connecting means, and a first linearly polarized light signal L1 traverses only ti~ first Faraday element 3 and a second linearly polarized light signal L2 traverses the first Faraday element 3 and the second Faraday element 4. Such embodiments are disclosed in the older German Patent Applications P 44 29 909.5 and CA 02238971 1998-0~-28 P 44 31 615.1, which were not published prior to the date of filing of the present application and whose content is also incorporated into the present application. Figures 4 to 6 show exemplary embodiments with Faraday elements 3 and 4 connected in this way in series.
In the embodiment in accordance with Figure 4, the first Faraday element 3 and the second Faraday element 4 are connected optically in series via a three-port coupler 18 as optical connecting means. An optical fibre which surrounds the conductor 2 in the form of a measuring winding is respectively provided both for the first Faraday element 3 and for the second Faraday element 4. It is preferable to provide annealed optical fibres which are distinguished by low linear birefring-ence and virtually negligible circular birefringence. Inaddition to the selection of different materials for the two Faraday elements 3 and 4, it is also possible, in this embodiment, to vary the number of the turns of the measuring windings in order to match the current measur-ing ranges. The measuring winding of the second Faradayelement 4 in this case preferably has more turns than the winding of the first Faraday element 3. A transmitting unit 10 generates linearly polarized measuring light L
which is preferably launched into the first Faraday element 3 via an optical fibre 50 which maintains polar-ization. A laser diode, or else a light source (for example LED) with a downstream polarizer can, for example, be provided as transmitting unit 10. A low-birefringence optical fibre (LoBi fibre) can be provided as the optical fibre 50 which maintains polarization. A
splice 53 preferably joins the optical fibre 50 to the measuring winding of the first Faraday element 3. The three-port coupler 18 is designed in such a way that a first portion of the measuring light L, which is launched into the first Faraday element 3 by the transmitting u~ t 10 and runs through the first Faraday element 3, is launched as first light signal Ll into the first evaluation unit 7, and a second portion of the measuring light L running through the first Faraday element 3 is CA 0223897l l998-0~-28 launched as second light signal L2 into the second Faraday element 4. A beam-splitting, semi-transparent mirror which is arranged at an angle of generally 45~ to the beam direction of the incident measuring light L, for example a beam-splitting splice, or else a Y-coupler, in particular a fibre coupler, can be provided as three-port coupler 18. A first port 18A of the three-port coupler 18 is optically connected to the first Faraday element 3, a second port 18B is optically connected to the first evaluation unit 7, and a third port 18C is optically connected to the second Faraday element 4. An optical fibre 70 which maintains polarization is preferably provided for transmitting the first light signal L1 from the three-port coupler 18 to the first evaluation unit 7.
The second light signal L2 traverses the second Faraday element 4 only once and is then directly launched into the second evaluation unit 8, preferably likewise via an optical fibre 60 which maintains polarization, and can be connected via a splice 64 to the measuring winding of the second Faraday element 4 and/or can be designed as a LoBi fibre. The second Faraday element 4 is thus operated in transmission mode with respect to the second portion L2 of the measuring light L.
The first evaluation unit 7 evaluates the Faraday rotation of the polarization plane of the first light signal L1 for a first measuring signal M1 which can be tapped at an output of the first evaluation unit 7. The first Faraday element 3 is thus operated in transmission mode. The second evaluation unit 8 evaluates the Faraday rotation of the polarization plane of the second light signal L2 for a second measuring signal M2, which can be tapped at an output of the second evaluation unit 8. The entire Faraday rotation of the polarization plane of the second light signal L2 is composed in this case of a first Faraday rotational angle component, produced in the first Faraday element 3, and a second Faraday rotational angle component, effected in the second Faraday element 4. If the Faraday rotation was performed in the same direction in the two Faraday elements 3 and 4, in CA 02238971 1998-0~-28 accordance with an identical sensor circulation of the transmitted light in the first Faraday element 3 and second Faraday element 4 relative to the direction of flow in the conductor 2, then the entire rotational angle is equal to the sum of the two individual rotational angles. In the case of opposite directions of rotation, the resultant rotational angle corresponds, by contrast, to the difference between the two individual rotational angles.
The advantageous embodiments, shown in Figure 4, of the two evaluation units 7 and 8 in each case carry out dual-channel polarization analysis of the associated light signal L1 and L2, respectively. Means 72 which àre optically coupled to the port 18B of the three-port coupler 18 via the optical fibre 70 which maintains polarization, for example a polarizing beam splitter, preferably a Wollaston prism, or else a beam splitter and two optically downstream analysers which cross at a prescribed angle, are provided in the first evaluation unit 7 for the purpose of splitting the first light signal L1 into two linearly polarized component light signals L11 and L12 with different polarization planes. It is possible, in particular, to provide as the optical fibre 70 which maintains polarization an optical fibre with a high linear birefringence (HiBi fibre), whose eigenaxes of the linear birefringence are adjusted to the eigenaxes of the Wollaston prism 72. Furthermore, the evaluation unit 7 contains photoelectric transducers 74 and 75 for converting the component light signals L11 and L12 into an electric signal R11 and R12, respectively, in each case as a measure of the intensity of the respective component light signal L11 and L12, respectively, and electronic means 76 for deriving the first measuring signal M1 from these two electric signals R11 and Rl2. As first measuring signal M1, the electronic means 76 preferably determine a quotient signal M1 = (Rll-Rl2)/Rll+Rl2) (20) from the difference R11-R12 and the sum R11+R12 of the two electric signals R11 and R12. This quotient signal is CA 0223897l l998-0~-28 largely freed from intensity fluctuations in the trans-mitting unit 10 or in the transmission links for the measuring light L and the first light signal L1, and it holds in general that:
Ml = sin(2 Ns Vs a + ~) (21) with the Faraday rotational angle a of the first light signal L1, the number NS of the turns of the first Faraday element 3, and the Verdet constant Vs ~f the material of the first Faraday element 3, as well as the constant offset angle ~. NS is typically between 1 and 3.
The component light signals L11 and L12 can be transmitted to the transducers 74 and 75 in a free-beam arrangement, or else via optical fibres. The outputs of the photo-electric transducers 74 and 75 are respectively connected electrically to an input of the electronic means 76.
The second evaluation unit 8 is constructed in a similar way to the first evaluation unit 7. Means 82, for example a Wollaston prism, which are optically connected to the second Faraday element 4 via an optical fibre 60 which maintains polarization are provided for the purpose of splitting into two linearly polarized component light signals L2l and L22 with different polarization planes the linearly polarized light component L2 transmitted through the second Faraday element 4. The optical fibre 60 is connected to the fibre of the second Faraday element 4 via a splice 64. As the optical fibre 60 which maintains polarization, it is possible, in particular, to provide an optical fibre with a high linear birefringence (HiBi fibre) whose eigenaxes of the linear birefringence are adjusted to the eigenaxes of the Wollaston prism 82.
Furthermore, the second evaluation unit contains photo-electric transducers 84 and 85 for converting these component light signals L2l and L22 into in each case an electric signal T21 and T22, respectively, as a measure of the intensity of he respective component light signal L21 and L22, respectively, and electronic means 86 for deriving the second measuring signal M2 from the two electric signals T21 and T22. As second measuring signal M2, a quotient signal CA 02238971 1998-0~-28 M2 = (T21-T22)/(T21+T22) (22) of normalized intensity-is preferably derived by the electronic means 86 from the difference T2l-T22 and the sum T21+T22 of the two electric signals T21 and T22. This second measuring signal M2 depends on the total Faraday rotational angle ~ of the second light signal L2 in the following way:
M2 = sin(2-(Ns-vs+NM-vM)-~B + ~) (23) with the number of turns NM~ the Verdet constant VM of the second Faraday element 4, and a constant offset angle ~. The number of turns NM of the second Faraday element 4 is generally selected to be between 10 and 50.
In a preferred embodiment, the electronic means 76 and/or 86 contain an analog-to-digital converter for digitizing the two signals R11 and R12 of the trans-ducers 74 and 75, or T21 and T22 of the transducers 84 and 85 and a downstream digital arithmetic unit for calculating the first measuring signal M1 or the second measuring signal M2.
In another embodiment, represented in Figure 5, the electronic means 86 contain analog arithmetic com-ponents. Quicker signal processing can be achieved thereby. Provision is made of a subtracter 31 and an adder 32 at whose inputs the two electric signals T
and T22 of the transducers 84 and 85 (not represented) are present in each case. The outputs of the subtracter 31 and of the adder 32 are connected in each case to an input of a divider 3 3. As second measuring signal M2, the divider 33 forms the quotient signal (T21-T22)/(T21+T22) from the difference signal T21-T22 at the output of the subtracter 31 and the sum signal T21+T22 at the output of the adder 3 2. A corresponding circuit can also be provided for the electronic means 76 of the first evaluation unit 7.
Figure 6 shows a further ~mbodiment of the measuring arrangement. The first Faraday element 3 and the second Faraday element 4 are connected optically in series via an optical four-port coupler 19 with four ports 19A to l9D as optical connecting means. The second evaluation unit 8 is optically connected to the port l9D
of the four-port coupler 19. Assigned to the second Faraday element 4 are optically reflecting means 40, for example a mirror, which retroreflect the second light signal L2 into the second Faraday element 4 after a single traverse of the second Faraday element 4. The retroreflected second light signal L2' traverses the second Faraday element 4 in the reverse direction a second time and is then fed to the second evaluation unit 8 via the four-port coupler 19. The port l9B con-nected to the second Faraday element 4, and the port l9D
connected to the second evaluation unit 8, of the four-port coupler 19 are optically coupled to one another for the purpose. Likewise optically coupled to one another are the port l9A, connected to the first Faraday element 3, and the port l9C, connected to the first evaluation unit 7, of the four-port coupler 19. A beam splitter, for example, with a semitransparent mirror arranged obliquely relative to the light propagation direction, or a fibre-optic coupler can be provided as the four-port coupler 19. In this embodiment in accordance with Figure 6, the second Faraday element 4 is operated in reflection mode. The second light signal L2' evaluated by the second evaluation unit 8 has a Faraday rotation of its polarization plane which is composed of the Faraday rotation in the case of a traverse of the first Faraday element 3 and twice the Faraday rotation in the case of a traverse of the second Faraday element 4.
In the embodiment in accordance with Figure 7, the first Faraday element 3 is operated in a reflection arrangement. An optical coupler 13 connects the first Faraday element 3 both to the transmitting unit 10 for transmitting linearly polarized measuring light L and to the first evaluation unit 7. This coupler 13 can be a Y-fibre coupler, or else a beam splitter formed by means 3fa semitransparent mirror. The optical connecting means 15 for optically coupling the two Faraday elements 3 and 4 are now designed in such a way that a portion of the measuring light L running through the first Faraday CA 02238971 1998-0~-28 element 3 is retroreflected as first light signal Ll into the first Faraday element 3, and another portion is passed as second light signal L2 and launched into the second Faraday element 4. For this purpose, the optical connecting means 15 preferably contain a semitransparent mirror 35 which is arranged essentially perpendicular to the beam direction of the incident measuring light L.
However, it is also possible to provide a beam splitter with a semitransparent mirror directed at an angle, for example 45~, to the beam direction of the incident measuring light L, and a further mirror, arranged in the beam path of the component measuring light reflected at this semitransparent mirror, for the purpose of retro-reflecting this component measuring light to the semi-transparent mirror. The first light signal Lretroreflected by the optical connecting means 15 tra-verses the first Faraday element 3 in the rever~e direc-tion a second time and is fed via the optical coupler 13 to the first evaluation unit 7. Because of the non-reciprocity of the Faraday effect, the first lightsignal L1 experiences a Faraday rotation which is twice as large as in the case of only a single traverse of the Faraday element 3. By contrast, effects of a possible circular birefringence in the first Faraday element 3 cancel one another out because of their reciprocal prop-erty. The evaluation unit 7 derives the first measuring signal M1 from the first light signal L1. By contrast, after the first Faraday element 3, the second light signal L2 passed by the optical connecting means 15 also traverses the second Faraday element 4, and is fed to the second evaluation unit 8 after traversing the second Faraday element 4. The second light signal L2 arriving at the evaluation unit 8 has a polarization plane which has been rotated both in the first Faraday element 3 about a first Faraday rotational angle and in the second Faraday element 4 about a second Faraday rotational angle. The resultant total rotational angle of the second light signal L2 is evaluated in the evaluation unit 8 for a second measuring signal M2.
CA 02238971 1998-0~-28 In the embodiment represented, a fibre spool surrounding the conductor 2 and made from an optical monomode fibre is respectively provided both for the first Faraday element 3 and for the second Faraday element 4. The optical connecting means 15 are then formed with the aid of a semitransparent coating which serves as a mirror 35 and is applied by sputtering or chemical deposition to the fibre end, preferably of the first Faraday element 3 and with the aid of mechanical connecting means for connecting the fibre end provided with the mirror to the neighbouring fibre end of the other Faraday element. The mechanical connecting means can be a detachable plug-and-socket connection, or else an undetachable splice, for example a capillary tube from the Nippon Electric Glass company.
The second evaluation unit 8 is designed in a fashion similar to Figure 4. By contrast, in the embodi-ment represented, the first evaluation unit 7 contains polarizing means lOB which are connected optically between the first Faraday element 3 and the coupler 13, and a photodetector 79 which is optically coupled to the coupler 13. A light source lOA which is optically coupled to the coupler 13 is provided for transmitting light L.
The coupler 13 can be connected to the polarizing means lOB via an optical fibre 50'. In this embodiment, this fibre 50' can be a simple teleco~mnnication fibre without properties of maintaining polarization, since the polarizing means lOB linearly polarize the light L only directly before its entry into the series circuit of the two Faraday elements 3 and 4 at the input of the first Faraday element 3. Consequently, it is also possible to provide a non-polarizing, simple light source as the light source lOA. The polarizing means lOB and the light source lOA together form the transmitting unit 10 for laun~hing linearly polarized measuring light L into the first Faraday element 3. The polarizing means lOB are provided at the same time as an analyser for the first light signal Ll which is reflected by the semireflecting connecting means 15 and has its polarization plane rotated. The analyser passes only the component Ll' of the first light signal L1-which is projected onto its set polarization axis. The light component L1', passing through the analyser, of the first light signal L1 is fed via the coupler 13 to the photodetector 79 and converted there into an electric signal as first measuring sig-nal M1. This first measuring signal M1 is proportional to the light intensity of the light component L1'. It there-fore holds that M1 = K cos2(0.5 Ns Vs ~) (24) with the Faraday rotational angle a of the first light signal L1, and a proportionality factor K.
It is also possible to carry out a method for compensating temperature and/or vibration because of the high measuring resolution required for norn;~l currents.
In principle, all analog and digital evaluation methods for detecting the polarization state of linearly polarized light can be used to evaluate the Faraday rotational angle in the first evaluation unit 7 and in the second evaluation unit 8. In particular, the embodi-ments of the evaluation units 7 and 8 in accordance with Figures 4, 5 and 7 can be combined with one another at will. Preferably, the two light signals L1 and L2 are evaluated by means of dual-channel polarization analysis.
However, it is also possible in each case to provide single-channel evaluation for each light signal Ll and L2. The measuring signals Ml and M2 obtained are then of the sinusoidal configuration as in the relationship (23) or (24) and thus fulfil, in conjunction with appropriately selected measuring sensitivities NS Vs of the first Faraday element 3 and NM-VM ~f the second Faraday element 4, the preconditions for deriving the third measuring signal M3 in accordance with one of the embodiments previously described.
The optical coupling of the various optical components of the measuring arrangement is preferably supported by collimator lenses (Grin lenses) for focusing the light.
In a particular embodiment (not represented), it CA 02238971 1998-0~-28 is also possible to use a plurality of linearly polarized measuring light signals of wavelengths which differ and are generally close to one another, in conjunction with wavelength-sensitive optical connecting means 15, 18 or 19. Crosstalk between protective and measurin~
channels can thereby be avoided.
Claims (14)
1. Method for measuring a measured variable (I) from a prescribed measuring range (MR), in which a) there is derived for the measured variable (I) a first measuring signal (M1) which is a unique function of the measured variable (I) over the prescribed measuring range (MR), b) there is derived for the measured variable (I) a second measuring signal (M2) which, at least over the prescribed measuring range (MR), is a periodic function of the measured variable (I) with a period (P2) which is shorter than twice the interval length (¦MR¦) of the measuring range (MR), and c) there is derived for the measured variable (I) from the first measuring signal (M1) and the second measuring signal (M2) a third measuring signal (M3) which is a unique function of the measured variable (I) in the prescribed measuring range (MR) and has at least the same measuring resolution as the second measuring signal (M2).
2. Method according to Claim 1, in which the first measuring signal (M1) is also a periodic function of the measured variable (I), at least over the prescribed measuring range (MR).
3. Method according to Claim 1 or Claim 2, in which the measuring range (MR) is subdivided with the aid of the first measuring signal (M1) into uniqueness ranges over which the second measuring signal (M2) is a unique function of the measured variable (I), and the third measuring signal (M3) is composed of the branches of the second measuring signal (M2) over these uniqueness ranges.
4. Method according to one of the preceding claims, in which the third measuring signal (M3) is derived from the first measuring signal (Ml) and the second measu~ing signal (M2) with the aid of a previously determined table of values.
5. Method according to one of Claims 1 to 3, in which the third measuring signal (M3) is calculated from the first measuring signal (M1) and the second measuring signal (M2).
6. Method according to one of the preceding claims for measuring an electric current (I) from a prescribed current measuring range (MR) in a conductor (2), having at least two Faraday elements (3, 4) surrounding the conductor (2), in which a) a first linearly polarized light signal (L1) traverses a first of the two Faraday elements (3) at least once, and the first measuring signal (M1) for the current (I) is derived from a rotation of the polarization plane of this first light signal (L1), b) a second linearly polarized light signal (L2) traverses at least a second of the two Faraday elements (4) at least once, and the second measuring signal (M2) for the current (I) is derived from a rotation of the polarization plane of this second light signal (L2).
7. Method according to Claim 6, in which the second linearly polarized light signal (L2) traverses both the first Faraday element (3) and the second Faraday element (4) at least once in each case.
8. Arrangement for measuring a measured variable (I) from a prescribed measuring range (MR), having a) a first measuring device (5) for generating a first measuring signal (M1) which is a unique function of the measured variable (I) over the prescribed measuring range (MR), b) a second measuring device (6) for generating a second measuring signal (M2) which is a periodic function of the measured variable (I) with a period (P2) which is shorter than twice the interval length (¦MR¦) of the measuring range (MR), and c) a signal processing unit (12) which is connected to the two measuring devices (5, 6) and derives from the first measuring signal (M1) and the second measuring signal (M2) a third measuring signal (M3) which is a unique function of the measured variable (I) in the prescribed measuring range (MR) and has at least the same measuring resolution as the second measuring signal (M2).
9. Arrangement according to Claim 8, in which the second measuring signal (M2) generated by the second measuring device (6) is a periodic function of the measured variable (I).
10. Arrangement according to Claim 8 or Claim 9, in which the signal processing unit (12) subdivides the measuring range (MR) with the aid of the first measuring signal (M1) into uniqueness ranges over which the second measuring signal (M2) is a unique function of the measured variable (I), and the third measuring signal (M3) is composed of the branches of the second measuring signal (M2) over these uniqueness ranges.
11. Arrangement according to one of Claims 8 to 10, in which the signal processing unit (12) contains a previously determined table of values for the purpose of deriving the third measuring signal (M3) from the first measuring signal (M1) and the second measuring signal (M2).
12. Arrangement according to one of Claims 8 to 10, in which the signal processing unit (12) contains arithmetic means for calculating the third measuring signal (M3) from the first measuring signal (M1) and the second measuring signal (M2).
13. Arrangement according to one of Claims 8 to 12 for measuring an electric current (I) from a prescribed current measuring range (MR) in a conductor (2), in which a) the first measuring device (5) contains a first Faraday element (3) surrounding the conductor (2) and contains a first evaluation unit (7) which derives the first measuring signal (M1) for the current (I) from a rotation of the polarization plane of a first linearly polarized light signal (L1) after the latter has traversed the first Faraday element (3) at least once, b) the second measuring device (6) contains at least a second Faraday element (4) surrounding the conductor (2) and contains a second evaluation unit (8) which derives the second measuring signal (M2) for the current (I) from a rotation of the polarization plane of a second linearly polarized light signal (L2) after the latter has traversed the second Faraday element (4) at least once.
14. Arrangement according to Claim 13, in which a) the first Faraday element (3) is a common component both of the first measuring device (5) and of the second measuring device (6), b) the two Faraday elements (3, 4) are connected optically in series via optical connecting means (14, 15, 16), and c) the second light signal (L2) traverses the first Faraday element (3) and the second Faraday element (4) at least once in each case.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE19544778A DE19544778A1 (en) | 1995-11-30 | 1995-11-30 | Method and arrangement for measuring a measured variable, in particular an electrical current, with high measurement resolution |
| DE19544778.6 | 1995-11-30 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2238971A1 true CA2238971A1 (en) | 1997-06-05 |
Family
ID=7778891
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002238971A Abandoned CA2238971A1 (en) | 1995-11-30 | 1996-11-18 | Process and device for measuring a quantity, in particular an electric current, with a high measurement resolution |
Country Status (5)
| Country | Link |
|---|---|
| EP (1) | EP0864098B1 (en) |
| JP (1) | JP2000501178A (en) |
| CA (1) | CA2238971A1 (en) |
| DE (2) | DE19544778A1 (en) |
| WO (1) | WO1997020222A2 (en) |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0963557A1 (en) * | 1997-02-28 | 1999-12-15 | Siemens Aktiengesellschaft | Circuit to measure electrical measurement variables by means of light signals of differing wavelengths |
| EP0905522A3 (en) * | 1997-09-30 | 2000-04-19 | Siemens Aktiengesellschaft | Device for measuring a measurement value from a predetermined measuring range |
| JP2002533682A (en) * | 1998-12-22 | 2002-10-08 | シーメンス アクチエンゲゼルシヤフト | Method and apparatus for optically detecting current with optical signals having different wavelengths |
| DE10021669A1 (en) | 2000-05-05 | 2001-11-08 | Abb Research Ltd | Fiber optic current sensor |
| DE10158836B4 (en) * | 2001-11-30 | 2007-06-06 | Infineon Technologies Ag | Method and device for calibrating a sensor system |
| DE112015000886T5 (en) * | 2014-02-21 | 2016-11-10 | Abb Schweiz Ag | Interferometric sensor with differential modulated phase detection |
| CN106062506B (en) | 2014-02-21 | 2021-06-01 | Abb电网瑞士股份公司 | Interferometric sensor |
| FR3067162A1 (en) * | 2017-06-06 | 2018-12-07 | Supergrid Institute | CONNECTING CABLE FOR HIGH-VOLTAGE DC-CONTINUOUS NETWORK, DEVICE AND METHOD THEREOF |
Family Cites Families (11)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE2653391C2 (en) * | 1976-11-24 | 1981-12-10 | Standard Elektrik Lorenz Ag, 7000 Stuttgart | Circuit arrangement for mapping electrical signals or functions with the aid of an electron beam oscilloscope |
| FR2492986A1 (en) * | 1980-10-29 | 1982-04-30 | Comp Generale Electricite | Faraday effect fibre optic ammeter - has optical fibre wound around conductor transmitting light with progressively rotating polarisation |
| DE3141325A1 (en) * | 1981-10-17 | 1983-04-28 | BBC Aktiengesellschaft Brown, Boveri & Cie., 5401 Baden, Aargau | Method for measuring the current on an electric conductor by means of the Faraday effect |
| US4564754A (en) * | 1982-03-08 | 1986-01-14 | Hitachi, Ltd. | Method and apparatus for optically measuring a current |
| FR2584810B1 (en) * | 1985-07-09 | 1987-10-16 | Enertec | METHOD AND DEVICE FOR EVALUATING AN ANGLE ON AN EXTENDED RANGE |
| US4698497A (en) * | 1986-05-22 | 1987-10-06 | Westinghouse Electric Corp. | Direct current magneto-optic current transformer |
| DE3829103A1 (en) * | 1988-08-27 | 1990-03-01 | Philips Patentverwaltung | OPTICAL SENSOR ARRANGEMENT |
| US4904931A (en) * | 1988-09-28 | 1990-02-27 | Westinghouse Electric Corp. | Electro-optical voltage measuring system incorporating a method and apparatus to derive the measured voltage waveform from two phase shifted electrical signals |
| DE4312184A1 (en) * | 1993-04-14 | 1994-10-20 | Siemens Ag | Optical measuring method for measuring an electrical alternating current with temperature compensation and device for carrying out the method |
| US5811964A (en) * | 1993-10-01 | 1998-09-22 | Siemens Aktiengelsellschaft | Method and device for measuring an electrical alternating quanity with temperature compensation |
| DE4337857C2 (en) * | 1993-11-05 | 1997-09-18 | Felten & Guilleaume Energie | Method for evaluating the rotation of the polarization plane of a transducer |
-
1995
- 1995-11-30 DE DE19544778A patent/DE19544778A1/en not_active Withdrawn
-
1996
- 1996-11-18 JP JP9520056A patent/JP2000501178A/en active Pending
- 1996-11-18 EP EP96945546A patent/EP0864098B1/en not_active Expired - Lifetime
- 1996-11-18 CA CA002238971A patent/CA2238971A1/en not_active Abandoned
- 1996-11-18 DE DE59604258T patent/DE59604258D1/en not_active Expired - Fee Related
- 1996-11-18 WO PCT/DE1996/002184 patent/WO1997020222A2/en active IP Right Grant
Also Published As
| Publication number | Publication date |
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| DE19544778A1 (en) | 1997-06-05 |
| EP0864098A2 (en) | 1998-09-16 |
| WO1997020222A3 (en) | 1997-07-31 |
| JP2000501178A (en) | 2000-02-02 |
| WO1997020222A2 (en) | 1997-06-05 |
| EP0864098B1 (en) | 2000-01-19 |
| DE59604258D1 (en) | 2000-02-24 |
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