WO2008030129A2

WO2008030129A2 - Sensor and procedure for measuring bus bar current with skin effect correction

Info

Publication number: WO2008030129A2
Application number: PCT/RS2007/000016
Authority: WO
Inventors: Radivoje Popovic
Original assignee: Radivoje Popovic
Priority date: 2006-09-06
Filing date: 2007-09-05
Publication date: 2008-03-13
Also published as: RS20060511A; WO2008030129A3

Abstract

The invention belongs to the field of devices and processes for measurement of electrical quantities and more precisely refers to measuring sensors and the procedures for measuring bus bar current with skin effect correction. The invention in the subject solves the problem of designing a measuring magnetic current sensor, the signal-primary current ratio of which is frequency independent in a relatively wide frequency range. This is achieved so that the invention in the subject explains the measuring procedures and the embodiments of the measuring sensor with skin effect correction in the bus bar as the following: with asymmetric positioning of the magnetic sensor with respect to the bus bar (1), reduction of lateral dimensions of the bus bar (1) in the vicinity of the magnetic sensor (2), and the implementation of a magnetic filter (4), the implementation of an open magnetic yoke (5), and the implementation of an electric filter (6) and the combination of two or more above mentioned methods.

Description

SENSORAND PROCEDUREFORMEASURING BUS BARCURRENT WITHSKINEFFECTCORRECTION

Field of Invention

Subject matter of invention in general belongs to the field of devices and procedures for measuring electric quantities, and which contain various measuring error compensation devices and more precisely it refers to a sensor for measuring electric current with compensating elements that decrease influence of the change of frequency and of the skin effect on the accuracy of measurement.

According to the Seventh Edition of the International Patent Classification (IPC⁷) this invention is classified and assigned the main classification symbol G01 R 3/00 that defines devices and procedures particularly adjusted for production of measuring instruments, and secondary classification symbols G 01 R1/38 that include instruments for altering the indicating characteristics, i.e. G 01 R 1/20 that refer to the modifications of basic electric elements for use in electric measuring instruments. Due to the specific method of operation, this invention may be classified with another secondary classification symbol G 01 R 33/07 defining Hall-effect devices.

Technical Problem

The subject of invention solves a technical problem that consists of the following: how to design a magnetic sensor for measuring electric current in a bus bar, where its transduction ratio, i.e. the signal- primary current ratio shall be frequency-independent and in a relatively wide frequency range and therewith of small dimensions and weight, and shall have a good linearity, i.e. it shall be reliable, easily applicable, and applicable for DC and AC with frequencies of several hundreds KHz and high electric current peaks.

Background of the Invention

Modern electronic systems are based on modules that include many functions (e.g. analog-digital elements and power electronics parts, all in one integrated circuit, the so called 'smart power') that are used for regulation of electric current and are even more implemented in electrically driven drives, for accurate positioning and moving control, and for regulation of force and moment. Moreover, they enable good control of these devices, but also contribute to energy saving. Functionality of such controllers is based on current sensors which have the ability of accurate current monitoring.

Magnetic current sensors that contain a primary conductor and a magnetic sensor are in most frequent use lately. There are two main classes of magnetic current sensors: sensors without magnetic feed-back (open - loop) and sensors with magnetic feed-back (closed-loop). (Those two classes of electric current sensors are described for example in the book by R.S. Popovic titled ¹HaII Effect Devices' published at the Institute of Physics Publishing, Bristol 2004, pages 316-317). Here, to sum up briefly, only magnetic current sensors without magnetic feed back (open-loop) shall be shortly described. Through a primary conductor there flows a measured electric current, that we call primary electric current. The magnetic sensor measures a magnetic field created by the primary electric current, that we call primary magnetic field. Thereto the output signal of magnetic sensors is proportional to a primary electric current within a certain frequency range. Hall-effect sensors are used as magnetic sensors, as are magneto-resistant sensors, coils or their combinations (Hall magnetic sensors are well-known, the above said book is an example, (Chapter 5). Hall or other magnetic sensors may be associated with ferromagnetic parts, that we call magnetic cores or magnetic concentrators (magnetic cores and magnetic concentrators are shown in the mentioned book on pages 316-317 and 292-293, respectively).

Electric currents in power systems usually flow through copper or aluminum ribbon-like conductors, called bus bars, therefore there exists a great practical interest that the measuring of electric current is carried out directly on them, whereas the bus bar is used as a primary conductor of an electric current sensor. There should be noted that the measuring of a magnetic field may be achieved in a way that measuring is carried out in the vicinity of the bus bar. Following the state-of-art, it is well-known that, due to the skin effect, distribution of electric current density in the bus bar depends on the frequency of primary electric current. In the bus bar with a cross section that is not circular, at very high frequencies, the electric current tends to concentrate next to the far edges of the cross section of the bus bar, thus the density of the electric current decreases in its central part. The paper titled "The lateral skin effect in a flat conductor", Philips tech. Rev.32, pg.221-231, no. 6/7, 18 from 1971, by V, Velevich, describes this phenomenon in more details. On the grounds of the state-of-art, the problem of eliminating errors at measuring, depending on the change of frequency is solved by averaging the magnetic signal along the closed path around the bus bar by the using several magnetic sensors, the Rogovski coil, or by application of a closed magnetic circuit. Examples of accomplishments of this kind of magnetic current sensors are given in the book titled "Hall Effect Sensors", by Edvard Ramsden, published by Elsevier, Amsterdam, 2006, pg.136-141. The most frequent application of a closed magnetic circuit in the form of a toroid that completely encloses the primary conductor. This method is in use with current transformer, flux-gate current sensors and Hall current sensors. In the last case, magnetic core has a quite small gap about 1mm wide, where the Hall magnetic sensor is located. Magnetic core always has approximately uniform cross section all along its circumpherence. However, according to the opinion of the author of the invention explained here, current sensors designed in this way have large dimensions and weight, and have a limited frequency and pulse response, and do not have sufficient linearity, and have the problem of perming (offset change upon strong electric current impulses), and they are expensive.

As the principle of functioning of the invention is partially based on a phenomenon of eddy currents, for better comprehension the author refers to the relevant theory and examples given in the book by Zoja Popovic and Branko D. Popovic, titled as "Introduction Electromagentics" published by Prentice Hall, Upper Saddle River, New Jersey, 2000, pages 247-248.

It is surprising that, besides detailed searching of domestic and international patent documentation relating to measuring devices with bus bars and shunts, neither a single patent file nor a patent application that would be of relevance to the subject of the invention, was found.

Summary of the Invention

Herewith described subject of invention completely solves the above defined technical problem. The subject matter of this invention is that, according to the author's idea, a magnetic current sensor is designed so that in itself it comprises of a bus bar, most favorably of rectangular cross section, and with at least one magnetic sensor that successfully solves the problem of frequency dependence in a relatively broad frequency range.

This problem solves the implementation of the following features that will be separately explained through the examples of embodiment: (one by one) I. Asymmetrical positioning of magnetic sensor with respec to the bus bar

II. Reduction of lateral dimensions of the bus bar in the vicinity of the magnetic sensor

III. Application of magnetic filter

IV. Application of open magnetic yoke

V. Application of electric filter

VI. Combination of two or several above mentioned features.

According to feature I, invention in the subject comprises a magnetic sensor set asymmetrically with respect to the axial axis of the wide side of the bus bar. Here, the change of magnetic field depends on frequency and due to the skin effect, it is less than in the case when the sensor is positioned symmetrically.

Even better effect is obtained when two magnetic sensors are used, where the first one is positioned next to the axial axis, and the other one next to the bus bar edge. Principle of functioning of this solution is based on the following: as that frequency of the primary electric current increases, the local magnetic field that acts on the first magnetic sensor, decreases, while the local magnetic field that acts on the other magnetic sensor, increases. In that case, the sum of output signals of two magnetic sensors remains relatively independent from the frequency of the primary current.

According to the feature II, of the subject of the invention, the cross section of a part of a flat bus bar, where the magnetic sensor is positioned, may be of reduced lateral dimensions. In that way, a part of the cross section has a shape approximate to a square, a circle or is divided into two parts. Such forms of cross section are less subject to skin effect than in the case of a flat rectangular shape. Changing of cross section may be achieved in such a way that one or more holes of certain size and shape are formed at an appropriate place in the bus bar.

According to the feature III the invention in the subject comprises a bus bar combined with a magnetic filter. The magnetic filter is made of a piece of electric highly-conductive material (for example, copper and aluminum). The magnetic filter in this case causes frequency dependent change of the primary magnetic field. This occurs as a result of induction of eddy currents in the magnetic filter. Now the magnetic sensor acts to a resultant magnetic field generated by superposing magnetic fields produced by primary current in the bus bar and eddy currents in the magnetic filter, Within this feature, there is a possible construction where the mutual positions of the magnetic filter and the magnetic sensor are parallel. The magnetic filter acts as a high-pass filter, so that it helps to compensate the reduced sensitivity of the current sensor to high frequencies. Also possible is an embodiment in which the mutual position of the magnetic filter and the magnetic sensor is perpendicular. In that case the magnetic filter acts as a low-pass filter and helps to compensate increased sensitivity of the current sensor to high frequencies.

As to the method IV, invention in the subject consists of a bus bar combined with an open magnetic yoke. The open magnetic yoke is at least one plate of ferromagnetic material that does not enclose the primary conductor entirely, but only a part of it. This plate is positioned almost parallel to a wide flat side of the bus bar. The presence of an open magnetic yoke decreases the influence of the skin effect in the bus bar on the result of measuring. The proper choice of shape of the magnetic yoke may help to achieve that the local density of magnetic flux, which the magnetic sensor measures, depends very little on the current frequency through the bas bar.

As in the case of the magnetic filter, there is a possibility of constructions where the mutual positions of the magnetic yoke and the magnetic sensor are both parallel and perpendicular. A parallel magnetic yoke functions also as a magnetic deconcentrator and may broaden a scope of functions of the current sensor and thus improve its linearity. The perpendicular magnetic sensor functions even as a magnetic concentrator and may increase the sensitivity of the current sensor. Both the parallel and perpendicular magnetic yokes may improve the immunity of the current sensor to the external magnetic interferences. According to feature V, invention in the subject includes a magnetic sensor combined with a standard electric filter (in the explanation of applicability a RC filter is shown). In the first case, if a magnetic signal that impacts the magnetic sensor decreases, due to skin effect in the bus bar, the output signal of the magnetic sensor is let through a high-pass electric filter, that compensates the loss in magnetic signal. In the second case, when a magnetic signal that acts on the magnetic sensor, increases due to the skin effect in the bus bar, the output signal of the magnetic sensor passes through a low-pass electric filter that compensates the increase of magnetic signal.

According to method Vl, subject of the invention comprises a combination of two or more above mentioned features. For example, a current sensor may have two asymmetrically positioned magnetic sensors, two magnetic filters, two magnetic yokes, and one electrical filter. By combination of several above mentioned features and methods, a frequency-independent transduction ratio can be achieveed in a more extended frequency domain than the case is with the implementation of only one of these features.

In comparison to existing solutions of current sensors, the current sensors manufactured according to the explained invention, have several advantages and the most important ones among them are the following:

- Small dimension and mass of measuring sensor

- Frequency-independent transduction ratio in a wide frequency range, from DC to high frequency AC.

- Successfulness of measurement even at high current peaks

- Great linearity and absence of or only a small perming

- Relatively simple construction for manufacturing and packaging.

Brief Description of the Drawings

For easier understanding of the invention as well as for demonstration of its practical realization, the author, only as an example, points at the enclosed figures of the drawing that refer to the application in subject and where:

Fig. 1 represents a cross section of the subject of invention with asymmetric position of the magnetic sensor,

Fig. 2 represents an appearance of the subject viewed from above,

Fig. 3 represents a magnetic current sensor with a hole in the bus bar,

Fig.4 represents a magnetic current sensor with a hole, viewed from above,

Fig. 5 represents the subject of the invention with a parallel magnetic filter,

Fig. 6 represents the subject of the invention with a parallel magnetic filter, viewed from above,

Fig. 7 represents a view of a DC magnetic flux with a parallel magnetic filter,

Fig. 8 represents a view of an AC magnetic flux with a parallel magnetic filter,

Fig. 9 represents a subject of invention for DC with a magnetic sensor and a magnetic filter positioned near the edge of the bus bar,

Fig. 10 represents the subject of invention for AC with a magnetic sensor and a magnetic filter positioned near the bus bar, Fig.11 represents a cross section of the subject of invention with a magnetic yoke,

Fig. 12 represents a magnetic current sensor with an electric filter and amplifier,

Fig. 13 represents a cross section of the measuring sensor with a hole and a magnetic filter,

Fig. 14 represents a view of the measuring sensor with a hole and a magnetic filter viewed from above,

Fig. 15 represents the subject of invention where a magnetic filter is achieved as a part of the bus bar,

Fig. 16 represents a cross section of the subject of invention with lateral notches and magnetic filter,

Fig. 17 represents a cross section of the subject of invention with lateral notches, magnetic filter and magnetic yoke,

Fig. 18 represents the lateral cross section of the subject of invention with lateral notches, a magnetic filter and a magnetic yoke the lateral projections of which are positioned in lateral recesses,

Fig. 19 represents a cross section of the subject of invention with lateral notches, a magnetic yoke and a magnetic filter set in the lateral notches,

Fig. 20 represents a view of the subject of invention with lateral recesses and magnetic filter viewed from above,

Fig. 21 represents a view of the subject of invention with lateral recesses and a magnetic yoke viewed from above.

Fig. 22 is a schematic representation of a measuring sensor where all five already described features (I-V) of the subject of invention are in use,

Fig. 23 represents a graph of the transduction ratio Vo/lp of the current sensor where the features illustrated in fig.22 are in use.

Detailed description of the invention

Example 1

As it can be clearly seen in fig. 1 of the drawings, which represents the current sensor viewed from underneath that contains a bus bar in the form of ribbon 1 and at least one magnetic sensor 2 set next to the flat surface of the bus bar 1. In figs. 1 and 2 there are two electric sensors of that kind represented and these are 2a or 2b. Bus bar 1 in this example of embodiment is made of copper, (for a maximum electric current of 500 A, the dimensions of the bus bar are approximately as follows: thickness T=5 mm, width 50 mm, and length at least 120 mm). In this example, a Hall or magneto-resistant magnetic sensor is used a magnetic sensor. Primary current Ip flows along the bus bar 1 creating a vector of primary magnetic flux density Bp. The corresponding function of the electric sensor, in this case, is to make the output signal of magnetic sensor 2 proportional to the primary current Ip in a defined frequency range, usually from DC up to the limit frequency fc ≥ 50KHz.

The magnitude and direction of vector Bp depends on the magnitude and frequency of the primary current

Ip and on its position with respect to the bus bar 1. Arrows Sa and Sb indicate the directions of the maximum magnetic sensitivities of the magnetic sensors 2a or 2b, respectively. It can be seen from the figure that Sa and Sb are approximately parallel to the nearest surface of the bus bar 1. At the position of sensor 2a, the local component of the magnetic flux density Bp, parallel to Sa, is the magnetic signal Bsa,

The magnetic sensor 2a converts the magnetic signal Bsa into an electric signal that represents the current Ip. The output of magnetic sensor 2b is related to magnetic signal Bsb through the appropriate analog function.

If the primary current Ip is a DC (direct-current) or a very low frequency AC (alternating current), the density of the current in a cross section of the bus bar is uniform. Then the largest density of magnetic flux

Bp occurs in the middle of a wide flat surface of the bus bar 1. In the figures, that place is represented by lines marked as z1 and z2, where the plane x=0 intersects with a wide flat surface of the bus bar 1. When the frequency of primary current Ip increases, then due to skin effect, the electric current tends to concentrate itself near the far edges of bus bar 1. As a result, the magnetic flux density on lines z1 and z2 decreases, and at the same time it increases near the edges of the bus bar 1 z3 and z4,

As it can be seen in fig. 1, to accomplish a current sensor according to this invention, the magnetic sensor

2 is used. So as to reduce the frequency dependence of magnetic signal Bsa that acts as magnetic sensor

2, it is positioned asymmetrically with respect to the axial axis z1 of the bus bar. As represented in this figure, the sensor is positioned close to the flat surface of the bus bar 1 , between the lines z1 of the edge of the bus bar z3.

The optimal position of the sensor depends on the relation W/T of the sides of bus bar 1 (for a bus bar having closed dimensions W= 50 mm and T = 5 mm, the optimal position is around the point Xa = 5mm,

Ya=2mm.

There should be noted that better results by far are achieved if two magnetic sensors 2 are used, in figure

1 represented as 2a and 2b. Double magnetic sensors contribute to the increase of the signal-noise ratio of the electric current sensor. Moreover, there should be mentioned that the use of two magnetic sensors, oppositely oriented, with sensitivities Sa and Sb respectively, that are positioned to two opposite sides of bus bar 1, contribute to reduction of the influence of external magnetic fields. Therewith, for the purpose of compensation of frequency dependence due to skin effect in bus bar 1 , the first magnetic sensor 2a is positioned near the symmetry line z1, while the other magnetic sensor 2b is positioned near the edge z3. As the frequency of primary electric current Ip increases, the magnetic signal Bsa that acts on the first magnetic sensor is reduced, while the magnetic signal that acts on the second magnetic sensor Bsh, increases. In that case, the sum of the output signals from these two magnetic sensors remains relatively independent from the frequency of the primary electric current. (For the bus bar having dimensions W=50mm and T=5mm, the best positions of two magnetic sensors 2a and 2b are approximately given by Xa = 5 mm, Ya = 2 mm, Xb= 20mm, Yb = 2 mm.

Example 2

Fig. 3 and 4 show a sensor made according to the invention, accomplished with a bus bar, where a hole of circular cross section exists. In this example, there is a magnetic current sensor where fig. 3 represents its cross section, and fig.4 represents a view from above of the wide flat surface of the bus bar. The magnetic current sensor contains a copper ribbon as bus bar 1 and a magnetic sensor 2 which is positioned near the flat surface of the bus bar 1. On bus bar 1, there is a circular hole 3 (even though it may be oval, polygonal and etc.). Hole 3 is intended to reduce lateral dimensions of cross section of the bus bar in the vicinity of the spot where the magnetic sensor is positioned. This part of the cross section is divided into two lateral segments having approximately square cross section 1a and 1b. Primary current Ip is then locally divided in two halves, lp/2. The skin effect does not alter the distribution of the current in parts of bus bar with reduced cross section 1a and 1b. Magnetic signal Bs that in this case acts on magnetic sensor 2 is in that way less frequency dependent than in the case of the same bus bar without the hole. The optimum dimension of the hole 3 (diameter D) is one that results in width W1 for parts 1a and 1b, approximately equal to the thickness of bus bar T.

Decrease in the lateral dimensions of the flat bus bar 1, may be achieved by punching one or more holes in the bus bar. As given in this example, hole 3 is positioned along the symmetry line of the bus bar 1. It is required to mention that two symmetrical holes may be punched near the edge of the bus bar 1 and then these may seem as two recessed sides of the bus bar 1. The role of the hole, notches or similar modifications of the bus bar, which may be of different shape, is to make that part of the bus bar similar to a cylinder or a multi-wire cable and thus the distribution of flux is less affected by the skin effect.

Example 3

In this example of embodiment, in figures 5 and 6, there is shown a magnetic current sensor that comprises a bus bar 1, in the form of a copper ribbon, and at least one Hall or magneto-resistant magnetic sensor 2 and at least one magnetic filter 4, As to the idea of the author of this invention, magnetic sensor 2 is positioned between the flat surfaces of the bus bar 1 and the magnetic filter 4.

The magnetic filter 4, as in this example, is a piece of electrically highly conductive material, such as copper or aluminum. The magnetic filter may also be made even of electrically conductive ferromagnetic material, such as iron, i.e. it may also consist of a combination of ferromagnetic and highly conductive, non-ferromagnetic material. It may have different forms (cylinder, prism, tube, ring, short-circuited coil or a combination thereof). The magnetic filter 4, as shown in figures 5 and 6, is positioned near the bus bar 1.

Therefore, it may be part of the bus bar 1 or not; and may be either electrically insulated from bus bar 1 , or not insulated.

The magnetic sensor 2 is positioned near the magnetic filter 4. From figs. 5 and 6 it can be seen that the magnetic sensitivity vector of the magnetic sensor, represented by arrow S, is parallel to the nearest surface of the magnetic filter 4. Such a mutual position of the magnetic filter and the magnetic sensor is called a parallel position.

In order to improve comprehension of the invention, an explanation of the principle of operation of the sensor, according to the example with a parallel non-ferromagnetic filter, is given.

In figures 7 and 8, a part of the magnetic current sensor is shown, in the arrangement of a magnetic sensor 2 and a magnetic filter 4, as it may be seen in fig. 5, above the horizontal symmetry line x. From these figures it may be clearly seen how the device works, by showing the distribution of the primary current Ip, the primary magnetic flux lines Φ and the eddy currents Ie.

In fig. 7, the case where the primary current Ip is a direct current (DC), is shown. Then, it is uniformly distributed over the cross section of the bus bar 1. The primary current creates a magnetic field around the bus bar 1 which is illustrated by the lines of magnetic flux Φ, which is also DC, so that its distribution is not influenced by the presence of a non ferromagnetic filter 4. The magnetic sensor 2, in this case, measures the local density of magnetic flux Bs.

In figure 8, the case where the primary current is an alternating current (AC), is shown. Due to the skin effect, the primary electric current Ip tends to concentrate at the peripheries of the bus bar 1. Therefore, if there is no magnetic filter, the local density of magnetic flux Bs, that acts on the magnetic filter 4 and induces eddy currents Ie that produce a secondary magnetic field (not shown in the figures) that is opposite to the primary magnetic field inside the magnetic filter 4,

This makes the magnetic filter 4 non-transparent for primary magnetic flux and therefore the lines of this flux circumvent the magnetic filter 4 and tend to concentrate in the space where the magnetic sensor 2 is placed. This means that when the magnetic sensor 2 is in parallel configuration, the magnetic filter 4 functions as a partial high-pass filter for the primary magnetic flux. In that way, the compensation of reduction of Bs at high frequencies, due to the skin effect in the bus bar, is achieved. With suitable geometry of the system, the bus bar 1 , the magnetic filter 4 and the magnetic sensor 2, and the function

Bs(f) may be made almost constant in small frequency range,

The described magnetic filter in this example of embodiment is given for the purpose of confirm the applicability of the invention for the following dimensions Tf = 5 mm, Lf = 25 mm, Wf = 25 mm.

If the magnetic filter 4 is made of ferromagnetic material, then the magnetic filter also acts as a magnetic yoke, described in the following example 4.

In figures 9 and 10 another way of embodiment of the subject of invention, according to example 3, is given. The magnetic sensor in figure 7 comprises a copper bus bar 1 in the shape of a ribbon, at least one

Hall or magneto-resistant magnetic sensor 2 and at least one magnetic filter 4, but now the magnetic sensor 2 and the magnetic filter 4 are positioned close to the edge of bus bar 1 and the mutual position of these two is different. Now, the magnetic sensitivity vector of the magnetic sensor 2, is represented by arrow S, and is perpendicular to the nearest surface of magnetic filter 4. Such a position of the magnetic filter 4 and the magnetic sensor 2 is called perpendicular.

And in this case, there is a need to explain the operation of this perpendicular non-ferromagnetic filter.

Figure 9 shows a case when the primary electric current Ip is a direct current (DC). Then, it is evenly distributed over the cross section of the bus bar 1. The primary electric current Ip creates a magnetic field around the bus bar 1 that is illustrated by the lines of magnetic flux Φ. The presence of a non- ferromagnetic filter 4 in this case has no influence on the distribution of the magnetic flux. The magnetic sensor 2 measures the local density of magnetic flux Bs.

Figure 10 represents a case when the primary electric current is an alternating current (AC). Due to the skin effect in this case, the primary electric current Ip tends to concentrate at the periphery of bus bar 1. Due to this, if there was no magnetic filter 4, the local density of the magnetic flux Bs would be higher than in the case of a direct primary electric current; however, when there is only a magnetic filter, the eddy currents Ie are induced in the magnetic filter 4 and generate a secondary magnetic field. This makes the magnetic filter 4 actually non-transparent for the primary magnetic flux. Hence, the lines of the primary flux Φ pass around the magnetic filter 4, and also partially pass around the magnetic sensor 2, also. Therefore, when magnetic sensor 2 is in perpendicular configuration, the magnetic filter 4 fulfills its function as a partial low-pass filter for the primary magnetic flux. This effect may be a compensation for the increase of Bs at high frequencies. Suitable geometry of the assembly of bus bar 1, the magnetic filter 4 and the magnetic sensor 2, the function Bs (f) may be made almost constant in a small frequency range.

Both parallel and perpendicular magnetic filters may act even as a shield against electric and electromagnetic fields. Therefore, the shape of the magnetic filter 4 may be adjusted for the purpose of electromagnetic protection. For that reason, the magnetic filter 4 may be electrically connected to a point of an electronic circuit of the electric current sensor which has a suitable electric potential, for example to the ground. Thus, the magnetic filter 4 may help to improve the immunity of the electric current sensor to , capacitive and parasite magnetic yoke conjunction of the bus bar/ magnetic sensor; and improve immunity of the electric sensor to external electromagnetic interferences.

Example 4

In this example of embodiment in figure 11, there is a magnetic sensor that consists of a bus bar in the form of a copper ribbon, at least one Hall or magneto-resistant magnetic sensor 2 (2a or 2b) and at least one magnetic yoke 5 (5a or 5b).

Magnetic yoke 5 is a piece of soft ferromagnetic material (material with a great permeability and small coercive field). It may be ferrite, iron, iron-nickel alloy, iron-silicon, and alike, and may have a homogenous structure or a shape of a package from thin metal sheets. It is recommended that the magnetic yoke 5 be a plate of approximately equal thickness, and a different shape, such as a rectangle, circle and ellipse. According to fig. 11 such a plate is positioned approximately parallel to the wide flat side of the bus bar. Dimensions (both the number and position, if there are more than one) of the magnetic yoke are such that it may cover the whole wide surface of the bus bar, or at least its one smaller part. Therefore, it is important that the magnetic yoke 5 is open, i.e. that it fills up only a part (around 90%) of the length of the magnetic circuit around the bus bar. At the example shown in fig. 11, it is achieved by making the distance between two magnetic yokes 5a and 5b longer than the thickness of the bus bar, and it is a few millimeters.

In this example, the magnetic sensor 2 (2a and/or 2b) is positioned close to the edge of bus bar 1 , and its magnetic sensitivity vector, is represented by a little arrow S (Sa and Sb), and is perpendicular with respect to the nearest surface of magnetic yoke 5. Such a position of the magnetic yoke 5 and the magnetic sensor 2 is called perpendicular, In the case of perpendicular configurations magnetic yoke - magnetic sensor, according to figure 11, it is favorable that the magnetic sensor 2 (2a and 2b) and the bus bar 1 are positioned between the flat surfaces of two magnetic yokes 5a and 5b. Similar to the case of a magnetic filter, it is possible to achieve a construction where the mutual positions of the magnetic yoke and the magnetic sensor are parallel. Therefore, figures 5, 6 and 11 illustrate at the same time the application of both the magnetic filter 4 and the magnetic yoke 5. In other terms, in these figures, identification 4 may be replaced by identification 5 and vice versa.

In the presence of an open magnetic yoke, the influence of the skin effect in the bus bar on the result of the primary current measuring is reduced. That happens due to the fact, that the magnetic yoke averages the magnetic field along its length in the direction of field. That effect is similar to the one that exists in already known electric current sensors with an open magnetic core. The significant difference lies in the fact, that in case of this invention, the magnetic yoke does not completely enclose the bus bar, i.e. here the magnetic core is wide open. In other terms, in this case, the magnetic circuit comprises relatively more twin-irons than in the case of known electric current sensors with a closed magnetic core. Therefore, the influence of the nonlinear magnetizing characteristic B(H) of the magnetic yoke on the linearity of the electric current sensor and the perming is insignificant.

Besides the favorable influence on the frequency response, an open magnetic yoke may have several other favorable effects on other characteristics of electric current sensors. For example, a parallel magnetic yoke as in fig. 5, functions as a magnetic de-concentrator. (A magnetic de-concentrator is an element the effect of which is opposite from the effect of a magnetic concentrator: magnetic de- concentrator rarifies lines of magnetic flux that pass through an associated magnetic sensor.) The effect of magnetic de-concentration results from the magnetic yoke 5 that focuses a part of the magnetic flux into itself, and therewith reduces the density of magnetic flux that acts on the magnetic sensor 2. This is favorable in the case of use of high sensitivity magnetic sensors, because this can reduce the problem of saturation of magnetic sensors and thus to extend the range of functions of the current sensor and improve its linearity. Opposite to that, the perpendicular magnetic sensor, as in fig. 11, acts as a magnetic concentrator and may increase the sensitivity of the current sensor. Both parallel and perpendicular magnetic yokes may act as a shield against external magnetic flux and therewith improve the immunity of electric current sensors to external magnetic interferences.

If magnetic yoke 5 is made of electrically conductive ferromagnetic materials, then the magnetic yoke acts as a magnetic filter, described above in Example 3.

Example 5

According to the inventor's idea, figure 12 shows an example of embodiment of a magnetic current sensor including a bus bar 1 in the form of a ribbon, a magnetic sensor 2, an electric filter 6 and an amplifier 7. Mutual positions and functions of the bus bar 1 and the magnetic sensor 2 are given in figures 1 and 2, in detail. The output terminals of magnetic sensor 2, are connected to the input terminals + and - of the amplifier 7 that may be of type INA217, production Burr-Brown. The electric filter 6 may have different forms, including a serially connected resistor R and capacitor C. The output voltage of the current sensor is given by

Vo=G x Vs (1) where G is the gain of the amplifier 7 and Vs is the output signal of the magnetic sensor 2. Usually, the gain of such amplifiers depends on a resistor Rg that is connected between the G1 and G2 input terminals of the amplifier. In the given case, the gain of such amplifiers is expressed by means of a expression

G= (1+ 10 kΩ) /Z (2) where Z is the impedance of a circuit connected between + and - terminals, where

Z=Rg II Zf (3) and where the symbol || stands for "connected parallel to", and Zf is the impedance of the electric filter 6. If the primary current is DC or very low frequency AC₁ the equations (2) and (3) determine the the DC gain

Gdc = 1 + 10kΩ/Rg (4)

If it happens that due to the increase of the primary current frequency the skin effect in the bus bar 1 causes the reduction of the magnetic signal Bs, however, the corresponding reduction of the output signal Vs of the magnetic sensor 2 would be compensated by the increase of the amplifier gain (equation (2)), which is achieved by reducing the impedance Zf of the electric filter 6. Therewith, function Vo (f) may be almost constant within a small frequency domain. For very high frequency values of primary current Ip, the equations (2) and (3) give the high frequency AC gain

Gac = 1 + 10kΩ / (Rg || R) (5)

In the example of embodiment of the invention for a bus bar with dimensions W = 50 mm and T= 5 mm, where the magnetic sensor is positioned close to a symmetry line Z1 and Rg = 4kΩ, and the optimum parameters of the filter are approximately R = 10 kΩ and C = 100 nF.

Example 6

Within this example of embodiment, several variants of the subject of invention are given, along with the use of two or more measurements shown in examples 1 to 5.

Example 6a

In figures 13, 14 and 15 there is given an embodiment of combinations of a measuring sensor and a bus bar described in examples 2 and 3. Figure 13 shows a cross section, and figures 14 and 15 show the appearance of the surface of the bus bar 1 viewed from above. According to this example of embodiment, the bus bar 1 has a hole 3 where a magnetic filter 4 is mounted. In the figures, the hole 3 has a circular cross section, with a diameter D that makes approximately 1/3 of the width of the bus bar W. Magnetic filter 4 is cylindrical in shape and its diameter is insignificantly shorter than the diameter of the hole 3, while its height is approximately equal to the thickness T of the bus bar 1. As illustrated in figure 11, the magnetic filter 4 is made of copper and is electrically insulated from the bus bar 1.

The magnetic filter 4 may be achieved also as a part of the bus bar 1, as it is shown in figure 15. In this case, the magnetic filter 4 is electrically connected to the bus bar 1 by means of at least one of the bridges

4a and 4b. The position of the bridges should be chosen so that the primary current Ip does not pass through the magnetic filter. The measuring magnetic current sensor shown in figures 13, 14 and 15 has a principle of functioning that is a combination of the principles explained in examples 2 and 3 along with the note that the magnetic sensor 2 and the magnetic filter 4 are parallel to each other. In this case both the hole 3 and the magnetic filter 4 have a tendency to reduce decrease of the magnetic signal for high frequencies of primary current Ip. Therefore the transduction ratio Vo/lp of a current sensor of this type may be approximately constant in a broader frequency range than it is in examples 2 or 3. Example 6b

Figures 16 and 20 show an example of embodiment of the invention that represents a combination of an earlier version of already explained examples 2 and 3 and where figure 16 represents a cross section and figure 20, a view from above on the flat side of the sensor in question. The primary conductor is the bus bar 1 and in the form of a ribbon with a symmetrically narrowed part 1c. The narrowed part 1c is made by two symmetrical notches 3a and 3b, oppositely set with respect to both lateral sides of the bus bar 1. The function of the notches 3a and 3b is to locally reduce the width of the bus bar 1 and thereby the influence of the skin effect. In the notches 3 there is positioned at least one magnetic sensor 2, and in the example of embodiment of the subject of invention, it contains two, also oppositely set sensors 2a and 2b. These sensors are approximately positioned to the main symmetry line of plain xz of the bus bar 1. The magnetic sensor 2 is positioned in-between two plate-like magnetic filters 4a and 4b. Figures 17, 18 and 19 represent a magnetic filter 4a and 4b that may be combined with a magnetic yoke 5a and 5b in the form of plates made of a soft ferromagnetic material 5a and 5b. In this case the magnetic sensitivity S of the magnetic sensor 2 is perpendicular to the closest surface of each magnetic filter 4 and each magnetic yoke 5. When using more than one magnetic sensor 2, each of these can have its pair of magnetic filters 4 (4a and 4b in figures 18 and 19). It is noted that several magnetic sensors may be included in the same magnetic filter.

The work of measuring electric current sensors shown in figures 16 and 20 is based on the following: due to the skin effect, the primary electric current Ip tends to concentrate at the periphery of the bus bar 1. Therefore if there were no magnetic filter 4, the magnetic signal Bs that acts on the magnetic sensor 2, would be larger than in the case of a direct primary current. However, in the presence of a perpendicular magnetic filter, the secondary magnetic field (due to eddy currents in magnetic filters) tends to reduce the magnetic signal Bs due to which it comes to compensation of increase of Bs for high frequencies. The role of additional magnetic yokes 5a and 5b is explained below in the example 6c. Example 6c

Similar to the above said in Example 6b, in figure 21 is given the example of embodiment of a magnetic current sensor which is made of a bus bar 1 in the form of ribbon with a narrow part 1c and two symmetrical notches 3a and 3b. In the notches 3 there is positioned at least one magnetic sensor 2, and in the example of embodiment, the invention in the subject includes two oppositely positioned sensors 2a and 2b. These sensors are positioned approximately in the main symmetry line of the plane xz of the bus bar 1. The magnetic sensor is positioned in-between two magnetic yokes 5, as in figure 11. The magnetic yokes are plates of elliptic shape, so that the longer axis of the ellipse, x, is approximately perpendicular to the longitudinal z axis of the bus bar 1.

When the primary current is AC, due to the skin effect, the primary current Ip tends to concentrate in the peripheral parts of the bus bar 1. If there was no magnetic yoke 5, the local density of the magnetic flux Bs (is not shown in fig. 21) would be higher than in the case of a DC primary current. However, in the presence of magnetic yoke 5, the magnetic field along the axis of the yoke is averaged, and thus the influence of skin effect on the field Bs is weak. Besides, the magnetic yoke 5 has two additional functions: first - to intensify the magnetic signal Bs; and second - to increase independence of interferences originating from external magnetic fields. The elliptic shape of the magnetic yoke 5 is particularly suitable due to the fact that such a shape is the least inclined to local magnetic saturations. Besides, such a magnetic yoke acts also as a magnetic capacitor that focuses the available magnetic flux onto the magnetic sensor set near the extremities of the ellipse. These properties are also common with similar (elongated, and in the middle wider) forms of the magnetic yoke, such as the elongated hexagonal shape shown in fig.22.

Example 6d

In fig. 22 there is a schematically represented embodiment of the measuring sensor that includes all earlier examples of embodiment of the subject of invention. The shape of the bus bar of this current sensor is similar to the example shown in fig. 20 and substantial properties are the following:

Property 1: Two magnetic sensors are positioned asymmetrically with respect to the main axis z of the bus bar 1. Their function is given in the description and explanation of figure 1 , When there is a gain in frequency of the primary current Ip, the magnetic signal Bsa increases and Bsb decreases, therefore the sum of the output signals of magnetic signals 2a and 2b (not shown) is less dependent on frequency than in the case of a magnetic signal in the vicinity of the axis of symmetry of the bus bar.

Property II: The magnetic sensors 2 are positioned near the part 1c with reduced width of the bus bar 1.

The narrow part 1c is obtained by means of oppositely achieved notches 3. The lateral skin effect in the narrow part 1c of the bus bar 1, is less significant than in the bus bar 1 with normal width.

Property III: The magnetic sensor 2a is associated with a magnetic filter 4 in the perpendicular combination so that, as in the above explained way, it adds to the correction of the increase of magnetic signal Bsa due to the skin effect in the bus bar 1.

Property IV: The bus bar 1, integrated with magnetic yoke 5 in perpendicular combination so that, in the above described way, the magnetic yoke helps to decrease the change of the magnetic signal Bsa due to skin effect in the bus bar 1.

Property V: The output signal of the magnetic sensor 2a (Vs), increased by means of the amplifier 7 where the (RC) electric filter 6 is located. The electric filter 6 is achieved as a high-pass filter and connected serially to a resistor for reverse feed-back Rg1. At high frequencies, the electrical filter 6 causes reduction of the amplifier gain and in that way it influences the correction of the increase of the magnetic signal Bsa due to the skin effect in the bus bar 1.

If, along with the increase of frequency of the primary electric current Ip, the skin effect in the bus bar 1 causes increase of the transduction ratio Vo / Ip of the current sensor, as illustrated in figure 23 (above), then the application of some of the given characteristics I - IV of the invention in the subject, or of their combination, causes the transduction ratio to become less frequency dependent, as it is explained in figure

23 (after).

Method for Industrial Application of the Invention

The application of the invention is absolutely possible and recommendable, first of all in the manufacturing of sensors for the measurement of electric current in a bus bar, where it is required that the relation between the output signal and the primary current is frequency independent in a relatively wide frequency range, the dimensions and weight of that sensor being small.

Industrial manufacturing of the invention in subject is possible in specialized factories for electrical measuring devices. Experts from the technical field in the subject may easily produce drawings and other production documentation by using a description and drawings from the application in the subject.

Claims

1. Sensor and procedure for measuring bus bar current with skin effect correction, is characterized by, the fact that it contains a bus bar (1) in the form of a ribbon made of a highly conductive material and at least one magnetic sensor (2) asymmetrically positioned with respect to the axial axis of a wide side of the bus bar (1), next to its flat surface.

2. Sensor and procedure for measuring bus bar current with skin effect correction, is characterized by, the fact that besides the bus bar (1) it includes two magnetic sensors (2), where the first magnetic sensor (2) 2a is positioned close to the axial axis of the bus bar (1), while the second magnetic sensor (2) 2b is positioned close to its edge.

3. Sensor and procedure for measuring bus bar current with skin effect correction, is characterized by, the fact that the cross section of the part of the flat bus bar (1), at the spot where a magnetic sensor (2) is positioned, has reduced lateral dimensions, where this part of the cross section may be shaped approximately as a square, circle or be divided into two or more parts.

4. Sensor and procedure for measuring bus bar current with skin effect correction, is characterized by, the fact that in the bus bar (1) there are formed one or more holes (3), where the holes may be approximately circular, elliptic or polygonal and etc.

5. Sensor and procedure for measuring bus bar current with skin effect correction, is characterized by, the fact that two holes may be achieved close to the edges of the bus bar (1), so that these form notches (3a) and (3b) in the sides of the bus bar (1) and the narrow part of the bus bar (1c).

6. Sensor and procedure for measuring bus bar current with skin effect correction, is characterized by, the fact that it comprises one bus bar (1), at least one Hall or magneto-resistant magnetic sensor (2) and at least one magnetic filter (4) made of electrically non-conductive non- ferromagnetic material, an electrically conductive ferromagnetic material or a combination of highly conductive material and ferromagnetic material of different shape, where the magnetic filter (4) may be a part of the bus bar (1) or not, and may be electrically insulated from the bus bar (1) or not.

7. Sensor and procedure for measuring bus bar current with skin effect correction, is characterized by, the fact that a magnetic sensor (2) and a magnetic filter (4) are positioned near the wide side of the bus bar (1) and positioned parallel one to another.

8. Sensor and procedure for measuring bus bar current with skin effect correction, is characterized by, the fact that a magnetic sensor (2) and a magnetic filter (4) are positioned near the edge of the bus bar (1) and positioned perpendicularly one to another.

9. Sensor and procedure for measuring bus bar current with skin effect correction, is characterized by, the fact that it comprises a bus bar (1), at least one Hall or magneto-resistant magnetic sensor (2) and at least one magnetic yoke (5) made of a soft ferromagnetic material, where the magnetic yoke (5) only partially encloses the cross section of the bus bar (1).

10. Sensor and procedure for measuring bus bar current with skin effect correction, as in Claim 9, is characterized by, the fact that a magnetic yoke (5) has a shape of a plate positioned near the wide side of the bus bar (1) and approximately parallel to the side.

11. Sensor and procedure for measuring bus bar current with skin effect correction, as in Claim 10, is characterized by, the fact that a magnetic sensor (2) is positioned near the wide side of the bus bar (1), and between the bus bar (1) and a magnetic yoke (5) where the magnetic sensor (2) and the magnetic yoke (5) are positioned parallel to each other.

12. Sensor and procedure for measuring bus bar current with skin effect correction, according to the Claims 9 and 10, is characterized by, the fact that the magnetic sensor (2) is positioned near the edge of the bus bar (1), where the magnetic sensor (2) and the magnetic yoke (5) are positioned perpendicularly to each other.

13. Sensor and procedure for measuring bus bar current with skin effect correction, is characterized by, the fact that it includes a bus bar (1), a magnetic senor (2) and an electric filter (6), where the electric filter (6) may have various forms, including a serial or parallel connection of a resistor R and a capacitor C.

14. Sensor and procedure for measuring bus bar current with skin effect correction, as in the Claim 13, is characterized by, the fact that the electric filter (6) is connected to an amplifier (7) so that it makes the gain of the amplifier (7) dependent on the frequency.

15. Sensor and procedure for measuring bus bar current with skin effect correction, as to the Claims 4 and 7, is characterized by, the fact that the bus bar (1) is achieved with a hole (3), a magnetic sensor (2) and a magnetic filter (4), where the magnetic filter (4) is inserted into the hole (3) and positioned parallel to the magnetic sensor (2).

16. Sensor and procedure for measuring bus bar current with skin effect correction, as in the Claim 15, is characterized by, the fact that the magnetic filter (4), being of a thickness approximately equal to the height of the bus bar (1) and electrically connected to it by at least one bridge (4a).

17. Sensor and procedure for measuring bus bar current with skin effect correction, as in the Claims 5 and 8, is characterized by, the fact that it includes a bus bar (1), magnetic sensors (2a) and (2b), magnetic filters (4a) and (4b), which are achieved so that there is the bus bar (1), where opposite to each other notches (3a) and (3b) are laterally positioned, and in them magnetic sensors (2a) and (2b) are positioned in such a way that they are located in-between two plate-like magnetic filters (4a) and (4b).

18. Sensor and procedure for measuring bus bar current with skin effect correction, as in the Claims 11 and 15, is characterized by, the fact that it includes a bus bar (1) achieved with a hole (3), a magnetic sensor (2), a magnetic filter (4) and a magnetic yoke (5), where the magnetic filter (4) is inserted in the hole (3), while the magnetic sensor (2), and the magnetic yoke (5) are positioned parallel to magnetic filter (5) so that the magnetic sensor (2) is positioned in-between the magnetic filter and the magnetic yoke (5).

19. Sensor and procedure for measuring bus bar current with skin effect correction, as in the Claims 5 and 10, is characterized by, the fact that it includes a bus bar (1), magnetic sensors (2a) and (2b), magnetic yokes (5a) and (5b), achieved so that the bus bar (1), whereon, opposite to each other, notches (3a) and (3b) are laterally positioned, and in them magnetic sensors (2a) and (2b) are positioned, and which are positioned in-between two plate-like magnetic yokes (5a) and (5b).

20. Sensor and procedure for measuring bus bar current with skin effect correction, as in the Claims 18 and 19, is characterized by, the fact that magnetic filters (4a) and (4b) and magnetic yokes (5a) and (5b) respectively are combined, and include at least two layers, one of which is of a strong electrically-conductive material, and the other of a soft ferromagnetic material.

21. Sensor and procedure for measuring bus bar current with skin effect correction, is characterized by, the fact that it includes a bus bar (1) with oppositely achieved notches (3a) and (3b) where the magnetic sensors (2a) and (2b) are inserted, and two externally positioned magnetic filters (4a) and (4b) and magnetic yokes (5a) and (5b), where the output signals of magnetic sensors (2a) and (2b) are amplified by means of an amplifier (7) and where the (RC) electric filter (6) is positioned.