EP1114429A1

EP1114429A1 - Current transformer with a direct current tolerance

Info

Publication number: EP1114429A1
Application number: EP99969529A
Authority: EP
Inventors: Detlef Otte; Jörg PETZOLD
Original assignee: Vacuumschmelze GmbH and Co KG
Current assignee: Vacuumschmelze GmbH and Co KG
Priority date: 1998-09-17
Filing date: 1999-09-16
Publication date: 2001-07-11
Anticipated expiration: 2019-09-16
Also published as: US6563411B1; DE59907740D1; JP2002525863A; EP1114429B1; JP4755340B2; WO2000017897A1

Abstract

The invention relates to a current transformer for an alternating current which has direct current tolerances. The current transformer consists of at least one converter core with a primary winding and at least one secondary winding, a burden resistance being connected parallel thereto and closing the secondary circuit with a low resistance. A closed annular core without an air gap is used as the converter core, said annular core consisting of a strip which in turn consists of an amorphous, ferromagnetic alloy which is almost free of magnetorestrictions and which has a permeability ν < 1400. The following alloys have been shown to be particularly suitable for an annular strip core of this kind: cobalt-based alloys, essentially of the following formula: Coa(Fe1-xMnx)bNicXdSieBfCg wherein X is at least one of the elements V, Nb, Ta, Cr, Mo, W, Ge or P, a-f are given in atom %, and a, b, c, d, e, f, g and x satisfy the following conditions: 40 ≤ a ≤ 82; 2 ≤ b ≤ 10; 0 ≤ c ≤ 30; 0 ≤ d ≤ 5; 0 ≤ e ≤ 15; 7 ≤ f ≤ 26; 0 ≤ g ≤ 3; with 15 ≤ d + e + f + g ≤ 30 and 0 ≤ x < 1.

Description

description

Current transformer with DC tolerance

The invention relates to a current converter for alternating current, in particular mains alternating current, with direct current components, consisting of at least one converter core with a primary winding and at least one secondary winding, to which a burden resistor is connected in parallel and terminates a secondary circuit with low resistance.

Energy meters are used to record the energy consumption of electrical devices and systems in industry and households. The oldest principle used is that of the Ferraris meter. The Ferraris meter is based on the energy counting via the rotation of a disk connected to a mechanical counter, which is driven by the field or current-proportional fields of corresponding field coils. For the expansion of the functional possibilities of energy meters such as B. for multi-tariff operation or remote reading electronic energy meters are used, in which the current and voltage detection is carried out via inductive current and voltage transformers. The output signals of these converters are digitized, multiplied in phase, integrated and stored. The result is an electrical quantity that is available for remote reading, among other things.

The electronic energy meters used for energy metering in industrial applications work indirectly because of the often very high currents, ie currents that exceed 100 A. Special current transformers are connected upstream of the current inputs, so that only purely bipolar, zero-symmetrical alternating currents have to be measured in the meter itself. For this purpose, current transformers are used, which are constructed with transformer cores made of highly permeable materials and to achieve low measurement errors via a small phase error with a very large number of seconds. därwindungen, ie typically more than 2500 secondary turns with a primary turn of 1, must be equipped. For use in household meters, which can also be used in small industrial plants, these are not suitable because the modern semiconductor circuits such. B. rectifier circuits or leading edge circuits, generated non-zero-symmetrical current profiles contain a DC component that magnetically saturates these current transformers and thus falsifies the energy count.

Current transformers which operate on the basis of open or mechanically introduced air gaps which are sheared and therefore low-permeable magnetic circuits are known for mapping such currents. However, since the requirements for interference immunity of such current transformers must be very high in order to enable verifiable energy metering, these designs are equipped with complex shields against external fields, which are material and assembly intensive and therefore economically unfavorable for a wide range of household applications .

Another known possibility for implementation is the use of current transformers with relatively low-permeable transformer cores, ie. H. Converter cores that have a permeability μ ~ 2000. Such a permeability avoids saturation with small DC components. It is difficult with these types of current transformers to strike a balance between the highest true, retransmitted effective value of the bipolar zero-symmetrical sine current to be measured and the highest, true, transmittable amplitude of a unipolar half-wave rectified sine current. A ratio of these two quantities of 1: 1 can be derived from the relevant international standard IEC1036.

In order to achieve this ratio, the lowest possible permeability is required, but the occurrence of a high phase when using manageable number of turns error between primary and secondary current. Since this has to be compensated in the energy meter, an appropriate electronic circuit is required.

In previously known versions of current transformers, the compensation range is limited to a phase error of 5 °. In design, this means that the highest transferable effective value has to be oversized. Ratios 3 - 4: 1 occur. This leads to very poor material utilization and thus to very high manufacturing costs.

Furthermore, it is necessary that a very high linearity of this phase error is maintained over the entire current range to be transmitted in order to keep the expenditure for the compensation as low as possible.

The object of the present invention is therefore to design a current transformer for alternating current with direct current components of the type mentioned at the outset in such a way that it has a high modulation capability with alternating current and direct current components.

Furthermore, it should have a high linearity of the transmission ratio for accurate current detection over a wide current range.

Furthermore, it should show a high immunity to external external magnetic fields without additional shielding measures, so that it should be inexpensive to manufacture with simple means, in particular with low converter core masses and number of turns, and in particular should be usable for recording the energy consumption of electrical devices and systems in the household.

According to the invention, the object is achieved by a current converter for alternating current with direct current components, consisting of at least one converter core with a primary winding and at least one ner secondary winding, to which a burden resistor is connected in parallel and terminates the secondary current with low resistance, which is characterized in that

a closed, air gap-free ring core made of a band (ring band core) made of an amorphous, ferromagnetic alloy is provided as the converter core,

- That the amorphous ferromagnetic alloy has a magnetostriction value | λg | <0.5 pp and a permeability μ <1400, and - that the alloy has a composition which consists essentially of the formula

Co _a (Fe ₁ _ _x Mn _x ) _b Ni _c X _d Si _e B _f C _g

where X is at least one of the elements V, Nb, Ta, Cr, Mo, W, Ge and P, ag are given in atomic% and where a, b, c, d, e, f, g and x are the following Satisfy conditions:

40 <a <82; 2 <b <10; 0 <c <30; 0 <d <5; 0 <e <15; 7 <f <26; 0 <g <3; with 15 <d + e + f + g <30 and 0 <x <1.

These measures provide a current transformer that has excellent controllability with both AC and DC components.

It is also characterized by a high linearity of the transmission ratio, so that accurate current detection is ensured over a very wide current range. Due to its air gap-free construction, it also has a high immunity to external magnetic fields, so that no additional shielding measures are necessary. Very low converter core masses can be achieved by the alloy systems according to the invention.

With a primary number of turns of n] _ = 1, current transformers can be provided with a number of secondary turns of about 1500 get by. Overall, according to the invention, a direct current-tolerant current transformer can be produced which is extremely inexpensive to manufacture and is outstandingly suitable for the above-mentioned applications in industry and household.

Particularly good current transformers can be achieved by using amorphous, ferromagnetic alloys that have a magnetostriction value | λg | <0.1 ppm and a permeability μ <1200, the alloy having a composition which essentially consists of the formula

Co _a (Fe ₁ _ _x Mn _x ) _b Ni _c X _d Si _e B _f C _g

where X is at least one of the elements, V, Nb, Ta, Cr, Mo, W, Ge and P, ag are given in atomic% and where a, b, c, d, e, f, g and x are meet the following conditions:

50 <a ≤ 75; 3 <b ≤ 5; 20 <c <25; 0 <d <3; 2 <e <12; 8 <f <20; 0 <g <3 with 17 <d + e + f + g <25 and x <0, 5.

The above-mentioned alloy systems are characterized by linear, flat bra loops up to a value of H = 1 A / cm or higher. The alloy system according to the invention is almost magnetostriction-free. The magnetostriction is preferably set by a heat treatment, the actual saturation magnetostriction being achieved by fine adjustment of the iron and / or manganese content. The saturation magnetization B _s from 0.7 Tesla to 1.2 Tesla is made possible by fine-tuning the nickel and glass former content. Glass former here means X, silicon, boron and carbon.

Alloys in which the parameters a + b + c> 77 are set to c <20 have proven to be particularly suitable in the amorphous, ferromagnetic cobalt-based alloy system according to the invention. This makes it easy to Magnetization B _s of 0.85 Tesla or higher can be achieved.

The permeability of less than 1400 is based on the physical connection that the permeability μ is inversely proportional to the uniaxial anisotropy K _u . The uniaxial anisotropy K _u can be adjusted by heat treatment in a transverse magnetic field. The higher the content of cobalt, manganese, iron and nickel, the higher the uniaxial anisotropy K _{u can be} set. The nickel content has a particularly strong influence on the uniaxial anisotropy K _u .

In order to achieve a low permeability, a thickness d <30 μm, preferably d <26 μm, has proven to be a favorable range of the band thickness of the ring band core.

In order to achieve a linear B-H loop that is as linear as possible, a band thickness of the ring band core d> 17 μm has been proven. In the case of the alloys according to the invention, the surface-related proportion of the interference anisotropy can surprisingly be greatly reduced.

Typically, the band of the ring band core has an electrically insulating layer at least on one surface. In another embodiment, the entire toroid has an electrically insulating layer. As a result, particularly low permeabilities are achieved and the linearity of the B-H loops is further improved. When selecting the electrically insulating medium, care must be taken that on the one hand it adheres well to the surface of the tape, but on the other hand does not cause any surface reactions which would damage the magnetic properties.

The alloys according to the invention have oxides, acrylates, phosphates, silicates and chromates of the elements calcium, magnesium, aluminum, titanium, zirconium, hafnium and silicon as particularly effective and compatible electrically insulating media.

Magnesia oxide is particularly effective and economical. It can be applied to the strip surface as a liquid magnesium-containing preliminary product and converts to a dense magnesium-containing layer during a special heat treatment that does not influence the alloy. The thickness D is between 25 nm and 400 run . The actual heat treatment in the transverse magnetic field then creates a well-adhering, chemically inert, electrically insulating magnesium oxide layer.

The invention is illustrated in the drawing, for example, and is described in detail below with reference to the drawing. Show it:

FIG. 1 shows an equivalent circuit diagram of a current transformer and the areas of the technical data, as can occur in various applications,

FIG. 2 magnetic fields in the current transformer without taking into account core losses,

FIG. 3 shows an oscillogram of the secondary current of a current transformer with half-wave rectified primary current,

FIG. 4 the permeability as a function of the induction amplitude,

FIG. 5 shows the change in permeability as a function of temperature,

FIG. 6 shows the change in permeability as a function of an aging period of alloys according to the invention, FIG. 7 shows a diagram with a possible temperature control during the heat treatment, and

8 shows a section through the surface of a body whose roughness depth is to be determined.

FIG. 1 shows the basic circuit of a current transformer 1. The primary winding 2, which carries the current ipr m to be measured, and a secondary winding 3, which carries the measuring current i _se k leads. The secondary current i _se k automatically adjusts itself so that the ampere turns primary and secondary are ideally of the same size and directed in opposite directions.

The current in the secondary winding 3 then adjusts itself according to the law of induction in such a way that it tries to prevent the cause of its formation, namely the change in the magnetic flux in the converter core 4 over time.

In the ideal current transformer, the secondary current multiplied by the ratio of the number of turns is negatively equal to the primary current, which is illustrated by equation (1): rideal _ _ j * (MI AT) sec pnm \ ^y pnm 'sec' (1)

This ideal case is never achieved because of the losses in the load resistor 5, in the copper resistor 6 of the secondary winding and in the converter core 4.

In the real current transformer, the secondary current therefore has an amplitude error and a phase error compared to the above idealization, which is described by equation (2): really ideal

F (0 = ^sec _dea phase error: φ = φ (/ ") -φ (-I _prim ) (2 [sec

An important area of application for current transformers is electronic energy meters in low-voltage AC networks with a network frequency of 50 or 60 Hertz. The evaluation electronics in such counters form the product of current and voltage at any time and use them to calculate the electrical power or energy consumption.

Inductive loads very often occur in AC networks, e.g. B. by transformers or motors. If such inductive loads are idle, the phase shift between current and voltage is almost 90 °. The active electrical power is therefore almost 0. In this situation, the phase error of the current transformer is particularly critical in the energy metering. For this reason, it is important to either achieve a phase error as low as possible, typically a phase error φ <0.2 °, or to achieve a higher phase error that is as constant as possible over the current measuring range and thus easily compensable.

In the ideal current transformer, the magnetic fields H of primary current and secondary current are canceled out according to equation (1). Accordingly, the converter core 4 would not experience any magnetic control. Even in real current transformers, the two fields almost compensate each other, so that the magnetic modulation of the transformer core 4 is very small in relation to the magnetic field of the primary current. These relationships are illustrated in FIG. 2. The lower the transmission errors of the current transformer, the lower the magnetic modulation of the transformer core 4 in relation to the magnetic field of the primary current. This is accompanied by the fact that a good current transformer can also transmit extremely high currents in relation to the saturation field strength of the transformer core 4 which is not terminated on the secondary side. The most important parameter of a current transformer 1 is the ratio of the ohmic resistance in the secondary circuit to the inductive resistance of the secondary winding, which can be seen from equation (3):

R

Q = - «tan (typically Q = 1/100 ... 1/500) (3)

CÜ ^• L

This quality factor Q of the current transformer 1, which determines the phase error in a first approximation, should be as small as possible. It also determines the ratio of the magnetic modulation B of the converter core 4 to the magnetic field Hp _r j_ _{m of} the primary current, which is illustrated by equation (4):

R B

Q = - = each with R = RC _U + RR, ω = 2 * π _* f, H = N »L / Lf _{e (} 4 ₎ ω - L μμ ₀ H _prm

For a more detailed view, the losses in the converter core 4 must also be taken into account. The core losses are dependent on the material properties of the converter core 4, i. H. for ring band cores made of the material, the band thickness and other parameters. They can be described by a second phase angle δ. The second phase angle δ corresponds to the phase shift between B and H in the converter core 4 due to the core losses. The complete relationships for the parameters of the current transformer 1 can be seen from equations (5), which describes the phase error, and (6), which describes the amplitude error:

Phase error: tan © = R - cosd with L = μμ ₍₎ - ^A —Fe - N9t. (5) ω - L l _Fe

R amplitude error F (I) = - are (6)

Cϋ ^■ L

Lp _e = iron path length (current circumference) Ap _e = iron cross section of the toroid The properties of the core material are contained in the relative permeability μ and the loss angle δ or the loss factor tan δ. These material properties are heavily dependent on the magnetic modulation B of the converter core and thus on the primary current. This is the reason for the non-linearity of the converter characteristics.

A so-called DC tolerance is very often required for electrical energy meters that are used for billing purposes in the household sector. This is not a real direct current, but an asymmetrical alternating current, as it is e.g. B. can be caused by a diode in the consumer circuit.

The international standard IEC1036 requires the functionality of the electricity meter, albeit with limited accuracy, even with fully half-wave rectified alternating current. This corresponds to a situation in which the entire primary current is conducted via a diode.

FIG. 3 shows an oscillogram of the primary current, the secondary current of a current transformer and the flux density B in a transformer core for a half-wave rectified primary current. As can be seen, the flux density B in the converter increases step-wise with each half-wave until the converter core saturates.

The effect of such a form of current on the converter can be described with reference to FIG. 3:

For half a period, the flux density in the transformer core is reduced by the value:

elevated. When driven with a symmetrical alternating current, exactly the same flux density will be used during the next half Period reduced again. If the driving force of the primary current is missing during this second half-period, the flux in the core can only be reduced very slowly. This degradation happens according to an exponential law:

B = ß ₀ (l-exp (-l / z)) (8) with the time constant τ = L / R of the secondary winding

This time constant is large if and only if the converter is of high quality in accordance with equation (3). With good current transformers, it is in the seconds range. At the start value B _Q , the flux density is approximately discharged through this for the duration of a period T = 1 / f = 2π / ω

R AB, = B _n - 2π- (9) ^ü ω - L

degraded, which is initially small compared to ΔB _] _.

That is, , the next period already starts with an increased flux density in the core, so that the core assumes a higher magnetic flux density B _Q from period to period. The mean flux density which occurs in the equilibrium state is calculated by equating equations (7) and (9) to:

B = ^ μμ ₀ H _pnm = ± -μμ ₀ -f ^ - (10)

71 TZ l _e

If the equilibrium value B is still in the linear region of the magnetization curve of the converter core, the half-wave rectified current is also transmitted without an increased error. However, this is the case with a very small current amplitude. At higher currents, the converter core gets into the transition area to saturation. There, the permeability μ decreases rapidly, so that a state of equilibrium in the kink of the magnetization curve occurs with a greatly increased error up to complete overdriving. With converter cores made of crystalline alloys and ferrites, there is no sensible solution to this problem.

Excellent results are however achieved according to the present invention with converter cores made of at least 70% amorphous, ferromagnetic, almost magnetostriction-free cobalt base alloys. These cobalt-based alloys have a flat, almost linear B-H loop with a permeability μ <1400. The converter cores are preferably designed as closed, air-gap-free toroidal cores in an oval or rectangular shape.

The table below shows two suitable alloy compositions

The amorphous, ferromagnetic cobalt-based alloys shown in Table 1 were first produced as an amorphous band from a melt using the rapid solidification technology known per se. The rapid starter technology is described in detail, for example, in DE 37 31 781 Cl. The band, which had a thickness of approximately 20 μm, was then wound without tension into a ring band core.

The setting of the linear, flat BH loop essential to the invention was then carried out by means of a special heat treatment the wound toroid in a magnetic field that was perpendicular to the direction of the ribbon. The heat treatment was carried out in such a way that the value of the saturation magnetostriction of the rapidly solidified (as quenched) band changed during the heat treatment by an amount depending on the alloy composition in a positive direction until it was within the ranges shown in the table.

For example, the heat treatment shown in FIG. 5 was found with which an alloy with the composition C072 7Fe4 _^ gSi5 5B] _7 _; 2 the strongly negative value of the magnetostication of λg ~ -45 x 10 ^{~ 8 of} the rapidly solidified (as quenched) band was shifted in the positive direction up to almost the zero crossing (λg = -2 x 10 ^{~ 8} ). At the same time, a highly linear F-loop occurs with an almost ideal permeability value of 1200 and a saturation induction B _s = 0.998 T. An F loop is understood to be a hysteresis loop which has a ratio of remanence B _r to saturation induction B _s <50.

However, if the cross-field temperature was lowered from 330 ° C to, for example, 310 ° C, the permeability dropped to the too low value of 1100, the magnetostriction being about -10 x 10 ^{~ 8} from the zero crossing. As a result, the linearity of the μ (B) characteristic curve suffers and the phase error increased by 10%.

On the other hand, when the cross-field temperature was increased to 370 ° C, the saturation magnetostriction increased to λg ~ +8 x 10 ^"8. At the same time, the permeability increased to a comparatively high value of μ ~ 1300 that reduced the DC tolerance. In addition, the first ripening processes started at this temperature of crystallization nuclei already present in the faster solidified band (as quenched), which led to a substantial disturbance of the linearity of the characteristic. During the heat treatment, the ring core was flushed with a protective gas so that no oxidation or other chemical reactions occurred on the surface of the band that would negatively influence the physical properties of the ring core.

The wound ring band core was heated under a magnetic field at a rate of 1 to 10 Kelvin / min to the temperatures of approximately 300 ° C, which are far below the specified Curie temperatures, and held in this transverse temperature field for several hours and then at a cooling rate of 0 , 1 to 5 Kelvin / min cooled again.

To achieve the linear and very flat B-H loop according to the invention, such a strong magnetic field was applied that the temperature-dependent saturation induction of the respective alloy was surely exceeded at every point within the ring band core. The toroidal cores treated in this way were finally solidified by covering them with a plastic.

The producibility of very small and yet very high-precision current transformers presupposes that the amplitude permeability μ of the current transformer core changes by less than 6%, preferably 4%, in the modulation range of 1 mT B B 0,9 0.9 B _s . This linearity requirement can be met via the described manufacturing process, provided that the strip material used has relative surface roughness R _{a re} .

The definition of the roughness depth Ra _re is explained below with reference to FIG. 8. The x-axis is parallel to the surface of a body whose surface roughness is to be determined. In contrast, the y-axis is parallel to the surface normal the surface to be measured. The surface roughness R ^ then corresponds to the height of a rectangle 7, the length of which is equal to a total measuring distance l _m and which has the same area as the sum of the areas 10 enclosed between a roughness profile 8 and a middle line 9. The related to the thickness of the band material on both sides surface roughness R _{a re} χ of the formula R _{a Re} = χ is then obtained in accordance with (R _a + ^R a top ^bottom) / ⁰ 'the thickness of the band mate rials ^¬ d is.

It was possible to produce a ring band core weighing only 4.7 g from the alloy Cθ72.8 ^Fe 4 7 ^S "-5 5 ^B 17, which was provided with a secondary winding with a number of turns n _sec of 1000. The current transformer produced in this way had a lineari - Opened the phase angle over a current range from <120 mA to 120 A of = 0.2 ° The permeability of this toroidal core was μ = 1150. The toroidal core had the dimensions 24.5 x 20.5 x 5.5 mm an iron cross section of Ap _e = 0.088 cm ² .

The current transformer manufactured with this toroid had a phase error of 8.9 ° +/- 0.1 ° in the entire current range. The ratio between the highest transferable effective value of the bipolar zero-symmetrical sine current to be measured and the highest transferable amplitude of a unipolar half-wave rectified sine current was 1.4: 1. Furthermore, the ring band core had a very good aging behavior at 120 ° C., which is shown in FIG. 6, which can be explained by the very high crystallization temperature and the high anisotropy energy of this alloy.

In the manufacture of the toroidal core, special emphasis was placed on carefully coordinating the winding technology and the heat treatment with the magnetic and metallurgical properties of the alloys. In particular, care was taken to ensure that the wound toroidal cores were reliably saturated at all points in the transverse direction, which was caused by stacking of several toroidal cores on top of one another was achieved. The directional deviation of the field lines from the rotational symmetry axis of the ring band core stack was approximately 0.5 °. It has been shown that a maximum deviation of 3 ° is permissible.

As can be seen in FIG. 4, permeability values between 500 and 1400 can be set with the alloy range used according to the invention. As FIG. 5 shows, extremely high temperature stability of the permeability can be achieved by using the claimed alloy system. For example, the typical change between room temperature and + 100 ° C is less than 5%.

It can additionally be seen from FIGS. 4 and 5 that the dependence of the permeability on the modulation or the temperature is considerably more favorable than ferrites (N67 or N27). Particularly striking is the strong dependence of the permeability on the modulation and on the temperature, so that in current transformers with converter cores made of ferrite, the linearization of the characteristic curve by shear and thus an air gap is unavoidable. An air gap in the transformer core, however, poses the risk of induction of interference voltages through externally interspersed magnetic fields. A temperature influence can also occur with sheared converter cores

Ferrite lead to an undefined change in the air gap and thus to a disproportionate change in inductance.

With the present invention can be inexpensive

Manufacture current transformers for industry and household in a compact design, which have secondary windings with number of turns n _sec <1500 with a primary winding n _pr j_ _m = 1 and are designed for a primary current ip _r j_ _m <120 A.

Claims

claims

1. Current transformer for alternating current with direct current components, consisting of at least one transformer core with a primary winding and at least one secondary winding, to which a burden resistor is connected in parallel, which terminates the secondary current with low resistance, characterized in that

- That as a converter core, a closed, air-gap-free ring core made of a band made of an amorphous ferromagnetic

Alloy is provided

- that the amorphous, ferromagnetic alloy has a magnetostriction value | λg | <0.5 ppm and a permeability μ <1400, and - that the alloy has a composition which consists essentially of the formula

Co _a (Fe ₁ _ _x Mn _x ) _b Ni _c X _d Si _e BfC _g

2. Current transformer according to claim 1, characterized in that a, b, c, d, e, f, g and x meet the following conditions:

50 <a <75; 3 <b <10; 5 <c <25; 0 <d <3; 2 <e <12; 8 <f <20; 0 <g <3 with 17 <d + e + f + g <25 and x <0.5.

3rd Current transformer according to claim 2, d a d u r c h g e k e n n z e i c h n e t that a, b and c meet the following conditions:

a + b + c> 77 and c <20.

4. Current transformer according to claim 3, characterized in that the amorphous, ferromagnetic alloy has a magnetostriction value | λg | <0.1 ppm and a permeability μ <1300.

5. Current transformer according to one of claims 1 to 4, characterized in that the amorphous, ferromagnetic alloy has a saturation magnetization B _s of 0.7 to 1.2 Tesla.

6. Current transformer according to one of claims 1 to 5, characterized in that the band has a thickness d of 17 microns <d <30 microns.

7. Current transformer according to one of claims 1 to 6, characterized in that the band is provided at least on one surface with an electrically insulating layer.

8. Current transformer according to one of claims 1 to 6, characterized in that the ring core is provided with an electrically insulating layer.

9. Current transformer according to claim 7 or 8, characterized in that a layer of magnesium oxide is provided as the electrically insulating layer.

10. Current transformer according to claim 9, characterized in that the magnesium oxide layer has a thickness D of 25 nm <D <400 nm.

11. Current transformer according to one of claims 1 to 10, characterized in that the secondary winding has a number of turns n _sec <1500, the primary winding having a number of turns np _r j_ _m = 1 and the current transformer for a primary current ip _r i _m <120 A. is designed.