EP1114429B1 - Current transformer with a direct current tolerance - Google Patents

Description

The invention relates to a current transformer for alternating current, in particular mains alternating current, with direct current components, consisting from 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 a secondary circuit terminates with low impedance.

To record the energy consumption of electrical devices and Energy meters are used in industrial and household systems. The oldest principle used is that of Ferraris meter. The Ferraris meter is based on energy metering about rotating one with a mechanical Counter connected disc by the current or voltage proportional Fields of corresponding field coils driven becomes. For expanding the functional possibilities of energy meters such as B. for multi-tariff operation or remote reading electronic energy meters are used at which the current and voltage detection via inductive current and Voltage converter takes place. The output signals of this Converters are digitized, multiplied in phase, integrated and saved. The result is an electrical one Size that is available for remote reading among other things stands.

The used for energy metering in industrial applications Electronic energy meters often work because of that very high currents, d. H. Currents that are more than 100 A, indirectly. The current inputs are special current transformers upstream, so that only purely bipolar, zero-symmetrical AC currents must be measured in the meter itself. To serve current transformers with transformer cores made of highly permeable Materials are built and to achieve small measurement errors about a small phase error with a lot of secondary turns, d. H. typically more than 2500 secondary turns with a primary turn of 1, must be equipped. For use in household meters, also in industrial Small systems can be used, they are not suitable, since the by modern semiconductor circuits, for. B. Rectifier circuits or leading edge circuits non-zero-symmetrical current profiles a DC component included, which magnetically saturates these current transformers and thus falsifies the energy meter.

Current transformers are known for mapping such currents, those based on open or mechanically introduced Air gaps sheared and therefore low permeable magnetic circuits work. Because the requirements for interference immunity such current transformers, however, must be very high in order to These constructions are to enable verifiable energy metering with elaborate shielding against external fields equipped, the material and assembly intensive and therefore for a wide range of household uses economically unfavorable are.

Another known way of realizing this is Use of current transformers with relatively low permeability Converter cores, d. H. Converter cores that have a permeability µ ≈ 2000. Such permeability saturation with small DC components is avoided. Balance is difficult with these types of current transformers between the highest true value that can be transferred without distortion of the bipolar zero-symmetrical sine current to be measured and the highest unalterably transmissible amplitude of a unipolar half-wave rectified sine current. Out The relevant international standard IEC1036 is a The ratio of these two quantities can be derived from 1: 1.

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

In previously known versions of current transformers Compensation range limited to a phase error of 5 °. This leads to the fact that the highest transferable effective value can be oversized got to. Ratios 3 - 4: 1 occur. this leads to very poor material utilization and therefore too much high manufacturing costs.

Furthermore, it is necessary to have a very high linearity this phase error over the entire to be transmitted Storm range is met to cover expenses for the To keep compensation as low as possible.

The object of the present invention is therefore a current transformer for alternating current with direct current components of the input mentioned type in that he has a high modulation equally with alternating current and direct current components having.

Furthermore, it is intended for precise current acquisition via a wide current range a high linearity of the gear ratio exhibit.

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

• daß als Wandlerkern ein geschlossener, luftspaltloser Ringkern aus einem Band (Ringbandkern) aus einer amorphen, ferromagnetischen Legierung vorgesehen ist,
• daß die amorphe ferromagnetische Legierung einen Magnetostriktionswert |λ_S| < 0,5 ppm sowie eine Permeabilität µ < 1400 aufweist, und
• daß die Legierung eine Zusammensetzung aufweist, die im wesentlichen aus der Formel Co_a(Fe_1-xMn_x)_bNi_cX_dSi_eB_fC_g

besteht, worin X zumindest eines der Elemente V, Nb, Ta, Cr, Mo, W, Ge und P ist, a-g in Atom% angegeben sind und wobei a, b, c, d, e, f, g und x die folgenden Bedingungen erfüllen:
40 ≤ a ≤ 82; 2 ≤ b ≤ 10; 0 ≤ c ≤ 30; 0 ≤ d ≤ 5; 0 ≤ e ≤ 15; 7 ≤ f ≤ 26; 0 ≤ g ≤ 3; mit 15 ≤ d + e + f + g ≤ 30 und 0 ≤ x < 1.According to the invention, the object is achieved by a current transformer for alternating current with DC 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 and terminates the secondary current with low resistance, which is characterized in that

• 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 | λ _S | <0.5 ppm and a permeability µ <1400, and
• that the alloy has a composition consisting 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 which has excellent controllability equally with AC and has DC components.

It is also characterized by a high linearity of the gear ratio off, so that accurate current detection is guaranteed over a very wide current range. By its air gap-free construction also has one high immunity to external external magnetic fields, so that no additional shielding measures are necessary. By the alloy systems according to the invention can be very small Transformer core masses can be achieved.

With a primary number of turns of n ₁ = 1, current transformers can be provided which can make do with a number of secondary turns of approximately 1500. Overall, according to the invention, a direct current-tolerant current transformer can be produced which is extremely inexpensive to manufacture and is excellently suited 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 | λ _S | <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 free of magnetostriction. The magnetostriction is preferably adjusted 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 the glass former content. Glass former here means X, silicon, boron and carbon.

Alloys in which the parameters a + b + c 77 77 are set to c 20 20 have proven to be particularly suitable in the amorphous, ferromagnetic cobalt-based alloy system according to the invention. As a result, saturation magnetizations B _S of 0.85 Tesla or higher can easily 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 set 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 .

For achieving a low permeability has proven to be favorable range of the band thickness of the toroidal core a thickness d ≤ 30 µm, preferably d ≤ 26 µm.

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 alloys according to the invention, this can be done the surface-related proportion of sturgeon anisotropies lower surprisingly strongly.

Typically, the band of the toroidal core at least instructs an electrically insulating layer on one surface. In In another embodiment, the entire toroid has one electrically insulating layer. This makes it special achieved low permeabilities and also the linearity of the B-H grinding is further improved. When choosing the It is important to ensure that the electrically insulating medium this adheres well to the belt surface on the one hand, on the other hand however, it does not cause surface reactions would damage the magnetic properties.

In the alloys according to the invention, oxides, acrylates, Phosphates, silicates and chromates of the elements calcium, Magnesium, aluminum, titanium, zirconium, hafnium and silicon as a particularly effective and compatible electrically insulating Media exposed.

Magnesium oxide is particularly effective and economical, which as a liquid magnesium-containing precursor the tape surface can be applied and during a special one that does not affect the alloy Heat treatment in a dense magnesium-containing Converts layer whose thickness D is between 25 nm and 400 nm lies. In the actual heat treatment in the transverse Magnetic field then creates a well-adhering, chemically inert, electrically insulating magnesium oxide layer.

Figur 1

ein Ersatzschaltbild eines Stromwandlers und die Bereiche der technischen Daten, wie sie in verschiedenen Anwendungen auftreten können,

Figur 2

Magnetfelder im Stromwandler ohne Berücksichtigung von Kernverlusten,

Figur 3

ein Oszillogramm des Sekundärstroms eines Stromwandlers bei halbwellengleichgerichteten Primärstrom,

Figur 4

die Permeabilität in Abhängigkeit der Induktionsamplitude,

Figur 5

die Permeabilitätsänderung in Abhängigkeit der Temperatur,

Figur 6

die Permeabilitätsänderung in Abhängigkeit einer Auslagerungsdauer von erfindungsgemäßen Legierungen,

Figur 7

ein Diagramm mit einer möglichen Temperaturführung während der Wärmebehandlung, und

Figur 8

einen Schnitt durch die Oberfläche eines Körpers dessen Rauhtiefe bestimmt werden soll.

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

Figure 1

an equivalent circuit diagram of a current transformer and the areas of the technical data as they can occur in different applications,

Figure 2

Magnetic fields in the current transformer without taking into account core losses,

Figure 3

an oscillogram of the secondary current of a current transformer with half-wave rectified primary current,

Figure 4

the permeability as a function of the induction amplitude,

Figure 5

the change in permeability as a function of temperature,

Figure 6

the change in permeability as a function of an aging period of alloys according to the invention,

Figure 7

a diagram with a possible temperature control during the heat treatment, and

Figure 8

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

FIG. 1 shows the basic circuit of a current transformer 1. The primary winding 2, which carries the current i _prim to be measured, and a secondary winding 3, which carries the measuring current i _sek , are located on a transformer core 4 which is constructed from a ring band core. The secondary current i _sek automatically adjusts itself so that the ampere turns primary and secondary are ideally of the same size and oppositely directed.

The current in the secondary winding 3 then turns after Induction law so that it is the cause of its emergence, namely the temporal change in the magnetic flux in the converter core 4, tried to prevent.

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): I ideal sec = - I prime * ( N prime / N sec )

This ideal case is due to the losses in burden resistance 5, in the copper resistor 6 of the secondary winding and in the converter core 4 never reached.

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): F ( I ) = I real sec - I ideal sec I ideal sec Phase error: ϕ =  ( I real sec ) -  (-I prime )

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 meters, the product is electricity and tension at any time and calculates the electrical power or energy consumption.

Inductive loads occur very frequently in AC networks on, e.g. B. by transformers or motors. Are such inductive loads at idle, so is the phase shift almost 90 ° between current and voltage. The electrical Active power is therefore almost zero in this situation the phase error of the current transformer is particularly critical in the Energy metering. Because of this, it is important to either the lowest possible phase error, typically one Phase error ϕ <0.2 °, or one over the Current measuring range as constant as possible and thus easily compensated to achieve higher phase errors.

In the ideal current transformer, according to equation (1) the magnetic fields H of primary current and secondary current straight on. Accordingly, the converter core 4 would not have a magnetic modulation Experienced. Compensate even in real current transformers the two fields are almost still, so that the magnetic Control of the converter core 4 in relation to the magnetic field of the primary current is very small. These relationships are in of Figure 2 illustrates. The lower the transmission errors of the current transformer, the lower the magnetic Control of the converter core 4 in relation to the magnetic field of the primary current. This goes hand in hand with a good one Current transformers also have extremely high currents in relation to the saturation field strength of the not completed on the secondary side Converter core 4 can transmit.

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): Q = R ω · L ≈ tanϕ (typically Q = 1/100 ... 1/500)

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 transducer core 4 to the magnetic field H _{prim of} the primary current, which is illustrated by equation (4): Q = R ω · L ≈ B μμ 0 H prime each with R = R Cu + R R , ω = 2 • π • f, H = N • L / L fe

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, ie in the case of toroidal core made of the material, the strip 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 found in equations (5), which describes the phase error, and (6), which describes the amplitude error: Phase error: tanϕ = R ω · L · Cosδ with L = µµ 0 A Fe l Fe · N 2 sec amplitude error F ( I ) = - R ω · L · sinδ L _Fe = iron path length (average circumference)
A _Fe = iron cross section of the toroid

The properties of the core material are above the relative ones Permeability µ and the loss angle δ or the loss factor tan δ included. These material properties are strongly dependent on the magnetic modulation B of the converter core and thus from the primary current. This is the cause of the non-linearity of the converter characteristics.

Very often it is used for electrical energy meters for billing purposes used in the household sector, a so-called DC tolerance required. This is not a real one Meant direct current, but an asymmetrical alternating current, how he z. B. arise from a diode in the consumer circuit can.

The international standard IEC1036 requires functionality of the electricity meter, however with limited accuracy, even with completely half-wave rectified AC. That corresponds to a situation in which the complete Primary current is passed through a diode.

FIG. 3 shows an oscillogram of the primary current, the secondary current of a current transformer and the flux density B in one Converter core for a half-wave rectified primary current. As you can see, the flux density B increases with the converter every half-wave in steps up to the converter core in the Saturation goes.

The effect of such a current form 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: Δ B 1 = 2 B = 2 R ω · L · μμ 0 H prime elevated. When driven with a symmetrical alternating current, exactly the same flux density is reduced again during the next half period. If the driving force of the primary current is now missing during this second half period, the flow in the core can only break down very slowly. This degradation happens according to an exponential law: B = B 0 (1-exp (-1 / τ)) 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 according to equation (3). With good current transformers, it is in the seconds range. At the start value B ₀ , the flux density is approximately discharged through this for the duration of a period T = 1 / f = 2 π / ω Δ B 2 = B 0 · 2π · R ω · L degraded, which is initially small compared to ΔB ₁ .

That is, the next period starts with an increased flux density in the core, so that the core assumes a higher magnetic flux density B ₀ from period to period. The mean flux density that occurs in the equilibrium state is calculated by equating equations (7) and (9) to: B = 1 π μμ 0 H prime = 1 π μμ 0 Î prime l Fe

If the equilibrium value B is still in the linear range of Magnetization curve of the transducer core is also the half-wave rectified current without increased error transfer. However, this is with a very small current amplitude the case. At higher current, the converter core gets into the Transition area to saturation. There the permeability increases µ rapidly, so that there is a state of equilibrium in the kink the magnetization curve with a greatly increased error up to for complete override.

With converter cores made of crystalline alloys and ferrites there is no sensible solution to this problem.

Excellent results are obtained according to the present invention meanwhile with converter cores of at least 70% amorphous, Ferromagnetic, almost magnetostriction-free cobalt base alloys reached. These cobalt base alloys have a flat, almost linear B-H loop with one Permeability µ <1400. The converter cores are preferred as closed, air gap-free toroidal cores in oval or rectangular shape.

The table below shows two suitable alloy compositions alloy saturation induction permeability Saturation magnetostriction λ crystallization temperature [at%] [T] μ as quenched heat treated [° C] Co _72.8 Fe _4.7 Si _5.5 B ₁₇ 0.99 1220 -32 * 10 ^-8 -1.6 * 10 ^-8 500 Co _55.6 Fe _6.1 Mn _1.1 Si _4.3 B _16.2 Ni _16.5 0.93 710 -110 * 10 ^-8 + 4.2 * 10 ^-8 432

The amorphous, ferromagnetic shown in Table 1 Cobalt based alloys were initially considered amorphous Tape from a melt using the known per se Rapid solidification technology manufactured. The rapid solidification technology is for example in DE 37 31 781 C1 described in detail. The tape, which has a thickness of approximately 20 µm, then became stress-free into one Ring core wound.

The setting of the linear, flat essential to the invention B-H loop was then carried out by a special heat treatment of the wound toroid in a magnetic field that stood perpendicular to the tape direction. The heat treatment was made so that the value of the saturation magnetostriction of the rapidly solidified (as quenched) band during the Heat treatment by one of the alloy composition dependent amount changed in the positive direction until it changed in the ranges listed in the table.

For example, the heat treatment shown in FIG. 5 was found with which, in the case of an alloy with the composition Co _72.7 Fe _4.6 Si _5.5 B _17.2, the strongly negative value of the magnetostication of λ _S ≈ -45 x 10 ^-8 of the rapidly solidified (as quenched) band was shifted in the positive direction up to almost the zero crossing (λ _S ≈ -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 and ) 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 λ _S ≈ +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, at this temperature the first ripening processes of crystallization nuclei already in the rapidly solidified band (as quenched) started, which led to a significant disturbance of the linearity of the characteristic.

During the heat treatment, the toroidal core was covered with a Flushed with protective gas so that no oxidation on the belt surface or other chemical reactions occurred that the physical properties of the toroidal core are negative would affect.

The wound ring band core was placed under a magnetic field at a rate of 1 to 10 Kelvin / min to far below the specified Curie temperatures are temperatures of approximately 300 ° C heated and several hours in the created transverse magnetic field held in this temperature interval and then at a cooling rate of 0.1 to 5 Kelvin / min cooled down again.

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

The manufacturability 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 and 0,9 0.9 B _s . This linearity requirement can be met via the manufacturing process described, provided that the strip material used has relative surface roughness R _rel .

The definition of the roughness depth Ra _rel 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 of the surface to be measured. The surface roughness R _A 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 surface roughness R _{a rel} on both sides related to the thickness of the strip material is then obtained according to the formula R _rel = (R _{a top} + R _{a bottom} ) / d, where d is the thickness of the strip material.

A ring band core weighing only 4.7 g could be produced from the alloy Co _72.8 Fe _4.7 Si _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 linearity of the phase angle over a current range from <120 mA to 120 A of ≈ 0.2 °. The permeability of this ring band core was µ = 1150. The ring band core had the dimensions 24.5 x 20.5 x 5.5 mm with an iron cross section of A _Fe = 0.088 cm ² .

The current transformer manufactured with this toroidal core had over a phase error of 8.9 ° +/- 0.1 ° in the entire current range. The ratio between the highest transferable RMS value of the bipolar zero-symmetrical sine current to be measured and the highest transmittable amplitude of a unipolar half-wave rectified sine current was 1.4: 1. The toroidal core also showed very good aging behavior at 120 ° C on what is shown in Figure 6 what with the very high crystallization temperature and the high Anisotropy energy of this alloy can be explained.

Particular importance was attached to the production of the ring band core on a careful coordination of the winding technology and the Heat treatment on the magnetic and metallurgical properties of the alloys. In particular, was on it paid attention to the fact that the wound toroidal cores in transverse Direction in every point were sure saturated by what stacking of several toroidal cores on top of each other achieved has been. The directional deviation of the field lines from the rotational symmetry axis of the ring band core stack about 0.5 °. It has been shown to be a deviation of a maximum of 3 ° is permissible.

As can be seen in FIG. 4, the device according to the invention can be used used alloy range permeability values set between 500 and 1400. As Figure 5 shows through the use of the claimed alloy system an extremely high temperature stability of the permeability realize. This is the typical change, for example between room temperature and + 100 ° C at less than 5%.

From Figures 4 and 5 can also be seen that the dependency of the permeability on the modulation or the temperature compared to ferrites (N67 or N27) is cheaper. The strong dependency is particularly striking the permeability of the modulation and of the Temperature, so that current transformers with transformer cores The linearization of the characteristic curve by shear and ferrite thus an air gap is unavoidable. Through an air gap in the converter core there is a risk of induction of Interference voltages due to external magnetic fields. Further can influence the temperature of sheared converter cores Ferrite to an undefined change in the air gap and thus lead to a disproportionate change in inductance.

With the present invention, current transformers for industry and household can be produced in a compact design, which have secondary windings with number of turns n _sec ≤ 1500 with a primary winding n _prim = 1 and are designed for a primary current i _prim <120 A.

Claims (11)

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 load resistor which closes the secondary current with low resistance is connected in parallel, characterised in that

   a closed toroidal core without an air gap made from a strip of an amorphous ferromagnetic alloy is provided as the transformer core,

   the amorphous, ferromagnetic alloy displays magnetostriction |λ_S| < 0.05 ppm and permeability µ < 1400, and

   the alloy has a composition consisting essentially of the formula Co_a(Fe_1-xMn_x)_bNi_cX_dSi_eB_fC_g, where X is at least one of the elements V, Nb, Ta, Cr, Mo, W, Ge and P, a-g are specified in atomic % 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.
2. Current transformer according to claim 1, characterised in that a, b, c, d, e, f, g and x satisfy 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.
3. Current transformer according to claim 2, characterised in that a, b and c satisfy the following conditions:
   a + b + c ≥ 77 and c ≤ 20.
4. Current transformer according to claim 3, characterised in that the amorphous, ferromagnetic alloy displays magnetostriction |λ_S| < 0.1 ppm and permeability µ < 1300.
5. Current transformer according to one of claims 1 to 4, characterised in that the amorphous, ferromagnetic ally displays saturation magnetisation B_s of 0.7 to 1.2 tesla.
6. Current transformer according to one of claims 1 to 5, characterised in that the strip has a thickness d of 17 µm ≤ d ≤ 30 µm.
7. Current transformer according to one of claims 1 to 6, characterised in that the strip is provided at least on one surface with an electrically insulating layer.
8. Current transformer according to one of claims 1 to 6, characterised in that the toroidal core is provided with an electrically insulating layer.
9. Current transformer according to claim 7 or claim 8, characterised in that a layer of magnesium oxide is provided as the electrically insulating layer.
10. Current transformer according to claim 9, characterised 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, characterised in that the secondary winding has n_sec ≤ 1500 turns, the primary winding having n_prim = 1 turn and the current transformer being designed for a primary current i_prim < 120 A.

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