Current sensor arrangement and method for measuring an effective primary current

Abstract

Current sensor arrangement for measuring an effective primary current (i P ) in a primary conductor (1); the current sensor arrangement comprises the following: a magnetic core (10) for magnetically coupling the primary conductor (1) to a secondary conductor (2); a controlled voltage source (Q) connected to the secondary conductor (2) and designed to apply a voltage (±Us) with adjustable polarity to the secondary conductor (2) so that a secondary current (i S ) flows through the secondary conductor (2); a measuring and control unit (20) coupled to the secondary conductor (2) and designed to generate a measuring signal (u SH ), continuously detecting the reaching of magnetic saturation in the magnetic core (10) and, upon detection of magnetic saturation of the magnetic core (10), reversing the voltage (±Us) in order to remagnetize the magnetic core (10), wherein the measuring and control unit (20) is further designed to evaluate a spectrum of the measuring signal and, depending thereon, to determine a frequency of a current flowing through the primary conductor.

Description

TECHNICAL FIELDThe present description relates to a flux gate current sensor that also provides the possibility of simple frequency measurement, for example a differential current sensor for use in residual current circuit breakers.BACKGROUNDFor the contactless and thus potential-free measurement of the strength of an electric current in a conductor, so-called direct-imaging current sensors are known, which detect the magnetic flux caused by the current, for example by means of a Hall sensor in a slotted magnetic circuit, and generate a signal proportional to the current strength. These sensors are very cost-effective, but have a relatively low accuracy. Direct imaging current sensors are so-called open-loop current sensors which do not contain a closed loop.Furthermore, so-called closed-loop current sensors are known, in which a magnetic opposing field of the same magnitude as the magnetic field of the current to be measured is continuously generated with the aid of a closed control loop, so that a complete magnetic field compensation is constantly effected and the magnitude of the current to be measured can be determined from the parameters for generating the opposing field. Closed-loop current sensors therefore belong to the class of compensation current sensors.A particular type of compensation current sensors, which do not contain a closed loop, however, are so-called flux gate sensors. Such current sensors include a magnetic core having a primary and a secondary winding. A compensation of the magnetic field which is generated by the current (primary current) to be measured through the primary winding takes place only at specific time intervals of a measurement cycle, wherein in each measurement cycle the magnetic core is driven into the positive and into the negative saturation with the aid of the secondary winding. A very precise current measurement is possible with the aid of such sensors because the influence of the hysteresis of the magnetic core can be eliminated by suitable signal processing. For this reason, flux gate current sensors are also suitable for differential current measurement. In this case, the primary winding consists of at least two partial windings; the difference between the currents through the two partial windings is measured. In the simplest case, the two partial windings are straight lines which are passed through an annular core. In the case of more than two partial windings, the currents in the partial windings subtract or add up depending on the current flow direction and orientation of the respective partial winding.Differential current sensors can be used in residual current circuit breakers. In some applications, it is necessary to check whether the differential current (residual current) to be measured has a direct current component (DC component). However, for the efficient calculation of the DC component, information on the frequency of the alternating current component (AC component) is necessary. For example, fault current sensors are necessary for securing charging stations for electric vehicles, which are capable of determining DC and AC components (cf. standard IEC 62752, "Charging line-integrated control and protection device for charging operating mode 2 of electric road vehicles", "In-cable control and protection device for mode 2 charging of electric road vehicles"). There is therefore a need for current sensors which are also capable of easily ascertaining the frequency of the alternating component of the current to be measured.SUMMARY OF THE INVENTIONA current sensor arrangement for measuring an effective primary current in a primary conductor is described. According to a first example of the invention, the current sensor arrangement has a magnetic core for magnetically coupling the primary conductor to a secondary conductor and a controlled voltage source which is connected to the secondary conductor and is designed to apply a voltage with adjustable polarity to the secondary conductor, so that a secondary current flows through the secondary conductor. A measurement and control unit coupled to the secondary conductor is designed to generate a measurement signal representing the secondary current, to continuously detect the achievement of magnetic saturation in the core and, when magnetic saturation of the core is detected, to reverse the polarity of the voltage in order to remagnetize the core. The measurement and control unit is further configured to evaluate a spectrum of the measurement signal and to determine a frequency of a current flowing through the primary conductor as a function thereof.Furthermore, a method for measuring an effective primary current in a primary conductor is described, which is magnetically coupled to a secondary conductor by means of a magnetic core. According to an example of the invention, the method comprises applying a voltage to the secondary conductor such that a secondary current flows through the secondary conductor, and generating a measurement signal representing the secondary current. The achievement of magnetic saturation in the core is continuously detected and the voltage is reversed upon detection of magnetic saturation of the core in order to remagnetize the core ( 10). The method further comprises evaluating a spectrum of the measurement signal and determining a frequency of a current flowing through the primary conductor depending on the spectrum of the measurement signal.The primary conductor can have a first and a second part, through which a first and a second primary current respectively flow in such a way that the magnetic field strength generated by the primary conductor and thus the effective primary current corresponds to the difference between the primary currents.BRIEF EXPLANATION OF THE DRAWINGSThe invention is explained in more detail below with reference to the examples shown in the figures. The representations are not necessarily true to scale and the invention is not limited only to the aspects shown. Rather, emphasis is placed on illustrating the principles underlying the invention. FIG. 1 is a block diagram of a known current sensor arrangement which operates according to the flux gate principle; FIG. 2, comprising FIGS. 2 aand 2 b, illustrates the signal profile (idealised) of the secondary current, the magnetization and the magnetic field strength in a free-running current sensor arrangement with a primary current of zero; FIG. 3, comprising FIGS. 3 aand 3 b, illustrates the signal profile (idealised) of the secondary current, the magnetization and the magnetic field strength in a free-running current sensor arrangement with a primary current greater than zero; FIG. 4 illustrates a sensor arrangement for measuring a current difference similar to the sensor arrangement according to FIG. 1 ; FIG. 5 shows the time profile and the spectrum of the secondary current at zero primary current; and FIG. 6 shows the time profile and the spectrum of the secondary current in the case of a primary current having a frequency of 50 Hz.In the figures, like reference numerals designate like or corresponding components having the same or similar meaning.DETAILED DESCRIPTIONFIG. 1 shows an example of a flux gate compensation current sensor without hysteresis errors on the basis of a block diagram. The current to be measured (primary current i P) flows through a primary winding 1 (primary conductor), which is magnetically coupled to a secondary winding 2 (secondary conductor) via a soft magnetic and, for example, unslotted core 10. The core 10 is also referred to below as a magnetic core, ferromagnetic core, soft magnetic core or magnetic core. The primary winding 1 can consist, for example, of a single winding, i.e. the primary winding 1 is formed from a conductor which is passed through the core 10 (number of windings 1). The secondary winding 2 (number of turns N) is connected in series with a controlled voltage source Q which generates the secondary current isthrough the secondary winding. For measuring the secondary current is, a shunt resistor R is connected in SH between the secondary winding 2 and the voltage source Q. The voltage U SH across the shunt resistor R SH is supplied to a measurement and control unit 20, which also provides a control signal CTR for driving the controlled voltage source Q.The mode of operation of the current measuring arrangement shown in FIG. 1 is described below with reference to FIGS. 2 (comprising FIGS. 2 aand 2 b ) and 3 (comprising FIGS. 3 aand 3 b ). FIG. 2 adescribes the ferromagnetic properties of the magnetic core 10 on the basis of a magnetization characteristic curve, wherein the magnetic field strength H is plotted on the abscissa and the magnetization M is plotted on the ordinate. The magnetization characteristic curve has an approximately rectangular hysteresis with a specific coercive field strength Hc and a specific saturation magnetization M SAT. According to Ampère's law, H=N·i S / I FE, applies in simplified form to the magnetic field strength H, wherein the parameter I FE denotes the effective magnetic path length of the magnetic field lines in the core 10.The voltage u i induced in the secondary coil 2 applies according to Fardy's law, wherein the parameter A denotes the cross-sectional area of the core 10, the symbol Φ denotes the magnetic flux through the core 10 caused by the secondary current is, and the symbol B denotes the magnetic flux density. The magnetic flux density B can be represented generally by the relationship B=μ 0 ·(H+M); it follows that during the remagnetization of the core 10 (corresponding to the left or right vertical branch of the magnetization characteristic in FIG. 2 a), the rate of change of the magnetization dM / dt is proportional to the induced voltage u i and the magnetic field strength H and thus also the secondary current i S are constant, i.e.It is also possible to say that the differential inductance of the secondary coil 2 is almost infinitely large during the remagnetization. As soon as the magnetization in the core 10 has reached the saturation magnetization M SAT the secondary current i S increases and is limited only more by the ohmic resistance of the secondary winding 2 and the shunt resistance R SH.The increase in the secondary current i S is detected by the measuring and control unit 20, for example with the aid of comparators (cf. FIG. 2 b ). As soon as the secondary current exceeds a positive threshold value +i SMAX or falls below a negative threshold value -i SMAX the measuring and control unit 20 generates a corresponding control signal CTR in order to polarity reversal of the voltage source Q and to initiate the next polarity reversal cycle.The time profile of the secondary current at a primary current i P of zero is shown in FIG. 2 b. During the remagnetization (cf. approximately vertical branches of the magnetization characteristic curve from FIG. 2 a), the secondary current is constant and corresponds to the magnetization current +i µ or -i µ. The magnitude of the magnetizing current i µ depends on the width of hysteresis in the magnetizing characteristic, i.e., on the coercive field strength Hc, i.e., i µ= I FE / N·H C. As soon as the magnetization in the core 10 reaches the positive or negative saturation magnetization, the secondary current is begins to increase, as already described above. Due to the symmetry of the hysteresis characteristic curve, the time profile of the secondary current is is also symmetrical about an average current value.FIGS. 3 aand 3 b show the same situation as FIGS. 2 aand 2 b, but for a primary current i P not zero. The magnetic field generated by the primary current i P is additively superimposed in the soft magnetic core 10 on the magnetic field of the secondary current i S, which can be represented as a shift of the magnetization characteristic along the abscissa. This situation is shown pictorially in FIG. 3a. The corresponding time profile of the secondary current is illustrated in FIG. 3 b. This is similar to FIG. 2 b with a primary current of zero, with the difference that the secondary current no longer runs symmetrically about the abscissa (i S=0) but symmetrically about the horizontal straight line (i S= i P / N). That is to say that during the remagnetization, the primary current and the secondary current have the same ratio k=1:N as the numbers of windings of primary winding 1 and secondary winding 2, with the exception of the hysteresis offset, at the level of the magnetizing current i µ. For current measurement, the secondary current signal i S, or precisely the voltage signal u SH at the shunt resistor R SH, is sampled during the remagnetization process. Thus, by sampling the secondary current signal, a current measurement value i S[ n-1]=(i P / N)+i µ is obtained in the first half of a period of the secondary current (measurement cycle), and a current measurement value i S[ n]=(i P / N)-i µ. is obtained in the second half of the period. By averaging, the hysteresis error caused by the magnetizing current can be eliminated, and the primary current at a sampling time n can be calculated as follows:Because the hysteresis of the magnetization characteristic curve has no influence on the measurement result, this current measurement method is very well suited for measuring very small currents. The measurement range ranges from a few milliamps to one kiloampere. During the magnetic reversal process in the core 10, the secondary current i S follows the primary current i P in accordance with the transmission ratio 1:k. The secondary current is sampled at least once during a magnetic reversal process in order to obtain a measured value (i S+ i µ or i S- i µ) for calculating the primary current. During the remagnetization, however, the sampling can also be carried out repeatedly at a sampling rate which is substantially higher than the oscillation frequency of the sensor f SENSOR. The secondary current is is approximately constant and equal to (i P / N)±i µ. during remagnetization and before magnetic saturation occurs in the core 10. This idealised consideration is true when the hysteresis characteristic of the magnetic core 10 is approximately rectangular.The measuring principle explained hitherto with reference to FIGS. 1 to 3 can also be used for differential current measurement with only slight modification of the sensor construction shown in FIG. 1. Differential current measurement is used, for example, in residual current circuit breakers. For this purpose, instead of a primary winding 1, a first partial winding 1 aand at least one second partial winding 1 bare coupled to the core 10. The primary current through the first partial winding 1a becomes Pa and the primary current through the second partial winding 1b becomes Pb. The partial windings can also each consist only of a single winding and are oriented in such a way that the magnetic fields caused by the currents i Pa and i Pb at least partially compensate (destructively overlap) and only the net primary current i Pa- i Pb( effective primary current) generates a corresponding net magnetic field in the core 10 (which is in turn overlapped by the magnetic field of the secondary current is). The modified sensor construction mentioned is illustrated in FIG. 4 and is substantially identical to the construction in FIG. 1 apart from the primary winding 1. in the example shown in FIG. 4, the two partial windings 1 aand 1 bare connected upstream and downstream of the load L, respectively, so that the difference i Pa- i Pb is not zero only if a leakage current which corresponds exactly to this difference flows away in the load. The differential current (effective primary current) is calculated from samples of the secondary current analogous to equation 3 as follows:The time intervals Δt + and Δt -( cf. FIG. 2 b ) are not constant, but depend on the magnitude of the primary current. From equation 2 it can be seen that the higher the amplitude U S of the voltage generated by the voltage source Q, the higher the speed of the magnetization reversal process; for from Eq. (2) follows:Consequently, the higher the amplitude U S of the voltage generated by the voltage source Q, the higher the oscillation frequency of the secondary current. The oscillation frequency f SENSOR of the sensor follows from Eq. (5): whereinThe parameter ΔM is the magnetization swing during a remagnetization process. From equations 6 ato 6 c, it can be seen that the oscillation frequency f SENSOR of the sensor depends on the primary current itself, on the one hand, as well as on the voltage amplitude U S the voltage generated by the voltage source Q, and on the magnetization swing ΔM.In particular in the case of differential current sensors, the geometric arrangement of the primary conductors with respect to the magnetic core 10 is asymmetrical and complete extinction of the resulting magnetic field does not occur, even if the difference i Pa- i Pb is zero. This results in local saturations in the magnetic core 10, which means a reduction in the effective cross-sectional area A of the core 10. This in turn results in a modulation of the magnetization reversal times Δt +, Δt -. This modulation is periodic and depends on the frequency f P of the primary current i P. This effect can also be deduced from equations 6b and 6c if it is assumed that the cross-sectional area A varies periodically with the frequency f P of the primary currents i Pa and i Pb.In order to measure the frequency of the primary current, the measurement and control unit 20 can be designed to sample the secondary current i S( i.e. the measurement signal U SH, which represents the secondary current), regularly and to calculate a spectrum from the sample values i S[ n] (e.g. by means of a fast Fourier transform (FFT) algorithm, optionally with windowing). The spectrum will have a distinct (global) maximum at the oscillation frequency of the sensor f SENSOR. Two further (local) maxima with a significantly lower magnitude (side maxima) are found at the frequencies f 1= f SENSOR- f P and f 2= f SENSOR+ f P. The sought frequency f P of the primary current can therefore be determined from the frequencies f 1 and f 2 of the two first side maxima, for example according to the equation.The diagrams in FIGS. 5 and 6 illustrate the above-described function of the current sensor. The upper diagrams of FIGS. 5 and 6 show the time profile of the primary current i Pa= i Pb of a differential current sensor (wherein Δi P= i Pa- i Pb=0) and of the corresponding secondary current i S over a time of approximately 50 ms. In the case of FIG. 5, the primary current i Pa= i Pb is equal to zero, whereas in the case of FIG. 6, the primary current has a sinusoidal profile with a frequency f P of 50 Hz. In both cases, the sensor measures the same differential current i Pa- i Pb, namely zero amperes. The lower diagrams of FIGS. 5 and 6 each show the spectrum of the secondary current i S. (depending on the primary current i Pa or i Pb respectively). The global maximum of the spectrum lies at the oscillation frequency f SENSOR of the current sensor (cf. FIG. 2 b), which in the present case is around 2 kHz. In the example shown in FIG. 5, the oscillation frequency f is SENSOR of the current sensor 1935 Hz. In the case shown in FIG. 6, it is 1970 Hz. The somewhat higher value of the oscillation frequency can be explained by the fact that the primary currents i Pa, i Pb in the magnetic core cause local saturations, as a result of which the effective cross-sectional area of the core decreases. The core can consequently be remagnetized more quickly and the frequency rises.If the primary current i Pa or i Pb does not have an AC component, the global maximum (main loop, main lobe) at the oscillation frequency f SENSOR of the current sensor is the only significant maximum in the spectrum. However, a primary current i Pa, i Pb with an AC component leads to the above-explained modulation of the period duration f SENSOR-1( cf. equation 6 a) of the sensor oscillation. This modulation manifests itself in the spectrum by local maxima at the frequencies f 1 and f 2( first side lobes) and f 1' and f 2' ( second side lobes) on both sides of the global maximum at the frequency f SENSOR. The frequency distances |f 1- f SENSOR| and |f 2- f SENSOR| of the two local maxima to the global maximum correspond in each case to the frequency f P of the primary current i Pa and i Pb. The frequency interval f 2- f 1 corresponds to twice the frequency f P of the primary current i Pa or i Pb( cf. equation 7). In the example shown in FIG. 6 (frequency f P of the primary current is 50 Hz), the calculation according to equation 7 results in a measured primary current frequency f P' of (2020-1920) / 2 Hz=100 / 2 Hz=20 Hz. Alternatively or additionally, the second side lobes may also be evaluated at frequencies f 1' and f 2'. as well. The second side lobes are twice as far away from the global maximum at the oscillation frequency f SENSOR of the current sensor as the first side lobes. There are therefore f 1'= f SENSOR- 2 ·f P and f 2'= f SENSOR+ 2 ·f P, and f 1- f 1'= f P and f 2'- f2=fP.

Claims

A current sensor arrangement for measuring an effective primary current (i P) in a primary conductor (1); the current sensor arrangement comprising: a magnetic core (10) for magnetically coupling the primary conductor (1) to a secondary conductor (2); a controlled voltage source (Q) connected to the secondary conductor (2) and configured to apply a voltage (±Us) with adjustable polarity to the secondary conductor (2) such that a secondary current (i S) flows through the secondary conductor (2); a measuring and control unit (20) coupled to the secondary conductor (2), which is designed to generate a measurement signal (u SH) representing the secondary current, to continuously detect the attainment of magnetic saturation in the magnetic core (10) and, upon detection of magnetic saturation of the magnetic core (10), to reverse the voltage (±Us) in order to remagnetize the magnetic core (10), wherein the measuring and control unit (20) is further designed to evaluate a spectrum of the measurement signal and, depending thereon, to determine a frequency of a current flowing through the primary conductor.The current sensor arrangement according to claim 1, wherein the primary conductor (1) comprises a first and a second part (1a, 1b), through which a first and a second primary current (i Pa, i Pb) respectively flow such that the magnetic field strength generated by the primary conductor (1) and thus the effective primary current corresponds to the difference of the primary currents (i Pa, i Pb).The current sensor arrangement according to claim 1 or 2, wherein the reaching of magnetic saturation in the magnetic core (10) is detected when the secondary current (i S) reaches a defined maximum or minimum value.Current sensor arrangement according to one of Claims 1 to 3, wherein the measurement and control unit (20) is designed to sample the measurement signal (u SH) and to calculate a digital spectrum of the measurement signal from the sample values by means of an FFT or DFT algorithm.The current sensor arrangement according to claim 4, wherein the secondary current (is) has an oscillation frequency, and wherein the spectrum has a main lobe at the oscillation frequency of the secondary current and two side lobes symmetrical to the main lobe, wherein the frequency of the current (i P) flowing through the primary conductor (1) is determined from the distance between main and side lobes or from the distance of the two side lobes.The current sensor arrangement according to claim 5, wherein the distance between the main lobe and an adjacent side lobe corresponds to the frequency of the current (i P) flowing through the primary conductor (1).A method for measuring an effective primary current (i P) in a primary conductor (1) magnetically coupled to a secondary conductor (2) by means of a magnetic core (10); the method comprising: applying a voltage (±Us) to the secondary conductor (2) such that a secondary current (i S) flows through the secondary conductor (2); generating a measurement signal (u SH); representing the secondary current (i S) detecting the achievement of magnetic saturation in the magnetic core (10) continuously; and reversing the polarity of the voltage (±U S) upon detection of magnetic saturation of the magnetic core (10) in order to remagnetize the magnetic core (10); and evaluating a spectrum of the measurement signal (u SH) and determining a frequency of a current (i P) flowing through the primary conductor (1) as a function of the spectrum of the measurement signal (u SH).Method according to claim 7, wherein the primary conductor (1) comprises a first and a second part (1a, 1b), through which a first and a second primary current (i Pa, i Pb) respectively flow in such a way that the magnetic field strength generated by the primary conductor (1) and the effective primary current correspond to the difference of the primary currents (i Pa, i Pb).Method according to claim 7 or 8, wherein the reaching of magnetic saturation in the magnetic core (10) is detected when the secondary current (i S) reaches a defined maximum or minimum value.Method according to one of claims 7 to 9, wherein the evaluation of a spectrum of the measurement signal (u SH) comprises: sampling the measurement signal (u SH); and calculating a digital spectrum of the measurement signal (u SH) from the samples of the measurement signal.

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