Flux-gate current sensor with additional frequency measuring

Abstract

A current sensor arrangement for measuring an effective primary current in a primary conductor having a magnetic core for magnetic coupling of the primary conductor to a secondary conductor and a controlled voltage source connected to the secondary conductor and configured to apply a voltage with adjustable polarity to the secondary conductor so that a secondary current passes through the secondary conductor. A measurement and control unit is coupled to the secondary conductor and configured to generate a measuring signal that represents the secondary current, to continuously detect the occurrence of magnetic saturation in the core, and to reverse the polarity of the voltage upon the detection thereof in order to reverse magnetization of the core. Furthermore, the measurement and control unit is configured to evaluate a spectrum of the measuring signal and determine a frequency of a current passing through the primary conductor based on the spectrum.

Claims

1. A current sensor arrangement for measuring an effective primary current through a primary conductor; the current sensor arrangement comprises: a magnetic core for magnetically coupling the primary conductor to a secondary conductor; a controlled voltage source that is coupled to the secondary conductor and is configured to apply a voltage with adjustable polarity to the secondary conductor so that a secondary current having an oscillation frequency passes through the secondary conductor; a measurement and control unit coupled to the secondary conductor, the measurement and control unit being configured to: generate a measuring signal that represents the secondary current, to continuously detect an occurrence of magnetic saturation in the core, and to reverse the polarity of the voltage upon the detection thereof in order to reverse the magnetization of the core; wherein the measurement and control unit is further configured to evaluate a spectrum of the measuring signal and determine the oscillation frequency of the secondary current flowing through the secondary conductor based on the spectrum and calculate a frequency of the effective primary current passing through the primary conductor using the determined oscillation frequency of the secondary current.

2. The current sensor arrangement of claim 1, wherein the primary conductor comprises a first and a second part, through which a first and a second primary current, respectively, pass in such that the magnetic field strength generated by the primary conductor, and thus the effective primary current, corresponds to the difference of the primary currents.

3. The current sensor arrangement of claim 2, wherein the first part and the second part of the primary conductor are connected upstream and downstream of a load and the difference between the first primary current and the second primary current is only unequal to zero in response to a leakage current being drained in the load that corresponds to the difference between the first primary current and the second primary current.

4. The current sensor arrangement of claim 1, wherein the occurrence of magnetic saturation in the core is detected when the secondary current reaches a defined maximum or minimum value.

5. The current sensor arrangement of claim 1, wherein the measurement and control unit is designed to sample the measuring signal and calculate the spectrum of the measuring signal from the sampled values using an FFT or DFT algorithm.

6. The current sensor arrangement of claim 5, wherein the spectrum has a main lobe at the oscillation frequency of the secondary current as well as two side lobes symmetric to the main lobe, and wherein the frequency of the effective primary current passing through the primary conductor is determined based on the distance between the main lobe and the side lobes or the distance between the two side lobes.

7. The current sensor arrangement of claim 6, wherein the distance between the main lobe and an adjacent side lobe corresponds to the frequency of the effective primary current passing through the primary conductor.

8. A method for measuring an effective primary current through a primary conductor that is magnetically coupled to a secondary conductor by means of a magnetic core; wherein the method comprises: applying a voltage to the secondary conductor so that a secondary current passes through the secondary conductor; generating a measuring signal that represents the secondary current; continuously detecting occurrences of magnetic saturation in the core and reversing the polarity of the voltage upon detection thereof in order to reverse the magnetization of the core; and evaluating a spectrum of the measuring signal and determining a frequency of the effective primary current passing through the primary conductor based on the spectrum of the measuring signal.

9. The method of claim 8, wherein the primary conductor comprises first and second parts through which first and second primary currents respectively pass in such a manner that the magnetic field strength generated by the primary conductor and the effective primary current correspond to the difference of the primary currents.

10. The method of claim 8, wherein the occurrence of magnetic saturation in the core is detected when the secondary current achieves a defined maximum or minimum value.

11. The method of claim 8, wherein the evaluation of a spectrum of the measuring signal comprises the following: sampling the measuring signal; and calculating a digital spectrum of the measuring signal from the scanned values of the measuring signal.

12. The method of claim 11, wherein the evaluation of a spectrum of the measuring signal further comprises the following: identifying a main lobe in the digital spectrum at the oscillation frequency of the secondary current and two side lobes symmetric to the main lobe; and determining the frequency of the effective primary current passing through the primary conductor using at least one of the distance between the main lobe and the side lobes and the distance between the two side lobes.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The invention can be better understood with reference to the following drawings and descriptions. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding or similar parts. In the drawings:

(2) FIG. 1 shows a block diagram of a known current sensor arrangement that operates according to the flux-gate principle;

(3) FIGS. 2a and 2b illustrate (idealized) waveforms of the secondary current, of the magnetization and of the magnetic field strength in a freely oscillating current sensor arrangement with a primary current of zero;

(4) FIGS. 3a and 3b illustrate (idealized) waveforms of the secondary current, of the magnetization and of the magnetic field strength in a freely oscillating current arrangement with a primary current greater than zero;

(5) FIG. 4 illustrates a sensor arrangement for measuring a current difference (residual current) similar to the sensor arrangement according to FIG. 1;

(6) FIG. 5 shows the waveform and the spectrum of the secondary current with zero primary current; and

(7) FIG. 6 shows the waveform and the spectrum of the secondary current with a primary current that has a frequency of 50 Hz.

DETAILED DESCRIPTION

(8) FIG. 1 illustrates, by means of a block diagram, an example of a flux-gate compensation current sensor without a hysteresis error. The current (primary current i.sub.P) to be measured passes through primary winding 1 (primary conductor), which is magnetically coupled to secondary winding 2 (secondary conductor) by a soft magnetic, non-gapped (i.e., having no air gaps) core 10. Primary winding 1 may be composed, for example, of a single winding; i.e., primary winding 1 is formed by a conductor run through core 10 (winding number 1). Secondary winding 2 (with N windings) is connected in series to a controlled voltage source Q, which generates secondary current i.sub.S through the secondary winding. In order to measure secondary current i.sub.S, shunt resistor R.sub.SH is connected between secondary winding 2 and voltage source Q. Voltage U.sub.SH across shunt resistor R.sub.SH is supplied to measurement and control device 20, which provides control signal CTR for controlling the controlled voltage source Q.

(9) The functioning of the current measurement arrangement shown in FIG. 1 is described in the following with reference to FIGS. 2 and 3. FIG. 2a describes the ferromagnetic properties of magnetic core 10 by means of a diagram showing the magnetization characteristics in which magnetic field strength H is entered on the abscissa and magnetization M is entered on the ordinate. The magnetization characteristic has an approximately rectangular hysteresis with a certain coercive field strength H.sub.C and a certain saturation magnetization M.sub.SAT. According to Ampère's (circuital) law, the (simplified) equation H=N.Math.i.sub.S/I.sub.FE applies to magnetic field strength H, wherein the parameter I.sub.FE designates the effective length of the path of the magnetic field lines in core 10.

(10) According to Faraday's law, the equation
u.sub.i=−N.Math.dΦ/dt=−N.Math.A.Math.dB/dt  (1)
applies to voltage u.sub.i, which is induced in secondary coil 2, wherein the parameter A designates the cross-section area of core 10, the symbol Φ designates the magnetic flux through core 10 caused by secondary current i.sub.S, and the symbol B designates the magnetic flux density. Magnetic flux density B can be generally represented by the relationship B=μ.sub.0.Math.(H+M); from this it follows that during the reversal of the magnetization of core 10 (corresponding to the left or right vertical branch of the magnetization characteristic in FIG. 2a), the change rate of magnetization dM/dt is proportional to induced voltage U.sub.i, and thereby the magnetic field strength H as well as the secondary current are constant; i.e.,
u.sub.i=−N.Math.A.Math.μ.sub.0.Math.dM/dt (during the reversal of magnetization).  (2)

(11) It can also be said that the differential inductance of secondary coil 2 is almost infinitely high during the reversal of magnetization. As soon as the magnetization in core 10 has achieved saturation magnetization M.sub.SAT, secondary current i.sub.S rises and is then only limited by the ohmic resistance of secondary winding 2 and shunt resistor R.sub.SH.

(12) The rise of secondary current i.sub.S is detected by measurement and control unit 20 by using, for example, comparators (cf. FIG. 2b). As soon as the secondary current exceeds positive threshold value +i.sub.SMAX or drops below negative threshold value −i.sub.SMAX, measurement and control unit 20 generates a corresponding control signal CTR in order to reverse the polarity of voltage source Q and trigger the next cycle of magnetization reversal.

(13) The secondary current's course through time (for a primary current i.sub.P of zero) is shown in FIG. 2b. During the reversal of magnetization (cf. the approximately vertical branches of the magnetization characteristic in FIG. 2a), the secondary current is constant and corresponds to magnetization currents +i.sub.μ and −i.sub.μ. The magnitude of magnetization current i.sub.μ is a function of the width of the hysteresis in the magnetization characteristic and is thus also a function of coercive field strength H.sub.C; i.e., i.sub.μ=I.sub.FE/N.Math.H.sub.C. As soon as the magnetization in core 10 achieves positive or negative saturation, the secondary current i.sub.S begins to rise, as already described above. Due to the symmetry of the characteristic hysteresis curve, the waveform of secondary current i.sub.S is also symmetric around an average current value.

(14) FIGS. 3a and 3b show the same situation as FIGS. 2a and 2b except that primary current i.sub.P is not equal to zero. The magnetic field caused by primary current i.sub.P is superposed in an additive manner in the soft magnetic core 10 with the magnetic field caused by secondary current i.sub.S, which can be regarded as a shift of the magnetization characteristic along the abscissa. This situation is graphically represented in FIG. 3a. The corresponding waveform of the secondary current is represented in FIG. 3b. This is similar to FIG. 2b, in which the primary current is zero, with the difference that the secondary current is not symmetrical around the abscissa (i.sub.S=0) but is rather symmetrical around the horizontal straight line (i.sub.S=i.sub.P/N). This means that during the reversal of the magnetization, the primary current and the secondary current have the same ratio k=1:N as the ratio of numbers of windings of primary winding 1 and secondary winding 2, respectively, disregarding the hysteresis offset at the level of magnetization current i.sub.μ. For the current measurement, secondary current signal i.sub.S, or more precisely voltage signal U.sub.SH across shunt resistor R.sub.SH, is sampled during the process of magnetization reversal. That way, a measured current value of i.sub.S[n−1]=(i.sub.P/N)+i.sub.μ is obtained by sampling the secondary current signal in the first half of a period of the secondary current (measuring cycle); in the second half of the period, a measured current value of i.sub.S[n]=(i.sub.P/N)−i.sub.μ is obtained. The hysteresis error caused by magnetization current i.sub.μ can be eliminated by the formation of the average value; the primary current at sampling time instant n is calculated as follows:
i.sub.P[n]=N.Math.(i.sub.S[n−1]+i.sub.S[n])/2.  (3)

(15) As the hysteresis of the magnetization characteristic has no influence on the measured result, this current measuring method is very well suited to measuring very small currents. The measuring range extends from a few milliamperes to one kiloampere. During the magnetization reversal process in core 10, secondary current i.sub.S follows primary current i.sub.P in accordance with the transfer ratio 1:k. The secondary current is sampled at least once during a magnetization reversal process in order to obtain measured values (i.sub.S+i.sub.μand i.sub.S−i.sub.μ) to calculate the primary current. However, during the reversal of magnetization, the sampling can also be carried out repeatedly with a sampling rate that is considerably higher than the sensor's oscillating frequency f.sub.SENSOR. Secondary current i.sub.S, equal to (i.sub.P/N)±i.sub.μ, is approximately constant during the reversal of magnetization and prior to the occurrence of magnetic saturation in core 10. This idealized consideration is applicable when the hysteresis characteristic of magnetic core 10 is approximately rectangular.

(16) The measuring principle previously explained with reference to FIGS. 1 to 3 can also be used to measure differential current (i.e., residual current) with only a minor modification to the sensor construction shown in FIG. 1. Differential currents are measured, for example, in residual-current circuit breakers. For the purpose of differential current measurement, a first partial winding 1a and at least one second partial winding 1b are coupled to core 10 instead of one primary winding 1. The primary current through the first partial winding 1a is denoted by i.sub.Pa (ingress winding), and the primary current through the second partial winding 1b (return winding) is denoted by I.sub.Pb. The partial windings may be composed of only a single winding and are oriented such that the magnetic fields caused by currents i.sub.Pa and i.sub.Pb compensate (destructively superpose) one another at least partially and only the net primary current i.sub.Pa−i.sub.Pb (effective primary current) generates a corresponding net magnetic field in core 10 (which is superposed with the magnetic field of secondary current i.sub.S). The aforementioned modified sensor setup is shown in FIG. 4 and is substantially identical to the setup of FIG. 1 except for primary winding 1. In the example shown in FIG. 4, the two partial windings 1a and 1b are connected upstream and downstream of load L so that the difference i.sub.Pa−i.sub.Pb is only unequal to zero when a leakage current is drained in the load that corresponds to this difference. The differential current (effective primary current) is calculated from the sampled values of the secondary current, similar to Equation 3, as follows:
Δi.sub.P[n]=i.sub.Pa[n]−i.sub.Pb[n]=N.Math.(i.sub.S[n−1]+i.sub.S[n])/2.  (4)

(17) The time intervals Δt+ and Δt− (cf. FIG. 2b) are not constant but are rather functions of the magnitude of the primary current. It can be seen from Equation 2 that the speed of magnetization reversal is greater the higher amplitude U.sub.S of the voltage generated by voltage source Q is, because it follows from Eq. 2 that
dM/dt=−u.sub.i/(N.Math.A.Math.μ.sub.0)=−(U.sub.S−R.sub.SH.Math.i.sub.S)/(N.Math.A.Math.μ.sub.0)  (5)

(18) Consequently, the oscillation frequency of the secondary current is higher the higher amplitude U.sub.S of the voltage generated by voltage source Q is. The sensor's oscillation frequency f.sub.SENSOR follows from Eq. 5:
f.sub.SENSOR=1/(Δt.sub.++Δt.sub.−),  (6a)
wherein
Δt.sub.+=(μ.sub.0.Math.ΔM.Math.N.Math.A)/(U.sub.S−i.sub.S.Math.R.sub.SH)and  (6b)
Δt.sub.−=(μ.sub.0.Math.ΔM.Math.N.Math.A)/(U.sub.S+i.sub.S.Math.R.sub.SH).  (6c)

(19) The parameter ΔM represents the magnetization swing during the magnetization reversal process. It can be seen from Equations 6a to 6c that the sensor's oscillation frequency f.sub.SENSOR is a function of the primary current itself, of voltage amplitude U.sub.S of the voltage generated by voltage source Q, and of magnetization swing ΔM.

(20) Particularly in the case of differential current sensors, the geometric arrangement of the primary conductors relative to magnetic core 10 is asymmetric, and a complete cancelling of the resulting magnetic field does not occur, even if the difference i.sub.Pa−i.sub.Pb is zero. This results in local magnetic saturations in magnetic core 10, which entails a diminution in the effective cross-sectional area A of core 10. This results in modulation of time spans Δt.sub.+ and Δt.sub.−. This modulation is periodic and depends on frequency f.sub.P of primary current i.sub.P. This effect can also be derived from Equations 6b and 6c if it is assumed that cross-sectional area A varies periodically with frequency f.sub.P of primary currents i.sub.Pa and i.sub.Pb.

(21) In order to measure the frequency of the primary current, measurement and control unit 20 can be configured to regularly sample secondary current i.sub.S (i.e., measuring signal u.sub.SH, which represents the secondary current) and to calculate a spectrum from sampled values i.sub.S[n] (e.g., using a fast Fourier transform (FFT) algorithm, if appropriate, with windowing). The spectrum will have a clear (global) maximum at the sensor's oscillation frequency f.sub.SENSOR. Two other (local) maximums with significantly lower magnitudes (maximums of the side lobes) are found at frequencies f.sub.1=f.sub.SENSOR−f.sub.P and f.sub.2=f.sub.SENSOR+f.sub.P. Therefore, the sought frequency f.sub.P of the primary current can be determined from frequencies f.sub.1 and f.sub.2 of the maximums of the first two side lobes according to the following equation, for example:
f.sub.P=(f.sub.2−f.sub.1)/2.  (7)

(22) The diagrams in FIGS. 5 and 6 illustrate the aforementioned function of the current sensor. The top diagrams of FIGS. 5 and 6 show a waveform of primary current i.sub.Pa=i.sub.Pb of a differential current sensor (wherein the differential current Δi.sub.P=i.sub.Pa−i.sub.Pb=0) and of the corresponding secondary current i.sub.S over a time of approximately 50 ms. In the case of FIG. 5, primary current i.sub.Pa=i.sub.Pb is equal to zero; in the case of FIG. 6, the primary current has a sinusoidal course with a frequency f.sub.P of 50 Hz. In both cases, the sensor measures the same differential current i.sub.Pa−i.sub.Pb, namely zero amperes. The bottom diagrams of FIGS. 5 and 6 show the spectrum of secondary current i.sub.S (dependent on primary currents i.sub.Pa and i.sub.Pb). The global maximum of the spectrum is at the current sensor's oscillation frequency f.sub.SENSOR (cf. FIG. 2b), which, in the present example, is approximately 2 kHz. In the example shown in FIG. 5, the current sensor's oscillation frequency f.sub.SENSOR is 1,935 Hz. In the case shown in FIG. 6, it is 1,970 Hz. The somewhat higher value of the oscillation frequency can be explained in that primary currents i.sub.Pa and i.sub.Pb cause local saturations in the magnetic core, as a result of which the effective size of cross-sectional area of the core drops. Consequently, the magnetization of the core can be reversed more rapidly and the frequency rises.

(23) If primary currents i.sub.Pa and i.sub.Pb have no AC component, the global maximum (main lobe) at the current sensor's oscillation frequency f.sub.SENSOR is the single significant maximum in the spectrum. However, if primary currents i.sub.Pa and i.sub.Pb have an AC component, this results in the modulation of the sensor oscillation's cycle period f.sub.SENSOR.sup.−1 (cf. Equation 6a), as explained above. This modulation entails local maximums in the spectrum at frequencies f.sub.1 and f.sub.2 (first side lobes) and f.sub.1′ and f.sub.2′ (second side lobes) on both sides of the global maximum at frequency f.sub.SENSOR. The “distances” |f.sub.1−f.sub.SENSOR| and |f.sub.2−f.sub.SENSOR| between the frequencies of the two local maximums and the global maximum correspond to frequency f.sub.P of primary currents i.sub.Pa and i.sub.Pb. Frequency interval f.sub.2-f.sub.1 corresponds to the double frequency f.sub.P of primary currents i.sub.Pa and i.sub.Pb (cf. Equation 7). In the example shown in FIG. 6 (when frequency f.sub.P of the primary current is 50 Hz), the calculation according to Equation 7 results in a measured primary current frequency f.sub.P′ of (2,020-1,920)/2 Hz=100/2 Hz=50 Hz. Additionally or as an alternative, even the second side lobes can be evaluated at frequencies f.sub.1′ and f.sub.2′. The second side lobes are twice as far away as the first side lobes from the global maximum at the current sensor's oscillation frequency f.sub.SENSOR. Therefore, f.sub.1′=f.sub.SENSOR−2.Math.f.sub.P and f.sub.2′=f.sub.SENSOR+2.Math.f.sub.P, and f.sub.1−f.sub.1′=f.sub.P and f.sub.2′−f.sub.2=f.sub.P.

(24) While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible within the scope of the invention. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents. With regard to the various functions performed by the components or structures described above (assemblies, devices, circuits, systems, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component or structure that performs the specified function of the described component (e.g., that is functionally equivalent), even if not structurally equivalent to the disclosed structure that performs the function in the exemplary implementations of the invention illustrated herein.