Current transducer of the Rogowksi type and arrangement for measuring a current

Abstract

The invention is about a current transducer of the Rogowski type, with a primary conductor winding having a first number of loops (N1) for carrying the rated current (I.sub.R) to be measured, with a secondary conductor winding having a pair of second terminals and a helical shape and a second number of loops (N2), said secondary conductor winding encircling the primary conductor in a toroidal manner, whereby a rated current voltage signal V.sub.S is induced between the pair of second terminals of the secondary winding, said rated current voltage signal being characteristic for the derivative of the rated current (dI.sub.R/dt), with a transducer electronics (IED) configured to receive the rated current voltage signal (V.sub.S), characterized in that the current transducer comprises a third conductor winding having a pair of third terminals with a third number of loops (N3), whereby the transducer electronics (IED) is configured to feed a calibration current signal (I.sub.Cal) into the third conductor winding, whereby in response to the derivative of the calibration current signal (dl-.sub.Cal/dt) an additional calibration signal (V.sub.cal) is created between the pair of second terminals of the second winding and whereby the transducer electronics (IED) is configured to process the rated current voltage signal (V.sub.S) and the calibration signal (V.sub.cal) to derive a corrected voltage signal (V.sub.S,corrected) with a calibrated sensitivity S.sub.cal.

Claims

1. An arrangement for measuring a current, with a current transducer of the Rogowski type and a transducer electronics, the current transducer comprising: a primary conductor for carrying the rated current to be measured, a secondary conductor winding having a pair of second terminals and a helical shape and a second number of loops, said secondary conductor winding encircling the primary conductor in a toroidal manner, a third conductor winding having a pair of third terminals with a third number of loops, wherein the third conductor winding is adapted to receive a calibration current signal, and wherein the secondary conductor winding is adapted to induce between its pair of second terminals a voltage signal, said voltage signal being a superposition of: a rated current voltage signal, being characteristic for the derivative of the rated current, and an additional calibration signal in response to the derivative of the calibration current signal, and the transducer electronics is: adapted to feed the calibration current signal into the third conductor winding, adapted to receive the voltage signal, and adapted to process the voltage signal to derive a corrected voltage signal with a calibrated sensitivity, wherein the transducer electronics is adapted to feed the calibration current signal with an amplitude which is smaller than the amplitude of the rated current, and with a frequency which is higher than the frequency of the rated current.

2. The current transducer according to claim 1, wherein the third conductor winding is arranged in close proximity to the second conductor winding.

3. The current transducer according to claim 1, wherein the third conductor winding covers the full perimeter of the second conductor winding.

4. The current transducer according to claim 1, wherein the current transducer comprises a shield surrounding the secondary winding and whereby the third conductor winding is placed between the secondary winding and the shield.

5. The current transducer according to claim 1, wherein the third number of loops is smaller than the second number of loops.

6. Arrangement according to claim 1, wherein the transducer electronics is adapted to apply several calibration current signals with several calibration frequencies sequentially.

7. Arrangement according to claim 1, wherein the transducer electronics is adapted to subsequently obtain and average values for the calibrated sensitivities in order to obtain improved accuracy.

8. Arrangement according to claim 1, wherein the transducer electronics is adapted to calculate the calibrated sensitivity continuously.

9. Arrangement according to claim 1, wherein the transducer electronics is adapted to calculate the calibrated sensitivity on a scheduled basis.

Description

(1) The invention will be described in greater detail by description of an embodiment with reference to the accompanying drawings, wherein

(2) FIG. 1. shows the principle of the state of the art Rogowski coil current transducer, and

(3) FIG. 2 shows the principle of the Rogowski coil current transducer design according to the invention.

(4) FIG. 1 shows the principle of a state of the art Rogowski coil current transducer 1, where the calibration coefficients are obtained by characterizing the sensors in the factory. The primary conductor 3 carrying the rated current i.sub.R(t) to be measured is passing through the centre of a conventional Rogowski coil 2. Between the pair of secondary terminals 4, 5 of the secondary conductor winding 6 there is a rated current induced voltage signal V.sub.S, which can be determined as V.sub.S=S×d(i.sub.R(t))/dt, where S is the sensitivity of the Rogowski coil.

(5) A temperature sensor 7 measuring the temperature is placed closed to the Rogowski coil 2. The rated current voltage signal V.sub.S and the temperature measurement is fed into the transducer electronics IED. In the IED, the temperature measurement is used to compensate the sensitivity S according to each Rogowski sensitivity temperature profile. The coefficients, which give the polynomial correction to apply to the signal in the transducer electronics, are stored in an EEPROM 8 placed in the sensor casing. The corrected sensitivity S.sub.corrected consists of a product multiplying the original coil sensitivity S and a correction polynom (α.sub.1×T+α.sub.2×T.sup.2+ . . . ). The corrected rated current voltage signal V.sub.Scorrected is then calculated to be
V.sub.Scorrected=Scorrected×d(i.sub.R(t))/dt=(α.sub.1×T+α.sub.2×T.sup.2+ . . . )×S×d(i.sub.R(t))/dt.

(6) The characterization of the coil is performed at the end of production of the coil. During this so called calibration, the Rogowski coil current transducers sensitivity at ambient temperature as well as its temperature dependency are measured. This solution allows temperature effect as well as initial error compensation, initial errors for example due to ambient temperature, but requires additional production effort, such as calibration and temperature characterization of each Rogowski coil current transducer.

(7) So the temperature sensor 7 and the EEPROM 8 containing the coefficients allow online temperature compensation, but no ageing, humidity or mechanical effect are taken into account since the coefficients cannot be dynamically updated.

(8) FIG. 2 shows the principle of a Rogowski coil current transducer (RCCT) 9 design according to the invention. It comprises a Rogowski coil 10, where the primary conductor 14 carries the rated current i.sub.R(t) to be measured and is passing through the centre of the conventional Rogowski coil 15 with the secondary conductor winding 16.

(9) The Rogowski coil current transducer 9 comprises a third conductor winding 10 having a pair of third terminals 11, 12 with a third number of loops (N3).

(10) The transducer electronics (IED) 13 is configured to feed a calibration current signal i.sub.Cal(t) into the third conductor winding 10. In response to the derivative of the calibration current signal (di.sub.Cal(t)/dt) through the third conductor winding 10, an additional calibration signal (V.sub.cal) is created between the pair of second terminals 17, 18 of the second winding 16: V.sub.cal=N3×dI.sub.cal/d.sub.t.

(11) The rated current voltage signal V.sub.S′ between the pair of secondary terminals 17, 18 of the secondary conductor winding 16, which is fed into the transducer electronics, can be determined to be V.sub.S′=S×(d(i.sub.r(t)+N3×i.sub.Cal(t))/dt), with S being the sensitivity of the Rogowski coil. V.sub.S′ can be understood to be a superposition of the rated current voltage signal V.sub.S=d(i.sub.r (t)/dt) and the calibration voltage signal V.sub.cal=N3×dI.sub.cal/d.sub.t.

(12) The transducer electronics 13 (IED) is configured to process the rated current voltage signal V.sub.S′ and the calibration signal V.sub.cal to derive a corrected voltage signal
V.sub.S,corrected=S.sub.cal.×d(i.sub.R(t))/dt, with a calibrated sensitivity S.sub.cal.

(13) So an additional winding N3 is designed for the RCCT, which is fed by a calibration current. The frequency of the calibration current is outside the frequency range of the rated current i.sub.R(t) to be measured. This additional winding offers the possibility to continuously measure within the transducer electronics 13 (IED) the corrected RCCT sensitivity S.sub.cal. without interrupting the rated current i.sub.R(t) measurement.

(14) An accurate calibration signal i.sub.Cal(t) is injected through the additional winding N3. Knowing precisely the number of loops N3 and the current i.sub.Cal(t) i.e. its amplitude and frequency, one can separate i.sub.r(t) and i.sub.cal(t) within the secondary voltage and detect any changes of the RCCT sensitivity. In this manner a way is provided to calibrate the sensitivity S.sub.cal without interrupting the measurement of the rated current i.sub.R(t).

(15) i.sub.cal(t) can be much smaller than i.sub.R(t) in amplitude, as smaller currents can be generated in the electronics more easily, and may have a higher frequency. For example for a value of I.sub.r=100 A (amplitude of i.sub.R(t)) and f.sub.r=50 Hz, one can choose the combination f.sub.cal>5 kHz, I.sub.cal=10 mA (amplitude of i.sub.cal(t)), N3=100 loops. In this manner one has: d(i.sub.R(t))/dt=N3.Math.di.sub.cal(t)/dt, so the signal of the calibration current creates about the same signal amplitude as the rated current. This is advantageous for achieving best resolution and accuracy in the calibration. f.sub.cal shall be selected outside the specified bandwidth of the primary current in the typical applications, i.e. higher than the highest harmonic to be measured.

(16) So if one needs to limit the amplitude I.sub.cal of the current i.sub.cal(t) to much less than the amplitude I.sub.R of the rated current i.sub.R(t), which may be required because i.sub.cal(t) has to be generated in the transducer electronics IED, i.sub.cal(t) can be applied at a frequency 10 to 100 times higher than the rated current signal. Since one can assume a very linear behavior of the Rogowski coil current transducer (RCCT) in its frequency domain, which is typically in a range between 10 Hz and 10 kHz, this frequency discrepancy between i.sub.R(t) and i.sub.cal(t) does not affect significantly the sensitivity estimation, which will be then calculated at a frequency 10 to 100 times higher than the rated current frequency. It is even possible to reach higher frequency, i.e. smaller current amplitude, while staying in the linear regime.

(17) In many cases there is an electric shield around the Rogowski coil in order to protect it from electric crosstalk from the primary conductor or from other sources. In this case it is advantageous to put the calibration windings between the shield and the winding N2 in order to protect it from external influences, thus making it easier to perform a clean calibration.

(18) In the case of a very high homogeneity of the winding N2, N3 can be a short cylindrical winding covering just a part of N2.

(19) However, in order to accurately take into account the possible localized nonhomogeneity of the winding N2 for the RCCT sensitivity calculation, it is advantageous that the winding N3 covers the full perimeter of the coil. A reasonable number of loops ratio can be chosen between N2/N3=10 and N2/N3=100. This number of loops contributes also to limit the necessary current amplitude I.sub.cal(t). So the number of loops N3 can be smaller than the secondary winding number of loops N2, but N3 should cover the entire RCCT perimeter in order to reduce the effect of winding inhomogeneities. The ratio N2/N3 depends on the expected N2 winding homogeneity. The more homogenous the winding N2 is, the higher N2/N3 (the lower N3) may be.

(20) The winding N3 could be placed between the winding N2 and the shield, thus protecting it from external perturbations.

(21) The signals of the calibration current and the current to be measured are separated in the transducer electronics IED based on frequency separating filters. The signal generated by the calibration current is removed from the output signal of the Rogowski coil sensor. The signal induced by the calibration current is thus used to correct the amplitude of the output signal of the Rogowski coil sensor.

(22) Several calibration frequencies can be sequentially used in order to obtain more accurate calibration coefficients. Alternatively, subsequent calibration coefficients can be averaged in order to improve the accuracy.

(23) The calibrated sensitivity S.sub.cal is calculated continuously, or on a scheduled-basis, in the transducer electronics IED and applied to the rated signal i.sub.R(t) ensuring highest accuracy on the rated current measurement over changing conditions, such as aging, temperature, mechanical strain or humidity. If the calibration current is precisely controlled, dimensional changes of the calibration winding due to these influences will not deteriorate the accuracy of the calibration process,

(24) The rated current i.sub.R(t) measurement can be never interrupted. It is not influenced by the continuously injected calibration current i.sub.cal(t). This principle is called continuous online calibration.

(25) In order to avoid damping and phase shift of the sensor signal, suited measures should be taken to prevent the primary current from inducing currents in the calibration winding. This can be done by inserting an additional effective impedance into the path of the calibration current. This can be achieved with a high pass filter or with an active control of the calibration current, which turns it into an ideal current source.

(26) It is obvious that the calibration procedure as described above in the context of FIG. 2 can be applied on-line or off-line. In case of an off-line calibration the transducer electronics 13 would be connected to the Rogowski coil transducer 9 in an off-line mode, for example in the factory at the end of production, or in the field during an in-field sensor maintenance operation.

LIST OF REFERENCE SIGNS

(27) 1 Rogowski coil current transducer 2 Rogowski coil 3 primary conductor 4 secondary terminal 5 secondary terminal 6 secondary winding 7 temperature sensor 8 EEPROM 9 Rogowski coil current transducer 10 third winding 11 third terminal 12 third terminal 13 transducer electronics 14 primary conductor 15 Rogowski coil 16 secondary winding 17 secondary terminal 18 secondary terminal