Current transformer and method for converting a conductor current flowing in an electrical conductor to an output signal

Abstract

A current transformer arrangement, and method, include a current sensor for producing a sensor output voltage that is proportional to the input current flowing in a conductor; a first processing branch including a voltage calculating device for calculating the effective voltage value of the sensor output voltage; a second processing branch including a polarity detecting device having an input terminal connected with the current sensor output terminal to produce a polarity signal having one polarity when the conductor input current is either an alternating current, a hybrid current, or a direct current flowing in one direction, and the opposite polarity when the conductor input current is a direct current flowing in the opposite direction; and a multiplier device for modifying the effective voltage value to produce a signed effective voltage having a polarity corresponding with the polarity of the polarity signal. A modifying circuit modifies the signed effective voltage.

Claims

1. A current transformer arrangement, comprising: (a) a current sensor having an input terminal connected with a conductor carrying a conductor input current, and an output terminal at which is produced a first output voltage that is proportional to the conductor input current; (b) a first processing branch including a voltage calculating device connected with said current sensor output terminal for calculating an effective voltage value of said first output voltage, said first processing branch having a first output terminal; (c) a second processing branch including a polarity detecting device having an input terminal connected with said current sensor output terminal, said second processing branch having a second output terminal at which a polarity signal is produced having: (1) a first polarity when the conductor input current is either an alternating current, a hybrid current, or a direct current flowing in one direction, and (2) a second polarity opposite to said first polarity when the conductor input current is a direct current flowing in the opposite direction; and (d) a multiplier device having a first input connected with said first processing branch first output terminal, and a second input terminal connected with said second processing branch second output terminal, said multiplier device being operable to modify said effective voltage value to produce a signed effective voltage value having a polarity corresponding with the polarity of said polarity signal.

2. The current transformer arrangement defined in claim 1, wherein said first processing branch includes a first low-pass frequency filter with high cut-off frequency, said first low-pass frequency filter being operable to filter high-frequency interference signals from said current sensor output voltage.

3. The current transformer arrangement as defined in claim 2, wherein said first low-pass frequency filter has an adjustable cut-off frequency.

4. The current transformer arrangement as defined in claim 2, wherein said first low-pass frequency filter has an adjustable step response time.

5. The current transformer arrangement as defined in claim 2, wherein said voltage calculating device has an adjustable time constant.

6. The current transformer arrangement defined in claim 2, wherein said second processing branch includes a second frequency filter operable to smooth out said current sensor output voltage, thereby to detect the sign of the direct current component of the conductor current input.

7. The current transformer arrangement defined in claim 1, and further including: (e) an output modifying circuit for modifying said signed effective value voltage for use by the user.

8. The current transformer arrangement defined in claim 7, wherein said output modifying circuit includes an adder for superimposing an offset voltage upon said signed effective value voltage.

9. The current transformer arrangement defined in claim 7, wherein said output modifying circuit includes an adder for adding a scaling factor upon said signed effective voltage.

10. The current transformer arrangement defined in claim 7, wherein said output modifying circuit includes a signal selecting device for selecting either a current output signal or a voltage output signal.

11. The current transformer arrangement defined in claim 1, wherein said current sensor is a Hall effect sensor.

12. The current transformer arrangement defined in claim 1, wherein said current sensor is a shunt resistor.

13. The current transformer arrangement defined in claim 1, and further including an analog/digital converter connected with the output of said output modifying circuit for supplying a control signal to an automation control system.

14. A method for converting a conductor current flowing in an electrical conductor into an output signal, comprising the steps of: (a) detecting the conductor current by means of a current transforming sensor to produce a sensor output voltage that is proportional to the conductor current; (b) calculating by means of a calculating device the effective voltage value of said sensor output voltage; (c) generating by means of a polarity detecting device a polarity signal as a function of the polarity of said sensor output voltage; (d) multiplying said effective voltage value by means of a multiplying device with a positive or negative factor corresponding with the polarity of said sensor output voltage, thereby to produce a signed effective value; and (e) modifying said signed effective value by an output modifying circuit to produce a modified output signal corresponding with the sign of said signed effective value.

15. The method as defined in 14, and further including the step of scaling said signed effective value by a scaling device by a scaling factor.

16. The method as defined in claim 14, and further including the step of adding by an adder device an offset voltage to said signed effective value.

17. The method as defined in claim 14, and further including the step of applying said modified output signal to an automation control system.

18. The method as defined in claim 14, wherein said polarity signal has: (1) a first polarity when said conductor current is a pure alternating current, a hybrid alternating current, or a direct current flowing in a first direction, and (2) a second polarity opposite to said first polarity when said conductor current is a direct current flowing in the opposite direction.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other objects and advantages of the invention will become apparent from a study of the following specification, when viewed in the light of the accompanying drawing, in which:

(2) FIG. 1a is a graphic representation of the transfer characteristic of a current transformer system of the Prior Art for measuring an alternating current flowing in a conductor, and FIG. 1b is a graphic representation of the transfer characteristic of a Prior Art current transformer system for measuring direct current flowing in a conductor;

(3) FIG. 2 is a graphic representation of the transfer characteristic of the current transfer system of the present invention; and

(4) FIG. 3 is a block diagram illustrating the current transformer system of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(5) Referring first to FIG. 1a, a transfer characteristic 3 is illustrated for a current transformer of the Prior Art for measuring the value of an alternating current flowing in a conductor. On the x-axis is plotted the conductor current value I.sub.AC,LOAD in Amperes (A), and on the y-axis is shown the output signal of an output current value I.sub.out in Milliamperes (mA). The output current is proportional to the effective value of the conductor current I.sub.AC,LOAD which produces the magnetic field in a Hall effect sensor, with a scaling factor k=I.sub.OUT/I.sub.AC,LOAD. Furthermore, an offset value I.sub.AC,OUT/OFF is superimposed on the output current, so that for a conductor current of 0 A, a measurable output current flows, being here 4 mA.

(6) Owing to the forming of the effective value, such current transformers are unipolar in design. Therefore, as shown in FIG. 1b, the measurement of a direct current I.sub.DC,LOAD as a conductor current always furnishes a positive output signal, regardless of the direction of the conductor current.

(7) FIG. 2 shows a transfer characteristic 3 of a current transformer 100 according to the present invention. On the x-axis is plotted the conductor current 1, I.sub.LOAD in Ampere (A), on the y-axis is shown as the output signal 2 an output current 2, I.sub.out in Milliamperes (mA). The output current 2, I.sub.out is proportional to an effective value of the conductor current 1, I.sub.LOAD. It is weighted with the scaling factor k=I.sub.out/I.sub.LOAD in analogous fashion to the output current value 2, I.sub.OUT of the current transformer known from the prior art (see FIG. 1(a)). Furthermore, the output current 2, I.sub.OUT is likewise superimposed by an offset value 6, I.sub.OUT/OFF, so that for a conductor current 1, I.sub.LOAD of 0 A, there flows a measurable output current 2, I.sub.OUT.

(8) The offset value 6, I.sub.OUT/OFF here is composed of a base value I.sub.GRUND, which corresponds here to the offset value 6 of a traditional current transformer (see. FIG. 1(a)) of 4 mA for example, plus the half value range being plotted:
I.sub.OUT/OFF=I.sub.GRUND+8 mA=12 MA

(9) The conductor current 1, I.sub.LOAD is therefore plotted as follows onto an output current 2, I.sub.OUT:
I.sub.OUT=12 mA+k*I.sub.LOAD/RMS
insofar as I.sub.LOAD is an alternating current, or a direct current flowing in the positive direction, as represented in a first region A, and
I.sub.OUT=12 mAk*I.sub.LOAD/RMS
insofar as I.sub.LOAD is a direct current flowing in the negative direction (see second region B).

(10) The current transformer 100 according to the invention thus works in bipolar fashion for a direct current as the conductor current and at the same time it is able to indicate an effective value for an alternating current as the conductor current. The overall information (effective value and the sign in the case of direct current) is put out in an output signal and can thus be detected by a single analog/digital converter 20 of an automation control system 22.

(11) FIG. 3 shows the current transformer 100 schematically. It comprises here a Hall sensor 10 for detecting the conductor current 1, I.sub.LOAD as a current sensor. At the output of the Hall effect sensor 10 there is measured an output voltage 4, which is generated by the magnetic field in the Hall sensor 10 produced by the conductor current 1, I.sub.LOAD. The output voltage 4 is a signed value. In an alternative embodiment, a shunt resistor can also be used as the current sensor, through which the conductor current 1, I.sub.LOAD flows. The voltage drop across the shunt resistor then represents the signed output voltage 4.

(12) The current transformer 100 has two processing branches I, II arranged in parallel with each other.

(13) In the first processing branch I there is provided a means 12 of calculating the effective value 5 of the output voltage 4 of the Hall sensor 10. In order to filter out interference signals, optionally a first frequency filter 11 is hooked up before the means 12 of calculating the effective value 5 of the output voltage 4, preferably being a low-pass filter with high cut-off frequency. The cut-off frequency of the first frequency filter 11 and/or its step response time are optionally adjustable. In this way, the user can choose whether the output signal 4 filtered in the first processing branch I will be used to detect essentially sinusoidal alternating currents, or also distorted alternating currents. Moreover, it can be provided that a time constant used in the means 12 for calculating the effective value 5 is adjustable. By selecting a short time constant, effective values can be put out with a high rate of variation, but then lower frequencies of the output voltage 4 will not be counted in the determination of the effective value. On the other hand, by selecting a longer time constant one can ensure that lower frequencies are also counted in the determination of the effective value, but the output of the effective value will respond correspondingly slowly to changes.

(14) In the second processing branch II there is provided a means 18 for detecting the polarity +/ of the output voltage 4. The detected polarity is produced as a signal which is assigned the value of +1 or 1. The means 18 is configured such that the value +1 is produced for a (pure) alternating current as the conductor current 1. A second frequency filter 17 is connected in front of the means 18 for detecting the polarity +/ of the output voltage 4, which smooths out the output signal 4. In this way, the smoothed output signal 4 receives the sign of a direct current component contained in the conductor current 1, I.sub.LOAD and not the sign of an alternating current component contained in the conductor current 1, I.sub.LOAD.

(15) The effective value 5 calculated in the first processing branch I is multiplied with a factor of +1 or 1 in a multiplier 13, depending on the polarity +/ detected in the second processing branch II, for example, a factor of +1 when a positive polarity is detected and a factor of 1 when a negative polarity is detected.

(16) The current transformer 100 is thus suitable not only for the detection of a pure alternating current as the conductor current 1, I.sub.LOAD or a pure direct current as the conductor current 1, I.sub.LOAD. It is also suitable for detection of hybrid currents as the conductor current 1, I.sub.LOAD, having both a direct current component and an alternating current component. For a pure alternating current as the conductor current 1, I.sub.LOAD the conductor current 1, I.sub.LOAD will be plotted in the first region A of the transfer characteristic 3 shown in FIG. 2.

(17) In order to modify the signed effective value 5 in a suitable manner for the user, the current transformer 100 furthermore contains an output modifying circuit 19. The output modifying circuit 19 comprises an adder 14, which superimposes an offset value I.sub.OUT/OFF on the weighted effective value 5. The superimposing of the offset value 6 and the magnitude of the offset value 6 are optionally adjustable by the user. Furthermore, the output circuit 19 comprises a means 15 of scaling the signed effective value 5 optionally superimposed with the offset value 6. The scaling factor k is also optionally adjustable by the user.

(18) Furthermore, one can adjust whether the signed effective value 5 optionally superimposed with the offset value 6 and/or scaled with the scaling factor k is produced in the form of an output current signal 2, I.sub.OUT or an output voltage signal 2, U.sub.OUT. For this, the output circuit 19 comprises an output signal selecting circuit 16 which can be adapted by the user.

(19) While in accordance with the provisions of the Patent Statutes the preferred forms and embodiments of the invention have been illustrated and described, it will be apparent to those skilled in the art that changes may be made without deviating from the invention described above.