GB2259150A

GB2259150A - Current measurements

Info

Publication number: GB2259150A
Application number: GB9118428A
Authority: GB
Inventors: William Frederick Ray; Rex Mountford Davis
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-08-28
Filing date: 1991-08-28
Publication date: 1993-03-03
Anticipated expiration: 2011-08-28
Also published as: GB2259150B; GB9118428D0

Abstract

A current measuring device comprises a Rogowski coil (Figure 3) which is connected to an integrator and feedback circuit by a coaxial cable. The transfer function VOUT/VIN of the integrator and feedback circuit is arranged (Fig 2 ii) such that the gain (Go) of the combined feedback and integrator at a frequency substantially lower than the frequency of the current to be measured is substantially less than the gain ( alpha Go) at a frequency for which the gain is a maximum. An input resistor Ro may be arranged on the Rogowski coil itself, i.e. rather than at the integrator end of the coaxial cable (Figures 6, 7 not shown). <IMAGE>

Description

Improvements In Current Measurement This invention relates to the measurement of current waveforms using the combination of a Rogowski coil and an electronic integrator and in particular to improvements in the circuit for the electronic integrator such that waveforms of lower current amplitude may be measured than has been possible hereto.

A Rogowski coil has the advantage that it may be looped around a current carrying conductor and may be used to provide an isolated or contactless measurement of the current. It is well known that a coil of uniformly spaced turns wound on a former of constant cross-sectional area and arranged to form a dosed loop will produce at any instant, a voltage which is proportional to the rate of change of the total current passing through the loop at that instant. Such a coil is commonly known as a Rogowski coil following the publication in 1912 by Rogowski W and Steinhaus W "Die Messung der Magnitischen Spannung" (Arch Elektrotech 1, pp 141150) and will be referred to as such in this specification.

The former is normally non-magnetic but it may be magnetic provided the relative permeability of the magnetic material used is sufficiently small such that the material does not magnetically saturate when used to carry a Rogowski coil.

In order to reproduce the waveform of the current linking the Rogowski coil, it is necessary to integrate with respect to time the voltage produced by the coil.

Some previous applications of Rogowski coils have used coil terminating resistors of small value to provide the integration in which case the coils are only suitable for measuring short duration pulses of very large (for example mega-amp) current amplitude. Previous applications which have used electronic integrators have used a feedback resistor around the integrator to limit the low frequency gain and thereby to avoid significant integrator drift. However, for this arrangement the gain of the integrator over the range of frequencies for which current measurement is required is less than the low frequency gain and hence, when used with a Rogowski coil, the sensitivity of the current measurement is low and the device is only suitable for measuring large currents for, example 100,000A.

The main object of the present invention is to increase the gain of the integrator over the range of frequencies necessary for the measurement of current waveforms of modest amplitude such as several hundred amperes associated with power circuits, whilst by reducing the relative gain at lower frequencies to avoid integrator drift effects.

According to the invention, a current waveform measuring device includes a former of substantially uniform cross-sectional area made of material which does not magnetically saturate, arranged to form a substantially dosed loop and carrying a uniformly wound coil, an electronic integrator the input of which is connected to the coil and feedback means responsive to the output voltage of the integrator whereby the input signal to the integrator may be adjusted such that the gain of the combined feedback means and integrator at frequencies substantially lower than the frequencies of the current to be measured is substantially less than the gain at the frequency for which the gain is a maximum.

The coil former may be a continuous ring made of plastic or some other suitable material and may be rigid or flexible. However, for ease of use it is desirable that the Rogowski coil may be looped around a conductor without disconnecting the conductor in which case the coil former needs to be flexible. The coil former may be a ring which is cut such that the ring can be opened and snapped shut around the conductor or may be a length of flexible material which can be bent into a loop around the conductor such that its ends are in close proximity.

There are a number of known electronic circuits which can be used for integrating a voltage; the circuit most generally used comprises an operational amplifier, an input resistor and a feedback capacitor. The input resistor is normally positioned close to the input terminal of the operational amplifier.

The integrator may be assembled in close proximity to the Rogowski coil.

However, it is generally more convenient to connect the integrator to the coil by a length of co-axial cable. Unless precautions are taken, this cable has disadvantageous effects on the operation of the current measuring device. The current waveforms to be measured may have short periods of time for which the rate of change of current is very high (several hundred amperes per microsecond). To measure these transients it is necessary that the bandwidth of the device extends to high frequencies, (for example in excess of 1MHz). Because of the capacitance of the cable and the inherent inductance of the Rogowski coil, there is a time delay before the induced coil voltage is applied to the input resistor of the electronic integrator. This time delay reduces the bandwidth of the rrreasuring device.From another viewpoint, it takes a finite time to charge the capacitance of the cable such that the coil induced voltage is seen by the integrator input resistor.

A further object of the present invention is to enable high rates of change of current to be reproduced in the measurement signal provided by the measuring device. The disadvantage of the cable capacitance may be avoided or substantially reduced by incorporating all or most of the input resistance of the integrator in the assembly of the nogowski coil. This has the effect that the voltage between the inner and outer conductors of the coaxial cable are virtually zero or substantially reduced compared with the case where the input resistance of the integrator is positioned at the integrator end of the coaxial cable.Since during transient currents of high rate of change the Rogowski coil generates large voltage pulses, positioning the integrator resistance entirely or mostly at the Rogowski coil end of the coaxial cable has the added advantage of reducing the voltage rating and hence the size of the cable.

The integrator input resistance incorporated in the assembly of the Rogowski coil may be a separate resistor, or resistors or the Rogowski coil may be wound with high resistivity wire to provide all or part of the resistance. Distributing the resistance in the winding of the Rogowski coil has the advantage that the voltage between turns is reduced thereby reducing the possibility of winding insulation failure.

In the case where a Rogowski coil is snapped or looped around a conductor it is necessary to make a connection from the coaxial cable to each end of the coil.

To avoid extraneous signals being induced in the coil other than due to the current being measured, it is necessary that any unscreened connecting wires should lie substantially along the centre of the coil and for this purpose the coil former requires either a hole along the centre of its axis or a slot along its length so that the connections can be so positioned. It is also preferable that any resistor assembled with the coil should also be positioned inside the former.

The Rogowski coil may have an earthed conducting screen outside the coil but arranged so as not to present a short-circuit turn around the coil. This is to minimise the presence of extraneous measurement signals due to capacitive coupling to the conductor whose current is being measured or to other conductors in the vicinity.

The screen however, has the disadvantage that it increases the capacitance of the coil and reduces its ability to measure high rates of change of current, and in applications for which high bandwidth is of more importance than high accuracy the screen may be omitted.

The principle of the invention will now be explained further with the aid of Figure 1 and Figure 2 which relate to the integrator and how its behaviour may be modified to advantage by application of feedback means in the form of a feedback circuit.

Figure I shows the basic block diagram where Vja, Vout and Vfb are the input, output and feedback signal voltages respectively. The integrator transfer function is I/(joT,) where co is the angular frequency of the signal and T1 is the integrator time constant.

The transfer function of the feedback, circuit 'H(jcl)) = Vtb/Vout preferably takes the form:

where T is a time constant and a and Go are gain factors, and where a is large and cc > 1.

The frequency response for the feedback circuit is shown as characteristic (i) in Figure 2. It will be seen that the gain of the feedback circuit is lower by a factor of a at angular frequencies in excess of Co = 1/T than for very low frequencies less than Co = 1/(aT).

By arranging that T takes a value near to T = GOT,(4a-2), the transfer function for the combined integrator and feedback circuit may be made to approach the form:

where for large values of a, the value of ,ss is close to 0.5.

The frequency response for the combined integrator and feedback circuit is shown as characteristic (ii) in Figure 2. For angular frequencies greater than the value of co = 2/T the combined circuit behaves as an integrator and the effect of the feedback circuit is negligible. For angular frequencies lower than Co = 2/T the gain of the combined circuit is reduced until at angular frequencies below Co = 1/aT, the gain is Go.

This reduction of gain is due to the effect of the feedback circuit which, as the angular frequency reduces below the value Co = 1/T, has a progressively higher gain and hence a progressively greater correction on the overall circuit.

The values for G, T,T, and a are chosen appropriately to give a suitable integrator gain and an acceptable integrator behaviour overthe range of frequencies for which current measurement is required (o > 2/T), whilst also reducing the gain for frequencies below this range such that at very low frequencies the gain is substantially reduced from its maximum value and integrator drift effects are thereby reduced or eliminated. By so doing the integrator gain may be set suitably high to enable the measurement of currents of modest amplitude which is the main object of the invention.

The invention will be described further with reference to Figures 3, 4, 5, 6 and 7 which illustrate various embodiments of the invention. In these figures the symbols Rl, R2 etc will be used to identify particular resistors in the circuits shown and these symbols will also be used to represent their respective resistance values. Similarly, the symbols C1, C2 etc will be used to identify various capacitors and also to represent their respective capacitance values.

Figure 3 shows the circuit for the first embodiment of the invention and includes three main units namely the Rogowski coil, the integrator and the feedback circuit. The integrator takes the well known form of an operational amplifier with an input resistor Ro and a feedback capacitor C,. In this embodiment the input resistor is assembled with the other integrator components at the receiving end of the co-axial cable connecting the integrator to the Rogowski coil. To avoid oscillation at the integrator input due to the interaction of the coil inductance and the cable capacitance and associated with sudden changes in the current to be measured, a damping resistor RD may be connected across the integrator input as shown.

The operation amplifier is connected to power supplies (not shown) such as batteries which provide positive and negative supply voltages +Vss and -Vb with respect to the zero reference voltage of the common line. The value of the resistors 1% and R4 should be ideally the same to reduce the effect of the input bias currents of the operational amplifier, but if their bias currents are sufficiently low R4 may be omitted and the non-inverting input of the operational amplifier connected directly to the common line. The operational amplifier may also have balancing and/or frequency compensation networks connected to it as recommended by its manufacturer, which are not shown in Figure 3.

The feedback circuit comprises resistors R1, R2 and R3 and capacitor C2. The voltage signal VN from the Rogowski coil produces a current in the resistor 1% substantially equal to VlN/Ro. The voltage signal Vx across the resistor-capacitor network R2 produces a current in the resistor R1 substantially equal to VX/R,. The sum of these currents largely flows through capacitor Cl such that

The voltage signal Vx is dependant on Volt, and when multiplied by the scaling factor Ro/RIr is equivalent to the feedback voltage signal Vfg shown in Figure 1.The feedback circuit adjusts the input to the integrator to produce the desired frequency response illustrated by characteristic (ii) in Figure 2.

It may be shown that the feedback circuit will provide the feedback transfer function HQw) referred to earlier. The resistance R3 is preferably made equal to resistance R1 in which case Go = 2R,/R, T = C2R2 a = 1 + (0.5Rl/R2) For the second embodiment of the invention the capacitor Cm shown in Figure 3 is replaced by two capacitors Cm and C4, where Ca = C3 + C4, as shown in Figure 4.

This enables polarised capacitors to be used in place of Ca which is helpful if the value of Cm is large. Cm and C4 are preferably the same value.

The feedback circuit may include non-passive components. In a third embodiment the feedback circuit includes an operational amplifier with associated resistors R1, R, R3 and Rg and capacitors C3 and C4 as shown in Figure 5. The resistors R, and R2 and capacitors Ca and C4 fulfil the same role and take the same values as for the first and second embodiments. Resistor Ra takes a value such that a = 1 + R3/R2. The feedback circuit of Figure 5 thereby has the same effect as and is equivalent to the feedback circuit of Figure 4.

For each of the three embodiments described above the integrator input resistor R,may be assembled with the Rogowski coil rather than at the receiving end of the connecting cable. In these cases the damping resistor RD shown in Figure 3 is not required.

Figures 6 and 7 by way of example,two forms of RDgowski coil incorporating the integrator input resistor Ro which would be omitted if the integrator resistor was included in the integrator assembly as shown in Figure 3. (It is also possible to subdivide the integrator input resistance by using two resistors, one assembled with the Rogowski coil and the other with the integrator).

Figure 6 shows a coil former, 11, of rectangular cross-section with a circumferential groove, 12, of such depth that the wire which rests at the bottom of the groove is substantially at the centre of the coil wound around the former. A co axial cable, 13, enters the groove (locally enlarged if necessary), the inner conductor of the cable passing around the groove in one direction and the screen (or outer conductor) passing around the groove in the opposite direction. The coil former is cut through at one point, 14, enabling it to be sprung apart sufficiently to pass over and encircle the cable (not shown) in which current is to be measured. Near the cut, 14, and on either side, two holes, 15, 16, are provided through which the coil winding wire, 17, passes from the groove to the uniform winding, 18, wound around the former throughout its circumference.The inner conductor is connected to an end of the integrator resistor, 1%, also accommodated within the groove (or a local enlargement thereof), the other end of the resistor being connected to the coil winding wire, 17, prior to passing through the hole, 15. The screen, or outer conductor is connected within the groove to the coil winding wire prior to passing through the hole, 16. The two windings 19, 20, continue around the circumference until they meet at the position of the co-axial cable, 13, where the two winding wires are joined together, thus completing the Rogowski coil and its resistors. If the winding wire has a sufficiently high resistance, the resistor, R,, can be reduced in value or omitted.

Figure 7 shows an alternative form of Rogowski coil where the former, 21, has a circular cross-section with a central hole, 22, (in place of the groove). An access hole, 23, is provided for the co-axial cable, 24, whereby its inner and outer conductors can be drawn through the central hole, 22, in opposite directions. The series resistor Ro, if located within the Rogowski coil, may require the local enlargement of the central hole, 22. Tic windings, 25, 26, begin at the cut, 27, in the former, and proceed around the former until they meet and are joined together preferably at tlle position of the co-axial cable.

Both forms of Rogowski coil may also incorporate an electrostatic screen surrounding, or substantially surrounding but not connected to, the coil, but not providing a shorted turn. This screen may be connected to the outer conductor of the co-axial cable, or it may be connected to a separate screen which surrounds the co-axial cable.

Claims

CLAIMS:

1. A current measuring device comprising a former, defining a loop of material which, in use, does not magnetically saturate when a conductor bearing a current to be measured is positioned extending through the loop, a Rogowski coil wound on the former, an integrator having an input connected to one end of the coil and an integrator feedback circuit connected to the output from the integrator, whereby the input signals to the integrator are adjusted such that the overall gain of the integrator and the feedback circuit, at frequencies substantially lower than that of the current to be measured, is less than the gain at a predetermined measuring frequency for which the gain is a maximum.

2. A device as claimed in claim 1 in which the transfer function of the integrator and feedback circuit is approximately:

where: Go is a gain factor greater than 1 a is a gain factor also greater than 1 T is a time constant equal to the reciprocal of the higher break frequency of the feedback frequency response, as defined in Figure 2 of the accompanying drawings p is a gain factor of approximately 0.5 for large a co is the angular frequency of the sensed signal.

3. A current measuring device comprising a former defining a loop of material which, in use, does not magnetically saturate when a conductor bearing the current to be measured is positioned extending through the loop; a Rogowski coil comprising a first winding, having a first portion passing circumferentially around one part of the loop and a second portion wound helically about the said one part of the loop and the first portion, and a second winding having a third portion passing circumferentially around another part of the loop and a fourth portion wound helically about the said other part of the loop and the third portion, ends of the first and third portions being respectively connected with adjacent ends of the second and fourth portions, the opposite ends of the second and fourth portions being connected together; an integrator having an input from the free ends of the first and third portions of the coil; and a feedback circuit associated with the integrator, the transfer function of the integrator and feedback circuit being approximately

where: Go is a gain factor greater than 1 a is a gain factor also greater than 1 T is a time constant equal to the reciprocal of the higher break frequency of the feedback frequency response, as defined in Figure 2 of the accompanying drawings.

p is a gain factor of approximately 0.5 for large a

4. A device as claimed in any of claims 1 to 3 in which the cross-section of the former is substantially uniform.

5. A device as claimed in any of claims 1 to 4 in which the coil is connected with the integrator by coaxial cable.

6. A device as claimed in any of claims 1 to 5 in which the bandwidth of the device is greater than one megahertz.

7. A device as claimed in any of claims 1 to 6 in which substantially all the input resistance of the integrator is associated with the coil.

8. A device as claimed in claim 7 in which the said resistance is substantially constituted by at least one separate resistive element.

9. A device as claimed in claim 8 in which the resistive element is mounted on or in the former.

10. A device as claimed in claim 7 in which the said resistance is provided by the windings of the coil.

11. A device as claimed in any of claims 1 to 10 in which the coil is electromagnetically screened.

12. A device as claimed in any of claims 1 to 10 in which the former is a contiguous loop.

13. A device as claimed in any of claims 1 to 11 in which the former is discontinuous, flexible and, in use, defines two ends in close proximity, the cable bearing a current to be measured being insertable into the loop by passing it between the two ends.

14. A device substantially as specifically described herein with reference to Figure 3, 4, 6 or 7 of the accompanying drawings.

15. A method of current measurement comprising placing a conductor bearing a current to be measured passing through the loop of a device as claimed in any preceding claim.