FORM 2
THE PATENTS ACT, 1970 (39 of 1970)
COMPLETE SPECIFICATION
(See section 10 and rule 13)
1. TITLE OF THE INVENTION
"A Current-Sensing and Associated Signal-Processing-System and Method for
Reducing Measurement Error"
2. APPLICANT(S)
(a) NAME: Larsen and Toubro Ltd.
(b) NATIONALITY: Indian
(c)ADDRESS: L&T House, Ballard Estate, Mumbai-400 001, India
3. PREAMBLE TO THE DESCRITION
COMPLETE
The following specification particularly describes the invention and the manner in which it is to be performed.

Title
A Current-Sensing and Associated Signal-Processing-System and Method for Reducing Measurement Error
Field of the invention
The present invention generally relates to a current-sensing and associated signal-processing-system and method for reducing measurement error, employed particularly in protection devices wherein a higher level of accuracy is required than in general metering devices. The system and method of the present invention provide an accurate and precise method of current sensing using PCB (printed circuit board) Rogowski coils.
Background of the invention
Current sensors are used in various devices for measuring current at various accuracy levels.
Generally, a current sensor is of the type of current transformer providing a proportional output current. However, there are sensors using Hall Effect or magneto-resistivity etc., which use a completely different phenomenon for gauging flow of electric current in a circuit. One such example is Rogowski coil as disclosed in US patent specification US7227441.
A further amendment of the Rogowski coil is disclosed in the US patent specification US5414400 which discloses a design of a Rogowski coil using printed circuit boards (PCBs) for forming the coil. This construction enables high manufacturing accuracy and reliability of output of the Rogowski coil due to high precision of PCB manufacturing process. US patent specifications US7902812 and US6680608 disclose further improvements of the PCB-based Rogowski coil. However, disadvantage of all these PCB-based designs of Rogowski coil is that the coil is designed to progress along the plane of the PCB with the conductor passing perpendicular through the plane of the PCB; this allows a very small area to get enclosed in a turn of the coil and so the output voltage is very feeble and difficult to detect.
US patent specification US6825650 discloses a design in which the coil is wound in the plane of the PCB with multilayer PCB construction and the PCBs are placed along a closed path encircling the conductor and parallel to the conductor. A base PCB perpendicular to the conductor forms a support to the coil PCBs. In such a construction the coils are coupled additively to form two closed loops around the conductor and the differential voltage of the two loops with certain ratios provides a signal voltage proportional to the current. The plane of the coil PCB being perpendicular to the flux generated by the current to be sensed, this type of

construction allows the output voltage to be significant as compared to the previously discussed arrangements. However, there is another disadvantage - due to the PCBs being placed discretely around the conductors, the contiguity of the turns of coil is lost and this introduces errors due to adjacent conductors. To overcome these errors, such a construction generally uses two distinct coils which are concentrically placed and the difference in the signal from the two coils can be adjusted to eliminate the error. However, limitation of this arrangement is that this arrangement is subject to certain minimum clearances between the conductor locations and leaves room for error during detection of short-circuit level fault currents.
In every type of Rogowski construction it is required to integrate the flux density along a closed loop. By forming multiple coils in series with each other the integration is an Euler approximation in which the assumption is that the flux density proportional to the discrete voltage of individual coil element is constant till the next coil element position. In other words, it is assumed that the discrete coil elements are spaced very close to each other allowing the integration length element to be small enough for Euler integration to be accurate.
In the wire-wound construction of the Rogowski coil the wire diameter is small enough to allow this assumption to be true; however, in the PCB-based Rogowski coil as disclosed in the US patent specification US6825650 {mentioned above) the arrangement has discrete coil placements and therefore the approximation error of Euler integration is significant. This necessitates addition of a second coil whose signal is to be subtracted from the first coil for eliminating external signal interference.
Therefore, there exists a need for a system that would overcome the aforesaid disadvantages and limitations of the prior art.
Objects of the invention
An object of the present invention is to present a system and method for current sensing and measurement, which integrates the signal in space by collecting magnetic flux density value at discrete points around the conductor and then uses a numerical procedure, which allows a greater accuracy over the prior art described in the US patent specification US6825650 and also eliminates the need for a second coil and subtracting external noise signals.
Another object of the present invention is to present for current sensing and measurement, wherein the signal need not be reduced due to subtraction of noise and hence the signal voltage available for sensing circuitry is higher as compared to the prior art techniques.

A further object of the invention is to provide a solution for efficiently and precisely sensing change in short-circuit level fault-currents in a protection device.
Another object of the invention is to provide a simpler and proficient system and method of eliminating errors introduced in a typical PCB Rogowski coil design and getting more accurate result.
Summary of the invention
In accordance with the above objects, the present invention involves a PCB-based Rogoswki coil design complemented by a system of integration of the current signals in space by collecting magnetic flux density value at discrete points around the conductor. As per the invention, coil PCBs are placed evenly around the current carrying conductor, within an optimized angle. The invention identifies the best span of angle between the two coils for optimal results. It then prescribes a specific kind of connection between the coil PCBs to obtain a cumulative result of the voltages at the discrete position which represent the magnetic flux at those positions.The next step involves integrating that magnetic flux by using an OP-AMP integrator. The system and the method of the invention accomplish production of an output signal strong enough for detection in the sensing stage, and also reduction in external noise errors, thereby ensuring enhanced accuracy.
Brief Description of Drawings
Fig. 1 shows Rogowski coil arrangement.
Fig. 2 shows conventional design of Rogowski coil.
Fig. 3 illustrates PCB Rogowski coil arrangement, with the PCB coils (14) placed around the conductor (13).
Fig. 4a and 4b illustrate the new approach to current sensing in accordance with the present invention, with an embodiment having with 12 coils arranged to form 3 subsets of 4 coils each.
Fig. 5a and 5b illustrate another embodiment of the invention, having 12 coils arranged to form 2 subsets of 6 coils each.
Fig. 6a and 6b illustrate yet another embodiment of the invention, having 6 coils arranged to form 3 subsets of 2 coils each.
Fig. 7a and 7b illustrate yet another embodiment of the invention, having 6 coils arranged to form 2 subsets of 3 coils each.

Detailed Description
The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, to thereby enable others skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omission and substitutions of equivalents are contemplated as circumstance may suggest or render expedient, but such are intended to cover the application or implementation without departing from the spirit or scope of the present invention.
The present invention improvises upon the Rogowski equation and coil arrangement based thereon as illustrated in Fig. 1.
Fig. 1 illustrates Rogowski coil arrangement, wherein Rogowski coil (10) formed by small loops (11) and a return loop (12) encircle the current carrying conductor (13). The coil has to form a complete closed loop around the conductor for the Ampere's Circuital Law to hold. The ends of the coil should ideally come to exact same spot. The small loops (11) have to be perfectly uniformly wound along the coil (10) for a good integration of the flux around the conductor (13). Higher turn density helps in maintaining better uniformity. The return loop (12) is required for negating effects of fields not in the same plane as the coil path, and is known as compensating turn.
V = voltage produced in Coil
A = Area of the small loops forming the coil
Fig. 2 illustrates effect of coil (10) wound around the conductor (13). Voltage is induced in each turn due to constant change in flux. Series addition of voltage in each turn provides Voltage integrated in space, integrating the voltage signal in time provides the rate of change of flux associated with the closed loop (11) around conductor (13). Integrated value divided by area of all turns gives JBavgdl. This is proportional to the current in the conductor (13). Thus, the Rogowski equation is:


I = Length of the coil encircling the conductor
μ - Permeability of medium (4π x 10-7 V.s/(A.m) for Vacuum or Air)
dl/dt = rate of change of current
Fig. 3 illustrates PCB Rogowski coil arrangement, with the PCB coils (14) placed around the conductor (13). In this arrangement, conventional loops (11) are replaced with PCBs (14) forming coils.
Fig. 4a and 4b illustrate the new approach to current sensing, in accordance with the present invention, which provides a system and a method for accomplishing requisite accuracy in current sensing measurements, especially in protective devices.
To obtain the desired result the coils (14) have to integrate the signal in space by collecting magnetic flux density value at discrete points around the conductor (13) and then using a numerical procedure spaced evenly and with an optimized angle. Too small an angle leads to constructional impracticality and also makes the solution too costly to implement. Too large an angle introduces possibility of errors due to nonuniform field distribution. Such non-uniformity can be due to proximity to another magnetic field source or to proximity of magnetic material to some PCBs of the Coil. A spacing angle from 30° to 6o is sufficient to get a good accuracy in integration. Hence the number of PCBs required to completely encompass the conductor varies from 8 to 12, but a construction of 6 PCBs is also possible with metering class accuracy.
In case of the embodiment involving 12 PCB coils, having 12 points of measurements can become problematic for signal acquisition; therefore the invention prescribes connecting in series sets of PCBs to form 3 series strings in such a way as to cumulatively represent the voltage of each position. These connections have to be made in such a way as to evenly space out the representative PCBs. Thus, for a set of 12 PCBs, a first subset is formed with a first PCB coil ("PCB1") connected in series with the fourth ("PCB4"), seventh ("PCB7") and tenth (PCB10); likewise, a second subset with PCB2 connected in series with PCBS, PCB8 and PCB11; and a third subset with PCB3 connected in series with PCB6, PCB9 and PCB12 - wherein each PCB coil is at an angle of 30° from the adjacent PCB coils.
In this arrangement illustrated in Fig. 4a and 4b, output voltage of each subset represents the magnetic flux at the respective locations. Once such voltages of discrete locations are available, these can be integrated using Simpsons rule or the three point Newton-Cotes quadrature rule to get the accurate flux density integration along the closed path around the conductor. This leads to an accurate application of the Ampere's Circuital law which is the guiding principle behind the Rogowski coil.

In another embodiment of the invention sets of PCBs are connected to form 2 series strings (instead of 3) cumulatively representing the voltage of each position, as shown in Fig. 5a and 5b. In this arrangement, a first subset is formed with a first PCB coil ("PCB1") connected in series with the third ("PCB3"), fifth ("PCBS"), seventh ("PCB7"), ninth ("PCB9") and the eleventh ("PCB11") coils, and a second subset is formed with PCB2 connected in series with PCB4, PCB6, PCB8, PCB10, and PCB12. In this arrangement, output voltage of each subset represents the magnetic flux at the respective locations. Once such voltages of discrete locations are available, these can be integrated using Trapezoidal rule of integration to get the accurate flux density integration along the closed path around the conductor. This leads to an accurate application of the Ampere's Circuital law which is the guiding principle behind the Rogowski coil.
As stated above, the invention can be worked with 6 PCBs (instead of 12), which gives metering class accuracy. For such an embodiment of the invention having 6 PCBs, in order to obtain accurate flux density using Simpson's rule of integration a first subset is formed by first (PCB1) and fourth (PCB4) coils connected in series, a second subset is formed by second (PCB2) and fifth (PCBS) coils connected in series, and a third subset is formed by third (PCB3) and sixth (PCB6) coils connected in series, as illustrated in Figures 6a and 6b.
For an arrangement with 6 PCBs, in order to obtain accurate flux density using Trapezoidal rule of integration the two series are formed with PCB1, PCB3 and PCB5 connected in series and PCB2, PCB4 and PCB6 connected in series, as illustrated in Figures 7a and 7b.
The effective radius 'r' of the circular path of integration around the conductor is unique for a particular construction and can be calculated as the radius at which the flux density due to the enclosed conductor is equal to the average fiux density over the coil PCB's surface area. This radius can be programmed into the signal processing unit to obtain the correct integrated voltage around the conductor.

We Claim:
1. A system for sensing current in a conductor, comprising a set of at least S PCB Rogowski coils ("coils") evenly spaced around the conductor, the coils being capable of being connected in series with one or more coils to form a subset, each such subset giving an output voltage representing magnetic flux density at respective locations of the coils in the subset, wherein integration of voltage outputs of the subsets and summing up the integrated voltage values gives accurate flux density along the closed path around the conductor, which, when used in Ampere's Circuital law, gives accurate value of the current in the conductor.
2. The system of Claim 1 having 12 PCB Rogowski coils spaced evenly around the conductor and forming three subsets of the coils, wherein a first subset is formed by first (PCB1), fourth (PCB4), seventh (PCB71 and tenth (PCB10) coils connected in series, another subset is formed by second (PCB2), fifth (PCBS), eighth (PCB8) and eleventh (PCB11} coils connected in series, and a third subset is formed by third (PCB3), sixth (PCB6), ninth (PCB9) and twelfth (PCB12) coils connected in series; wherein integration of voltage outputs of the subsets using Simpson's rule of integration and summing up the integrated voltage values gives accurate flux density along the closed path around the conductor, which, when used in Ampere's Circuital law, gives accurate value of the current in the conductor.
3. The system of Claim 1 having 12 PCB Rogowski coils spaced evenly around the conductor and forming two subsets of the coils, wherein a first subset is formed by first (PCB1), third (PCB3), fifth (PCBS), seventh (PCB7), ninth (PCB9) and eleventh (PCB11) coils connected in series, and the other subset is formed by second (PCB2), fourth (PCB4), sixth (PCB6), eighth (PCBS), tenth (PCB10), and twelfth (PCB12) coils connected in series; wherein integration of voltage outputs of the subsets using Trapezoidal rule of integration and summing up the integrated voltage values gives accurate flux density along the closed path around the conductor, which, when used in Ampere's Circuital law, gives accurate value of the current in the conductor.
4. The system of Claim 1 having 6 PCB Rogowski coils spaced evenly around the conductor and forming three subsets of the coils, wherein a first subset is formed by first (PCB1) and fourth (PCB4) coils connected in series, a second subset is formed by second (PCB2) and fifth (PCBS) coils connected in series, and a third

subset is formed by third (PCB3) and sixth (PCB6) coils connected in series; wherein integration of voltage outputs of the subsets using Simpson's rule of integration and summing up the integrated voltage values gives accurate flux density along the closed path around the conductor, which, when used in Ampere's Circuital law, gives accurate value of the current in the conductor.
5. The system of Claim 1 having 6 PCB Rogowski coils spaced evenly around the conductor and forming two subsets of the coils, wherein a first subset is formed by first (PCB1), third (PCB3) and fifth (PCB5) coils connected in series, and the other subset is formed by second (PCB2), fourth (PCB4) and sixth (PCB6) coils connected in series; wherein integration of voltage outputs of the subsets using Trapezoidal rule of integration and summing up the integrated voltage values gives accurate flux density along the closed path around the conductor, which, when used in Ampere's Circuital law, gives accurate value of the current in the conductor.
6. A method for sensing current in a conductor, comprising the steps of:
i. Placing at least 6 PCB Rogowski coils ("coils") evenly around the conductor, the coils being capable of being connected in series with one or more coils to form a subset, each such subset giving an output voltage representing magnetic flux density at respective locations of the coils in the subset;
ii. Collecting output voltages from individual coils or from the subsets; and
iii. Integrating the output voltages, summing up the integrated voltages to get flux linked with the coils, and converting the linked flux to current enclosed by integration path.
7. A method for sensing current in a conductor as claimed in Claim 6 having 12 coils placed evenly around the conductor and connected to form three subsets each having 4 coils, wherein the output voltages are integrated using Simpson's rule of integration.
8. A method for sensing current in a conductor as claimed in Claim 6 having 12 coils placed evenly around the conductor and connected to form two subsets each having 6 coils, wherein the output voltages are integrated using Trapezoidal rule of integration.
9. A method for sensing current in a conductor as claimed in Claim 6 having 6 coils placed evenly around the conductor and connected to form three subsets each

having 2 coils, wherein the output voltages are integrated using Simpson's rule of integration.
10. A method for sensing current in a conductor as claimed in Claim 6 having 6 coils placed evenly around the conductor and connected to form two subsets each having 3 coils, wherein the output voltages are integrated using Trapezoidal rule of integration.