FORM 2
THE PATENT ACT 1970
&
The Patents Rules, 2003
COMPLETE SPECIFICATION (See section 10 and rule 13)
1. TITLE OF THE INVENTION:
Current Sensor with Flexible Printed Circuit for Smart Grid Application
2. APPLICANT:
(a) NAME: Larsen & Toubro Limited
(b) NATIONALITY: Indian Company registered under the
provisions of the Companies Act-1956.
(c) ADDRESS: LARSEN & TOUBRO LIMITED,
L&T House, Ballard Estate, P. 0. Box: 278, Mumbai 400 001, India
3. PREAMBLE TO THE DESCRIPTION:
COMPLETE
The following specification particularly describes the invention and the manner in which it is to be performed.

Current Sensor with Flexible Printed Circuit for Smart Grid Application
Field of the invention
The present invention relates to current sensors, and more particularly, to a low secondary volt ampere air cored current sensors to be used for digital metering and microprocessor based release and protection unit used in circuit breaker.
Background of the invention
It has been a practice over several decades of using a current transformer for powering up, metering and protection purpose. The current transformer essentially consists of a hollow ferromagnetic core, which surrounds a primary conductor centrally located in to it Over the hollow ferromagnetic core, a secondary winding is uniformly wound whose start and end winding is connected to the external device or burden. The device such as measuring instruments, for example, ammeter and energy meter are mostly of analogue type and hence demands more VA consumption. Similarly, over current and short circuit protection unit are mostly of electromagnetic type, which demands higher VA for its healthy operation. The current transformer, which develops higher secondary output VA, is the ideal powering up solution for such devices; However, due to the presence of ferromagnetic core material, the current transformer's magnetic characteristics are highly nonlinear at low and high current. Hence, at such current, the device loses its linearity.
Further, the ferromagnetic material of the current transformer depending upon its thickness and magnetic loop is subjected to high core loss such as eddy current and hysteresis loss which is responsible for higher temperature of the core and current transformer. In order to maintain the linearity and . low temperature rise, the current transformer demand increase in its size,

volume and weight. In case a secondary circuit is open, the current transformer develops dangerous high voltage which can result into fatal accident. Also, the current transformer's performance gets affected due to external electromagnetic field. The ratio, phase angle and composite error of the current transformer are very high and depending upon its class, vary.
With the advancement in measuring and protection technology towards smart grid application, the VA consumption of metering and protection devices has drastically reduced. This demands a current sensor with lower VA consumption.
US Patent No., 6414475 discloses a current sensor which suffers from poor sensitivity and output voltage. In the current sensor disclosed in '475 patent, inner turn area of a sense coil has not been fully utilized for the same primary current rating. Further, the current range mentioned in the '475 patent limits it to maximum 200A for energy meter application. Furthermore, secondary windings of the '475 patent are tracked upon PCB with thickness of the order 1.6 mm, due to which the overall size and volume of the current sensor is high.
Another US patent application 2005726846 discloses a trip device comprising at least one current transformer. The '846 patent discloses an iron cored current transformer with a shunted path suitable for powering up an electronic circuit of the trip device used in a current transformer.
Yet another US patent 7355381 discloses a current sensor with reduced sensitivity to parasitic magnetic fields.
Still another US Patent No. 6108185 and 7327133 discloses a circuit breaker having Hall Effect sensors. The disclosed patents use a primary conductor shaped in such a way that it enhances the flux density so as to increase the

output voltage and VA rating of the current sensor. However, it is subjected to saturation at very high current application and will clamp the peak.
One more US Patent No. 7230413 discloses a flexible current sensor. The '413 patent discloses a current sensor and method for determining the current flow in conductors to be evaluated.
Accordingly, there exists a need for a current sensor which overcomes drawbacks of the prior art.
Object of the invention
An object of the present invention is to overcome saturation problem in a current transformer.
Another object of the present invention is to provide a consistent performance of the current sensor.
Yet another object of the present invention is to provide differential change in secondary output voltage which quantifies variation in the primary current.
Summary of the invention
Accordingly, the present invention provided a current sensor to be used for digital metering and microprocessor based release and protection unit of a circuit breaker. The current sensor comprises an insulated enclosure having, a plurality of secondary windings stacked together. Each secondary winding of the plurality of secondary windings includes a sense winding and a cancellation winding printed/etched on an insulated polymer and wound in anticlockwise and clockwise direction respectively. Further, each secondary winding is connected to other secondary winding of the plurality of secondary windings in series. The current sensor also includes a primary

conductor mounted on a protruded member of the insulated enclosure coplanar to the plurality of secondary windings.
Brief description of drawings
Figure 1 shows an assembly of air cored current sensor;
Figure 2 shows a front and back sides of a secondary winding;
Figure 3 shows assembled insulated enclosure having a plurality of
secondary winding;
Figure 4 shows an insulated enclosure of the current sensor, in accordance
with the present invention;
Figure 5 shows a primary conductor, in accordance with the present
invention;
Figure 6 shows a basic integrator diagram with low pass filter circuit;
Figure 7 shows AC waveform with and without DC offset;
Figure 8 shows a phase shift with and without corrector;
Figure 9 shows a current distribution in the primary conductor of the current
sensor of the present invention;
Figure 10 shows a field intensity distribution in adjacent air column;
Figure 11 shows a flux density distribution in air column;
Figure 12 shows a primary current and secondary output voltage waveform
for sample 1 of the current sensor of the present invention;
Figure 13 shows output characteristics of flexible current sensor for sample
1, in accordance with the present invention;
Figure 14 shows a primary current and secondary output voltage waveform
for sample 2 of the current sensor of the present invention;
Figure 15 shows output characteristics of flexible current sensor for sample
2, in accordance with the present invention;
Figure 16 shows a linearity error for a 40 mm diameter current sensor (sample-2) of the present invention;

Figure 17 shows a RC filter circuit along with the current sensor of the
present invention; and
Figure 18 shows an improvement in the linearity error due to RC filter circuit
Detailed description of the invention
The foregoing objects of the present invention are accomplished and the problems and shortcomings associated with the prior art, techniques and approaches are overcome by the present invention as described below in the preferred embodiments.
The present invention provides a current sensor with flexible printed circuit for smart grid application. The current sensor is an air cored current sensor which offers very low Volt ampere suitable for high burden devices like digital meters and micro-processor based releases and protection unit.
Referring now to figure 1-5, there is shown current sensor and various components thereof, in accordance with the present invention. Specifically, figure 1 shows the current sensor (100) (hereinafter "the sensor (100)"], in accordance with the present invention. Specifically, the sensor (100) is an air cored current sensor. The sensor (100) includes a primary conductor (10) and an insulated enclosure (40).
The insulated enclosure (40) includes a plurality of secondary winding (20) stacked together therein. Specifically, the plurality of secondary windings . (20) is placed inside an insulated enclosure (40). Each secondary winding of the plurality of secondary windings (20), for example, the secondary winding (22) as shown in figure 2, and also referred to as flexible printed circuit, includes a sense winding (22a) and a cancellation winding (22b) printed/etched on an insulated polymer (24).. The sense winding (22a) and the cancellation winding (22b) are wound in anticlockwise and clockwise

direction respectively on the insulated polymer. Further, each secondary winding, for example the secondary winding (22) is connected to other secondary winding (not shown) of the plurality of secondary windings (20) in series to form a stacking.
The insulated enclosure (40) further includes a protruded member (30) configured centrally thereon. The protruded member (30) includes the primary conductor (10) mounted thereon as shown in figure 1. Unlike the current sensor of the prior art, the primary conductor (10) and the secondary windings (20) are in the same plane.
In an embodiment, the primary conductor (10) is made out of diamagnetic material such as copper. The primary conductor (10) is made in any one of rectangular, triangular and circular shape. The primary conductor (10) is made upon current carrying parts, especially for spreader link in the protection devices such as moulded case circuit breaker (MCCB), air circuit breaker (ACB) and the like. In an embodiment, the primary conductor (10) is individually formed in current carrying parts used in switchboards and the like. Shape of the primary conductor (10) depends upon the inner area covered by the sense winding (22a) to increase the flux distribution which links with the sense area of the secondary winding (22).
The secondary winding (22) is made on any of polyester and polyimide polymer in 25 - 100 micron upon which the sense winding (22a) with negligible inner diameter and the cancellation winding (22b) is either printed or etched. The inner turn area of the sense winding (22a) is fully utilized so as to produce higher output voltage. The sense winding (22a) and cancellation winding (22b) are normally wound with definite pitch in anticlockwise and clockwise direction respectively. The turn area of sense winding (22a) is always maintained very near to that of the cancellation winding (22b). Specifically, an end winding (26b) of the sense winding (22a)

is connected in series to that of the cancellation winding (22b). This prevents interference of external magnetic field upon performance of the air cored current sensor (100].
Specifically, inner turn of the sense winding (22a) and that of outer turn of the cancellation coil (22b) is the start and end of the secondary winding (20) per layer respectively. A start winding (26a) is normally routed out through the back side of the insulated polymer (24) and the end winding (26b) is routed through the front side of the insulated polymer (24). Figure 2 shows the front and back side of one layer of such printed or etched circuit of secondary winding (22).
Each winding of the plurality of secondary windings (22) is stacked one above the other in the insulated enclosure (20) depending upon the output VA and sensitivity requirement of the current sensor (100). Each of the secondary winding, for example the secondary winding (22a) of the plurality of secondary windings (22) is masked with insulation along both its surface to prevent shorting between the plurality of secondary windings (22). The end turn of the first secondary winding (not shown) of the plurality of secondary winding (22) is connected to a start turn of a subsequent secondary winding (not shown) the plurality of secondary winding (22) lying beneath it and several such secondary windings are connected in series with each other.
While stacking the layers of printed plurality of secondary windings (22) one above the other, the secondary windings are placed coplanar with respect to each other, so as to have a common axis to avoid interference of unwanted external magnetic field. The entire stacked unit i.e. the plurality of secondary windings (22) is then either cold moulded or wrapped with insulation tape so as to make it a rigid assembly. Figure 3 shows the assembled secondary winding (22) of the current sensor (100) of the present invention, which is

placed inside an insulated enclosure (20). Figure 4 shows the insulated enclosure (20) along with the stack of secondary windings (22) upon which the primary conductor (10) is held.
In an operation, when a time varying current is made to flow through the. primary conductor (10), the primary conductor (10) produces alternating magnetic field which varies with time in accordance to Ampere's circuital law. The rate of change of flux induces an electromotive force across the shorted sense coil (22a), in accordance to Lenz's law. The electromotive force induced opposes the very cause producing it. Due to shorting of the secondary winding (22) through the external burden, current flows there through. The secondary output voltage is then fed to an integrator circuit so as to reject the high frequency noise and act as an amplifier for low frequency signal. The integrator circuit in actual practice relates the output voltage in exact proportional with the primary current. While doing so, due to parasitic DC offset of the integrator circuit, the AC waveform gets distorted which is avoided by means of using a capacitor coupling circuit. Figures 6 and 7 show a basic integrator diagram and waveform with and without DC offset.
Inclusion of the capacitive coupling circuit leads to phase shift error, which is further fine tuned using a low pass filter circuit. Figure 8 shows the phase shift error with and without correction.
Following are the specifications of the current sensor (100), in accordance with the present invention.

Sample 1 (20mm) Sample 2 (40mm)
Primary current range(A) 10-300 10-300
Sensitivity(mV/A) 0.0707 <1.2
Operating temperature(°C) -40 to +85 -40 to +85
Saturation effect Negl gible
Secondary winding diameter(mm) 20 40

Number of layer 24 38
Total number of turns .600 1900
Number of sense turns / layer 15 31
Number of cancel turns / layer 10 19
Track width (mm) 0.2
Track thickness [mm] 0.018
Flex PCB thickness [mm] 0.1
Air gap between tracks 0.2
Air gap between sense & cancel coil 0.2

Hg is the preliminary or guess magnetic field (input parameter)
Effect of primary conductor (10) profile upon the flux distribution in air can be understood by conducting a 3 - D Magnetostatic analysis. It is known that ANSYS solves the magnetostatic analysis using Ampere's circuital law and Biot-Savart law. The primary independent degree of freedom in magnetostatic analysis is the scalar potential for a known quantity of current or current density. The governing equations which are solved to find the scalar potential are equation 1 to 3. Figure 9 shows the distribution of current in the primary conductor (10), which is self explanatory. Corresponding to this, the flux distribution in the adjacent air column is shown in figure 10. Due to the special hollow circular geometry of the primary conductor (10) as shown in figure 1, it is observed that the flux adjacent the primary conductor (10) is converging whereas towards the right and left away from centre it is diverging. This converging of the flux at the centre increases its distribution around the sense coil (22a), due to which the flux density distribution as shown in figure 11 is very high across the sense coil (22a) as compared to the cancellation coil (22b). Hence the electromotive force induced by the sense coil will be quiet high as compared to outer coil.

φg is the generalized potential (output parameter) Hg which is the initial guess made after calculating the magnetic scalar potential shall satisfy Ampere's circuital law as given in equation - 1. The
remaining part of the field can be calculated from gradient of φg. The selection of Hg can be obtained from Biot-savart law, which satisfies ACL.

Where
Js is the current source density vector at d(volc)
R is the position vector from current source to node point
Vole is the volume of current source
Solving equation 2 & 3, we get the g from which other quantities of the magnetic analysis can be derived.
Testing of the current sensor (100) is done by mounting two primary conductors upon the current sensor (100] at the top and bottom surface. Current from lower to higher current made to flow through the primary conductor which gets equally shared in the parallel path. At different primary current, secondary output voltage of the current sensor (100) noted down and sensitivity of the current sensor calculated.
Figure 12 shows a waveform of 314A, 50 Hz primary current and secondary output voltage for the current sensor (100) of sample 1, whose outer diameter is of 20 mm and number of turns are 600. Figure 13 shows an output characteristics of the current sensor (100), which provides relationship between the primary current and secondary output voltage.
Further, figure 14 shows a waveform of 300 A, 50 HZ primary current and secondary output voltage for the current sensor (100) of sample 2 , whose

outer diameter is of 40 mm and number of turns are 1900. Figure 15 shows its output characteristics at different primary current, which is linear from lower to higher current.
Unlike a current transformer output of the prior art, V - I characteristics of both the current sensors (sample 1 and sample 2) are linear from lower to higher current. Corresponding sensitivity of samples -1 and 2 is observed to be 0.0707 and 1.14 mV/A respectively. Further, increase in the sensitivity is obtained by means of increasing the number of turns. Due to the presence of noise in the form of high frequency spikes, surges and harmonics in the primary conductor (10] and the secondary winding (22), the output voltage especially at lower current observed to be highly distorted. Due to this the linearity of the current sensor (100) is drifted by an appreciable amount. Figure 16 shows the linearity error at lower current for sample 2 current sensors with 40 mm diameter. This has to be taken care by means of connecting the output of the flexible current sensor to an integrator circuit, which is shown in figure 6. A simple integrating circuit can also be a RC filter circuit as shown in figure 17, which will reject the high frequency signal. Figure 18 shows the reduction in the linearity error from 28 % to 5 % at lower current due to rejection by the RC filter circuit of the high frequency signal.
Advantages of the invention
1. The current sensor (100) provides high linearity and operating current range.
2. The current sensor (100) provides lower secondary open circuited voltage and Low temperature rise of sensor.
3. As the current sensor (100) never saturates being air cored, the current sensor (100) eliminate residual magnetism and core loss.

4. The current sensor (100) provides moderate short time withstanding. capability with fast acting or semiconducting fuses and higher flux linking.
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, and 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 omissions and substitutions of equivalents are contemplated as circumstances may suggest or render expedient, but such omissions and substitutions are intended to cover the application or implementation without departing from the spirit or scope of the claims of the present invention.

We Claim:
1. A current sensor to be used for digital metering and
microprocessor based release and protection unit of a circuit breaker, the
current sensor comprising:
an insulated enclosure having:
• a plurality of secondary windings stacked together, each secondary winding of the plurality of secondary windings having a sense winding and a cancellation winding printed/etched on an insulated polymer and wound in anticlockwise and clockwise direction respectively, each secondary winding being connected to other secondary winding of the plurality of secondary windings in series, and
• a protruded member configured centrally on the insulated enclosure; and
a primary conductor mounted on the protruded member of the insulated enclosure coplanar to the plurality of secondary windings.
2. The current sensor as claimed in claim 1, wherein a start winding of the sense winding is normally routed out through the back side of the insulated polymer.
3. The current sensor as claimed in claim 1, wherein end winding of the cancelation winding is routed through the front side of the insulated polymer.
4- The current sensor as claimed in claim 1, wherein the plurality of secondary windings is any one of cold moulded and wrapped with insulation tape to form a single unit.