The present subject matter described herein, in general, relates to current sensors 5 for trip unit of circuit breakers (MCCB & ACB), and more particularly relates to a composite current transformer (CT) with rogowski coil assembly for better accuracy.
BACKGROUND OF THE PRESENT INVENTION
10 The electrical systems in residential, commercial and industrial applications usually include a panel board for receiving electrical power from a utility source. The power is then routed through overcurrent protection devices to designated branch circuits supplying one or more loads. These overcurrent devices are typically circuit breakers which are capable to interrupt the electrical current in
15 case of fault. Interruption of the circuit reduces the risk of injury or the potential of property damage from a resulting fire.
Circuit breakers are commonly used in electrical circuits for the purpose of making and breaking the circuits as per the requirements. Usually circuit breakers 20 have current transformers (CT) to sense faults and help in issuing trip instructions.
A Composite Current Transformer assembly consists of an Iron Core CT and a Rogowski Coil. In a composite CT an Iron core does the work of giving required amount of power to the trip unit whereas the Rogowski coil does the work of
25 sensing the current through the primary conductor. As circuit breakers are required to measure currents going into short circuit zones with precision using a CT core constructed out of soft magnetic material has its limitation for current sensing due to inherent magnetic saturation. Hence a rogowski coil comprising of air core is used for current measurement by processing the voltage output across
30 its terminals.

The effect of electro-magnetic interference on Rogowski coil due to the Iron CT is a prominent problem, which affects its accuracy. Further, the rogowski coil output is found to be disrupted due to presence of iron core CT windings placed at proximity in an assembly. This deviation of voltage output from intended value is dependent on the distance of separation of both components and on the current flowing through the Iron Core CT windings.
For conventional current transformer, reference is made to US5726846 which discloses a trip device comprises at least one current transformer for supplying power to electronic circuits. The current transformer comprises a magnetic circuit, surrounding a primary conductor, a secondary winding wound onto a part of the magnetic circuit forming a core, and a magnetic shunt branch connected on the magnetic core. The magnetic shunt comprises an air-gap. When the current flowing in the primary conductor is of low value, the magnetic flux stopped by the air-gap flows essentially via the core of the secondary winding. At high current levels the induction is greater and a large part of the magnetic flux passes through the shunt via the air-gap. The current transformer has a non-linear current response which limits excess power supplied to the electronic circuits and dissipated in the transformer. The trip device is useful, for example, in a circuit breaker.
Reference is also made to US6335673 which discloses that the current transformer comprises a magnetic circuit and a secondary winding. The magnetic circuit is formed by stacked metal plates. Each plate comprises a cut-out without an air-gap. The magnetic circuit comprises a fixed first part enabling the secondary winding to be received and a flexible second part able to be momentarily deformed. The current transformer is advantageously used in protection relays and electronic trip devices of circuit breakers.
Thus, the conventional current transformers comprise the following drawbacks:

• Since the output of existing Iron CTs is higher than around 50% than invented CTs at rated current, hence a greater electromagnetic interference from Iron CTs was found to affect rogowski output and hence disrupt its accuracy.
• Rogowski coil is directly placed over the Iron CT windings in the existing transformers. It has been observed from FEM analysis that increasing the distance of separation between these two minimizes the electromagnetic interference of Iron CT on Rogowski. Hence spacers were designed to get the intended distance of separation between them.
• Existing current transformers have rogowski formers with small bending radius hence the winding machine is not able to distribute turns uniformly in the bending zone. Hence the winding density is non-uniform which will affect the rogowski output.
Hence, the in view of the prior arts and the drawbacks in the existing current transformers, the inventors of the present invention have felt a dire need to provide a better and more efficient composite current transformer.
SUMMARY OF THE INVENTION
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the present invention. It is not intended to identify the key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concept of the invention in a simplified form as a prelude to a more detailed description of the invention presented later.
An objective of the present invention is to provide a to current sensing in a circuit breaker assembly with improved accuracy of current metering by Rogowski coil.

Another objective of the present invention is to provide a combination of an optimized isolation of rogowski coil with respect to an iron CT and introduction of a Ferromagnetic shield designed so as to limit the electromagnetic interference on the Rogowski coil output.
Yet another objective of the present invention is to provide rogowski Coil wound over an oval cross-section former to ensure uniform winding density and wire tension so that a better coil hygiene is achieved.
Accordingly, to one aspect, the present invention provides a composite current transformer for at least a circuit breaker, wherein said composite current transformer comprising: at least an iron core current transformer (CT) adapted to provide at least a power requirement to a trip unit; and a Rogowski Coil adapted to sense at least a current through a primary conductor. The iron core CT comprises a main core having laminations in rolling direction such that it is aligned with at least a magnetic field path, and a main core windings and a main core windings, and a secondary core and a secondary core windings. The main core and said main core windings adapted to meet said power requirement to said trip unit; and said secondary core and secondary core winding adapted to limit at least an output of said main core once said power requirements of the trip unit is met.
In one implementation, the main core is made of material selected from Cold-Rolled Grain Oriented (CRGO) material. The secondary core is made of material selected from at least a Cold-Rolled Non Grain Oriented(CRNGO) material.
In one implementation, a ferromagnetic shield is positioned between said iron core and said Rogowski Coil, to limit at least an external electromagnetic interferences.

In one implementation, a rogowski coil is wound over an oval cross-section to maintain an uniform turn tension and hence a uniform winding density.
In one implementation, the main core winding and said secondary core winding connected such that net current transformer secondary current generated is the difference of induced currents in both.
In one implementation, the main core and said secondary core are made of two different type of materials.
According to the various implementations of the present invention, electromagnetic interference by effective design of iron core current transformer and optimized isolation of the rogowski coil within the limit of Composite CT assembly is achieved.
Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The above and other aspects, features, and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings in which:
Figure 1 illustrates the front isometric view of the composite current transformer, according to one implementation of the present invention.
Figure 2 illustrates the rear isometric view of the composite current transformer, according to one implementation of the present invention.

Figure 3 illustrates the magnetic flux density distribution, according to one implementation of the present invention.
Persons skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and may have not been drawn to scale. For example, the dimensions of some of the elements in the figure may be exaggerated relative to other elements to help to improve understanding of various exemplary embodiments of the present disclosure. Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary.
Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope of the invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness.
The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.

It is to be understood that the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.
By the term "substantially" it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those of skill in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide.
Features that are described and/or illustrated with respect to one embodiment may be used in the same way or in a similar way in one or more other embodiments and/or in combination with or instead of the features of the other embodiments.
It should be emphasized that the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.
In one implementation, the present invention discloses a current sensor that consists of a iron core current transformer and Rogowski coil placed inside an enclosure. The Iron current transformer used in this disclosed invention is a unique design with a combination of multi 'core materials' and their corresponding windings.
The present invention is intended to increase the distance of separation between the Iron CT windings & Rogowski coil winding as well as reduce the iron CT secondary current in order to improve the Rogowski Coil Accuracy.
According to the present invention Intended distance of separation between the iron CT windings and Rogowski winding means an optimum distance at which

the interference of iron CT windings on Rogowski is minimum and also satisfies the space constraints of CT within the circuit breaker.
In addition to above mentioned factors for improving Rogowski accuracy, uniform winding density and ferromagnetic shielding are requisites for better accuracy, which has been incorporated in this invention. When a rogowski former is subjected to machine winding, it is intended that that there should be no gap between adjacent turns, in other words turns per unit length should be constant.
In one implementation, the rogowski former shape has been designed in the shape of an oval cross-section, the machine winding on the former would be more smooth due to absence of sharp corner. Also, the bending radius at corners of the rogowski former has been increased so that winding machine is able to distribute turns more uniformly in that region and air gaps are less.
Reference is made to figures 1 & 2 which shows front isometric view and rear isometric view of the composite CT, respectively. The components of the composite CT are as follows:
A. Main Core
B. Ferromagnetic Shield
C. Rogowski Coil
D. Secondary Core
E. Main Winding
F. Secondary Winding
In one implementation, the iron core CT has a Main core (A), its corresponding main winding (E) and also a secondary core (D) and secondary winding (F) on it The purpose of the main core and the main windings associated to it is to meet the power requirement of the trip unit as early as possible. The purpose of the secondary core is to limit the output of the main core once the power requirements

of the trip unit is met. The Iron cores, main and secondary are made of two different type of materials.
In one implementation, the main core having laminations designed in the rolling direction such that it is aligned with the magnetic field path. Winding is done after laminations are completely rolled and therefore no open shapes present. Rolling in direction of magnetic field enhances performance of transformer at low currents.
In one implementation, FEM analysis and calculations with respect to core material helped deduce that Cold-Rolled Grain Oriented (CRGO) when used in main core shows better power up of trip unit, here the laminations are designed in rolling direction. Reference is made to the figure 3, which shows the magnetic flux density distribution. The details of FEM analysis are as follows:
1. Model was designed and later imported in FEM software.
2. CRGO steel was custom made in FEM library by plotting it's BH curve and assigned to main core.
3. CRNGO steel was present in FEM library and assigned to the secondary core.
4. No of turns were calculated based on power requirement of the trip unit.
5. Output was to be checked during the power-up & saturation zone.
In one implementation, the main core winding and secondary core winding are connected such that net CT secondary current generated is the difference of induced currents in both. The secondary core will not participate when the primary current is lower because of relatively higher reluctance path to the magnetic circuit that the secondary core is involved, hence rate of rise of secondary current generated in the main core winding would be high. However, as the primary current through the CT increases the reluctance of the magnetic path of the main core reduces and becomes comparable with that of the secondary core. Thus the flux gets shunted through secondary core as primary current of the CT

increases and hence there is a secondary current that gets developed in the secondary core winding as well. Since the direction of the current developed in secondary winding is opposite to that of the main core winding, the net output is forced to saturate with respect to an increasing primary current. A better saturated Iron CT output at higher primary current ensures that the electromagnetic interference on the Rogowski coil is reduced to a great extent thus ensuring a more linear rogowski output resulting in a better accuracy.
In one implementation, the optimized isolation of the rogowski coil within the limit of Composite CT assembly was determined with the help of FEM analysis. Also an effective introduction of ferromagnetic shield in-between both the Iron CT and the Rogowski coil (as shown in the fig) combined together limited the external electromagnetic interferences.
In one implementation, optimized isolation of rogowski coil as said earlier is decided by the reducing electromagnetic interference of rogowski coil and the assembly constraints of CT inside the circuit breaker. Determination of rogowski accuracy through FEM analysis is done in the following manner:
• Composite CT model was developed based on optimized isolation of rogowski coil.
• Ferromagnetic shield (MS) was placed between Iron CT and rogowski coil.
• The rogowski output was plotted through FEM analysis at 25%, 50%
& 100% of rated current.

Ip Actual Rog 0/p(V) Linear Rog 0/p(V) %
linearity
error
A A A
200 127.41 127.86 0.35
400 255.29 255.73 0.17

800 511.45 511.45 0.00
In one implementation, rogowski coil makes use of a plastic former on which the rogowski coil winding was made. The shape of the former plays a pivotal role in maintaining a good coil winding hygiene, which intern is very important for achieving a desired output from the rogowski coil. A former with an oval cross section as shown in figures 1 and 2, was found to have a better scope with respect to the space and coil winding hygiene as it helps in maintaining uniform turn tension and hence a uniform winding density.
Some of the noteworthy features of the present invention, are as follows:
• Unique Iron CT design which force saturates the CT secondary current at higher primary current.
• A combination of an optimized isolation of Rogowski coil with respect to Iron CT and introduction of a Ferromagnetic shield designed so as to limit the electromagnetic interference on the Rogowski coil output.
• Rogowski Coil wound over an oval cross-section former to ensure uniform winding density and wire tension so that a better coil hygiene is achieved.
Some of the non-limiting advantages of the present invention, are as follows:
• The present invention provides an iron CT design that achieves a reduced iron core CT output through forced saturation, which in turn improves the accuracy of rogowski coil due to reduced electromagnetic interference.
• The present invention provides current transformer with early power up and early saturation using CRGO material in the main core.

• The present invention provides an optimized placement isolation of Rogowski coil and introduction of an effective ferromagnetic shield helps to better the Rogowski accuracy.
• The present invention better linearity in rogowski output owing to better coil winding hygiene achieved by oval rogowski formers.
Although the composite current transformers for molded case circuit breakers has been described in language specific to structural features and/or methods, it is to be understood that the embodiments disclosed in the above section are not necessarily limited to the specific features or methods or devices described. Rather, the specific features are disclosed as examples of implementations of the composite current transformers for molded case circuit breakers which facilitates better conduction of heat and better electrical insulation.

WE CLAIM :

1. A composite current transformer for at least a circuit breaker, wherein said
composite current transformer comprising:
At least an iron core current transformer (CT) adapted to provide at least a power requirement to a trip unit; and
a Rogowski Coil adapted to sense at least a current through a primary conductor;
wherein, said iron core CT comprises:
a main core having laminations in rolling direction such that it is aligned with at least a magnetic field path, and a main core windings; and
a secondary core and a secondary core windings; wherein, said main core and said main core windings are adapted to meet said power requirement to said trip unit; and
said secondary core and secondary core winding adapted to limit at least an output of said main core once said power requirements of the trip unit is met.
2. The composite current transformer as claimed in the claim 1, wherein said main
core is made of material selected from at least a Cold-Rolled Grain Oriented
(CRGO) material and the secondary core is made of material selected from at least
a Cold-Rolled Non Grain Oriented(CRNGO) material.
3. The composite current transformer as claimed in the claim 1, wherein a ferromagnetic shield is positioned between said iron core and said Rogowski Coil, to limit at least an external electromagnetic interferences.
4. The composite current transformer as claimed in the claim 1, wherein said rogowski coil is wound over an oval cross-section to maintain an uniform turn tension and hence a uniform winding density.

5. The composite current transformer as claimed in the claim 1, wherein said main core winding and said secondary core winding connected such that net current transformer secondary current generated is the difference of induced currents in both.