Embodiments of the invention in general relates to the field of current sensing in a circuit breaker assembly, more particularly to powering up of a trip unit as early as possible along with early saturation of the current transformer.
BACKGROUND OF THE INVENTION
Apparatus used for controlling, regulating and switching on or off the electrical circuit in the electrical power system is known as switchgear. Circuit breaker are commonly known as interrupters that opens and closes the contacts to stop and start the flow of electrical current to the circuit. Circuit breakers are required to have a current transformers connected to trip unit in order to sense faults and issue trip instructions. The Trip unit is also required to display the current flowing through the contact system.
Current transformer assembly comprises of Iron core current transformer which is required to match the trip unit power requirement. The output of Iron Core Current transformer primarily depends on core material, area of cross-section of core and the number of turns of secondary winding. Optimization of the following aspects ensured that the trip unit gets the required power at a much lower value of primary current. Theoretical study regarding BH curves of various material helped in gaining insight about significance of core material in CT output. Assembly space constraint mainly with respect to depth reduction of circuit breaker for purpose of compactness, which required reduction in CT cross-section area, was a challenge.
Another challenge was limiting the output of current transformer once the power requirements of the trip unit is being met. The rate of rise of secondary current was required to be as minimum as possible at the rated current, otherwise the components on trip unit would get affected due to high secondary current.

Calculation and analysis helped in material selection for core and also at arriving optimized core cross-sectional area, the number of turns so as to maximize the CT output initially and forced saturation of the CT output later on.
In view of the prior art, there is a dire need of a Current Transformer with improved power up of trip unit, reduced secondary current at rated and higher primary currents and saturated secondary current at short circuit primary currents.
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.
According to one aspect of the present invention, a Current Transformer to power up a trip unit of switchgear comprising a plurality of adjoining iron cores with a small air gap between them, the iron cores comprising a main core (A) and a secondary core (B), wherein the main core (A) is configured to meet the power requirement of the trip unit, and the secondary core (B) is configured to limit the output of the main core (A) once the power requirements of the trip unit is met, a plurality of secondary windings comprising a main winding (C) disposed around the main core (A) and a secondary winding (D) disposed around the secondary core (B), and a notch on the secondary core (B) to indicate the direction of the winding.
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 ACCOMPANYING DRAWINGS
The above and other aspects, features and advantages of the embodiments of the present disclosure will be more apparent in the following description taken in conjunction with the accompanying drawings, in which:
Fig. 1 illustrates an isometric view of Iron core Current Transformer to power up trip unit of switchgear, according to an embodiment of the present invention.
Fig. 2 illustrates a front view of Iron core Current Transformer to power up trip unit of switchgear, according to an embodiment 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
The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of various embodiments of the present disclosure as defined by the claims and their equivalents. 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 various embodiments described herein can be made without departing from the scope and spirit of the present disclosure. In addition, descriptions of well-known functions and constructions may be 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 present disclosure. Accordingly, it should be apparent to those skilled in the art that the following description of various embodiments of the present disclosure is provided for illustration purpose only and not for the purpose of limiting the present disclosure 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. Thus, for example, reference to "a component surface" includes reference to one or more of such surfaces.
The terminology used herein is for the purpose of describing particular various embodiments only and is not intended to be limiting of various embodiments. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising" used herein specify the presence of stated features, integers, steps, operations, members, components, and/or groups thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, members, components, and/or groups thereof. Also, Expressions such as "at least one of," when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list.
The present disclosure will now be described more fully with reference to the accompanying drawings, in which various embodiments of the present disclosure are shown. The present disclosure may, however, be embodied in many different forms and should not be construed as being limited to the various embodiments set forth herein, rather, these various embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the present

disclosure. Furthermore, a detailed description of other parts will not be provided not to make the present disclosure unclear. Like reference numerals in the drawings refer to like elements throughout.
The subject invention lies in a Current Transformer to power up trip unit of switchgear. According to an embodiment, comprises of two sets of secondary winding made on different iron core, both parallel to each other, and their winding directions reversed and connected in series.
Fig (1) shows Iron core CT where A is the main core, B is the secondary core, C is the main winding & D is the secondary winding. The notch is made on secondary core for indicating the direction of winding.
The purpose of the main core and the 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. FEM analysis and calculations with respect to core material helped deduce that CRGO (Cold-Rolled Grain Oriented) when used in main core shows better power up of trip unit, here the laminations are designed in rolling direction. Power Up of trip unit was improved by 100% when CRGO steel was used instead of CRNGO (cold rolled non grain oriented) steel.
Hence, main core, which is primarily responsible for power up, is made of CRGO material. Also, as rate of rise of secondary current should be as minimum as possible after power up, forced saturation was required to be employed. Hence a secondary core made of CRNGO material is employed which is placed adjoining the main core with small air gap between them (as shown in Fig 1, 2). 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.
This forced reduction in output of CT helps in optimizing the wire gauge of the CT windings and the ratings of the components used in trip units, hence optimizing the sizes effectively.
In the disclosed invention, the main core is made of CRGO steel & the secondary core of CRNGO steel. Here as seen in Fig the secondary core is accommodated in between the primary core limbs, which is shaped in the form of a trapezium. The primary current flows through the primary conductor, which passes through center of the main core of CT. The primary current sets up magnetic field in the region around it. The magnetic flux density is the measure of concentrating magnetic field lines per unit area at a particular value of magnetic field strength.
At low magnetic field strength (Power Up Region), when the primary current is low, the permeability of CRGO material very large as compared to CRNGO material. Also due to presence of air gap, the secondary core path is of high reluctance whereas the main core path is of low reluctance. Hence the flux does not get shunted to secondary core during power up.
However, at high magnetic field strength, as saturation sets in both the materials, the permeability of both materials drop and their reluctances becomes comparable,

hence flux now gets shunted through both the cores and the net opposing current flows through the secondary winding. Hence we get a much reduced secondary current at rated or higher primary currents.
For the disclosed invention, the working principle remaining similar, multiple types of designs were generated and tested. In one of the design, the secondary core is placed at right angle to the main core.
The same philosophy can be extended to multiple core designs not restricting to just a main and secondary core to generate a favourable CT output as per different requirements.
Some of the non-limiting advantages of the Current Transformer to power up trip unit of switchgear are:
1. Improved power up of Trip Unit ensures that current detection and display is available at a lower value of primary current.
2. Improved protection of circuit breaker due to decreased chances of Trip Unit failure due to reduced secondary current at short circuit Fault.
3. Reduction in copper used due to increased SWG of copper wire as a result of decreased CT secondary current at rated and higher primary currents.
4. Temperature Rise of CT winding reduced due to decreased CT secondary current at rated primary current.
5. Current Transformer with saturated secondary current at short circuit primary currents.
6. Current transformer made of multiple core materials. Main Core, secondary core, tertiary cores etc. constructed by using different cores material having relatively different B-H curves.
7. Current transformer with multiple air gaps introduced between cores to offer a high reluctance path during lower primary currents.

8. Help in making a cost effective and size optimized design of CT and trip unit for the same application.
9. Improves the rogowski accuracy in proximity of iron CT winding. An oval cross-section rogowski for better winding hygiene and uniform turn tension. Introduction of ferromagnetic shield between rogowski and Iron CT windings for shielding against interference from Iron CT windings. Rogowski accuracy improved due to improvement in Iron CT saturation at higher primary currents.
Although a Current Transformer to power up trip unit of switchgear has been described in language specific to structural features, it is to be understood that the embodiments disclosed in the above section are not necessarily limited to the specific methods or devices described herein. Rather, the specific features are disclosed as examples of implementations of a Current Transformer to power up trip unit of switchgear.

WE CLAIM :

1. A Current Transformer to power up a trip unit of switchgear comprising:
a plurality of adjoining iron cores with a small air gap between them, the iron cores comprising a main core (A) and a secondary core (B), wherein the main core (A) is configured to meet the power requirement of the trip unit, and the secondary core (B) is configured to limit the output of the main core (A) once the power requirements of the trip unit is met;
a plurality of secondary windings comprising a main winding (C) disposed around the main core (A) and a secondary winding (D) disposed around the secondary core (B); and
a notch on the secondary core (B) to indicate the direction of the winding.
2. The current transformer as claimed in claim 1, wherein the main core and the secondary core are made of two different type of materials, and the secondary core is accommodated in between the primary core limbs, which is shaped in the form of a trapezium.
3. The current transformer as claimed in claim 2, wherein the main core is made of CRGO (Cold-Rolled Grain Oriented) steel and the secondary core is made of CRNGO (Cold-Rolled Non Grain Oriented) steel.
4. The current transformer as claimed in claim 1, wherein the main core winding and the secondary core winding are connected such that net CT secondary current generated is the difference of induced currents in both.
5. The current transformer as claimed in claim 1, wherein 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.

6. The current transformer as claimed in claim 1, wherein the plurality of secondary windings are disposed parallel to each other.
7. The current transformer as claimed in claim 1, wherein the secondary core is placed at right angle to the main core.
8. The current transformer as claimed in claim 1, wherein the current transformer has multiple cores, and the multiple cores are made of different materials having relatively different B-H curves.
9. The current transformer as claimed in claim 1, wherein the current
transformer has multiple air gaps introduced between cores to offer a high
reluctance path during lower primary currents.