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FIELD OF THE INVENTION
The present invention relates to a sensor for detecting the circulating currents in
the transformer core due to routing LV winding and tap changer of the
transformer under load condition. More particularly, the invention relates to a
non-contact, modular core current sensor with ac linear output characteristic.
BACKGROUND OF THE INVENTION
A power transformer is one of the complex static power equipment and consists
of windings, iron core, solid and liquid insulation. The iron core provides the
magnetic path, made from insulated magnetic laminations, ands enables
transferring the power between the primary and secondary windings of the
transformer. The flux generated in the iron core links magnetically the primary
and secondary windings in the transformer. The mutual flux developed through
the core, transfers the power. The leakage flux through the air may induce
voltage and might result in the circulating currents in the core through shorted
electrical path, if any. These currents cause over heating, sparking, vibration,
noise, and gas bubble in the transformer affecting operation of the transformer.
Identification and control measure results in improved performance of the
transformer, optimizing the routing of the leads and improve the quality of the
core assembly. No study or publication exits to determine the characteristics and
behaviour of the circulating currents in the core and the method for calculating

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their effects. Thus, measuring the core currents during the operation under load
condition at test bed during core assembly stage is the only alternative. The
measurement of core current has been carried out, using an electromagnetic
sensor based on Ampere's law.
A non-magnetic sensor has been reported (Patent US 4611191-1996). This
device employed a ring - shaped core through which a primary conductor runs
and supports a secondary winding. A magnetic shield encloses the core, whereas
a non-magnetic conducting shield is inserted between the conductor and the
core with the winding.
A Rogowski coil has been reported (US 6313623, 2001).This coil measures a
current in a conductor and includes first and second loops electrically connected
in series with each other. The first loop is wound with a substantially constant
winding density in a first direction around a first core that has constant cross
section. The second loop is wound with constant winding density in a second
direction around second core, which also has constant cross section. The
direction of the first loop is perpendicular to the normal of the first core cross
section. Similarly second loop is perpendicular to the normal of the second cross
section.
A current sensor has been developed and reported (US 2006/0232263A1, 2006).
The sensor is provided including a Rogoswski coil in an assembly that is easily
positioned about a conductor without cutting the line. In some arrangements,
the coil is formed about a split helical structure such that the ends of the coil are

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not coincident. The current sensor includes suitable insulation such that it can be
placed around energized, high voltage lines.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a non-contact, modular core
current sensor with ac linear output characteristics, which eliminates the
disadvantages of the prior art.
Another object of the invention is to propose a non-contact, modular core
current sensor with ac linear output characteristics, which is capable to
accurately detect and control the circulating currents in the transformer core.
A further object of the invention is to propose a non-contact, modular core
current sensor with ac linear output characteristics, which enables prevention of
overheating, sparking, vibration, noise, and gas bubble in the transformer.
A still further object of the invention is to propose a non-contact, modular core
current sensor with ac linear output characteristics, which is capable to
contribute improved performance of the transformer.
SUMMARY OF THE INVENTION
Accordingly, there is provided a modular current sensor for measurement of
transformer core circulating currents from core assembly stage on test bed up to
field operation. The sensor is rectangular and non-contact type construction,

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encompassing the core section. The sensor is wound with class F electrical
copper wire in a single layer over an insulating former. The sensor is designed
such that it attenuates both electromagnetic and electrostatic signals induced by
any other circuits except the enclosed core current. The sensor is compatible
with transformer oil at higher temperatures. Since no power is being drawn from
the sensor and heat generation inside the coil is insignificant. It is installed on
the core which is at earth potential. The ac output signal of the sensor is of order
of few mV depending on the magnitude of core current. The sensor is detachable
and it can be easily mounted in the transformer.
Modular insulating formers have been used for the sensor to retrofit the
clearance available between the core with winding and yoke in the power
transformer for installation of the sensors. It is a segmental, robust, portable
insulating structure. Copper wire has been wound over insulation formers
covered with heat sinkable insulating sleeve for mechanical and electrical
protection of the sensor. Thermo setting encapsulation was carried out and post
cured at its class temperature. Two ends of winding have been brought oit
through the shielded cable to connect to the measurement system. All the
segments of the sensor formers are interconnected and the winding terminals
are connected in series along the entire length of the former. The assembled
sensor is in rectangular shape.
The sensor consisting of segmental insulating formers interconnected in
rectangular shape suiting the transformer core section. This wound former
assembly pickup induced magnetic field due to circulating core currents.

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The sensor is a compact, modular, and non-contact rectangular type sensor
retrofit encircling the transformer core assembly.
The sensor helps the designer to properly route the winding leads to minimize
the core current during manufacturing stage.
The sensor eliminates the stray electro-magnetic and electro-static stray signals
without the employing magnetic material. This results in linear low output signal.
The sensor constitutes a part of the transformer for on -line measurement of the
core current at site during service.
The sensor is mounted on the grounded core assembly, which it is suitable for
HV power transformer application.
BRIEF DESRIPTION OF THE ACCOMPANYING DRAWINGS
The invention is described in detail with the help of the accompanying drawing
given as Fig.l.
Fig.l: Details of the Sensor assembly according to the invention.

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DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
As shown in figure - 1, the main components of the sensor is toriodal insulation
formers (1) compatible with transformer oil, rugged with high temperature
withstand capability and high dielectric strength. Basically Hylam tubes with
26mm (OD) and 12 mm (ID) and lengths 615mm of each segment were used in
sensor construction. Six such segments have been used in total sensor assembly
in rectangular form. There is a provision for extension of length and width of
sensor by adding small former segments to retrofit the transformer core of any
size. In view of small clearances available around the transformer core for
mounting the sensor, 25 SWG super enameled copper wire (6) has been
employed for winding. The selection of the wire is based on mechanical and
thermal compatibility with transformer oil. This wire (6) has been continuously &
closely wound along the length of the each segment of the former (1) in a single
layer. This winding is encapsulated using polyolefin insulating sleeve. The wound
former along with sleeve is post cured at 135°C. This winding sense the magnetic
field generated due to circulating core current while in operation and induces
proportional ac signal across the standard resistor. A Hylam stud (3) with
M12X1.75 pitch and 40 mm length is used as inter connector of the segmental
formers (1) in rectangular shape. A stud (2) with M12X1.75 pitch and 60 mm
length is used as end connectors between the arms of the formers(l) at the
corners. A Hylam round nut (4) with 260, M12 inner thread and 10 mm long is

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employed for mechanically tightening at the four corners of the rectangular
sensor assembly. 25 SWG super enameled copper wire is used as a return
conductor (5) through the entire sensor assembly to encounter the electro
magnetic stray signals. A first end of the return conductor (5) and a first end of
the winding (6) is connected together. A second end of the return conductor (5)
and the winding (6) are used as measuring terminals. These two ends of the
winding (6) are brought out from the transformer unit through a shielded twisted
cable for core current measurement using FFT Analyzer. To avoid the
electrostatic signals during measurements one of the end terminals of the wiring
is grounded externally.
A known resistor is connected across the measuring terminal of the sensor.
Voltage crossing this resistor is calibrated to give the measure of the core
current. The external resistance value is more than the resistance of the sensor
coil.
The invention is not only limited to the embodiments only more specially deals
with

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WE CLAIM
1. A core current sensor assembly comprising an insulating tubular frame (1)
with solenoid winding (6) constituted by turns, and wound on the entire
periphery forming a secondary winding; a return wire (5) running along
the axis of the sensor acting as electromagnetic shields, the terminal of
the return wire (5) grounded to provide the electrostatic shield; and the
core of the transformer constitutes a primary winding.
2. The sensor assembly as claimed in claim 1, wherein the electromagnetic
shield is constituted by the return wire (5) running through the periphery
of the former (1) in opposite direction to the progression of the secondary
winding.

3 The sensor assembly as claimed in claim 1 or 2, wherein said insulated
tubular frame (1) comprising a plurality of segmental insulating formers
interconnected in a rectangular shape corresponding to the shape of the
transformer core section.
4 A core current sensor assembly as substantially described herein with
reference to the accompanying drawings.

This invention relates to a core current sensor assembly comprising an insulating
tubular frame (1) with solenoid winding (6) constituted by turns wound on the
entire periphery forming a secondary winding; a return wire (5) running along
the axis of the sensor acting as electromagnetic shields, the terminal of the
return wire (5) grounded to provide the electrostatic shield; and the core of the
transformer constitutes a primary winding.