FIELD OF THE INVENTION
The present invention relates to a high voltage current transformer (CT) and
more particularly to a dead tank current transformer with parallel primary cable
configuration.
BACKGROUND OF THE INVENTION
The current Transformer (CT) finds application in every segment of the power
system including generation, transmission and distribution networks. The broad
functions of the CT are metering and protection. The CT is configured to
accurately measure the line current for billing purposes and protection function
through different secondary windings. The output terminals on the secondary
side are marked accordingly to which metering instruments / relays are
connected. The CTs are designed as per the voltage, current ratings, besides
other specific electrical and mechanical parameters. Primary conductor is the first
component of the CT that makes the contact with the high voltage line carrying
the load current. The primary conductor is an important component of the
equipment as it produces the ampere-turns for the induction of corresponding
current in the secondary circuit.
In high voltage circuits, the primary conductor of a CT is normally a copper /
aluminum tube of a particular cross-section corresponding to the current
specification. A tubular section is mostly preferred in place of a flexible cable due
to mechanical considerations. In a conventional dead tank oil filled CT [Fig.1.0],

the primary tube takes the shape of a hairpin wrapped with full line voltage
insulation, traveling from a top camber [1] of the CT to a bottom tank [4] and
then back to the top chamber [1].
When the primary current is found to be higher than the current carrying
capacity of a tube within the available range, the size/cross-section of the tube is
correspondingly increased to be able to carry the additional current. However,
sometimes there could be a limitation on the size of the tube due to constraints
of space and electrical clearance. The problem becomes more serious in case of
a CT where there are restrictions imposed on the dimensions of the insulator,
bottom and top chambers. Also at times, a current density limit is specified by
the consumers to avoid excess heating of the primary conductor. Under those
circumstances, the design of CT becomes difficult.
Conventionally, the primary conductor of a dead tank CT is a single aluminum or
copper tube shaped in the form of a hairpin. Its dimensions are decided based
on technical parameters such as continuous, short time current ratings, current
density etc.
When the current rating of the CT is large, it calls for a higher cross-sectional
primary tube and the consequent insulation build-up for the specified voltage
class.

There is sometimes a space constraint within the CT which per forces
configuration of the CT with a higher current density. But, there is also a
limitation to which the current density can be increased due to constraints of
desired specification and heating. Thus, the CT is to be configured with enlarged
inner dimensions of all the components. Thus, there is a restriction imposed by
the technical specifications and also the space availability. Even if the tube
dimensions are enlarged, it calls for a larger bore insulator which inter alia
escalate the cost of the CT. Additionally, the insulation build-up over the primary
tube enhances the criticality of electrical clearances.
US 3686598 discloses a technique where two tubes are run in parallel to increase
the current rating of the CT. But such an arrangement requires additional space
in the top chamber and the insulator including insulation build-up and cost.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention is to propose an improved dead
tank current transformer for power systems, which eliminates the need to
increase the cross-section of the primary tube or using multiple primary tubes.
Another object of the invention is to propose an improved dead tank current
transformer for power systems which effectively utilizes the vacant space of the
tube to save space, insulation and related components.

A further object of the invention is to propose an improved dead tank current
transformer for power systems, which ensures controlling the current density on
parallel conductors without over-heating.
SUMMARY OF THE INVENTION
Accordingly, there is provided an improved dead tank current transformer for
power systems comprising at least a primary tube (2) circumscribing a top
chamber (1), an insulator (5) and a bottom tank (4), the primary tube (2) having
a bore; and one each incoming and outgoing terminals (P1, P2), the
improvement is characterized in that an additional cable (6) is provided to
parallely connect the primary tube (2), the additional cable (6) being passed
through the bore of the primary tube (2) which allowing a sharing of the
transmitting current between the additional tube (6) and the primary tube (2)
corresponding to the ratio of their resistances.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1.0 : Dead tank Current Transformer with primary tube in parallel with
an additional cable according to the invention.
Section A-A (Fig. 1.0): Primary tube and additional cable of figure 1.

Configuration-A (Fig. 1.0 : Primary tube and additional cable connected in parallel
configuration of figure 1.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention, and as shown in figure 1.0, the current transformer
comprises a primary tube [2] taking the shape of a hairpin traveling from a top
chamber [1], an insulator [5], bottom tank [4] and back to the top chamber [1],
The proposed invention offers a viable solution to the problems discussed above
by providing an additional cable [6] that is in parallel and runs through the bore
of the primary tube (2). When the additional cable [6] is employed in parallel
with the tube [2], the two parallel conductors share the primary current
proportionately. The sharing of the current between the tube [2] and the cable
[6] allows restricting the current density within the allowable limits in both the
conductors. P1 and P2 represent the incoming and outgoing terminals of the CT
respectively.
Section A-A of figure 1.0 shows the additional cable [6] passing through the bore
of the tube [2]. Configuration-A of figure 1.0 shows that the primary tube [2] is
connected in parallel to the cable [6]. The current rating of the CT being large,
the improved configuration of the CT provides a better solution in the form of an
additional cable connected in parallel to the primary tube. The configuration-A
prevents the primary tube from getting over-stressed which may otherwise lead

to development of higher current density / extra heating. The additional cable
[6], when connected in parallel with the primary tube [2], shares the
transmission of current depending up on the ratio of resistances. This
arrangement avoids the need for enlarging the CT dimensions and also restricts
the current density within the specified limits. The new CT does not require any
additional space within it as compared to the prior art.
The additional cable [6] which is designed to pass through the bore of the tube
[2] is provided with an insulation to ensure that it does not make a galvanic
connection with the tube [2] being in a parallel conductor configuration
[Configuration-A]. Since the tube [2] and the cable [6] are at the same potential,
the category of insulation is not a serious issue. Also, since the flexible cable [6]
passes through the bore of the tube [2], its movement is obviously restricted and
the CT remains unaffected even under the sustained mechanical forces at the
time of fault. The new concept has been implemented successfully on a 220 kV
oil filled dead tank CT [figure 1.0] which passed all the type/routine tests
prescribed as per IS-2705 (part-1) and IEC44-1 standards. The successful
clearance of the prototype CT with additional parallel cable for the short time
current test (STC) for a duty cycle of 40 kA rms / 3 sec with 100 kA peak is a
proof of technical validation where the primary tube [2] experiences high
dynamic forces too. The test was conducted as per clause 9.6 of IS 2705
(part-l)-1992.

ADVANTAGES OF THE INVENTION
• Effective utilization of the bore of the primary tube with insertion of
parallel cable.
• No additional space requirement for enhancement of current rating.
• Limiting the current density of the primary tube for the given current
rating.
• Economical solution with parallel connection of tube and the additional
conductor to carry larger currents.
• Implementable flexible configuration irrespective of the voltage class of
theCT.
• Sturdy design for Short Time Current (STC) test with tubular conductor
with embedded parallel cable.

WE CLAIM:
1 An improved dead tank transformer for power systems, comprising at
least a primary tube (2) circumscribing a top chamber (1), an insulator (5)
and a bottom tank (4), the primary tube (2) having a bore and one each
incoming and outgoing terminals (P1, 92), the improvement is
characterized in that an additional cable (6) is provided to parallely
connect the primary tube (2), the additional cable (6) being passed
through the bore of the primary tube (2) which allowing a sharing of the
transmitting current between the additional tube (6) and the primary tube
(2) corresponding to the ratio of their resistances.
2. The transformer as claimed in claim 1, wherein the additional cable (6) is
provided with an insulation to prevent galvanic connection with the tube
(2).
3. The transformer as claimed in claim 1, wherein the primary tube (2) is
configured in the shape of a hairpin /U-pin.
4. The transformer as claimed in claim 1, wherein the primary tube (2) is
connected in parallel with the cable (6) ensures adherence to
current density limitations.

4. The transformer as claimed in claim 1, wherein the primary tube (2) is
connected in parallel with the cable (6) ensures proper sharing of line
current.

ABSTRACT

The invention relates to an improved dead tank current transformer for power
systems comprising at least a primary tube (2) circumscribing a top chamber
(1),an insulator (5) and a bottom tank (4), the primary tube (2) having a bore;
and one each incoming and outgoing terminals (P1, P2). The improvement is
that an additional cable (6) is provided to parallely connect the primary tube (2),
additional cable (6) being passed through the bore of the primary tube (2) which
allowing a sharing of the transmitting current between the additional tube (6)
and the primary tube (2) corresponding to the ratio of their resistances.