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 split primary tube.
BACKGROUND OF THE INVENTION
The Current Transformers (CT) find application in every segment of the power
system comprising generation, transmission and distribution networks. The
purpose of the CT is broadly two pronged namely metering and protection
functions. It is designed 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 proper cross-section as per the current specification. A tube is
mostly preferred in place of flexible cable due to mechanical considerations. In a
conventional dead tank oil filled CT (shown in figure-1), the primary tube takes
the shape of a hairpin provided with full line voltage insulation, traveling from a
top chamber [1] of the CT to bottom tank[4] and then back the to top chamber
[1]. The primary tube is normally a single conducting tube that carries the
current. When the primary current is high enough, it produces adequate ampere-

turns to result in a feasible design. In a typical case where there are a few
ampere-turns on the primary side (low current rating), the size of the secondary
cores [3] become bulky for the given specification, rendering the same as
unviable.
Conventionally, the primary tube of a dead tank CT is a single aluminum or
copper tube in the form of a hairpin. Its dimensions are designed as per the
continuous, short time current ratings, current density etc. When the current
ratio of the CT is small, the dimensions of the secondary cores need to be
invariably higher to reduce the magnitude of magnetizing current to ensure the
ratio and phase angle errors within the specified limits and also to satisfy the
knee point voltage requirements. Obviously, the increase of magnetic core size
will further enhance the quantum of copper, insulation, overall weight and cost.
In physical terms, this will further aggravate the space constraint and handling
problems. Usage of low loss magnetic materials like Ni-Fe and Nano-crystalline
cores is of some help in reducing the size of metering cores for the same
magnetizing current, but it comes with a premium because of the high
differential cost between CRGO steel and others superior materials.
US 3686598 teaches a technique to overcome the prior art disadvantages by
increasing the primary ampere-turns by using multi-turn primary configuration.
But, this takes larger space, bigger insulator, higher cost and bulkier fabrication
items.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose an improved dead
tank current transformer to reduce the magnitude of magnetizing current to limit
the ratio and phase angle errors and the knee-point voltage.

Another object of the invention is to propose an improved dead tank current
transformer to reduce the magnitude of magnetizing current to limit the ratio
and phase angle errors and the knee-point voltage, in which the primary
ampere-turns are doubled through splitting of the primary tube into two equal
halves and connecting the halves in series.
A still another object of the invention is to propose an improved dead tank
current transformer to reduce the magnitude of magnetizing current to limit the
ratio and phase angle errors and the knee-point voltage, by doubling the primary
ampere-turns linking with the secondary magnetic cores.
A further object of the invention is to propose an improved dead tank current
transformer to reduce the magnitude of magnetizing current to limit the ratio
and phase angle errors and the knee-point voltage, which saves the cost of
copper, insulation and the space inside the CT.
SUMMARY OF THE INVENTION
Accordingly, there is provided an improved dead tank current transformer to
reduce the magnitude of magnetizing current and limit the ratio and phase angle
errors, comprising a copper or aluminum tube acting as a primary tube and
circumscribing a top chamber, an insulator, and a bottom tank; one each
incoming and outgoing terminals; the improvement is characterized in that the
primary tube is split lengthwise to form two equal halves which are separated by
an insulating layer and connected in series, and in that the two halves forming
two turns / loops through the secondary cores of the transformer which doubles
the ampere-turns on the primary side to reduce the ratio and phase angle errors.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1.0 - Dead tank Current Transformer with split primary
tube, according to the invention.
Figure 1 (Section 'A-A') - Secondary view of primary tube split into two halves
with insulation separation
Figure 1 (Section 'B-B') - Sectional view if insulated two halves of primary tube
connected in series configuration.
DETAILED DESCRIPTION OF THE INVENTION
Figure 1 shows an improved current transformer with low current on the primary
tube[2] taking the shape of a hairpin travelling from a top chamber[1l], a
insulator[5], a bottom tank[4] and back to the top chamber[l]. The
copper/aluminum tube [2] is split length-wise so that it is divided into two equal
halves and separated by an insulation layer [6']. A thin layer of insulator is
employed between the split halves to separate the turns such that the tube
halves are at the same potential. Each half will have a cross sectional shape
similar to a half ring. They are connected in series [Section 'B-B'] so that two
turns/loops are formed through the secondary cores [3]. P1 and P2 represent the
incoming and outgoing terminals respectively. This type of arrangement doubles
the ampere-turns on the primary side, effectively reducing the percentage
magnetizing current to nearly half of what it was earlier, thereby drastically
reducing the ratio and phase angle errors for the given class of metering core.
Thus, the proposed configuration eliminates the need to use a full tube for 2-turn
configuration which is a difficult proposition with regard to profile bending, inter-
connection and also the space limitations. In the disclosed invention, the low
current rating of the CT is exploited by conventional splitting the tube into two

halves and using them as different conductors in series configuration [Section %B-
The kind of arrangement offers a viable/optimum solution for all low current /
low ratio CTs within the available space. The new concept has been tried out
successfully on a 132 kV oil filled dead tank CT [Figure 1.0] which has passed all
the type/routine tests prescribed as per IS 2705(part-l) and IEC44-1 standards.
The clearing of the prototype (with split primary design) for the short time
current test (STC) for a duty cycle of 31.5 kA rms / 1 sec with 80 kA peak is a
proof of design validation where the primary conductor experiences high
dynamic forces too. Another point of validation is the exhibition of nearly the
same characteristics of routine test results before and after the STC test. The
test has been conducted as per clause 9.6 of IS 2705(part 1)-1992.
Advantages of the invention
• Usage of a single primary tube to achieve 2 turn configuration to get double
the ampere-turns.
• Effective utilization of the space of inside the CT".
• Optimum utilization of the primary tube for a given current rating.
• No major additional insulation requirement.
• Implementable flexible design concept irrespective of the voltage class of the
CT.
• Sturdy design in spite of the split tubular conductor to withstand dynamic
forces at the time of system faults.

WE CLAIM
1. An improved dead tank current transformer to reduce the magnitude of
magnetizing current and limit the ratio and phase angle errors,
comprising:
- a copper or aluminum tube acting as a primary conductor and
circumscribing a top chamber, an insulator, and a bottom tank;
- one each incoming and outgoing terminals;
- the improvement is characterized in that the primary tube is split
lengthwise to form two equal halves which are separated by an
insulating layer and connected in series, and in that the two halves
forming two turns / loops through the secondary cores of the
transformer which doubles the ampere-turns on the primary side to
reduce the ratio and phase angle errors.

2. The transformer as claimed in claim 1, wherein the two halves acting as
two different conductors in a series configuration.
3. The transformer as claimed in claim 1, wherein both the split halves are
operative at the same potential.
4. The transformer as claimed in claim 1, wherein the primary tube is
configured in the shape of a hairpin.

ABSTRACT
The invention relates to an improved dead tank current transformer to reduce
the magnitude of magnetizing current and limit the ratio and phase angle errors,
comprising a copper or aluminum tube acting as a primary tube and
circumscribing a top chamber, an insulator, and a bottom tank; one each
incoming and outgoing terminals; the improvement is characterized in that the
primary tube is split lengthwise to form two equal halves which are separated by
an insulating layer and connected in series, and in that the two halves forming
two turns / loops through the secondary cores of the transformer which doubles
the ampere-turns on the primary side to reduce the ratio and phase angle errors.