FIELD OF THE INVENTION
The present invention generally relates to protection circuits having current
transformers and used in switchgear. More particularly, the present invention
relates to a low output over-current sensor adaptable to protection circuits in a
switchgear.
BACKGROUND OF THE INVENTION
A switchgear is basically an electrical switch having modular and cubical type of
construction. The switchgear is used for connecting a power source to a cable
that transmits power to a load. The switchgear performs the basic function of a
power circuit switching using a circuit breaker, and further having protection
circuits with necessary controls that can cause tripping of the circuit breaker
during a fault condition. The protection circuit receives input from the power
circuit, which is usually at a high voltage, through instrument transformers.
There are two types of instrument transformers viz., i) voltage transformer (VT)
which provides the voltage input and ii) current transformer (CT) that supplies
current input to the protection circuit. While a voltage transformer (VT) has a
normal transformer construction, a current transformer (CT) consists of a
primary current conductor generally with rectangular cross-section surrounded
by a secondary coil wound over an iron core. In practice, the output signals are
of standard values. For example, the output current from a current transformer
(CT) is either 5 Amperes or 1 Ampere. Similarly, the output voltage from a
voltage transformer (VT) is also of standard values viz. 63 Volts 110 Volts. The

signals are fed into a plurality of electro-magnetic relays for operating the
protection circuit. In addition, these outputs are used for the purpose of
metering, i.e. measuring input voltage and current.
However, use of a current transformer (CT) in a switchgear has the following
disadvantages:
The output current from a current transformer (CT) is in amperes, which has to
be again reduced to small signals by using step-down transformers, in order to
meet the requirement of a microprocessor relay, when such relays are used in a
protection circuit.
Further, a current transformer (CT) employs a magnetic (iron) core, which can
develop "magnetic saturation", when the input current exceeds certain limits.
This may lead to an error in output measurement.
A current transformer has to be designed for each current rating, as the output
of the current transformer (CT) is always fixed, hence no single current
transformer can cater to a given primary current range.
The existing current transformers are bulky and costly because of the iron core.
Patent EP 0678950 - year 1995 discloses a combined voltage and current sensor
which employs a resistance divider for measuring the high voltage and a
Rogowski coil i.e. an air core current transformer (CT) to measure the current in
a high voltage bus. The said sensor is directly connected to the circuit breaker.

Further, the signals are used to operate electronic relays. However, such sensor
being directly connected to the circuit breaker, the impact of mechanical shocks
is higher leading to short life of the sensor.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a low output over-current
sensor adaptable to protection circuits in a switchgear, in which the input-output
magnetic link is deliberately kept weak, by replacing the magnetic iron core into
a solid insulation core.
A further object of the invention is to use a two-winding coil as secondary, which
increases the secondary output and at the same time nullifies the effect of
external electrical noises.
Another object of the invention is to propose a low output over-current sensor
adaptable to protection circuits in a switchgear, which ensures that the
relationship between the primary current and the output milli-volt is always linear
and that there is no saturation effect during fault condition.
Yet another object of the present invention is to propose a low output over-
current sensor adaptable to protection circuits in a switchgear, which eliminates
the employment of main and intermediate current transformers.
A further object of the present invention is to propose a low output over-current
sensor adaptable to protection circuits in a switchgear, which is adaptable to
different current ratings.
A still further object of the present invention is to propose a low output over-
current sensor adaptable to protection circuits in a switchgear, which is less
costly and compact having less weight.

A still another object of the present invention is to propose a low output over-
current sensor adaptable to protection circuits in a switchgear, in which the
insulation between the primary and the secondary coils is so selected to
withstand the corresponding class of applicable voltage.
Another object of the present invention is to propose a bw output over-current
sensor adaptable to protection circuits in a switchgear, which eliminates the
disadvantages of the prior art by adopting two-winding coil construction and not
connecting to the breaker directly.
SUMMARY OF THE INVENTION
Accordingly, there is provided a bw output current sensor adaptable to
protection circuits in a switchgear, comprising a central conductor, surrounded
by a coil having two windings that is wound on an insulation core, and housed in
a homogeneous epoxy casting. The windings are wound in two layers and
connected in series to maximize the output.
Further, the external noises are cancelled because the direction of windings of
both the coils is exactly opposite. In the new sensor, no insulation spacer is
provided to support the coil around the central current conductor, i.e. the copper
flat, and only the epoxy cast forms the homogenous insulation between the
conductor and the coil. This significantly improves the basic electrical insulation
level. The size of the coil is optimized to get a maximum transformation ratio
(mV output / input current in ampere) and further to maintain the required
insulation level. The sensor is not connected to the circuit breaker but placed in
the busbar. This reduces the impact of mechanical shocks produced by the
circuit breaker and thus increases the life of the sensor.

In addition to measuring the current in the electrical circuit, the new sensor
senses fault current or over-load current and operates a microprocessor based
relay, which eventually trips the switchgear.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention can be described in detail with the help of the accompanying
drawings in which
Figure 1 - shows the details of a current sensor according to the present
invention.
Figure 2 - shows the details of the coil of the current sensor according to the
present invention.
Figure 3 - shows the current sensor in assembled condition according to the
invention.
DETAILED DESCRIPTION OF A PREFERRED EHBODIMENT OF
THE PRESENT INVENTION
The present invention provides a current sensor, which replaces the current
transformer of the prior art device.
As shown in Figure - 1 a primary conductor (1) that carries the current is
provided which constitutes a copper flat section of 50 mm x 12 mm. At each end
of the conductor (1), at least two holes of 10 mm diameter are arranged that can

facilitate the low resistance connection to an external circuit. A circular coil (2) is
fixed around the copper conductor (1) at the centre. The coil (2) is made of
enamelled copper wire wound over a circular insulator core, made of phenol
formaldehyde. The coil (2) consists of two concentric windings wound one over
other with glass tape insulation in between them. The glass tape also covers the
outer of the two concentric windings. Both the windings are wound in the
opposite direction so that any induction due to external disturbances is nullified.
Further, both the windings are connected in series in such a way that the
induced voltages caused by primary current in both the windings aid each other
to maximize the output voltage. The assembly (1,2) is encapsulated in an epoxy
resin casting (3). The leads (4) of the coil (2) are taken to a terminal box (5),
which is configured as a part of the epoxy casting (3).
Since the proposed sensor is used in a power circuit, it is caused to pass through
high voltage tests as prescribed by the standards. Such tests generally are (a)
one minute AC over voltage and (b) Impulse voltage. For the 11 kV class, these
are 28 kV and 75 kV peak respectively. Since the dimension of the proposed
sensor is already known being retrofittable in the space meant for the replaced
current transformer (CT), the epoxy insulation plays a crucial role in taking the
voltage. During the process of fixing the coil (2) around the conductor (1), no
insulation spacers is used in between, thereby avoiding reduction in the
homogeneity of total insulation which otherwise would have caused by the
spacer through introducing interface junctions.
Further, the sensor has been tested with the microprocessor over-current relay
and found compatible.
WE CLAIM :
1. A low output current sensor adaptable to protection circuits in switch
gears, the switch gear performing power circuit Switching through a
circuit breaker, and tripping the circuit breaker via the protection circuit
under fault condition, the current sensor comprising :
a central conductor (1) that carries the current and connected to an
external circuit;
- a coil (2) wound over a circular insulator core, the coil (2) having leads
(4); and
- an epoxy resin casting (3) configured with a terminal box (5), the coil-
conductor assembly (1 & 2) being encapsulated in the epoxy resin casting
(3), and the leads (4) of the coil (2) extended to join the terminal box
(5), characterized in that:
- the coil (2) comprises at least two concentric windings wound in opposite
direction one over the other with glass tape insulation interposed in
between which enables nullification of external noises due to induction,
and
the at least two concentric windings are connected in series to maximize
the output voltage due to addition of the induced voltage in both the
windings caused by the primary current.
2. A low output current sensor adaptable to protection circuits in a
switchgear as substantially described herein with reference to the
accompanying drawings.