FIELD OF INVENTION
This invention generally relates to monitoring of insulation current of high
voltage bushings (HV bushings), i.e. the insulation current passing through an
insulation body of a high voltage bushing. More particularly, the present
invention relates to an on-line monitoring system to identify a faulty bushing
including classification of the nature of defect in a bush-bank adaptable to high
voltage transformer or reactor.
BACKGROUND OF THE INVENTION
A HV bushing is a device which is used to take a high voltage, high current lead
into electrical equipment like a transformer and a reactor. It consists of a current
carrying conductor which is centrally located and surrounded by high voltage
insulation. Usually the insulation is an oil-paper combination with metal foils in
between. This assembly is housed in a casing, which is made of either porcelain
or an epoxy-silicon composite insulator. In service, the insulation has to take the
entire high voltage. In addition to the rated high voltage, the insulation also
withstands the abnormal over voltages that are generated in a power system as
well as the heat generated by the load current which passes through the central
conductor of the bushing.
Failure of a bushing, which generally occurs as a result of the complete
breakdown of its insulation may lead to oil catching fire and results in total
burning of the transformer. Hence, a bushing is tested for its insulation
conduction periodically. With the introduction of very high system voltages in the

recent times, the reliability of the bushing has become more important. An
unplanned outage of an electrical supply system might cause a heavy revenue
loss. Therefore, the bushing insulation requires a continuous monitoring for its
condition, which can be achieved by analyzing the current through the bushing
insulation. For taking the current lead from the bushing, a special tap is provided
in the bushing. By using a suitable adapter that can be fixed on to the bushing
tap and a sensor, the insulation current can be measured and further analyzed to
ensure the bushing healthiness.
Publication no. US 2003/0184325 discloses an invention entitled 'Device and
method for monitoring a capacitor bushing', according to which a permanent
fixed impedance is used and that is connected in between the bushing insulation
and ground. Across the impedance, a capacitor is connected to a series switch.
The signal is measured across the capacitor.
Publication No. US 2003/0160602 Al describes an invention entitled 'power
factor/Tan delta testing of high voltage bushing on power transformers, current
transformer and circuit breakers'. This prior art invention employs a capacitor
divider principle to get a measurable signal. A surge arrester is used across the
low voltage capacitor. Further, the signal is taken through a choke coil and a
fuse to a transformer and the output from the secondary coil is used for
measurement purposes. It means that the measurement system is protected as
well as electrically isolated.

International patent Publication No. 03/019211 A1 discloses an invention entitled
Tap switch for frequency response and partial discharge measurement'. This
invention employs a permanent capacitor at the ground point of the bushing
insulation to avoid any spark over. The method employs two switches which
operate exclusive to each other. One switch connects the lower end of the
bushing insulation to ground while the other connects directly to a measurement
circuit. During the measurement the permanent capacitor along with the bushing
insulation forms a capacitive divider'.
The following inventions employs a capacitor device principle to derive the
primary signal.
US patent 3,710,242 discloses an invention entitled 'Service bushing test',
US Patent no. 6,028,430 discloses an invention entitled 'Method for monitoring a
capacitor bushing and monitoring system',
US Patent 2002/0079903 Al describes an invention entitled 'Electrical system
with a stress shield system for partial discharge on - line monitoring of the state
of high-voltage insulation', and
Patent No, RU 2328009 C1 discloses an invention entitled 'Device for monitoring
of high-voltage bushing and annunciation of their insulation condition'.

US patent no. 4,914,38 describes an invention entitled "High voltage measuring
circuit coupled to the capacitive grounding tap bushing of an HV device'. This
invention uses a capacitor divide circuit, and a current transformer connected
across the lower end capacitor of the capacitor dividing circuit. The signals from
the secondary of the transformer are measured and analyzed.
Thus, the method and apparatus for monitoring the bushing current through
bushing tap is generally known from the prior art. In all the existing methods,
the ground connection of the bushing capacitance is disconnected and suitable
impedance in the form of a resistor or a capacitance is introduced. The voltage
that appears across this impedance is taken as a measure of the bushing
insulation current. However, incase this connection is discontinued there is an
over voltage that appear across the impedance, the measurement circuit
experiences a high voltage that can damage the circuit and is also unsafe to the
operating personnel. In such cases, a protection circuit and also an isolation
circuit are employed to minimize the effect of the over voltage. However, such
protection/isolation circuit is always not full-proof, and further increases the cost.
OBJECTS OF THE INVENTION
It is therefore an object of a present invention to propose a adapter-sensor
assembly, which is part of on-line monitoring system to identify a faulty bushing
including classification of the nature of defect in a bush-bank adaptable to high
voltage transformer or reactor.

Another object of the present invention is to propose an adapter-sensor
assembly for an on-line monitoring system to identify a defaulted bushing
including classification of the nature of defect in a bush-bank adaptable to high
voltage transformer or reactor, without disconnecting the ground connection of
the bushing capacitance and introduce impedance.
A still another object of the present invention is to propose an adapter-sensor
assembly for an on-line monitoring system to identify a defaulted bushing,
including classification of the nature of defect in a bush-bank adaptable to high
voltage transformer or reactor, which eliminates the possibility of developing
over voltage because of circuit opening.
A further object of the present invention is to propose an adapter-sensor
assembly for an on-line monitoring system to identify a defaulted bushing
including classification of the nature of defect in a bush-bank adaptable to high
voltage transformer or reactor, in which an adapter is configured so as to be
fixed on the bushing tap to ensure that the bushing current passes through a
measuring unit where a non-contact type sensor is used for measurement. This
sensor must be capable of measuring a very small bushing current even though
it is away from the central current conductor of the adapter-sensor assembly.
A still further object of the present invention is to propose an adapter-sensor
assembly for an on-line monitoring system to identify a defaulted bushing
including classification of the nature of defect in a bush-bank adaptable to high
voltage transformer or reactor, which is capable to output from the sensor a pure
sinusoidal wave by adapting a filter for better measurement accuracy.

SUMMARY OF THE INVENTION
According to the invention, the ground connection is not disturbed and is
maintained as it is. A non-contact type current sensor which is capable of
sensing the insulation current is placed concentrically around the ground
conductor. This eliminates the possibility of any over voltage being developed
due to the ground circuit opening, during operation of the bushing. An adapter
for fixing on to the bushing tap is also provided as a part of the unit. This
facilitates the bushing current to pass through the measuring unit. Further, it is
necessary according to the invention to get a pure sinusoidal wave output, for a
better analysis. Hence a filter circuit is provided at the output of the sensor to
eliminate high frequency noises.
The adapter - sensor device of the system which initially measures the bushing
insulation current comprises a non- contact type sensor capable of measuring
few tens of milli-amps current. The device can be directly fixed on to a HV
bushing tap for on-line measurements.
The adapter can be directly coupled to a bushing tap to make the bushing
current to flow through the central conductor of the unit. According to the
invention, the bushing insulation is directly grounded so that no additional
component is introduced between the bushing insulation and ground.

The sensor of the invention is very compact and also designed to nullify the
effect of external noise, and the sensor output is obtained as an amplified and a
pure sinusoidal wave without any harmonics by deploying a filter circuit. The
measured and output data is available at a BNC connecter for further processing.
BRIEF DESCRIPTION OF THE ACOMPANYING DRAWINGS
The invention is described in detail with the help of the accompanying drawing
shown as
Fig. 1 - shows a block diagram of an adapter-sensor system to identify a faulty
bushing including classification of the fault nature in a bush-bank adapted to
high voltage transformer or reactor.
Fig. 2 - indicates a calibration graph indicating the bushing insulation current &
sensor output voltage of an adapter - sensor device of the system.
DETAIL DESCRIPTION OF PREFERRED EMBODIMENTS OF THE
INVENTION
As shown in Figure - 1, the device comprising an adapter housing (01) which
has an internal threading to match with the threading on the bushing tap. Inside
the adapter housing (01), a groove is provided in which an insulation support
(02) is placed. The insulation support (02) comprises a threading on its side so
that it can be screwed into the adapter housing (01) and loosely disposed in the

groove. At the centre of the insulation support (02), a central conductor (03) is
fixed. One end of the central conductor (03) is designed in such a way that it can
be directly fixed on to the conductor of the bushing tap. When the adapter (01)
is fixed over the bushing tap, the central conductor (03) exerts a pressure over
the bushing tap, establishes a connection with the current conductor of the
bushing tap and also disconnects the bushing tap from ground. The central
conductor (03) of the adapter-sensor passes through a main housing (04), which
is made of a non magnetic metal. This metal housing (04) acts as a shield
against the external noises. However, the central conductor (03) is insulated
from the main housing (04) by means of at least two insulating bushes (05). This
makes the bushing current to compulsorily flow through the central conductor
(03) but not through the body of the main housing (04). A sensor coil (06) is
able to pick up a small current of a few tens milli - amperes. The coil (06) being
multi-wound it increases the measurement sensitivity and at the same time
nullify the effect of any external noise. A measurement housing (07) keeps the
coil (06) in position and also holds a filter circuit (08) including an output point,
which constitutes a BNC connector (09). The filter circuit (08) makes the output
voltage exactly at 50 Hz AC voltage. Further this filter (08) enhances the signal
magnitude. The central conductor (03) is tightly fixed by means of at least two
nuts (10) which are fixed over the measurement housing (07). A signal output
for further analysis is available from the BNC connector (09) which enables the
on-line monitoring of the sensed data in respect of the insulation current passing
through the high voltage bushing in order to identify the faulty bushing in a bank
including the nature of defects.

WE CLAIM
1. An adapter-sensor unit for online monitoring for identifying a faulty
bushing including classification of the nature of defect in a bush-bank
adaptable to high voltage transformer or reactor, the system comprising :
a measuring device having an adapter (01) releasable and attachable
to a bushing tap, an insulation support (02) which is fixed inside the
adapter (01), a conductor (03) centrally disposed on the insulation
support (02), a first end of the conductor (03) being fixable on the
bushing tap such that the conductor is enabled to establish a
connection with the bushing tap on exertion of a pressure while the
adapter (01) is fixed on the bushing tap, the conductor (03) passing
through a metal housing (04) and insulated by disposing at least two
insulating bush (05) in the metal housing (04) which ensures that the
bushing current flows via the conductor (03); a sensor (06) in the form
of a coil disposed into a measurement housing (07), the sensor (06)
being capable to acquire current data of a magnitude of tens of
milliamperes, a BNC connector (09) disposed at the output point of the
measurement housing (07), and a filter circuit (08) connected at the
output (09) to ensure that the output voltage exactly maintained at
50Hz pure AC.

2. The system as claimed in claim 1 wherein a non contact type sensor is
used to detect the bushing insulation current, which is of very small
value. Further the sensor winding is such that it does not allow the
external noise from entering the measurement circuit.
3. The system as claimed in claim 1, wherein the metal housing (04)
comprises a non-magnetic material.
4. The system as claimed in claim 1, wherein the measurement housing (07)
is fixed to the central conductor (03) by means of at least two nuts (10)
thus is eliminating deliberate breaking of ground connection and
introduction of an additional impedance.
5. The system as claimed in claim 1, wherein the filter circuit (08) is
externally held by the measurement housing (07).
6. The system as claimed in claim 1 or 4, wherein the filter circuit (08) is
enabled to amplify the output from the sensor and to convert the output
to a pure sinusoidal wave without the harmonics.

7. An online monitoring system to identify a faulty bushing including
classification of the nature of defect in a bush-bank adaptable to high
voltage transformer or reactor, as substantially described and illustrated
herein with reference to the accompanying drawings.

This invention relates to an adapter-sensor online monitoring system to identify a
faulty bushing used in a bush-bank adaptable to high voltage transformer or reactor. The system comprising a measuring device having an adapter (01) which is releasable and attachable to a bushing tap, an insulation support (02)
fixed inside the adapter (01), a conductor (03) centrally disposed on the insulation support (02), a first end of the conductor being fixable on the bushing tap such that the conductor is enabled to establish a connection with the bushing tap on exertion of a pressure while the adapter (01) is fixed on the bushing tap.
The conductor (03) passes through a metal housing (04) and insulated by disposing at least two insulating bush (05) in the metal housing (04) which ensures that the bushing current flows via the conductor (03). A non contact
type sensor (06) and disposed into a measurement housing (07), the sensor being capable to acquire current data of a magnitude of tens of mili-ampers. A BNC connector (09) is disposed at the output point of the measurement housing
(07), and a filter circuit (08) connected at the output (09) to ensure that the output voltage exactly maintained at 50Hz AC. The central conductor kept in position by two nuts (10).