FIELD OF APPLICATION
The present invention relates generally to a bushing tap device. In particular it relates to on-line condition monitoring of a transformer bushing by a bushing tap device that isolates the test tap in a bushing tap from ground to enable on-line measurement of the current which flows through the insulation that determines the condition of bushing insulation.
BACKGROUND OF THE INVENTION
A bushing is a device that is used in a transformer or in a reactor to take the high voltage lead from outside to the windings, which are kept in the transformer oil and housed, inside a metal tank. Basically, it consists of a conductor at high voltage surrounded by insulation, which electrically insulates the high voltage conductor from the metal tank of the transformer that is always connected to substation grounding system. This insulation is a Kraft paper rolled around the conductor and impregnated with transformer oil. In order to have equal voltage distribution throughout the radial length, the paper insulation is made as a series of equal capacitance by providing metal foils at regular intervals. The entire oil¬paper insulation is housed inside a porcelain shell filled with oil. The porcelain shell is surrounded by a metal flange at center, which is connected to the transformer tank.
A bushing is always stressed with high voltage during the service. Also, it is subjected to transient over voltages that can arise due to system abnormalities. Due to these conditions the bushing insulation gets aged and looses its properties in course of time. An unexpected bushing failure is a serious matter as this culminates in the blasting of bushing,
which may lead to the burning of whole transformer. Hence, periodically a bushing is tested for its insulation property. For this, the bushing insulation is subjected to the test voltage. Since the central end is connected to the high voltage and the other end (flange) is already grounded whereas the other end should be connected to the measuring equipment, a bushing tap assembly is provided. The bushing tap is connected to the last capacitance (metal foil) and there is a provision that the other end of insulation is either connected to ground or isolated from ground for measurement purposes.
For testing purpose, a bushing tap assembly is provided at the outermost metal foil. It is a standard accessory for a bushing. In order to determine the condition of the bushing, the insulation between the central conductor and a test tap in the bushing tap assembly is subjected to various electrical tests. A contact plug is provided, surrounding the test tap. This contact plug is movable and normally kept connected to body of the bushing tap assembly and thereby to transformer ground by the action of a spring. This also makes the test tap to ground. Presently, whenever, the bushing is under test, the test tap is disconnected from ground by pressing the contact plug against the spring action. After the testing, the pressure is removed and the contact plug comes back to the original position, thereby connecting the test tap to ground. A metal cover is provided over bushing tap assembly as a protection.
Presently, bushing up to 400 kV rating are widely used and 765 kV bushings are under development. The operational reliability of such bushings is very important for the safe operation of the transformers. Hence, the bushings are subjected to continuous monitoring, in order to detect faults at incipient levels and thereby avoid unexpected outages. In such cases temporary disconnection of test tap from earth may not be advisable from safety point of view.
A need therefore exists for a device which can disconnect the test tap permanently from ground and connect to a convenient remote grounding point during continuous monitoring of high voltage bushings.
SUMMARY OF THE INVENTION
Thus an object of the present invention is to provide a bushing tap device which when directly connected to a bushing tap assembly, the contact plug is permanently kept under pressure and the test tap is disconnected from immediate ground.
Another object of the present invention is to provide a bushing tap device which is capable of connecting the test tap to a remote ground point through a current lead. This enables online monitoring of the current through bushing insulation while the bushing is in service, by mounting a monitoring system viz., a current transformer around the lead.
Yet another object of the invention is to provide the bushing tap device which can be connected to all bushings and bring out the terminals to a common measurement platform so that the measurement can be carried out continuously on all bushings without climbing on to the transformer, thereby reducing the total test duration.
These and other objects can be achieved by bringing out a remote grounding of the test tap when the bushing tap device is connected to a high voltage bushing tap assembly. This enables the measurement of the current through bushing insulation. This is useful as a monitoring parameter of the bushing. Essentially the device comprises a brass stud housed inside an insulator support. The insulator support is housed inside a brass cap. When the brass cap is fixed onto the bushing tap assembly, the brass stud presses the contact plug of the bushing tap assembly against a spring action. This releases the test tap from the immediate ground and connect it to a current lead which help in remote grounding of the test tap.Thus the present invention provides a bushing tap device connectable to a bushing tap assembly for online monitoring of a transformer bushing, said device comprising a brass stud arranged in a brass cap, with one end of said brass stud connected to a cable for onward connection to a remote ground and the other end provided with a whole for receiving a test tap when said device is connected to the bushing tap assembly, thereby isolating said test tap from the immediate ground.
The device is suitable for the standard bushing tap assemblies and enables the monitoring of current and other signals through the bushing insulation and hence useful for on-line monitoring. The device is also useful in electrical measurements on several bushings simultaneously, thereby reducing the test duration.
STATEMENT OF INVENTION
According to this invention there is provided a bushing tap device, connectable to a bushing tap assembly, for online monitoring of a transformer bushing, said device comprising a brass stud arranged in a brass cap, with one end of said brass stud connected to a insulated copper wire for onward connection to a remote ground and the other end is provided with a hole for receiving the test tap when said device is connected to the bushing tap assembly, thereby isolating the said test tap from the immediate ground, the brass stud is fixed into an insulator support which insulates the brass stud from the brass cap, the brass stud is freely kept inside the space of the brass cap so that fixing of the said device on to the bushing tap is possible by providing inner threads in the brass cap to suit the threading on the bushing tap.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 shows details of a high voltage bushing.
Figure 2 shows the bushing tap assembly of the present invention.
Figure 3 shows the bushing tap assembly and details of the bushing
tap device of the present invention. Figure 4 shows the bushing tap device of the present invention in
assembled condition. Figure 5(a) shows the photograph of a bushing tap assembly. Figure 5(b) shows the photograph of bushing tap device of the present
invention. Figure 5(c) shows the device of figure 5(b) filted to a bushing assembly.
The invention will now be described in detail with the help of the accompanying drawings.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
As shown in Figure 1, the high voltage bushing comprises a central conductor 1, oil-paper insulation 2 between metal foils 4 in the form of equal capacitance and a bushing tap assembly (here shown as a test tap 3).
Figure 2 is a drawing indicating the working principle of the bushing tap assembly. The bushing tap assembly consists of a test tap 3, which is connected to the other end of the bushing insulation arranged inside a metal contact plug 5 and both are electrically connected. The contact plug 5 with test tap 3 are housed inside an outer metal body 6 that is already connected to the flange and the outer body is connected to the earth. The contact plug 5 is connected to the outer body by the action of a spring 7. In this condition the test tap 3 is connected to earth. When the contact plug 5 is pressed against the spring 7, it is electrically disconnected from the outer body 6 and thereby the test tap 3 is isolated from earth. A metal cap 6 made of mild steel 6 is screwed over the outer body 6 and thus protects the test tap 3. When the bushing undergoes a test, the metal cap 6 is removed and the contact plug 5 is manually pressed against the spring 7 and held in this position by temporary means so that the test tap is isolated from earth and connected to the measuring instruments.
The details of the bushing tap device of the present invention are shown in Figures 3,4 and 5. The device mainly comprises a brass stud 10, on one side of which a teflon insulated copper wire 20 is soldered firmly. This wire acts as a current lead and connects
the test tap 3 to a remote earth. Teh other side of the brass stud 10, has a hole in which

the test tap 3 goes in during the fixing of bushing tap device to the bushing tap assembly.
The outer diameter of the brass stud 10 is exactly same as the contact plug 5. This brass stud 10 presses the contact plug 5 during fixing of bushing tap device. The brass stud 10 is screwed into an insulator support 30. This insulator support 30 prevents immediate grounding of test tap 3 during fixing. The insulator support 30 is screwed into a brass cap 40 totally and the entire insulator support 30 comes out of the screw of brass cap 40 and sits freely inside the available space SO. The brass cap 40 has the same screw size as that of the bushing tap assembly. When the brass cap is screwed into the bushing tap, the brass stud presses the contact plug and at the same time maintakis electrical contact with the test tap. Thus the test tap is disconnected from the immediate ground and can be connected to a remote ground through the teflon insulated copper wire. This bushing tap device is tested for its performance and can be used in all the tests involving high voltage bushings at sites.

WE CLAIM:
1. A bushing tap device, connectable to a bushing tap assembly, for online monitoring of a transformer bushing, said device comprising a brass stud (10) arranged in a brass cap (40), with one end of said brass stud (10) connected to a insulated copper wire (20) for onward connection to a remote ground and the other end is provided with a hole for receiving the test tap (3) when said device is connected to the bushing tap assembly, thereby isolating the said test tap (3) from the immediate ground, the brass stud (10) is fixed into an insulator support (30) which insulates the brass stud (10) from the brass cap (40), the brass stud (10) is freely kept inside the space (50) of the brass cap (40) so that fixing of the said device on to the bushing tap is possible by providing inner threads in the brass cap (40) to suit the threading on the bushing tap.
2. The bushing tap device as claimed in claim 1, wherein said cable connected to said brass stud (10) for connection to a remote ground is Teflon insulated copper cable (20).
3. The bushing tap device as claimed in claim 1, wherein said brass stud (10) is screwed on to an insulator support (30) for preventing immediate grounding of the test tap (3) during fixing of the bushing tap device to the bushing tap assembly.
4. The bushing tap device as claimed in claim 3, wherein said insulator support (30) is arranged to rest freely in an available space (50) inside side brass cap (40).
5. The bushing tap device as claimed in claim 4, wherein said insulator support (30) is provided with threads matching the threads on the inside of said brass cap (40) for being screwed down completely for arranging the insulator support (30) inside the brass cap (40).
6. The bushing tap device as claimed in claim 5, wherein said brass cap (40) has same threads as those provided on outer body 6 of said test tap assembly (3) for fixing said device to said test tap assembly.
7. A bushing tap device, connectable to a bushing tap assembly for online monitoring of a transformer bushing, substantially as herein described and illustrated in the accompanying drawings.