FIELD OF THE INVENTION
The present invention relates to a safe and reliable method of factory testing of Power transformer fitted with Cable Box suitable for ground run cables. More particularly, the invention relates to a method of testing power transformers fitted with cable box at manufacturing location prior to delivery for erection at installation site.
BACKGROUND OF THE INVENTION
A power transformer is a voltage transforming electrical machine, and having a magnetic circuit known as core and two different coils one each for incoming and outgoing of the power. The core and the coils are separated from each other by providing insulating materials in between them. The core-coil assembly is kept inside a transformer tank.
The HV and LV winding (meant for incoming and out going of power) terminals of the transformer are taken out from the tank and connected to external circuits. Even through there are a number of ways to take out the terminations, the actual method of termination is decided based on applications for example, the HV/LV termination of the transformer are taken out through Cables.
For taking out the termination through cables, an oil/Air filled metallic box is used; which is known as Cable Box. The Cable box comprises an insulated conducting copper connection (bar/rod); that is housed inside a steel box (circular or rectangular). One end of the insulated copper bar/rod is connected to a bushing of the transformer and the other end is connected to a ground run/laid cable coming from the switch yard (ref-Fig-1). The insulated conducted copper

connection is supported (or hung) from the housing of the steel box (circular or rectangular) and full with oil or Air. The cable box is at ground potential (being earthed) where as the insulated copper bar/rod is at the full voltage of the winding, to which it is connected through the bushing of the transformer.
So, there is a large voltage difference between the insulated copper bar/rod connecting the transformer bushing and it's housing made of rectangular/circular steel box. Since the cables coming from the out side are laid at the ground level and the bushing of the transformer are at an elevated height, the cable box takes the shape of "Inverted U" (figure 3).
The transformers according to prior art are tested at manufacturing stages for many high voltage electrical tests to verify their suitability for working at site. During testing of the transformer at the manufacturing arena, it is not always possible to run high voltage live testing cables (connection from a test deck) due to human safety reasons. It is further not possible to test the transformer with connecting cables or cable box. Accordingly, the transformers are conventionally tested at manufacturing stage without the cable box (figure 2).
As a result, the cable box is not tested for required high voltage electrical test. Any non-conformance to design parameters, or deficiency in manufacturing of the cable box and cable box assembly does not get detected at the point of manufacture. The un-tested cable box is sent as loose item to site and fitted onto the transformer. Since, the high voltage electrical tests can not be done during installation at site, the cable box is not tested at site during installation. As a result, there is always a potential risk regarding the healthiness of the cable box assembly which in-turn leaves a potential risk on the reliability/suitability of the transformer fitted with a cable box during working at site.

During operation of the transformer, if there is a voltage break down (failure) between, insulated the copper bar/rod (connecting to HV/LV bushing of the transformer) and it's housing (made of rectangular/circular steel box the transformer terminal) inside the cable box, the transformer is likely to cease operation and the power supply is disrupted. The failure of the transformer also results short circuiting of the transformer generating a high current in the transformer winding (Line to ground fault) leading to premature catastrophic damage of the power transformer windings. The short circuiting can also lead to blast inside the cable box that can break the supporting insulator and generate heavy arcing leading to damage and fire hazard in the cable box causing burning of the transformer and the adjacent associated equipments.
To overcome the above problems, a method has been invented to tests the transformer fitted with "Inverted U" shaped cable box in the factory. In this process, the transformer and the cable box is fully tested for all the high voltage electrical tests in factory before dispatch. The testing of the transformer fitted with cable box when tested for all the high voltage electrical tests in the factory guarantees regarding the suitability of the transformer with cable box at site. In this method, the reliability of the transformer for working at site increases considerably in comparison to the transformer fitted with un-tested cable box at site.
OBJECTS OF THE INVENTION
It is therefore, an object of the invention is to propose a method of testing power transformers fitted with cable box at manufacturing locations prior to delivery for erection at installation site.

Another object of the invention is to propose a method of testing power transformers fitted with cable box at manufacturing locations prior to delivery for erection at installation site, which still maintains safety standards.
A still another object of the invention is to propose a method of testing power transformers fitted with cable box at manufacturing locations prior to delivery for erection at installation site, which allows protection of the transformer against Line to ground short circuit fault including that of the transformer winding from permanent damage.
Yet another object of the invention is to propose a method of testing power transformers fitted with cable box at manufacturing locations prior to delivery for erection at installation site, which prevents possible blasting inside the cable box including fir hazard and burning of the transformer.
A further object of the invention is to propose a method of testing power transformers fitted with cable box at manufacturing locations prior to delivery for erection at installation site, which prevents possible damage to other sub-station equipments operating connected to the power transformer.
A still further object of the invention is to propose a method of testing power transformers fitted with cable box at manufacturing locations prior to delivery for erection at installation site, which provides reliably and safety to the transformer including the electrical sub-station associated with the transformer.

SUMMARY OF THE INVENTION
Accordingly, there is provided a method of testing power transformers fitted with cable box at manufacturing locations prior to delivery for erection at installation
site.
According to the invention, the healthiness and suitably of a transformer fitted with a cable box for delivering safe, reliable, and rated performance at site, is determined at manufacturing location. Thus, all the potential hazards associated with possible failure of the cable box are avoided and safety and reliability of the transformer and the sub-station, is substantially improved.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Shows a Transformer fitted with Cable box at site.
Figure 2 - Conventional method of testing a transformer in factory without Cable
box.
Figure 3 - Shows the details of the "Inverted U" shape of Cable Box.
Figure 4 - Method of testing a transformer alongwith the Cable box at
manufacturing location.
DETAILED DESCRIPTION OF THE INVENTION
In many sub-stations, the HV/LV bushings of the transformer is connected to ground laid Cables (which is a sub-station accessory). As shown in figure 1; the connection from a transformer to the ground laid cables (1) is done through an insulated copper conductor connector (2). The cable (1) coming from the ground

level is connected to the insulated copper conductor connector (2) at one end of the ground level. The other end of the insulated copper conductor connector (2) is joined to a bushing (4) on the transformer. The insulated copper conductor connector (2) is housed inside a cable box (3).
Conventionally the cable box (figure - 3) is a "inverted U shape "steel box (round/rectangular shape) that houses the insulated copper conductor connector (2). The cable box (3) is manufactured in 2 halves (3a and 3b). A first half (3a) is connected to a hood of the bushing (4) of the transformer. The ground run cable enters the cable box through part (3b) .The two halves (3a,3b) are bolted onto each other to form an inverted U shaped metallic structure (3).
According to prior art, the insulated copper conductor connector (2) inside the cable box (3) is made in two parts (2a) and (2b). The parts 2a and 2b are bolted onto each other. Each part 2a and 2b is supported from the metallic cable box (3) by supporting insulators (5).
During testing of the transformer in factory, the transformer is tested in the factory without the cable box (as shown in figure 2). The untested cable box is connected on to the factory tested transformer at site. This leaves a potential danger for failure of the cable box, transformer and other adjacent equipments of the sub-station.
According to the invention, the transformer is tested along-with the cable box in the factory as per the arrangement shown in figure 4. Conventionally the cable box (3) is an "inverted U shape "steel box (round/rectangular shape) that houses

the insulated copper conductor connector. The cable box is manufactured in two halves 3a and 3b. The part 3a and part 3b are bolted on to each other. The insulated copper connector (2) housed inside the cable box, is also manufactured in two parts (2a) and (2b). The parts (2a) and (2b) are bolted onto each other.
During the testing of the transformer in the manufacturing location, the cable box part - 3b is turned upside. So, the bottom side of the cable box 3b is shifted to top side; much above the ground level. The insulated copper conductor connector part 2b (housed inside the cable box part 3b) is also turned upside. So, the bottom side of the copper conductor connector 2b is shifted to top side; much above the ground level. In this arrangement testing cable coming from the test deck is connected to the insulated copper conductor connector (2b) of the cable box (3). This arrangement is shown in figure 4.
High voltage live testing wires can easily be connected from the testing deck to the insulated copper conductor connector part (2b) inside the up-turned cable box during testing. In this arrangement, the connection is much above the ground level (beyond the height of the transformer) without violating safety norm( no risk) . In this way the transformer along with the cable box is tested. Any defect/non-conformance in the cable box or the insulated copper conductor connector can be detected during the testing. Thus, the healthiness or reliability of the transformer along with the cable box for meeting the performance at site is ensured.

WE CLAIM :
1. A method of testing power transformers fitted with cable box at manufacturing locations prior to delivery for erection at installation site, the power transformer connected to ground laid cables constituting accessory of an electrical sub-station through a first end (2b) of an insulated copper conductor connector (2), a second end (2a) of the insulated copper conductor connector (2) attached to a bushing (4) of the transformer, the first end (2b) and the second end (2a) of the connector
(2) being bolted together and housed in an inverted U-shaped cable box
(3) formed of a first section (3a) accommodating the second end (2a) and a second section (3b) housing the first end (2b) of the insulated copper conductor connector (2), both the first and second sections (3a,3b) of the cable box (3) being bolted together, the method comprising the steps of :

- turning upside the second section (3b) of the cable box (3) so as to locate the bottom side of the cable box (3) on top position above the ground level;
- turning upside the first end (2b) of the conductor (2) to locate the first end (2b) at a top portion above the ground level; and
- connecting a high voltage testing cable between said first end of said insulated copper conductor connector (2) and a test deck for testing of the cable box (3), the high voltage testing cable running high above the ground level to eliminate the risk of safety of the operators.

2. The method as claimed in claim 1, wherein the first end and the second end (2b, 2a) of the insulated copper conductor connector (2) are provided with support by the cable box (3) via a plurality of supporting insulators (5).