FIELD OF THE INVENTION
The present invention relates to a reliable and safe method of grounding the terminals of Unloaded Tertiary winding of Power transformer. More particularly, the invention relates to an improved method of grounding the terminal of stabilizing winding of a transformer to eliminate bushing failure of the stabilizing winding due to high transient voltages including flow of high short duration short circuit current.
BACKGROUND OF THE INVENTION
A power transformer is a voltage transforming electrical machine, and having a magnetic circuit known as Core and two (or more) set of coils 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 the transformer tank.
The HV and LV winding terminals (meant for incoming and out going of power) of the transformer are taken out from the tank and connected to external circuits. In many cases, the transformers are also provided with an additional non-power transmitting winding known as Tertiary winding. The tertiary winding is connected in Delta and is primarily meant for (a) Improving the quality of power viz. Suppression of Harmonics, improving power factors by connecting capacitor, and (b) providing additional strength to the transformer during fault and transient condition of the power transmitting system. Hence, the normally un-loaded tertiary winding is also known as Stabilizing winding.

The stabilizing winding is not connected to the power transmitting net-work and remains isolated from the power system. It is always connected in Delta. For grounding the terminals of the stabilizing tertiary windings, according to prior art, one corner of the Delta connected Stabilizing is taken out through an out door bushing and connected directly to the ground mat of the sub-station through copper flats or cables as per requirement. This is known as Solidly grounding of the terminals of the stabilizing tertiary winding.
A current transformer (CT) is some times mounted on the terminal to detect the current flowing this ground connection which also contributes in protection of the transformer.
In a conventional method of grounding the terminal of the stabilizing winding it, is connected to the bushing inside the transformer full of oil. The external terminal of the busing is connected to the copper cable or copper flat. The cable or copper flat is made to run towards the bottom of the tank. At the bottom of the tank, the cable or the flat is connected to the ground mat of the sub-station.
During working of the transformer, the transformer is often subjected to high transient voltages due to lightning or Switching on transmission lines connected to HV/LV winding terminations. The stabilizing winding is also subjected to high voltages (due to transferred surges), during the transient conditions. The transferred surge on the tertiary winding when exceeds the rated voltage of the tertiary winding bushing, the tertiary bushing fails. The failure of the bushing leads to breakage of the bushing, breakage of adjacent other sub-station equipments owing to blasting of the bushing, disruption of power transmission, oil leakage, and also fire hazard.

OBJECTS OF THE INVENTION
It is therefore, an object of the invention is to propose an improved method of grounding the terminal of stabilizing winding of a transformer to eliminate busing failure of the stabilizing winding due to high transient voltages, including flow of high short duration short circuit current.
Another object of the invention is to propose an improved method of grounding the terminal of stabilizing winding of a transformer to eliminate busing failure of the stabilizing winding due to high transient voltages, including flow of high short duration short circuit current, which eliminates the failure of tertiary bushings and the associated problems of oil leakages and fire hazard .
A still another object of the invention is to propose an improved method of grounding the terminal of stabilizing winding of a transformer to eliminate busing failure of the stabilizing winding due to high transient voltages, including flow of high short duration short circuit current, which eliminates the possibility of the stabilizing winding bushing being blasted thereby damaging the adjacent sub¬station equipments.
Yet another object of the invention is to propose an improved method of grounding the terminal of stabilizing winding of a transformer to eliminate busing failure of the stabilizing winding due to high transient voltages, including flow of high short duration short circuit current, which enhances reliability of the power supply.

A further object of the invention is to propose an improved method of grounding the terminal of stabilizing winding of a transformer to eliminate busing failure of the stabilizing winding due to high transient voltages, including flow of high short duration short circuit current, which enables reduction in size and cost of the transformer.
SUMMARY OF THE INVENTION
According to the invention, there is provided an improved method of grounding the termination of the stabilizing winding. The terminal of the stabilizing winding is not brought out. The cable connections of the stabilizing winding terminal is bolted on to a boss welded onto the inner surface of transformer tank inside the transformer through a disconnecting link . The link is mounted on the tank cover and it can be closed/opened from outside without disturbing/opening the transformer. On closing the link , the winding is connected to the boss and on opening the link the winding is isolated from the boss . Under working conditions, the link is closed causing the tertiary winding connected to the ground through the boss inside the tank. The tank is always grounded. So the winding terminal is also grounded. The result of this type of grounding is exactly identical to the solidly grounding of the stabilizing winding termination. In this type of grounding connection, the tertiary bushings are not required (since there is no need of it). So there is no probability of failure of the tertiary bushing. As a result, all the hazards that can be caused by the failure of the bushing are eliminated. A current transformer (CT) can be conveniently mounted on the cable connecting the tertiary winding and the boss, to detect the current flowing through it. The

terminals of the CT are brought out for connecting to the protection circuit as per requirement. For any testing at sites on the tertiary winding, the disconnecting link is kept open thereby isolating the tertiary winding termination from ground. After the testing, the disconnecting link is closed resulting solidly grounding of the tertiary winding.
During the fault condition, the tank also acts as a shorted path providing a passage to Zero sequence fault current by virtue of the Zero sequence impedance of the transformer. In the disclosed method, the tank carries the fault current of Stabilizing winding. Hence there is no potential hazard to the transformer.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows a schematic diagram of grounding the terminal of stabilizing winding of a transformer according to prior art.
Figure 2 - shows an improved method of grounding the terminal of Stabilizing winding internally through a disconnecting LINK eliminating the provision of bushing, according to the present invention.
Figure 3 - shows disconnecting Link in Open (3a) and Closed (3b) condition.
DETAILED DESCRIPTION OF THE INVENTION
A transformer has primarily 2 (or more) sets of windings for power inflow and out flow. In addition, some times a non-power flow winding is also provided inside the transformer; which is known as tertiary Stabilizing winding. The stabilizing winding is always connected in Delta. A corner of the Delta stabilizing

winding is brought out through a bushing (1) mounted on the transformer tank cover (2) and grounded as per requirement as shown in figure 1. Conventionally this bushing outer terminal is brought up to the ground level through copper flat or cable (3) and connected to a ground mat (4) of the sub-station.
The stabilizing winding is substantially affected by the transferred high voltage stresses and short time fault current, which increases risk of failure of this bushing. The failure of the stabilizing bushing leads to oil leakage from the transformer tank, fire hazard, breakage of adjacent sub-station equipments and also disruption of power supply service.
In order to avoid this potential danger, and improve reliability of the transformer including the power supply system, it is proposed that the terminal of the stabilizing winding is internally bolted onto the tank wall/cover, as shown in figure 2. A metallic boss (1) made of steel is welded on to the inner surface of tank wall/cover (2). A disconnecting link , mounted on tank cover (as shown in figure 3) has 2 terminals; one terminal connected to the metallic boss (1) welded on to the inner surface of tank wall/cover (2), and the other terminal is connected to the terminal of the Delta connected stabilizing winding. The link can be opened/closed from outside without disturbing/opening the transformer. When the link is opened (as shown in (3a)), the tertiary winding is isolated from the ground. When the link is closed (as shown in figure (3a)), the tertiary winding is grounded through the boss (1) which is welded onto the tank (2). Since the transformer tank (2) is always connected to the ground

mat (4), the corner of the stabilizing winding is also grounded as per requirement.
In this method, the provision of bushing is eliminated and hence all the hazards related to the failure of the bushing are completely eliminated.

WE CLAIM :
1. An improved method of grounding the terminal of stabilizing winding of a transformer to eliminate busing failure of the stabilizing winding due to high transient voltages including flow of high short duration short circuit current . The transformer comprising at least one each high voltage and low-voltage windings for incoming and outgoing of power; a transformer tank filled with oil; a tank cover; a magnetic circuit separated from the windings with the provisions of insulating material, terminals of the windings taken out from the tank; and a stabilizing tertiary winding connected in delta without being connected to power supply network associated with the transformer, and grounded as per requirement. The method is characterized by comprising the steps of :-
providing a metallic boss welded on to the inner surface of tank wall/cover;
locating a disconnecting link being operable from out side and closable with two terminals without opening/disturbing the transformer;
connecting the first terminal of the link with the metallic boss and the second terminal of the link to the terminal of the delta connected stabilizing winding;

opening the link to isolate the stabilizing tertiary winding from the ground, and closing the link to connect the tertiary winding to the metallic boss welded on to the inner surface of tank wall/cover which remaining grounded .