The invention relates to a novel method of protecting the high voltage winding of a power transformer, equipped with surge arrestor in a substation. More particularly, the invention is related to the protection of the high voltage winding of the said transformer, primarily protected by a surge arrestor and connected to the grid.
BACKGROUND OF THE INVENTION
In a substation bulk AC power is generally handled by high voltage large oil filled power transformer installed at outdoor switch-yards, provided with high voltage bushings at the high voltage side and connected to the power transmission grid through long transmission line. There are other equipment like Reactors, circuit breakers, CT, PT, Isolators and many other components connected to the transmission line. Depending upon the load requirements and the system disturbances these equipment are made to be ON or OFF generating different types of high voltage surges. This many happen locally or at a distance from the transformer. When this happens at a distance, the surges caused by the disturbances travel through the transmission line and reach up to the

transformer. Depending upon the distance travelled, the amplitude of the surge at times increases to a still higher level causing still larger stress to the transformer. These surges are typically known form their high amplitude of voltage which lasts for very short time in the tune of microsecond. Besides there are also occurrence of lightning surges (due to thundering) that are generated by the lightning in the sky.
The surges that are generated by lightening (thundering) or by any system disturbances travel through the transmission line and strike the transformer causing higher stress in the transformer core, winding, winding connections and the bushing.
The bushings and the transformers are tested for stipulated level of surges at the factory by applying stipulated no. of shots (voltage applications) and stipulated time duration (the no. of shots and their duration of application are decided as per the governing standards IS-2026 and IEC-60076). But there is no control on the number of surges or level of surges (or their duration) appearing in the system during working of transformer and hence this surge appearing in the system can generate high voltage stress on the transformer which could damage the core or the winding or the bushing or any other items connected to the transformer.
At times the bushing of the transformer also breaks/blasts due to the stress generated due to these phenomena. The broken parts of the blasted

bushing can cause damage to the adjacent equipment and can also harm people working around. The breakage of the bushing can cause oil leakage form the bushing and the transformer. Associated catastrophic failures like fire hazard can also take place which leads to burning of the transformer and associated other equipment in the vicinity of the transformer in the sub-station. The failure of the bushing and the transformer causes interruption of power supply. The replacement of the bushing and the transformer huge costs in terms of time and money and discomfort to the utilities.
Lightning arrestors are always provided in the sub-station, which are connected to the transmission lines near the transformer. The lightning arrestors gives a bye-pass path to the surges caused by Lightening, when the level exceeds a certain pre-determined voltage level of the surge.
But the lightening arrestors have a limited role. The lightening arrestors can also fail when a thundering is of a voltage level higher than for which it is selected. In a situation where the arrestor fails but the thundering continues, the transformer remains in a stage of totally unprotected and exposed to stress caused by the surges. The arrestor can also fail due to water ingress into it resulting reduction in the IR (Insulation Resistance) thereby increasing the leakage current. This increase in leakage current results Thermal instability leading to break down of the arrestor. The lightening arrestor can also fail due to ageing (resulting increase in leakage current) with time.

Hence the lightening arrestors have a limited level of protection.
The failure of a particular phase (line to ground) has a tendency to swing the
voltages of other 2 phases (of the 3 phase supply) which can also fail.
OBJECTS OF THE INVENTION
It is therefore an object of the invention is to propose a method for providing protection to transformer in the event of failure of lightening surge and continuation of thundering.
It is another objective to protect the Lightening arrestor (and the transformer) against increase in the leakage current (resulted by the moisture ingress and ageing) and generation of Thermal in-stability, which is responsible for the break down of arrestor.
Another object of the invention is to propose a method for protecting the fire hazard in a switch yard caused by the failure of the transformer and bushing due to high voltage surges.
A still another object of the invention is to propose a method for protecting the transformer against repeated lightening surges and maintain un-interrupted power supply.

A further object of the invention is to propose a method for providing safety to the people working in the switchyard at the time of occurrence of the surge.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1 – Shows the arrangement of power transformer connected to the system Fig.2 – Shows the arrangement of conventional surge arrestor Fig.3 – Shows the arrangement of modified surge arrestor
DETAILED DESCRIPTION OF THE INVENTION
A power transformer is an oil filled out door installed electrical machine to increase/reduce the voltage of the generation or transmission line located in the switch yard of a sub-station or generating station. Large power transformers are meant for handling large bulk of power. It is either connected at the generating station (5) to boost the generation voltage level or to inter¬connect two different transmission lines of different voltages so as to facilitate flow of power.
Since the transformers are directly connected to the transmission lines and placed outdoors. It is subjected to over voltages spikes caused by lightening (thundering) and the phenomena is known as lightening impulse voltage.

A 220 kV transmission line is rated for lightening impulse voltage of 950-1050 kV peak and the time duration is 1.2/50 micro-second. A 400 kV transmission line is rated lightening impulse voltage of 1300-1425 kV peak and the time duration is 1.2/50 micro- second.
This high voltage spikes are transmitted through the transmission lines (2) and reach the transformer terminals which in-turn is connected to the winding of the transformer. As a result the transformer termination and the winding are over stressed. The windings of the transformer may fail due to the over stress and the terminations (mostly bushing) may fail or even break or blast under the over voltage (unless the surge arrestor absorbs the complete surge voltage) causing oil spillage and fire, which will not only interrupt the power supply but also cause fire resulting damage to the transformer, switchyard and other associated equipment in the vicinity. The blasting of the bushing can also damage the other equipment and the people working near-by.
During the manufacturing, power transformer is made suitable to the above impulse voltages to a level as laid out in the governing standards. A lightening arrestor is provided near the transformer so that a lightening impulse voltage above the test level will be bye-passed to ground and will not reach the transformer. A lightening arrestor is a semi-conductor device that is sensitive to voltage. It does not conduct at normal voltage and acts as an insulator. But at lightening voltage it acts as a conductor. The arrestor acts as a semi-conductor

switch that is open during normal voltage but it is closed under the application of lightening voltage.
However, this has a limitation because the lightening arrestor does not stop the lightening. Higher level of lightening (impulse) voltage can also break/damage the lightening arrestor and thundering can still continue even after. As a result the lightening arrestor has a limited scope to protect the transformer against lightening.
The lightening arrestor has inherent potential threat of increase in the leakage current caused by moisture ingress (associated with reduction in Insulation Resistance value) ageing and accelerated ageing (due to internal partial discharge). This current insets higher temperature in the arrestor which again causes higher leakage current. This phenomena of Thermal instability leads to break down of arrestor.
To improve the system, an additional small coil (4) made with solid copper conductor with very small number of turns (5-10) can be provided in series with the surge arrestor (3). In this process, the total surge voltage is applied across the arrestor and the coil. So, the arrestor is subjected to a part of the surge voltage (in-lieu of the total surge voltage) appearing in the system, there by the chance of failure of the surge arrestor reduces.
Also, in the event of water ingress and reduction of Insulation resistance value, the leakage current is restricted owing to the additional small

coil provided in series with the arrestor. Thus, the arrestor is provided with additional protection against failure.
The number of turns in the High voltage winding of a power transformer (1) (which is aimed for protection) are very high in the tune of 600– 800 turns or even more. So, the impedance (inductance/resistance) offered by this winding is considerably much more high in comparison to the additional small coil (4) with 5-10 turns; provided along with the arrestor (3). So, the path resistance of the surge (in series with coil with high cross section and less turns) will be much less in comparison to that of the winding enabling the arrestor to act fast in diverting the surge to ground and protecting the transformer.

WE CLAIM
1) A method of protection of a transformer including its core, bushing and winding from repeated high amplitude surge voltage due to lightening or any system disturbance, or both by incorporation of an additional coil in series with the lightening arrestor
2) A method for protecting the fire hazard in a switch yard caused by the failure of the transformer and bushing due to frequent occurrence high voltage surges due to lightening.

3) A method for providing safety to the people working in the switchyard at the time of occurrence of the surge.
4) A method of protecting the transformer and the switchyard causing saving of huge financial property owing to frequent Lightening.
5) Reduction in frequency of replacement of Lightening arrestor owing to ageing
there by increasing reliability of the transformer and the sub-station.