2 FIELD OF INVENTION
This invention relates to a novel design of yoke shunt to control stray losses in power transformers installed in electrical substations.
BACKGROUND OF THE INVENTION
As per prior art normal power transformer consists of yoke shunt as illustrated in fig 3, which is made up of cold rolled grain oriented core laminations covering the yoke at the top & bottom. The yoke shunt consists of 4 sections and it does not coyer the complete winding area.
For a 100% impedance transformer stray losses are very high for larger rating transformer with higher impedance value than the normal value.
Controlling such huge stray losses by means of conventional yoke shunt has been found to be difficult and costly. More over the prior art leads to more eddy current loss, generation of more heat resulting gas formation of oil.
The present invention of yoke shunt (ring shunt) is made up of core laminations packed in epoxy and formed in to ring with insertion as illustrated in fig (4).
This specially designed yoke shunt helps in covering all the stray/fringing flux emanating through the windings and results in minimization of stray loss.
Moreover modified design overcomes are the manufacturing problems of cutting, optimising shape and size, stacking, moulding and curing.

3 OBJECTS OF THE INVENTION
- It is therefore the object of the present invention is to reduce the stray
loss of power transformer to its minimum.
- Further object of the present invention is to reduce the stray loss through
loaded and unloaded windings, clamping frame work and transformer tank
which are dependent on leakage flux and magnetic field surrounding the
leads.
- Yet another object of this invention is to control stray losses in power
transformer with nearly 100% impedance between primary and secondary
windings
- Also the object of this invention is to avoid all manufacturing and
assembly problem of previous art.
- Finally, the object of this invention is to reduce the eddy current loss to
minimum.
SUMMARY OF THE INVENTION
The stray loss in power transformer is generally made up of the following components:
- Stray loss in transformer tank due to leakage flux form the windings.

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- Stray loss in the tank due to leads.
- Stray loss in clamping framework due to leakage flux from the windings.
- Stray loss in the clamping framework due to leads.
- Stray loss in the loaded windings.
- Stray loss in the unloaded windings.
In a power transformer with approximately 100% impedance between primary winding and secondary winding, the very high leakage impedance and special disposition of the winding make the stray losses increase many fold. Special methods like use of yoke shunts as per the present invention and providing them on the strategic locations with the help of accurate magnetic field analysis were employed to control the stray losses. It is important to note that successful implementation of these methods would reduce the stray losses to about 50% of I2R losses compared to stray losses of 630% of I2R losses without these methods.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS.
Figure 1. shows magnetic field plot without yoke shunt
Figure 2. shows magnetic field plot with yoke shunt
Figure 3. shows conventional yoke shunt shown in 4 sections.

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Figure 4. shows ring type yoke shunt. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The load losses in the transformer consist of losses due to the ohmic resistance of winding and stray losses.
As per present invention a minimum stray loss design has been achieved by analyzing systematically, the source of leakage flux and its path and subsequent development at magnetic yoke shunt. Figure 1. shows magnetic field distribution of power transformer without yoke shunt. From the field plot it is evident that in the absence of yoke shunt, there is maximum fringing of flux at the top and bottom.
This stray flux is responsible for creating very high losses in the adjacent structure and tank. This is also evident that such high stray losses in the adjacent structures and tank will not allow the satisfactory operation of power transformer and would lead to temperature rises beyond guaranteed values.
Figure 2 show the magnetic field plot of power transformer provided with yoke shunt. From the field plot it is evident that, the flux path has become almost parallel to windings and fringing at the ends have reduced to minimum.
A very high percentage of flux has been collected by yoke shunts and pumped to core. Similarly the stray flux near the tank passing through the wall shunts and entry of flux in the tank wall has been reduced to a great extent.

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A normal power transformer consists of a yoke shunt as shown in figure 3 having 4 sections. A large portion of the axial flux is fed back into the yoke. On the contrary, 100% impedance transformer as per present invention consists of a yoke shunt as shown in fig 4. Newly designed (ring shunt) is made up of core laminations packed in epoxy, formed in the shape of a ring with insertion.
Like a 100% impedance transformer, stray losses are very high for large rating transformers with higher impedance value than the normal, controlling such huge stray losses by ring type yoke shunt is cheaper solution for the utility to minimize unwanted energy losses in the transformers and overall economy of the cheaper generation /transmission system.
Ring shunt can be manufactured through the following steps.
(a) Each strip of core laminations is of 3 to 5mm width depending on the
diameter of the ring.
(b) For example, 600mm wide shunt is made up of 120, 200strips
respectively for 5mm and 3mm width strip.
(c) For each strip, the lamination are cut to same length.
(d) The laminations are stacked one by one in a circular mould of desired
diameter.
(e) After completion of stacking, the laminations are resin impregnated under
vacuum and processed in oven to make it solid.

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With the above process followed during manufacture and a assembly was used in 50MVAr, 400K.V class controlled shunt Reactor transformer for substations and reduction in stray loss has been found to be about 50% of I2R loss compared to stray losses of 630% of I2R loss without these shunts.

8 We Claim
1) A novel ring type yoke shunt comprising core lamination sheets as per
figure 4 and the methods thereof to manufacture the same to reduce the
stray losses in power transformers.
2) The methods as claimed in claim 1 involve the following steps.

(a) Core lamination sheets, each of (3-5)mm. width are cut to same
length depending on the diameter of the ring to be formed.
(b) The laminations are stacked one by one in a circular mould of
required diameter.
(c) The stacked laminations are resin impregnated under vacuum and
processed in oven to make it a solid one.
3) The novel ring type yoke shunt manufactured by the methods as claimed
in claim numbers 1 and 2 reduces the stray loss to about 50% of I2R loss
compared to stray losses of 630% of I2R loss without these shunts and
coverage area of ring type of yoke shunt is more that the conventional
type.