FIELD OF THE INVENTION
The present invention relates to an outside interwound helical type of tapping
winding in single group with high voltage center line lead for power transformer.
BACKGROUND OF THE INVENTION
Transformers with linear impedance pattern i.e. maximum impedance at
maximum tap voltage and minimum impedance at minimum tap voltage are
generally known in the art. This impedance pattern involves less system fault
level/ current, and consequently less expensive switchgears to handle the level/
current. However, configuration of such impedance pattern of the transformer
winding entails various constraints.
The following prior art configurations of the transformer winding are known and
used in the art:
1) Low voltage winding - High voltage winding with tap from its body as shown
in figure 1.
This pattern normally not be preferred, because of large ampere turn unbalance
between the windings, which results in higher electro- magnetic forces on the
windings in case of short circuit.
2) Low voltage winding - High voltage winding with separate Tapping winding as
shown in figure 2.

This pattern shall be most preferred, because of its good ampere turn balance
between the windings and thus have lesser electro magnetic forces on the
windings in case of short circuit. This pattern is achieved by comprising the
windings by following two ways -
2 a)
- Low voltage winding in single parallel group
- High voltage winding in single parallel group & top line lead
- Tap winding is single parallel group (Interwound type coil)
Disadvantages of the prior art arrangement 2 a), as shown in figure 3(a):-
Al. In this arrangement, high voltage lead is taken out from top and thus
nearer to top yoke of core (which is an earthed part) and thus needs higher yoke
distances to keep it safe from high voltage stresses.
A2. As the high voltage lead travels nearer to tapping leads as well as adjacent
to the earthed top yoke of core, this arrangement needs costly mounded
component to meet the electrical stresses between the High voltage lead,
tapping leads and top yoke.
A3. Due to Higher yoke distance, the leg length of the core increases, which
results in higher mass of core material and increased transformer cost. This
arrangement also leads to fringing i.e. bending of flux at top yoke area. This
fringing of flux produces higher short circuit forces in the top portion of the
winding and additional stray loss in core clamp plate.

A4. Due to proximity of the high voltage winding to the tapping winding,
height of the tapping winding needs to be reduced for obtaining requisite
electrical clearance of the high voltage lead from the top of tapping winding. This
result in ampere-turn unbalances and increased short circuit forces.
2b)
- Low voltage winding in single parallel group
- High voltage winding in two parallel groups & center line lead
- Tap winding is four parallel groups (Disc type coil)
Disadvantages of the prior art arrangement 2 (b), as shown in figure 3(b):-
Bl. As the high voltage lead is taken out from center of the tapping winding,
both the windings needs to make as a composite one, i.e. on a single mould.
Due to higher rotating volume and weight of both the windings, it is extremely
difficult to maintain them in aligned condition.
B2. As the tapping winding is made by discs, some of the discs may be
positioned out of the circuit, when the transformer runs at a different tap from
its normal tap position, which results in higher unbalanced ampere-turn and
increased short circuit forces.
B3. To avoid the unbalanced ampere-turn, the tapping winding is made in four
groups. Due to this, the number of tapping leads are large i.e. (n+1) x 4, where
n is the number of steps (e.g. when the number of steps = 8, the number of
tapping leads = 36). Hence, to handle these large number of tapping leads, a
complicated terminal gear arrangement needs to be designed. This further
increases the possibility of error.

B4. To avoid the inherent part-resonance phenomenon of this disc type of
tapping winding, the tapping winding is made interleaved disc type, which
requires skilled labour and expertise resulting in lower productivity, and higher
cost.
B5. These disc type taping winding have smaller width, and lesser Turn per
disc i.e. 2.5 to 3.5, and require break at every two disc due to taking out of
tapping lead, or for interleaving. This results in loosening of the winding
conductor, less mechanical strength to withstand short circuit forces.
OBJECT OF THE INVENTION
It is therefore an object of the invention to propose an outside interwound helical
type of tapping winding in single group with high voltage center line lead for
power transformer.
SUMMARY OF THE INVENTION
The present invention provides an outside interwound helical type of tapping
winding in single group with high voltage center line lead for power transformer.
The innovative interwound helical type tapping winding is capable of
withstanding all the electrical tests and other parameters satisfactorily.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Shows a conceptual configuration of a low voltage and high voltage
with tap from the body of high voltage winding of a high power
transformer.
Figure 2 - Shows a high voltage, low voltage, and tap winding of a
transformer according to prior art.
Figure 3a - Shows a low voltage, high voltage, and a tap winding in single
parallel group with top line lead in a transformer of prior art.
Figure 3b - Shows a transformer of prior art with low voltage winding in single
parallel group, high voltage winding in two parallel groups with
center line lead, and the tapping winding in four parallel groups.
Figure 4 - Shows a transformer with low voltage winding, high voltage
winding in two parallel groups and outside interwound helical type
of tapping winding in single group with high voltage center line
lead.
DETAIL DESCRIPTION OF THE INVENTION
As shown in figure 4, a high voltage winding is made in two parallel groups and
the high voltage lead is taken out from the center of the tapping winding,
wherein the tapping winding is formed as interwound helical tapping such that
each turn shall encompass the total height of the tapping winding. Accordingly,

the total height of the tapping winding is accommodated within the circuit. This
results in better ampere-turn balance.
The moulded components used in this invention are simple in shape and thus
available at lower cost.
As the high voltage lead is disposed far way from the top yoke, the yoke distance
can be kept lower, which results in lower leg length of the core, reduced core
weight and lower cost of the transformer.
Flux fringing at the top portion of the winding is avoided due to lower top yoke
distance which reduces stray loss in the core clamp plate.
The height of the tapping winding is kept equal to other windings to achieve an
optimum ampere-turn balance and reduced short circuit forces.
The number of tapping leads are less i.e. nx2, where n is the number of steps,
(e.g. for no. of steps = 8, no of tapping leads = 16). Accordingly, terminal gear
of the tapping winding becomes easier.
As the tapping winding is made as an interwound helical tapping, the risk of
inherent part resonance phenomenon is avoided.
Moreover, winding of the interwound coil is easier than prior art disc type coil,
and needs less expertise, less time and a single winding process. Unlike prior art,
joints between the winding is eliminated. It also avoids any loosening of the
winding conductor and chance of error in winding work.
The tapping winding has more mechanical strength to withstand short circuit
forces because less number of joints.

WE CLAIM :
1. An outside interwound helical type of tapping winding in single group with
high voltage center line lead for power transformer, wherein the tapping
winding is formed as interwound helical tapping such that each turn shall
encompass the total height of the tapping winding.
2. An outside interwound helical type of tapping winding in single group with
high voltage center line lead for power transformer, as substantially
described and illustrated herein with reference to the accompanying
drawings.

The invention relates to an outside interwound helical type of tapping
winding in single group with high voltage center line lead for power
transformer; wherein a high voltage lead brought out from the center of
said tap winding and disposed away from the top yoke, wherein the tap
winding is formed as interwound helical tapping such that each turn of the
winding covers the total height of the tap winding whose total height is
accommodated within the circuit, and wherein the height of the high
voltage winding, the low voltage winding, and tap winding is substantially
the same.