FIELD OF INVENTION :
The present invention relates to an improved three phase shunt unit for phase shifting
transformer with independent tapping winding with ±100% tapping range.
BACKGROUND OF THE INVENTION :
Tapping windings are provided in the power transformers to vary the HV winding
voltage as per site/ line requirement. The variation in tapping winding voltage is done
with the help of on load/off load tap changers (OLTC).
a) In most of the power transformers the change in HV voltage is around ±10% in
maximum seventeen steps of the rated HV voltage.
b) In majority of the power transformers the tap winding is connected to the HV
winding and is small in comparison to the size of the HV winding in terms of
height, lateral width and weight.
c) In some of the transformers, the requirement exists for tap winding with ±100%
change in the tapping voltage within 35 steps.
A requirement of ±100% change in tapping voltage in 35 steps is known to be
technically impossible as the prior art tap winding is integrated with HV winding. Any
attempt to be achieve ± 100% charge in tapping voltage within a maximum of 35 steps,
particularly when the tap winding is integrated with HV winding, shall lead to increase
in the size & weight of HV winding with a large number of HV-turns, making the
manufacture of the HV-winding virtually impossible.

OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved three phase shunt unit
for phase shifting transformer with independent tapping winding with ±100% tapping
range.
Another object of the invention is to propose an improved three phase shunt unit for
phase shifting transformer with independent tapping winding with ±100% tapping
range, which allows easy variation of voltage in tap winding upto ± 100%.
A still another object of the invention is to propose an improved three phase shunt unit
for phase shifting transformer with independent tapping winding with ±100% tapping
range, which provides an optimum cooling of the windings.
A further object of the invention is to propose an improved three phase shunt unit for
phase shifting transformer with independent tapping winding with ±100% tapping
range, which is enabled to vary transformer voltage between 52KV to ± 52KV within
thirtyfive equal steps.
SUMMARY OF THE INVENTION :
Accordingly, there is provided an improved three phase shunt unit for phase shifting
transformer with independent tapping winding with ±100% tapping range. Both the
windings has their own terminations.
According to the invention, a modified tap changer is connected to the tap winding for
±100% variation in tap winding voltage. The voltage is varied from -52kV to +52kV in
35 equal steps. The results of the same were verified by testing at works at all the 35
tap positions.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS :
Figure 1. Shows an Elevation (front) view of tap winding terminal gear showing routing
of Tap leads.
Figure 2. Shows a Plan (top) view of tap winding terminal gear and interconnection to
OLTC.
Figure 3. Shows an End view of tap winding terminal gear.
Figure 4. Shows a Connection of tap winding leads to the OLTC of 3-phase shunt unit of
Phase Shifting transformer according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT :
The present invention provides a power transformer with an independent tap winding
and for voltage variation of ±100% over the entire tapping range to vary voltage from -
52kV to +52kV.
The transformer as shown in figure 1, consists of LV, Tap, IV and HV windings. Terminal
markings at lead take outs are marked on Figure 1.
The tap changer consisted of 35 tap positions per phase i.e. total of 105 leads were
routed from OLTC to three phases. The leads are routed throughout terminal gear by
providing spacing between the leads.
Figure 2 shows that the interconnection has been done maintaining the required
clearances from earth parts.

Figure 3 shows routing of tapping leads and individual bushings 3U,3V,3W for each
phase.
The OLTC support was also strengthened to cater to such huge weight of the terminal
gear.
OLTC is selected frequent operations. Conventional metallic contact tap changers are
not suitable for frequent operation, as these contacts are prone to wear and tear due to
continuous tap changing operations, one of static type or vacuum type of OLTC is
selected for smooth, trouble free & unlimited (lifelong) operation.
Following are the main advantages of independent tap winding:
- ±100% variation in tap winding voltage with ease.
- Optimum size & weight of HV and Tap windings.
- Better cooling of both the windings due to sufficient radial gap between
the windings.
- Better ampere turns balance.

WE CLAIM :
1. An improved three phase shunt unit for phase shifting transformer with
independent tapping winding with ±100% tapping range, the phase-shifting
transformer essentially consists of Low-voltage windings, tap windings, high
voltage windings, Intermediate voltage windings, terminal gear for tap windings
at least one vacuum type OLTC for the shunt unit of the transformer, a plurality
of bushings assigned to each phase, a plurality of tapping leads, the
improvement is characterized in that t he OLTC is formed of thirtyfive tap
positions per phase totaling one hundred and five tap positions, in that the
corresponding number of tapping leads are routed throughout the terminal gear
by maintaining an accurate and mixture spacings between said leads, and in that
the OLTC is provided with additional support members to cater for substantial
increase in the weight of the terminal gear.

ABSTRACT

The invention relates to an improved three phase shunt unit for phase shifting
transformer with independent tapping winding with ±100% tapping range, the
phase-shifting transformer essentially consists of Low-voltage windings, tap
windings, high voltage windings, Intermediate voltage windings, terminal gear for
tap windings at least one vacuum type OLTC for the shunt unit of the transformer, a
plurality of bushings assigned to each phase, a plurality of tapping leads, the
improvement is characterized in that t he OLTC is formed of thirtyflve tap positions
per phase totaling one hundred and five tap positions, in that the corresponding
number of tapping leads are routed throughout the terminal gear by maintaining an
accurate and mixture spacings between said leads, and in that the OLTC is provided
with additional support members to cater for substantial increase in the weight of
the terminal gear.