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TITLE : NOVEL MEHTOD OF WINDING PLACEMENT FOR A SPLIT
WINDING TRANSFORMER
FIELD OF INVENTION
This invention relates to a method of winding placement in a split winding power
transformers. More particularly, the present invention relates to a method of
winding placement is transformers with split windings belong to special type of
power transformers with more windings. They usually contain several pairs of
input and output windings. All input windings are connected in parallel and all
output windings are independent, galvanically separated from one another.
Function of input and output windings may be interchanged.
BACKGROUND OF THE INVENTION
A Normal power transformer consists of 2 windings one HV and other LV with
power output of both the winding being same. This results in fixed power output
and fixed output voltage and fixed percentage impedance between the pair of
windings. However in a split transformer the LV is divided in to two with a
common HV. The LV can be arranged either concentrically or axially split.
In case of concentric split LV winding the power output between HV and
different pair of LV is different and also the percentage impedance between the
pairs eg/ 63/28/35 MVA, 33/11.5/6.9 kV, 3-ph Station Transformer having 1 HV

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and 2 LV windings. % impedances at normal tap is 9%(max.) for HV/LV1 pair,
13%(Min.) for HV/LV2 pair and 20%(Min.) for LV1-LV2 pair.
In case of axially split LV winding the power output between the two pairs is
same but half of the rated output of HV but the percentage impedance between
the pairs is same eg. 90/45-45 MVA, 132/11.5-11.5 kV, 3-ph Station Transformer
having 1 HV and 2 LV windings. % impedances at normal tap is 21% ±10% Tol.
For HV/LV1 pair or HV/LV2 pair, 37% min for LV1-LV2 pair at 90MVA base.
Thus by the help of split winding transformer, various combinations of power
output and % impedances can be achieved in order to suit the requirement of
the utility. Any one of these split design is adopted.
OBJECTIVES OF THE INVENTION
It is therefore, an object of the present invention to propose a method of
placement winding in a split winding power transformers which substitute two
double-winding transformers of half the power. This leads to savings in place and
instrumentation (disconnecting switches and circuit breakers) on the side of the
input voltage.
Another object of the present invention is to propose a method of placement of
winding in a split winding power transformer which decreases of values of the
short circuit currents in the individual supplied circuits thus enable use of low
capacity circuit breaker.

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A further object of the present invention is to propose a method of winding in a
split winding power transformer which lowers the fluctuations of voltage in one
supplied circuit in dependence on the magnitude of load connected to the second
circuit.
A still further object of the present invention is to propose a method of winding
or in a split winding power transformer which makes possible operation of each
of the branches of Split winding with other winding disconnected.
An yet further object of the present invention is to propose a method of winding
or in a split winding power transformer which is more economical being a 2-in-l
transformer.
SUMMARY OF THE INVENTION
Split winding transformers considered here are those in which LV winding is split
into 2 parts but a common HV winding. These transformers are exclusively used
as station transformers in thermal power plants to feed power to Boiler feeding
pump motors, induced draught fans, forced draught fans etc., Three, 1-phase
step-up transformers with split winding can work for 2 generator supplying
power into one network. Hence these transformers play an important role in the
thermal power plants.
In the split winding transformer, since low voltage windings are split into 2 parts,
they limit the interrupting capacity of circuit breaker. An useful transformer
arrangement with the dual low voltage winding permits two supplies to each half

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of a split bus bar and thus limits short circuit level whilst ensuring continuity of
supply in the event of a transformer failure.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1. shows the winding disposition of a 2-wdg. transformer. This
transformer consists of LV, HV and Tap winding. In this transformer the LV and
HV are rated for same power output i.e. the rated MVA. The percentage
impedance between LV and HV winding is dependent on the radial placement of
the windings ie the radial gap between the main windings.
Figure 2. shows the winding disposition of a split winding transformer.
This transformer consist of LV in two halves. LV coils 1 and 3 form LV1 and LV
coils 2 and 4 form LV2. In this transformer LV coils 1 and 2 are at same radial
distance from core and LV coils 3 and 4 are at same radial distance from LV coils
1 and 2. The axial gap between the LV1 and LV2 coils, HV coils 5 and 6, and Tap
coils 7,8 and 9,10 plays a major role in deciding the percentage impedance
between HV-LV1, HV-LV2 and LV1-LV2.
The rated MVA of each of the LV ie LV1 and LV2 is half of the rated MVA of HV
winding and the percentage impedance for HV-LV1 is same as for HV-LV2.
It can also be termed as two transformers working in parallel.

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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
This specification is being filed for the axially split LV winding of the station
transformer. BHEL has successfully manufactured and type tested following such
station transformers (ST):
-80/40-40MVA, 400/11.5-11.5 kV ST for KPCL Bellary,
-80/40-40MVA, 132/11.5-11.5 kV ST for NTPC Rihand and Vindhyachal,
-90/45-45MVA, 132/11.5-11.5 kV ST for NTPC Sipat.
Of these, STs for NTPC Rihand and Sipat have also been successfully Short
Circuit tested at CPRI Bangalore.
Following are adopted in design for meeting the impedance requirement:
- Empirical formula of BHEL design data for impedance calculation.
- Gap between top and bottom split winding is fixed to achieve the
impedance pattern.
- Methodology to simulate axial and radial forces for split winding.
- Methodology to simulate all short circuit forces for split winding.

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WE CLAIM
1. A method of winding placement for a split winding power transformer
comprising:
- at least two LV windings and at least one common HV winding;
- LV can be disposed either concentrically or axially split;
- LV windings maintains a definite axially split gap with core or LV windings
maintains a definite axial gap in between them;
- LV windings and HV winding and Tap coils also maintain a definite gap in
between them to control impedance between HV-LV1, HV-LV2 and LV1-
LV2;
- The rated MVA of each of LV ie LV1, and LV2 is half of the rated HV
winding and the percentage impedance for HV-LV1, is same as for HV-LV2
Characterized in that said split winding power transformer are exclusively
employed in station transformers in Thermal power plants to feed power
to boiler appliances.
2. The method of winding placement for a split winding power transformer
as claimed in claim 1 wherein a low capacity circuit breaker is placed in LV
winding.

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3. The method of winding placement for a split winding power transformer
as claimed in claim 1 wherein in low fluctuation of voltages in one
supplied circuit because of segregation of loads.
4. The method of winding placement for a split winding power transform as
claimed in claim 1 wherein it makes possible operation of each of the
branches of split winding with other windings disconnected.

A method of winding placement for a split winding power transformer
comprising:
- at least two LV windings and at least one common HV winding;
- LV can be disposed either concentrically or axially split;
- LV windings maintains a definite axially split gap with core or LV windings
maintains a definite axial gap in between them;
- LV windings and HV winding and Tap coils also maintain a definite gap in
between them to control impedance between HV-LV1, HV-LV2 and LV1-
LV2;
- The rated MVA of each of LV ie LV1, and LV2 is half of the rated HV
winding and the percentage impedance for HV-LV1, is same as for HV-LV2
Characterized in that said split winding power transformer are exclusively
employed in station transformers in Thermal power plants to feed power
to boiler appliances.