FIELD OF THE INVENTION
The present invention relates to power transformers, in particular a 40 MVA
transformer adaptable for testing of power transformers of different ratings from
11 kV to 400 kV.
BACKGROUND OF THE INVENTION
Testing of a power transformer provides a certificate of reliability and proves its
design/proper manufacturing. For testing power transformers of different ratings,
for example, from 11 kV to 400 kV and 15 MVA to 600 MVA, the requisite testing
transformer is recommended to have voltage variations from 4 to 156 kV that is
97.4% on the secondary side of the transformer.
Designing a testing transformer with such a high voltage variation for example,
97.4%, and having 40MVA rating is complex. In the normal manufacturing
range, the power transformers are manufactured with ± 10% variation on the
high voltage side. Further, there exists no general standard, which governs /
guides manufacturing of such type of transformers, except for the guidelines
provided in IS: 2026, and other general parameters.
The design of such a testing transformer is therefore unique and challenging, as
it requires addressing plurality of constraints for example, space limitation, low
noise level, large voltage variation, non-availability of tap changer for required
variation, capability of withstanding short-circuit at lower impedance, and over
voltages in hanging portion of the winding at lower taps.

Accordingly, JP 7326521 provides a transformer in which when the voltage of a
testing power supply regulated by a voltage regulator is applied to a test lap of
the secondary winding of a test transformer, a desired test voltage is induced at
the primary winding, and withstandable voltage test is performed. Thus,
withstandable voltage test is performed without using a testing transformer, if
the capacity of test tap is set to a large value by adding the condensive capacity
due to electrostatic capacity of another equipment connected to the primary
winding to own excitation capacity, no compensation reactor is required, so that
the withstandable voltage test at site can be effected with ease.

OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a 40 MVA transformer
adaptable for testing power transformers of different ratings from 11 kV to
400kV.
Another object of the invention is to propose a 40 MVA transformer adaptable for
testing power transformers of different ratings from 11 kV to 400kV which can
operate with large voltage variations for example, 97% without generating swing
voltage.
An yet another object of the invention is to propose a 40 MVA transformer
adaptable for testing power transformers of different ratings from 11 kV to
400kV which eliminates the use of an off circuit tap switch.
A still another object of the invention is to propose a 40 MVA transformer
adaptable for testing power transformers of different ratings from 11 kV to
400kV in which the coil space should be reduced to ensure uniform impulse
distribution.
SUMMARY OF THE INVENTION
Accordingly, a transformer developed, which is adaptable for testing power
transformers of different ratings from 11KV to 400 KV. The improvement
comprising: a HV-winding configured in two sections, the HV termination being
taken out through two sets of bushing terminal; an OLTC is disposed at the

neutral end. LV winding designed as helical and a tapping winding is configured
in two layers. The first section of HV-winding is designed as standard helical, and
the second section of HV-winding is configured as partial interleaved disc to
ensure uniform impulse distribution and lower cross-sectional area. The first
section of HV-winding (HV1) including the tap winding provides a voltage
variation as 42 KV ± 38 KV to be obtained from terminals 2U, 2V, 2W, and, the
first and second sections of the HV-winding together including the tap winding
provide a voltage variation as (118 KV ± 38 KV).
The voltage variation requirement being quite high, the overall variation has
been proposed in steps, for example from 4kV to 156 kV. Thus, 40 steps of
approximately 3.8 kV that is (156-4)/3.8=40 steps, almost 97.4% variation on
HV side have been decided. But the number of steps being very large, no single
OLTC could be identified to cater for the requirement of 40-steps during voltage
variation of 3.8 kV in each step. The existing OLTCs are capable to cater for
maximum 35-tap position.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows a line diagram of a first embodiment of an off circuit tap switch
of prior art.
Figure 2 - a line diagram of a second embodiment of an off circuit tap switch of
prior art.
Figure 3 - shows a flow-chart according to the invention, describing the steps of
achieving large voltage variation without using an off circuit tap switch.

Figure 4 - shows the coil configuration of a test transformer according to the
invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF
THE INVENTION
The first experiment has been envisaged to configure a new device which has
the main HV winding in 4 sections (HV1, HV2, HV3, HV4) with one OLTC (2) and
one off-circuit tap switch (1) as shown in Figure 1. As per this embodiment, all
the four sections of the HV winding (HV1, HV2, HV3, HV4) alongwith the off
circuit tap switch (1) is disposed at line end and the OLTC (2) is disposed at the
neutral end. For this embodiment, the specification of the off circuit tap switch
(1) was envisaged as under: -
Application - Linear off circuit tap changer for delta connection
Type - U
Rated through current - 600 Amp
Highest voltage for equipment - 170 kV
Rated impulse withstand - 650 kVp
Rated PF withstand for 1 min-230 kVrms
Phase - 3
Operating tap positions - 4
Power frequency step voltage - 42 kV
The above specification as initially decided was tried with various manufacturers
of, off circuit tap switch / OLTC and it was revealed that an off circuit tap switch

(1) such as described hereinabove is not suitable for the purpose of the invention
because of its high BIL and power frequency levels.
During a further experiment, a second embodiment has been attempted, in
which the main HV winding has 4 sections (HV1, HV2, HV3, HV4) with one OLTC
(2) and one off circuit tap switch (1) as shown in figure 2. As shown in this
figure, the off circuit tap switch (1) is not directly exposed to the line voltage of
156kV or BIL of 650 kVp, but connected between the HV winding and the tap
winding, the OLTC (2) being disposed at neutral end.
In this arrangement, while carrying out a transient voltage analysis that is
lightning impulse distribution at lower voltage taps, it was found that the upper
section of the HV winding remains floating and an across the voltage range of
the off circuit tap switch is found to be more than 650 kVp. Thus, the designed
tap circuit switch was found to be unsuitable for commercial manufacture.
Due to non-applicability of the first two embodiments to solve the prior art
problems, a new method for achieving large voltage variation (as shown in
Figure 3) has been proposed. The new method altogether avoids the use of an
off circuit tap switch (1), which was proving to be an unsolvable problem in the
design of the required transfer. The method proposes a HV winding in two
sections (that is HV2 and HV1) and taking out the HV termination through two
sets of bushing terminal (1U1 and 2U1).
Thus, the HV1 and the tap winding shall provide the voltage variation as 42kV ±
38kV (that is 4 kV to 80 kV). This variation can be obtained from terminals 2U,
2V and 2W. HV1 + HV2 and the tap winding shall provide the voltage variation

as 118 kV ± 38 kV (that is 80 kV to 156 kV). This variation can be obtained from
the terminals 1U, 1V and 1W. At 4 kV to 40 kV level, the current at those taps is
restricted to 600 Amp.
While designing the transformer, it was found that the number of tapping turns
is very high. Moreover, due to thermal consideration, it is necessary to restrict
the current density in tapping winding near to 3A/mm2, which resulted in
reduced space factor. Hence the accommodation of the large number of turns
with higher cross sectional area (to limit the gradient) in axial direction posed
constraint to design the coil. To overcome the problem, the tapping winding has
been segregated in two layers (as shown in figure 4) and a Glued Continuously
transposed conductor has been proposed for the tapping winding for better
space utilization.
The most severe problem arises due to the requirement that the testing
transformer has to be installed in a hall that is indoor, hence there remains no
possibilities of lightning impulse. Keeping in view a frequent switching
requirement, it has been proposed to meet the lightning impulse level as per IS:
2026 at all taps. A further problem arises in that while carrying out the impulse
distribution in windings at minimum tap, it was found that some sections of the
HV winding remain floating and a swing voltage occurs in the floating portion of
the winding, which causes the impulse voltage of the floating terminal with
respect to earth, to increase. In order to overcome the problem, the winding
configuration has been selected as shown in figure 3 and 4 with selection of
correspondingly suitable ducts in axial direction between the turns in the winding
including adequate interleaving at line end to equalize the capacitance
distribution, and an optimum selection of radial ducts between the windings.

The coil configuration has been selected as shown in figure 4. Helical coil (3) has
been used for Low Voltage (LV) coil. The tapping winding has been made in two
layers (4, 5) as described hereinabove. The HV1 (6) winding has been selected
to be helical due to lower number of turns being susceptible to high current. The
HV2 (7, 8) winding configured as a partial interleaved disc due to large number
of turns, to ensure uniform impulse distribution and lower cross sectional area
due to less current in comparison to HV1 winding.

WE CLAIM:
1. A transformer for testing power transformers of 11KV to 400 KV ratings,
comprising :
- a high voltage (HV) winding configured in two sections (HV1, HV2), the
HV termination being taken out through two sets of bushing terminal (1U1,
2U1);
- an online tap changers (OLTC (2) disposed at the neutral end;
- a coil means having a low-voltage (LV) coil in helical form (3), tapping
winding (4,5) being configured in two layers, the HV1-winding (6) designed as
standard helical, and HV2-winding (7,8) configured as partial interleaved disc to
ensure uniform impulse distribution and lower cross-sectional area, characterized
in that the first section of HV-winding (HV1) having the tap winding provide a
voltage variation between 42 KV + 38 KV obtained from terminals 2U, 2V, 2W,
and in that the first and second HV- sections together (HV1 + HV2) having the
tap winding provide a voltage variation between (118 KV ± 38 KV).
2. A transformer for testing power transformers of different ratings from
11KV to 400 KV as substantially described herein with reference to the
accompanying drawings.

ABSTRACT

TITLE: A TRANSFORMER FOR TESTING POWER TEMPERATURES OF 11
to 400 KvA RATINGS".
A transformer for testing power transformers of 11KV to 400 KV ratings,
comprising : a high voltage (HV) winding configured in two sections (HV1, HV2),
the HV termination being taken out through two sets of bushing terminal (1U1,
2U1); an online tap changers (OLTC (2) disposed at the neutral end; a coil
means having a low-voltage (LV) coil in helical form (3), tapping winding (4,5)
being configured in two layers, the HV1-winding (6) designed as standard helical,
and HV2-winding (7,8) configured as partial interleaved disc to ensure uniform
impulse distribution and lower cross-sectional area, characterized in that the first
section of HV-winding (HV1) having the tap winding provide a voltage variation
between 42 KV ± 38 KV obtained from terminals 2U, 2V, 2W, and in that the first
and second HV- sections together (HV1 + HV2) having the tap winding provide a
voltage variation between (118 KV ± 38 KV).