Fifteenth National Power Systems Conference (NPSC), IIT Bombay, December 2008
Transformers for 1200 kV Testing Station at Bina
M. M. Goswami, Additional General Manager, POWERGIRD, New Delhi
and V. K. Lakhiani, Vice President,Vijai Electrical Ltd., Hyderabad
Abstract—Indian power system is poised to grow at an accelerated pace. Peak demand is expected to increase more than
500GW by 2026 and beyond from present level of about 105 GW
for which installed capacity of about 685GW is required. To meet
the long-term power transfer requirement by 2025 and beyond
as well as optimal utilization of right of way, large capacity
transmission corridors are being planned interconnecting the
generating resources/pooling stations with load centers. In this
direction, next higher voltage level at 1200kV is being planned. To
develop 1200 kV AC Transmission System and associated equipment indigenously, a joint initiative is taken by POWERGRID,
CPRI and Equipment Manufacturers to set up a 1200 kV Testing
Station and an experimental line at BINA in Madhya Pradesh.
The paper describes the considerations of the specifications of the
1200 kV transformers for the Testing Station. These prototype
transformers shall pose several challenges to designers and shall
pave way for the futuristic commercial transformers for the 1200
kV UHVAC systems.
equipment and obtain first hand experience of field testing,
trial operation etc. and build “A grid for tomorrow”.
This paper presents challenges of 1200 kV Power Transformers and discusses at length, the technical specifications of
1200 kV Prototype Transformer adopted for the 1200 kV Test
Station at Bina.
II. D EVELOPMENT OF UHV T RANSFORMERS
W ORLD - WIDE
A. Experiments in Japan, Italy & USA
1) 1100 kV, 3000 MVA, Transformer Bank of TEPCO,
Japan: The basic specifications of the 1100 MVA single phase
auto transformers forming a bank of 3000 MVA is given
in Table I [2]. Three transformer manufacturers, two coreTABLE I
BASIC S PECIFICATIONS OF T RANSFORMER BANK OF TEPCO
I. I NTRODUCTION
Rated Power
Voltage Ratio
Rated frequency
Tertiary power
On load tappings
at neutral point
% Impedance
Cooling Method
BIL - LI
PD -
1
200 kV AC system is being envisaged as next transmission
voltage to meet the long term power transfer requirement in the country. Ultra High Voltage (UHV) transmission
schemes are driven by the need to transfer large amounts of
electrical power from the generation resources to major load
centres. This need is typical of large geographical regions with
a strongly growing electrical power consumption in one part
of the region and natural resources that are far away from the
major load centres.
India is one of the most notable examples where in future
several UHV systems at 1200 kV AC shall be needed to
transfer large quantum of power from various generation
complexes in Chhattisgarh, Jharkhand, Orissa to load centres
in Northern and Western regions. Starting from next 5-6 years,
1200 kV AC system in due course, is expected to emerge
as main transmission level along with 800 kV, 6000MW
HVDC integrated with National Grid at 765kV AC, 400kV
AC supported by ± 500 kV HVDC Network [1].
Research on ultra high voltage transmission systems in
different parts of world started in seventees and eightees.
Experimental lines and UHV test stations were built in USA,
USSR, Italy, Japan, China to develop the technology. However,
because of reduction in the requirement of power transfer,
commercialization of UHV AC systems have not taken place.
POWERGRID has taken lead to establish a 1200 kV UHVAC
Test Station at Bina with experimental line (1 km) to provide
a platform to Indian Manufacturers to develop their 1200 kV
M. M. Goswami is Additional General Manager/Engg., POWERGRID,
New Delhi. e-mail: mgoswami@powergridindia.com.
V. K. Lakhiani is Vice President/Engg, Vijai Electricals Ltd., Hyderabad
e-mail: Virendra.lakhiani@vijai.co.in
3000/3 MVA, Single Phase
1050
√
√ / 147 kV
/ 525
3
3
50 Hz
1200/3 MVA
+7% to √-7%, 27 taps
√ for voltage range
1133.6/ 3 - 986.5/ 3 kV
18
ODAF
HV 1950√kVp, IV 1300 kVp , LV 750 kVp
1.5 Um/ 3 (Um = 1100 kVp)
form and one shell-form produced each single phase auto
transformer.
In view of the stringent transport limitations (4.1m H
x 3.1m W) 1000 MVA transformer was divided into two
units per phase. To be able to load these transformers, onload tapchangers were provided in each unit to supply the
circulating current between two units of the single phase
transformer using the difference of their tap positions. The
two units were designed to be connected in parallel by a Ttype oil filled cable box having an oil to SF6 bushing at one
end.
Challenges in the design of UHV transformers were mainly
related to 1100 kV insulation technology. Insulation structure
used sub-divided oil gaps and highest possible electric stress
value in the coil to meet the stringent transport limitations.
A multi-barrier insulation structure was also adopted for the
1100 kV lead insulation to reduce the insulation distance.
Some of the special tests conducted at Works comprised
to carry out heat-run test with DGA and temperature scan
for 110% rating, neutral leakage current measurement to
verify absence of static electrification phenomenon, transferred
surges between windings, FRA etc.
504
Fifteenth National Power Systems Conference (NPSC), IIT Bombay, December 2008
√
√
2) 200 MVA, 1000/ 3 / 400/ 3 single phase auto transformers for ENEL, Italy: Table II lists the auto transformer
ratings and basic specification developed for the Italy’s 1000
kV Project [3].
TABLE V
BASIC S PECIFICATION FOR UHV AC T RANSFORMERS
√
√
Voltage Ratio
1050/ 3 / 500/ 3 / 110 kV
Rated frequency
50 Hz
% Impedance
HV- IV
18%
HV-LV
62%
IV- LV
40%
Voltage Regula- VFVV Mode, Tappings at Neutral end
tion
Cooling System
OFAF
1100 kV Bushing Oil / Air
Transport Limita- 12 m long, 4.15 m wide, 4.9 m high
tion
BIL LI
HV 2250 kVp
SI
HV 1800 kVp
√
PD
1.5 pU x 1 hr. (1.0 pU = 1100
kV)
TABLE II
BASIC T RANSFORMER S PECIFICATIONS FOR I TALY ’ S 1000 K V P ROJECT
Rated Power
Voltage Ratio
Rated frequency
% Impedance
Cooling Method
BIL - LIL
SIL
PD
400/400/50 MVA, Single Phase
1000
√
√ / 12,2 kV
/ 400
3
3
50 Hz
15
ODAF
HV 2400 kVp, IV 1300 kVp , LV 125 kVp
HV 1950√kVp
1.5 Um/ 3 (Um = 1050 kVp)
3
On-load voltage regulation was considered not necessary.
Oil/air bushing was adopted. Interleaved winding configuration
was adopted for series and common coils.
√
√
√
√
3) 333 MVA, 1500/ 3/765/ 3/300/ 3/120/ 3 kV auto
transformer for AEP, USA: The basic specifications of the
transformer for AEP-ASEA Project are listed in Table III
below [4]:
TABLE III
BASIC T RANSFORMER S PECIFICATIONS FOR FOR AEP, USA
Rated Power
Voltage Ratio
Rated frequency
% Impedance
Connection
333/333/200
MVA, Single
√
√ Phase
√
1500/
/ 765/ 3 / 300/ 3 /
√ 3
120/ 3 kV
60 Hz
15.5% on 200 MVA base
YnaYo
TABLE IV
BASIC S PECIFICATIONS OF 667 MVA SINGLE PHASE AUTO TRANSFORMER
SI
UHV transformers are essentially large rating transformers
both in terms of voltage and current. Transport limitations
pose the biggest challenge. Problems related to dielectric
design of UHV Transformers are to be addressed meeting
the dimensional limitations for the transport. Because of high
current magnitudes, the problems related to dynamic stability
under effects of short circuit currents, hot spot generation etc.
get magnified and are to be taken care of adequately. And,
above all, “Reliability is a key word” for such transformers.
A. Transport Limitations
B. USSR’s Commercial Line
The transformer specifications for the 1150 kV Ekibastuz
- Kokchotav - Kustanai transmission system commissioned in
1985 are as per Table IV [5].
Rated Power
Voltage Ratio
Rated frequency
% Impedance
BIL - LI
III. C HALLENGES OF UHV T RANSFORMERS
667 MVA,
√ Single Phase
√
1150/ 3 / 500/ 3 kV
50 Hz
13
HV 2550 kVp FW/2800 kVp CW
IV 1800 kVp FW / 1950 kVp CW
HV 2100 kVp
The 411 km, 1150 kV line was in operation for about 2
years. After split of USSR, it was operated at 500 kV. Efforts
are being made to revive the line for 1150 kV operations.
C. China’s first experimental line and commercial line at 1100
kV
China’s 1100 kV test station√located at Wuhan HV Research
Institute used 40 MVA 1050/ 3 / 35 kV single phase transformers equipped with OLTC for voltage regulation in ±16 x
.625% steps.
The first 1100 kV UHV AC pilot project of China includes
654 km overhead transmission lines, two substations and
one switching station. The basic parameters of the UHV AC
transformers are given in Table V as under [6]:
In India, road transport is the established mode now.
Hydraulic trailers on modular concept are used for heavy
consignment. Suitable number of axles are assembled to form
the trailers of desired capacity. Pay load per axle allowed is
13.5 tonnes and requisite number of axles can be combined
longitudinally. Permissible transport height is the most important dimension as this decides the winding height which is
critical since 1200 kV lead entry is at the centre.
Standard axle height is 1.1 m. Allowing for some packing,
a max consignment height (transport height of tank containing
active part) can be 5 - 5.3 m for a max. height from ground
level <6.5 m. 4.5 m width of consignment should also be
permissible. In case of higher width, basic axle units can be
combined side by side. These units being single phase, length
is not the issue. However, in some cases, spreader plate may
be required to distribute the load. Transport weight of the
prototype shall be of the order of 250 T sans oil. A size of
7.5m(L) x 4.5m(B) x 5.3 m(H) consignment appears to be
feasible. It may, however, be desirable to get a road survey
done to assess the maximum moving dimensions en-route
before designing such massive transformer.
During transport, a transformer experiences shocks which
lead to undue forces at different heights and may cause damage
to active part fixing points. They have similar effects as in
case of seismic conditions. The intensity of these forces may
be limited below 2g in x, y, & z directions.
B. Di-electric Design
The insulation design presents many challenges since it
cannot be simply achieved by extrapolating designs for lower
505
Fifteenth National Power Systems Conference (NPSC), IIT Bombay, December 2008
voltage ratings. Due to transport restrictions there is not
much freedom on insulation distances. The di-electric design
therefore calls for much more controlled stress distribution.
The first problem is that for 1200/400 kV auto transformer,
the winding end insulation clearance is large and minimizes
the possible length of the series & common winding. The 400
kV common winding is made with line end entry insulation but
the 1200 kV winding line end stresses are so high that centre
entry insulation is exclusively adopted. This, then requires 400
kV class clearance at top and bottom of the winding. For a
maximum practical core height of 5 m (for the permissible
transport height) and resultant window height of typically 3.5
m, the series winding lengths for a centre entry arrangement
may be only of the order of 1 m. The stresses along the
winding for both AC and impulse tests are exceptionally high
and special stress control techniques are required to provide
adequate voltage distribution.
As far as winding design philosophy is concerned, full
interleaved disc configuration seems to be the ideal choice.
The interleaved winding, however, involves complications as
regards the electric stresses between turns and between discs
due to the service voltage stresses that are present during the
normal operation. The problem turns out to be of considerable
importance both because the HV terminal is provided at the
centre of the winding and because transportability requirements made it impossible to choose a higher winding height.
Special studies are therefore required :
• Insulation between turns and between adjacent discs.
• Insulation between windings and towards earth
• Insulation towards earth of the outgoing connections
The insulation design between windings and towards earth
is not a simple scaling up from 765 kV techniques. The
oil insulation has much larger volumes than in conventional
insulation structures. It is, therefore, necessary to understand
the performance of such large gaps and stressed oil volumes
to allow an optimum dimensions of the insulation. The voltage
stress distribution in each of the oil ducts and along the
pressboard barriers is calculated precisely and the insulation
is optimized to give stresses within proven acceptable PD
free stress levels. This allows the insulation structure to be
optimized without compromising quality and reliability. The
electrical breakdown strength depends upon the quality of
oil and particle content. The voltage withstand of oil is
reduced with increasing number of particles. The particles are
inherently produced from the insulation materials which are
to be controlled in addition to ensuring absolute cleanliness in
manufacturing and processing technology.
C. Thermal Design - Hotspots
Conductor insulation may require thicker paper to achieve
adequate inter-turn, inter-disc voltage withstand strengths because of the limited winding lengths. This presents additional
problems for thermal performance. It is required to calculate
precisely the losses and flux distribution in every turn/disc of
the windings to optimally design the cooling ducts to control
hot spot temperatures. Since, the insulation arrangements for
UHV class transformers are very complex with the use of
several barriers, ducts, angle rings, angle caps etc., provision
of adequate cooling paths becomes increasingly difficult. The
cooling performance can be enhanced by resorting to fully
directed oil flow cooling but the oil speed in ducts must not
lead to any electrostatic charging effects. These transformers
have a powerful leakage field linking with metallic parts of
tank. A very elaborate tank shielding is required to control
hot spot generation.
D. Short circuit withstand capability
A high power UHV transformer would mean high fault
currents during system short circuits with consequent higher
magnitudes of electromagnetic forces calling for an adequate
mechanical design. Both dynamic & static forces are considered resulting from flux distribution obtained from finite
element studies. The transformer is carefully designed and
ampere-turn balance is achieved as precisely as may be
practical. Manufacturing processes have to meet strict design
tolerances. Coil stabilization and sizing attains a very high
importance. Epoxy bonded conductors and high proof stress
copper are used to obtain mechanical strength, rigidity and
stability.
IV. POWERGRID’ S 1200 K V T EST S TATION AND
E XPERIMENTAL L INE
POWERGRID has planned to develop a 1200 kV UHVAC
test station at existing 400 kV substation at Bina (MP) where
two 1200 kV test lines (one single circuit and one double
circuit) each of about a km in length shall be constructed with
two 1200 kV test bays.
The 1200 kV UHVAC test station is being developed
by the joint efforts of POWERGRID, participating agency
(CPRI) and Equipment Manufacturers. Participating Transformer Manufacturers shall develop,
sup√design, manufacture,
√
ply, install and commission 1150/ 3 / 400/ 3 / 33 kV, 333
MVA single phase auto transformer at the 1200 kV UHVAC
Bina s/s. Establishment of 1200 kV UHV AC test station
shall help in development of 1200 kV transmission lines
and substation equipment for indigenous requirement. The
feedback from the field tests shall facilitate Indian Equipment
Manufacturers in improvisation and optimization of their 1200
kV equipment design.
V. 1200 K V T RANSFORMER S PECIFICATION FOR T EST
S TATION
The Working Group of the participating transformer manufacturers along with POWERGRID decided
√ the basic
√ parameters & specifications of 333 MVA, 1150/ 3 / 400/ 3 / 33
kV single phase auto transformer as listed in Table VI given
here under :
A. Basis for the specification of the auto transformer for the
test station
1) Rated power: 1200 kV transmission corridors are capable of transferring 6000-8000MW power. Commercial transformer banks of 3000 MVA (3x1000 MVA single phase)
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Fifteenth National Power Systems Conference (NPSC), IIT Bombay, December 2008
TABLE VI
S PECIFICATION OF 333 MVA T RANSFORMERS FOR THE 1200 K V T EST
S TATION
Rated Power
Voltage Ratio
Rated frequency
Connection
(3
ph.)
% Impedance
Cooling type
Cooling
equipment
Temp. rise
BIL - LI
SI
PD
333/333/111
MVA,
Single Phase
√
√
1150/ 3/400/ 3/33 kV
50 Hz
Ynaod11
HV-IV
18%
HV-LV
40%
IV-LV
20%
ODAF or OFAF
4 x 33.3% Unit OFAF Coolers
450 C : Winding, 400 C : Top oil
HV 2250 kVp, IV 1300 kVp , LV 325 kVp
Neutral 95 kVp
HV 1800 kVp, IV 1050√kVp
1.5 pu (1.0 pu = 1200/ 3 kV)
transformers are envisaged. 1000 MVA single phase shall be
the maximum rating that could be transported in the country.
A 1000 MVA single phase transformer shall comprise three
wound limbs in parallel. Max. MVA capacity of a single limb
can reach to 333.3 MVA (in auto connection). The transformer
shall actually be of 5 limbed construction with two return
limbs. It is unanimously decided by transformer manufacturers
to build one limb of the commercial 1000 MVA transformer
i.e. 333 MVA so that future adaptation to 1000 MVA single
phase rating becomes easier - connecting three limbs of 333
MVAs in parallel. Since 1200 kV transformers would be
essentially large rating transformers, consensus was not in
favour of small rating transformer like Chinese 40 MVA. 333
MVA rating was decided to prove both 1200 kV voltage and
high current (333 MVA).
2) No tappings: From reliability point of view, tappings
which are always vulnerable parts of transformer (owing to
part-winding resonance etc.) are not envisaged. Since on 400
kV side in India, voltage regulation facility is available on
down stream side transformers, need of voltage regulation
of 1150 kV was not felt necessary. Further, provision of an
additional tapping coil affects the transport dimensions of the
transformer which is a real challenge for such large rating
consignments. Since a tertiary winding is provided there will
always be a possibility to feed a regulating transformer which
by buck & boost connection can add/subtract voltage to neutral
point to achieve voltage regulation if required.
3) Voltage Ratio: 1200 kV is the system highest voltage
and 1150 kV is the nominal voltage finalized in the country.
No load voltage ratio for these transformers has been kept
as ratio of nominal voltages of the interconnecting systems.
(1200 kV & 400 kV systems in case of BINA s/s). Most
of the 1200 kV lines are expected to feed power to 400 kV
system and hence 1150/400 kV auto transformer banks have
been conceived. However, there may be few interconnections
to 765 kV systems also where 1150/765 kV auto transformer
banks shall also be required. These transformers may pose
more challenges from transport and insulation design point
of view since winding to yoke distance shall be for 765 kV
instead of 400 kV.
4) % Impedance: Considering the system short circuit level
limit, manufacturing capability, cost economies and transport
limitations, 18% short circuit impedance between HV & IV
windings has been finalized. The impedance to tertiary shall be
40% from HV side and 20% from IV side (without reactors).
5) Tertiary Rating: In line with the 765 kV class auto
transformers, tertiary voltage has been kept as 33 kV. This will
facilitate transformer manufacturer to test these transformers
(no load tests, PD test etc.) without resorting to additional
testing transformers as most of the manufacturers have interconnecting test transformers to feed power supply at 33 kV
level. Static compensation from tertiary is not envisaged at
the moment for these transformers. If required, compensation
equipment at 33 kV shall be economical compared to the case
when tertiary voltage is chosen as 132 kV or higher.
Further, if regulating transformer is externally added to
achieve voltage regulation, a 33 kV voltage of tertiary shall
be more desirable. As is the established practice, tertiary is
designed for 1/3rd rating so that a robust tertiary from short
circuit point of view is achieved. Rated BIL of tertiary shall be
based on transferred voltage from 400 kV side during impulse
test condition. A 66 kV class insulation (325 kVp BIL) is
envisaged.
6) Insulation co-ordination: Based on insulation coordination studies conducted for the 1200 kV systems and
based on experience in other parts of the world particularly
latest Chinese project, 2250 kVp lightning impulse level and
1800 kVp switching impulse level is decided for 1200 kV
windings. BIL of 420 kV winding is based on established
level currently in vogue is 1300 kVp (LI) & 1050 kVp (SI).
Transformer bushing BIL, however, shall be one level high i.e.
2550 kVp LI and 1950 kVp SI.
VI. C ONCLUSION
UHV transformers form part of very critical transmission
links and therefore need to combine high reliability with a
permissible transport requirement. Reliability can be built in
the technical specifications & parameters. While deciding the
specification of prototype 1200 kV transformer for the test station, reliability has been the central idea. Indian Manufacturers
and Utilities have risen to the occasion to fulfill the dream of
developing 1200 kV Technology indigenously.
R EFERENCES
[1] R.N. Nayak etal, Power Grid Corporation of India, “Integration of 1200
kV AC systems for future Indian Grid”, IEC/CIGRE Beijing, China, July.
2007.
[2] Y. Shirasaka (Japan AE Power Systems), Y. Ebisawa (Toshiba), H. Murakami (MELCO), T. Kobayashi (TEPCO) and T. Kawamura (University
of Tokyo), Japan, “Development and long term field tests for UHV, 3000
MVA transformers in Japan”, IEC/CIGRE Beijing, China, July. 2007.
[3] Italian Panel on UHV, Italy, “Open aspects and possible alternative
technologies following the UHV 1000 kV Italian experience” IEC/CIGRE
Beijing, China, July. 2007.
[4] Anders Lindroth et al, ABB Transformers, Sweden, “Important considerations for the development of insulation structures for UHV transformers
1000 kV AC and 800 kV DC”, IEC/CIGRE Beijing, China, July. 2007.
[5] Andrei K. Lokhanin, VEI Moscow, and Victor Sokolov, “Insulation
problems of UHV transformers”, VEI-ZTZ-Service, Russia. IEC/CIGRE
Beijing, China, July. 2007.
[6] Li Peng et al, “Type selection and test technology of UHV transformers”,
CEPRI China, EPRI, IEC/CIGRE Beijing, China, July. 2007.
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