CG”s 1200 KV transformer development : A success story

Topography of India is very peculiar. The generation of the electric power is concentrated in east and north east, whereas the load centers are concentrated mainly in the west and the north, where the large cities and industrial centers are situated.

This requires transmitting the power over very long distances, 1000 km and more. For such distances, the power must be transmitted at ultra high voltages (UHV). UHV reduces the cost of transmission lines and improves efficiency of transmission.

At present, the highest transmission voltage in India under commercial operation is 800 kV. In order to improve the transmission efficiency further, PGCIL was studying the feasibility of 1200 kV system in India. It decided to set up a 1200 kV test station at Bina, where an 1150 kV line of short length would be set up with 1000 MVA transformers in 3X333 MVA formation. Research on 1200 kV system would be done on this experimental line.

Just for records, all over the world, China, Italy, Japan, USA and former USSR are the only countries who have commissioned UHV system. India is the recent addition to the list of countries which have UHV transmission. CG was the first to sign the MOU with PGCIL for supply of the 333 MVA single phase transformer to PGCIL, and eventually became the first company in India to manufacture and supply India’s first 1200 kV class transformer! Below is the story. The box below shows the brief specifications of the transformer.

In the final commercial implementation of 1200 kV system, the transformer would be 3X1000 MVA, with each single phase 1000 MVA transformer having 3 legs with a 333 MVA winding system placed on each one of them. Thus, providing a 333 MVA single phase transformer effectively ensures a life like prototype in terms of electromagnetic, dielectric, thermal and short-circuit conditions.

The specification above is a very tough to realize, and CG, as also the other manufacturers, had huge challenges to tackle in design, manufacture, testing and transportation.

MVA              : 333 Phases           : 1 Voltage ratio    : 1150/?3/400//?3 / 33 kV Cooling           : ODAF, with heat exchangers Impedance      : HV-LV: 18% LV-TV             : 20% minimum HV-TV            : 40% minimum Test voltages   : LI: 2250/1300/250 kVp SI                   : 1800 kVp LDAC             : 1.5. Um//?3 kV

Challenges in design:
333 MVA on 1 core limb is close to upper limit of MVA that can be delivered by any core type transformer. Such huge rating produces very high leakage flux density in and around the windings, which produces thermal and short-circuit mechanical stresses, that are insurmountable by conventional design methodology. In order to avoid the adverse thermal effect such as hot spots in windings, very special pre-transposed conductors with small strand dimensions were used. Adequate cooling ducts were provided in the windings to allow abundant oil circulation, to bring the hot spot temperature well below dangerous limit.

The tank and the coil clamping structure had to be thoroughly shielded by CRGO steel shunts to divert the leakage flux into these shunts. Accurate estimation of the flux in the tank and clamping structure required a computer analysis. The snapshot of the flux in and around the windings is shown below.

Any transformer is expected to withstand short-circuits, and for such a large one, it is really most difficult to achieve the short-circuit strength. Large forces developed in this transformer during short circuits could not be withstood by ordinary winding conductors that are employed conventionally. In order to overcome this problem, special enameled conductors with low-chip™ epoxy bonding had to be used. In addition, the clamping structure had to be made extremely rigid. The picture below shows the stresses and deflections in the clamping structure.

Adequate safety factors, in line with CG design rules were ensured for windings and the clamping structure. Cooling of such large transformers has always been an important consideration; keeping the temperature of oil and windings below their safe operating temperature ensures a long and trouble-free life for the transformer. In order to reduce the foot print of the transformer and the cost of making foundation, it was decided not to use radiators but provide compact oil-air heat exchangers, which substantially reduce the foot print for given losses. The cooled oil coming out from these compact coolers was directed on transformer windings, rather than just feeding it into the tank. This facilitated efficient collection of heat developed in the windings. Such a procedure would however lead to an electro-static charging in oil, due to the friction between the moving oil and the windings and insulation thereof. This phenomenon, which has been a cause of failure of some large transformers, can be eliminated only by reducing the oil velocities in the windings below 50 cm/sec. This required special barriers and oil guides in the winding system.

For 1200 kV transformers, the most difficult challenge in design is the dielectric design of the transformer, so that the transformer can withstand the severe over-voltages such as lightning and switching impulses during testing and in service.

Most critical aspects in dielectric design are controlling the electric fields at the high voltage lead exit and the control of impulse voltage distribution within the windings. CG’s established design rules had been implemented in 800 kV class transformers, and it was really a big step to develop and implement the rules for 1200 kV transformers. Collaborative research carried by our Global R&D and other high voltage companies of international repute was the key to achieve this.

The design and Global R&D teams developed the complete dielectric structure of the windings to ensure very high dielectric strength against the over-voltages. CG has a state of the art analytical capability for calculating the dielectric stresses and strength of transformers. For ensuring the dielectric strength of the 1200 kV lead exit, help was taken from the leading company manufacturing specialized transformer insulation components, and the 1200 kV lead exit system was jointly developed. A schematic view of this system is shown below.

External clearance         : HV-E: 7.3m                                       IV-E: 3.5m                                       TV-E: 0.53m Overall dimensions, m    : 10.0L X 9.1B X 15H Total mass                    : 310 ton Transport dimensions, m: 6.0L X 5.15W X 5.15H Transport mass              : 206 ton

This lead exit system ensured a reliable and partial discharge free routing of the 1200 kV lead from the winding to the bushing. Making the transformer free of partial discharges was ensured by providing subdivided oil ducts and adequate protection of the electrodes by contouring and careful insulation. This much about the design. Whereas design was very critical, the manufacturing and testing of the 1200 kV transformer required special attention and investment.

This transformer represented the largest core diameter and the coils with highest weight and dimensions. In order to manufacture and process the transformer in the Bhopal plant, CG did a large investment. In order to manufacture and handle the large coils, new winding formers and cranes were installed in the winding bay. Special dust control facility was commissioned in the winding/assembly area to exclude very tiny dust particles, which can prove lethal to the transformer insulation. Modifications were carried out to the vapour phase drying plant to accommodate the height of the active part of the transformer.

The drying out of the transformer and processing of the oil was done in line with the “One-World-Quality” norms which are established and followed in all the CG plants globally. This was a key success factor to make the transformer partial discharge free, and to impart it the necessary dielectric strength. In the box below, we can see the various physical parameters of the transformer.

CG manufactured this 1200 kV transformer meeting all the stringent quality norms and subjected the transformer in their completely shielded test laboratory at the Bhopal factory. The picture below shows the transformer under test.

The transformer successfully passed all tests. The tests were witnessed by global experts from CG as well as the PGCIL QA. The culmination of everybody’s efforts into a successful testing was really a high point. CG’s, and India’s first 1200 kV transformer was ready for dedicating to the nation.

The transformer was then made ready for transportation and was flagged off to its final destination, Bina, on 05th December, 2011. Transporting such a huge weight and bulk on the bad roads from Bhopal to Bina was a challenge in itself; but our partners in transportation boldly negotiated all these challenges and brought the transformer to Bina in time.

CG engineers erected the transformer and made it ready for commissioning from the 400 kV sides, which was available at the Bina substation. The transformer was charged in the presence of the MD of PGCIL on 26 May 2012, and created history. CG made this transformer at its own expense, without any cost to PGCIL, and showed its interest and concern in the development of Indian power system. The transformer is now in service without interruption, and CG and PGCIL are doing a lot of research activities to establish 1200 kV systems for commercial application.

By Vikrant Joshi & Manish Yadav

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