Developing Technology for 1200 kV Transformer – TARIL Experience
UHV Transformers are essentially large rating transformers and require facilities to handle larger assembly weights and dimensions.TARIL have major infrastructure available to manufacture such heavier components e.g.20 tonne vertical winding machine, 150 tonne core building platform, a large VPD autoclave, 250 tonne active part handling capacity, 2.8 MV Impulse generator rating etc. However, balancing facilities like winding mandrel, increased battery of capacitors for temperature rise test, particle counting facility etc. were added to manufacture such high rating transformer.
Development of 1200 kV technology has been a great experience for Transformers & Rectifiers India Ltd. UHV technology is not yet established in India. It is still in the nascent stage. A few experimental test lines were setup in USA, Japan, Italy, USSR, China etc. Commercial operation is limited to Chinese 1100 kV line commissioned in 2009 or erstwhile USSR 1150 kV line which operated for a short period.
1200 kV AC Transmission corridors are essential for India considering that excess generation in Eastern and North Eastern blocks is to be transmitted to load centres located far away in Northern and Western regions. In view of lead time available before commercialization, POWERGRID embarked upon the idea of developing a 1200 kV test station along with a short 1200 kV transmission line on Public Private Partnership (PPP) model. First phase comprising 3 single phase Auto Transformers forming a bank of 1000 MVA, 1150 / 400 / 33 kV has been commissioned on No Load and dedicated to Nation.
UHV transformer is the largest and the most critical link in the power system. Reliability of the grid highly depends to a larger extent, on the reliable transformers. Reliability is the sum total of reliable specification, reliable design, reliable material, reliable manufacturing and of course reliable O&M at site. Developing a 1200 kV Class reliable transformer for TARIL, who are specialized in special transformers and power transformers up to 400 kV range of transformers, was a great challenge which has been met with successfully.
Various critical design issues related to dielectric design, thermal design, control of a powerful leakage field, short circuit withstand capability,etc. during design of this transformerwere handled successfully. State-of-art software were used to get precise results and design margins.
Longitudinal insulation was more challenging owing to transport height limitations. Permissible transport height en route limited the allowed window and winding heights. Winding with shielded turn arrangement was adopted to get near uniform voltage distribution across the winding.
1200 kV calls for larger radial clearances which enhance criticalityof dielectric stresses due to stressed oil volume effect. Larger oil gaps have to be divided to smaller gaps based on 2Dimensional electrostatic field study to increase factors of safety.
Large rating transformers have a powerful leakage field, which if not controlled properly lead to hot spot temperatures in the structural parts. Appropriate shielding measures were taken to avoid hot spot temperatures. Higher value of leakage flux also generates eddy currents responsible for hot spots in the transformer windings. Continuously transposed cables with epoxy bonding were employed to control winding hot spot temperatures as well as improve the dynamic short circuit withstand capability. State-of-art softwarebased on field solution was used to analyze the strength and rigidity of windings and clamping structure integrity towards axial and radial components of mechanical forces during short circuit conditions.
UHV Transformers are essentially large rating transformers and require facilities to handle larger assembly weights and dimensions.TARIL have major infrastructure available to manufacture such heavier components e.g.20 tonne vertical winding machine, 150 tonne core building platform, a large VPD autoclave, 250 tonne active part handling capacity, 2.8 MV Impulse generator rating etc. However, balancing facilities like winding mandrel, increased battery of capacitors for temperature rise test, particle counting facility etc. were added to manufacture such high rating transformer.
A reliable manufacturing of this UHV transformer requireda concept of design to manufacture, qualitative materials from approved quality vendors, good manufacturing infrastructure including all required tools like vertical winding machines of higher capacity, core building platform, vapour phase drying ovens, iso-static pressing of winding stabilization, higher capacity cranes, air casters, shielded test station and above all clean rooms for windings and active part assembly..All such things were made available during manufacturing of this transformer. It is mandatory to define all process steps in anexhaustive normative documentation in order to have a total control over a process during manufacturing of such transformers. Various check lists mentioning all required technical parameters need be filled up during various manufacturing processeswere developed and followed religiously.
After completion of manufacturing under thorough technical supervision, transformer was tested for all required routine and type tests. A corona shield of bigger diameter which was also a challenge to manufacture was developed indigenously for conducting partial discharge test. Factory acceptance tests included Impulse Test, Partial discharge test and temperature rise test and transformer passed all the tests successfully with fairly compared parameters with designed values.
Developing technology for a 1200 kV (UHV) transformer has been truly a great satisfying experience for TARIL! This Transformer shall form part of Phase II of the 1200 kV test station and shall be used for loading the transformers of Phase I on 400 kV side.
