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DEVELOPMENT OF INDIA’S FIRST 1200 KV TRANSFORMER FOR TESTING OF LARGE RATING 800 KV SHUNT REACTORS

1. Introduction

1200 kV UHVAC system is being envisaged as next transmission voltage to meet the future high power transfer requirement in India. Development of 1200 kV UHVAC Transformers in India is taking into shape with BHEL first to develop 1200 kV Transformer. BHEL is progressing step by step in basic issues for development through development and improvement of design analysis tools and particularly insulation design technology required for such ultra high voltage Transformers. The experience in development of 1200 kV Transformer for testing purpose is being further extended for development of 333 MVA, 1150/400/33 kV single phase Auto Transformer, which is also under development at BHEL, for India’s first 1200 kV Experimental Test Station of Powergrid India at BINA for field studies and trials.

800 KV AC transmission line has already been commercially established in the country for bulk power evacuation. 800 KV Transformers, Shunt Reactors and Instrument Transformers have been developed. Power Transformers, Shunt Reactors and Instrument Transformers are vital equipments in the complex UHV transmission system.

Testing of 800 KV Shunt Reactors necessitates higher voltage rating testing Transformer.

Although for testing transformer the transient voltage requirement similar to outdoor field transformer is not envisaged , however in order to have the hands on experience of manufacturing and testing of 1200 KV transformer, impulse voltage calculation in the windings of transformer has been introduced to ascertain the withstand capability as per test requirement in the specification. This leads to refinement and establishment of design analysis software for transient voltage test requirement for 1200 KV UHVAC Transformers. Other design considerations viz. short circuit forces ability and heat transfer characteristics are also evaluated. Extremely high voltage level and large capacity of UHV Transformer lead to more technical difficulty and innovative technology in main insulation design, lead exit design, core clamp

plate and tank structure design, transportation etc. needed. Presently very few manufacturer worldwide have made power transformers for 1200 KV system voltage.

The major aspects considered and challenges faced during design and engineering of 1200 KV Transformer are discussed in this paper.

2. Transformer’s Parameter

This transformer will be installed in a limited space and will be employed for testing of 800 KV 1- phase Shunt Reactor. Hence it is a single-phase transformer. Due to transport limitation and available technology, UHV Transformers are generally single phase. The 1200 KV class transformers in former Soviet Union, Italy, Japan and China are all single phase. As only one voltage level is desired, voltage regulation of 1200 kV is not required hence no tapping has been specified.

Important specification parameter features are given in table I.

TABLE I Specification features of 1200 kV Transformer

Rated Power (MVA) HV/LV 180/180

Rated no load voltage (kV) HV/LV

1200/√3/138/√3

Rated Frequency & no. of phase 50 Hz, 1

% Impedance HV-LV 12%±10% tol.

Insulation Level

i) Separate source power frequency voltage withstand HV/LV

ii) Induced over voltage withstand with PD meas. HV/LV

iii) PD level at 1040 kV (1.5Um/√3) for 60 sec.

iv) Full wave Lightning impulse voltage HV/LV

v) Switching impulse voltage HV/LV

70/275 kVrms

1040/275kVrms

<100 pC

2300/550 kVp

1675/ - kVp

R. K. Singh* S. K. Gupta J. S. Kuntia R. K. Tiwari

Transformer Engineering Division, Bharat Heavy Electricals Limited, Bhopal, India

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Type of Cooling, Cooling Equipment

OFAF, 4x33% OFAF coolers

Top Oil/ Winding by resistance Tempr. rises 0C

50/ 55

3. Design

Challenges in the design of this 1200 kV ransformer is mainly related to 1200 kV insulation technology. The development of reliable 1200 KV Transformer requires a well developed and proven design concept. Reliable and proven design for extra high voltage and high power rating require good basic and theoretical understanding regarding physics of transformer and extensive experience in transformer design, manufacturing, testing and field trial.

The design concept used by us is based on technical knowledge acquired over 40 years of experience in transformer design and development ranging from 1 MVA to power transformers up to 500 MVA 800 KV class 1-phase auto transformer (forming a bank of 1500 MVA), HVDC converter transformer, Shunt Reactors up to 80 MVAR, 800 kV class 1-phase (forming a bank of 240 MVAR) etc. Prototype manufacturing of 800 kV equipments is under progress.

Computer aided design tools are extensively employed in the following areas:

i) Optimization of basic parameters like core dimensions, magnetic flux in core, number of turns in the windings, current density in windings, cable dimensions in the windings etc.

ii) Study and analysis of electrostatic and electromagnetic fields.

iii) Simulation of windings to study voltage distribution under transient over voltages like lightning and switching over voltages.

iv) Evaluation of electro-dynamic forces in windings and on various parts during external short circuit.

v) Thermal performance and analysis.

Transformer insulation is subjected to the continuous influence of operating voltage and over voltages. The over-voltages may consist of lightning over-voltages (aperiodic surges-duration from ones to tens of μsec), switching over-voltages (oscillation surges with considerable attenuation-duration up to several thousand μsec) and temporary over-voltages at or near power frequency (voltage rises lasting up to several minutes). Dielectric strength of transformer should be sufficient to withstand dielectric stresses due to these over voltages.

During insulation design of transformer all such over-voltages are converted into equivalent design insulation level (DIL) for calculation of major insulation distances. Clearances between windings and other major insulation distances are calculated based on our empirical formulae for worst DIL condition.

The basic parameters of this 1200 KV Transformer like core frame dimensions, magnetic flux in core, number of turns in windings, current density in windings, conductors dimensions, winding dimensions etc. has been optimized with the state of the art computer software.

Low voltage to high voltage winding insulation, windings end insulations, disc to disc insulation and between turns insulation are most critical and complex part of transformer insulation arrangement. Di-electric design calls for much more controlled stress distribution. Sub-divided oil-barrier insulation system is used between the windings and between the winding ends and the yoke (major insulation). This type of major insulation is reliable and has proved effective during the long term operational experience. In HV winding sufficient disc to disc insulation and insulation paper covering on turns has been kept to withstand operating voltages and over voltage stresses as decided by DIL.

Sub-divided oil-barrier insulation structure is also envisaged for the 1200 kV lead exit to optimise insulation distance and obtain sufficient uniform voltage distribution.

After optimization of design important technical performance parameters like no load losses, load losses, short circuit forces, winding temperature gradients etc. has been calculated by the computer software.

Verification of withstand capability of transformer for following areas has been done at the design stage

a) Electric strength of insulation of transformer

The calculation of electric fields and strength of main insulation are performed for representative areas such as top, middle and bottom areas of windings and along the windings.

b) Short circuit withstand

Magnetic flux distribution in different parts of windings and transformer parts and corresponding mechanical forces and withstand capability of windings, clamping structures etc.

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c) Thermal design

Thermal distribution in windings, core clamping structures, tank wall etc. are obtained and windings temperature gradients are calculated. The cooling performance is made more effective by directed oil flow washer and oil guiding rings in the windings. The oil speed in the winding ducts kept within limits so that it must not lead to any electrostatic charging effects.

4. Investigation of Insulation Electrical Strength

The insulation arrangement is determined by all dielectric test requirements and voltage conditions during operation. Tests by power frequency voltages up to 1.5 Um and of one hour duration are intended for checking partial discharge behaviour of insulation under continuous working voltage over a designed life and over voltage applications[1].

For the calculation of transient voltage stresses in windings a special sophisticated computer software has been used. For all specified voltage conditions the stresses have been calculated at each and every location of windings. The withstand capability has been checked and main insulation configuration for the transformer has been designed. The dielectric strength of winding edge insulation is determined by oil spacing between the rounded edge of the static ring and barrier adjacent to the winding. Electrostatic field distribution and stresses have been analyzed by 2D field plots. High voltage connections (1200 KV line lead) outside the windings are also verified by field plots to check electrical clearances. Fig.1 shows the schematic diagram of the transformer windings. It can be seen that the HV winding is split in two parts. The major part is with two group design and minor portion with helical design placed close to the core. The LV winding appears in between the two portions of HV winding. In order to carry out impulse voltage distribution in HV winding, which is considered more critical compared to LV winding, an appropriate number of sections have been chosen. The inductances and capacitances associated with these sections have been calculated and electric network has been derived. The network is solved using in-house developed programme to compute voltage in any portion of the HV winding [2]. Fig.2 shows the impulse voltage distribution in the HV winding of transformer at selected nodes.

Fig. 1: Schematic Diagram of 1200 kV Transformer

Fig. 2 : Impulse Voltage Distribution in HV Winding

Fig. 3: 1200 kV Transformer. Impulse Level=2300 kVp. Maximum Stress close to HV Winding

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Fig. 4 : Power Frequency Field Distribution for 1178 kVrms (Symmentrical Half Winding considered)

5. Short Circuit Electro Dynamic Forces Strength

High fault currents during system short circuits with consequent higher magnitudes of electrodynamic forces in high power UHV transformer required an adequate mechanical design. BHEL already performed full scale short circuit tests on more than 20 nos. high rating power transformers including two designs of 200

MVA 420 KV class single phase generator transformer (forming a bank of 600 MVA). Experience of these tests has given very good base for establishment of proven calculation methodology for short circuit forces. Short circuit withstand capability of 1200 KV transformer is analyzed with the help of computer softwares. The transformer is carefully designed and ampere-turn balancing is done. Epoxy bonded CTC conductor and proof stress copper conductors are used to have mechanical strength, rigidity and stability.

6. Conclusion

UHVAC Transformers are critical part of transmission and therefore needs high reliability. Successful design, manufacturing, testing and installation of this 1200 KV transformer shall establish a milestone for technology development in India in the field of Ultra High Voltage equipments for Ultra High Voltage transmission system and future development of 1200 kV Transformers and Shunt Reactors for commercial installation.

7. Acknowledgement

The authors are thankful to BHEL Management for permission to publish this paper.

Reference 1) LI Guang-fan et al, “Type selection and test technology of

UHV transformers”, IEC/CIGRE UHV Symposium Beijing July 2007.

2) Gupta, S. C. & Singh, B. P., “Determination of impulse voltage distribution in windings of large power transformers”, Electric Power Systems Research, pp. 183-189, 1992.

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