Conversion of AC line into HVDC

Conversion of one circuit of a double circuit 220 kV AC line into 100 kV, 100 MW, 6 pulse monopolar HVDC ground/metallic return while the other circuit of the same line remaining at 220 kV AC was planned in 1983, with a view to develop the in-house HVDC technology. Many options were discussed and finally concluded to go for 100 kV 100 MW, 6 pulse monopolar HVDC ground/metallic return. During the period the focus was to develop in-house HVDC technology & products. The HVDC technology was mature at that time but had an access to only few manufacturers and users of the globe. It was then decided that National HVDC project could be taken up in three stages. Stage-I 100 kV, 100 MW, monopolar, ground/metallic return; Stage-II 200 kV, 200 MW, monopolar, ground/metallic return ; Stage-III +200 kV, 400 MW, bipolar, ground return in-house development in this direction by taking up stage-I of the project, paved the way to commission India's first HVDC line project with the involvement of multiple agencies and funding from the Government of India in October 1989. The project was taken into commercial operation in 1990 after resolving some teething problems. After the success story of development of the stage-1 of the project with analog controls and 55 mm thyristor device, the focus then shifted towards the development of digital controls and thyristor valve with 75 mm device, which could be used for any commercial project, with the rating of up to 1500 MW + 500 kV, bipole and a back to back block of up to 500 MW. Decision on development of stage-2 came in 1993, with a funding proposal from multiple agencies. Stage-II with 200 kV, 200 MW, monopolar, ground/metallic return was successfully commissioned in the year 2000, keeping the other circuit of the line as parallel AC. The experience gained during the conversion of AC line into DC and operational experiences are discussed in this paper