Nepal's NEA studies a bypass line to free 176 MW stranded by a lost substation

What happened

The Trishuli-3B hub substation stood at Pairebesi in Kispang Rural Municipality, Nuwakot. It worked at three voltages: 220, 132 and 33 kV. It received power from the Chilime hub substation in Rasuwa over a 28 km, 220 kV double-circuit line. From there, power travelled on to the Matatirtha substation in Kathmandu. When the flood washed the hub away, the whole chain broke. According to the Post, the flood stopped 256 MW of operating plants in the area.

Some plants were destroyed. Others were not touched by the water but still cannot send out power. On the Chilime-Trishuli 220 kV system these are the 78 MW Salasungi (Sanjen), 42.5 MW Sanjen, 14.8 MW Upper Sanjen, 14.8 MW Upper Mailung and 5 MW Mailung Khola projects. Three more plants on the Tadi Khola corridor, with 21 MW together, are also cut off. Some plants now run in isolated mode and supply only nearby settlements.

The NEA asked its subsidiary, NEA Engineering Company, to study a direct line from the Chilime hub to Matatirtha. Ratopati English reported on 9 October that the terms of reference give the company two months for all technical study and design. The NEA aims to restore the grid connection within six months. Acting managing director Dirghayu Kumar Shrestha told the Post that even half of these plants' output helps the Kathmandu Valley through the dry season.

The engineering behind it

A hub substation is a meeting point. Several lines arrive, transformers change the voltage, and switchgear sends power onward. Planners build hubs so that many plants can share one large line to the main load centre. This saves money, but it concentrates risk. If the hub fails, every plant behind it loses its route, even when the plants are fine. Engineers call this a single point of failure. A bypass line removes the lost point by joining the two remaining ends directly.

One proposal in the Post report is a double-circuit line, with one circuit at 220 kV and the other at 132 kV. A double-circuit line carries two separate three-phase circuits on the same towers. Two voltages on one line is less common, but it fits this case, because the Samundratar area already runs at 132 kV. In general, a design like this needs careful spacing between conductors and the right insulators for each voltage.

Towers are the second half of the problem. Chirantan Rana, chief executive of NEA Engineering Company, gave preliminary counts to the Post. He listed 22 damaged towers on the 220 kV route from the Trishuli-3B hub to Matatirtha and 15 on the 132 kV line from Samundratar. He also counted 24 on the 66 kV Chilime-Trishuli line and 4 on the 66 kV Trishuli-Balaju line. He said these numbers may change, because damaged roads limit the survey.

Every tower stands on foundations designed for its soil, slope and wind load. A river that moves its banks can wash out that ground, so the new route must avoid the places where the flood did its damage. The final report from the company is expected to give exact routes and tower positions. Only then can construction start.

What it means in Nepal

Ratopati English put the preliminary loss in the electricity sector at Rs 74.23 billion. Of this, Rs 33.47 billion is on structures owned by the NEA and Rs 40.76 billion on structures of its subsidiary companies. About 300 MW of capacity is fully damaged and may take one to two years to rebuild. About 150 MW could return once a transmission route is repaired. This is why the bypass matters. It can bring back power from plants that already exist, long before the destroyed plants are rebuilt.

The work is not only electrical. The NEA has asked the government for a fast-track supplementary environmental impact assessment (EIA), for permission to cut trees in forest areas, and for land acquisition for the new route. Ratopati English described the EIA as the largest obstacle. Machines are still hard to move in the flood area. A transmission project depends on approvals, survey access and land as much as on circuit design.

The timing is hard. The plants on this corridor usually supply the Kathmandu Valley in winter, the Post noted, so each month without a line makes winter grid management more difficult. Meanwhile, the companies that own the stranded plants earn nothing while the line is down. Studies are also under way on whether the destroyed powerhouses can be rebuilt on the same sites.