Water and Energy International

India is a vast country with varied geography & topography having wide range of weather conditions across different parts. Ladakh, a region of Jammu and Kashmir (J&K) state in the Northern part of India is geographically and climatically a peculiar region. It is located at an altitude ranging between 2400 m and 4500 m above mean sea level with rugged topography having steep slopes. The ground access to the region is cut off every winter due to avalanche activities along the roads. The Ladakh region is presently not connected to Indian power grid and for meeting the electricity demand, the region is dependent upon local small hydro and DG generation.

Power Grid Corporation of India Ltd (POWERGRID), a Govt. of India Enterprise & the Central Transmission Utility of the country is responsible for planning and coordination of inter-State transmission system and implements transmission system for evacuation of central sector projects & various Independent Power Producers (IPPs) and grid strengthening schemes across India. POWERGRID presently owns & operates approx. 135,000 ckt kms of transmission lines in the country forming National Grid.

As an endeavour to provide electrical connectivity to Ladakh region, POWERGRID has taken up construction of 220 kV transmission line from Srinagar (Alusteng) to Leh via Kargil, Drass and Khaslti. The transmission line is approx. 350 km long passing through difficult mountainous terrain having high altitude and extreme climatic conditions (heavy winds, snow & temperature going below - 45^OC). The line traverses through two major mountain ranges (viz. Great Himalyas & Zanskar) encountering some of the high altitude passes (viz. Zozi-la -3529 m & Fatu-la – 4049 m) and heavy snow & avalanche prone areas (Gagangir, Zozi-la, Gumri, with more than 5 m standing snow). The transmission line also passes through high security, military zones close to LOC with Pakistan. The construction of the line is in progress.

Appropriate selection of transmission line components, design parameters & developing detailed designs has been as challenging as construction of transmission line. The line route was selected using satellite imagery. Due to inaccebility in some places along the route, digital terrain modelling & fly through techniques have been used to identify most suitable route.

Special features have been considered in design of conductors, OPGW, towers and foundations, insulators & hardwares. As the line passes through the highest wind zone of the country (basic wind speed of 55 m/sec) and heavy snow zone areas, towers has been designed for the high wind speed & ice loading (25 mm radial ice deposition on conductor and earthwire). Tower geometry has been configured for additional electrical clearances due to high altitude and cross arm of towers have been specially designed and staggered to take care of conductor galloping. Special kind of steel for towers has been used to mitigiate the effect of low temperature. High strength conductor & OPGW has been considered instead of conventional conductors used in normal 220 kV lines. In addition to this, high strength OPGW and double strength insulator strings with additional insulators have been considered.

All efforts has been made to select towers locations free of snow avalanches, despite, some locations between Minamarg and Zoji La are falling in snow avalanches prone area which may be subjected to impact of fast moving snow alongwith strong wind. Besides envisaged use of general avalanche protection control measures (such as snowrakes, deflecting and retarding structures), foundations of the towers which are to be constructed in these avalanches prone area are designed to take care of snow impact loading alongwith creeping and gliding of snowpack. Special type of chimneys of about 10 m height are designed for these towers to withstand the avalanche load and to provide sufficient height to cables above ground level to avoid avalanche impact. To protect the high raised chimney foundation from snow glide and snow avalanches forces, tiebeamsat two intermediate levels have been provided. These tiebeams have been securely tied with chimney with sufficient reinforcement anchorage to make a perfect monolithic structure. The paper covers in detail, salient design aspects and special design features of the under construction 220 kV Srinagar-Leh transmission line in high altitude, extreme weather, snow bound areas including analysis & design of foundations for lattice transmission towers in avalanche prone areas.