The Power System in India has a Huge Shortage of Energy. The margin between demand and supply is increasing as the annual growth in demand continues to rise at 8% per annum. The installed generation capacity has increased from 1362 MW in 1947 to 132,330 MW in 2007, and steps are being taken to achieve a plan of electricity for all by 2012. This allows for opening the market to private participation in generation, transmission and distribution, and seeking increased energy imports from neighboring countries.

Under consideration, a major step toward solving the energy-deficit problem is the formation of the South Asian Association for Regional Cooperation (SAARC) power grid, designed to strengthen regional cooperation on energy with neighboring countries, namely Pakistan, Bangladesh, Nepal and Bhutan. Joint-venture generation projects undertaken by the Royal Government of Bhutan and the Government of India include the 1020-MW Tala Hydroelectric Plant (THEP) and the 336-MW Chukha Hydroelectric Plant. These two projects have the potential to export surplus energy to regions of India that have an energy deficit.

BHUTAN TO INDIA TRANSMISSION LINES

THEP is a run-of-the-river-scheme hydro plant in Bhutan equipped with six 170-MW generators and an underground powerhouse that accommodates 12-MVA to 70-MVA 13.8/400-kV transformers, 12 bays of a 420-kV gas-insulated substation (GIS) and a bus reactor (63 MVAR). The design work, completed by WAPCOS (India) Ltd. in association with the Central Electricity Authority (India), included a direct connection from THEP to the Indian grid at 400 kV with a 220-kV voltage step-down for connection to the Bhutan grid.

The results of the geological survey found it was not possible to site a 400-kV substation at the THEP site. This resulted in a design change and created the need to establish Bhutan's first 400/220-kV interconnecting substation equipped with air-insulated devices, four 66-MVA 400/220-kV transformers at Malbasse some 90 km (56 miles) by road from THEP. This substation has transmission-line interconnections to the Indian (400-kV and 220-kV) and Bhutan (220-kV) grid systems.

TRANSMISSION-LINE DESIGN

The transmission system interconnections from THEP comprised two 400-kV double-circuit lines: the 141-km (88-mile) Tala-Khogla-Siliguri circuit and the 147-km (91-mile) Tala-Pagli-Siliguri strung with twin Moose aluminium conductor with steel reinforcement. Tala Hydroelectric Project Authority (THPA) was responsible for the construction of THEP and the 400-kV transmission lines in Bhutan, and the Power Grid Corporation of India Ltd. was responsible for the construction of the 400-kV transmission line from the Indian border to the 400/220-kV Siliguri Substation.

The design and route planning for the 400-kV circuits erected in Bhutan's Himalayan terrain, which is prone to landslides, was a demanding challenge. This formed the basis of a two-year turnkey contract awarded to Indian firm Larsen & Toubro Ltd. The 400-kV transmission-line towers are designed in accordance with IS:802-1995 (part 1, section 1) using the relevant climatic and isokeraunic conditions. The standard technical parameters were adopted for the towers and accessories, conductor, ground wire/optical ground wire and insulators.

The route in the Himalayan terrain resulted in span lengths in excess of the 400 m (1312 ft) used in normal Indian terrain, so it was necessary to design and subject these double-circuit towers to testing at India's test facility. As a result of the tests and spotting requirement at the site, it was necessary to strengthen or add extra members to the crossarms on the lattice steel tower structures.

The average span length was more than 500 m (1640 ft), but many spans were in the range of 900 m to 1100 m (2953 ft to 3609 ft). The longest span was 1412 m (4633 ft), but because of the hilly terrain only a 9-m (30-ft) increase in tower height was required to accommodate the excessive conductor sag when operating at 75°C (167°F). The terrain and reserve forest areas in Bhutan encountered along the route of the 400-kV transmission lines presented the construction teams with a demanding challenge.

THE MALBASSE SUBSTATION

Due to Bhutan's mountainous terrain, it proved difficult to obtain a level site suitable to accommodate the 400-kV Malbasse Substation, which has five operational voltages: 400 kV, 220 kV, 66 kV, 33 kV and 11 kV. Therefore, the bays with different voltage levels are accommodated on two levels.

The upper level is occupied by the 400-kV equipment and sections of the 220-kV equipment, including the 400/220-kV transformers. The remainder of the 220-kV switchyard and the lower-voltage switchbays and control room are located on the lower level. The top floor of the three-story control room is at the same level as the 400-kV switchyard. This floor contains the control and protection for the 400-kV, 220-kV and 66-kV systems, including the power-line carrier communication panels and the remote tap-changer cubicle for the substation transformers.

The 400-kV switchyard is designed on the basis of a circuit breaker and a half, and the 220-kV level is a standard double-bus bar arrangement with a bus coupler. The transformers, four single-phase 66.67-MVA 400/220/33-kV units, offer the opportunity to use a spare unit, without the need to physically move units, in the event of a single-phase transformer failure.

System earthing proved to be a major problem due to the high soil resistivity and lack of site space, so the earth mat is designed for a soil resistivity of 1200 Ω-m and an assumed fault current of 40 kA (1 sec). These resistivity and fault-level values made it almost impossible to limit the step and touch voltages to acceptable limits.

Ultimately, the decision was made to consider this fault current value to determine the cross-section of the earth mat. The step and touch voltages have been determined on the basis of a 10-kA fault current, the value expected to flow to earth following an earth fault after due consideration of the distribution of the earth fault current throughout the network.

In view of the high soil resistivity and space limitations, it was not possible to reduce the ground resistance below 1 Ω. With the installation of a satellite grid comprising concrete-encased electrodes on the 400-kV entry point and the 220-kV line entry position, the ground resistance was reduced to about 1.3 Ω and the ground potential rise to 13 kV. Additionally, auxiliary mats were installed 0.3 m (1 ft) below ground level at the position where operating personnel stand to operate isolators and earth switches. Finally, the ground surface is covered by gravel and the whole area treated with weed killer, a design feature known to improve the safety of operational staff.

The substation is protected from direct lightning strikes by overhead earth wires, and the substation equipment is protected from the effects of traveling waves by the installation of metal oxide, gapless-type surge arresters.

The turnkey contract for substation construction specified completion within a period of 24 months. Awarded to one of India's leading contractors, Crompton Greaves Ltd. supplied most of the equipment.

CONTROL AND PROTECTION

The ABB-manufactured control and protection system was installed to minimize the risk of damage to the transmission lines, autotransformers and bus bars in the event of system faults and abnormal conditions. The design and construction of this project coincided with the advent of the new generation of relays conforming to the IEC 61850 protocol.

Following due consideration and the apprehension associated with the reliability, performance, cost and familiarity of the users with the new-generation numerical relays, the decision was made to install a combination of static and numerical relays with a conventional stand-alone event logger (able to record 512 events) and time synchronizing equipment.