THE KINGDOM OF BHUTAN IS LOCATED ON THE SOUTHERN SLOPES OF THE EASTERN HIMALAYAS. It is bordered by China in the north and the three Indian states of Sikkim, Assam and Arunachal Pradesh. By virtue of its geographical location with elevations ranging from 150 m (490 ft) to more than 7500 m (24,500 ft), Bhutan has a substantial annual precipitation and abundant water resources. Until the 1960s, the water resources were solely used for drinking, land irrigation, providing mechanical power for cereal grinding and, in ancient times, the turning of prayer wheels.

Global technological advances in the development of hydropower led Bhutan to realize that these water resources could be harnessed for immense socioeconomic development. The hydropower generation would not only satisfy Bhutan's internal use, but it would also allow the country to earn revenue from energy sales to neighboring countries. The first hydropower plant (360 kW) was built and commissioned in 1967 across the River Samtelingchhu in Thimphu Valley, but to date, only less than 2% (342 MW) of the theoretical hydro potential has been harnessed in Bhutan. Hence, a master plan based on a topography study identified 89 sites having a potential installed capacity of 12,000 MW. The sites were mainly along the major river basins, with a few reservoir schemes near Bhutan's southern border. The largest identified storage scheme of some 2800 MW was located near the exit of the River Manas.

ENERGY POTENTIAL DEVELOPMENT

Bhutan has a population of about 600,000 spread over 46,500 sq km (17,950 sq miles), and so far, only a small percentage of the identified hydropower potential density has been harnessed. Chukha Hydropower Plant (336 MW) is the largest generation plant in operation, although the hydropower plant at Tala (1020 MW) in the southern part of Western Bhutan is nearing completion. With these two plants located in Western Bhutan, a large sector of Eastern Bhutan was practically devoid of electric power until the early 1990s. Some towns in Eastern Bhutan — such as Kilikhar, Trashigang and Pemagatshel — were supplied with power from local mini-hydro schemes, and some areas adjacent to the Indian border such as Gelephu were fed power that was imported from the Indian power grid.

The long-overdue need to boost economic and social standards in Eastern Bhutan resulted in the decision to construct a run-of-the-river hydropower plant at the River Kurichu with a capacity of four 15-MW units. Commissioned in November 2001, this first major plant in Eastern Bhutan was envisaged to satisfy the power requirements of the area and earn substantial revenue from the export of surplus power to India.

To efficiently use the power generated at the Kurichu Hydropower Plant for the development of the eastern region of Bhutan and to establish a transmission system capable of exporting surplus power to India, a 132-kV transmission system with a local 33-kV feeder network was finally selected based on detailed power-system studies. Load forecasts, spatial distribution over different periods of time, system load flows, transient stability and short-circuit studies were conducted by the Water & Power Consultancy Services (India) Ltd. (WAPCOS) — a leading government of India consultancy enterprise — following a request from the royal government of Bhutan (RGoB).

The Kurichu Hydropower Project, including the switchyard at Kurichu and the first 132-kV line section of Kurichu-Nangkhor-Nangalam, was constructed by the Kurichu Power Authority, established by the RgoB with the help of the National Hydroelectric Power Corp. (NHPC) Ltd. The remaining associated transmission system was executed by the Department of Power (DOP) of RGoB using the expert design, engineering and construction agencies previously mentioned (WAPCOS, NHPC and DOP).

During the latter stages of the project, a new public enterprise, the Bhutan Power Corp. (BPC), was established to handle all works relating to the development of power supplies and distribution in Bhutan, responsibilities formerly executed by the DOP.

While the design, engineering and supervision of the various activities were undertaken by WAPCOS or NHPC, reputable contractors were responsible for construction of the transmission lines and substations.

132-KV SYSTEM FEATURES

The design specification of the 132-kV transmission lines used standards applicable in India as a basis (Table 2). Detailed consideration was given to the topography and geographical features of Bhutan. For example, horizontal and vertical conductor clearances were increased to allow for the combination of deep valleys that can create high-tunneled wind speeds and conductor motions caused by ice shedding. The design loadings on transmission line components were specified in accordance with the wind-speed zones adopted in India, and temperature limits were specified after due consideration of Bhutan's geographical location and meteorological conditions.

The six 132-kV substations were constructed to distribute the power generated at the Kurichu Hydroelectric Project in the eastern region of Bhutan. All of these substations were equipped with two 132-kV/33-kV transformers with ratings of 3 MVA or 5 MVA, the capacity being determined from long-term load projections. In addition to the new substations, the existing 66-kV substation at Gelephu was upgraded to 132 kV to supply the Gelephu area and to facilitate interconnection to the 132-kV substation at Salakati (Bongaigaon, Assam) in India. This interconnection is designed as a two-way power flow circuit. It will be used to transmit the surplus power from Kurichu Power Station to the Indian grid, a distance of nearly 300 km (186 miles).

Substation sites were selected on the basis of the most favorable parameters such as nearness to load-growth centers. Sites were also considered for optimum position with respect to the 132-kV transmission system. The need for local infrastructure, such as roads for construction equipment, to the substation sites also had to be considered.

Each substation was equipped with 132-kV/33-kV transformers and 33-kV/11-kV level main and transfer bus bars. Under this scheme, one feeder at a time — without regard to voltage level — can be taken on the transfer bus for maintenance without interruption of power supply.

The 3-MVA, 5-MVA and 7-MVA 132-kV/33-kV transformers are ONAN (oil natural air natural, referring to circulation method) rated. Future rating enhancement can be obtained by provision of ONAF (forced air) facilities whenever needed.

Ground-mat design is for high soil resistivity, although this could not be measured at most sites due to monsoon conditions. Soil resistivity for various substations was calculated based on the available data resulting in design values that ranged from 400 ohm-m to 3500 ohm-m. As the ground resistivity on some sites is fairly high, special efforts had to be made to provide a suitable grounding system design even though the fault levels at most of the substations were fairly low.

The lightning protection at each switchyard is provided by a shield wire composed of seven 3-mm (0.118-inch) strands, which is tied between the switchyard towers. The angle of protection adopted is 60 degrees internally and 45 degrees externally. Additional lighting masts have been provided if and where required. The protection of the switchyard equipment against indirect strokes, traveling waves and surges is taken care of by the lightning arrestor being duly coordinated with the equipment insulation.

SPECIAL CHALLENGES

The planning, design, engineering and construction of the transmission lines and substations that comprise the Eastern Bhutan transmission grid posed many special problems. Some were related to the nonavailability of reliable data on load growth, weather conditions, soil resistivity, soil parameters/condition and firm future plans of development of generation/transmission in the Eastern region of Bhutan. Sufficient margins of variation were therefore incorporated in the estimation of load forecasts, transformer capacities and the development of the local power networks.

The design and engineering of the transmission lines and substations posed an even larger challenge due to the need to minimize the total project cost while simultaneously ensuring a fault-free performance under somewhat uncertain conditions. The instability of excavated soil during the digging and preparation of substation sites and transmission line foundations was mainly due to the geological nature of the soil and rock strata of the Himalayas, which are considered to be comparatively young. There was also difficulty in locating towers in some of the steep valleys due to highly undulating terrain, and special designs of grounding systems at some substation sites were required due to high earth resistivity. Special attention was also given to designs of protection systems due to very low fault levels and a weak network

The project engineering teams used technical and economic solutions to solve all of these problems. Local contractors and labor were used to construct the Eastern Bhutan grid where possible and the system commissioned within a 36-month period. The overall cost of this project was the equivalent of US$26 million.

The Eastern Bhutan transmission grid has been operating without any major problems for the past three years, satisfying the project's twin objectives to distribute the energy produced by Kurichu Hydropower Plant to supply the energy requirements of Eastern Bhutan and transmit surplus energy to the Indian power grid via the 132-kV Salakati Substation.

---

R.N. Ray earned a BSEE degree and a post-graduate diploma from the Operational Research Society of India. Ray was appointed Water & Power Consultancy Services (India) Ltd.'s deputy chief engineer in 1989 and was promoted to chief engineer and general manager (power) in 2001. He is responsible for the planning, design and engineering, co-ordination and monitoring of consultancy services for the transmission lines and hydropower projects in India and abroad. wapcos@vsnl.com

Inder Singh is general manager (electrical) of Water & Power Consultancy Services (India) Ltd., where he has worked since 1993. Singh gained extensive experience in the operation and maintenance of thermal power plants in India, Libya and Zimbabwe while employed by the Central Electricity Authority and the Electricity Supply Commission (Libya). He was awarded a BSME degree from Birla Institute & Technology and a post-graduate diploma in industrial engineering at the IIT, Delhi. wapbcp@yahoo.co.in

D.V.S.N. Raju received a BSEE degree from the Regional Institute of Technology Jamshedpur, Ranchi University, and joined Water & Power Consultancy Services (India) Ltd. (WAPCOS) 12 years ago, following appointments with Balaji Gears (Pvt.) Ltd., Hyderabad and the Central Electricity Authority. During his career at WAPCOS, Raju has held a series of appointments linked to design studies, specifications and contracts for hydropower plants. He currently serves as deputy chief engineer and is responsible for the management of extra-high-voltage and high-voltage transmission line projects in Bhutan, Afghanistan and Zimbabwe. dndraju@yahoo.com

V.N. Rikh holds a BE (electrical) degree and a Ph.D. in engineering and technology. He worked with the U.P. State Electricity Board, India's largest Public Sector Power Utility, for some 36 years, retiring as its chairman. Rikh is a fellow of the Institution of Electrical Engineers (London) and a fellow of the Institution of Engineers (India), and has published more than 110 technical papers. For his contribution to electrical engineering, he was awarded the President of India Prize in 1968, the CBI&P Diamond Jubilee Award in 1991 and the Lifetime Achievement Award by the Meerut Engineers Association in 2005. vnrikh@yahoo.com

Table 1. Main features of 132-kV substations in Eastern Bhutan.

No.	Substation name	Power transformers		Number of bays
		132/33 kV	33/11 kV	132 kV	33 kV	11 kV
1	Kurichu HEP Swichyard	—	—	8	—	4
2	Tintibi	2 × 3 MVA	2 × 1.5 MVA	6	7	4
3	Deothang	2 × 5 MVA	2 × 5 MVA	4	7	4
4	Nangkor	2 × 5 MVA	2 × 5 MVA	6	7	4
5	Nanglam	2 × 3 MVA	1 × 5 MVA	5	7	4
6	Kanglung	2 × 5 MVA	2 × 5 MVA	4	7	4
7	Kilikhar	2 × 5 MVA	2 × 5 MVA	5	7	4

Table 2. Key parameters of 132-kV transmission line.

Transmission line component	Specification details
Tower structures	Self-supporting, latticed barrel-type mild steel towers
Overhead line conductors	Panther 30/7 ACSR, each strand is 3 mm (0.118 inch)
Ground wire	One 7#9 AWG galvanized steel wire installed at tower top
Ground wire shield angle	30 degrees
Conductor-phase configuration	Right-angle triangle — two lower conductors at same height
Minimum ground clearance	6.10 m (20 ft)
Minimum clearance between power lines	66 kV- 3.05 m (10 ft); 132 kV — 3.05 m (10 ft)
Wind velocity/pressures	Basic wind speed 3 sec average; 47 m/sec (105 mph)Average wind speed — 10 min value; 34.18 m/sec (76 mph)
Conductor/ground wire tension	Maximum 70% ultimate tensile strength
Temperature limits	Minimum — 0°C (32°F); Everyday — 32°C (90°F); Maximum — 40°C (104°F)
Maximum conductor temperature	75°C (167°F)
Normal design span	325 m (1060 ft)
132-kV insulator strings	9 — Standard glazed porcelain (suspension) 10 — Standard glazed porcelain (tension) All insulators 225 mm by 145 mm (8.9 inches by 5.7 inches)