Georg Woeber* describes the refurbishment work taking place at one of Thailand’s most famous hydro power plants

Named after King Bhumibol, and claimed as one of the highest in Southeast Asia, it is not surprising that Bhumibol, 480km north of Bangkok, is one of the most well-known dams in Thailand. Construction of the power station and dam started in 1952 and was completed in 1964, with the facility’s namesake presiding over the inaugural ceremony.

The power station consists of eight generators and has an installed power capacity of 736MW — almost one third of Thailand’s total hydro capacity. The reservoir has a maximum storage capacity of 12,400M m3, with an active capacity of 8600M m3. Dam height is nearly 150m and it is 600m long, so a visit to Bhumibol is a unique experience.

An uprating experience

Uprating and refurbishing hydro plants has been of considerable interest for plant owners in Thailand since the late 1980s. The Electricity Generating Authority of Thailand (EGAT), which owns Bhumibol, decided then that it should renovate units 1 and 2. Contracts were awarded to Elin Energieversorgung of Austria for generators and power plant equipment, and to Voest Alpine Machinery and Construction (VA-MCE) for turbines and auxiliary equipment.

In 1995, a follow-up order was received by the same companies for the renovation of units 3 and 4. Scope of supply covered the same equipment as for units 1 and 2, but with the additional supply of the station service system. After successful completion of the project at units 3 and 4 EGAT decided to finish the upgrade by refurbishing units 5 and 6.

The evaluation of the upgrade of units 5 and 6 was carried out in January 1998 — after the beginning of the financial crisis in Southeast Asia. But even after the worst phase of the crisis, in 1997, when the Thai exchange rate against the US dollar dropped from 25:1 to 41:1, the investment was still deemed to be profitable. EGAT decided to use its own money to finance the entire scheme.

Work at units 5 and 6 will be similar to that at units 3 and 4. It includes uprating the generators from 73.6MVA to 80.3MVA — an increase of around 9%. The generators will meet guarantees of output and a guaranteed efficiency of 98.77%. Other work includes:

•Replacing the active parts of the generator and the generator bus ducts.

•Incorporating a new SF6 generator circuit breaker.

•Providing new digital excitation and digital protection.

•Introducing low voltage power cables for the power plant equipment.

•Modernising the existing plant control system to be fully computerised.

For units 1 and 2 only the low voltage main distribution board was replaced, but for units 5 and 6 the low voltage sub-distribution boards will also be replaced.

Work started in February 1998, since then Elin has concentrated on electrical and mechanical design. The company will perform acceptance tests for the first unit in the presence of EGAT in November.

For the installation, shutdowns of unit 5 and unit 6 are scheduled for May 1999 and March 2000, respectively. All installation work will be performed by EGAT under Elin’s supervision and work is scheduled to last for about eight months, from shutdown until the restart of commercial operation. During this period one complete unit will be disassembled, the new equipment installed, and dry and wet testing of the unit will be completed.

EGAT will remove the old equipment in the power plant, make the modifications to the foundation and install the new equipment to Elin’s design. All cables will be replaced by new ones and new cable trays will be installed. EGAT will perform the installation and test the wiring.

In the power plant, the generator will be modified, under Elin’s supervision, as follows. After the old stator winding and stator core have been removed, the stator frame will be modified to the new design. The new dove tail bars will be adjusted and welded onto the stator frame. The lower clamping plates will be screwed onto the stator frame and the stator lamination sheets will be stacked with intermediate pressing. The upper clamping plate will be mounted and the core pressing bolts tightened. To ensure uniform pressure on the stator core the bolts will be elongated with hydraulic cylinders. After the core induction test the stator winding will be inserted using Elin’s elastic winding embedding system. The bars will be connected by brazing and high voltage tests will be performed.

In parallel to the work at the stator, EGAT will change the pole winding by reusing the existing pole bodies. Simultaneously the pole body insulation and insulation frames will be renewed.

VA-MCE, supplier of the turbine, will design a new runner based on the amended rated head from 100m to 96m. A model test of this new design was performed successfully in June 1998.

The new digital excitation system to be installed at units 5 and 6 is the Elin Thyne 5 system, a full static shunt field excitation system with a two-channel digital automatic voltage regulator. In order to protect the generator and main transformer, Elin will install a DRS digital protection system.

EGAT’s specific technical requirements for the equipment, as well as the shared responsibility during different project phases, will be a challenge for the project management team. However it is anticipated that all parties will work well with each other. In fact, during previous contracts for renovation of units 1-4 EGAT engineers took a special training course on generator assembly at the Elin factory in Austria, as well as advanced training on the computer system which allows EGAT to set up the site facilities.