Power for processing

The Larona river provides most

of the energy consumed at

the PT Inco processing facility

on the Indonesian island of Sulawesi. The catchment receives plenty of rainfall and there is natural storage in

the headwaters of Lake Matano and

Lake Mahalona. A third lake, Lake Towuti, was regulated by the construction of Batubesi dam in 1979 and a

long canal and surface penstocks

were built to supply the 167MW Larona power house. In the 22km reach between Batubesi dam and the coastal delta,

the Larona river falls more than

300m. Much of this reach lies in a

steep-sided gorge.

Nickel production has increased over

the years and PT Inco has already added oil-fired and diesel generation to supplement the energy supply to the large arc furnaces. However, when a large expansion of the mine and processing plant got under way in 1994, it

was decided to develop additional

power from a new dam and hydroelectric generating station on the Larona river. Pacific Rim Power (PRP), a joint venture of SNC-Lavalin and Klohn-Crippen Consultants, based in Vancouver, Canada, was retained to design these facilities.

System load

The type of development was strongly influenced by the unique characteristics

of the local electrical system. The

system is isolated and large arc

furnaces dominate the electrical load. Each furnace consumes

over 60MW and uses three large suspended electrodes

to generate arcs to the molten nickel below.

The long power canal

and penstocks limit the ability of Larona power house to respond to load changes, so in the past the system frequency usually varied between 40Hz and 60Hz every few minutes.

The system will now be stabilised by giving the new installation two key characteristics:

• A responsive generation system with short penstocks.

• The flywheel effect of high inertia generators.

Reconnaissance

The owner and consultant made a joint reconnaissance of the steep, jungle-covered river valley downstream of the Larona power house, and identified several potential dam sites in the river gorge. The pre-feasibility study found that developing two of these sites would capture the full hydroelectric potential of the river with minimal flooding, no displacement of population and little impact on existing infrastructure. One of these dam sites was near the midpoint of the river reach, close to the village of Balambano. The other site was at the lower end of the river gorge, near the village of Karebe. Development at the latter site is planned for a future date.

The feasibility study began with site investigations, preliminary design, detailed layout of the facilities and estimation of costs. Preliminary results of the study were favourable and only four months into the work, PT Inco decided to advance the work schedule. While work on the study continued, PRP prepared tender documents for preliminary site work and for supply and installation of long lead equipment items.

Contract organisation

Work on procurement of contracts started in April 1995 with the prequalification of contractors for supply and installation of the main generating units and the preliminary civil works package. Work proceeded concurrently with preparation of the tender documents.

This was the first stage of a project implementation programme that would ultimately involve seven contract packages of significant size:

•Preliminary contract (access roads, a river crossing and diversion tunnel).

•Turbines, governors, generators, exciters and power house crane.

•Main civil contract (including the penstocks).

•Mechanical and electrical power house auxiliaries (including the generator transformers).

•Switchyard and transmission line.

•Hydraulic gates and gantries.

•Overall hydro power control system.

Construction started at the site in October 1995 and was managed by PT Fluor Daniel Indonesia. PRP was responsible for technical management.

Project area

The project is located in South Sulawesi, near the town of Malili and the port of Balantang on the northeast shore of the Gulf of Bone. A paved main road connects the port to the process plant and the town of Soroako on the shores of Lake Matano. The dam is only 1km from this main road.

The soil is poor in the project area but the mean annual rainfall is high, so the jungle is dense with indigenous trees and the undergrowth is difficult to penetrate because of plants like rattan and

vine bamboo.

The Larona river valley has steep sides, so there are no settlements except the village of Balambano (which overlooks the river valley) and Karebe, which lies on the Malili river immediately downstream of the confluence of the Larona and Pongkeru rivers.

The water in Lake Towuti is extremely soft and soft water attack is evident on limestone aggregates at Batubesi dam and on the Larona power canal. Since the same aggregate was selected for all conventional concrete at Balambano, the concrete mixes for the water passages incorporate fly ash to resist soft

water attack.

The Larona river is almost devoid of fish life, which is probably due to the water chemistry and the steep bed gradient. The steep-sided valley results in a reservoir that is narrow (generally less than 300m wide) and the inundated area is only 115ha. The large regulated storage of Lake Towuti means that Balambano only needs a 1.5m operating range for balancing storage. These factors result in a project with an unusually low environmental impact.

Geology

The country rock is peridotite, an ultrabasic igneous rock, highly jointed near Balambano with clay gouge, talc and some serpentine. This formation is overlain in areas by conglomerate, and there is some limestone in the surrounding hills which reduces the water softness between the Balambano and Karebe sites.

The topography is fairly young, and slumps and slides are common on the natural slopes and in road cuts after high rainfalls. The lateritic overburden is shallow and there is no significant potential for landslides to generate displacement waves in the reservoir.

There are shears in the dam foundation, but these features have limited continuity. The region lies on the rim of the Pacific Ocean, in the seismically active ‘Ring of Fire’. Both regional and fault-specific seismic studies were carried out and the controlling source was identified as a potentially active fault, some 7km from Balambano dam. This could produce a maximum credible earthquake (MCE) with a peak ground acceleration of 0.6g at the site (this value was used when designing the dam). The dam structure amplifies the ground motion, and equipment near the dam crest would experience a peak acceleration of 1.78g during an MCE event.

Site investigation involved exploratory drilling, down-hole testing, test pitting, excavation of exploratory adits, plate bearing tests, flat jack tests and a direct shear test, geological mapping, environmental studies, topographic survey and precise levelling.

Hydrology and floods

The Larona river drains an area of some 2800km2 in South Sulawesi. The upper basin has three lakes in a cascade. These are: Lake Matano at el390m; Lake Mahalona at el325m; and Lake Towuti at el320m. The average annual rainfall varies from 2700mm to 3000mm, with larger rainfall on the higher catchment elevations.

There was no river gauging at the Balambano site, but the flow is generally dominated by the regulated releases from Batubesi spillway and Larona power house. These releases have been recorded daily since 1979.

Diversion floods were estimated for the ungauged sub-catchment between Batubesi dam and Balambano site by frequency analysis of rainfall records and catchment modelling. A regulation rule curve was defined for Lake Towuti storage to control contributions to the flood from Batubesi dam during construction of Balambano. The tunnel was designed for a discharge of 500m3/sec, but the 1998-99 El Niño produced a drought and it was never used to capacity.

The probable maximum precipitation (PMP) for the project was estimated by the Hershfield statistical procedure from 38 years of recorded maximum daily rainfalls. The estimated PMP has a 12-hour duration and rainfall depth of 493mm. The flood hydrograph resulting from this PMP was added to a Larona river base flow of 20m3/sec and the 1:1000-year storm spillage from Batubesi dam to yield a probable maximum flood (PMF) with a 4450m3/sec peak discharge into Balambano reservoir. There is little attenuation of this flood when it is routed through the limited storage of Balambano reservoir.

Plant characteristics

Power studies were carried out on the Larona river system using the SNC-Lavalin program, PROSPER. The installed capacity was optimised for various combinations of hydro development at Balambano, Karebe and Lake Matano sites, together with the existing Batubesi dam and Larona

power house.

Balambano was eventually sized as a stand-alone addition to the system. It operates in cascade with Larona power house and has an installed capacity of 137MW, from two units.

The reservoir level at the Balambano site was selected to reach the tailrace at Larona power house, with an allowance for flood surcharge. The future Karebe reservoir will reach the tailrace of Balambano power house, and complete the development of the full hydroelectric capacity of the Larona river.

The layout at Balambano is conventional. A surface power house was located on the flatter left bank of the river, close to the dam to keep the penstocks short. The switchyard was located on a natural river terrace about 1km downstream of the dam.

River diversion

A tunnel was used for river diversion, because the valley bottom was too narrow for surface works. This was excavated in the right bank of the river, which was shorter than the left bank alternative and avoided the power house work area.

Estimated unit costs for tunnel excavation, concrete lining and cofferdam construction were used for hydro-optimisation of the tunnel size. This resulted in a concrete-lined tunnel with a 6.4m finished diameter, set below the grade of the river.

The tunnel inlet was equipped with an efficient bellmouth intake and a gate for impounding the reservoir. Permanent closure is provided by a concrete plug which was installed on the dam sealing plane on completion of the project.

Upstream of the plug location, the tunnel has a circular cross section and is reinforced for full reservoir head. Downstream of the plug, the tunnel is D-shaped and the concrete lining is drained and only designed for nominal external water pressure.

The river was diverted by building the downstream cofferdam and using this crossing for access to the upstream cofferdam site. An RCC cofferdam was built upstream of the dam site, behind a temporary fill cofferdam.

RCC dam

Balambano dam was built under a single main civil contract. The site topography and foundations were suitable for either a rockfill dam or a concrete gravity structure. The gravity dam was found to have a lower cost and a shorter construction schedule because of the shorter diversion tunnel, the mass production techniques used for roller compacted concrete (RCC), and the ability to incorporate the spillway and power intake in the dam body.

The maximum height of the dam is 99.5m (measured from the dam crest el167.0m to the lowest point in the foundation, el67.5m). The crest length is 351.6m. The design crest width was 6m, but this was increased to 8m on the left bank when it was decided to prefabricate the spillway radial gate leaves, and install them with a 150t mobile crane. The dam has a vertical upstream face and a stepped downstream face that forms a 1V:0.8H slope.

The tender design of the RCC mix was based on testing at the US Corps of Engineers’ laboratory in Troutdale. Aggregate, cement and water were shipped from Indonesia for the trial mixes. Stresses in the RCC dam were calculated from dynamic and pseudo-static analysis of the dam structure and time-dependent thermal analysis. The RCC must resist stresses due to static and dynamic load and thermal contraction. Increasing the cement content of the mix increases the compressive and tensile strength of the material, but it also raises the peak temperature developed in the body of the dam. The design called for a compressive strength of 13.7MPa at one year. To prevent premature tensile or shear failure at bedding planes, a mortar bedding mix was applied to 20% of

the upstream and downstream RCC

lift surface.

The contract required further testing of RCC mixes at the site, and a lean mix was selected for Balambano dam from this programme. This mix uses 127kg of cementitious material per m3, of which 40% by mass is pozzolanic fly ash.

Peridotite aggregate was quarried from the country rock at the site and crushed to a specified size envelope ranging from sand to 75mm stone. The RCC was mixed in a continuous process in a pug mill and delivered to the placing area on the dam by a Rotec conveyor system, where it was spread by conveyor and bulldozer and compacted with vibratory rollers in lifts up to 420mm thick. Fibreboard was inserted during RCC placement to develop vertical contraction joints. These were spaced 38m to

45m apart.

The dam is sealed below the foundation with a grout curtain, drilled and installed from a concrete plinth at the upstream heel of the dam. The selected lean RCC mix is permeable, so the dam face was sealed with a membrane system supplied and installed by carpi Tech of Switzerland. This comprises a PVC liner supported by vertical stainless steel channels embedded in the upstream face of the dam. The system incorporates its own drainage system, comprising a geofabric thermally bonded to the back of the membrane, the vertical support channels and a system of box drains.

A drainage curtain was installed in the dam and foundations downstream of the grout curtain and membrane. Drainage holes were drilled from two galleries in the body of the dam, and from adits that extend the galleries into the abutments. Flows from the box drains and drainage curtain are monitored. Both systems carried some flow even before the reservoir was impounded due to condensation on the membrane and groundwater from the abutments. These flows increased when the reservoir was filled, but the total seepage into the galleries is only 25 litre/sec, which is less than the design allowance.

Thermocouples were installed in the dam to monitor heat build-up and dissipation. The highest temperature recorded in the RCC was 49°C. Survey monuments were installed on the dam crest to monitor deflection. The greatest downstream deflection recorded was 27mm but as the heel of the dam settled this reduced to 19mm. Joint meters were installed to monitor differential movements between monolith blocks

but no distinguishable movements

were observed.

Spillway

Any construction on the steep valley sides required large cuts, so the dam was the most economical location for the spillway. The geometry was designed to minimise deviation in the dam section, since RCC relies on mass production techniques to achieve a low unit cost. The spillway structure was founded directly on

the RCC.

The spillway has the capacity to pass half the PMF at full supply level (FSL). If a PMF enters the reservoir, the routed outflow at the peak of the flood will cause overtopping and some erosion of the dam toe, but the duration and discharge of the overtopping flow is limited and there is no risk of dam breach.

The spillway has three ogee crests with 8m by 15.6m radial gates operated by independent automatic control systems. A standby diesel generator was installed to ensure a reliable power supply. Stoplogs are provided for servicing and testing the spillway radial gates. A travelling dam crest gantry is provided to handle these stoplogs as well as the intake trash racks and intake bulkhead gate. The gates and gantries for the project were supplied and installed by Voest-Alpine of Austria.

Apart from a system of post-stressed Dywidag high yield steel bars for the trunnion loads, the spillway concrete has conventional steel reinforcement.

There are walls separating the chutes and flip buckets, so there is no need to co-ordinate flows in the three chutes. Each chute invert has a step to aerate the flow. The steps were placed where the wide spillway piers join the narrower chute walls, as flow separation at these set backs creates vertical air passages to ventilate the aeration steps. Flip buckets at the toe of each chute throw the discharge into the river downstream of the dam. In the normal range of spillway flows, the throw is approximately 90m.

The plunge pool was partially excavated before the reservoir was impounded, and has now been deepened and extended further downstream by erosion. The reservoir has very little storage relative to the high river flow, so the reservoir filled in only two days and the spillway operated almost continuously for three months during wet testing and commissioning of the generating units.

Power intake and waterways

Like the spillway, the intake is a reinforced concrete structure founded on the dam. The layout of the intake and penstocks was designed to minimise disruption to RCC construction. The intake has two rectangular bellmouth inlets equipped with trash racks and slots for a bulkhead maintenance gate. Each intake has an emergency closure gate that can close under flow if a unit reaches the mechanical overspeed trip, or in the unlikely event of a penstock rupture. An access shaft downstream of each closure gate serves as an air vent.

Two steel penstocks are mounted on concrete anchor blocks on the downstream face of the dam. There is an expansion coupling at the lower end of each penstock. Transducer feed-through fittings were installed at the lower end of the penstock, upstream of the lower bend for flow measurement equipment supplied by Accusonic Technologies. The penstocks are short, so there are no turbine inlet valves.

Power house

Balambano power house and the adjoining control building are situated in a large excavation cut on the left bank

of the Larona river at the toe of

Balambano dam.

The power house has a reinforced concrete substructure and a structural steel superstructure with coated steel cladding and roofing. It is approximately 36m wide from the upstream edge of the transformer pit to the downstream edge of the draft tube deck, and 18.8m wide inside the steel structure. It is 57m long, including the service bay and control building. The unit spacing is 15.1m, controlled by the two, large, high-inertia generators. The turbine runners are set 3.3m below the tailwater level at one unit full load, and the walls around the substructure provide freeboard for operation during a spill of approximately 1000m3/sec. These requirements resulted in a substructure a little over 25m high. The generator floor is 5.2m lower than the service bay.

Mechanical and electrical

The power house is equipped with two vertical Francis turbines coupled to synchronous generators. The turbines, generators, governors, exciters and the power house gantry crane were supplied and installed by a consortium led by General Electric Canada and including Elin and MCE of Austria.

The basic turbine parameters are as follows:

•Rated net head of 84.5m.

•Rated power output of 70MW.

•Rated speed of 214.3rpm.

•Rotation - clockwise looking down.

The governors are microprocessor-based, programmable digital controllers of the PID type, with hydraulic actuators. Control is based on a digital algorithm. The governor control parameters for power output and speed can be modified, and the wicket gate limit position and power limit position can be adjusted.

The synchronising scheme required a unique design approach by the vendor because the system frequency varies so widely and so rapidly that it is not possible to regulate the high inertia units to follow the system. The governor and AVR therefore set the unit at the rated speed and system voltage. The synchroniser supervises the generator phase, frequency and voltage. When these parameters match the corresponding line parameters, the synchroniser closes the generator circuit breaker to put the unit on line.

The three-phase 50Hz AC generator is rated at 80.6MVA at a rated power factor of 0.85 lagging. The generators were specified with a high inertia to help stabilise the system frequency, so they have a relatively large diameter and a heavy rotor.

All the equipment was barged to the Port of Balantang and transported to the site by road, so it was necessary to ship components of limited size and to build both the stator and rotor on site. The capacity of the power house crane was reduced to 120t by lifting the rotor separately from the steel inertia rings.

The basic speed regulation parameters for the generating units are as follows:

• Generator WR2 of 9 595 Mg-m2.

• Mechanical start time of 17.27sec.

• Water start time of 1.25sec.

• Effective governor time of 3.95sec.

Each phase of the generator main leads is carried in isolated phase bus ducts, designed for a line-to-line voltage of 15.8kV and rated for a current of 5000A. Each generating unit has a set of static type excitation equipment that draws its power from the generator main leads through an exciter transformer connected to the isolated phase bus.

The exciter is equipped with an automatic voltage regulator (AVR) function that can regulate generator output from 85% to 115% of its rated voltage (11kV). The set point for the AVR and the joint VAR control can be set remotely. The voltage regulator has a stabilising signal, which is proportional to the integral of the active power and operates in accordance with an adjustable transfer function.

The 11kV generator circuit breakers are of the spring-operated type with a rated interrupting time of less than 50msec and SF6 insulation. It is fully supervised and remotely operated.

Surge arresters are of the metal oxide gapless type. The neutral grounding transformers are of the dry, encapsulated, self-cooled type and have a rated voltage of 11kV - 230/110V. The generator transformers have secondary 11kV delta-connected windings and primary 150kV Wye-connected windings.

Lines from the transformers are connected to a dead end gantry mounted on the downstream face of the RCC dam and then to the Balambano switchyard on a 1km double-circuit transmission line. This is finally linked to the nickel processing plant over a 26km double-circuit line. The transmission lines are three-phase AC 150kV double circuit steel lattice tower, with a single 795 kcmil ACSR Tern conductor. The station auxiliaries, generator transformer, switchyard and the transmission line were supplied and installed by ABB Canada.

An overall hydro power control system was supplied and installed by SAT (Systeme Für Automatisierungstechnik GmbH) of Austria. This system operates over a wide area network on optical fibres in buried cable and in the transmission line ground wires. It provides control and supervision of both Larona and Balambano power houses

and switchyards from the Larona

power house or from the process plant

at Soroako.

Commissioning

The diversion tunnel was closed on 5 September 1999 and the reservoir filled to the spillway crest level in approximately 1.5 days. Water was allowed to flow freely over the crest of the service spillway as work continued on the spillway gate equipment and while the dam instruments were monitored. The monitoring confirmed that the dam was behaving safely as expected.

On 17 September, the spillway gates were closed, and on the following day the reservoir reached full supply level el165.5m. Once the reservoir was full, the penstocks were watered up and the generating units were tested and commissioned.

Unit 1 was placed into service on 31 October and Unit 2 followed on 28 November 1999.