T he Xiaowan dam will be located in the northwest of the Yunnan Province on the middle stretch of the Lanchang river in China. The site is located 1500m beyond the mouth of the tributary river Heihui in a V-shaped part of the deep valley, where both flanks reach a height of more than 1600m asl. The walls of the valley are steep and have an average slope of 40-42?.

The dam will be used for sourcing clean water, flood control and power production.

The main dam will be a twin spherical arc construction, 292m high, with a total arc length of 893m at the top. At the bottom, the base will be 73m thick and, at the crown, 12m. The crown will be 1245m asl.

A total volume of 8.733Mm3 of concrete will be used in the construction of the dam and, after its completion, an artificial lake containing more than 15.2Bm3 of water will be formed at the side of the hydro station, which will have a total capacity of 4200MW.

As the topography of the site is extremely complicated, concrete for the dam construction will be supplied by cable cranes. During construction, the amount of concrete required will vary according to the progress of the civil works. These require a maximum annual supply of approximately 2.2Mm3, with the highest monthly quantity reaching 220,000m3. To transport such a large amount, five cable cranes will be used, each with a lifting capacity of 30t.

In November 2001, China Yunnan Lanchang River Hydropower Development Company, the developer and owner of the Xiaowan project, placed an order with Krupp Fördertechnik’s Materials Handling Business Unit (formerly PWH) of St Ingbert-Rohrbach, Germany, for the design, supply, erection, supervision and commissioning of five cable cranes for the construction of the dam and hydro station.

In order to work in confined spaces, and so that they can be used for other purposes, the five cranes will be arranged in two levels. In this formation, they can travel closely together and discharge all five buckets of concrete within a 20m area.

Design features

Because of the topography of the site, the rail tracks for both the machine car and counter carriage sides will have to be built partly on concrete foundations and partly on a manmade structure of steel, up to 65m high. The two upper cranes, with a span of 1158m, will travel approximately 50m above the three lower level cranes, which will have a span of 1048m.

All five cranes will be interlocked to each other by encoders via the PLC system. These will also monitor the minimum spacing between the cranes. However, for flexibility of operation, the upper level cranes will be able to pass over the lower level cranes, under non-load and special travel conditions, to reach parallel positions for simultaneous operation.

Each crane will comprise a machine car on the left bank and a counter carriage on the right. These will be connected by a track rope on which a carriage, equipped with a lifting block, will travel over the river. Hoisting and travel winches will be installed on the machine car, together with a powerhouse containing electrical equipment and control systems. Power will be supplied to the machine car by a medium-voltage trailing cable and cable reel, with a feed point in the middle of the rail track. The design will be the same for the counter carriage, except that a low-voltage cable will be used.

Both the machine cars and counter carriages will be equipped with variable speed AC drives, with acceleration and deceleration along the track controlled by a frequency converter. The mechanical brakes will act as holding brakes for the cranes when they are not operating, either in high winds or during emergency power failures. Hydraulically-operated rail clamps will also be installed for when the cranes are not operating.

Both the hoisting and traversing winches will be equipped with thyristor-controlled DC drives. These will regulate all movements of the buckets under the carriage during operation in order to achieve precise positioning of the loads and to enable the cranes to accelerate quickly and maintain the required transportation capacity.

While the crane is lowering with a full load or hoisting with a partial or no load, the hoisting motor will provide a higher speed by fully utilising the drive torque potential, depending on the changes of the loads and efficiency.

Control system

Each cable crane will be controlled from a mobile operator’s cabin, which will be located close to the relevant crane to provide the best visibility, either by radio control or through cable-connected control panels.

The cranes will also be equipped with a master/slave control function, which will allow two neighbouring cranes to operate in tandem while being controlled from one operator’s cabin. This special design feature will enable the cable cranes to transport heavy loads of up to 60t each.

As well as being controlled from the operator’s cabin, each crane will be equipped with a portable remote control device which will operate all movement, including master/slave tandem operation.

Each crane will be equipped with a target control loop and an automatic pendulum control system. The crane’s starting position, its destination and any obstacles in between will be entered into the system via an on-screen menu in the operator’s cabin. A PLC programme, which will have all the starts, targets and obstacles recorded, will then enable the crane to reach its destination correctly, without any hindrance caused by obstacles.

Transportation of the concrete from the loading station to its destination will be carried out using special concrete buckets, hanging from the lifting block. A hydraulic system requiring no external power supply will open and close the buckets

The potential energy of the bucket and concrete will be utilised as work potential for the hydraulic reservoir feeding the opening and closing cylinder, enabling a controlled discharge of the concrete in the correct location.

According to the time schedule for the construction of the dam, the first crane should be ready for operation by the end of 2003. This crane will be used as a hoisting appliance for transportation of construction machines and materials and, from 2005, in services for concrete transportation. The remaining four cranes are to be delivered to site in 2004 and erected in 2005, in order to be ready for operation when the high demand period for the supply of the concrete commences.

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