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Making Better Equipment Choices for Small Hydro

By Lars B. Meier and Dieter E. Hoffmann

When selecting turbines and generators for small hydro sites, developers seek optimal performance and low costs. Choosing equipment that best fits a particular site involves an evaluation of hydraulic conditions as well as an understanding of how the project will be operated. A review of the process of selecting equipment for a small project in Sri Lanka provides insight into the most important parameters to consider.

Many hydro sites being developed throughout the world are of a capacity of 10 mw or less. Choosing the optimal equipment for such sites requires a systematic evaluation of electro-mechanical parameters versus civil works constraints.

A description of the process the authors used to select equipment for a small, run-of-the-river hydro project in Sri Lanka, the Way Ganga project, illustrates how the evaluation works. Such an evaluation can be used at other small hydro developments throughout the world.

The situation at Way Ganga

The Way Ganga site is in the Ratnapura region, about 125 kilometers southeast of Sri Lanka’s capital Colombo. A call for bids was released in 2001 as a water-to-wire package. Table 1 on page 36 lists hydraulic conditions in the bid.

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In addition to the basic project parameters listed in Table 1, the call for bids included a flow duration curve, based on more than 30 years of flow data. The developer intended for the plant’s operation to be based on this flow duration curve. The authors used this flow information as the basis for evaluating different plant layouts with respect to annual energy calculations.

Selecting a turbine

The head of Way Ganga power plant suggests the opportunity to consider two different types of turbines: either Francis or Kaplan type turbines of different specific speeds (characteristics).

Furthermore, the plant could be equipped with different numbers of units. The alternatives for Way Ganga, analyzed in detail, are shown in Table 2 on page 37. The authors derived the specifications for each alternative via a model turbine.

At the Way Ganga project, high flooding of the river is expected. Consequently, the powerhouse is to be set to a reasonable (positive) reference level. For reduction of maintenance efforts, the shaft centerline of the turbine shall have an elevation so that the draft tube elbow can be removed at nominal tailwater level without draining the draft tube itself. This leads to a required setting of about +1.5 meters.

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The Kaplan unit was waived due to the deep setting of the turbine, which is necessary to avoid cavitation damage. To improve the required setting of the Kaplan turbine solution to approximately -5 meters, it would be necessary to reduce rotational speed to the next lower synchronous speed, which is 500 revolutions per minute (rpm). This would have a significant effect on lower part load limit and, therefore, also on the annual energy production.

A Kaplan solution with two units was not considered, owing to high costs and submergence reasons (the setting is nearly independent on turbine size and speed for relatively comparable hydraulic applications).

Alternative 4 from Table 2 also failed due to submergence.

Based on the flow duration curve, about 25 percent of the time the flow is below 4.5 cubic meters per second (cms). About 10 percent of time, the flow is below 3.0 cms. Due to this desire for part-load operation, the alternative of two large plus one small turbine combination (alternatives 3 and 4 in Table 2) was included in the evaluation.

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After an economic evaluation taking into account all factors driving cost and performance (see Table 3 on page 38), the authors identified the following as the optimal turbine solution for the Way Ganga plant: three identical Francis units with 3,400 kva and 750 rpm. As shown in Table 3, this configuration provided a significant annual energy advantage compared to a two-machine option.

Selecting a generator

The turbine, as the prime mover for the generator, delivers mechanical power, at a system typical speed defined by the turbine application.

The generator input is the mechanical torque, simply calculated by:

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