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:

    Equation 1:
      M 5 (9550 3 P) / n

    • M is torque, Nm;
    • P is power, kw; and
    • n is rotational speed, 1/min.

    The generator output is electrical power on a system-specific voltage and frequency. But, it is important to understand that the torque mainly defines the size of the generator. For example, a 3.5-mva generator with a rotational speed of 1,500 1/min is much smaller than a generator of the same power with a rotational speed of 300 1/min.

    Selection of the generator also depends on the parameters of frequency, voltage, and power factor (PF) as defined by the transmission grid.

    The influence of the frequency

    The frequency of the grid and the turbine speed defines the number of generator pole pairs, as shown in Equation 2:

      Equation 2:
        p 5 f 3 60 / n

      • p is number of pole pairs (only pairs of poles are possible, because each pole pair is a magnet and has a north and a south pole);
      • f is frequency, Hertz (Hz); and
      • n is rotational speed, 1/min.

      The number of poles of a generator often is indicated in the generator type. Typical small hydro applications have generators with the number of poles ranging from four up to more than 56.

      Gearboxes or belt drives sometimes may be used if the rotational speed of the turbine is less than 500 1/min, but a fast running standard generator should be used. This solution reduces the cost of the unit. However, resulting consequences — including higher noise level, mechanical stress, bearing requirements, and maintenance — must be taken into consideration.

      The influence of the voltage

      Although lower voltages require higher currents and create more external losses between generator and transformer, they are an advantage in the design of the generator if the nominal current is lower than approximately 3,000 to 3,500 amperes (A). That’s because a simple winding process and a higher grade of copper filling in the stator slots is possible.

      In 50- or 60-Hz grids, the following typical voltage ranges are applicable:

      • Low voltage, 400 to 990 volts (V); and
      • Medium voltage, 3.0 to 4.16 kilovolts (kv), 6.0 to 7.2 kv, or 10 to 15.8 kv.

      When selecting the voltage, consider the generator power and whether the plant will use a unit transformer or be directly connected to an existing grid.

      If rectangular coil or bar windings will be used in small hydro generators, the voltage is limited to less than 16 kv. Cable windings may shift the voltage limit to higher levels, but this technology is still in an experimental status.

      Voltages higher than 30 kv are impossible because of the size of the generators in small hydro applications. To contain equipment costs, it is important to not choose a voltage any higher than needed.

      The effect of the power factor selection in feasibility studies

      Nominal apparent power, Sn, and nominal current, In, are direct proportionally, if the nominal voltage, Un, is constant. A part of this current is a reactive current only, but the losses are direct proportional to In. The generator efficiency (etagenerator) is inversely proportional to the losses. Furthermore, the surrounding components (switches, cables, etc.) must be designed according to In.

      These concepts are described in the following equations:

      Equation 3:

        S n 5 3 0.5 3 U n 3 I n

      Equation 4:
        S n 5 P n / PF

      Equation 5:
        P n 5 P turbine 3 eta generator

    The type of power typically sold by an energy-producing company is effective power, P. Under optimal conditions, P equals Pn, nominal power. If a lower power factor (PF) will be specified, Sn and the generator cost increases and Pn decreases because of a reduced etagenerator.

    Simply said, the investment and the losses increase, and the suitable benefit decreases. The additional cost is only feasible if the installation of reactive power, Q, also will be given financial compensation.

    The basic selection for the generator is now possible. To optimize the selection, the authors recommend considering additional mechanical parameters:

    • Coupling concept (direct, flexible coupling, coupling and gearbox, or belt-drive) and the resulting bearing and frame construction;
    • Overspeed (the system must be safe under all speeds up to maximum overspeed) and the resulting bearing and rotor construction;
    • Plant design (horizontal, vertical, etc.) and the resulting machine construction and erection process; and
    • Required inertia to limit overspeed in case of load rejection by adaptation of the generator design or the installation of a flywheel. For standard generators, the additional flywheel is a common solution. It is more cost-effective than a generator design with a higher inertia. Only the design of the shaft line must be revised and modified.

    Ambient parameters to consider:

    • Altitude — output reduction if installation is above 1,000 meters because of lower density of the cooling air;
    • Temperature — reduction needed if the ambient temperature is above 40 degrees Celsius because of the reduced difference from ambient temperature to the allowed temperature limit of the electrical insulation system.
    • Humidity or special conditions such as polluted air, insects, fungus, or external vibrations — presence of these may require modifications in design. Examples of such modifications are enclosure and cooling, winding insulation, frame construction, or terminal box sealing.

    Specifications that could influence equipment selection include:

    • Over heating class B limit (such as recommended in International Electrotechnical Commission (IEC) Standard 61116. and .2) creates a 25K thermal reserve if an F insulation system is used. But this is not always needed. For example, if 40 degrees Celsius ambient temperatures are present, the water flow may be low (summer season) thus providing limited power output, etc. If the over heating is not strictly limited to class B level, up to the maximum generator output power required, a smaller generator could be chosen.
    • Specifications created for large hydro but adapted for small hydro can significantly increase the cost of generators, as well as the technical risk. It is important to understand that the construction and production processes for building a small machine and a large machine are different in many ways, including stator core design, windings and insulation, and rotor construction. Modifications to a well-designed and approved standard increase technical risk.
    • Accessories specified often require too many sensor installations. For example, a small generator does not need temperature sensors in the stator core. Such a sensor can reduce the reliability of the generator owing to a defect in the sensor or an unclearly designed limit definition.

    Other considerations:

    • The selection of the basic speed range is important for the generator size. If the speed goes down from 1,000 rpm to 600 rpm for the same generator output power of 3.55 mva, the frame size of a standard generator increases from 560 or 630 up to 800. Consequently, the cost of the generator also increases.
    • A larger machine (larger frame size) of the same speed typically has a higher efficiency.
    • An optimally designed system has a better efficiency. Not all “cold machines” have the best efficiency; for example, maybe too much power will be used for the cooling system.
    • Direct ventilated machines, cooled by ambient air, are the lowest in cost, but the losses heat up the ambient temperatures and the noise emission is high. Silencers or air ducts can be used to reduce the noise and to handle the cooling air, but this equipment increases the cost. The most silent machines are closed machines, with air-water-cooler. The ambient will not be heated up, but the cost increases more significantly.
    • Salient pole machines typically have a better efficiency and a lower price but a limited overspeed capability, compared with full pole and asynchronous machines because of the rotor design.

    Choosing a generator for Way Ganga

    After considering all of the above, the authors chose to couple each turbine at Way Ganga with a generator type 710-M/8. Each generator is a horizontal, medium long frame size 710 machine with eight poles and the following basic data: 3,400 kva, 0.85 power factor, 6.3 kv, and 50 Hz.

    Adapting equipment to the civil design

    The next step at Way Ganga was to adapt the hydropower equipment to the civil design without drastically affecting the hydraulic behavior and/or to adapt the civil design to the selected equipment without affecting the civil costs. Hence, it was necessary to modify the design of the draft tube due to civil constraints as:

    • The tested model included two piers in the draft tube; and
    • The draft tube was rotated with respect to the vertical centerline.

    In addition, it is common practice for small hydro turbines to reduce the number of sections in the spiral case from the homologous model. Typically, the base hydraulic design was developed for large units, where the number of sections is orientated at the maximum size of commercially available metal sheets. Attention was paid that the performance was not influenced negatively due to this design change. A similar adaptation procedure was also done for the draft tube elbow to reduce steel and concrete work to a reasonable amount.

    Typical hydraulic guarantees in accordance to IEC guidelines were provided (e.g., mean weighted efficiency level, power outputs, and maximum material loss due to cavitation).


    The equipment for Way Ganga was placed into operation in April 2004 and is operating successfully.

    Messrs. Meier and Hoffmann can be reached at Voith Siemens Hydro Power Generation, Alexanderstraße 11, 89522 Heidenheim, Germany; (49) 7321-372565 (Meier) or (49) 7321-372058; E-mail: or

    Lars Meier, Dipl.-Ing., is head of the Hydraulic Application Department and Dieter Hoffmann, Dipl.-Ing., provides technical sales support for small hydro generators, at Voith Siemens Hydro Power Generation GmbH & Co. KG in Germany. The authors performed the selection and optimization of the electro-mechanical equipment for the Way Ganga project described in this article.

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