## 2 The Case Study
### The Rebolluelo Micro-Hydropower Plant  
The Rebolluelo micro-hydropower plant is located in the town of Rincon de Soto in La Rioja (Spain) (latitude 42\({}^{\circ}\)14\({}^{\prime}\)04\({}^{\prime\prime}\) N, longitude 1\({}^{\circ}\)51\({}^{\prime}\)03\({}^{\prime\prime}\) W, and altitude of 261 m. It is a run-of-the-river power plant. The Rebolluelo micro-hydropower plant operates at fixed-speed. Figure 1 shows a Google Earth image of the Rebolluelo micro-hydropower plant location. The Rebolluelo micro-hydropower plant is mainly comprised of the following components: a semi-Kaplan turbine, an asynchronous squirrel cage generator, and a speed multiplier (gearbox). The semi-Kaplan turbine (THEE, model THEE D-644-AET) has an automatic blade drive, fixed guide vanes, and an output of 60.60 kW. The nominal flow rate is 1495 L/s and the minimum start-up flow rate is 200 L/s. The rated speed of the semi-Kaplan turbine is 584 rpm. This type of turbine is a reaction turbo-machine, has a vertical axis and is fed by a penstock connected to the Ebro river upstream. The net head is 4.80 m. The turbine shaft is connected to the generator shaft via a gearbox. The gearbox has a ratio of 1:1.72, to match the speed of the semi-Kaplan turbine to the speed of the electric generator. The characteristics of the electric generator (Leroy Somer, model PLSG280 SC-T [27]) are: 70 kW, 1000 rpm and 400 V.  
The mini-hydropower plant uses the difference between the level of the irrigation canal and its outflow into the river Ebro. The flow enters the loading chamber through an automatic screen cleaner that prevents plant debris from entering the turbine. Figure 2 shows the water supply to the micro-hydropower plant.  
Figure 1: The Rebolluelo micro-hydropower plant.  
Figure 2: Irrigation canal. Water supply to the micro-hydropower plant.  
There is an oil-hydraulic gate in the load chamber through which the flow enters the turbine. The plant has a side spillway with a hydraulically-operated gate (see Figure 3). The turbined flow is channelled to the river Ebro via a 31 m long outflow channel. Figure 4 shows a diagram of the channel, the load chamber, and the turbine.  
The Rebolluelo micro-hydropower plant uses an \"all-nothing\" type of water level control with hysteresis (3%) and actuation only on the position of the blades. The control algorithm is run every 25 s and the action on the blades applies opening and closing pulses lasting 2 s and 1 s respectively for each control cycle. Although the automatic blade regulation makes it possible to maintain acceptable efficiencies above 40% of the design flow rate, the following problems were observed:  
1. As flow rates below the turbine's design flow were recorded, efficiency became low and even forced shutdowns due to minimal power. Figure 5 shows the efficiency-flow and power-flow curves for the micro-hydropower plant. According to Figure 5a, the efficiency of the turbine significantly decreases at low flow rates;
2. The low flows caused the blades to be positioned at minimal angles in order to maintain the water level in the load chamber. The decrease in suction head coupled with the high turbine speed (rated speed) resulted in cavitation.  
Figure 4: Diagram of the channel, the loading chamber, and the turbine.  
Figure 3: Water inlet to the loading chamber and side spillway.