Šebelův mlýn small hydropower plant

In 1995, four of us formed a partnership to operate a small hydropower plant at Šebelův mlýn in Husinec on the River Blanice.

The other founding members were:

Pavel Schiezsl, a self-employed joiner; Petr Schieszl, an electrician qualified under sections 6, 7 and 8 of the former Czech Decree 50 for LV and MV work; and Petr Zaumüller, a municipal building inspector.
We commissioned the plant in 1998. The following description follows the direction of water flow:

1. The installation begins with the construction, or preservation, of the fish pass on the left side of the weir. Its dimensions were calculated using the following formula:
   
   On-site water-velocity measurements showed that the existing fish pass met the condition with a margin at an ecological flow of 622 l/s.
2. The existing weir belongs to the Šebel family. Mature trees were removed, the structure was secured with quarried stone and the concrete slab on the crest was repaired.
3. The millrace intake was rebuilt in its original position and to the dimensions shown on a 1934 drawing. It has a screw-operated gate with a handwheel accessible from the path and lockable by chain. The gate dimensions are 2500 × 1000.
4. The entire millrace was cleared and cleaned to carry 1.3 m³/s at a velocity of 1 m/s.
5. Bypass spillway No. 1 at the municipal pond had to be modified so its crest matched the existing concrete spillway crest at the plant. The No. 70 U-sections were extended by 20 cm on each side. The timber boards are secured by a lockable through-rod to prevent unauthorised changes to the pond level.
6. An approximately 2.5 m safety spillway was built next to spillway No. 1, discharging downstream of it. If the plant fails, most of the water passes through this spillway, whose crest is around 4 cm above the normal reference water level.
7. At the end of the pond, a mechanical fine screen stands before the turbine intake. Its electric drive has PLC-controlled running and stopping intervals.
8. A manual gate is fitted at the turbine intake, with its handwheel secured by a chain.
9. The plant building’s roof was repaired to stop water ingress. The badly deteriorated turbine was removed and replaced by elbow-type Francis turbine HG1. Unlike the old machine, its elbow and draft tube face into the turbine hall. The corroded original draft tube was sealed, and the new one was made from rolled and welded sheet-metal sections. HG2 is a cased Francis turbine located behind HG1 along the turbine hall’s longitudinal axis.
   Rated outputs:
   HG1 – 35kW
   HG2 – 11kW
10. The entire tailrace was cleared and cleaned to prevent an unwanted rise in downstream water level.
11. The control system developed over time. After commissioning in 1998, it was optimised using practical operating experience. Once operation had become stable, the 2002 floods destroyed the entire electrical installation, including the PLC. Early in 2003, the plant was provisionally restarted at minimum cost to use the spring flows that are important at this site.
12. At the beginning of 2005, a new control system added:
    
    
    1. Extended measurement of power, speed, water levels, temperatures and operating cycles of the plant supply, generators, screens and turbine guide-vane mechanisms.
    2. Remote monitoring and control from any PC equipped with a modem.
    3. Automatic fault notifications by SMS.
    4. Automatic daily operating reports by SMS.
    5. Automatic weekly operating reports by SMS.
    6. Automatic quarterly operating reports by SMS.
    
    
    
13. At the end of 2012, the entire electrical installation was replaced alongside improved control of the two Francis turbines, adding:
    
    
    1. Water-level sensors in the pond and tailrace.
    2. Automatic PLC data collection into a database for 840 operating points representing all combinations of HG1 and HG2 operation in 5% steps.
    3. PLC group-control software managing start-up, shutdown and optimum load allocation during parallel operation of hydrogenerators HG1 and HG2.

We apply this experience to refurbishing civil, mechanical and electrical parts of small hydropower plants, including instrumentation and control, for existing and future operators.

A recurring challenge was finding the optimum turbine speed. Empirical formulas and software offered values, but did not establish with certainty which speed gave the best efficiency and maximum generation. We therefore bought a regenerative variable-frequency drive. It allows us to measure a turbine at different speeds and openings from 0 to 100%. The method can optimise different turbine types in the 1–45 (60) kW range. An advantage is that the owner can see the gain directly on their own metering, as the drive connects between the generator and the existing plant circuits.