Cam correlation analysis using Acoustic discharge measurement

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Courtesy of Courtesy of HydroVision GmbH

The low head hydro power plant in Wettingen, Switzerland, is in operation since 1933 and no effective cam optimization has been carried out up to now. The goal of the measuring campaign was to provide data enabling an economic study for a retrofit and upgrade project of the plant and the measurement of the efficiency improvement of turbines after refurbishment. The contribution discusses cam tests using the acoustic transit time method in the intake for discharge measurements. A cam correlation analysis and an efficiency test of the installed Kaplan turbines were performed as a first measurement campaign showing the actual status. While one turbine operated at optimum cam, the other two turbines operated off-cam. After rehabilitation works, based on the recommendations of the first measurements, a second measurement campaign was performed. The results showed the expected efficiency increase due to overhaul of the regulating mechanics (connection between the piston stroke in the machine hall and the regulating ring / guide vane stem at the turbine) of the governor.

With the set of data from the second measurement campaign an optimization study for the operation of the three Kaplan units were conducted. Especially the transition zone from one to two and from two to three machine operation could be optimized due to the knowledge of the individual efficiency curves.


The hydro power plant (HPP) Wettingen is a run of river plant located at the river Limmat downstream of Zurich. There are three Kaplan units installed with a rated capacity of 8.5 MW each. The head varies in a relatively small range between 21.6 and 22.8 m. In a typical year the plant produces about 145 Mio kWh.

In the context of a planned retrofit and upgrade of the HPP Wettingen, 8-path acoustic transit time (ATT) discharge measurements from HydroVision GmbH were installed in each of the three intakes. The original Winter-Kennedy measurement devices were not anymore reliable and a Winter-Kennedy measurement thus not practicable, Obermoser et al. [1]. The new discharge measurements were also foreseen to be implemented in the governor system of the HPP. Discharge measurements based on the ATT principle are known to be long term stable and a good reproducibility can be achieved therefore. A decision criterion for the HPP operator was - among others - to verify the economic analysis for the planned retrofit

The ATT discharge measurement is described in the appendix of the IEC Standard 60041 [2]. But in the case of the limited installation possibilities in the HPP Wettingen an unconventional solution for the positioning of the transducers had to be found. The only possible location for installation was identified to be in a convergent section, downstream of a central pier in the intake. In order to get an overview on the flow field that had to be expected in the measuring section numerical flow simulations (CFD) were carried out by the Hochschule Luzern, Switzerland. Further goal of these flow simulations was to determine the positions of the transducers and the weighting of the individual acoustic paths. The simulation domain with the measuring section is depicted in Figure 1. The sensitivity analysis for the weighting of the paths and a discussion of the expected integration uncertainty of discharge based on these simulations are described in Aakti et al.

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