Stability of tank level control systems in hydropower plants

In the operation of a run-of-river hydropower plant, flow control to ensure a suitable head in the forebay is a key issue and is often performed by a regulation system combining a level sensor, a controller and a downstream valve. The stability assessment of this system is performed to avoid filling or emptying of the upstream tank. In the past, stability assessment was performed by analytical means but neglecting the transients in the conduits, the uncertainties of the sensor, the delays in the PLC, the finite velocity downstream valve and the backlash in mechanical parts. However, these mechanisms are real-life phenomena and assessing stability considering them has gained attention nowadays. For this purpose, a stability assessment is here performed on a hydropower plant including these mechanisms. The analysis is performed using numerical simulations where the behaviour of the plant is reproduced and then the oscillations of the level in the forebay are studied to determine stability. The obtained results include the analysis of the effect of each mechanism on the stability of the system, the effects of the chosen stability criteria and stability curves showing when a system is stable under certain combinations of variables of the system.