Model Predictive Control of Skeboå Water system

Sammanfattning: This thesis is a study of model predictive control of water levels and flows in a water system. The water system studied includes five lakes and six dams that are regulated manually by sluice-gates. The water is used in the papermaking process at Holmen Paper Mill in Hallstavik. The aim of this thesis is to find out how to control the water system when all dams are automated and to minimize the discharge of water from the system without risking production stops due to water shortage. To fulfil the aim, a simulation is made during a dry period with low amount of rain. The simulation is then compared to the same period but when the system was manually controlled. In this thesis two models of the water system are constructed, a simple linear model and a more complex non-linear model. In the linear model the channels between the lakes are assumed to be delays of water flow. In the non-linear model the same channels are described by Saint Venant equations of changes of flow and Manning’s equation on how water flow and the cross-section of a channel are related. In both models, the lakes are modelled as the change in volume with respect to time due to inflow to and outflow from the lake. The non-linear model is verified against measured water levels, flows, sluice-gate heights and precipitation to ensure that the model describes the water system well enough. The linear model is used in the model predictive controller to calculate the optimal outflow from the dams. The optimal outflows are then converted into optimum gate heights in the dams, which in turn are used as input to the non-linear model. The non-linear model is used to simulate the water system. The results from the simulation show that the control of the water system can significantly be improved. The conclusion of this thesis is that a lot more water can be saved when the system is automated and that the water levels in the lakes can be kept more stable with respect to a set reference level. The recommendation if only one dam is to be controlled initially is to start with the dam at Närdingen.