Two-Layer Control of Integrated Quality and Quantity in Dynamic Water Distribution Systems

Abstract

An integrated approach to control of quantity and quality in water supply and distribution systems is proposed. The integrated control consists in optimising the operational cost, meeting a demand on water of desired quality and maintaining the constraints. This constrained optimising control problem is complex due to nonlinearities, large dimension, output constraints, mixed-integer structure of the variables involved, at least two time scales in the system dynamics and an uncertainty. A sub-optimal two-level hierarchical control structure is proposed that allows incorporating the desired controller functions and yet making the synthesis of these functions possible. The control trajectories optimising the operational cost over one day to one week horizon are generated by the upper layer. The upper layer controller operates according to a repetitive control strategy. Due to model-reality differences the prescribed bounds on chlorine residuals may not be met when the generated controls are applied to a real systems. The main objective of the lower control layer is to adjust on-line actions of the booster stations determined by the upper control layer so that the chlorine residuals are kept within the limits. The lower layer controller operates almost continuously as opposed to the upper layer one. Its design is very difficult as the dynamics between the chlorine injection nodes and monitored nodes involves time-varying delays and parameters that can abruptly change. A decentralisation of the lower layer controller into a number of independent local controllers allocated to selected sub-areas of a distribution system by using the interaction trajectories determined by the upper layer is discussed. The design of the lower layer controller and its performance are illustrated on an example water system.