Optimization and simulation of decoupling algorithm for cascade drainage system control

Coupling characteristics of a series channel system refers that the regulation action of a single control gate can affect the water level of adjacent pools in upstream/downstream reaches. Without decoupling, there is often a dramatic decline in the performance of the control system, even instability in the optimal controller for a single channel pool. In particular, the coupling effect is more complex, because there are significant differences in the length and capacity of adjacent channels in various irrigation districts of China. Decoupling can allow the pools to run independently of channel control, making it easier to change the individual channel. However, it is still lacking in the design principle of decoupling coefficient in complex canal systems with different lengths and scales. In this study, an optimization was proposed for decoupling algorithm in the upstream direction using Proportional-Integral-Differential (PID) feedback control. Firstly, an efficient range was determined for the decoupling coefficient. Secondly, an amplification factor was introduced to improve the decoupling effect in various lengths of adjacent channels. Finally, a simulation was carried out under different water intake flows and geometric structures between the upper and lower reaches of the channel. Three engineering examples were also selected with different decoupling amplitude to verify the simulation. The results show that the optimization scheme presented a significant improvement in the control performance. The improved range of system increased first and then decreased, with the addition of amplification factor. There was also an excellent enhancement of coefficient interval with a gentle change of the improved range in the middle, indicating that the reasonable value range of amplification factor. Specifically, 1) the specific range of basic decoupling coefficient should be between 0.8 and 1.0. 2) A calculation formula was proposed, while the amplification factor was selected in the area near the ratio of water surface area of adjacent channels and pools. 3) The selection and correction of the amplification factor significantly improved the control performance, when the upstream and downstream designs of a series channel system differed greatly in the flow capacity. 4) The amplification factor was directly determined by the length ratio for the convenience of engineering application when there was no obvious change in the flow rate and section size in the channel. Whereas, it was best to determine the ratio of water surface area when the operating flow varied greatly. Furthermore, the optimized decoupling adjusted the value of amplification factor, according to the change of water surface area ratio caused by different channel flow, responding more accurately and rapidly to the disturbance of channel pool, while reducing the degree of coupling between the channels. Consequently, the algorithm can be applied to the decoupling controller design in the complex channel systems in different scales, further to realize the independent control of drainage pools. The finding can provide a sound reference for the intelligent scheduling of water transmission systems in the irrigation district and water diversion projects. More importantly, it can be suggested to consider the characteristics of specific channels for the optimized coefficient, according to the response characteristics and application of channels in engineering applications.