Study On Multi-objective Optimization Decision-making Of Load Rejection And Start-up Collaborative Fuzzy Control Of Pumped Storage Units

The security and stability of the power system confront significant issues due to the ongoing transformation of the energy structure and the intermittent clean energy sources connected to the grid and continuing to play a role.Pumped storage systems can effectively resolve the multi-scale space-time contradiction between clean energy absorption and electric energy supply as an energy storage technology for absorbing intermittent renewable energy sources,and as a tool for peak frequency and phase regulation of the power grid,enabling rapid development and large-scale construction.However,the complex water transmission system structure and large unit capacity of pumped storage power stations,as well as the reverse "S" region and frequent operation characteristics of units based on reversible bidirectional operation design,have brought great challenges to unit regulation and stable operation of power stations.In light of the foregoing,this study develops the nonlinear simulation model and characteristic line distribution parameter model of the speed governing system,and investigates the multi-objective optimization decision-making method of the unit control strategy.It focuses in particular on the regulation performance of typical load rejection and start-up processes.In order to form a multi-objective optimal decision-making research system of unit transient process control strategy,the optimal decision-making method of unit closing rule and the open-closed-loop collaborative fuzzy control strategy of unit are designed,respectively,for a variety of typical load rejection and complex start-up conditions of a "one-tube-two-unit" annular structure pumped storage power station.The following are some of the paper’s primary research findings and inventive contributions:(1)The mathematical description method of each link of the speed governing system of pumped storage unit is systematically studied.Aiming at the problems of interpolation diversity and simulation iteration non-convergence caused by the "S" area of the complete characteristic curve,the unit model based on the improved Suter-Lagrange interpolation method and the logarithmic curve project-BP neural network fitting method is proposed,which effectively alleviates the phenomenon of crossing and twisting in "S" area.Furthermore,cascading the mathematical model of each link of speed governing system fully takes into account the nonlinear characteristics such as dead zone,saturation,and unit inverse "S",the nonlinear simulation model and characteristic line distribution parameter model of the pumped storage unit speed control system is established,which provides the model basis for the subsequent research.(2)Aiming at the problems of hydraulic oscillation and mechanical instability of pumped storage units under successive load rejection conditions,a multi-objective optimization decision-making method for the control strategy of pumped storage units is proposed to improve the regulation performance of the unit during the transient process of successive load rejection.Firstly,the influence law of successive load rejection interval time on multiple units sharing the same hydraulic pipelines is deeply explored.Multiple constraints such as regulation and maintenance calculation requirements,governor dead zone,and oil pressure restriction are introduced,and a multi-objective optimization model of unit operation strategy is established,which balances the hydraulic and mechanical characteristics of units.An improved non-dominated genetic algorithm,which combines Latin hypercube initialization,chaotic mapping mutation,and adaptive punishment mechanism,is proposed.On this basis,a multi-level decision-making model of the control strategy of the unit is constructed,and intuitionistic fuzzy analytic hierarchy process is introduced to evaluate the alternative scheme Pareto set.The specified optimal control law makes the maximum speed of the two successive load rejection units decrease by 4.4%and 5.19%,the maximum volute internal water pressure decrease by 64.4m and 50.31 m,and the minimum water pressure of the tailpipe increase by 26.97 m and 32.52 m,which significantly improves the performance of the transient process of the unit under successive load rejection condition.(3)Aiming at the problems of the deficiency of conditions adaptability of the existing operation strategy and the mismatch between the control strategy and working conditions,a mang-objective optimization strategy for typical load rejection conditions of pumped storage units is proposed.Three extreme load rejection conditions are considered as a whole,and uniform closing rules of the unit are selected.Firstly,the transient process hydraulic characteristics of typical load rejection conditions are investigated,and the mechanism of guide vane delay-ball valve co-operation rule on improving the dynamic quality of load rejection units are revealed.Furthermore,the optimization objective function and constraint conditions are extended to typical load rejection conditions,and the bi-modal characteristics representing the steady-state and dynamic-state of the transient process,are taken into account to establish the many-objective optimization model of the operation strategy.In order to solve the dilemma that the dominance relationship deteriorates rapidly with the increase of the optimization objective dimension,a many-objective evolutionary algorithm based on reference vectors and ε-constraints is proposed to efficiently solve the many-objective optimization problem of operation strategy.The obtained control law balances the unit speed,hydraulic pressure of the volute and tailpipe under three typical load rejection conditions,and the transient process is more stable,which provides scientific guidance for ensuring the safe and stable operation of pumped storage units.(4)Aiming at the problems that the rapidity and stationarity of pumped storage units start-up are contradictory and the robustness of existing start-up control strategies is poor,an open-closed-loop collaborative fuzzy control optimization strategy based on variable universe fuzzy PID control is proposed.First of all,the influence law of controller pitching-in frequency,interval time,and working water head on the start-up process of the unit is explored,and the most unfavorable start-up operating parameters are determined.Aiming at the problems of frequency oscillation,large overshoot of frequency disturbance and long regulation time,a variable universe fuzzy PID controller and its operation rules for speed governing system are designed.Furthermore,taking the overshoot of rotation speed to characterize start-up stability and the unit rise time to characterize rapidity as optimization objectives,the variable universe fuzzy PID controller is introduced to replace the traditional PID controller,and the open-closed-loop cooperative fuzzy control multi-objective optimization strategy is established.The simulation results show that the open-closed-loop cooperative fuzzy control of the unit can effectively improve the dynamic quality of the start-up process regarding the "one-tube-two-unit" pumped storage power stations,which have good control parameter robustness and working condition adaptability.The research results have significant engineering application value to improve the control quality of pumped storage units and the safe and stable operation of electric power systems.