Hydraulic Performance To Different Types Of Surge Tank In A Hydroelectric System

Hydraulic transient is a common phenomenon happening during the operation of the hydroelectric generating system(HGS).Many control devices are advised for controlling hydraulic transient.Among these,the surge tank is an essential device to control the hydraulic transient of HGSs.However,various types of surge tanks reflect differently in reducing water hammer and improving the performance of HGS.This paper aims to analyze the different performances of Air Chamber Tank(ACT)and Orifice Surge Tank(OST)during the successive load rejection process(SULRP).To simulate the hydraulic transition characteristics during successive load rejection process,this paper applies the theory of pipe flow transient and adopts the method of characteristics(MOC).Moreover,Shitouxia Hydropower Station consisting in three parallel unit systems and one orifice surge tank at upstream is used to validate the established model.After the result of the simulation agrees well with the physical experiment data,the mathematical model of air chamber tank is derived to replace the existing orifice surge tank in the model.There are four cases are proposed to analyze the performance of both types of the surge tank,such as:the first case is the diameter of the orifice surge tank and air chamber are the same,the second cases is the diameter of air chamber is designed to control a specific maximum water head.The other two cases are the optimization process by applying a modified NSGA-Ⅱ algorithm to perform multiple objective optimization to minimize the air chamber properties and operating conditions.As the simulated results show that,the diameter of air chamber tank and the orifice surge tank is 13m and initial air heigh(Ho,air)is 0.45m.The water head in orifice surge tank increase 11.57m and air chamber tank increases only 7.81m.By the way,HGS with air chamber increases the water head at each turbine inlet higher than HGS with orifice surge tank.When the diameter of air chamber tank is 11m with H0,air is 0.45m and diameter of orifice surge tank is 13m,both types of surge tank can control the maximum water head 41.07m.But,the air chamber tank can decrease the fluctuation period from 130 seconds to 110.7 seconds.The water head at each turbine inlet is slightly different but the fluctuation period is shorter than those orifice surge tank..The increasing delay time of the closing guide vane of the last unit(ΔTd)decreases the water fluctuation in ACT and OST and the maximum water fluctuation in OST and ACT reduce with slope-6.2%and-15%when ΔTd is in range of[14,25]seconds.The increasing ΔTd produces both positive and negative impact on water pressure at the inlet of units and overspeed.Lastly,from the Pareto optimal solution of the two scenarios,the best solution of air chamber dimension and the operation conditions are achieved that provides essential guidance for decision-makers to deal with hydraulic transient in SULRP.