Safety Assessment Of A Hydropower Generation Unit And Its Regulation Effects On Benefits Of A Multi-energy Power System

Under the background of low-carbon electricity transformation,the hydropower generation unit,i.e.conventional hydro and pumped hydro,as a stable and flexible resource will provide more opportunities for the penetration of wind and solar renewable energy resources.Hydropower generation units will face harsh transient processes due to the huge peak-valley difference of power load and the hydro-mechanical-electrical coupling characteristic.During such transient processes,the large fluctuation of critical indices,e.g.vibration of upper bracket,swings of bearing,and water pressure pulsation,seriously threatens the safe operation and hydro regulation.For these reasons,this paper aims to establish a dynamic safety assessment framework,which focuses on two key scientific problems:i)revealing the complex hydro-mechanical-electrical coupling mechanism,and 2)understanding the potential risk law behind complex fluctuation of multiple indices.Moreover,we put the above two problems under the scenario of the ever-increasing pentration of renewable-based electricity in order to optimize the dynamic hydro regulation and complementary benefit.The specific research achievements include the following three aspects.1.The target of the first key scientific problem is to overcome the shortage of conventional models of hydro-turbine governing system and shaft system in describing hydro-mechanical-electrical coupling mechanism.For this aim,the paper attempts to establish a dynamic coupling model from some proper cut-in points.This research part includes two specific contents,as follows:（1）Take the conventional hydropower generation unit with two bifurcation pipes as a research object.Its hydraulic unbalance force of runner is deduced from the equation of turbine torque.Then,using the hydraulic unbalance force to link the generator unbalanced magnetic force,damping force,rubbing force and the nonlinear oil film force of hydro guide bearing.The close relationship between hydraulic subsystem and mechanical-electrical coupling subsystem is build.The characteristic line is employed to solve transfer functions from penstock to draft tube,and the four-order Runge-Kutta method is employed to solve force equations.In this way,a coupling model between hydro-turbine generating system and shaft system is established.Using the validated coupling model to further study the effects of structure or operating parameters on vibration charateristics in the start-up transient process.These critical parameters are also optimized.As a result,the unit can improve its transient stability with a more economical and simpler optimization method.The results show that the relationship between vibration amplitude of rotor and self-regulating coefficient is approximately described as a quadratic equation while the the relationship between vibration amplitude of rotor and diameter ratio of inlet-outlet runner is approximately described as a quintic equation.When the distance of bearing eccentricity is approximately equal to 1×10^-6,the unit can lose sensitivities to vibrations.The optimal value of the diameter ratio of inlet-outlet runner is nearly equal to 0.8,and the optimal value of the self-regulating coefficient is nearly equal to 3.（2）Take the pumped hydropower generation unit with two bifurcation pipes as a research object.Its abnormal water pressure in pumping phase modulation is considered as a Gaussian random excitation or step excitation.These external excitations can influence the output of active power,and then further influence the characteristic of reactive power.From this cut-in point,a close relationship between hydraulic subsystem and mechanical-electrical coupling subsystem is linked.The characteristic line is employed to solve transfer functions from penstock to draft tube,and then the Matlab/Simulink block is employed to link models of pipe,excitation device and electromotor.In this way,a complete multi-machine phase modulation model is established.Using a validated simulation model to study the transformation mechanism between leading and late phases as well as the reactive flow between multiple engines under different external excitations.Take the failure of emergency shutdown in an existing hyropower as an example,some risk mitigation suggestions are provided.The results show that the exticted unit is easily to reduce the leading phase depth of parallel unit and even turn to late phase operation.It is also shown that the step excitation has more influence on phase transformation than the Gaussian random excitation,and the auxiliary regulation of field current can mitigate phase modulation instability cased by a large step excitation.2.The target of the second key scientific problem is to overcome the difficulties in risk feature extraction and risk performance classification.For this aime,the paper attempts to present some dynamic risk quantification methods to explore the correlation law among indices and the correlation law between index and risk level.This research part includes two specific contents,as follows:（1）To precisely define the prohibitive load area and mitigate the risk in recommended load area,the dynamic entropy method is used to improve static methods.Then,some dynamic risk assessment methods are presented,including the dynamic entropy-fuzzy method and grey-entropy correlation method.Based on revised critical indices,these dynamic risk assessment methods are further used to explore the potential risk law in prohibitive load area and recommended load area.The real-time risk of unit is quantified in the form of probability,and high-risk indices are also sorted.The detailed results show that the prohibitive load area can be reduced from 0 MW～121 MW to 0 MW～100 MW,and 21MW additional adjustable capacity for flexible dispatching will be supplemented.It is also shown that there are obvious differences in the risk rank of indices under different operating heads in the recommended load area,and this mean that high-risk components are different under different operating heads.（2）To mitigate load rejection risk of pumped hydropower generation unit in turbine working condition,an advanced Fisher discriminant method is presented to explore risk evolution characteristics of indices.The advanced Fisher discriminant method is improved by large sets of training data and corresponding risk discrimination criteria.The real-time risk of unit is quantified in the form of probability under two regulation modes.These two regulation modes involve the separate mode only to include a guide vane（mode 1）and the linkage mode that includes a guide vane and a ball valve（mode 2）.The results show that the unstable operation probabilities for the regulation modes 1 and 2 are 0.23 and 0.16.This means some serious stability problems（e.g.water pressure）occurs after the load rejection of unit under the guide vane linear closing mode.If the regulation mode is not optimized,long-term operation will cause fatigue damage to components.It is also shown that the risk evolution performs a bimodal characteristic under the two regulation modes.In this situation,the first risk peak occurs in the initial stage of load rejection,while the second risk peak occurs in the late stage of load rejection.The auxiliary regulation of ball valve plays a minor role in improving safety level in the first risk peak,but it plays an extremely important role in reducing risk probability in the second risk peak.3.A progressive study is conducted by considering the adverse impact of high pentration of renewable-based electricity on safety operation and regulation performance of a hydropower generaiting unit.This research part overcomes the defect that the existing economic objective function lacks the quantification of regulation costs of flexible hydropower units.Firstly,some dynamic hydro regulation indicators,including overshoot,rise time,regulation time and response peak,are presented to estimate the optimization effect of the PID control parameter,energy ratio and transmission line lenghth on complementary characteristics of a hydro-photovoltaic power system.Secondly,take a hydro-wind power sytem as a research objective.Extracting the most sensitive indicator,i.e.overshoot,to quantify the hydro regulation cost.Comprehensively considering some input-output factors such as the power loss cost,investment cost and profit from selling electricity,a power benefit assessment method is finally presented.The presented method is further used to investigate the effects of wind type,wind penetration ratio and electricity price scheme on power benefits.The results show that the wind benefit has a chance to exceed the hydropower benefit under the most unfavorable wind condition if the wind penetration ratio is more than 54.5%.It is also shown that the total benefit generated from time-of-use price is higher than that from the fixed price,approximately ten thousand yuan per day.