| The pumped-storage plant(PSP)plays a major role in the primary and secondary frequency regulation of the power grid.These regulations,being unpredictable and never-ending in nature,result in varies transients within PSPs and consequently lead to complicated situations.To this day most PSPs constructed or designed adopt a shared-conduit,two-unit layout,which combines the hydraulic coupling with the existing electric coupling,introducing countless new engineering problems,and among them is the small signal stability problem.From the system modeling perspective,a shared-conduit,two-unit layout involves many more modeling components and complex relationships between them,ultimately gives high order equations too hard to solve and analyze.From the case study perspective,there is a significantly larger amount of parameter combinations for a PSP of such layout,and many of these cases are unique to the two-unit system.A few researches have been done towards the subject of synchronized operation and stability of PSPs,however there is no integrative,all-in-one model for PSPs of shared-conduit,two-unit layout proposed,and existing models are only supported by computer simulations,without real experiment data to prove them.These drawbacks inevitably prevent the owners of PSPs from more efficient but radical operation strategies,restricting the usage and limiting the advantages of PSPs of shared-conduit,two-unit layout.To eliminate the weakness of former studies,this thesis aims to probe into the small signal stability problem of PSPs of shared-conduit,two-unit layout by combining theoretical analysis,computer simulation and experiment data,reveal the mechanism of the transient process and interactions between components,The whole procedure can be divided into:(1)Designing and compilation of a method of characteristics(MOC)simulation program.Based on the classical MOC method for one-dimensional hydraulic transient problems and Preissmann four-point finite-difference implicit method,every pipe segment in the system is converted to an elementary object to be used alongside other components.All non-pipe components are analyzed and modeled conforming to the requirements of the MOC method,and for the pump-turbine the B-spline surface fitting method is used to allow unhindered interpolation in the program.Topology theories are applied to establish the relationships between components as well as their respective equations used in paralleled computation.The resulting MOC simulation program is able to simulate most of the transient situations in a PSP of shared-conduit,two-unit layout,and enables extensive comparison with results from other methods such as the simplified transfer function(STF)method,improving their modeling accuracy.(2)Small signal stability analysis of PSPs of shared-conduit,two-unit layout.Taking the impedance approach,transfer functions of PSP components can be composed into a set of equations which completely covers every characteristic of interest of the system.The accuracy of this approach is then verified through comparison against experiment data from a real PSP and MOC simulation results.For each particular operation conditions,specialized equations are derived from the original ones and root loci analysis as well as frequency margin calculation are conducted to obtain the stability of these systems with respect to governor parameters Kp and Ki.From these time and frequency domain analysis,the stability of systems in different operation conditions are assessed and conclusions are made,which can serve as guidance to the operation in real PSPs.(3)Experiment and analysis on a model PSPs of shared-conduit,two-unit layout.With the help of a well-established model PSP of shared-conduit,two-unit layout,a series of experiments are devised and carried out to determine the stability criterion of the system operating in one-unit condition.The stability criterion from the test result is compared with that deduced using the transfer functions,and real time domain transients are also compared with those from the MOC simulation.High degrees of consistency is seen in both comparisons,and thus proved the correctness as well as accuracy of theoretical analysis and computer simulation.In addition experiment results of two-unit operation are provided,while no comparison is made between test results and simulations,general conclusions drawn with the theoretical analysis are verified.Verified by MOC simulations and experiment data,the integrated transfer function model of PSP of shared-conduit two-unit installation is ready for use in the small signal analysis of such type of PSPs commissioned or in construction.This model can be further utilized in the small signal analysis of PSPs with more than two units when properly modified,making it an effective tool in both theoretical research and engineering practices. |
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