Research On Safety Of USC Steam Turbine Under Severe Conditions

With the development of national power industry and increase of the electrical grid capacity,large capacity thermal power units are forced to frequently participate in peak load operation due to the gradual wideness of peak-valley difference.In China,more and more large Ultra-Supercritical (USC) participate in peak load operation.As the important factors of restricting the peaking capacity,the rotor thermal stress and life loss especially the security of USC unit under severe conditions have become more and more concerned by people.In addition,there exists unexpected possibility of Sub-Synchronous Oscillation (SSO) in the wide use of series capacitor compensation and HVDC. This oscillation caused by the special electromechanical coupling action may cause damage to the shaft of the large steam turbine generator set. So the torsional vibration on the shaft of the turbine generator set induced by SSR is an important research topic to ensure the security of the power system.Firstly,a 1000 MW USC thermal power unit steam turbine is taken as the object of the study.The thesis sets the thermal boundary conditions of high pressure (HP) steam turbine rotor working under the extreme conditions by thermal calculation method,followed by the heat transfer coefficient between rotor and steam based on the Alston formula. Then,the paper establishes the FE (finite element)analysis model according to the flowing passage of the HP rotor.Secondly, according to the established finite element model,this paper studies the start-up process,identifies dangerous sections of the rotor in different extreme operating conditions,and analyzes thermal stress on the isolation body by FE.The calculation results show that:1)In the start-up process, the part of HP gland seal and the first stage of HP reach the maximum stress;2) The whole complex rotor structure has many stress concentration positions and the maximum stress location is not fixed in a certain position but changes during the process because steam parameter varies;3)In view of the whole start-up process,the maximum thermal stress value appears in the transitional fillet of the front gland seal.This is mainly because the high steam parameters are most variable,and the transition fillet causes a certain degree of stress concentration.Thirdly,according to the established model,the paper analyzes the load rejection and FCB(fast cut back)conditions by FE,and tests the maximum thermal stress at 50% load rejection,75% load rejection and 100% load rejection as well as the value at 50% load FCB and 100% load FCB.Calculation shows that:l)Under the same load,the maximum stress at rejection load is slightly larger than the corresponding FCB process;2)In load rejection or FCB process,maximum thermal stress value also appears at the wheel root of the HP last stage where stress is prone to be concentrated;3)In load rejection,the thermal stress can be effectively controlled by artificial intervention to reduce the life loss of the rotor;4) Using the control variable method,it is concluded that sensitivity of thermal stress to thermal conductivity,linear expansion coefficient and elastic modulus is on maximum,more sensitivity to the density and specific heat,and minimal sensitivity to Poisson's ratio.Fourthly,this paper studies the low cycle fatigue crack life of the rotor. The formation process of fatigue is mainly microscopic.From the perspective of the formation mechanism,the fatigue life arises from the imbalance of the force change of metal materials.Based on the calculation of centrifugal stress under various working conditions in the key position of the rotor,combined with thermal stress simulation,the results of low cycle fatigue crack life loss in startup, load rejection and FCB conditions are as follows:1) In dynamic change of load, centrifugal stress value reaches maximum in the low pressure cylinder impeller, which is in the range of safety; 2)In start-up,low cycle fatigue crack life loss is calculated under different temperature rise rates;3)In FCB process,due to rapid changes of load,the corresponding steam parameters in the rotor temperature field change dramatically,causing large thermal stress inside the rotor,which will lead to certain amount of life loss.The study calculates the value of low cycle fatigue crack life lost at 50% load FCB process and 100% load FCB process.Finally, according to the actual state of power system,the thesis constructs the sub-synchronous oscillation analysis model and SSR impedance analysis model. By establishing the model for sub-synchronous oscillation analysis in PSCAD system,the study finds that three-phase output voltage and three-phase current fluctuate violently after the SSR fault occurs. Strong unstable oscillations appear in the shafting torques between the two low-pressure rotors,the generator and the exciter. In the study of impedance characteristics of SSR,the research confirms the accuracy of the established model by comparing the theoretical value with the simulation value and compiles early warning procedures for the Sub-Synchronous Oscillation combined with the Sub-Synchronous Oscillation simulation model and impedance scanning circuit in PSCAD/EMTDC.