Mechanism Research And Troubleshooting Of Faults Caused By Partial Arc Admission

With the development of power industry, many large and medium turbo-generatorsparticipate in power dispatching, and the biggest difference between peak and valley ofload can reach 50%. A type of rotor-bearing system fault occured at some units whichare in the use of nozzle control (under partial admission) to regulate load when operate atintermediate-load, suffered by bearing pad temperature rise and shaft vibration increases.Some other units have to interrupt valve switching process due to excessive shaft vibrationwhen switch from single valve mode to sequential valve mode, thus can not operate underpartial admission. Mechanism of the fault is not clear, and there is no effective solutionto it. To ensure safety, these units have to use throttle control (under full admission),lead to a great deal of economic loss. Therefore, diagnostic studies of the fault to find thecause and the mechanism, and effective solutions, have important theoretical and practicalvalues to improve fault diagnosis theory and to enhance operation safety and efficiency ofpower plant.In this paper, to deal with faults occur in high-pressure rotor-bearing system andduring valve switching process when units operate under partial admission, we carry outa series of fault diagnosis, mechanism analysis, numerical simulation, process simulationand experimental research, and study the corresponding solutions. This paper mainlyincludes the following:Firstly, a type of rotor-bearing system faults occurred at a 200MW unit and a600MW unit under partial admission (partial admission fault for short) is analyzed, sum-mary their typical fault features, and then select the case of 200MW unit to diagnose. Inorder to obtain sufficient fault information, an algorithm based on dynamic time warpingtechnique (DTW) is proposed for fast correlation analysis. It can be used to search forfault-related signals automatically among a large number of signals in the DAS system.Based on the fault-related signals, fault cause is inferred to be asymmetric steam ?owforces (partial arc steam force for short) acting on governing stage rotor blades gener-ated by partial admission. The model of partial arc steam force with variable load of the200MW unit is established by thermal calculation of governing stage. Correlations be-tween partial arc steam force and fault features are analyzed and the inference result is confirmed that the fault is caused by partial arc steam force.Secondly, nonlinear dynamic modeling of the high and medium pressure rotor-bearing system of the 200MW unit is performed. Based on the model of the unit andpartial arc steam force, numerical simulation research on rotor-bearing system dynamicbehavior under the action of the partial arc steam force is carried out with damped new-mark method, to investigate the variation of shaft centerline position, bearing load, rotor-bearing system stability and vibration characteristics with different load. The results ofnumerical simulation are verified by the actual variation of rotor-bearing system underthe action of partial arc steam force through a field experiment. The research reveal themechanism how partial arc steam force in?uences shaft centerline position, bearing padtemperature and shaft vibration, confirm the diagnosis conclusion. For rotor-tilting-pad-bearing system whose stability and horizontal stiffness are higher than rotor-oval-pad-bearing system, an experiment is carried out and illustrate that the impact extent on bothof them are similar.Thirdly, partial arc steam force is considered as an restriction in valve managementprogram design. All 50% minimum admission nozzle control programs are studied withnumerical simulation. The advantage of diagonal admission compared to sequential ad-mission and double-diagonal admission is comprehensively analyzed when consideringpartial arc steam force and throttle loss. The law that diagonal admission in?uences shaftcenterline position, bearing pad temperature and shaft vibration is revealed through exper-imental studies. The feasibility and necessity of controlling partial arc steam force withvalve management are investigated through field experiments, to achieve the purpose ofeliminating partial admission fault, regulating bearing load or increase rotor-bearing sys-tem stability. The loss of efficiency of 1000MW ultra-supercritical unit which use 75%minimum admission in valve management is analyzed. Two indicators are proposed tomeasure the disturbance capacity of partial arc steam force, and are utilized to demon-strate the feasibility of the application of 50% minimum admission nozzle control. Valvemanagement based on a beveled diagonal admission structure is designed to reduce partialarc steam force and can improve the safety of 1000MW ultra-supercritical units. Throttleloss of air-cooling unit in?uenced by seasonal change is analyzed. A multi three-valve-point valve management program is designed to improve the efficiency of air-cooling unitcorresponding to different seasonal conditions as well as ensure safety. Finally, diagnosis is performed for abnormal shaft vibration during valve switch-ing. The cause is analyzed to be the transient impact of steam forces generated by valveshaking, and then the response of steam forces transient impact on the rotor is studied bynumerical simulation. A valve position control loop model is established for the simu-lation of valve switching, and the cause of valve shaking is inferred to be that the valvemanagement ?ow characteristics do not match the actual condition. A method to identifyvalve management ?ow characteristics based on operation data is proposed. By applyingthe method to practice, the characteristics are accurately identified and corrected, and theabnormal shaft vibration during valve switching is effectively solved.