Dynamic Modeling And Simulation Analysis Of The Shaft System For Large Hydro-turbine Generator Units

Vibration is one of the most important factors affecting the safety and stable operation of hydro-turbine generators. Excessive vibration can cause fatigue damage to the unit equipment and shorten the life of units, and may even lead to a serious damage accident, resulting in significant economic losses. With the continuous improvement in water head and single-machine capacity, vibration problems of the hydro-turbine generator units are increasingly prominent and have become key issues to be solved in the hydropower industry. An appropriate dynamic model of the shaft system of a hydro-turbine generator unit is of great significance for the clarification of vibration mechanism, the diagnosis of vibration failure and for the implementation of technology of the condition based maintenance.Taking 125 MW hydroelectric units in Gezhouba as example, a comprehensive rotordynamic modeling and simulation of the shaft system of a large hydro-turbine generator unit is investigated in this study. The dynamic model is built up taking full account of the structural characteristics of the unit to ensure its feasibility.In accordance with the structural characteristics, dynamic relevant factors are divided into four modules of shaft, boundary conditions, exciting forces and operating conditions logically. While Shaft is the essential part of a shaft system including all the rotating part of the unit. Both boundary conditions and exciting forces, which are affected by the operating conditions like rotating speed, field current and load etc., are important factors affecting the dynamic response of the shaft system.Finite element (FE) method is selected to model the shaft system. The modeling process of the shaft system by FE method is discussed in detail along with the solutions to the problems encountered in the process of modeling. The boundary conditions and the exciting forces are discussed in detail respectively, both of which are very important factors affecting the dynamic response of shaft system. The boundary conditions, including guide bearing, thrust bearing and seal etc., are closely related to the dynamic response of the shaft system, and meanwhile, have important influence on the dynamic response. Numerical method could be useful to obtain accurate bearing coefficients, but coefficients could not be updated with the operating conditions due to the oversized computational cost. To overcome this problem, a simplified and practical coefficient database method is proposed. The database method takes much less computational cost than numerical method and the accuracy could be guaranteed by a suitable database. Effect of the rotating speed and bearing clearance on dynamic response could also be considered by the method. The exciting forces, like unbalanced force due to mass eccentricity, hydraulic force, unbalanced magnetic pull (UMP) and exciting forces induced by the failure etc., are another type of important factors affecting the dynamic response of shaft system. Some strong individuality features, behaved in the operation of hydro-turbine generator unit, are probably caused by the exciting forces due to operating environment. Hydraulic force and UMP are induced by the fluid-structure interaction around runner and electromagnetic field around rotor respectively. Numerical method such as FE method could be useful to obtain proper solution for the induced force. However, problems due to oversized computational cost also exist. Therefore, an analytical method for unbalanced magnetic pull taking advantages of the no load characteristic curve of a generator is proposed. The method takes account of field current and its saturation effect on UMP, which would be more suitable than treating the UMP as a negative stiffness coefficient. To simulate the hydraulic imbalance on the runner, unbalanced moments are exerted on the rotational coordinates.Finally, a simulation model describing dynamic characteristics of the shaft system is established by systematically analyzing the factors mentioned above. The simulation model, which is described by high dimensional second order differential equations, is a linear dynamic system with local nonlinearity only at several nodes. The dynamic response of the model can be obtained within a reasonable time by a numerical integration method. Taking the advantage of the model, dynamic response of the unit are calculated under rotating speed test, excitation test, variation of operating conditions and opening uneven of the runner blades and the guide vanes. Comparative analysis between the simulation with the the vibration monitoring information acquired by the Optimal Maintenance Information System for Hydropower plants (HOMIS), many meaningful results about vibration fault mechanism are obtained. Although not all the factors are considered, the current simulation model still shows its application value for illustrating the fault mechanism and diagnosis. If all the factors are considered properly, a more applicable simulation model could be obtained. And higher accuracy, wider application range and greater application value of the model would be obtained.