【作者】 徐永；

【导师】 李朝晖； 赖喜德；

【作者基本信息】 华中科技大学 ， 系统分析与集成， 2012， 博士

【摘要】 振动是影响水轮发电机组安全、稳定运行的最重要因素之一。过大的振动会导致材料的疲劳损坏,缩短设备的使用寿命,甚至可能引发严重的机组毁坏事故,造成重大的经济损失。随着水轮发电机组单机装机容量和水头的不断提高,机组的振动问题日益突出,已经成为水电行业亟待解决的关键课题之一。建立一个合适的水轮发电机组轴系动力学模型对于振动机理的阐明、振动故障的诊断以及状态检修技术的实现都具有重要的研究意义。本文以葛洲坝电厂125MW机组为例,深入研究了大型水轮发电机组轴系的动力学建模与仿真。为了建立更准确的轴系动力学模型,详细分析了机组的结构特性。针对机组的结构特性,将机组轴系的动力学相关因素按逻辑分为轴系本体、边界条件、外部激励、运行工况四个模块。轴系本体指机组的旋转部分,为主要的研究对象。边界条件和外部激励是影响轴系动力响应的重要因素,而运行工况参数如转速、励磁电流及负荷等对边界条件和外部激励都具有重要的影响。在认真总结转子动力学建模理论的基础上,选用了有限元方法建立机组轴系本体模型。详细讨论了利用转子动力学有限元方法建立轴系本体转子模型的过程,以及实际机组建模过程中的问题及解决办法。在此基础上,分别详细论述了对机组轴系动力响应具有重要影响的边界条件和外部激励。其中边界条件主要包括导轴承、推力轴承及密封等,与轴系的动力响应密切相关,同时对轴系的动力特性具有重要影响。数值计算的方法可以较精确地求解轴承动力特性系数,但是因为过大的计算量而无法在轴系响应分析时实时更新系数。为了解决这一问题,提出了简洁而实用的基于动力特性系数数据库的轴承分析方法。既解决数值计算方法计算量过大而无法实用的问题,又改进了传统的轴系响应分析时轴承分析过于简化的缺陷。该轴承分析方法还可以考虑转速及轴承间隙对轴承力的影响,使模型能够仿真分析轴承参数对轴系动力响应的影响。外部激励指质量不平衡力、水力、不平衡磁拉力及故障因素引起的受力等。水轮发电机组在运行中表现出的一些“个性”较强的特征,往往是由运行环境所引起的外部激励所造成。轴系所受的水力及不平衡磁拉力分别由流固耦合作用及电磁场作用引起,通过有限元等数值计算方法能够计算得到较精确的受力。但是,同样存在计算量过大而无法适用于以轴系响应分析为目的的模型的问题。为此,提出了利用发电机空载特性曲线分析计算不平衡磁拉力的方法。该分析方法考虑了励磁电流及其饱和效应对不平衡磁拉力的影响,改进了以往轴系响应分析中用一个负刚度系数简化不平衡磁拉力作用的做法。提出了施加不平衡力矩于轴系转角自由度的方法模拟水力不平衡。通过对机组轴系各模块的系统分析,得到一个描述机组轴系动力学特性的仿真模型。仿真模型是一个只在若干结点自由度上具有局部非线性因素的线性动力系统,由一个高维二阶微分方程组描述。利用数值积分方法求解该系统运动方程,可在合理的时间内得到轴系的动力响应。利用动力学模型仿真分析了机组在变转速试验、变励磁试验、工况变化以及轮叶和导叶开口不均下的振动特性。与葛洲坝水电站最优维护信息系统(HOMIS)采集到的振动监测信息进行对比分析,得到一些有意义的振动故障机理推断知识。分析结果显示,仿真模型在振动机理的阐明、振动故障的诊断方面已显示出其应用价值。在该轴系仿真模型框架下,不断完善相应结点下各种影响因素的考虑,可发展成为更加成熟的仿真模型。模型精度将更高,应用范围将更广,应用价值也将更大。更多还原

【Abstract】 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.更多还原