Arefieva M. Methods of increasing the vibration resistance strength and reliability of hydrogenerators due to the improvement of the design of the elements of the rotor and the crosses according to the criterion of own frequencies

The dissertation addresses improving the vibration resistance and reliability of hydrogenerators by enhancing the design of support-guide cross-arms and determining critical rotor frequencies. A causal relationship between cross-arm design, its non-uniform stiffness, and resonant phenomena in the shaft line was established. Natural frequencies and vibration mode shapes of the shaft line were calculated in three dimensions, accounting for bearing compliance. The combination of radial, axial, and tangential loads on a bridge-type cross-arm with insufficient rigidity causes the natural frequency to coincide with operating speed, leading to excessive vibration amplitudes. Based on vibration tests, a transition to a radial-type cross-arm with increased stiffness and shifted critical frequencies is proposed. The goal of the thesis is the development of methods and algorithms for the study of a stress-strain state of the support-guide cross-arm (cross-piece) of a large-power hydrogenerator in a three-dimensional formulation that accounts for technological, geometric, and oil-film contributions. Within this framework, analytical assessments of the stiffness of the cross-arm’s main elements and the equivalent stiffness of the bearings and thrust jacks are integrated as boundary-condition parameters for determining the rotor’s eigenmodes and critical speeds. The results are then used to justify and implement design solutions that increase the cross-arm stiffness, thereby reducing shaft-line vibrations and improving the unit’s reliability. The dissertation provides scientifically substantiated methods for determining stiffness characteristics and stress-strain states of hydrogenerator support-guide cross-arms. Results quantify how design parameters affect shaft line dynamics and serve as a basis for assessing existing unit reliability. Scientific novelty of the work: 1. For the first time, a new method has been developed for calculating the stress–strain state of the hydrogenerator support–guide cross-arm (cross-piece) which, unlike existing approaches, accounts for short-circuit forces and the non-uniform mass component of the hydrogenerator, with stepwise refinement of load factors corresponding to critical operating regimes. 2. Existing engineering methods for calculating the eigenfrequencies of hydrogenerator rotors have been improved in terms of the exact specification of structural geometry and the inclusion of the equivalent compliance of supports and thrust jacks in a three-dimensional formulation with prescribed first-kind (Dirichlet) thermal boundary conditions; spatial bending patterns (mode shapes) have been obtained in three planes. 3. The method for calculating the compliance of bearing and thrust units of the hydrogenerator has been further developed, enabling—unlike existing methods—a three-dimensional assessment of the respective contributions of the technological, geometric, and lubricating film components. Scientific and practical significance of the results: 1. The new design of the modernized cross-arm (cross-piece), which differs from the existing additional power zones, which provide the necessary rigidity of the design for high-power Hydrogenerators, the necessary supply of strength and vibration resistance according to existing normative documents. 2. Developed methods allow to develop support-guide cross-arm (cross-piece) with optimal mass-dimensions characteristics, with the necessary rigidity of the design, which includes the design of the spacing elements namely jacks, for damping actions from the moment of short-circuit poles. Results are applicable to research organizations, design bureaus, energy sector organizations, universities, and specialized power equipment research and operation centers.