ANALYSIS OF GENERATOR FOUNDATION OF A HYDRO-ELECTRIC POWER HOUSE

Abstract

Scroll case and generator foundation are the most important parts of a hydro-electric power house whose irregular shapes render their structural design rather complicated. However in almost all the designed organisations of the world these parts of a power house are designed by dividing them into simpler elements amenable to two - dimensional analysis. A review of the practices adopted by various design organizations shows that sometimes they are inconsistant with each other. The design of the structure at present is usually based on empirical rules and engineering judgement rather than on rational theory. The photoelastic study carried out on a small size model for vertical generator loads by Nigam in 1972 is the only major investigation carried out on this type of structure. The present investigation has been carried out to analyse the generator foundation and scroll case of a hydro electric power house considering all possible load combinations. It gives a real knowledge of stress concentration in concrete around scroll case and generator foundation. A simple and efficient method of slices for geometric representation of the structure has been developed. The method which divides the structure into radial slices in plan, helps in pre-processing and « post-prcessing (i.e. it makes the input data preperation very simple and output interpretation easier). Two computer programes of 3-D mesh generation and piloting have been developed to evolve the method of slices for three dimensional solutions. IV A full 3-D finite element analysis of scroll case and generator foundation including the barrel shaft at the top and mass concrete upto the top of the elbow portion of the draft tube at the bottom has been carried out. This gives the effects of participation of the foundation and barrel with scroll case. The main objective of the three dimensional FEM study is that it gives a firm basis for establishing a design/analysis methodology based on plane strain and axisymmetric finite element analysis so that the order of accuracy for various load cases are identified. Approximate analysis of the structure has also been carried out using plane strain, axisymmetric and boundary element idealizations. The same method of slices has been used in all of the above analyses. Pre-processors for 3-D FEM, Plane strain and axisymmetric cases are able to generate the same number of elements in all the slices resulting simplicity in the comparison of the results from diffrent analyses. Results from different analyses are compared with Nigam's photoelestic study for vertical generator loads only. In conclusions it has been found that axisymmetric and plane strain analyses of different slices is a suitable replacement for 3-D finite element analysis of the structure, so that a design methodology as proposed can be used.