Eddy-current problems associated with screening and negative-sequence heating in large turbo-generators

This thesis is concerned with calculating and measuring eddy-currents and their associatedlosses which are induced in the solid conducting parts of large synchronous generators during periods of abnormal operation. Two methods of measuring loss density are examined: one based upon measurement of surface current density and the other on rate of rise of temperature. The methods are calibrated against theoretical predictions on steel and aluminium blocks and on a circular copper rod. It is found that the current-probe method is more reliable than the rate of temperature rise method in predicting loss density in massive materials. An experimental investigation of eddy-currents, flux densities, and loss densities on a small conventional rotor model are presented. The effects of the wedges, end rings and dc. rotor field have been studied. The experimental results are compared with the theoretical prediction obtained from numerical analysis by Jack (1977). A numerical finite-difference technique has been used to obtain the current distribudon near the joint between two wedges and the damper winding in a rotor slot of large machine. A theoretical investigation has been carried out on the application of an aluminiumscreen in a superconducting turbogenerator. Solutions are obtained by solving the numerical finite-difference equations in two dimensions (r, z), while the third component (0) is assumed to vary sinusoidally. An experimental aluminium screen model was assembled so that measurements could be made for comparison with the theoretical results. The eddy currents, flux density and screening property of this aluminium screen have been studied for variable screen position and with the generator exicted by both straight and helical stator windings.