The determination of the transient temperature distribution in a turbo-alternator rotor by means of an analogue

Owing to the difficulty in the direct measurement of temperature in the winding and steel of turbo-alternator rotors, indirect methods, requiring a knowledge of the boundary conditions, can be used to advantage. In the present work the electrical-analogue approach is used, i.e. the replacement of thermal quantities by analogous electrical ones, giving both steady-state and transient solutions. The latter is of interest when considering overload capacity and unbalanced loading. The rotor studied was that of a 60 MW 3000r.p.m. 11kV 50c/s turbo-alternator with indirect cooling by hydrogen at a pressure of 41b/in2 (gauge). Design and constructional features of a resistance-capacitance analogue representing two-dimensionally the thermal conditions in a `slice¿ of rotor 1 in thick and containing the hottest slot are given. By simple adjustment any slice in the rotor length may be simulated. Incorporated in the analogue are electronic current-input units which vary the `copper-loss¿ input currents to allow for the change in winding resistance with temperature. Transient temperature rises at 20 points in and around the hottest slot in sections at the rotor end and at the axial centre are obtained for the application of step-functions of excitation currents of 500 A and 550 A and various values of surface loss. The mean winding temperature obtained is compared with that measured on the actual machine using the change-of-resistance method and good agreement is found. Variations in the coolant temperatures are seen to have small effect on the analogue temperature-rises.