The rotor thermal sensitivity is a phenomenon that often affects the dynamic behavior of power unit generators. Changes in the field current, and other process parameters correlated to it, cause changes in the machine vibration. Some different causes can give rise to a rotor thermal sensitivity. A&nbsp;local deterioration of the rotor winding insulation causes an uneven circumferential distribution of the rotor temperature. Moreover, the axial thermal expansion of the copper bars may not happen uniformly in the circumferential direction. These phenomena cause a shaft bow whose magnitude depends on the field current.A generator thermal bow can also be caused by the large difference in the thermal expansion coefficient between copper bars and the steel alloy rotor. Changes in the machine thermal state, due to changes in electrical load and reactive power, cause different thermal expansions of these components and, in turn, friction forces between the bars and the slots. Sometimes, local jamming can occur between these two components. This phenomenon gives rise to a rotor bow whose magnitude can be affected by different process parameters.This fault mainly causes changes in magnitude and phase of the generator synchronous vibration vectors. This symptom can also be caused by many other common malfunctions that affect rotating machines. Therefore, diagnostic techniques that are able to identify the actual fault are very useful for optimizing maintenance activities.&nbsp;The study of the evolution over time of the effects of the rotor thermal sensitivity on the machine vibration is also very important to schedule maintenance actions, timely.This paper shows the results obtained applying an original diagnostic method for the analysis of generator vibrations caused by rotor thermal bows. This technique is based on multiple linear regression models. The goal of this method is to model the relationship between a suitable set of independent variables, represented by process parameters, and the dependent variable, which consists of the generator vibration caused by the malfunction.Regression models consider more terms in the linear regression equation, with each term representing the effect of different physical parameters. The optimization process of the regression model, performed to increase the accuracy of the fitting between predicted and observed data, points out the contribution of each process parameter to the vibration ascribed to the investigated malfunction.Moreover, changes over time in the regression coefficient associated with each independent variable denote an evolution of the sensitivity of the machine vibration to single process parameters. This information can be very useful for optimizing maintenance activities.To simulate non-linear relationships between process parameters and machine vibration, fictitious independent variables can be defined. For instance, owing to the Joule effect, the heat generated by the current flowing through the rotor bars can be expressed as a function of the square of the field current. This phenomenon is emphasized in occasion of the first rise of the electrical load at the end of a unit runup. In this case, the generator rotor is subjected to a significant heating that produces a temperature gradient in radial direction, between the shaft core and the outer part that contains the slots and copper bars, where the heat is introduced. Further parameters, like temperature of the cooling fluid and reactive power also affect the shaft thermal bow and 1X vibrations.Moreover, the high thermal capacity of large generator rotors may cause a significant delay between quick considerable changes in process parameters and the corresponding changes in the machine vibration. A&nbsp;method based on the response to a unit step input has been developed to define fictitious independent variables of the regression model that better simulate the relationships between process parameters and the dynamic behavior of the generator.The capabilities of the diagnostic technique used to simulate generator vibrations caused by a rotor thermal sensitivity have been validated with the analysis of experimental data collected in a power plant. The results of this investigation are shown and discussed in the paper.

An Unconventional Method for the Diagnosis and Study of Generator Rotor Thermal Bows

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2021/volume/84898

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