Numerical impulse response tests to identify dynamic induction-machine models

OF DOCTORAL DISSERTATION HELSINKI UNIVERSITY OF TECHNOLOGY P.O. BOX 1000, FI-02015 TKK http://www.tkk. fi Author Anna-Kaisa Repo Name of the dissertation Numerical impulse response tests to identify dynamic induction-machine models Manuscript submitted 7.12. 2007 Manuscript revised Date of the defence 4.4. 2008 Monograph Article dissertation (summary + original articles) Faculty Faculty of Electronics, Communications and Automation Department Department of Electrical Engineering Field of research Modelling of electrical machines Opponent(s) Prof. Thomas A. Lipo Supervisor Prof. Antero Arkkio Instructor Abstract The aim of the study is to identify dynamic models for induction machines using two-dimensional finite-element analyses. Attention is given mainly to cage-induction machines with closed rotor slots and deep rotor bars. However, cage-induction machines with semi-closed rotor slots and slip-ring machines are considered as well. Two new applications of numerical impulse response tests are presented. The tests are performed within a time-stepping finite-element analysis. First, an impulse excitation is applied to the stator voltage and the response of the stator current is studied. Next, an impulse excitation is applied to the rotor position angle and the response of electromagnetic torque is studied. The results of the impulse tests are used to estimate parameters for small-signal models in the frequency domain. The estimation is performed by fitting the results of an impulse response test to the corresponding analytical small-signal model. The analytical models are based on the theory of electrical machines and their parameters have a physical meaning. Based on the study, it is concluded that the skin effect in rotor bars has a significant influence on the results of the impulse response tests. In order to obtain a good fit, the skin effect has to be taken into account in the structure of the analytical model. In addition, the saturation of iron has a prominent effect. The effects of saturation can be detected as harmonics in the frequency response and in the values of parameter estimates. A model for saturation can be included in the small-signal models. In the case of this more advanced model, the parameter estimates obtain a clear physical interpretation. The applicability of impulse response tests is based on the assumption of linear time-invariant behaviour of the system. The assumption is studied by several means. Within the tests, the amplitude, length, and initial time of the impulse excitation is varied and the results are compared. It is concluded that, in the case of normal operation points, the assumption can be made. The reasonability of the parameter estimates is evaluated by comparing the values with steady-state circuit parameters. The steady-state parameters are estimated using a time-harmonic finite-element analysis. When the impulse is applied to the rotor position angle and the torque response is computed, the results predict negative electromagnetic damping within some frequency ranges. This phenomenon is also studied by measurements. It is observed that, within the range of predicted negative damping, self-excited torsional oscillations occur.