Diagnostics of stator insulatin by dielectric response and variable frequency partial discharge measurements : a study of varied low frequencies in stator insulation, with particular attention to end-winding stress-grading

Stator insulation is a critical to the reliability of electrical generators and motors. It is common industrial practice to use electrical measurement of partial discharges (PD) and some form of current-voltage measurement as part of the condition assessment of stator insulation at maintenance times. Extension of these methods by the recently investigated methods of high-voltage dielectric spectroscopy (HV-DS) and variable frequency phase- resolved partial discharge analysis (VF-PRPDA) may offer r usefully increased information about the condition of a stators insulation while requiring less power from the test voltage source than with conventional power-frequency measurements. HV-DS and VF-PRPDA have independent variables of the amplitude and frequency of a sinusoidal voltage applied to the insulation system. The dependent variables are the smooth currents of HV-DS and the discharge pulse charges of VF-PRPDA; these may be analysed in many ways, typically as complex capacitance, the harmonic spectrum of currents when there is PD activity or other non-linearities in the insulation system, and various measures of PD pulse distribution. The methods provide complementary information and have a common need of a variable frequency high voltage driving source. This makes the simultaneous use of these methods a matter of interest, as further information can be gained without extra time and with a total equipment size and cost smaller than that of both separate systems. In this thesis, results are presented from several directions of work relevant to the application of the low frequency diagnostic methods, HV-DS and VF-PRPDA, to machine insulation. The contribution to measured frequency domain dielectric response from the current into the non-linear stress grading of stator end-windings has been studied from physical and numerical models. As well as the effecects on the dielectric response (complex capacitance), the harmonic spectrum of the current into the grading, and the distribution and waveform of the potential along the grading is shown from the numerical models. The frequency and amplitude dependent response of the stress-grading is of importance due to its significacant contribution to the measured dielectric response of a whole stator and to the harmonic currents due to other non-linear phenomena such as partial discharge currents. Short dielectric response measurements have been made on a complete hydro-generator before and after a period of maintenance, giving a better idea of the practical limitations that time constraints and a large test-object put on possible amplitudes and frequencies for driving the test object. Two new epoxy-mica stator coils have been studied with both DS and VF-PRPDA before and after accelerated thermal aging, as a preliminary step for seeing what changes can be detected electrically and for studying how the PD and DS methods differer in their measurement of PD.QC 2010112