M. Farahani

Behavior of machine insulation systems subjected to accelerated thermal aging test

Accelerated aging tests have been extensively used on motor and generator insulation systems to simulate in a short time period the deterioration mechanisms occurring during normal operation of many years, to identify insulation system designs with longer life time and to support the qualification of a new system. The contribution presents results from extensive investigations on the behavior of two VPI machine test insulation systems (reference and candidate) with thermal class F (155°C), which were subjected to three thermal overstresses in repeated cycles. After each thermal cycle the insulation system is subjected to mechanical stress, moisture, and voltage. In order to check the condition of the insulation systems in a non-destructive manner several properties of insulation like dissipation factor, insulation resistance and partial discharge quantities were measured after each thermal cycle. After that the end of life of the insulation systems were checked with ac voltage and impulse voltage. The results support the qualification by comparative evaluation and they indicate, how the changes in electrical and dielectric properties of insulation take place during the accelerated aging test.

Dielectric response studies on insulating system of high voltage rotating machines

A significant method to characterize the electrical properties of materials and to evaluate the condition of the insulating systems is the dielectric response study which can be done in the time or frequency domain. The dielectric response measuring results can be affected by different parameters like measuring voltage, temperature, water content and aging condition of insulating system. In this work numerous measurements are presented, thus to understand, how the dielectric response measurement results at the insulating system of high voltage rotating machines can be affected by these parameters in the time and frequency domain. Because of the importance of the dielectric response application as a non-destructive diagnostic method the possibilities and limitations of this method to reflect the deterioration processes due to thermal and electrical stress were also investigated. In addition the possibility to transfer the measuring data from the time to frequency domain was investigated and the calculated and measured data were compared. For the test object actual stator bars were used that were manufactured with epoxy VPI technology.

Partial discharge and dissipation factor behavior of model insulating systems for high voltage rotating machines under different stresses

This article describes the change in partial discharge (PD) pattern of high voltage rotating machines and the change in the tan /spl delta/ as a function of the applied test voltage during the aging processes as caused by the application of different stresses on stator bars. It also compares the PD patterns associated with internal, slot, and end-winding discharges, which were produced in well-controlled laboratory conditions. In addition, the influence of different temperature conditions on the partial discharge activities are shown. The investigations in this work were performed on model stator bars under laboratory conditions, and the results might be different from those obtained for complete machines, as rotating machines are complex PD test objects, and for example, the detected PD signals in a complete machine significantly depend on the transmission path from the PD source to the measurement device.

Dielectric spectroscopy in time and frequency domain on insulation system of high voltage rotating machines

In this work the relations between the time and frequency domain parameters of the dielectric spectroscopy will be discussed. Then the results of dielectric response measurements in time and frequency domain on the insulation systems which are used for high voltage rotating machines will be reported. As test object, actual stator bars were used based on the epoxy VPI technology. The linearity of the insulation system will be checked and the possibility to transfer the measuring data from one domain to another investigated. The paper compares the calculated and measured data and shows the influence of difference parameter.

Calculation and measurement of dielectric response function in insulation systems of high voltage rotating machines

In this work the possibility of the dielectric response measurement for the evaluation of the actual status of insulation system of high voltage rotating machines are investigated. As test object, model stator bars were used, which are close to real stator bars of high voltage rotating machines. An accelerated ageing procedure was applied to the model bars and the dielectric response was measured in time domain. The dielectric response function was calculated from depolarization current measuring results and the change of this function was evaluated.

A method for the evaluation of insulation systems for high voltage rotating machines

Partial Discharge Pattern of three important types of defects in insulation systems of high voltage rotating machines (the internal defect, the damaged semi-conductive screen and the defect on grading coating) were studied. As test object, model stator bars with artificial defects similar to practical ones on the ground wall and end-winding of bars were used. The internal defect in insulation was furthermore realized with a thermal ageing procedure on machine bars. The comparison of PD patterns, that were caused by different defect on stator bars presents clearly that the different defects generates different partial discharge types and that the defect type can be recognized and used for diagnosis of the insulation of high voltage rotating machines. For the interpretation of off-line and on-line measurements, the partial discharges were also measured at different temperatures in order to generate a basis for the on-site measurements, which can be only carried at given temperatures depending on the load conditions.

Experience with partial discharge, dissipation factor and recovery voltage measurements for the evaluation of insulation systems of high voltage rotating machines

Possibilities and limitations of partial discharge (PD), recovery voltage (RV) and dissipation factor measurement for evaluation of the actual status of insulation systems of high voltage rotating machines are presented. An accelerated cycling thermal ageing procedure was applied on a model insulation system similar to a real machine insulation system and during the ageing procedure the development of the recovery voltage phenomena, partial discharge activities and dielectric dissipation was investigated.

Investigations of diagnostic tools for insulation systems of high voltage rotating machines

In this work the results of extensive investigations on the behaviour of a solid insulation system for rotating high voltage machines with thermal class F (155 °C) will be reported, which was subjected to a thermal overstress (215 °C) in repeated cycles. The test parameters should represent the thermal deteriorating effects of the insulation system in service on an accelerated basis, in order to evaluate the condition of the insulation system in a non-destructive manner. The results indicate how the changes in electrical and dielectric properties of insulation take place during the accelerated aging test.

Partial discharge and dielectric response behaviour of insulation systems for high voltage rotating machines under electrical stress

In order to understand how the partial discharge (PD), dissipation factor and dielectric response measurements can reflect the changes in the groundwall insulation of high voltage rotating machines due to electrical stress, which lead to insulation degradation, an electrical stress has been applied to the insulation systems of high voltage rotating machines and the change of PD pattern, dissipation factor as a function of the applied test voltage and dielectric response in the time and frequency domain were studied. As test objects, actual stator bars were used, which are based on the epoxy VPI technology, according to the manufacturers standard.