Test setup and methods for the electrical characterization of semiconducting powders

Summary

In insulation system of high voltage rotating machines, different insulating tapes consisting of mica, resin and glass cloth carrier were used. In the winding head, where the generator bar leaves the slot, the electric field has to be graded through the so-called end winding tapes. Ideally, these tapes present a nonlinear characteristic. For this reason, semiconducting fillers were added into the polymer matrix during the tapes production process. One important aspect of this study was the electrical characterization of semiconducting powders. Therefore, a test setup was constructed and different measurement methods were compared. The challenge of the construction was the performance and durability at high temperatures (class F), whereby only heat resistant materials for the test cell and the measuring cables (high voltage, as well as test signal) could be used. To enable a high sensitivity, a test arrangement with guard ring electrodes and a thermocouple inside the test cell was the best solution. The used materials were polytetrafluorethylene (PTFE) for the test chamber and brass for the electrodes. The measurement cables were coaxial and double shielded with PTFE coating. For the electrical characterization, several methods were compared, as polarization/depolarization currents, insulation resistance and alternating polarity method. The differences between the applied methods is shortly explained in the paper. Only for the insulation resistance method, an external DC source was installed and the measurements were performed by an electrometer. The comparison was done over the whole temperature range. To guarantee a statistical significant result, a minimum of five measurements and double repetition at each semiconducting powder was accomplished. The evaluation and comparison of the test results were done in time domain representations.

Additional informations

Authors

Jia Yongyong, Hackemack Klaus, Neumann Detlef, Liu Ning, Vaterrodt Klaus, Brüggemann Martin