Effect of HVDC polarity reversal stress on life expectancy of insulation-empirical approach and circuit model

Abstract

Life estimation is an important aspect in the assessment of the reliability of power equipment. Life estimation under DC fields did not receive as much attention as that of AC, which has been dealt with for several ages. Further, polarity reversal is a distinct operating condition in conventional line-commutated converter-based high-voltage DC systems, complicating the life analysis. In literature, a few semi-empirical or phenomenological models have been proposed for life estimation with certain limitations. In the thesis, carefully designed experiments are reported, which indicate that, the change in frequency of PRs results in a change in the slope of 𝑉−𝑡 characteristics on a log-log scale (known as inverse power law). The power law parameters are shown to be systematic functions of frequency of PRs. Based on these results, the authors have proposed a comprehensive inverse power law that is valid for both DC and PRs stress application, in place of conventional inverse power law. The proposed modification is examined with experimental data of authors apart from that available in literature and found to be working well. The model provides the minimum duration (maximum frequency) of a reversal for maximizing the life of power equipment which may pave the way for deciding frequency of the 'planned reversals'. Further the mechanism that leads to insulation degradation under DC and PR is investigated with conduction current measurement. In the early phase of current measurement for investigating LDPE conductivity, an anomalous current profile was detected —specifically peaks were observed at different time instants for different voltages and temperatures. These anomalous current profile characteristics are reproduced by replicating the commercial electrometer. LDPE thin films are subjected to different electric fields and temperatures and the experimental results are matched with analytical calculations based on an LDPE equivalent circuit model, and the values are tabulated. The erroneous interpretation of actual current characteristics caused by the conventional electrometer is identified, and a solution to eliminate the error in the interpretation of current was provided based on proposed dielectric model. The peak current characteristics are distinguished from the total current and its characteristics under electro-thermal stress are derived. It is considered very important to correct this erroneous interpretation and arrive at a proper interpretation of the conduction current, due to the reason that the current profile is considered important for understanding space charge phenomenon. Further, the experiments are performed under PR conditions. The continuously measured current of each reversal interval is sequentially split into three sections and compared. A significant difference in instantaneous polarization and relaxation current is observed. Also, dissimilar current trend with different time intervals (different frequency of reversal) is observed. These results were correlated with the charge dynamics in LDPE to explain the difference in times to break down with different frequencies of PR.