Koochack Zadeh, Masoumeh (2011)
Field Emission Current Analysis for the Assessment of Dielectric and Switching Performance of Vacuum Interrupters.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

Vacuum circuit breakers (VCB) are dominantly used worldwide in medium voltage levels. However, in the recent years, there is a marked increase in interest to use VCBs also in sub-transmission voltage levels. This is mainly due to the environmental issues. However, the existing medium voltage VCB technology cannot be directly applied to the high voltage VCB due to the different design features which face an increase in difficulties as the voltage rises. One very important issue in this regard could be the lack of an appropriate diagnostic tool for the evaluation of the vacuum interrupters’ internal pressure. The users in the transmission voltage levels often want to know more about the up-to-date state of the high-voltage devices during their whole service life. Although in case of vacuum interrupter, it is difficult to decide upon the necessity of such diagnostics, and further discussion is required. According to current findings of the Cigré working group A3.27 “The impact of the application of vacuum switchgear at transmission voltages” (established 2009; work not yet finished), besides some technical and economical concerns, the non-availability of vacuum quality diagnostic tools might be one of the reasons for transmission system operators not to apply vacuum circuit breakers in their high-voltage transmission systems. Therefore, more investigation should be performed to develop an applicable diagnostic method for the internal pressure verification and vacuum quality tests, without demounting the interrupter from the switchgear. This investigation is the first part of the presented work, which is abbreviated in the whole work as “DVQ” for the diagnostic of vacuum quality. In this regard, a promising method, based on the measurement of field emission current immediately after arc-polishing of the contacts (named here as “FEA” method) is studied, which was firstly proposed by Frontzek and König. The results show that for a model vacuum interrupter with inhomogeneous electrode configuration (tip-plate), the decay rate of the field emission current after arc-polishing can be used for identifying the vacuum quality. However, for commercially available vacuum interrupters having more complex gap and contact geometries, it was not possible at all to define a test method or any test parameter configuration based on the chosen method, respectively, which would reproducibly cause a decay of the field emission current after arc-polishing. For statistically reliable evaluation on commercial interrupters extremely large number of trials would be necessary, which is totally unacceptable. Therefore, the investigated method has no practical meaning for monitoring purposes. Though the effects, which were published earlier, are basically present, the reproducibility is unfortunately too low. The search for a practical on-site diagnostic tool must therefore go on. Another challenging issue with regard to the applicability of VCBs in high voltage systems, is the performance of the interrupter during capacitive switching test duty and the avoidance of dielectric breakdowns (restrikes). Even in the medium voltage level, there are severe requirements for this test duty according to the IEC circuit breaker standard. To design an interrupter with very low probability of restrikes, especially at higher voltages, it is necessary to understand the physical origins of restrikes occurrence. Therefore, systematic research is performed to identify different electrical activities (especially field emission current) in the vacuum gap during the recovery phase of the interrupter after capacitive switching. The influence of pre-arcing during closing as well as arcing during opening on the dielectric performance and field emission characteristics of the gap for different types of vacuum interrupters are studied. This investigation is the second part of this dissertation, which is abbreviated as “ICS” for the impact of capacitive switching. The results of the investigation show that the capacitive arc current and its time duration have significant influence on the field emission current during recovery. The impact of the inrush (making) current on the field emission current during recovery is observed to be large and to be clearly related to its amplitude. It was also found that the average value of the field emission current (according to the proposed procedure “(10x10)measurement”) is a good measure for dielectric instabilities of the interrupter. However, no solid one-by-one relationship can be established between breakdown probability and field emission current in the available types of the interrupters. Furthermore, it is observed that the likelihood of restrikes during capacitive switching is a proper figure to control the production quality of vacuum interrupters. Hence, by using this method the stability of the production process of vacuum interrupters can be evaluated. In both part-projects (DVQ and ICS), measurements, analysis and modeling of very small field emission currents (microampere up to milliampere range), as prognosticated by the quantum mechanical Fowler-Nordheim theory, in a high voltage environment under severe electromagnetic interferences, are the basis of the investigations. For this purpose, an especial measurement system as well as the required circuit for compensation of the capacitive current component, caused by the interrupter stray capacitance, are developed.

Frage zum Eintrag

Redaktionelle Details anzeigen