A study of re-ignition phenomena and arc modelling to evaluate switching performance of low-voltage switching devices

Abstract

This thesis deals with the prediction, improvement and simulation of switching performance of low-voltage switching devices (LVSDs). A literature review of arc characteristics, interruption principle, switching performance and arc modelling of LVSDs has been conducted. The experimental investigations of switching tests, arc imaging measurement and arc spectra measurement are also discussed. Switching tests have been carried out with 10kA, 20kA, 55kA and 100kA test circuits using either miniature circuit breakers or moulded case circuit breakers to investigate re-ignition phenomena and re-ignition evaluators. It is found that the ratio of the recovery voltage to exit arc voltage, where the exit arc voltage is defined as the value of the arc voltage immediately prior to the current zero point, is a reliable evaluator for the prediction of re-ignition in the switching tests of LVSDs. It is also noted that there are no occurrences of instantaneous re-ignition where this voltage ratio lies in the range of 1.0 to -1.0 and there is a threshold of the voltage ratio at approximately -2.0, which can distinguish the successful interruption and instantaneous re-ignition. Arc imaging measurement has been conducted through an array of total 109 optical fibres to allow observation of the overall quenching chamber of the flexible test apparatus. This experiment reveals that arc motion fluctuation (repeat of back- and forwards-motion) in the splitter plate region leads to the instability of the arc voltage. Moreover, the arc moves further as well as more quickly in the case of the larger vent size. The well distributed vent contributes to an increase in an arc motion velocity and reduction in a total arc duration. Arc spectrum is captured by a spectrometer to calculate the arc temperature when the arc is ignited by copper wire in a narrow enclosed chamber. It is found that the arc light intensity measured by the arc imaging system is directly related to the arc temperature: the light intensity increases as the arc temperature rises. 3-D arc modelling has been implemented, based on the magnetohydrodynamics theory. Lorentz force, plasma properties depending on temperature as well as pressure, contact motion, radiation loss, arc root voltage, and external circuit are considered in this modelling. It is observed that the simulated results have a similar trend with the experimental data and it is able to predict current limitation and exit arc voltage, which are key features of switching performance.

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ORCID for John Mcbride: orcid.org/0000-0002-3024-0326

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