Ablation-assisted load current interruption in medium votage switchgear
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- Institutt for elkraftteknikk 
The switching of loads and feeders in medium voltage (1-52 kV) networks is essential to the operation of distribution networks. Thus, medium voltage load-break switches are abundant in networks today and could become even more important in the future, as the complexity of distribution network operation increases. Because of the market size, there is a strong pressure to reduce their cost, without compromising on reliability or compactness. In the past decades, gas-insulated switchgear filled with SF6 has provided a compact solution, but due to environmental concerns, manufacturers of these products are now seeking alternative ways to achieve compact and low-cost products. One approach is to review and improve the technologies that were applied before the introduction of SF6, one of which is the use of arc-induced ablation of polymers that release hydrogen compounds, also known as hartgas and wall gassing. Some older switchgear products take advantage of the arc-quenching effect of this ablation, also in the medium voltage product range, but there is no comprehensive publication that explains the mechanisms involved. In high voltage circuit breakers, ablation of hydrogen-free polymer (polytetrafluoroethylene/ PTFE) is used for interruption of the highest currents by employing the self-blast principle. The ablation vapors are then contributing to the pressure buildup inside a heating volume during the high current phase of the interruption, and around current zero (CZ), the compressed gas creates a gas blast quenching the electric arc. The present work relates to medium voltage load current interruption, exploring the possibility to improve current interruption performance by having certain polymer materials exposed to the arc, allowing for simpler and inexpensive switchgear designs. Two paths have been explored; one based on low voltage experiments showing that exposing certain hydrogen-containing polymers to the arc increases the interruption performance, the other is based on the self-blast principle used in high-voltage circuit breakers. Experiments in a medium voltage test circuit show that exposing polypropylene (PP) to the arc contributes to arc-quenching around CZ. A sharp decrease in current (current chopping) has been observed a few tens of microsecond before the natural CZ crossing, indicating intense arc cooling at this instance. However, the conductance in the contact gap after current chopping still allows a low current to pass, which in many cases leads to re-ignition when the transient recovery voltage reaches high values. The current chopping is explained by the extraordinarily high thermal conductivity in hydrogen in a narrow temperature interval around 4000 K, efficiently cooling the arc core. The remnant conductance is explained by the fact that the cooling effect vanishes once the temperature is below this interval, while the contact gap is filled with hot gas. This method of current interruption is thus not suited for medium voltage switchgear, and especially not at the higher levels of 24-36 kV unless combined with some other techniques. The outcome of the interruption experiments with PTFE nozzles is fundamentally different. The mere presence of PTFE, without forced gas-flow, does not quench the arc the way PP does, and no current chopping is observed. When there is forced gas-flow, PTFE nozzles perform well and bring the conductance down faster to lower levels than PP. Several model switch designs have been developed and tested in this project and, even though not systematically compared, the observable trend is successful interruptions at higher voltages for every new design through the project. The last model switch is an adaption of a self-blast switch to medium voltage load current interruption and can successfully interrupt currents in a 24 kV circuit, as long as the current is above a critical current of approximately 200 A. The interruption of low currents is a challenge in all ablation-assisted current interruption because the arc energy can become too low to obtain the required ablation. These current interruption techniques must, therefore, be combined with some other method to cope with the low currents. This could be an additional airflow source, like the puffer mechanism in high voltage circuit breakers, or ways of boosting the ablation at lower currents, like introducing more or different ablation materials to the arc zone.