Ablation-Assisted Current Interruption in a Medium Voltage Load Break Switch: An Experimental Study with Polypropylene as Ablation Material
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Near an electric arc some polymers can change the properties of an arcing medium in a beneficial manner. Such polymers, known as ablation material, decompose and evaporate when exposed to heat from an electric arc. The evaporated polymer improves current interruption capability by increased pressure and heat dissipation. The target is to use arc quenching properties of ablation material to reduce the size of load break switches (LBSs) that interrupts current in atmospheric air. Consequently, the air insulated LBSs can be more competitive to more compact SF6 insulated breakers. The present thesis is an experimental study of polypropylene (PP) as ablation material. Experiments are performed in a medium-voltage (MV) laboratory to determine the interruption limits with regards to current level and shape of transient recovery voltage (TRV). Little has earlier been published in this field of study so the design of the test switch is developed through experimental work. The test switch interrupts the thermal part of the current interruption for all tested currents, up to 800 A. Even for hard TRVs the switch interrupted the thermal part to an impressive degree. The dielectric strength in the arcing zone after the thermal phase is however a larger challenge. For a current of 800 A, the test switch struggles to withstand a recovery voltage higher than 6 kV the first 300 µs after current zero (CZ) crossing. The ablation effect is more or less self-regulated since the arc intensity is directly related to current magnitude. Experiments indicate that the test switch struggles with low currents since the heat becomes too low. However, the thermal part is still interrupted for a current of 80 A, but recovery voltages above 2 kV the first 200 µs after CZ crossing µs is likely to cause a re-ignition.