Untersuchungen zur Hochfrequenzkonditionierung von Vakuumschaltkammern

摘要

The vacuum switching technology is preferred used in the medium voltage level due to its high reliability, the high number of switching operations, its free-maintenance as well as its long term durability. Contrary to gas-insulated switches, the contact surface of a vacuum circuit breaker has a substantial influence on the withstand voltage. Charge carriers, escaped from the contacts due to an electrical field stress, will be accelerated in the contact gap, whereas the absorbed energy could not be reduced significantly, because of the marginal number of collisions. Caused by the impact on the contact surface, additional charge carriers may be generated with a subsequent breakdown. The required withstand voltage of a vacuum circuit breaker is reached by a conditioning of the metallic surfaces inside the vacuum circuit breaker. A mechanical handling alone is not sufficient. All established procedures for conditioning vacuum circuit breakers to reach a required withstand voltage use current limited breakdowns to remove emission centers which are the origin of a breakdown. In this work, a conditioning process for industrial type vacuum circuit breakers is investigated, using high-frequency currents in the kHz-range. Due to high current rises before and high voltage rises after a current interruption during a current zero crossing, a reignition of a vacuum insulation can be enforced. On the one hand the high-frequency conditioning process has been evaluated if the required lightning impulse withstand voltage can be reached reliably, on the other hand it has been investigated, how far the lightning impulse withstand voltage can be increased beyond it. It is investigated, to what extend the voltage and current distribution, measured during a high frequency conditioning process, can be used to specify the reached lightning impulse withstand voltage. The conditioning process is successful only, if every critical emission centre is removed. Therefore the spatial distribution of the arcs is an important information for the evaluation of the conditioning process. Here, a circuit breaker model is used for optical investigations of the conditioning arcs. The surfaces of the contacts are investigated using scanning electron microscopy and atomic force microscopy.