天然黒鉛質ブラシ/鋼スリップリングシステムにおける摺動接触通電特性に関する研究(博士論文概要)--(電子情報メディア工学専攻)

In the brush/slip-ring system of turbine generators, many brushes are connected in parallel to a ring of the same polarity, which optimizes the current carrying capacity. The multiple brushes are connected under the same conditions, but the currents run thorough the brushes can become imbalanced or non-uniform. When a heavy current imbalance exists, abnormal overheating can happen and the brushes have to draw a huge current beyond their current collection capability. This excessive current can cause flashover accidents that result in a short-circuit between positive and negative terminals, disruption of electric power generation, and eventually whole plant shut-down. Therefore, the flashover accidents are a significant risk that can make a power system instable and develop into a large-scale power outage. Regarding the a-spot radius expansion caused by softening of steel, we adopted the φ-θ theory developed by R. HOLM to predict the drop in contact voltage and the rise in contact temperature at the a-spot. The temperature rise was theoretically estimated according with the theory, and experimental measurements on the temperature rise near the sliding surface of the brush were obtained using thermography in order to examine the agreement of the theory and the experiment. Referring to another prior study on steel softening, the temperature at which steel lost its elasticity was found to coincide the temperature of the a-spot predicted by the φ-θ theory when the gradient of the V-I characteristics changed abruptly. The theoretical and experimental analyses on the peculiar saturation of the V-I characteristics revealed that the saturation caused the current imbalance in two parallel brushes. Conventionally, the countermeasure against the sparking at the brush and slip- ring contact was a simple application of a sand-stone treatment, which was deemed a temporary but practical fix. This study developed a fundamental countermeasure technology to improve brush-gear design and maintenance. With the design proposed, the number of brushes were decreased and the current imbalance was relaxed. The platinum brush plating proposed in this dissertation was found to be efficient to improve the reliability of slip-rings.