Criteria for renovating Circuit-Breakers of 500 kV MTS

This paper describes the replacement criteria and the expected service life for the second‑generation GCBs based on the deterioration surveys.

In highly aged GCBs, it has been confirmed that ageing degradation of the grease applied to the sliding surface of the arc‑quenching chamber leads to progressive wear of the silver plating during operations, and this has become the dominant factor determining the GCB’s service life.

However, there are few reports on the quantitative relationship between grease degradation and the slide friction, the amount of silver-plating wear, or on how to set an expected service life.

Therefore, a deterioration investigation was carried out: second-generation GCB field units with roughly 40 years of service were removed to examine functional and performance degradation, determine their expected service life, and identify optimal replacement timing.

For the life assessment, the relationship equation between ageing and grease oil content was established by formalizing the grease oil content based on the results of a degradation survey using GCB field units. Next, a rotary sliding model test to determine the limiting oil-content value was conducted to calculate the service life at which the field units’ oil content would reach that limit. As a result, the grease oil content was found to reach the limit at 50 years of ageing. It should be noted that the deterioration condition of the O-rings applied to second‑generation GCBs was also measured, revealing a seal lifespan of approximately 60 to 70 years. From the above, the service life of second-generation GCBs was set at 50 years, and they are planned to be gradually updated in the future aiming for 50 years.

The replacements will follow the same approach used previously: without heavy lifting equipment and without de-energizing the busbars, dismantle the GCB, move the replacement unit on transport carts, and reassemble it while reusing equipment and the foundations other than the GCB.