INVESTIGATION OF THRASHER COMPRESSION FITTINGS ON BPAS DIRECT-CURRENT TRANSMISSION-LINE

Failure of a "take down" Thrasher compression fitting from BPA's direct current (DC) transmission line section to pass the industry standard heat cycle test and similar thermal failure of 30 compression fittings on the companion DC line section initiated this investigation. Primary project goals were to predict Thrasher fitting's future performance and develop a reinforcement design. The project correlated fitting test data, field experience, and design parameters into an analytical tool, Bennett Number, capable of ranking relative fitting performance. Bennett Number also has potential as a design tool for establishing minimum fitting diameter and electrical size. Bennett Number, laboratory data, and field experience identified the Thrasher fittings, plus five other fitting designs used at BPA, as suspected or demonstrated marginal performers over their design lives. The Thrasher fittings could, as a population, exhibit unacceptable performance before the end of their expected life. Based on laboratory tests, the Thrasher fittings have an estimated remaining life of approximately 30 years. This investigation concluded the best way to mitigate a failed or marginal Thrasher fitting is by reinforcement. The reinforcement design is a helical shunt which acts both as a current shunt and heat shed enhancer.