Experimental Simulation Research On Running States Of Plane Contact Type Cu/Al Terminal Connectors Used In Transformer Substation

Cu/Al terminal connectors are widely used in electric substations and transmission lines of power plants. Scientific assessment on the Cu/Al terminal connectors is concerned with power transmission and distribution safety. Because Cu/Al terminal connectors are usually running in the field condition for decades, neutral salt corrosion in seashore areas and atmospheric corrosion in industrial pollution areas are main damage styles. In addition, the Cu/Al terminal connectors are possibly damaged when long time operating at high voltage and high current condition. In this work, soldering plane contact type Cu/Al terminal connectors common-used in electric substations and transmission lines were chosen to study the corrosion behaviors and the states after servicing at high-temperature for a long-term through experimental stimulation methods. The research results are beneficial to the operation maintenances of the Cu/Al terminal connectors, theoretically and technologically.Salt spray experiments of the Cu/Al terminal connectors were carried out with two different corrosion media of the NaCl aqueous solution with a concentration of50±5g/L and the NaCl+NaHSO3aqueous solution with a concentration of O.Olmol/L NaCl+O.lmol/L NaHS03to study their corrosion behaviors of the Cu/Al terminal connectors in the seashore and industrial pollution areas, respectively. Corrosion mechanisms of the Cu/Al terminal connectors were determined via the observations and analyses of phases, micro structures and fractrographies of corrosion products, and the electrical property measure. The Cu/Al terminal connectors were also annealed at150℃for times till720h to investigate their microstructures, fractographies and their effects on the bonding strengths and electrical properties. And, failure mechanism of the Cu/Al terminal connectors was then discussed.The neutral salt corrosion and the atmospheric corrosion of the Cu/Al terminal connectors both originate from the galvanic corrosion between the Al plate and the Sn-Pb alloy solder, resulting to the formation of the chaps and pittings of the corrosion layer at the Al side and a little Sn-Pb alloy solder corrosion at the Cu side, respectively. Compositions of these corrosion products are complicated, and the Cu/Al terminal connector’s corrosion increases with increasing the time. Therefore, part of the Cu/Al terminal connectors crack at the Al plate/the Sn-Pb alloy solder interface. Moreover, electrical resistivities of the Cu/Al terminal connectors increase slowly in the initial corrosion stage and then accelerate. For the Cu/Al terminal connectors, the atmospheric corrosion is more severe than the neutral salt corrosion.The150℃annealing results of the Cu/Al terminal connectors indicate that an intermetallic (IMC) layer of Cu6Sn5and Cu3Sn forms at the interface between the Cu terminal and the Sn-Pb alloy solder. The IMC layer follows the diffusion-controlled kinetics of a parabolic growth law with a growth rate constant, k=3.5×10-17m2/s. Meanwhile, shear strengths of the Cu/Al terminal connectors steadily decrease as the aging time prolonged. After the Cu/Al terminal connectors aged for720h, the shear strengths decrease nearly20%, the fracture path changes from the Sn-Pb solder to the Pb-rich layer, and the fracture mode changes from the toughness fracture to the brittle fracture. Moreover, the electrical resistivities of the Cu/Al terminal connectors also increase obviously with the aging time. The aging failure of the Cu/Al terminal connectors results from the formation and growth of the IMC layer, the Pb-rich layer, the Kirkendall voids and the microcracks in them.