161 / 2018-08-25 18:35:22

Study on Gaseous Products in the Aging Process of Silicone Oil in Cable Terminals

silicone oil; aging; discharge; gaseous products

Cable is widely used in urban power network in recent years. In the cable system, terminals are more likely to break down because of its complex manufacturing technology and weakness in insulation. Oil-filled terminal, which is highly applied, is filled with insulation material between the epoxy casing and the stress cone to improve the distribution of electrical field inside the terminal. Low-viscosity silicone oil is used as insulating material and coolant in terminals for its low environmental impact, excellent insulation strength and fire-safe capability, so it is necessary to study the aging properties of the silicone oil. Gas generation is always observed in the aging process of silicone oil. However, there are few systematic researches on the gaseous product of the silicone oil, while the research of this kind on mineral oil is relatively mature. Dissolved gas analysis (DGA) is used in the diagnosis of the incipient fault in mineral oil-immersed transformer. Despite the difference between the silicone oil and the mineral oil, the idea of the DGA method is trialed in this study.
This paper focuses on the relationship between the gaseous product generated by spark discharge and the cumulative discharge quantity. The gas dissolved in the oil and escaping from the fluid are separately measured and summed, because there are differences between the solubility coefficients of each gaseous product to silicone oil. The electric parameters of silicone oil, such as dielectric loss tangent and volume resistivity, are also measured to evaluate the aging state of the silicone oil. The method to estimate the state of silicone oil by the gaseous product is proposed.
Silicone oil TR50 produced by Wacker Corporation is used for the sample of the experiment. The major component of TR50 is polydimethylsiloxane. A needle-plane electrode model under AC high voltage is used to simulate the non-uniform field inside the terminals of cable. The distance between the needle and the plane is 5mm. A copper multi-needle electrode which consists of 13 needles evenly embedded in the upper copper plate to accelerate the aging process of silicone oil is used. The experimental circuit is based on the standard GB/T 7354-2003, using a RC circuit as the acquisition module of discharge quantities.
The aging process of the silicone oil is the major reason for the accidents occurred on the oil-filled terminals. This paper focuses on the relationship between the gaseous product generated by spark discharge and the aging status of the silicone oil. The results are drawn as follows:
Hydrogen, methane, ethane, ethylene, acetylene, carbon monoxide and carbon dioxide are found. Carbon monoxide and hydrogen occupy the main proportion of the gaseous products in spark discharge. Spark discharge is the main cause of the generation of hydrogen and hydrocarbon products, while the main factor that influences the volumes of carbon monoxide and carbon dioxide needs further study. The method for mineral oil is not suitable for silicone oil, so the electric parameters of silicone oil is used to characterize the aging status. The appearance of acetylene and the proportion of hydrogen reaching 6% in total gaseous product are the symbol of the aging of silicone oil.