Electrical insulation systems are typically used to isolate components having different electrical potentials in power transmission or distribution equipment, which especially serve to electrically insulate high voltage components from ground, and prevent electric current flow from the high voltage components to ground. Transient overvoltage conditions caused by a system disturbance may lead to power equipment flashover, resulting in a system outage and potential damage to the power equipment.
To reduce or eliminate power equipment flashover, a surge arrester is typically used in parallel with the power equipment. Surge arresters are typically connected to the high voltage terminal to carry electrical surge currents to ground, and thus, prevent damage to the power equipment. Conventional surge arresters typically include an elongated outer housing made of an electrically insulating material, such as porcelain or polymer, a pair of electrical terminals at opposite ends of the housing for connecting the arrester between a high voltage conductor and ground, and an array of electrical components in the housing that form a series path between the terminals. These components typically include a stack of voltage-dependent, nonlinear resistive elements. These nonlinear resistors or varistors are characterized by generally offering high resistance to normal voltage across distribution or transmission lines, and providing very low resistance to surge currents produced by sudden high voltage conditions, such as those caused by a lightning strike, and thereby reducing the risk of power equipment flashover during surge events. Depending on the type of arrester, it may also include one or more electrodes, heat sinks, or spark gap assemblies housed within the insulated housing and electrically in series with the varistors.
The voltage gradient, or voltage distribution, along the surge arrester is generally uneven between the high potential and ground connections. When the electric field at a point in the high voltage apparatus exceeds a critical threshold, significant discharge activity can be initiated, which may result in the degradation of or damage to the materials, eventually leading to apparatus failure. Since the electric field across the surge arrester and power equipment is concentrated at the ends, in an overvoltage condition, the end insulating units will break down first. A substantially uniform voltage gradient along the elongated electrical devices is generally obtained by using grading devices, or within the arrester housing a high number of small capacitors which are connected physically and electrically in parallel to the nonlinear resistive elements. The grading devices are usually in the form of grading rings and are ring-shaped conductors and securing means surrounding the high potential end of elongated electrical devices. By distributing the electric field more evenly, grading devices also minimize discharge activity.
Conventional grading devices are generally constructed from metal, such as aluminum, copper, or galvanized steel. Metal has always been used in grading devices due to its conductive properties, ability to withstand voltage surge currents, corona activities, and ability to withstand exposure to ultraviolet (UV) rays without breaking down in the environment that the grading devices are placed. In the past, manufacturers have not looked to wholly nonmetallic materials, such as plastics or composites, for the construction of the grading devices, because the electrical conductivity of nonmetallic materials is not as good as metallic materials, and the required conductive properties of suitable materials for a grading device are not known. Moreover, the behavior of nonmetallic materials exposed to high voltage surges is also not known.