This contribution focuses on the characterization of field grading materials (FGMs) for DC applications. Such materials are often composed of a filler material in powder form, which is embedded in a polymer like epoxy resin or silicone rubber, thus forming a compound. Examples of these fillers are nonlinear resistive particles like microvaristors or Iriotec 7000® particles.
In order to efficiently “tailor” a nonlinear FGM for a certain application, a forecast of the expected electrical characteristics of the resulting FGM before the process of compounding would be helpful, e.g. by conducting measurements on the filler alone. However, no guidelines or standards are actually available for these measurements. This causes not only an additional and unnecessary challenge when it comes to the measurements. It is also very difficult today to write a technical specification, which could be used to contact a potential manufacturer of filler materials with nonlinear electrical characteristics. Therefore, one goal of this contribution is to show processes of powder specimen preparation and measurement approaches that have been developed and proven feasible.
Main influencing factors on the measured conductivity of powders are temperature, humidity and electric field strength. But also contact pressure and the compactness of the powder can affect the measurements. In addition, the electrical behavior may depend on measurement time and electro- thermal aging.
Results of the DC measurements of fillers in powder form and of the compounds manufactured from them are compared and supplemented by additional investigations under impulse voltage stress in order to characterize the FGM over the full interesting range of electric field strength. To further simplify the description of the electrical characteristic, a mathematical model is used. Applications for the nonlinear FGMs investigated here may be DC power cable accessories, DC bushings or DC gas-insulated systems (GIS/GIL).