Nonlinear Electric Filler for Field Grading in HVDC applications

The development of HVDC electrical power lines is aiming for higher voltage levels and increased capacities, because they tend to minimize loss over longer distances compared to conventional AC systems. To overcome the huge... [ view full abstract ]

The development of HVDC electrical power lines is aiming for higher voltage levels and increased capacities, because they tend to minimize loss over longer distances compared to conventional AC systems. To overcome the huge electric fields and transient effects (polarization, room charges) on the insulation material, new material concepts must be developed that are specially designed for these applications. The typical nonlinear resistive fillers like silicon carbide or zinc oxide micro varistors were not accepted over the time due to low chemical stability or incompatibility to the polymeric matrix. Varistors (“variable” and “resistor”) show non-linear electrical properties and are used in high voltage AC-applications to prevent surge voltages in HV power lines.

Merck is developing new microscopic oxide-based ceramic fillers, with non-linear electric and field-grading properties, to meet the new requirements and overcome the constraints of e.g. zinc oxide materials. By using a core-shell approach particle size, form and density of the particles can be easily adjusted by selecting a convenient substrate. A layer of doped oxidic material is precipitated around e.g. alumina-flakes in a wet chemical process. After sintering at high temperatures, the electrical properties like resistivity and especially it’s non-linear behavior can be adjusted in a wide range.

Employing SEM imaging the grain size and shape is analyzed while using XRD the different phases of the materials are distinguished. UV-Vis measurement is used to estimate the band gap. A near-application like preparation is done by preparing an oxide filler – silicone composite and measuring current density vs. electric field strength. The non-linearity at high voltages and polarization effects can be observed.