Partial Discharge Detection in Dielectric Elastomer Actuator Systems
Muffoletto, Daniel Paul
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Dielectric elastomers are a class of field-driven electroactive polymers which utilize a soft dielectric and compliant electrodes to couple electric fields to mechanical strains. As actuators, the charge stored on opposing electrodes generates a compressive force on the soft elastomeric dielectric, and when used as a generator, the changing capacitance resulting from cyclic stretching of the dielectric can be used harvest electrical energy. In both cases, a major limiting factor to either the output force of an actuator or the energy per cycle of a generator is the maximum field strength across the dielectric. Other factors include the elastomer's dielectric constant and the stiffness both of the dielectric and of the electrodes. If the maximum field strength is exceeded the dielectric will fail, generating a conductive path between electrodes, which renders the dielectric non-functional. This event is known as dielectric breakdown. Partial discharges are instances of an electrical discharge partially bridging the dielectric in response to an applied voltage stress, typically at points of concentrated electric field or at material defects or other inhomogeneities in the dielectric. Depending on the discharge amplitude, partial discharge activity typically results in damage to the dielectric. Since partial discharge degradation is a slow process that worsens with time, partial discharge detection is a common method to assess the health of an insulating system. Several unique attributes of dielectric elastomers make the study of their dielectric breakdown particularly interesting, and will be touched on in this work. Due to their intentionally low elastic modulus, electromechanical effects are more apparent in the causes of breakdown and degradation in comparison to the more rigid dielectric materials traditionally used as high voltage insulators. In particular, electrical tree propagation resulting from internal partial discharges is a function of the material's elastic modulus. Also, in contrast to many other high voltage systems where operation below the rated voltage is not worthwhile, such as in power distribution applications, dielectric elastomer systems will still operate when lower voltages are applied, albeit at lower output energies. This can allow for real-time sensing for symptoms of degradation in the elastomer to provide feedback to the elastomer's controller, preventing further damage to the dielectric. The goal of this research is to investigate partial discharge activity in the acrylic elastomers (VHB4910 films from 3M) commonly used in dielectric elastomer actuators and generators. Since partial discharge activity can occur due to internal defects or due to external factors such as electrode material or geometry, a series of breakdown studies are presented in this work, starting first with ideal electrodes in an environment free of external partial discharges and progressing to the more realistic scenario of a powdered carbon electrode where discharges between electrode particles are expected. Data of the partial discharge amplitudes that accompany the breakdown event and images of the electrical trees that form at the breakdown site are presented. The presence of partial discharge activity prior to breakdown and the electrical trees observed indicate that degradation due to internal partial discharges is a viable breakdown mechanism for dielectric elastomers, even in the most ideally prepared test environment. These results show that sub-picoCoulomb sensitivities are required if a detection system were to react to signs of damage due to partial discharges in dielectric elastomers, and that partial discharge activity due to surface discharges in a powdered electrode system can mask the detection of internal discharges that indicate the growth of an electrical tree.