title: A 3D Faraday Shield for Interdigitated Dielectrometry Sensors and Its Effect on Capacitance.
abstract: Interdigitated dielectrometry sensors (IDS) are capacitive sensors investigated to precisely measure the relative permittivity ( ϵ r ) of insulating liquids. Such liquids used in the power industry exhibit a change in ϵ r as they degrade. The IDS ability to measure ϵ r in-situ can potentially reduce maintenance, increase grid stability and improve safety. Noise from external electric field sources is a prominent issue with IDS. This paper investigates the novelty of applying a Faraday cage onto an IDS as a 3D shield to reduce this noise. This alters the spatially distributed electric field of an IDS affecting its sensing properties. Therefore, dependency of the sensor's signal with the distance to a shield above the IDS electrodes has been investigated experimentally and theoretically via a Green's function calculation and FEM. A criteria of the shield's distances= s 0 has been defined as the distance which gives a capacitance for the IDS equal to 1 - e - 2 = 86.5 % of its unshielded value. Theoretical calculations using a simplified geometry gave a constant value for s 0 / λ = 1.65, where λ is the IDS wavelength. In the experiment, values for s 0 were found to be lower than predicted as from theory and the ratio s 0 / λ variable. This was analyzed in detail and it was found to be resulting from the specific spatial structure of the IDS. A subsequent measurement of a common insulating liquid with a nearby noise source demonstrates a considerable reduction in the standard deviation of the relative permittivity from σ unshielded = ± 9.5% to σ shielded = ± 0.6%. The presented findings enhance our understanding of IDS in respect to the influence of a Faraday shield on the capacitance, parasitic capacitances of the IDS and external noise impact on the measurement of ϵ r .
authors.affiliation: School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington 6012, New Zealand. alex.risos@vuw.ac.nz.|Robinson Research Institute, Victoria University of Wellington, Lower Hutt 5010, New Zealand. alex.risos@vuw.ac.nz.
authors.forename: Alex
authors.initials: A
authors.lastname: Risos
authors.affiliation: Robinson Research Institute, Victoria University of Wellington, Lower Hutt 5010, New Zealand. nick.long@vuw.ac.nz.
authors.forename: Nicholas
authors.initials: N
authors.lastname: Long
authors.affiliation: Robinson Research Institute, Victoria University of Wellington, Lower Hutt 5010, New Zealand. arvid.hunze@vuw.ac.nz.
authors.forename: Arvid
authors.initials: A
authors.lastname: Hunze
authors.affiliation: School of Engineering and Computer Sciences, Victoria University of Wellington, Wellington 6012, New Zealand. gideon.gouws@vuw.ac.nz.
authors.forename: Gideon
authors.initials: G
authors.lastname: Gouws
keywords: FEM; Green’s function; capacitance measurement; co-planar electrodes; electric field distribution; guard; interdigitated dielectrometry sensor; noise reduction; shielding