Monitoring insulator contamination level under dry condition with a microwave reflectometer

—Build-up of surface contamination on high voltage insulators can lead to an increase in leakage current and partial discharge, which may eventually develop into flashover. Conventional contamination level monitoring systems based on leakage current, partial discharge, infrared and ultraviolet camera are only effective when the contamination layer has been wetted by rain, fog or condensation; under these conditions flashover might occur before there is time to implement remedial measures such as cleaning. This paper describes studies exploring the feasibility of applying microwave reflectometry techniques to monitor insulator contamination levels. This novel method measures the power generated by a 10.45 GHz source and reflected at the insulator contamination layer. A theoretical model establishes the relationship between equivalent salt deposit density (ESDD) levels, dielectric properties and geometry of contamination layers. Experimental results demonstrate that the output from the reflectometer is able to clearly distinguish between samples with different contamination levels under dry conditions. This contamination monitoring method could potentially provide advance warning of the future failure of wet insulators in climates where insulators can experience dry conditions for extended periods

Feasibility study on application of microwave radiometry to monitor contamination level on insulator materials

This paper introduces a novel method for monitoring contamination levels on high voltage insulators based on microwave radiometry. Present contamination monitoring solutions for high voltage insulators are only effective in predicting flashover risk when the contamination layer has been wetted by rain, fog or condensation. The challenge comes where the pollution occurs during a dry period prior to a weather change. Under these conditions, flashover can often occur within a short time period after wetting and is not predicted by measurements taken in the dry period. The microwave radiometer system described in this paper measures energy emitted from the contamination layer and could provide a safe, reliable, contactless monitoring method that is effective under dry conditions. The relationship between equivalent salt deposit density and radiometer output is described using a theoretical model and experimentally verified using a specially designed X-band radiometer. Results demonstrate that the output from the radiometer is able to clearly distinguish between different levels of contamination on insulator materials under dry conditions. This novel contamination monitoring method could potentially provide advance warning of the future failure of wet insulators in climates where insulators can experience dry conditions for extended periods

Monitoring contamination level on insulator materials under dry condition with a microwave reflectometer

Current techniques used for monitoring the levels of contamination on high voltage insulators, such as leakage current and infrared, are not effective in dry conditions since they require the surface of the insulator to be wetted by fog, rain or snow. If a buildup of contamination occurs during a prolonged dry period prior to a weather change there will be a significant risk that flashover may occur before there is time to implement preventative maintenance. Previous work has demonstrated the use of microwave radiometry to determine the levels of contamination on an insulator material under dry conditions, however practical applications are limited by low sensitivity. This paper reports the development of a novel technique based on microwave reflectometry to detect the power levels reflected from the surface of the insulator material. The level of contamination is then determined as a function of received power. A theoretical model establishes the relationship between equivalent salt deposit density levels on insulator surface and the dielectric properties of the contamination layer. A Finite Difference Time Domain (FDTD) model is used to simulate the total loss as a function of the contamination level. Experimental results verify the FDTD model and demonstrate the sensitivity of the reflectometer system to be approximately 100 times greater than the radiometer system. Therefore, the reflectometry system has considerably greater potential for practical applications to provide advance warning of the future failure of insulators under dry conditions for both HVDC and HVAC systems

EFFECT OF SALINITY ON THE DIELECTRIC PROPERTIES OF GEOLOGICAL MATERIALS : IMPLICATION FOR SOIL MOISTURE DETECTION BY MEANS OF REMOTE SENSING

International audienceThis paper deals with the exploitation of dielectric properties of saline deposits for the detection and mapping of moisture in arid regions on both Earth and Mars. We then present a simulation and experimental study in order to assess the effect of salinity on the permittivity of geological materials and therefore on the radar backscattering coefficient in the [1-7GHz] frequency range. Dielectric mixing models were first calibrated by means of experimental measurements before being used as input parameters of analytical scattering models (IEM, SPM). Simulation results will finally be compared to field measurements (Pyla dune, Death Valley, Mojave Desert) and will be used for the interpretation of SAR data (AIRSAR, PALSAR)

Performance of Electrical Spectroscopy using a Resper Probe to Measure the Salinity and Water Content of Concrete or Terrestrial Soil

This paper discusses the performance of electrical spectroscopy using a RESPER probe to measure the salinity s and volumetric content {\theta}W of the water in concrete or terrestrial soil. The RESPER probe is an induction device for spectroscopy which performs simultaneous and non invasive measurements of the electrical RESistivity 1/{\sigma} and relative dielectric PERmittivity {\epsilon}r of a subjacent medium. Numerical simulations establish that the RESPER can measure {\sigma} and {\epsilon} with inaccuracies below a predefined limit (10%) up to the high frequency band (HF). Conductivity is related to salinity and dielectric permittivity to volumetric water content using suitably refined theoretical models which are consistent with the predictions of Archie's and Topp's empirical laws. The better the agreement, the lower the hygroscopic water content and the higher s; so closer agreement is found with concrete containing almost no bonded water molecules provided these are characterized by a high {\sigma}. A novelty of the present paper is the application of a mathematical- physical model to the propagation of errors in the measurements, based on a sensitivity functions tool. The inaccuracy of salinity (water content) is the ratio (product) between the conductivity (permittivity) inaccuracy, specified by the probe, and the sensitivity function of salinity (water content) relative to conductivity (permittivity), derived from the constitutive equations of the medium. The main result is the model's prediction that the lower the inaccuracy for the measurements of s and {\theta}W (decreasing by as much as an order of magnitude from 10% to 1%), the higher {\sigma}; so the inaccuracy for soil is lower.Comment: 45 pages, 5 figures, 1 tabl

Electrical methods of determining soil moisture content

The electrical permittivity of soils is a useful indicator of soil moisture content. Two methods of determining the permittivity profile in soils are examined. A method due to Becher is found to be inapplicable to this situation. A method of Slichter, however, appears to be feasible. The results of Slichter's method are extended to the proposal of an instrument design that could measure available soil moisture profile (percent available soil moisture as a function of depth) from a surface measurement to an expected resolution of 10 to 20 cm

SYNTHETIC APERTURE RADAR OBSERVATIONS AT SALAR DE PAJONALES, CHILE

Remotely sensed microwave radars provide the spatial and temporal coverage needed to improve our understanding of the relationship between moisture content and salt pan mineralogy and, ultimately, climate variability. Moisture content in the surface and near-surface crusts found in salt pan environments, such as salt pan, has a significant impact on the backscatter values recorded by synthetic aperture radar (SAR) systems. This is because moisture affects the dielectric constant and surface roughness of the saline surface, which in turn influences the amount of electromagnetic energy reflected back to the SAR sensor. Changes in backscatter values are attributed to seasonal and interannual variations in salar surface properties (dielectric constant and surface roughness) and correlate with variations in regional climate trends. To better understand the spatial and seasonal dynamics of a salt pan (also known as salar), this study interprets a series of Sentinel-1 SAR images collected over Salar de Pajonales, Chile between 01 January 2019 and 31 December 2021. A total of 171 images were collected at 6-day intervals and processed using the Alaska Satellite Facility’s Hyp3 pipeline. An image stack was compiled and a time series was explored with the open-source, cloud-based platform, OpenSARLab. The time series of a mixed evaporite-mineral surface (composite surface) revealed that seasonal changes in dielectric properties and surface roughness drive variations in backscatter values at Salar de Pajonales. Rougher surfaces had stronger backscatter values in areas with higher surface roughness, except in wet conditions when increased soil moisture led to higher dielectric properties and, consequently, increased backscatter values. Mean backscatter values varied across the salar, with greater variability for the composite surface. These results underscore the significance of both dielectric properties and surface roughness when interpreting SAR data in salt pan environments, such as Salar de Pajonales. Future field studies on different salar surfaces are needed. Those studies should include in situ surface and near-surface water samples, the composition of sediment samples, and the installation of climate stations. These surface data would enable precise dielectric constant and surface roughness models and subsequently, better remotely sensed soil moisture measurements

The planning of a South African airborne synthetic aperture radar measuring campaign

Bibliography: leaves 153-163.This thesis sets out the results of work done in preparation for a South African Airborne Synthetic Aperture Radar (SAR) measuring campaign envisaged for 1994/5. At present both airborne and spaceborne SARs have found a niche in remote sensing with applications in subsurface mapping, surface moisture mapping, vegetation mapping, rock type discrimination and Digital Elevation Modelling. Since these applications have considerable scientific and economic benefits, the Radar Remote Sensing Group at the University of Cape Town committed themselves to an airborne SAR campaign. The prime objective of the campaign is to provide the South African users with airborne SAR data and enable the Radar Remote Sensing Group to evaluate the usefulness of SAR as a remote sensing tool in South Africa

Estimation of change in soil water nitrate-nitrogen concentration using impedance spectra

A fast and reliable method for in situ monitoring of soil nitrate-nitrogen (NO3-N) concentration is vital for evaluation of N management practices focused on reduction of NO3-N losses to ground and surface waters from agricultural systems. Using dielectric measurements at multiple frequencies can help to estimate several physical and chemical soil properties simultaneously. Hence the goal of this study is to examine the feasibility to estimate changes in pore water NO3-N concentration together with volumetric water content (VWC) from the dielectric measurements obtained at multiple frequencies below several MHz where conductive behavior of soil dominates. An initial experiment with two off-the-shelf capacitance probes showed that at a relatively high frequency response of the probe was primarily correlated with VWC, while measurements at a lower frequency made by the second probe also incorporated the effect induced by changes in pore water ionic concentration. These results confirmed that using measurements at multiple frequencies can provide information about several soil properties, including NO3-N concentration. Consequently two follow-up laboratory experiments used impedance spectroscopy to estimate changes in NO3-N concentration in pure solutions and soil water, respectively, using a multivariate chemometric analysis, particularly partial least squares (PLS) regression. The results showed that change in NO3-N concentration could be estimated with sufficient accuracy when its concentration was greater than concentration of other anions (chloride in our case). In addition, estimation of NO3-N in soil water improved significantly with increasing VWC. A good agreement was found between actual and estimated NO3-N concentration when the PLS model was built using permittivity data obtained at VWC ≥ 0.20 m3 m-3. R2 and the root mean square error (RMSE) of NO3-N estimation for the best model (VWC ≥ 0.20 m3 m-3 and concentration of chloride \u3c 500 mg L-1) were 0.84 and 28 mg L-1, respectively. In general, the study demonstrated that PLS regression method coupled with the dielectric measurements obtained at multiple frequencies below several MHz can be used to indirectly estimate VWC and NO3-N concentration, but after the proper calibration equally covering the expected variations in VWC and NO3-N. For in situ application other environmental variables such as temperature should also be incorporated into the calibration process