The article emphasizes the ability of new nonlinear resistive materials to be tuned for specific applications, which opens up new possibilities in designing electric field control of products for medium and high voltage applications.

Nonlinear resistive electric field grading Part 2: Materials and applications

For electric power transmission the overhead power line monitoring system plays an important role in operating security for grid operators as well as high supply availability at optimised cost. In this study, an approach to harvesting energy from the electric field around the conductor to supply the wireless sensor nodes was investigated. Special requirements in functionality, reliability and limited accessibility at the power line result in the development of a specific energy management strategy. A prototype of the power module was realised and tested under high voltage laboratory conditions. Then, a field test was carried out under real operating conditions. The test results confirm that the power module can collect enough energy to power the sensor nodes. The contribution in this study presents theoretical considerations of energy availability from the electric field, design of energy management and results from measurements with the prototype in a high voltage laboratory and in field test. The field test results demonstrate that the energy harvester can collect enough energy to power the sensor nodes by 16 mW.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-gtd.2012.0152

Energy harvesting for a wireless-monitoring system of overhead high-voltage power lines

Accurate knowledge of the soil hydraulic properties is a prerequisite for reliable modelling of soil water dynamics. As a consequence, many methods have been developed to derive these constitutive relationships either under field or laboratory conditions. Among these methods, the simplified evaporation method conducted on soil samples in the laboratory has found widespread use and application, mainly due to its relative ease of implementation and its straightforward evaluation of the experimental data. This method, however, relies on various simplifying assumptions. A common approach to assess the validity of these assumptions and to explore potential linearization errors associated with them is the use of synthetic data. In the past, such synthetic data were generated using rather simplistic models considering liquid water flow in capillaries only. In this study, we reinvestigated the accuracy of the simplified evaporation method using a more realistic process description of evaporative drying of the soil sample, including both liquid water flow in capillaries and films, as well as isothermal water vapour diffusion. In contrast to previous results reported in the literature, our results show that the simplifying assumptions used to evaluate the experimental data may result in biased estimates of the soil hydraulic properties, particularly for coarse textured soils. The bias typically increased progressively during stage-two evaporation, which is characterized by the development of a dry surface layer in which water flow is dominated by diffusion of water vapour, resulting in strongly nonlinear pressure head and water content profiles. We investigated various strategies for correcting for this bias caused by simplifying assumptions.

/pdf/simplified-evaporation-method-for-determining-soil-hydraulic-2rxct2gf7f.pdf

Simplified evaporation method for determining soil hydraulic properties: a reinvestigation of linearization errors

A method and apparatus are provided to acquire direct thermal measurements, for example, from a LiDAR collecting vehicle or air vessel, of an overhead electrical conductor substantially simultaneous with collection of 3-dimensional location data of the conductor, and utilize temperature information derived from the direct thermal measurements in line modeling, line rating, thermal line analysis, clearance analysis, and/or vegetation management.

Thermal powerline rating and clearance analysis using thermal imaging technology

Epoxy-based model insulators were manufactured and fluorinated under a F2/N2 mixture (12.5% F2) at 50 °C and 0.1 MPa for 15 min and 60 min. Surface charge accumulation and decay behavior were studied with and without dc voltage application. The effect of direct fluorination on surface charge migration as well as on flashover voltage was verified. The obtained results show that the charge decay of epoxy-based insulators is a slow process, but the decay rate increases when an outer dc electric field is applied. The surface charge distribution is changed when a streamer is triggered on the insulator surface. The existence of heteropolarity surface charges can decrease the dc surface flashover voltage to some extent, while the surface flashover voltage is almost unchanged when charges of the same polarity accumulate on the insulator surface. The short time fluorinated insulator can modify the surface resistivity, and the rate of surface charge dissipation is greatly increased under a dc electric field.

https://iopscience.iop.org/article/10.1088/1361-6463/aa5207/pdf

Surface charge migration and dc surface flashover of surface-modified epoxy-based insulators

Various diagnostic methods-either on-line or off-line-for monitoring of metal-oxide surge arresters in power systems are known. In most cases they are based on the measurement of the leakage current. This paper describes the most important ones in detail, such as measuring of the V-I-characteristics or analysis of the third harmonic content of the leakage current. Their advantages and weak points, respectively, are discussed, based on the most recent investigation results on the change of the V-I-characteristics due to electrical and nonelectrical stresses. It turns out that almost none of these commonly applied methods really work satisfactorily under either technical or economical aspects. As an alternative the temperature of the MO-insert is proposed to be used for diagnostics purposes. This is made possible by newly developed passive temperature sensors, which can be remotely accessed without any hardware connection to the environment. It is shown how the temperature can be used for monitoring of all kinds of degradation, energy absorption, thermal stability or even pollution performance.

Diagnostics and monitoring of metal-oxide surge arresters in high-voltage networks-comparison of existing and newly developed procedures

Water droplets on hydrophobic surfaces of polymeric insulators oscillate under applied high voltage electric field stress. Deformation of the droplets increases local electric field stress in the triple zones, causing lower partial discharge inception voltage and consequently affecting the ageing performance of the insulator. For this contribution dominant modes of single water droplet oscillation on hydrophobic surfaces were investigated in a wide range of droplet volume and frequency of applied electric field using a high speed camera. 3D frame analysis showed for all volumes an increasing trend of mode number with frequency in the range of 20 to 100 Hz. Experimentally found resonance frequencies for water droplets on a hydrophobic surface approximately correlate with published frequencies of free water droplets. Conductivity showed no effect on the pattern of water droplet oscillation. Partial discharge inception voltages on two different hydrophobic surfaces (silicone rubber and epoxy resin) with conductive and non-conductive (distilled) water droplets were measured. These measurements showed that inception voltage makes a step whenever the oscillation mode of water droplet changes. Conductivity of the water droplets was found to have no effect on the inception voltage on silicone rubber surfaces having a high degree of hydrophobicity, whereas conductive droplets on epoxy resin with lower hydrophobicity lead to lower inception voltages.

Experimental investigations on water droplet oscillation and partial discharge inception voltage on polymeric insulating surfaces under the influence of AC electric field stress

Metal-Oxide Surge Arrester: Fundamentals

This contribution discusses different aspects of impulse energy handling capability of metal-oxide (MO) varistors used in high-voltage surge arresters. The investigations were performed on more than 3000 MO varistors of different manufacturers worldwide. It can be confirmed that the single-impulse energy handling capability increases with increasing current density of the energy impact, a finding that was also published earlier but based on a very limited number of test samples. In addition, the impact of the MO varistor diameter is discussed. Thereafter, the differentiation of single and multi-impulse energy handling capability is addressed. Finally, investigations with a repeated energy injection (up to 100 prestresses per MO varistor at time intervals that allow the MO varistors to cool down to ambient temperature) are reported. At none of the investigated repeated energy stresses, any change of the V-I characteristic in the leakage current region was observed that would affect the arrester's overall thermal stability.

Recent Experimental Findings on the Single and Multi-Impulse Energy Handling Capability of Metal–Oxide Varistors for Use in High-Voltage Surge Arresters

In the deregulated market of electric transmission power systems today and as the energy demand continues to increase, electric utilities are faced with the challenge of moving more power via existing lines while at the same time obtaining permission for the erection of new lines and upgrading of existing lines becomes increasingly more difficult. As a result, power utilities and system operators are currently looking for opportunities to increase the capacity of the existing OHTLs without increasing the risk of equipment or system failure due to higher loading and an accelerated aging of the transmission infrastructure. One approach to manage the reliable operation of these systems is to utilize modern monitoring techniques as a method to prevent unexpected system outages. On OHTLs the sag of the individual line is an operational and security concern, which directly relates to the conductor operating temperature. The conductor operating temperature is influenced by various conditions eg. line losses, ambient temperature, wind speed and direction, solar radiation and conductor material properties. During periods of high ambient temperatures, low wind speed and high electrical system load conditions, monitoring and analyzing regional areas of the transmission network can be essential to evaluate the actual conductor operating temperature for the purpose of optimizing the line capacity and to prohibit potential sag problems. For measuring the conductor temperature of an overhead transmission line a sensor, which will have approximately the same thermal behaviour as the conductor, can be mounted on the conductor. Due to the high electrical and magnetic stresses in the area surrounding the overhead transmission line, a purely passive SAW sensor is used. The temperature of the conductor at the point of installation can be measured and the data transmitted wirelessly by means of a radio frequency backscatter (radar principle) to a data collection point. Along the overhead transmission line, installed passive sensors observe the temperature of critical line locations allowing continuous monitoring of the line behaviour. The measuring principles of the SAW sensor, the involved radar system components as well as current applications are presented.

/pdf/temperature-measurement-on-overhead-transmission-lines-ohtl-1pnxasu6d1.pdf

Volker Hinrichsen

Papers

Diagnostics and monitoring of metal-oxide surge arresters in high-voltage networks-comparison of existing and newly developed procedures

Experimental investigations on water droplet oscillation and partial discharge inception voltage on polymeric insulating surfaces under the influence of AC electric field stress

Metal-Oxide Surge Arrester: Fundamentals

Recent Experimental Findings on the Single and Multi-Impulse Energy Handling Capability of Metal–Oxide Varistors for Use in High-Voltage Surge Arresters

Temperature Measurement on Overhead Transmission Lines (OHTL) utilizing Surface Acoustic Wave (SAW) sensors