M. Farzaneh

Dynamic modeling of DC arc discharge on ice surfaces

A dynamic model for predicting DC arc behavior and critical flashover voltage of ice-covered insulating surfaces is presented. The model takes into consideration insulating geometry, pre-contamination level, and characteristics of ice layers. Assuming arc behavior as a time dependant impedance, it is possible to determine various arc characteristics such as time histories of leakage currents, potential gradient, channel radius, trajectory, propagation velocity and the energy injected into the zones free of ice (also called air gaps). The simulated results provided by the model are in agreement with those obtained experimentally using a simplified ice-covered cylinder as well as a short string of five IEEE standard porcelain suspension units covered with artificial ice.

Applications of Plasma Technology in Development of Superhydrophobic Surfaces

Superhydrophobic surfaces, originally inspired by nature, have gained a lot of interest in the past few decades. Superhydrophobicity is a term attributed to the low adhesion of water droplets on a surface, leading to water contact angles higher than 150°. Due to their vast variety of possible applications, ranging from biotechnology and textile industry to power network management and anti-fouling surfaces, many methods have been utilized to develop superhydrophobic surfaces. Among these methods, plasma technology has proved to be a very promising approach. Plasma technology takes advantage of highly reactive plasma species to modify the functionality of various substrates. It is one of the most common surface treatment technologies which is widely being used for surface activation, cleaning, adhesion improvement, anti-corrosion coatings and biomedical coatings. In this paper, recent advances in the applications of plasma technology in the development of superhydrophobic surfaces are discussed. At first, a brief introduction to the concept of superhydrophobicity and plasma is presented, then plasma-based techniques are divided into three main categories and studied as to their applications in development of superhydrophobic surfaces.

Modeling of icing and ice shedding on overhead power lines based on statistical analysis of meteorological data

Statistical analysis of meteorological data for icing and ice shedding on overhead power-line conductors was performed. The data studied were recorded during the 57 icing events (1659 h), which occurred in the period between February 1998 and January 2000 at the Mont Be/spl acute/lair icing test site in Quebec, Canada. The analysis aims at establishing shape and statistical parameters of the transfer functions representing the correlations between hourly icing rate and the variations of the following meteorological variables: ambient temperature, hourly number of ice-rate meter signals, wind speed and direction, and freezing precipitation rate. Two different regimes of the ice formation process, in-cloud icing, and freezing precipitation icing, are considered for characterizing ice events. The set of fitting regression curves obtained is the statistical base for creating an empirical probabilistic icing, and ice shedding model for studying and forecasting atmospheric-icing loads on overhead power-line conductors.

Study of discharge in air from the tip of an icicle

Electric discharge between the tip of an icicle and a plane electrode has been studied because of its relevance to the flashover of high voltage ice-covered insulators. A photomultiplier was used to scan the high voltage rod simultaneously with current measurement technique. The basic properties of streamers, namely the onset voltage, leakage current, mean propagation velocity and breakdown voltage, have been measured and compared with propagation from a metal rod as a reference. The effects of several experimental parameters such as freezing water conductivity, surrounding temperature and air gap length on the discharge characteristics have been examined. Possible mechanisms, which control discharges from ice points, have also been discussed and empirical relationships to predict breakdown voltage have been derived.

Linear stability analysis of ice growth under supercooled water film driven by a laminar airflow

We propose a theoretical model for ice growth under a wind-driven supercooled water film. The thickness and surface velocity of the water layer are variable by changing the air stream velocity. For a given water supply rate, linear stability analysis is carried out to study the morphological instability of the ice-water interface. In this model, water and air boundary layers are simultaneously disturbed due to the change in ice shape, and the effect of the interaction between air and water flows on the growth condition of the ice-water interface disturbance is taken into account. It is shown that as wind speed increases, the amplification rate of the disturbance is significantly affected by variable stresses exerted on the water-air interface by the air flow as well as restoring forces due to gravity and surface tension. We predict that an ice pattern of a centimeter scale in wavelength appears and the wavelength decreases as wind speed increases, and that the ice pattern moves in the direction opposite to the water flow. The effect of the air stress disturbance on the heat transfer coefficient at the water-air interface is also investigated for various wind speeds.

New method for in live-line detection of small defects in composite insulator based on electro-optic E-field sensor

Preliminary results based on experimental tests in laboratory and numerical investigation concerning the live-line detection of a conductive defect in HV composite insulator are depicted. The proposed method is based on E-field sensing using a non-invasive and compact pigtailed electro-optic (EO) single-ended probe. Measurements and numerical FEM modeling were done on a clean dead-end 28 kV composite insulator on which conductive defects of different size and position were simulated. Based on these simulations, the optimal location and orientation of the EO probe were determined regarding the conductive defect length and position. Laboratory tests were performed to validate these results and demonstrate the efficiency of the new detection method. The experimental results have demonstrated that using the optimal EO probe orientation, the E-field sensor was able to detect and locate conductive defect as short as 15 mm × 1 mm in contact with the HV electrode and of 26 mm × 2 mm between sheds. Moreover, the experimental tests have demonstrated the ability of the EO sensor to detect the presence of corona discharges induced by the conductive defects.

Equivalent surface conductivity of ice accumulated on insulators during development of AC and DC flashovers arcs

Surface conductivity of ice was investigated during the development of AC and DC arcs on ice-covered insulators. To determine this parameter, an approach based on fluid mechanics combined with experimental measurements of the water film flow rate and conductivity was used. In particular, the variation of the thickness and volume conductivity of the water film as well as the mechanisms of discharge initiation and arc development on the surface of an ice-covered post insulator were studied. The effect of ice surface conductivity on arc propagation velocity was evaluated for different freezing water conductivities using highspeed video camera techniques. Empirical models were proposed to account for equivalent surface conductivity dynamics during the flashover process. The derived equivalent surface conductivity was used to improve existing dynamic models to predict the minimum flashover voltage of the ice-covered insulator. The computed results from the models were in good agreement with those obtained experimentally.

Improving electrical performance of ehv post station insulators under severe icing conditions using modified grading rings

This paper presents some mitigation solutions for improving the performance of EHV post station insulators under severe icing conditions using modified grading rings. For this purpose, numerical simulations using the finite element method (FEM), implemented by the commercial software Comsol Multiphysics, were performed, in parallel with experimental tests to study and evaluate the influence of the proposed solutions on ice-covered insulator performance during melting period. The solution proposed consists in adding a fine metallic and conductive mesh on the standard grading ring. In addition to improving the potential grading, the metallic mesh allows the grading ring to behave like a booster shed by created a large air gap under it. Experimental tests under severe icing conditions reveals that using two modified standard grading rings permits to improve maximum withstand voltage by 17% compared to the standard grading ring alone. Also, numerical simulations permits to demonstrate that increasing the diameter of the modified grading ring helps to improve both maximum withstand voltage and uniformity of potential distribution of the EHV post station insulator. The results obtained will also be useful to improve general knowledge on the use of grading rings for the EHV post station insulator under clean and severe icing conditions.

Influence of air gaps on the DC withstand voltage of ice-covered UHV insulators

The influence of the number and position of air gaps on the maximum withstand voltage of a typical 735-kV ice-covered station post insulator under DC voltage was investigated both experimentally and numerically. Numerical results obtained by the Finite Element Method (FEM) with the presence of a water film on the ice surface showed that the number of air gaps has a direct effect on the distribution of potential along the iced insulator. Moreover, experimental results showed that the number and position of air gaps have a significant effect on the maximum withstand voltage of ice-covered insulators. In the same way, for the three air gap configurations tested in this study, the maximum withstand voltages obtained showed the lowest value when two of the air gaps are located at the top and bottom of the insulator. The results obtained will be helpful in the design of HVDC insulators in regions subjected to atmospheric icing.

Simulation analysis of the effect of booster sheds on post insulators under icing conditions

The main objective of this paper is the numerical simulation of the potential distribution along ice-covered EHV post station insulators equipped with booster sheds during a melting period. Numerical simulations were carried out using the finite element method (FEM), implemented by the commercial software Comsol Multiphysics™ to calculate the voltage drop distribution along different air gap configurations. In particular, it was shown that following the addition of 4, 5 or 6 booster sheds, more that 50% of the applied voltage was dropped along the closest air gap to the HV electrode, which was also the longest one. Finally, based on previous experiments carried out at CIGELE, numerical simulations and empirical equations, the effect of booster sheds for improving the electrical performance of EHV post insulators under sever icing conditions, was analyzed.