S Farokhi

Arc Energy and Temperature During Its Propagation Over Ice-Covered Surfaces

The voltage and current waveforms during the propagation of an arc over an ice surface were measured and analyzed. The V-I characteristics of the discharge driven by ac power supply were studied. The instantaneous and per-cycle energy delivered to the arc column were calculated. The spectra of the emitted light during the arc propagation were collected and analyzed using a spectroscopic system. The rotational temperature was derived from the relative intensities of two groups of rotational lines corresponding to the peaks of the OH (A-X) (0, 0) vibrational band. The rotational temperatures of the experimental spectra for different current levels were derived. The relationship between the delivered energy to the arc column and the rotational temperature was studied and discussed.

Mechanisms and processes of arc propagation over an ice-covered surface

The general objective of this paper is to investigate the physical mechanisms and processes involved in the propagation of arc over the surface of ice. In the present research work, high-speed imaging and optical emission spectroscopy techniques were synchronized with the electrical measurements to study the various parameters of arc on an ice surface, i.e. channel radius, propagation velocities, arc column temperature and electron density. These results, together with observation of the propagation patterns and arc foot geometries, calculation of arc energy and the examination of the state of thermal equilibrium provide background information for understanding the ice-covered insulator flashover phenomenon. Arc root structure, current profile, propagation pattern and velocity were investigated under different type of applied voltage and different polarities. Water film conductivity and thickness, relative air humidity and the direction of arc propagation were also analyzed as influencing parameters of arc propagation. The effects of different possible mechanisms, e.g. collision and thermal ionization, buoyancy force and electrostatic force, on different stages of arc propagation has been discussed.

Effect of cold fog on leakage current characteristics of polluted insulators

Due to the wide range applications of high voltage insulators in power network systems, the reliable performance of insulators is challenged by different climate conditions including soluble and non-soluble contaminations. High voltage insulators installed near coastal areas are heavily contaminated with sea salt. In winter, due to the combined effect of sea salt and cold fog deposition, leakage current (LC) may flow on the insulators and surface degradation may take place, which may lead to surface flashover and consequently, to premature failure of power network systems. The leakage current waveform has a strong relationship with the contamination level, fog conditions and applied voltage. The investigation of LC before the flashover is essential to the reliability and secure operation of outdoor insulators. To determine the leakage current (LC) performance of high voltage silicone rubber insulators, laboratory investigation was carried out in an environmental chamber to understand this phenomenon and to simulate different behavior on the insulator surface. The performance of LC was investigated in laboratory during three different periods i.e. wetting period, drying period and dry band arcing period. Also for better understanding of insulator leakage current response an electrical model was used for computer based simulation using EMTP software. The relationship was analyzed on experimental and simulation work on the bases of magnitude and total harmonic distortion (THD).

Mechanism of saline deposition and surface flashover on outdoor insulators near coastal areas part II: Impact of various environment stresses

This is the second in a two-part paper series dealing with sea salt transportation and deposition mechanisms, and discussing the serious issue of degradation of outdoor insulators resulting from various environmental stresses and severe saline contaminant accumulation near the shoreline. The deterioration rate of outdoor insulators near the shoreline depends on the concentration of saline in the atmosphere, influence of wind speed on the production of saline water droplets, moisture diffusion and saline penetration on the insulator surface. This paper comprises two parts. The first part, deals with the impact of different environmental stresses on insulator surface degradation, including wind speed and direction, cold fog and rainfall. The second part concerns the flashover process related to saline contamination of the surface under constant and variable cold fog wetting rates and equivalent salt deposit density (ESDD). The experiments were performed on high voltage insulators based on the model presented in Part-I. Based on the proposed model, the influence of wind speed and direction on the pollution accumulation rate and impact of wetting rate on discharge current and surface flashover process were investigated. The equations S=S0e(Vdep0/αh)[e(−αx/v)−1] and D=Doe(Vdep0/αh)[e(−αx/v)−1] are derived from the model for saline concentration and deposition show good reliability and well represent the results obtained. Test results also show that due to the different wetting and contamination deposition rate, surface discharge current characteristics of tested insulator in rain are different with that in cold fog, which lead to different surface flashover voltages. An experimental setup was mounted for artificial saline contamination deposition. The proposed model can be therefore used to investigate insulator flashover near coastal areas and for mitigating saline flashover incidents.

Prediction of surface degradation of composite insulators using PD measurement in cold fog

In this paper, surface degradation detection and analysis was investigated for composite insulators under cold fog. A composite insulator was tested with different contamination levels under various environmental stresses. The surface degradation stresses considered were cold fog, surface resistance, contamination levels, as well as interior and exterior temperatures on insulator surface. Characteristics of partial discharge and discharge current of surface discharge were studied to determine the effects of dense cold fog on the surface degradation of composite insulators for different contamination levels. AC high voltage was used for the tests. Wavelet transform analysis at different decomposition levels was carried out to recover the discharge signals from surface leakage current signals. The internal temperature of the insulator is directly related to the severity of electric discharges on its surface. The interior temperature was measured by heat propagation tests. Thermal imaging camera was used to determine the exterior temperature of the insulator. The results showed that the surface discharge signals are strongly correlated with the contamination level, water conductivity and significant damage on the insulator surface caused by some of the imposed degradation stresses.

Velocity and electric field of the arc root propagating over an ice surface

Power outage caused by flashovers of ice-covered insulators is a challenging problem in cold climate regions. In order to investigate the physical mechanisms and factors which influence arc propagation over ice surface, a simplified ice model is used. A model was developed to calculate the electric field of the arc head and the velocity of arc propagation. In order to validate the model, high-speed photography using an ultra-high speed camera was employed. A simplified physical setup which enables us to observe the movement of arc head over an ice surface was used. Arc velocity over the ice surface was measured experimentally from the captured images and the results have been compared to the calculated value from mathematical model.

Overview of on-line and off-line ampacity identification techniques of bare overhead transmission line

The economics of power transmission requires operators to maximise the power through the network. Increasing knowledge of the impact of environmental factors on the current carrying capabilities of bare wire overhead lines is, therefore, important. Cost savings resulting from enhancing thermal transfer capabilities of transmission lines has motivated many utilities and power systems research centres to investigate techniques for maximising operational parameters. Enhancing power ratings of bare power conductors brings, alongside improved generated power dispatch flexibility, cost savings and economical and technical challenges. The aim of this paper is to examine some of the existing techniques employed to compute the ampacity of bare power lines and discuss practical limitations of these techniques. In addition, the impact of each technique will be considered from an operational prospective. On-line and off-line techniques are summarised and compared based on the nature and quality of power sources connected to the system, e.g. power output of windfarms tends to be of intermittent nature.

Impact of wind on pollution accumulation rate on outdoor insulators near shoreline

The present paper is an experimental investigation of the impact of wind speed on the pollution accumulation rate on outdoor insulators near coastal areas. Outdoor insulators near shoreline suffer from rapid saline accumulation due to heavy wind coming from the seashore, which is more dangerous in foggy weather conditions. A method was developed in laboratory to evaluate the impact of wind velocity and direction on the pollution accumulation rate on outdoor insulators and subsequently to determine a suitable mitigation method. In order to replicate seashore conditions on outdoor insulators an experimental setup was designed and installed inside an environmental chamber and was equipped with a wind generator and shoreline specification salts (NaCl, CaSO4), as well as a kaolin powder injection system. ESDD and NSDD were measured on the top and bottom of insulator surfaces at different wind speeds. Useful observations were made, measuring wind speed and salt deposit density on energized and non-energized insulators. It was found that pollution accumulation rate increases as speed increases up to 8 m/s but that it decreases when the wind speed is higher than 8 m/s. Moreover, the pollution accumulation rate was different in energized and non-energized states.

Observation of surface flashover process on high voltage polluted insulators near shoreline

This paper attempts to provide a relevant information on the serious threats of contamination surface flashover and related discharges on insulators near coastal areas. The reliability of power network systems can be improved by reducing the hazard of contamination surface flashover. A test method was adopted to examine the surface flashover process. To replicate typical coastal area contamination, a modified solid layer contamination deposition method was used in the experimental work. Two configurations were studied: 1) by turning the artificial contamination setup and cold fog generation continuously “ON” and, 2) with “ON/OFF” cycles of contamination and cold fog. The contamination and cold fog were used under different conductivities. The insulator was hung vertically in an environmental chamber simulating natural climate conditions. The results showed that flashover voltage decreased as cold fog-water conductivity and contamination level were increased. It was found that the extreme period as when surface flashover was at its lowest, after twenty minutes of cold fog generation, at a cold fog conductivity of 250 μS/cm. Visual observation results indicated that partial discharge propagated increasingly with leakage current before the occurrence of surface flashover, and that the latter occurred during “OFF” conditions when cold fog generation was suspended for a moment.

Effect of uneven wetting on E-field distribution along composite insulators

The aim of this paper is to study the influence of uneven wetting conditions on the E-field distribution and leakage current along composite insulators under dense cold fog. The tests were carried out in an environmental chamber to simulate natural climate conditions. In the experimental investigation two conditions were considered: the presence of uniform contamination levels and uneven wetting pattern of that contamination under cold fog along insulator surface. The changes in the E-field distribution with uneven wetting on the insulator surface were measured with a spherical probe and field meter. For the E-field distribution, the probe was moved parallel to the axis of the composite insulator. The obtained results suggest that the E-field distribution based on uneven wetting can be an effective tool to evaluate the performance of composite insulators.