David Birtwhistle

Condition assessment of EPDM composite insulators using FTIR spectroscopy

Two new sampling and analysis techniques for assessing the condition of EPDM (ethylene-propylene diene monomer) composite insulators are presented. Polymer oxidation is assessed by removing small amounts of surface polymer by swabbing with xylene and analyzing this material by FTIR (Fourier transform infrared) emission spectroscopy. A measure of the amount of surface chalking is obtained by scraping a small amount of degraded surface material with a blade and analyzing by FTIR absorption spectroscopy. Numerical indices quantifying the amount of oxidation and surface chalking are obtained by calculating the ratios of absorption or emission peak heights in the infrared spectra. These indices are named the oxidation index and the chalking index. Three types of field-aged 275 kV EPDM composite insulators are investigated and the results from the new techniques compared with analyzes by XPS (X-ray photoelectron spectroscopy) with good agreement. It is found that for two types of insulator that the oxidation index is increased in regions near the HV conductor.

Breakdown between water drops on wet polymer surfaces

This paper describes the results of experiments made to examine the nature of discharge phenomena between water drops on polymer surfaces. Small samples of high temperature vulcanised silicone rubber were subjected to electric stress and exposed to clean and salt fog. The resulting surface activity was observed with a high-speed video camera. Water drops on the surface were found to vibrate in a variety of ways and were pushed away from the high electric stress regions to low stress regions. The distribution of the water drops on the surface of the samples appeared to follow equipotential lines between the electrodes. Water drops on the surface grew larger by absorbing droplets from the fog or by coalescing together. Depending on the size of water drops, the process of water drop coalescence took up to several cycles of the 50 Hz supply voltage. During this period the shape of water drops was observed to change to shapes more likely to intensify the electric field around them and thus initiate discharges at the solid/liquid/gas interface. When several water drops simultaneously coalesce, they can bridge substantial lengths of insulation and there may be an increased risk of flashover. The conductivity of the fog is shown to have A marked effect on the mode of discharge development. Salt crystals deposited on the surface by salt fogs greatly increase the conductivity of water drops during drying and wetting of the polymer surface.

Noninvasive on-line condition monitoring of on load tap changers

The paper describes a condition monitoring and diagnostic monitoring system developed for power transformer on-load-tap-changers (OLTCs). The system has been installed in distribution substations to continuously monitor the operation of OLTCs on 33/11 kV power transformers. The field trials have demonstrated that the system can provide reliable indication regarding the actual condition of OLTC contacts. Also presented in the paper are the details about an industry survey on failure modes of commonly used OLTCs in Queensland electricity transmission and supply industries.

Surface discharges on polymeric insulator shed surfaces

This paper describes results of an experimental investigation into surface discharges occurring on the surface of polymeric insulators. Discharges on partially hydrophobic surfaces were found to occur across well defined dry bands, while discharges on hydrophobic surfaces appeared as small points of light occurring between discrete water drops. These discharges were observed to produce a localized loss of hydrophobicity. It is shown that these discharges are stable atmospheric-pressure glow discharges. The discharges are characterized by a high cathode voltage fall, dependent on the electrical conductivity of the water drop, and a voltage gradient in the positive column, dependent on the discharge current. A technique is presented for measuring the rate at which surface hydrophobicity of polymeric materials is lost due to the action of this type of discharge. It was found that the loss of hydrophobicity occurred predominantly in the region near the cathode.

Modelling of infectious spreading in heterogeneous polymer oxidation I. Development of a stochastic model

The heterogeneous features of the oxidation of polypropylene in the amorphous region are consistent with infectious spreading from a small number of sites, such as catalyst residues. A stochastic model was developed for demonstrating the spatial and temporal development of oxidation from these sites. In this model, the probability of passing the infection from one site to an adjacent site could be used in place of conventional rate constants. The probability of spreading was assumed to be temperature dependent, and modelled with Arrhenius behaviour. Each site was given a predefined lifetime such that the maximum probability of spreading coincided with the maximum rate of oxidation at a particular site. This has enabled visualisation of the spatial development of oxidation within the amorphous region. The epidemiological fractions corresponding to the remaining (unoxidised) polymer, the dead (oxidised) polymer and the infectious (oxidising) polymer, were calculated and used to test the limitations of the simple epidemiological model.

Corona Ions from Overhead Transmission Voltage Powerlines: Effect on Direct Current Electric Field and Ambient Particle Concentration Levels

Along with their essential role in electricity transmission and distribution, some powerlines also generate large concentrations of corona ions. This study aimed at the comprehensive investigation of corona ions, vertical direct current electric field (dc e-field), ambient aerosol particle charge, and particle number concentration levels in the proximity of some high/subtransmission voltage powerlines. The influence of meteorology on the instantaneous value of these parameters and the possible existence of links or associations between the parameters measured were also statistically investigated. The presence of positive and negative polarities of corona ions was associated with variation in the mean vertical dc e-field, ambient ion, and particle charge concentration level. Though these variations increased with wind speed, their values also decreased with distance from the powerlines. Predominately positive polarities of ions were recorded up to a distance of 150 m (with the maximum values recorded 50 m downwind of the powerlines). At 200 m from the source, negative ions predominated. Particle number concentration levels, however, remained relatively constant (10(3) particle cm(-3)), irrespective of the sampling site and distance from the powerlines. Meteorological factors of temperature, humidity, and wind direction showed no influence on the electrical parameters measured. The study also discovered that e-field measurements were not necessarily a true representation of the ground-level ambient ion/particle charge concentrations.

Electrohydrodynamic phenomena related to water drops on polymer surfaces

The present study provides new insights into discharge phenomena between water drops on polymer surfaces through observations with a highspeed video camera.

In-situ condition monitoring of EPDM composite insulators using Fourier transform infra-red spectroscopy

Procedures for determining the amount of polymer oxidation and surface chalking occurring on composite insulators are developed using FTIR spectroscopy. Numerical indicators of surface oxidation and chalking are obtained from ratios of peak heights in the IR spectra. Results of analyses of samples from three different types of EPDM 275 kV insulators suggest that oxidation is affected by surface discharging and weathering and that the amount of surface chalking is related to leakage current.

EHV Circuit breaker condition monitoring using antenna-based coupling methods

Catastrophic failures of modern SF6 circuit breakers have been reported during shunt reactor and capacitor bank de-energisation. In those cases, evidence of destruction of the interrupter nozzle by cumulative re-strikes is found. Monitoring of voltage waveforms during switching using established method would provide information about the magnitude and frequency of small re-ignitions and re-strikes. However, waveform measurements at a moderately high frequency require plant outages to connect equipment. A non-intrusive measurement technique has been developed to measure the high and low frequency voltage waveforms during switching operations without the need of an outage. The principles of this measuring technique are discussed and an example of its application in both shunt reactor and capacitor switching is presented.

A Noninvasive Method for Detecting Restriking: Application to the Switching of HV Shunt Reactors

Modern circuit breakers (CBS) may be susceptible to catastrophic failure possibly because they are designed to be more highly stressed than earlier CB designs with multiple interrupters per pole. Improved performance is obtained with dielectrically well-designed interrupters by taking advantage of advanced controlled switching to achieve a reignition-free window during opening to minimize the risk of nozzle puncture. On occasion, CB owners may wish to check whether equipment is performing satisfactorily and whether controllers are producing reliable and repeatable stress control. The monitoring of voltage waveforms during switching using established methods will provide information about the magnitude and frequency of small reignitions and restrikes. However, waveform measurements at a moderately high signal frequency require plant outages to connect specialized equipment. A nonintrusive measurement technique has been developed to measure the high- and low-frequency voltage waveforms occurring during reactor switching operations without the need for an outage. Results of tests performed in the laboratory and in a 275-kV substation demonstrating the capability of this new method to detect reignitions are presented.