Christos Zachariades

A coastal trial facility for high voltage composite cross-arms

A trial site has been developed within a substation on the North East coast of Scotland for electrical testing high voltage composite cross-arms. A 400 kV transformer energizes two cross-arms installed on a lattice tower oriented at 90° from each other. A custom made protection system has been designed to keep the equipment within operating limits and minimize potential damage from faults. The purpose of the trial is to monitor the electrical performance of the cross-arms by measuring the leakage current on the surface of the insulating members. Weather monitoring equipment that includes a weather transmitter, a present weather detector and a solar sensor is used to correlate electrical behavior with specific weather phenomena. Cameras overlooking the cross-arms provide information regarding pollution accumulation and snow accretion. A data acquisition and control platform is responsible for recording the measurements. The monitoring system is capable of compiling and transmitting wirelessly a summary of the leakage current and weather information every five minutes.

Electric field computation for a 400 kV composite cross-arm

If not well designed, high voltage composite insulators are prone to ageing and failure due to corona and electric field stress. Particular care is required designing areas where the metal end connections meet the insulator core, otherwise a large electric field enhancement is observed. It is difficult to completely eradicate corona on any insulator, thus manufacturers try and minimise this phenomenon by designing the assembly to increase the corona inception voltage across the end connections and other metallic components. Often this involves introduction of corona rings, especially at the high voltage end of the composite insulator, to manage the magnitude of the electric field. The use of FEA to design corona/grading rings and optimise their position has recently been of great interest. The present paper reviews the use of FEA simulation to design stress relief devices across the high voltage end of a composite insulating cross-arm. Design work has been realised in practice and verified using corona inception measurements and compared to previous FEA results.

Optimization of a High-Frequency Current Transformer Sensor for Partial Discharge Detection Using Finite-Element Analysis

High-frequency current transformer (HFCT) sensors are widely used for partial discharge detection due to their versatility, high sensitivity, and wide bandwidth. This paper reports on a finite-element analysis (FEA) methodology that can be employed to optimize HFCT performance. The FEA model consists of accurate 3D representations of the sensor components. Two different FEA software modules were used in order to cover the wide operating frequency range of the sensor. The simulation computes the frequency response of the sensor in the range 0.3-50 MHz for various HFCT geometric and material parameters, specifically the number of winding turns, spacer thickness, aperture size, and core material. A prototype HFCT was constructed and the measured response compared with that of the simulation. The shapes of the responses were similar, with the simulated sensitivity being higher than the measured sensitivity by 1 dB on average. The measured low-frequency cutoff of the sensor was found to be only 0.05 MHz lower than that of the simulation.

Real-time monitoring of leakage current on insulating cross-arms in relation to local weather conditions

Two insulating cross-arms have been installed within a trial site near the coast of Northeast Scotland, energised at 231 kV. The trial aims to observe the electrical behaviour of the cross-arm by monitoring in real time the leakage current of the insulating members. The weather monitoring equipment installed at the site records wind speed and direction, precipitation, atmospheric pressure, temperature, relative humidity, visibility and solar irradiance. The instrumentation is complemented by cameras that provide visual verification regarding the conditions at the site. The results from the trial during June 2012 show that the cross-arm performs according to design expectations. The leakage current profile of the novel compression insulators is similar to that of the industry standard tension insulators. A relative humidity threshold of 90% has been identified which if exceeded, results in increased leakage current activity. Additionally it has been found that the prevailing wind affects the south facing cross-arm more than the west facing one. Also, the effects of precipitation have been found to be more prominent on the tension members.

Development of Electric-Field Stress Control Devices for a 132 kV Insulating Cross-Arm Using Finite-Element Analysis

Insulating cross-arms (ICAs) allow compaction or upgrading of transmission lines. The process of designing and verifying the performance of electric-field grading devices is reported for rigid cross-arms on a 132 kV lattice tower. For the grounded end, traditional grading devices resembling rings which follow the general shape of the insulators were designed. For the high-voltage end, an iterative process yielded a novel grading device which is a unibody piece of cast aluminium that manages the field on all four ICA members. Finite-element analysis simulations show that the electric-field magnitude at the triple junctions of the insulating members meet the design criteria of 3.5 kV/cm. Also, the field magnitude on the metallic end-fittings and electric-field grading devices is maintained below 18 kV/cm. The corona extinction test was performed on ICA assemblies showing that the grading devices can effectively control the electric field at voltages up to 132 kV since the average corona extinction voltage was 173.7 kV, well above the required value. The complete ICA assemblies were installed on an existing line in Scotland in August 2013. This paper provides a set of recommendations for use of FEA in the design of complex insulation geometries.

Electric field analysis of 132kv EPDM insulator and correlation with ageing features

A COMSOL multiphysics electric field analysis of an ethylene-propylene rubber (EPDM) insulator is presented. The 132kV insulator is an inverted post insulator that is taken from a jumper loop. It has been in service in a severely polluted environment for eight years. The insulators surfaces are asymmetrically aged with organic dark deposits covering the insulators surface, and with additional surface markings. The insulator has a manufacturing artifact, known as the mold line, which forms an elongated protrusion along the length of the insulator. This artifact is introduced into the COMSOL model to investigate its effect on the local electrical field. The reason for this study is that the area surrounding the mold line is lighter in color, with markings that suggest discharge activity. The regions of the localized field enhancements are identified and correlated with the ageing features, such as hydrophobicity loss, surface roughness and discoloration observed after eight years in service. A discussion whether the environment or the electric field are dominating the ageing process of the insulator is included.