Vidyadhar Peesapati

Lightning protection of wind turbines — A comparison of real lightning strike data and finite element lightning attachment analysis

Wind turbine lightning protection systems have been developed to the point where lightning damage is relatively rare. However, with windfarms moving offshore, manufacturers are striving to continuously improve lightning protection systems while ensuring that they comply with relevant IEC standards. The case of offshore wind farms is particularly important due to the difficulties faced in accessing a wind turbine should this be required owing to lightning damage. The paper details work done to model upward propagating lightning strike attachment on a wind turbine. A 3D electrostatic model of a full scale wind turbine has been modelled using available Finite Element Analysis software. This full scale model is subjected to high electric fields comparable to those created by a charged cloud. Results from these simulations are then compared with those found from analysis of real lightning strike data taken from wind turbines and windfarms across the world.

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.

Impact of thermal cycling on high voltage coils used in marine generators using FEA methods

Premature failures of stator insulation account for a large percentage of repairs of marine generator systems. The failure mechanisms of such faults have been presented in many parts of the literature. Partial discharge activity, thermal degradation, thermal cycling, harmonics and transients are some examples of such failure mechanisms. Whilst there has been an insight into the failure mechanisms, there is still no definite answer to how these defects manifest in the first place. Most of the failures that have been identified within literature are on end windings, especially slot ends. Some failure mechanisms have also been linked with thermal cycling. Frequent and rigorous stop/start cycles stress coils by inducing mechanical forces between elements of the coil and housing owing to differential thermal expansion. This differential expansion is dependent on the rate of rise of temperature and also the different coefficients of thermal expansion of the materials. The present paper will evaluate the thermal degradation of insulation systems used on marine generators using Finite Element Analysis (FEA) methods. On board temperature measurements of stator coils during a high speed run are used as one of the parameters within the FEA simulations, to investigate if there is any risk of differential thermal expansions during such an operational cycle. Different ramp rates are also analyzed within the FEA simulations to understand the effect of uneven thermal expansions and the risk of material degradation of the insulation in coils on marine systems. A brief review of the standards available for thermal cycling and testing are also presented within the paper.

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.

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.