Ian Cotton

Dissolved gas analysis of alternative fluids for power transformers

Ester-based transformer fluids have the same DGA fingerprints as mineral oil. However, with lower volumes of gas produced, they will demand more precise dissolved gas measurements and modified or new ratio criteria to allow fault detection and diagnosis.

Stray current control in DC mass transit systems

Stray current control is essential in direct current (DC) mass transit systems where the rail insulation is not of sufficient quality to prevent a corrosion risk to the rails, supporting and third-party infrastructure. This paper details the principles behind the need for stray current control and examines the relationship between the stray current collection system design and its efficiency. The use of floating return rails is shown to provide a reduction in stray current level in comparison to a grounded system, significantly reducing the corrosion level of the traction system running rails. An increase in conductivity of the stray current collection system or a reduction in the soil resistivity surrounding the traction system is shown to decrease the corrosion risk to the supporting and third party infrastructure.

Higher voltage aircraft power systems

Modern aircraft designs are focused on minimising the use of engine bleed air, eliminating gearboxes, and reducing the reliance on hydraulic systems. Instead, many of these mechanical systems will be replaced with electrical versions. The increasing amounts of electrical power that will be shipped through an aircraft means that it is increasingly difficult to rely on low voltage power systems owing to voltage drop and weight constraints. This discusses the challenges that relate to the safe use of higher voltage systems within aircraft.

A Simulation Tool to Predict the Impact of Soil Topologies on Coupling Between a Light Rail System and Buried Third-Party Infrastructure

The production of stray currents by DC light rail systems leads to the corrosion of the supporting and third-party infrastructure in close proximity to the rail system. This paper simulates two parallel tracks that are occupied by two trains: one on each track. This type of modeling constitutes a case study that is utilized to investigate the effect of soil topologies on the corrosion performance of a floating DC light rail system focusing on the supporting and third-party infrastructure. The modeling technique used involves the accurate computation of the shunt and series parameters for use in a resistive-type model using a commercially available software package. The results demonstrate the importance that soil resistivity has on the corrosion risk to traction system and third-party infrastructure. Such information could ultimately be used to vary the level of stray current protection across a light rail system to ensure a consistent lifetime across the whole system.

Partial discharge control in a power electronic module using high permittivity non-linear dielectrics

High electric fields at the edge of the substrate metallization can give rise to partial discharge within power electronic modules and can lead to eventual failure. This paper examines the use of silicone gels filled with barium titanate to reduce the electric field enhancement at the edge of substrate metallization and therefore increase partial discharge inception voltages. The barium titanate filled gel produces a dielectric in which the relative permittivity is increased over a plain gel and that also exhibits a dependence on electric field. The theoretical electric field reduction that can be achieved in a power electronic module through the use of filled gels is demonstrated and compared against experimental measurements including the trial of the technique in some commercial modules. As promising results are achieved, consideration is also given to the effect of the barium titanate filler on the viscosity of the gel and the thermal conductivity, two key manufacturing issues.

Induced Voltages on Long Aerial and Buried Pipelines Due to Transmission Line Transients

In a previous paper , the voltage induced onto a 1-km above-ground pipeline by transmission line transients was shown to be significant in comparison to the induced voltage resulting from power system currents. This paper enhances the previous work in three distinct areas. First, both aerial and buried pipelines are considered. Above-ground pipelines are shown to be more at risk from transient-induced voltages. Second, parallelisms of up to 10 km are simulated. The results show that increasing parallelisms do not result in higher induced voltages once a critical distance has been reached. Third, a backflashover from a tower in the vicinity to a pipeline is modeled. This allows conductive coupling to take place at the same time as inductive and capacitive coupling. Backflashovers are shown to be an important consideration in determining the maximum voltages observed on a nearby pipeline.

Reliability Impact of Dynamic Thermal Rating System in Wind Power Integrated Network

Electrical networks across the globe are gearing towards environmentally friendly operation with many renewable sources being introduced. Among them, the popularity of electrical generation by wind has gained the most ground. Wind, as one of the meteorological conditions, also has high influence on line ratings and such manner of operating the line is known as dynamic thermal rating (DTR) system. Seeing this relationship, it is intuitively important to investigate the reliability of a electrical network incorporating DTR and wind farm. Hence, the objective of this paper is to propose a methodology that can perform this task. In the methodology, sequential Monte Carlo (SMC) simulation was performed due to time dependencies of line ratings and wind power. As a result, modeling of these time-series data was performed using the auto-regressive and moving-average (ARMA) model. Moreover, correlations between line rating and wind power were also considered to provide a practical approach to the modeling. Results have shown that DTR system is able to increase network reliability and allows for higher wind energy penetration.

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.

Comparison of Transient and Power Frequency-Induced Voltages on a Pipeline Parallel to an Overhead Transmission Line

An analysis of the voltages induced on a 1-km pipeline by a parallel overhead transmission line has been carried out when the transmission line is carrying power frequency (50 Hz) current and when it is subject to the propagation of a lightning or switching transient. A frequency-based circuit modeling technique coupled with forward and inverse Fourier transforms is used to carry out this analysis. The relative severity of the induced voltages from power frequency current or transient (lightning/switching) overvoltages is illustrated using the simulation results. The results demonstrate the high relative magnitude of induced pipeline voltages that result from the propagation of lightning transients down overhead lines. The need to model the full overhead line for such an analysis is investigated as is the variation of the level of transmission line/pipeline coupling as a function of the local soil resistivity. Analysis of the level of induced voltage as a function of length of parallelism is also carried out.

Power transfer capacity improvements of existing overhead line systems

The increased demand for power transfer in combination with environmental and economic issues which set constraints to building new lines, force the implementation of new technologies into the existing system in order to improve its power capability. Such methods involve re-tensioning, re-conductoring, or modifying the tower design to utilize composite cross-arms. It is hypothesized that a composite cross-arm and a novel conductor together provide an insulating significant opportunity to increase the overhead line voltage. The paper explores the range of options that could be implemented on an L3 overhead line tower typically used at 275kV in the United Kingdom, and demonstrates clear improvement in power capacitiy through the implementation of new technologies.