Richard Chippendale

Numerical modelling of thermal decomposition processes and associated damage in carbon fibre composites

Thermo-chemical degradation of carbon fibre composite (CFC) materials under intensive heat fluxes are modelled. The model couples together heat diffusion, polymer pyrolysis with associated gas production and convection through partially decomposed CFCs, and changes in transport properties of the material due to the damage. The model is verified by laser ablation experiments with controlled heat input. The numerical predictions indicate that the thermal gas transport has a minimal effect on the decomposition extent. On the other hand, the model shows that the internal gas pressure is large enough to cause fracture and delamination, and the damage extent may go far beyond the decomposition region as witnessed from experimental verification of the model.

Induced Losses in Three-Core SL-Type High-Voltage Cables

It is considered that the present IEC 60287 standard overestimates the induced losses in SL-type armored cables, which are commonly used to connect offshore windfarms to the onshore grid. This paper addresses the issue through a series of equivalent-circuit models for the estimation of circulating-current, eddy-current, and armor losses in these cables. Finite-element models are used for further validation. Both the finite-element and the equivalent-circuit models use an equivalent-material representation of the armor layer, for which two calculation methods are developed. The results demonstrate clearly that the equation for armor loss in such cables as presently used in IEC standards is overly conservative.

Model of structural damage to carbon fibre composites due to thermo-electric effects of lightning strikes

The study concentrates on structural damage caused by the thermal effects of lightning strike to carbon fibre composites (CFC). The main objectives are to construct a model of the major physical effects involved, and to understand the correlation between the damage mechanisms and the damage witnessed in modern CFC. Model verification will be done by experimental decoupling of damage mechanisms, e.g. the real Joule heating from a lightning strike is replaced by a high power laser beam acting on composite surface. Outcomes from this study can then be used for further investigations and optimisation of lightning strike protection methods.

Cyclic Load Profiles for Offshore Wind Farm Cable Rating

By its nature, the power generated by offshore wind farms varies substantially with time. Despite this fact, the cable systems which connect such wind farms back to the onshore grid are typically sized based on a maximum, continuous current rating. In order to develop techniques by which the sizing of such cable connections might be optimized, this paper presents a method for deriving equivalent cyclic load curves from realistic, time-variant generation data. These cyclic load profiles may be used with the conventional IEC 60853-2 cable rating calculation, allowing the cable to be sized for a more realistic duty cycle. The example calculations presented show that this reduction in conservatism permits the use of smaller cable sizes, helping to drive down project costs. The implications of using such methods are discussed, highlighting the tradeoff between cost and conservatism.

Transport properties and current flow patterns in homogeneous strongly anisotropic materials

Purpose – The purpose of this paper is to investigate and explain the unexpected current flow patterns and twisting equipotential surfaces observed in strongly anisotropic materials. Design/methodology/approach – Potential distributions and current flow paths in highly anisotropic composite materials were studied via numerical simulation and experimentally. Simplified composite panels with two plyes were analysed using a finite-element model; the predictions were then confirmed experimentally. Findings – The unexpected twisting equipotential surfaces and current flow patterns were found to be consistent with minimising of Joule heat release in the material. Numerical modelling suggests that the twisted profiles of the potential are highly sensitive to the anisotropic electrical conductivity. Originality/value – This paper discusses the reverse current flowswitnessed in a two-layer anisotropic system. Such behaviour has never been predicted or observed experimentally before. The reported results will be of interest to anyone who is considering using anisotropic materials such as carbon fibre composites which might experience applied potential difference, such as lightning strikes.

Analytical Thermal Rating Method for Cables Installed in J-Tubes

One possible thermal pinch point along the route of a wind farm export circuit is a J tube, commonly used to provide mechanical protection to cable sections between the sea floor and the offshore platform. Current ratings for such cable sections are not covered by the scope of IEC 60287, while the existing publications covering such systems have limitations. This paper presents an updated 2-D analytical method and a 3-D extension for the rating of J tubes with short air section lengths. Continuous rating comparisons have been made against a 3-D finite-element model which shows a 4.5% variation in rating from the 2-D analytical model for air section lengths greater than 10 m, rising to 13% for short air section lengths. With the addition of longitudinal heat transfer within the new 3-D analytical approach, this variation decreases by 2.5%. Both methods proposed can be solved readily using conventional spreadsheet tools and are broadly compliant with the IEC 60287 methodology.

Dielectric properties of environmental friendly cooling fluids

This paper presents an investigation of the dielectric properties of two environmental friendly cooling fluids, hydrofluoroethers and fluorinated ketone. The AC and DC dielectric response of both cooling fluids have been measured at different temperatures. The experimental results indicate that temperature is positively correlated with DC conductivity and imaginary permittivity, in contrary it is negatively correlated with real permittivity of both two fluids. Dielectric strength at 295 K has also been measured. The recorded data indicate that the dielectric strength of both hydrofluoroethers and fluorinated ketone is approximately 10 kV mm-1.

Bubble motion in high voltage thermosyphon fluid under non-uniform electric field

In recent years, Fluorinated ketone (FK), [C<inf>2</inf>F<inf>5</inf>C(O)CF(CF<inf>3</inf>)<inf>2</inf>] has been considered as an alternative fluid to replace Hydrofluorocarbons (HFCs) in high voltage thermosyphons due to their environmentally friendly properties.