James Pilgrim

Effect of Sediment Properties on the Thermal Performance of Submarine HV Cables

The thermal performance of high-voltage submarine cables is controlled by the effectiveness of heat transfer through the sediments in which they are buried. This paper assesses the influence of sediment parameters on this heat transfer using finite-element simulations. Particular attention is paid to the role of convective heat transfer, with a method being developed to define installation scenarios where this may become significant. This paper shows that the permeability is the dominant factor and can have a significant impact on cable temperatures, even though it is typically neglected in conventional models.

Quantifying the operational benefits of new HV cable systems in terms of dielectric design parameters

In recent years, a significant volume of work has been undertaken on the development of novel dielectric materials which offer improvements in a variety of properties. Questions are frequently asked as to how particular properties can be improved by more intelligent dielectric design. However from the point of view of the network operator it is not just the dielectric design which is of interest - rather, how the finished product performs in the network. This paper seeks to demonstrate what quantifiable improvements in cable performance could be gained through advances in the dielectric properties of cable systems. The analysis is set in the context of the operational current rating of the cable circuit and the associated losses per unit of power transferred. Analysis of the results shows that the greatest improvement in cable operational performance can be made from increasing the dielectric thermal conductivity, even if it comes at the expense of a slight decrease in other properties.

Dynamic cable ratings for smarter grids

This paper discusses the shortage of static current ratings in the increasingly stressed, yet intelligent, power grid. Using online monitoring systems, real-time data can be obtained to perform a more accurate and reliable Dynamic Rating. A comparison of base thermal models is presented, demonstrating that the widely used IEC60853 approach is not always the most accurate. Load current prediction algorithms based on support vector regression are examined, along with the requirements for ambient temperature data. Through considering data from operational transmission cable circuits, the value of taking a network level approach is demonstrated.

Rating Independent Cable Circuits in Forced-Ventilated Cable Tunnels

Over the last decade, there has been a notable rise in the number of forced ventilated cable tunnel schemes in the U.K., with new construction at transmission and distribution levels. The ability to accurately calculate continuous and emergency circuit ratings for these installations is vital in ensuring that their full operational benefit can be realized. While the Electra 143 calculation method in present use is fast and easy to use, it relies on several simplifying assumptions which make it unsuitable for application to tunnels with multiple independent cable circuits. This paper details a series of modifications to the present method which allow the direct calculation of ratings for tunnels containing multiple independent cable circuits. Significant benefits can be obtained from using this approach to calculate emergency ratings in these circumstances, as demonstrated by the example calculations provided. Implementation of an axially varying ac resistance also improves the accuracy of loss calculations. A number of key tunnel design considerations are illustrated through the results of the example calculations.

Impact of corrosive sulfur in transformer insulation paper

In recent years a significant volume of research has been undertaken in order to understand the latest failures in oil insulated power apparatus due to deposition of copper sulfide on the conductors and in the insulation paper. Dibenzyl Disulfide (DBDS) has been found to be the leading corrosive sulfur compound in the insulation oil. As there are currently no nondestructive techniques available to analyze the degree of damage from sulfur corrosion, electrical plant operators are faced with a choice of either replacing an asset which might still have several years of operational life or keeping a transformer operational with an increased risk of failure. This paper investigates permittivity and loss factor curves of transformer oil impregnated insulation paper, at different corrosive sulfur degradation stages. Three types of transformer conductors are investigated; new, artificially aged and service aged. To increase the understanding of the electrical behavior of Cu2S this paper demonstrates how the electrical properties change with moisture content. Dielectric spectroscopy is used to demonstrate some of the electrical changes across a wide frequency range, from 1mHz to 1MHz.

An Investigation of Thermal Ratings for High Voltage Cable Joints through the use of 2D and 3D Finite Element Analysis

The operation of power transmission systems requires accurate current ratings for each component within the system. With improvements in computational power, the application of modelling techniques such as Finite Element Analysis (FEA) has become viable for rating calculations. The influence of modelling assumptions implemented in FEA models for directly buried and force-cooled cable circuits has been previously quantified, but without direct consideration of the joint bay. To provide a rating which covers all potential hotspots requires the ability to model the temperature distribution through a joint bay. With the eventual aim of creating 3D FEA models for water cooled joint bays, this paper reports the development of FEA models for rating joints. By extension of modelling techniques developed previously for rating high voltage cables, a steady state 2D axi-symmetric FEA heat transfer model for a 400 kV cable joint has been developed. This allows examination of the sensitivity of the model to variations in the model parameters. Modelling has been subsequently extended and a 3D FEA model of a single joint built, allowing more realistic representations of the boundary conditions experienced by cable joints in service. Knowledge gained from this process informed the development of a 3D model of an un-cooled straight joint bay. Results confirm that such joint bays are not the limiting factor on circuit rating where sufficient core to core spacing has been provided.

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.

X-ray fluorescence as a condition monitoring tool for copper and corrosive sulphur species in insulating oil

Currently there are no standard techniques to detect copper sulphide deposits in transformers to act as an early warning system. This paper develops and tests a procedure using X-Ray Fluorescence (XRF) spectroscopy to tackle this problem and to monitor dissolved copper in oil, which can have a major impact on the dielectric properties of insulating oil. Initially calibration curves for sulphur and copper demonstrate the XRFs limits of quantitation. Subsequently, a laboratory scale sample preparation procedure is developed to demonstrate that corrosive sulphur depletion trends may indirectly monitor copper sulphide formation. To confirm depleting sulphur species are corroding copper samples are analysed with Energy Dispersive XRay spectroscopy. Finally XRF demonstrates that corrosive oil has a low impact on the long term concentration of copper in oil. An XRF condition monitoring technique feasibility study is conducted and recommendations are given in how to implement it in the field.

Current rating methodology for mass impregnated HVDC cables

Calculating the current rating of HVDC cable systems is less straightforward than for ac cables, which are always thermally limited. While the permissible current in an HVDC cable circuit may be limited by the operating temperature of the dielectric, it may also be restricted by the electric stress within the dielectric. Conventional current rating calculations such as IEC 60287 are thus ill suited to HVDC applications. This paper demonstrates the importance of the electric stress constraint through the application of finite element modeling techniques which permit a coupling of the thermal and electrical properties within the cable. The results obtained demonstrate that changes within the installation environment cause the limiting factor to switch from thermal to electrical under steady state conditions, demonstrating the importance of developing calculation methods which can represent both cases.

Assessment of the Impact of Joint Bays on the Ampacity of High-Voltage Cable Circuits

Improvements in the cost and availability of computational power in recent years has led to numerical techniques such as finite-element analysis (FEA) becoming viable for cable-rating calculations. This analysis has previously been completed for the case of directly buried and force-cooled cable circuits, but without direct consideration of the joint bay. This paper presents methods by which conductor temperature profiles within joint bays may be obtained through the use of FEA. Results are presented for a number of joint bay configurations common to the U.K. for naturally and force-cooled installations. Implications of the results on circuit-rating practice are discussed and the benefits of using tools, such as FEA, are considered versus other existing methods.