1,720,975 research outputs found
Methods for rating directly buried high voltage cable circuits
No full textTo manage a cable network effectively it is desirable to rate cable assets precisely. To investigate the conservatism of cable ratings provided by accepted modelling approaches several models have been employed to provide comparative continuous ratings for a directly buried cable group arrangement. Operational ratings are performed using expedient cable rating calculations, typically employing analytical or one dimensional (1D) finite difference (FD) methods. The case for a directly buried cable group is considered; ratings are obtained using three methods including the international standard analytical approach (IEC 60287), 1D FD and a 2D finite element analysis (FEA) approach. A continuous equation for resultant heat-flux has been derived to more precisely model the dielectric loss in 2D and 3D FEA models. Comparing results shows that all the models have good agreement when modelling an isothermal ground surface condition and validate the 2D FEA model against established approaches. The FEA model developed has been used to investigate more realistic ground surface boundary conditions. Results for different cable burial depths and ground surface boundary conditions show that using the established methods can lead to overestimation of cable ratings in the case of shallow buried cables
The use of Finite Element Analysis Modelling to Improve the Precision of High Voltage Cable Ratings
No full textZero phase filtering for lightning impulse evaluation: a K-factor filter for the revision of IEC60060-1 and-2
No full textThe next revision of the international standard for high voltage measurement techniques, IEC 60060-1, is planned to include a new method for evaluating the parameters associated with lightning impulse voltages. This would be a significant improvement on the loosely defined existing method which is in part reliant on operator judgment and would ensure that a single approach is adopted worldwide to determine peak voltage, front and tail times, realizing standardization in measured parameters across all laboratories. Central to the proposed method is the use of a K-factor to attenuate oscillations and overshoots that can occur with practical generation of impulse voltages for testing on high voltage equipment. It is proposed that a digital filter that matches the K-factor gain characteristic be implemented and used for this purpose. To date causal filter designs have been implemented and assessed. This paper is concerned with the potential application of a non-causal digital filter design to emulate the K-factor. The approach has several advantages; the resulting design is only second order, it can be designed without using optimisation algorithms, it is a zero phase design and it matches the K-factor almost perfectly. Parameter estimation using waveforms from the IEC 61083 -2 Test Data Generator and experimental impulse voltages has been undertaken and obtained results show that the zero phase filter is the ideal digital representation of the proposed K-factor. The effect of evaluating parameters by the proposed method is compared to mean-curve fitting and the challenge of effective front time evaluation is discussed
Effects of modelling assumptions on the rating calculation for externally forced cooled high-voltage cables
No full textTo increase the rating of a high-voltage cable circuit the cable group can be externally forced cooled, using additional coolant pipes in proximity to the buried cable group. This complicates modelling of the heat transfer problem to obtain ratings as coolant temperature and therefore heat transfer coefficient varies along the cable route. The most common approach for obtaining the circuit rating is the finite difference (FD) method outlined in Electra 66. This method is computationally efficient and quick to solve. To investigate the assumptions underlying this approach and provide confidence over a range of model parameters, this paper presents the development of an extended 2-D heat-transfer finite element method (FEM) model. The ratings of two cable circuits have been modelled using this approach and are compared with results from Electra 66. Cable ratings from the two methods are consistent in trend but offset favourably by 2.6% using the FEM model for all burial depths tested. With the FEM model verified for standard assumptions the model provides a useful tool for rapid investigation of sensitivity to model assumptions. A sensitivity analysis to changes in ac resistance, burial depth, dielectric loss, soil thermal resistivity and surface boundary condition is presented
Thermal performance of high voltage power cables
No full textThe UK high voltage electricity transmission network continues to face annual rises in demand, with ever greater volumes of power supplied to load centres throughout the country. To operate this network effectively, it is vital to accurately calculate the maximum allowable electric current which can be safely carried by each component in the power system. In high voltage power cables, this limit is defined by the maximum operating temperature of the cable insulation. Specify this current rating to be too low and the cable asset will never be used to its full potential; conversely setting the rating to be too high risks damage to the asset as the excessive heating can cause premature failure. Thus the rating calculation must be optimised to maintain security of supply by minimising the risk of cable failure, while also maximising the returns from capital investment on the power network. This project has employed a variety of mathematical techniques to improve the methods by which current ratings are calculated. Modern computational techniques such as finite element analysis (e.g Figure 1) and computational fluid dynamics are used to create more advanced circuit rating techniques. These have been compared and refined with input gained from field data. By eliminating simplifications from existing methods, it has been possible to identify ways of increasing the utilisation of the existing network. In addition the new techniques allow examination of the potential benefits of future developments in cable technology. Benefits are being derived from this work on both a day to day and strategic planning levels. For instance, by re-evaluating the current rating method for cables installed in tunnels, it has proved possible to consider the benefits from co-locating more cables in one tunnel to best use these expensive assets. The application of this method has allowed the quantification of the benefits which might be available from next generation cable technologies, enabling the prioritisation of future research effort in cable materials. Upon completion, the knowledge gained from this work is to be used to revise the international standard on calculating current ratings in cable tunnels. Techniques such as these underpin the concept of smart grids with improved operational flexibility and capability. Simultaneously the requirement to build expensive new components into the network is limited, whilst still meeting the need to supply ever increasing volumes of power across the country
A Comparison between 2-D Isothermal Region Matrix and Finite Element Analysis Cable Rating Methods for Water Cooled Circuits
No full textCryogenic dielectrics and HTS power apparatus: research at the University of Southampton
Full text linkVariable pressure and temperature liquid nitrogen cryostat for optical measurements with applied electric fields
No full textA cryostat with a high-voltage bushing, optical observation ports, variable controlled temperature and pressure has been designed to further the study of liquid nitrogen as a dielectric medium. The novelty in this design lies in the simultaneous achievement of these functions in a single design with a sufficiently large vessel to accommodate realistic geometries for high temperature superconducting cable termination prototypes. In addition, a commercial single-stage helium expander cryo-cooler is integrated into the apparatus to achieve steady state temperatures down to 63.5 K, without the need for sacrificial loss of liquid nitrogen to maintain vessel temperature. The cryostat inner vessel is certified for operating up to 2 MPa. A custom-made filled-resin bushing provides an electrical feed-through rated to 76.2 kV ac. For optical measurements with a range of sample geometries four optical ports are incorporated into the vessel utilizing sapphire windows and indium seals to form the inner pressure vessel. A technique employing a copper-vapour laser light source and high-speed digital camera for stroboscopic image capture of density change streamers and bubble dynamics with synchronized collection of electrical discharge data has been developed. This design has been used to study pre-breakdown phenomena, bubble dynamics with applied electrical fields and electrical breakdown. General construction, mode of operation and initial results are presented
Rating methods for cables installed in unventilated and partially ventilated surface troughs
No full textCables installed in surface troughs are a critical part of many transmission circuits. This paper presents a method for calculating the rating of such circuits using computational fluid dynamics techniques, applied to two common trough designs. One design has solid concrete lids while for the second some lids are replaced by grilles to encourage natural air circulation. For the case of covered troughs with solid concrete lids, the IEC 60287 method was found to be conservative owing to its failure to consider the thermal impact of the ground. Good agreement was observed between the CFD model and an improved analytical formulation by Anders. Modeling of partially ventilated troughs, which at present cannot be rated analytically, shows continuous rating increases of up to 31% over the covered design owing to greater air circulation
Application of Finite Element Analysis to externally forced water cooled cable circuit ratings
No full textIn order to increase the available current carrying capacity of high voltage cables, utilities may employ forced water cooling within power transmission networks to remove the heat from cable groups. For example the transmission network of England and Wales includes externally forced cooled circuits, i.e. cooled by pumping water through pipes buried in the vicinity of high voltage transmission cables. At the present time a simple and expedient computer program is used to rate these circuits. This model is similar to that of Electra 66. To attribute confidence limits to cable ratings attained using this method an extended 2-D heat-transfer FEA model has been constructed to allow the calculation of the cable core temperature for a typical water-cooled three-phase circuit of three singlecore cables buried in flat horizontal arrangement within a backfilled trough. A sensitivity analysis of the model to changes in ac resistance, burial depth, dielectric loss, soil thermal resistivity and surface boundary condition has been performed and is presented
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