S.T. Larsen

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.

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.

Rating of Cables in Unfilled Surface Troughs

Cable circuits installed in unfilled troughs must often support high current ratings. To achieve higher ratings in unfilled troughs in the U.K., trough lids can be replaced by ventilated grilles, provided that the trough is within a substation site. While several methods exist for rating the traditional covered trough design, no standard method exists for naturally ventilated installations. To examine the possible uprating available, a coupled numerical model has been created for cable trough installations. Following successful benchmarking tests where the covered trough was modeled, the method has been extended to troughs with full natural ventilation. The results have been compared to commonly used engineering assumptions in order to validate simpler analytical methods. It was found that by allowing full natural ventilation of existing covered troughs, the continuous rating could be increased by as much as 28%.

Use of finite element analysis to obtain thermal ratings for high voltage cable joint bays

Recent computational advances have increased the viability of using numerical modelling techniques such as finite element analysis (FEA) to compute the rating of cable circuits. Such analysis has previously been completed for directly buried and force cooled cable circuits, but without direct consideration of the joint bay. In previously presented work a 2D axially symmetric finite element model of a 400kV straight joint was created, allowing for a sensitivity analysis of the model parameters to be performed. Modelling has been subsequently expanded into 3D to allow the calculation of the temperature profile within a 3 phase circuit force cooled cable joint bay. This paper presents a discussion of results obtained from a model of the force cooled cable joint bay and the implications on the circuit rating.

Thermal rating implications of the co-location of HV cable circuits in tunnels

Within the past decade, the number of operational cable tunnels in the UK has increased significantly. As the quantity of available tunnel space has increased, the way in which the installations are used is beginning to evolve, notably with regard to the installation of transmission and distribution circuits within the same tunnel environment. This complicates the calculation of the thermal rating, making it difficult to apply standard methods. To address this problem a new rating method has been developed which facilitates a more in depth analysis of the applicable current ratings. This paper presents an analysis of the considerations to be made for both continuous and post-fault rating scenarios, identifying several key trade-offs which allow optimization of the cable asset use.