Increasing power demand in urban areas combined with the goal of evergreater system reliability mean that many utilities will need to add new underground cable transmission. The challenge will be how to build such lines in the most cost-effective manner while also assuring high performance of this expensive asset.
Total cost of new underground cable lines of the same length, voltage and capacity can vary widely. Still, even with the latest technologies, installation costs in typical urban terrain run from at least 4 to as much as 15 times higher than for an equivalent overhead line. While a large share of this cost differential comes from the cable itself, some 60% of total installed cost typically relates to the civil work. Cable accessories account for only 2% or less of this cost.
Joints are required along the route of most underground lines simply because cable is supplied in fixed lengths determined by drum size, cable diameter, weight and transport constraints. Joints are usually installed every 500 to 800 m, meaning their numbers and related bay infrastructure can be considerable, especially for long lines. Trenching work and joint bay construction cause great soil disturbance. In suburban and rural areas without space restrictions, direct burial has been found to be the most economic option and joint bay size plays little role in total infrastructure cost.
But if there is a requirement for underground lines to cross major roads or pass through densely populated areas, the situation is different. Now there is considerable cost for large-scale excavation as well as traffic management. Added problems relate to restrictions on noise, dust, etc. that only further increase costs and add to projects delays.
Utilities and contractors building new underground cable transmission lines in urban centres aim for as compact joint bays as possible. These not only help reduce the civil costs but also shorten project completion times. Horizontally directed drilling and cable installation in pipes represents the most cost effective approach to minimize excavation and trenching work. Here, the necessary joint bay infrastructure can be accomplished on site or, preferably, be of the pre-fabricated types available today. The main advantage is significant reduction in total time to complete the civil work and end disturbance to the public.
Joint bay size is a dominant factor in optimizing transport and installation costs and, with this in mind, an innovative vertical split-box outer protection design has now been developed. When combined with existing gas cushion installation systems for pre-moulded joint bodies, this design allows up to 50% reduction in size and direct cost of joint bay infrastructure. Practical comparisons confirm the advantages of this new cable joint design versus standard joint bays for systems up to 245 kV.
Impact of Outer Protection Design on Joint Bay Size
Selection of joint bay locations along underground cable lines is usually made with the aim of maximizing cable section lengths. Joint bay size will then be determined by density of existing services, minimizing disruption to traffic and space requirements for cable drums and cable pulling equipment. Final joint bay infrastructure dimension is governed by space required for joints to be constructed within the bay as well as by minimum bending radius of the cables. As such, joint design has direct impact on joint bay size and cost of related civil works.
Joint bay width and depth are determined by necessary minimum clearances required for installation and safe operation. However special care has to be devoted to the distance needed to position components of the joint’s outer protection system in the parking position and also for the installation tools along the cable prior to final joint assembly. Given this, the total free length 4 / 9 required for assembly, and therefore for any joint bay, is determined primarily by design of the joint’s outer protection. The new vertical split-box design demonstrates advantages in this regard, while avoiding problems.
To better understand and quantify these benefits, a comparison was made of the new joint’s outer protection versus a traditional joint type with cylindrical metal casing and horizontal split-box design filled compound. The case involved a typical 245 kV cable circuit joint bay installation.
During assembly, traditional joint outer protection design starts with bringing the metal casing as well as the split-box joint outer protection into a parking position along the cable. After assembly of the main joint body over the prepared cable distance, the metal casing as well as split-box is assembled to its final position over the joint area. Considering total joint length of approx. 2.8 m, the space required to assemble the joint is about 6 m, given the need for final cable fixing into the joint bay using a cable clamp. With installation of 3 joints per bay, a total of at least 12 m is necessary (see Figs. 2a and 2b).
The new joint outer protection concept represents an improvement due to its simpler innovative vertical split-box design for the joint’s outer protection. No parking position is required in this case and total joint installation length is determined only by cable preparation distance needed. Total installation length per joint is thus reduced to practically the joint length of only meters, determined by the space needed for final cable fixing after joint assembly.
Accordingly, total joint bay length can be reduced to only 6 m, representing a 50% reduction in length compared to the earlier outer protection design (see Figs. 3a and 3b).
Based on these joint pit dimensions, total volume saved is approximately 27 m3 per bay. The new vertical split-box design offers proportionate savings in installation and infrastructure costs due to less excavation work and soil transport, less reinforced concrete for the joint bay, less need for back-filling and less collateral damage to paved areas. Further advantages include a state-of-the-art installation technique, without need for time consuming plumbing or soldering work. This allows even faster and easier installation.
Among the requirements for such an installation is suitable clean conditions to be established at the joint bay location, including temporary power supply conditioning. For this purpose, a 20 ft container can easily be adapted to the joint bay cabin, allowing dust and rain protection. The container is then transported to site and placed by crane above the joint bay. The advantage of joint pit size optimization, combined with a pre-fabricated joint pit technique, is that the container cabin can now fit joint bay dimensions.
The new vertical split-box design offers cost savings in infrastructure and installation time compared to traditional horizontal outer protection designs. It is therefore expected to find growing application during construction of new underground HV transmission lines. In view of the compact dimensions, future standardization of a pre-cast joint bay becomes possible for cable systems from 72.5 kV to 245 kV.