Influence Of The Joint Design

With the steady increasing demands of electric power in major cities around the globe combined with the challenge for much greater system reliability of supply, utilities throughout the world are grappling with the inevitable increase in demand for undergrounding high voltage cable transmission lines and the need to improve the overall performance of this expensive asset. The challenge is how to perform cable underground installations in the most cost effective manner.

The total costs of underground cable systems can vary widely even for the same voltage, power and length, making it difficult to generalise. Using modern cable techniques, it still costs approximately 4 to 15 times more compared to an equivalent overhead line through normal / urban terrain. A major element of this cost differential is accounted by the cable itself. On the other hand around 60% of the installation costs result from the civil works required for the cable installation, while cable accessories themselves represent less than 2% of the total costs of the cable transmission lines.

For most installations, joints are required at intervals along the route. This is because cable is supplied in fixed lengths given by the cable drum diameter and weight. The amount of cable on a drum is in turn dictated by the cable diameter and transport options for the cable drum. For these reasons joints are required approximately every 500 m to 800 m of cable length. For long cable transmission lines then the number of joints and resulted joint bay infrastructure is considerable. The trenching work and joint bay construction of underground transmission lines causes great soil disturbances. In suburban and rural areas where no space restrictions are a major problem, direct burial result the most economical option. In such cases the joint bay size normally play no a major infrastructure role in the total cost (Picture 1).

Picture 1: cable and joint installations in open trench…

In contrast installations in urban or central city areas, where there is a requirement to cross major roads or go through high density populated areas, the costs of major excavation work in terms of traffic management and construction can be considerable. Further limitations for construction activities that are imposed for reasons of noise, dust and traffic impacts must also be considered since often derive in further increase of the total cost as well as delays in project execution plans. For these reasons, power utilities and contractors are striving for compact joint bays able to considerably reduce the civil work costs as well as to reduce the total installation time of high voltage cable transmission lines. In order to minimize excavation and trenching work horizontally directed drilling and cable installation in pipes represent a cost effective alternative. The necessary joint bay infrastructure can then be performed on site or preferable today be pre-fabricated type (Picture 2). The advantage of the latter solution is that the total execution time for civil work as well as disturbances to public traffic, dust and noise are drastically reduced.

Picture 2: horizontal directional drilling and pre- fabricated joint bay…

For further cost optimizations for transportation and installation work, the size of the joint bay becomes the dominant improvement factor. To solve the problem Brugg has developed a new innovative vertical split-box outer protection design, which, combined with existing the gas cushion installation system for pre moulded joint bodies, allows up to 50% reduction of the necessary infrastructure and therefore direct costs for the joint bay infrastructure. A practical comparison illustrates the advantages of the new joint type as well as presents a recommendation to a standard joint bay for cables systems up to 245 kV.

Impact of joint outer protection design in the size of a joint bay

For underground cable projects the joint bay location is selected in order to maximize each section length of cable. The size of the joint bay will be determined by the density of existing services, likely disruption to traffic and space requirements for cable drums and cable pulling equipment. Therefore, are normally located within or adjacent to public roads. The ultimate dimensions of a joint bay infrastructure are majorly governed by the space requirement of the joints to be constructed within the bay and the minimum bending radius of the cables. As a consequence, the joint design has a direct impact in the final join bay size – and therefore direct impact in the costs of the necessary civil work. While the joint bay width and depth are determined by the necessary minimum clearance distances required for installation and safety operational reasons, a particular attention is to be made to the necessary length for positioning major components of the joint outer protection system in a parking positon as well as installation tools along the cable prior the final joint assembly. As a result the total free length required for the final joint assembly and therefore for the joint bay is majorly affected by the joint outer protection design. A new vertical split-box design demonstrates to be advantageous while eliminating the problem (Picture 3).

Picture 3: Joint type MPFP with vertical split-box design…

For a better understanding and quantification a comparison between two types of joint outer protection is presented in the case of a 245 kV cable circuit joint bay installation. A traditional joint type with cylindrical metal casing and horizontal split-box design filled compound (Picture 4) is considered.

Picture 4: Installation space for joint type MPSP with cylindrical metal casing and horizontal split-box design…

During the assembly this joint outer protection design requires first to bring the metal casing as well as the split-box joint outer protection into a parking position along the cable – and therefore the joint bay. After the assembly of the main joint body over the prepared cable distance, the metal casing as well as split-box is then assembled to its final position over the joint area. Considering a total joint length of approx. 2.8 meters, the necessary space to assemble the joint results approx. 6 meters considering necessary space for final cable fixing into the joint bay with a cable clamp. Considering the installation of 3 joints per bay, a total length of at least 12 meters resulted minimum necessary (Pictures 5).

Picture 5: Joint bay dimensions for joint with metal casing and horizontal split-box design…

The new joint outer protection concept presents as improvement a simpler innovative vertical split-box design for the joint outer protection, which does not require any parking position at all. The total joint installation length required in this case is determined mainly by the necessary cable preparation distances. As a result, the necessary total installation length per joint is reduced to practically the joint length of about only meters considering necessary space for final cable fixing after the joint assembly (Picture 6).

Picture 6: Installation space for joint type MPFP with vertical split-box design…

Accordingly, the total joint bay length can then be reduced to only 6 meters, thus representing a 50% length reduction compared to the previous outer protection design (Picture 7)

Picture 7: Joint pit cabin made by 20 feet container…

Based on the joint pit dimensions presented above, the volume saving of the joint bay is approximately 27 m3 per joint bay. As a result and proportional to this volume difference the new vertical split-box design offers further considerable installation and infrastructure costs savings due to less soil excavation work and its transportation, less necessary reinforced concrete for the joint bay, less back filling material as well as less collateral damage to public pavements. Further advantages are presented the state of the art installation techniques without needing time consuming plumbing or soldering work, allowing further faster and easier installations. This is particularly important in view of maintenance and repairmen work in case of unexpected failure of the cable network.
For the joint installation, suitable clean conditions must be established at the joint bay location, including temporary power supplies conditioning. For this purpose a 20 feet container can easily be adapted to the joint bay cabin thus creating a protective dust and rain protection to the joint bay. The container is then transported to site and placed above the joint bay with help of a crane (Picture 7).

Picture 7b: Joint bay dimensions for a joint with vertical split-box design…

Based on the joint pit dimensions presented above, the volume saving of the joint bay is approximately 27 m3 per joint bay. As a result and proportional to this volume difference the new vertical split-box design offers further considerable installation and infrastructure costs savings due to less soil excavation work and its transportation, less necessary reinforced concrete for the joint bay, less back filling material as well as less collateral damage to public pavements. Further advantages are presented the state of the art installation techniques without needing time consuming plumbing or soldering work, allowing further faster and easier installations. This is particularly important in view of maintenance and repairmen work in case of unexpected failure of the cable network.

For the joint installation, suitable clean conditions must be established at the joint bay location, including temporary power supplies conditioning. For this purpose a 20 feet container can easily be adapted to the joint bay cabin thus creating a protective dust and rain protection to the joint bay. The container is then transported to site and placed above the joint bay with help of a crane (Picture 7). The advantage of the joint pit size optimization, combined with pre-fabricated joint pit technic, is that the container joint pit cabin fits perfectly to the suggested joint bay dimensions.

Conclusions

The new vertical split-box design offers further cost savings in infrastructure and installation times compared to traditional horizontal outer protection designs. In future, this innovative design will majorly influence the execution of underground high voltage transmission lines. In view of the highly compact dimensions, a future standardization of a pre-cast joint bay is possible for voltages systems from 72.5 kV and up to 245 kV.

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