Overhead line
CASE STUDY: CONNECTION TO A NEW HV SUBSTATION
CASE STUDY: CONNECTION TO A NEW HV SUBSTATION
Ring main connection of a new 132kV HV substation to existing overhead lines.
Scope of works
- Finalise location of the proposed new substation.
- Survey the possible routes and identify tie-in points on the existing overhead lines.
- Evaluate the connection options; underground cables vs overhead lines.
- Prepare drawings and documentation.
Considerations
It is established practice to have parallel connections between HV substations in a ring main to ensure continuity of supply. Expanding a ring main to accommodate a new substation requires four connections; two to each of the adjacent substations in the ring as shown schematically below where a new substation C is added to the ring main between existing substations A and B.
The location of the tie-in point is normally chosen to minimise the distance between the tie-in point and the new substation. However, when considering proposals for routing, constraints arise due to obstacles, residences, infrastructure layout and soil conditions to name but a few. There is a choice between overhead lines and underground cables.
Overhead lines are a much more economical solution however there are considerations of visual impact, foundation positions, road crossing design including for example high vehicles or oil rigs and proximity to airports. Underground HV cables generally require a number of joins since the maximum length on a cable drum is limited to 250 to 300 m because of cable thickness and weight. Link boxes mounted in pits with access from above are needed to allow phase transposition of earthing screens at the joins. Underground cable route preparation can be very costly if there are existing underground obstacles. Soil conditions such as flooding, termites, acidity and conductivity need to be allowed for.
Implementation
The design proposal for the project was to connect from the tie-in points to the new substation, a distance of 4 kilometers, by underground cables. Due to the particular geography, the only viable route was constrained over the first half of the route by a large storage facility on the one side and a mixed industrial and residential area on the other, leaving a narrow corridor approximately 1 km wide to be crossed. The second half of the route was constrained by future recreational and existing administrative and residential infrastructure. A survey of the first half of the route was carried out and the results depicted on a drawing which identified a multitude of underground obstacles including large diameter oil pipelines, underground HV cables and other services which had to be crossed. The underground route would involve trenching up to 5 m deep in places to cross underneath these existing services and also micro-boring under a new 70 metre wide road corridor.
Given these considerations, overhead lines became a more practical solution for the first half of the route. Careful consideration had then to be made of where tower foundations could be placed while still meeting the maximum span requirement. In the event, it proved possible to route overhead lines across the corridor thereby eliminating all the underground obstacle constraints. The route chosen crossed a road used by oil drilling rigs which required high towers and it crossed the main road at the limit of the permitted span distance.
A further proposal to complete the entire route with overhead lines was discarded due to visual impact and environmental considerations and also because the available width of the second half of the route was insufficient to accommodate the clearances required between overhead line towers and the boundaries of the route. Nevertheless, the use of overhead lines over the first half of the route considerably reduced the capital and installation costs originally budgeted for.
Similar overhead line connection studies can be carried out to optimise the economic and environmental factors. For further details contact This email address is being protected from spambots. You need JavaScript enabled to view it.
