How many cables are required when undergrounding transmission lines?

One circuit formed by three single core 380 kV XLPE 2500 mm2 cables laid directly buried in trefoil formation can sustain a continuous load of approx 1000 MVA (depending on the type of cable). This means that two or three circuits in parallel can sustain continuously a load of around 2500-3000 MVA. This takes into account the continuous load (24 hours per day) corresponding to the maximum thermal rating of the overhead line. This in practice is unrealistic because the rating of the OHL is dependent on other factors (e.g. voltage reductions) and generally the rating is lower and not continuous. As the maximum load is not constant, and the cable has a much higher overload capability due to the higher thermal constant, some economical considerations on the possibility of reducing the number of cables per circuit and/or the conductor size may be taken into account. This was the case for the Danish Eltra 400 kV cable project installed in rural areas in Jutland where the rating of the cable was selected for the effective load to be transported and not in accordance to the rating of the overhead line.

It should be noted that overhead lines have a standard design and a 1000, 2000 or 3000 MVA rating does not affect significantly the overall cost. On the contrary the cost and feasibility of underground cables is strongly affected by the electric load to be transmitted.

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