Hybrid overhead power lines for Switzerland

In the context of the Energy Strategy 2050, the grid infrastructure will face a significantly increased share of renewables, especially in remote locations far away from traditional load centres. The required grid reinforcement is therefore a crucial factor for the success of the Energy Strategy 2050. However, acceptance for new transmission lines is very low, hence, delaying the reinforcements and possibly leading to congestion.

Through a hybrid AC/DC conversion of existing multi-circuit AC overhead lines, with AC and DC on the same tower, the transmission capacity can be significantly increased without the need for new corridors. In this way, delays in construction can be avoided thanks to higher acceptance.

The implementation of a hybrid line in Switzerland can contribute to higher grid availability and make it possible to use cheaper and more remote energy sources. The project team demonstrated that a hybrid line conversion is generally more acceptable than the construction of a new transmission line. However, public acceptance of this new technology has proven to be extremely sensitive to uncertainties and potential environmental impacts. It was shown that the main criteria for the public rejection of overhead lines are potential health risks, the disruption of landscapes and the audible noise. Based on the measurements and simulations, these effects can be predicted more accurately and reduced to an acceptable minimum. Two possible North-South corridors have been identified as potential routes, and the conversion was shown to be feasible for a typical Swiss tower, although the existing small bundles pose a considerable challenge as regards the reduction of corona effects.

Implications for research

This project has given a much better understanding of AC and DC corona effects, such as ground level electric fields and audible noise, especially regarding a hybrid tower environment. Moreover, new methods have been developed to model an HVDC line and converters in an AC power flow simulation as well as the benefit of variable speed drives connected to an HVDC converter. Several criteria were devised to study the techno-economic impact of a line conversion. The acceptance study has shown how the perception of new technology is very reliant on information campaigns and easily influenced by vague fears.

Implications for practice

Based on the acceptance surveys, the Swiss public generally favours the implementation of a hybrid line compared to a new overhead line. The utility companies and authorities can make their decision based on the discussion of technical and economic benefits taking into account the importance of clearly communicating the potential impact on humans and the early involvement of the public. Furthermore, a better understanding of the generated electromagnetic fields and audible noise leads to more accurate predictions as well as a more sound basis for discussions with regulators and members of the public. The construction of a Swiss out-door test line specifically designed to test corona effects of AC and DC lines can be used to build future prediction models based on a larger and more realistic pool of data. Furthermore, the project has shown that the outage of a DC line does not substantially threaten system security.

Hybrid HVAC / HVDC overhead lines in Switzerland