Hybrid AC/DC overhead line (OHL) is a technology used to increase the transmission capacity of existing mainly AC OHLs through converting an existing alternating current (AC) circuit into a direct current (DC) circuit. This creates also a by-pass to the AC grid with better power flow control.
The major factors in designing of hybrid AC/DC OHL are the internal and external air clearances as well as emissions (noise, electric and magnetic field, ions). Depending on those factors the voltage rating and design of DC system can be done.
The hybrid AC/DC OHL use the same components as pure AC or pure DC lines. While there are no special requirements concerning the conductors and towers, the insulators should be designed for hybrid AC/DC voltage stress.
Components & enablers
Advantages & field of application
Due to the need for new transmission line capacity and public concerns about the erection of new transmission lines, increasing transmission capacity by converting one of the existing HVAC lines to HVDC can be an interesting option.
The conversion of existing overhead lines can be done by replacement of insulators only and keeping the conductors and towers as they are. However, one must choose the maximum operating voltage properly, in order to keep the given internal and external electric clearances and to avoid corona associated levels of noise and ionization of air.
Technology Readiness Level
2020: TRL 7
2025: TRL 9
2030: TRL 9
Research & Development
Current fields of research: Hybrid AC/DC corona effects, insulator design.
Other: Long term full-scale outdoor pilot projects are necessary to conclusively verify the theoretical developed approaches.
Best practice performance
Maximum capacity: Based on Ultranet project the increase of transmission capacity of about 10-20% can be expected. It results in ca. 2200 MW per converted 380 kV circuit.
Voltage rating: 420 kV
Current rating: 2.7 kA
Best practice application
North Rhine-Westphalia and Baden-Württemberg, Germany
planned for 2021
An existing 3-phase HVAC OHL will be converted to a bipolar HVDC system on the same tower. Converter stations are built at either end of the transmission line.
Full Bridge Technology converters will be applied based on requirements concerning fast clearing of inter-system faults, frequent performance of auto-reclosing actions and better reactive power support to the AC system.
The project is making use of the existing pylons to increase the capacity of the network in an efficient and resource-friendly way.
 ETH Zurich, Dr. Martin Pfeiffer. Ion-Flow Environment of HVDC and Hybrid AC/DC Overhead Lines. [Link]
 ABB. Feasibility study for converting 380 kV AC lines to hybrid AC / DC lines. [Link]
 Amprion. Project Description: Ultranet. [Link]
 Transnet BW. New technology to make power transmission more secure. [Link]
 Cigre. Guide to the conversion of existing AC lines to DC operation. [Link]
 Cigre. Impacts of HVDC Lines on the Economics of HVDC Projects. [Link]