Hybrid alternating current/direct current (AC / DC) overhead lines (OHLs) carry at least two systems: one conventional AC circuit comprising three conductor bundles for each AC phase, and one DC circuit comprising three conductor bundles for a plus pole, minus pole, and neutral metallic return. Since the number of conductor bundles for AC and DC is the same and the air insulation is insensitive to voltage type, it is possible to convert the technologies from AC to DC, and vice versa. With a choice of appropriate insulators and voltage level, such voltage swipe can be realised immediately.

Existing masts can be converted to carry AC and DC currents simultaneously while being technologically safe.

Hybrid AC/DC OHL technology has been introduced to optimise the use of existing infrastructure and speed up the realisation of long high voltage direct current (HVDC) links, which frequently cross urban areas, where the realisation of lines or cables might be difficult and time-consuming. Converting an existing AC circuit into a DC circuit can be an interesting option.

For the conversion from AC to DC, it is not necessary to realise the whole DC link at once, which can have a length of 200-400 km. Shorter sections of AC system can be converted step by step through the replacement of insulators and – where necessary – an increase in the tower height. After these modifications, the system continues to operate under AC. This increases the realisation flexibility without excessively weakening the AC system. Finally, all sections can be connected to a DC circuit.

The circuits converted from AC to DC can carry about 10-30% more active power and do not need reactive power compensation. A further advantage of DC link is bypassing the AC network and delivering electrical energy directly to the load area.

Converted DC circuit can also be operated according to the present weather conditions, enabling an additional increase of the transmission capacity. The principle is well-known in AC systems and is called dynamic line rating (DLR).

The major factors in the design of hybrid AC / DC OHLs are the internal and external air clearances as well as emissions (noise, electric and magnetic field, ions). The permitting of hybrid AC/DC lines tends to take almost the same amount of time as a new AC line due to public acceptance and the degree of innovation. It is expected that after the realisation of the first project, many basic questions will be clarified and the realisation of the following lines will be quicker.

The use of OHLs for energy transmission using DC technology requires special AC/DC converter types (full bridge) or capable DC switching equipment due to the possibility of frequent insulation failures caused by thunderstorms or lightning strikes. The air insulation is self-healing, meaning that the energy transmission can be restored after clearing the fault and a very short pause.

Several steps should be taken to further increase the technology readiness level (TRL) of hybrid AC/DC OHLs:

• Converting existing AC circuits into DC circuits in the same tower increases the probability of inter-system AC-DC failures, which need to be detected and cleared appropriately in both systems. Depending on the converter technology used, different protection schemas can be applied. For the appropriate choice, numerous digital simulations of system behaviour through the standardised DC system and HVDC converter models are necessary.
• The combination of DC-converted OHLs with DC cables raises many questions about the insulation coordination of both transmission technologies, since the failures behave and propagate differently. The control procedures of HVDC converters need to be studied carefully.
• In the future, multi-terminal and multi-vendor HVDC systems will be considered. The combination of hybrid AC/DC OHLs with cables with pure DC OHLs in wider system has not yet been considered, as the interoperability of all DC components needs to be assured.
• Hybrid AC/DC lines require special insulators, whose design can differ depending on the operating conditions (e.g. salty environment). Appropriate tests and dimensioning are necessary.
• Developing international standards and experience in urban areas with operation for hybrid AC/DC OHLs.
• To increase the energy availability and assure the AC system stability, the DC components must be able to clear frequent faults in a very short time (some faults within one minute), which might be very challenging. Moreover, the power of converters needs to be originally designed for the use of DLR. Finally, refurbishment is current not possible.

The technology is in line with milestone “Integration of dynamic ratings and AI-based renewable power forecasts” under Mission 1, milestones “Alignment of requirements for HVDC, cable and monitoring systems” and “HVDC system planning criteria and identification of possible new interconnectors” under Mission 2 of the ENTSO-E RDI Roadmap 2024-2034.

Examples

TRL 6 for hybrid AC/DC OHL technology that has been validated in an laboratory environment. Expected to reach TRL 7 via a pilot project by 2030.

1. Amprion, “Hybridleitung”

2. Amprion, “Ultranet”