HVDC cable systems with metallic return line (DMR)

HVDC with DMR

Some transmission system operators (TSOs) opt for a dedicated metallic return conductor (DMR) in their bipolar HVDC cable projects. This means an additional cable that is only in operation when one of the two cable poles is out of service.

For mega-infrastructures such as the German HVDC corridors, it is common practice to design the system based on the most unfavorable thermal conditions. How does this approach affect HVDC projects? The answer is quite simple: you have to calculate the total space required for the system to function at full load in emergencies. At first glance, it seems perfectly logical to choose this approach, which will certainly keep the system functional, but is it worth it? Taking a two-pole HVDC system as a reference, adding a third cable (DMR) and calculating the minimum distance between the cables so that the system does not reach its maximum temperature in emergencies results in doubling the width of the system. This results in additional land and permits being required for hundreds or even hundreds of kilometers of HVDC connection, which has a significant impact on project costs. Bynoc has conducted several studies to investigate the effects of reducing the current carrying capacity in asymmetric (emergency) operation, with the cable spacing and system configuration being the same as in symmetric (normal) operation. In addition, Bynoc has also performed transient calculations to determine how long it would take for the DMR to reach a stable state, which in the worst case is approximately 50 days.

Of course, there is uncertainty about the duration of the repair time for a failed mast. Further information will be gathered as more experience with 525 kV HVDC cable systems in operation becomes available. At the moment, we can only exchange ideas, and we would like to stimulate discussion on the following question: Would it be better to optimize the system spacing for normal operation and reduce the load (approx. 5% load reduction) for a limited period of time, or should everything be calculated so that the full load can be handled in an emergency, making the system more robust but probably more expensive?