Towards Robust Qualification Procedures for Metallic Return Cable

In recent years, the use of HVDC systems for long-distance and offshore power transmission is rapidly increasing. A bipolar configuration of converters with a so-called Metallic Return (MR) provides enhanced system operation reliability. In normal bipolar operation, the MR cable is not expected to carry rated load and its insulation is expected to be stressed with negligible voltage. However, during (asymmetrical) monopolar operation, the MR carries the full current up to the rated load of the pole, which continues to remain in service. The MR cable is in operation for a limited time during the life-time of the HVDC system. In both CIGRE recommendations and IEC standards, the guidelines on how to verify the ability of MR to remain reliable during the life-time of the HVDC system are either limited or not present at all. This stems from the limited service experience of both manufacturers and utilities with MR. Consequently, an agreement between the two aforementioned parties must be reached on the construction and final form of the qualification of the MR. In this contribution, the focus is on the operational and construction characteristics of the MR cable in the first turn. In the second turn, the current requirements regarding the type testing of such cable, also in comparison to the existing medium voltage AC cable systems, will be presented. Furter, the type test sequences described in chosen international standards and guidelines towards MR testing, will be discussed. The paper will conclude with identification of gaps currently existing in the type test regimes and accordingly recommendations on the electrical type test sequence to be performed on a MR to ensure its availability for the entire service life-time of the bipolar HVDC system.