Proposition of qualification procedure for MVDC cables

Abstract

The medium voltage direct current (MVDC) electric networks are emerging in power distribution and collection, following the expansion of the high voltage direct current (HVDC) in power transmission. The benefits of MVDC can be analyzed in terms of energy efficiency, power dispatch, power capacity, power supply range, raw material consumption, CO2 emissions, network resilience and ancillary services to the AC grid. MVDC transmission has many advantages such as the control of the load flow and the higher transmission capacity, which can be up to 5.7 times in comparison to MVAC systems.

The emergence of MVDC systems relies on the development of electric components including power electronic converters, DC switchgear and DC cables. The requirement for these components depends on the MVDC system architecture, control and protection.
The development of MVDC systems faces several challenges, including:
• Standardization: There is currently no international standard for MVDC systems. This lack of standardization can hinder the development of MVDC systems and make it difficult to compare different technologies.
• Cost: MVDC systems are currently more expensive than AC systems. However, the cost of MVDC systems is expected to decrease as the technology matures.
• Acceptance: There is some resistance to the adoption of MVDC systems, due to concerns about safety and reliability. However, these concerns are likely to be addressed as the technology matures.
Miscellaneous working groups are currently working on these challenges in the industrial community. One can cite the efforts for standardization of DC devices on MV, such as the IEC Technical Committee (TC) 17 on the DC switchgears and the IEEE Std 1709 on MVDC applications in ships. CIGRE WG C6.31 MVDC Grid Feasibility Study has published the brochure and a similar brochure is available from CIRED.

An ongoing CIGRE working group C6/B4.37 Medium Voltage DC distribution systems is further surveying the development of MVDC systems and newly launched WG B1.82 will study specifically the requirements to MVDC cable systems. Despite the absence of standards, utilities (T&D system operators and renewable producers) start running techno-economic studies and equipment manufacturers (switchgear, transformers, converters, cable systems, …) start developing products, for example Siemens MVDC Plus and ABB.

MVDC electric networks offer several potential benefits over AC systems. However, there are also several challenges that need to be addressed before MVDC systems can be widely adopted. In this paper, a full qualification procedure for MVDC cables system is proposed. This contribution starts with a comparison between stresses applied from AC and DC systems on the cable. The main difference will be the electrical field stabilization constant that is dependent on the temperature. Then, Specific transients in MVDC systems are simulated to extract the wave shape that will be applied to the cable system. Different MVDC architecture and converter types are used to cover a wide range of stresses. Harmonics and losses are discussed and their impact on the cable system is clarified. Therefore, a full qualification procedure is proposed covering the electrothermal stresses (type test, prequalification test, thermal stability test, wet aging test, …) as well as transients such as lighting impulses and damped/transient over-voltages.

Amjad Mouhaidali, Ludovic Boyer, Raphaël Guffond (Nexans), Lina Ruiz (Nexans).

Presented at CIGRE 2024

HAL reference : hal-04708849v1