We have a number of recognized transmission and distribution experts serving on our board of ‘Grid Masters’. Several times each month we’ll post what we judge to be the toughest questions that also have high interest to our readers. At least one of our experts will respond. Want to challenge our Grid Masters?

This week’s question and answer:

Q: Submarine cables have limited capacity, the maximum that was put in operation is that of 800 MW and of plus/minus 500 KV, what do you think is the alternative technology to be used for such an application please?

--M. Wasfi

A: Cable power handling capacity is determined by its voltage and current rating. Current rating depends on the conductor material (copper or aluminum), conductor cross-sectional area, and maximum operating temperature. Conductor area is limited by the manufacturing capability and possibly transport or installation limitations. Maximum operating temperature depends on the insulation material and the service conditions. The power handling capacity of AC cables diminishes with distance due to the cumulative effect of charging current. Therefore, high-power, long-distance cables utilize HVDC where the entire cable ampacity is available for power transmission regardless of distance. There are HVDC systems in operation with 800 MW capability on a single 500 kV dc submarine cable.

The capacity of HVDC cable transmission can be increased by use of alternative configurations with two high voltage cables, e.g. bipolar or symmetrical monopole. The former uses two HVDC converters connected pole-to-neutral at each terminal. The latter uses a single HVDC converter connected pole-to-pole at each terminal. Transmission capacity can also be increased by raising the transmission voltage. For mass-impregnated (MI) submarine DC cables, the maximum voltage is 600 kV and maximum operating temperature is 50 degrees C. For extruded DC cables the maximum operating voltage currently available is 320 kV and maximum operating temperature is 70 degrees C. This voltage will increase in the future.

The Western HVDC link currently under construction is a 2200 MW, +/- 600 kV dc bipole with MI submarine cables. The NorNed HVDC link uses a +/- 450 kV dc symmetrical monopole configuration with 900 kV converters with MI cables. There are a number of different HVDC projects under construction with symmetrical monopole configuration using a pair of 320 kV extruded dc cables and voltage-sourced converters.

Michael Bahrman
Manager HVDC Business Development
ABB Grid Systems

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