Power Transfer Capability of HVAC Cables for Subsea Transmission and Distribution Systems

This paper provides a methodology to estimate power versus distance envelops of high voltage alternating current (HVAC) transmission systems for subsea applications. Alternating current (AC) technology is mature and is proven for land-based applications and relatively short offshore tiebacks. However, for long tiebacks, due to increased conductor capacitance, large reactive power needs to be supplied, leading to higher cable current rating, losses, and expensive umbilical. Subsea ac cables are limited in their capability to transmit power beyond a certain distance, depending on cable characteristics, installation conditions, and system operating mode. Power transfer boundary charts for ac technology based on analytical methods are presented. Analytical calculations and computer simulations of HVAC power cables were performed on more than 30 cables with rated voltage between 35 and 150 kV and cable cross sections between 95 and 400 mm². Effects of reactive compensation on system tieback distance have been analyzed, as well as low-frequency transmission and different modes of system operation. Cable models employing multiple pi sections and distributed parameters were used. Well-known power flow and simulation tools were used for validation. AC transmission boundaries were estimated based on the voltage and current limits for various cables under various operating modes.