MVAC submarine cable, impedance measurements and analysis

Due to environmental concerns an increase in off-shore windfarms has been observed in recent years, leading to an increased demand for three-core-wire-armoured submarine cables. However, the IEC Standard 60287 used to calculate the ampacity of these cables is widely recognized as being not accurate. Several recent studies have shown that the standard overestimates the cable losses, which results in over-dimensioning the cables and hence higher costs. The studies claim that the over-dimensioning might be due to the collective wire-armour which electromagnetic behaviour is not considered accurately by the standard. This paper investigates the standard inaccuracies, by conducting comprehensive measurements on a three-core-wire-armoured submarine cable. Firstly, DC measurements are conducted in order to investigate the connections between cable components (screen-screen and screens-armour) as this may influence the losses. Secondly, cable parameters such as component impedances, positive-, zero-sequence impedances and losses are measured for varying currents and frequencies. Zero sequence is measured for two setups, one with armour grounded, and with armour open to match CIGREs formula assumption that no currents flows in armour. Finally, the results are compared to a PSCAD model and calculations based on CIGRÉ and IEC formulas. It is found that IEC 60287 overestimates the losses, as expected, but the difference is much smaller than found in other studies. A deviation in the calculation of zero-sequence impedance is found between the measurements and the CIGRÉ formulas. It is concluded that the formulas are not accurate for the specific cable under test.