Electricity has started to flow through the 2.2 GW Western Link between Scotland and Wales but further work is needed at Hunterston before full capacity is reached, National Grid told S&P Global Platts.
The cables can transfer up to 900 MW under current conditions, National Grid said, but "further work is required at Hunterston and there may be times when the power flow will need to be taken out of the system" before the full 2.2 GW capacity is available.
"All activities are expected to be completed during 2018," the transmission system operator said.
The high voltage direct current link was originally scheduled to be operational this year, but has suffered construction and commissioning setbacks.
In July, energy regulator Ofgem postponed allowances to the interconnector because of manufacturing problems with the cable.
Then in September a component at the link's Scottish converter station in Hunterston failed, with fire crews attending the site, although there were no casualties.
A consortium of Siemens and Prysmian is building the 420 km link, running from Flintshire Bridge in Wales to Hunterston in Scotland. It is the first submarine interconnector to use a DC voltage level of 600 kV.
When fully operational in 2018, the GBP1 billion ($1.34 billion) joint venture between National Grid and ScottishPower Transmission will increase capacity between Scotland and England to over 6 GW, easing transmission bottlenecks between Scotland and England/Wales and reducing constraints on Scottish wind farms.
Scotland has over 7,400 MW of installed wind capacity. Analysis by WWF Scotland found that during the first six months of 2017, wind turbines produced 6.6 TWh, an increase of 24% year on year and enough to meet 57% of Scottish demand.