Northern Ireland's electricity link with Great Britain has developed a fault, halving its capacity to import power ahead of peak winter demand.
Mutual Energy, which operates the undersea Moyle Interconnector, said the cause is still being investigated, and it is not yet clear when it will be repaired.
However, grid operator System Operator NI (SONI) said that even with the fault, it expects sufficient power generation to meet demand this winter.
Publishing details in its annual winter outlook, SONI's base-case scenario assumes the interconnector will return to service and that Northern Ireland's gas-fired power stations will not be constrained from operating for environmental reasons.
It added that if the interconnector does not return to full capacity this winter, the risk to the system increases, but remains within acceptable safety thresholds set by the Department for the Economy.
SONI Director of Operations, Emma Morris, said this year's assessment shows that, under normal operating conditions, "we expect there will be sufficient generation available to meet electricity demand throughout the winter", provided the "anticipated portfolio of generation remains available".
The outlook emphasised there is no risk of a system-wide blackout solely due to insufficient generation under any circumstances this winter.
"Other conditions would have to be present, such as multiple significant failures, to cause a system-wide blackout," it added.
Outages are 'rare'
A spokesperson for Mutual Energy said: "The Moyle Interconnector experienced an unexpected technical fault which has reduced its capacity to 250MW.
"Our recovery protocols were immediately activated, and our team is working hard to investigate and resolve the issue."
He added it is not known at this early stage when the Moyle Interconnector will be back to full capacity, but said they were in close communication with industry partners and government to minimise any potential disruption.
"Outages like this are rare, and rest assured our team is working tirelessly to return the Moyle Interconnector to full capacity."