Power transmission through a 600 megawatt (MW) undersea cable between Sweden and Poland was expected to resume by midnight on Monday after suffering an unplanned outage on Sunday evening, Polish power grid operator PSE reported.

Planned maintenance of the cable began last month. The cable started operating again on Sunday evening, but later a technical fault caused another outage, a PSE spokesperson said.

The outage was not due to a fault with the cable itself but because low oil levels had been discovered at a substation in Sweden, a spokesperson for Swedish power grid operator Svenska Kraftnat said.

The outage started at 11 p.m. (2100 GMT) on Sunday, according to a market message published by PSE on the Gas Inside Information Platform late on Sunday. The cable is set to be back on line at midnight on Monday, it said.

The power line crosses the damaged Nord Stream pipelines beneath the Baltic Sea and had been checked last week by Polish and Swedish grid operators and was found to be intact.