By Tim McLaughlin

July 16 (Reuters) - New York Governor Kathy Hochul's administration on Wednesday blasted an ongoing outage on the Champlain Hudson Power Express transmission line, a $6 billion project she championed to reduce reliance on dirty oil-fired plants while boosting the state grid's razor-thin buffer against unforeseen events.

The 1,250-megawatt Champlain Hudson Power Express transmission line is expected to be offline through the end of July, according to grid operator New York ISO's latest transmission outage schedule. Previously, New York ISO data indicated CHPE would be out of commission through the end of this week. New York ISO did not return a message seeking comment.

“The ongoing CHPE outage is unacceptable, said Ken Lovett, a spokesperson for Hochul. "At the governor’s direction, administration officials have been in daily contact with the developers as they work to identify and resolve the problem."

CHPE can deliver up to 20% of New York City's electricity. It is currently offline due to a cable issue, Hydro-Quebec told Reuters on Monday. The current cable issue on the U.S. side of the line is unrelated to a shutdown that happened on July 1, Hydro-Quebec said.

Hydro-Quebec developed the project with private-equity giant Blackstone Inc BX.N.

Hochul last month said CHPE will help replace the power lost with the closure of the Indian Point nuclear plant. Since then, New York City has had to rely more on fossil fuel generators, increasing downstate pollution.

This month, as temperatures have climbed toward 100 degrees Fahrenheit (38 degrees Celsius), oil fired-generators, which produce CO2 at about twice the rate of natural gas plants, have been ramped up to meet the surge in energy demand from increased air conditioning use, according to NYISO generation data.

The 339-mile (546 km) power line stretches along the entire length of New York State, from the Canadian border to a converter station in Astoria, Queens, where energy enters the New York City grid.

Earlier this week, New York ISO said it was not relying on CHPE this summer, even as grid reserves have been "extremely tight."