NEW DELHI- India's federal investigator will probe top state-run utility NTPC NTPC.NS and state-owned company MMTC MMTC.NS , over whether they were involved in a scheme to import inferior quality coal from Indonesia and pass it off as superior grade.

The Central Bureau of Investigation (CBI) has registered a case against unnamed officials of NTPC and MMTC, as well as the owner of a private firm Coastal Energy Pvt. Ltd for entering "into a criminal conspiracy amongst themselves with a view to cheat," according to a report on the CBI's website.

The CBI report said Coastal Energy, based in the southern city of Chennai, imported 147 consignments of coal from Indonesia between 2011 and 2015 and entered into agreements with NTPC, MMTC and some of their joint ventures to pass off the inferior quality of coal as superior grade through a "fraudulent modus operandi."

The scheme resulted in 4.87 billion rupees ($76.4 million) in "excess" charges being paid for the imported coal, the CBI added.

NTPC declined to comment, while MMTC and Coastal Energy did not immediately respond to request for comment.

India generates four-fifths of its power generation through coal-fired plants and imports the commodity from countries such as Australia, Indonesia and South Africa. Coal imports, however, have fallen for the past two fiscal years due to increasing local supply. ($1 = 63.7700 Indian rupees)

(Reporting by Sudarshan Varadhan, editing by Louise Heavens) ((Sudarshan.Varadhan@thomsonreuters.com ; +911149548059; Reuters Messaging: sudarshan.varadhan.thomsonreuters.com@reuters.net))