NEW YORK, June 18 (Reuters) - Crews at Exelon Corp's (EXC.N: Quote, Profile, Research) Commonwealth Edison subsidiary continued to repair power lines south of Chicago damaged by a tornado on June 7, a spokesman for ComEd said Wednesday.

The damaged lines, coupled with heavy West-East power transfers, have resulted in increased congestion in the ComEd and NIPSCO systems, PJM, the Mid-Atlantic/Midwest power grid operator, said in a June 17 report.

NIPSCO, or Northern Indiana Public Service Co, is the neighboring power company in Indiana. The damaged cables feed into some of the lines that connect the two systems.

NIPSCO is a unit of NiSource Inc (NI.N: Quote, Profile, Research), of Merrillville, Indiana.

The ComEd spokesman said the company was doing everything it could to restore the damaged lines but could not say when the equipment would return.

Electricity traders pointed to a daily PJM transmission report that said the damaged lines - the East Frankfort-Crete and East Frankfort-University Park lines in Illinois - could return June 25-28.

Last week, ComEd said in a June 12 release that its crews would continue to make repairs to the damaged equipment over the next six weeks.

The June 12 release was about the reopening of all lanes of Interstate-57 in Illinois after traffic was interrupted June 7 when the tornado damaged five transmission towers, leaving seven spans of wire blocking the highway.

Interstate-57 is a north-south interstate highway from Miner, Missouri to Chicago.

Exelon, of Chicago, owns and operates more than 38,000 MW of generating capacity. It transmits and distributes electricity to 5.4 million and distributes natural gas to 480,000 customers in Illinois and Pennsylvania. (Reporting by Scott DiSavino; Editing by John Picinich)