BISMARCK, N.D. (AP) ¿ Otter Tail Power Co.'s North Dakota customers will be paying slightly lower rates under a settlement of an electric rate case. The utility's 57,000 North Dakota electric customers also will be getting a small refund, with interest, for overpaying their electric bills during the past year. An average residential customer using about 836 kilowatt hours of power each month will get a bill credit of about $6 within the next three months. North Dakota's Public Service Commission approved the settlement Wednesday. Otter Tail customers also will be paying less to finance the cost of new wind farms in northeastern and east-central North Dakota. Now, a typical residential customer is paying $3.82 a month for the wind energy. The new rates put the payback amount at $2.65 a month.