WARWICK, R.I. — The Rhode Island Public Utilities Commission on Wednesday approved a power purchase agreement for a proposed wind farm off the coast of Block Island, ruling over the objection of critics who slammed the arrangement as a sweetheart deal meant to benefit one developer. Attorney General Patrick Lynch vowed to appeal the decision to the state Supreme Court, saying in a statement that the deal makes ratepayers buy "grossly overpriced electricity." The agreement between Deepwater Wind, LLC, a New Jersey-based developer, and National Grid, the state's dominant utility, involves a proposed eight-turbine pilot project connected by a transmission cable to the mainland. The three-member commission, a quasi-judicial body, approved the agreement after weighing economic and environmental benefits and whether the terms were reasonable for ratepayers.