The Independent System Operator - New England (ISO-NE) is responsible for planning and managing the regional bulk power system in six states, with advice from the Reliability Committee of the New England Power Pool (NEPOOL). The 462-MW Cape Wind offshore wind project applied to ISO-NE for interconnection to NSTAR's 115-kV transmission system at Barnstable substation on Cape Cod, Massachusetts. As part of this planning process, ISO-NE required system impact studies to assess Cape Wind's impact on thermal limits, steady-state voltage performance, short-circuit duties, dynamic stability, transient overvoltages, and harmonic resonances. NSTAR carried out these impact studies with cooperation from Cape Wind Associates and with assistance from several consultants. The results included requirements for shunt reactor compensation for the submarine and underground 115-kV cables that would connect the wind park to Barnstable substation, as well as reconductoring certain transmission lines and replacement of certain circuit breakers. The 115/33-kV step-up transformer impedance's were changed from 9% to 12% based on impact study results, and specific procedures for energizing the 115-kV cables were identified. It was noted that many of the new circuit breakers need to be definite-purpose for switching the 115-kV cables, and that circuit breaker transient recovery voltages require a detailed evaluation after the offshore Gas Insulated Substation (GIS) design has been finalized. On October 6, 2005, ISO-NE approved the Cape Wind interconnection, as recommended by the Reliability Committee, with appropriate conditions based on the system impact study results.
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