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Green Gridworks

Case studies on integrating renewable resources.

Fortnightly Magazine - February 2012

in place, the risks faced by both CAISO and Imperial Irrigation District—with their mutually beneficial upgrades—might have been reduced, thereby allowing the transmission projects to proceed with less risk to both parties.

Hawaii: Island Economics

Hawaii is blessed with sunshine, beautiful beaches, lush lands, and wind. Indeed, the economics of wind power in Hawaii are compelling; Hawaii has the highest retail power costs in the U.S. due to the isolation of the relatively small systems on each island and their heavy dependence on oil-fired generation. The island of Oahu is the most populous island and accounts for about 75 percent of Hawaii’s load and peak demand. Wind resources are limited on Oahu, but plentiful on the nearby islands of Molokai, Lanai, and Maui (see Figure 8) . Hawaii Electric Co. (HECO) and its subsidiaries serve about 95 percent of Hawaii’s load, including these islands.

In 2007, HECO released a solicitation of interest announcing plans to issue a renewable resource RFP for projects up to 100 MW. The RFP would allow HECO and its subsidiaries to meet the Hawaii RPS law that requires renewable resources to supply 10 percent of their net electricity sales by 2010, 15 percent by 2015, 25 percent by 2020, and 40 percent by 2030. HECO issued the RFP in 2008 and received PUC permission to negotiate with two much larger (400 MW) wind proposals on the nearby islands of Molokai and on Lanai that would deliver their power to Oahu by undersea cable. The two wind and cable projects, plus necessary upgrades to the HECO system on Oahu, are referred to as the Big Wind project.

In a parallel effort, the U.S. DOE’s National Renewable Energy Laboratory commenced a technical study to examine the technical feasibility and cost for the undersea AC or HVDC cable systems from Molokai and Lanai. Additional wind development on Maui will be considered in the future. Released in February 2011, the National Renewable Energy Laboratory Oahu Wind Integration and Transmission Study (OWITS) reached several conclusions.

AC wasn’t viable because the three-core XLPE cable, which would minimize the magnetic field and losses, would be too heavy and limit its depth. An AC cable could be utilized for shallower applications, however, as was recommended in the Phase 2 report issued later. HVDC was preferred, and voltage source converters were found to be “the only practical converters” because they “create a stable AC supply for [the] wind turbine generators” and “offer a significant buffer to AC system faults.” A 200 MW cable system was determined to be the maximum rating consistent with HECO’s normal operating practice of 180 MW of spinning reserves. Adjusted budgetary cost estimates for two 150-kV, 200-MW cable systems ranged from $466 to $709 million. Roughly one-half of the costs were for the converter stations and one-half for the cables.

The report concluded that the system must be designed to avoid problems “when the DC cable is tripped out of service at its receiving (Oahu) end.” Interconnecting the wind farms to the small local loads on Molokai and Lanai wouldn’t be practical; the