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The growth in combustion turbine ratings has been a market driver for larger air-cooled generators. Open-aircooled (OAC) generators have a capability versus ambient temperaturc characteristic that closely parallels that of combustion turbines. Combining this with technology developments such as thinner. higher temperature capability high voltagc stator insulation. improvcd ventilation analysis such as computerized fluid dynamic (CFD), and high efficiency filters have led to economics favoring large air cooled generators over hydrogen for many applications. ECONOMIC TRADE-OFFSEntrepreneurs designing today's merchant power plants equipped with large combustion turbines must choose among three options for generators. These are hydrogen cooled generators. TEWAC (Totally Enclosed Water to Air Cooled) generators, and OAC (open-air-cooled) generators.Initial installed costs for hydrogen cooled generators and supporting auxiliaries are typically higher than for air coolcd generators, and hture costs of operating these auxiliar.ies are not negligible.Initial installed costs for TEWAC generators and auxiliaries are typically lower than for hydrogen cooled generators, and projected auxiliary operating costs are lower. However, providing adequate generator cooling water can be as expensive as for hydrogen cooled units; and even when a supply of cooling water is available, the temperature rise of cooling water above ambient air and the tcrnperature rise of generator cooling air above cooling water can lead to specifying TEWAC generators to be 20% to 30% larger than OAC generators of equal output power capability. Bccause cooling watcr and associated heat exchangers and water piping are not needed for OAC generators, because OAC generators are smaller than TEWAC generators for a given output power at a given ambient air temperature, and because OAC generator power output capabilities closely match combustion turbine capabilities over a broad ambicnt temperature range (Figure 1' ). OAC generators provide an attractive option for combustion turbines. 0-7803-5293-9/99 $10.00 0 1999 IEEE 758 COMPARISON OF LARGE COMBUSTION TURBINE 81 OPEN AIR COOLED GENERATOR CAPABILITIES i I 260

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