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Problem Statement: Useful life of winding isolation is about 30 years. It can be reduced when it is submitted to overloads or when it is worked in aggressive environments. Approach: When retrofit is done, an increasing to superior isolation class is recommendable. So, generator capacity can be increased and pay back could be excellent justifying the investment. Result: Several Brazilian’s experiences successfully showed the retrofit with repower, for example many GE generators at Henry Borden Power Plant in Cubatao, Sao Paulo, whose power had increased up to 50%. This study aimed at presenting two cases of rewound with repower, changing the old insulating materials by other modern ones in substantial increases of power about 30%. The discussion also included how to determine the new power obtained after the repower and before the load tests. In large machines, the load tests are accomplished with machine connected with network power system and they needed special attention and care. In other words, they are critical tests. The calculations of the field current of full load generator for determining field temperature elevation, indirect tests (no load) are done. Which are the methods that present reliable results? Conclusion: This study showed a confrontation among three studied methods, ASA, IEEE 115 and General Method with their theoretical analyze. ASA method was normalized in many countries; in Brazilian showed in NBR-5052. In IEEE 115, the method is named “Phasor Diagram Analysis-Salient-pole Machines’’. General Method is an academic treatment with better theoretical explanation. |
