Electric power systems, which include many renewable energy resources for worldwide lowcarbon path, have a lot of problems and many studies are made for its solutions. Co-generation systems which consist of gas engine etc. have also been expected to make low-carbon path come true. One of difference between gas engines and renewable energy resources is their controllability. In this study, the operating reserve is considered as the ancillary service from distributed generator side. We have

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... ered whether distributed generators are able to alternative thermal power plant when the peak demands. As the result, we have realized the generating cost of distributed generator possible to able to reduce against existing thermal power plant.