I. Introduction
Due to the challenge of the energy shortage and environmental pollution and because of its advantages of consuming no fossil fuels, infinite reserves, and harmlessness for the environment, the renewable energy source grid-connected generation system (RES-GGS) has attracted more and more attention [1] –[3]. At present, the initial investment cost of the RES-GGS is still high and the competitive edge of the RES-GGS is reduced. Therefore, how to enhance the power extraction ability and increase the utilization rate of the RES is a key issue in the RES-GGS. The solution is helpful to shorten the payback period and the competition of the RES-GGS will be enhanced significantly.