## 1 Introduction  
There are many small islands with residential areas in which power systems are often small-scale and isolated. In most cases, electrical power sources in such power systems are supplied from diesel engine-driven power plants. However, such plants have a negative environmental impact. Therefore, effective utilization of renewable power on islands is a crucial matter because there are often abundant wind energy sources there. However, because of vacillating renewable power due to dependence on weather conditions, the electricity quality of small-scale power systems with renewable power sources may decline. Hence, even a small amount of renewable power sources may not be introduced. Against this background, various design methods of microgrids, which include diesel power plants, energy storage systems and renewable energy sources, have been proposed that consider various viewpoints. Most of these studies are premised on the utilization of energy storage devices, and stabilization control studies using batteries for the balance between supply and demand in the microgrid have also been reported [1; 2; 3; 4; 5; 6]. In some cases, flywheel energy storage systems are introduced into diesel power generator-based microgrids instead of chemical batteries [7; 8]. The problem which requires an energy storage device can be significant if the main electrical power source is supplied by a conventional fixed speed synchronous generator (FS-SG). According to such situations, several kinds of strategies focusing on the main generator operation have been reported to regulate a microgrid system incorporating renewable power sources [9; 10; 11]. Most of these systems are composed of a conventional synchronous generator driven by a diesel engine as the main power source, and an energy storage system is considered in some cases. On the other hand, if the main generator can control its output quickly, then a balance between the generation and consumption in the power system will be effectively maintained. Under such consideration, applications of an adjustable speed diesel generator have been proposed in several reports [12; 13; 14]. However, there are few reportsthat provide detailed analyses of a small power system composed of a diesel power plant and a wind power station as a renewable power source, as well as their cooperative control system performed under natural variable wind conditions. In this paper, a new control strategy for the design and operation of a small-scale power system is proposed in order to improve its frequency stability against vacillating power from a wind power station. A diesel engine driven adjustable speed generator (ASG) employing a doubly-fed induction generator (DFIG) and a cooperatively controlled battery are adopted. In our previous reports [15; 16], we presented that frequency stabilization can be achieved effectively by the installation of an ASG. As only two profiles of fluctuating power were considered there; however, the effectiveness of the ASG control was not sufficiently evaluated. Moreover, in these reports, a conventional proportional-differential (PD) controller with fixed control parameters was applied to the frequency stabilization by using the ASG. Therefore, it was unclear whether the ASG could sufficiently regulate the grid frequency under the variations of the fluctuating power or not. This was because the conventional PD controller with fixed gains could not always achieve excellent performance under various conditions of the controlled system. Accordingly, in this study, a fuzzy logic control based PD controller was applied to the frequency control system by using the ASG. In general, fuzzy logic control is robust and can be expected to achieve sufficient performance under a variety of controlled system conditions, such as randomly fluctuating power from renewable power sources.