## 1 Introduction  
Due to the problem of global warming, utilization of distributed generation systems, which are connected with a distribution grid system, has been of interest and has received considerable attention in recent years. As the distributed generation can be located close to load consumers, it can have some merits: increasing the available power, improving the overall system reliability, lowering cost, reducing emissions and expanding energy options [1]. It is well known that wind power is one of the distributed resources. However, connecting wind turbine generators to a distribution grid system leads to stability problems due to the output power fluctuation. Therefore, it is very important to analyze a suitable control system for wind generators connected to the grid. The wind turbines can be Fixed Speed Wind Turbines with Induction Generator (FSWT-IG) or Variable Speed Wind Turbines with Permanent Magnet Synchronous Generator (VSWT-PMSG). The FSWT-IG has the advantages of mechanical simplicity, low specific mass, robust construction, and cost efficiency [2]. However, its disadvantages are a limited ability for power quality control and terminal voltage fluctuation under steady state condition, due to the uncontrollable reactive power consumption [3]. The VSWT-PMSG is a promising and attractive type of wind turbine concept, in which PMSG can be directly driven by a wind turbine and is connected to the power grid system through the AC/DC/AC power converter. The advantages of VSWT-PMSG are: (1) No gearbox and no brushes, and thus higher reliability; (2) No additional power supply for excitation; (3) The converter permits very flexible control of active and reactive power in cases of normal and disturbed grid conditions [4,5]. Therefore, a combined installation of VSWT-PMSG and FSWT-IG in a wind farm can be efficient due to reduced system investment cost. However, the PMSG has a more complex generator construction and more complicated controller system compared with FSWT-IG. Hence, the design and analysis of the power converter controller system still needs to be improved. The AC/DC/AC converter of PMSG consists of a stator side converter and a grid side converter linked by dc circuit. The grid side converter has an important role in ensuring the active and reactive power delivered to the network effectively. Parameter change in the grid system can lead to a significant impact on the stability of the control system performance, especially under fault conditions. The deviation of grid system impedances can cause change in the stability gain margin and phase margin of the control system. In addition, the converter is operated at high switching frequencies between 2-15 kHz resulting in high order harmonics, which can disturb sensitive load on the grid and generate power losses [6,7]. To reduce harmonic currents injected to the grid, LC filter is an attractive solution because of its many potential advantages, such as higher harmonic attenuation and smaller inductances compared with an L filter [8]. However, the resonance frequency of the filter can cause stability problems in the control system performance. Hence, determination of gain parameters should be performed carefully in the design process. Traditionally, the conventional PI controller is a very common in the control of the power converter of PMSG because of its simple structure and good performance in a wide range of operating conditions. PI controllers are simple but can not always effectively control systems with changing parameters or strong nonlinearities, and they may need frequent online retuning of their parameters [9]. Integration of a fuzzy logic control with a conventional PI controller could be an effective way to solve the problem of system parameter change. The fuzzy logic control can be used to adjust the gainparameters of the PI controller for any operating conditions. Hence, a good control performance can be achieved. However, the membership function of the fuzzy set should be carefully determined in the controller design.