Abstarct: Voltage control within the proper limits in islanding mode of operation is one the most important challenges in the microgrids. In this thesis, the design of a voltage control system in an islanded microgrid is investigated during uncertain variations of loads and renewable energy resources. The general approach in this thesis is to provide a multi-purpose local control system, taking into account the sudden and uncertain changes in loads and generation of DG units. This issue has been considered while using the Hierarchical Control structure at local control levels. By using current-baxsed droop equations and performing steady-state analysis, the Current-baxsed Steady-state Capacity Curve (CSCC) in the form of a circle has been introduced. baxsed on these curves, the upper and lower limits of droop equations have been calculated. Then, the idea of microgrid operation area has been introduced by intersecting the capacity curves of DG units with each other. The effects of possible variations of effective parameters and the corresponding variation in CSCC circles have been investigated. By performing dynamic analysis, the other droop limits for frequency and voltage have been calculated. These limits have been derived from elliptic shape of voltage-frequency curve. Along with these added features, a harmonic compensation is added to the voltage loop in order to reduce the amount of voltage harmonic contents. In the next approach, an inner controller in a hierarchical structure has been designed baxsed on direct Lyapunov stability theory. By using the proposed controller, proper switching functions have been derived in order to achieve a stable performance of local controller as well as overall proper operation of DG unit in the microgrid. The main contribution of this section is the use of the proposed Lyapunov-baxsed controller in the hierarchical structure along with voltage harmonic compensation that create a multi-purpose control system. To compensate the uncertain variations in the DC-side voltage caused by generation and load intermittent nature, an additional complementary control loop has been added to the hierarchical structure and the stability of the loop has been investigated. The use of this complementary control loop in the previously proposed methods and its integration in the hierarchical multi-purpose structure are the main contributions of this section.