چكيده به لاتين
In recent years, due to important issues such as energy security, environmental pollution and low efficiency of conventional electricity networks, the tendency to use distributed generations (DGs) has increased. In new energy systems, due to the use of DGs, the pressure on the transmission system and power losses are reduced. However, the use of distributed generation sources leads to the complex structure and challenges to the operation, control, stability and security of power distribution networks. Therefore, the microgrid has been proposed to address the technical and economic shortcomings of distributed generation systems and to realize the use of these resources under high reliability and power quality conditions. Subsequent to the emergence of the concept of microgrid, issues such as control and operation became apparent, including power management between sources and power quality, especially in island mode, which are important parameters that have to considered to control and operation of the microgrid. Therefore, in this thesis, the control of interfaced converters of DGs in order to improve microgrid power quality with emphasis on voltage unbalance is considered. The control scheme of these converters is coordinated so that the DGs contribute to voltage Unbalance compensation and reactive power sharing proportional to their rated power by considering the residual capacity of each DG. The control structures presented in this thesis are classified into two categories: voltage-controlled mode (VCM) control scheme and current-controlled mode (CCM) control scheme. VCM units using capacitive virtual impedance contribute to voltage unbalance compensation, while in the control scheme of CCM units, a virtual admittance is implemented in order to compensate voltage unbalance with considering the residual capacity of the inverter. Also, VCM and CCM units contribute to reactive power sharing by droop and reverse droop control methods, respectively. Droop coefficients are adjusted considering the limited capacity of these inverters and the unbalanced power. The results were then presented to show the suitability of the proposed control structure in the Voltage unbalance compensation and proper power sharing of reactive and unbalanced powers among the DG units.
