English title

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To achieve accurate reactive power and harmonic currents sharing among inverter interfaced distributed generation (DG) units in islanded microgrids, a hierarchal control scheme consisting of primary and secondary levels based on instantaneous circulating currents is proposed in this paper. Firstly, fundamental and main harmonic components of each inverter output current are extracted at the primary level and transmitted to the secondary controller. Then, using this information, instantaneous circulating currents at different frequencies are calculated and to generate proper control signals are applied to the primary controller. Consequently, these signals are inserted as voltage references after passing control blocks. In contrast to the conventional virtual impedance schemes, where reactive power and harmonic currents sharing are realized at the expense of introducing additional voltages drop and harmonic distortions, the proposed control strategy effect on the amplitude and waveform quality of DGs’ voltage is negligible. Meanwhile, it is able to provide accurate harmonic currents sharing even if nonlinear load(s) is directly connected at the terminal of DG unit(s). Control system design is described in detail and simulation results for four voltage-controlled DG units are provided to demonstrate the effectiveness of the proposed control method. 

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This paper provides a novel hierarchical control for VSI-based microgrids. The advantage of the provided control scheme is to maintain the frequency and voltage stability and load sharing against large-signal disturbances. A hierarchical control, consisting of three levels, is described. A new control loop based on PI controller, is presented. The new control loop has a great impact on increasing the stability margins, by moving the poles. In next steps, secondary and tertiary control levels are described. Then, the voltage droop equation is improved by a fuzzy controller. This controller generates a floating reactive power reference value, by a fuzzy logic. The role of floating reactive power reference value is to compensate the drastic changes in the voltage amplitude by changing the reactive power reference value. To verify the performance of the provided control scheme, a microgrid including four VSI-based DGs is simulated in islanded and grid-connected modes, by MATLAB/ SIMULINK. The simulation results show that the micogrid can maintain the frequency and voltage stability, in term of large-signal disturbances such as 3-phase and 1-phase short circuits. With this method microgrid load sharing is not altered after disturbances.

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In this paper, an optimal integrated inner controller is designed for the microgrid primary control level. The main task of the primary control level is to maintain stability and proper power sharing in microgrids. Non-optimal controllers have been generally used to design the inner controller in this level in the majority of researches. On the other hand, accurate and complete models of microgrids can be obtained. Therefore, it is an advantage to use the model based optimal control approaches. In this paper, an optimal inner controller is designed to simultaneously control of the microgrids voltage and current using a technique called linear quadratic tracking controller (LQT). In order to evaluate the performance of the proposed controller, it is applied to a microgrid including two distributed generators (DGs) in MATLAB/Simulink. Results of the simulations illustrate the proper performance of the controller in comparison with conventional methods.