الفهرس 
IntroductionOnly 14 pages are availabe for public view
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Abstract
Most of Distributed Generators (DGs) use Voltage Source Inverter (VSI) for the purpose of interfacing DG with the grid and producing electricity at the desired voltage and frequency. By applying different control strategies for the inverter, we can control the DG as a current source or as a voltage source depending on the controlled variable in the control strategy.
DG is controlled as a current source in the grid connected mode and the controlled variable in this mode is the grid current to control the active and reactive power injected to the grid. In contrast, DG is operated as a voltage source in the islanded mode, and the controlled variable will be the voltage across the local load to keep its amplitude and frequency within the allowable limits. Therefore, the DG operates in two steady state modes: grid connected mode and islanded mode, and two transient modes: transition from grid connected to islanded, and from islanded to grid connected. During these transient modes, the voltage and current of the critical local load connected to the DG suffer from destructive spikes, as we need to switch the control strategy of the inverter. To solve this problem a seamless control strategy for the inverter of the DG is used with no need for switching between different control schemes. The strategy is called the “Unified Control Strategy”. The strategy is implemented in the Synchronously Rotating reference Frame (SRF), where AC quantities, that need to be regulated, are converted to DC quantities in this frame so conventional Proportional Integral (PI) controllers can be used which are simple and can achieve zero steady state error.
Hysteresis Current Controller (HCC) is used for generating the gate signals for the inverter switches which is based on comparing output currents of the inverter with their reference values, and then the error signals pass into Hysteresis comparator to decide which switch that will be given a signal on its gate to operate, so the controlling variable is always the inverter output current.
In this thesis, a seamless control strategy in the SRF is applied for the inverter to achieve smooth transition between different modes of operation and to improve the power quality. The same control structure is used for all modes, and there is no need for transition between different control structures. The DG is controlled as a current source in the case of grid connected mode by the means of current loop to control active and reactive power injected to the grid, and as a voltage source in the islanded mode by the means of voltage loop to control the voltage of local critical load . The controller proposed in the strategy is the Set Membership Affine Projection Algorithm (SMAPA) based Proportional Integral (PI) controller whose proportional gain (Kp), and integral gain (Ki) are chosen based on optimization technique to get the better dynamic performance during transitions.
The proposed control strategy is tested using PSCAD/EMTDC simulation environment under different loading conditions, and different types of loads linear, and non-linear loads. Simulations show the effectiveness of the control strategy during different modes of operation. The superiority of the proposed method is achieved through comparison with another method based on PWM technique.