In this paper, a complete control scheme is developed for an inverter-based Distributed generation (IBDG) unit to produce high quality AC voltages in unit terminals. The unit uses a three-phase, four-leg inverter. The control scheme includes a feed-forward current control path and two nested loops, the outer loop for regulating the DG output voltage and the inner one for controlling the inverter current. Proportional-Resonant (PR) controllers are used in the voltage loop to track a sinusoidal reference voltage and eliminate low-order voltage harmonics. The inner current loop is used for over-load protection of the inverter, and improvement of the control system response. The current controller is designed by the state feedback method. The feed forward current control path is used to reduce impacts of load disturbances on the IBDG output voltages. Regarding the role of the inverter output filter and controllers in voltage quality, the filter design procedure is reviewed, the approach to choosing the controllers parameters is explained, and stability conditions of the system are investigated. The performance of an IBDG that supplies a local load is studied under unbalanced and nonlinear loading, and sudden changes in the load amount and voltage amplitude.

In this paper, a new approach is proposed for voltage and current harmonics compensation in grid-connected microgrids (MGs). If sensitive loads are connected to the point of common coupling (PCC), compensation is carried out in order to reduce PCC voltage harmonics. In absence of sensitive loads at PCC, current harmonics compensation scenario is selected in order to avoid excessive injection of harmonics by the main grid. In both scenarios, compensation is performed by the interface converters of Distributed generation (DG) units. Also, to decrease the asymmetry among phase impedances of MG, a novel structure is proposed to generate virtual impedance. At fundamental frequency, the proposed structure for the virtual impedance improves the control of the fundamental component of power, and at harmonic frequencies, it acts to adaptively improve nonlinear load sharing among DG units. In the structures of the proposed harmonics compensator and the proposed virtual impedance, a self-tuning filter (STF) is used for separating the fundamental component from the harmonic components. This STF decreases the number of phase locked loops (PLLs). Simulation results in MATLAB/SIMULINK environment show the efficiency of the proposed approach in improving load sharing and decreasing voltage and current harmonics.

In this paper, two control strategies are proposed for Distributed generation inverter ((DGI)) to control 1) active power exchange with the grid, 2) load voltage and 3) grid current. The main goal in these strategies is supplying local load with a standard sinusoidal voltage in the presence of grid voltage distortions and load current nonlinearity. In conventional (DGI)s, the load is directly and without intermediate impedance connected to the grid; thus, the possibility of load feeding with appropriate voltage quality is hindered in the grid connected mode. In order to overcome this deficiency, (DGI) with an LC-L filter topology and two control strategies are presented in this paper to supply the local load with standard voltage and to exchang power with the grid, simultaneously. In the first control strategy, the local load is supplied with pure sinusoidal voltage and grid current quality is not important. In the second control strategy, load voltage waveform and grid current waveform are controlled by adding appropriate harmonics to load voltage. For the implementation of the mentioned control strategies, a cascade power-voltage-current control structure has been proposed. A new controller has been introduced in the stationary reference frame for the voltage loop which gives the opportunity for compensation and tracking of harmonics without the need for complex and vast calculations. Test results under nonlinear load and non-ideal grid conditions validate that the proposed method can supply the local load with a standard voltage, inject power to the grid, and also control the grid current.

Nowadays the control and stability of DG system are important topics that researchers in both academia and industry have been addressing. Small and large signal analyses for stability studies on various systems have been done in papers and books. In this paper, at first models of an inverter-based Distributed generation (DG) subsystems are created and after the linearization if be required the small-signal stability analysis of the DG which is controlled with a voltage and frequency control scheme based on model predictive control (MPC) that has been used previously is established. In this control scheme, load currents at the point of common coupling (PCC) of the DG are considered as disturbances and used as feed-forward signals. This technique enhances the performance of the DG control system in transient and steady-state conditions for a wide range of loads. The stability of the DG system under various loads (such as one phase load as imbalanced load, rectifier load as nonlinear load and induction motor load as dynamic load) is demonstrated by the eigenvalues trajectory. Also the sensitivity analysis and robustness assessment of the control scheme are conducted and discussed. For more performance consideration, the DG system is simulated with MATLAB/SIMULINK software and implemented in the lab and then suitable performance of the system is demonstrated by the simulation and experimental studies.

Islanded operation of Distributed generators is a problem that can happen when they are connected to a distribution system. In this paper an islanding detection method is presented for inverter based Distributed generation (DG) using under/over voltage relay.The method is an adaptive one and is based on the change of DG active power reference (Pref) in inverter control interface. The active power reference has a fixed value in normal condition, if the Point of Common Coupling (PCC) voltage changes, Pref, is determined as a linear function of voltage. The slope of Pref depends on the load active power (Pload) and it should be changed if Pload changes. The Non-Detection Zone (NDZ) of the proposed method is zero. If changing of the PCC voltage is sensed, islanding will be detected when it occurs. Also, it does not have any negative effects on the distribution system under normal conditions. The proposed method is evaluated according to the requirements of the IEEE 1547 and UL 1741 standards, using PSCAD/EMTDC software.