Microgrid instability poses critical issues to the power delivery following a load change or a tripping event. In island operating mode lack of grid intensifies this challenge. This study aims at controlling several converter-based distributed generations (DG) sharing the power in an island microgrid (MG). At first, the microgrid model including virtual impedances and phase-locked loop (PLL) is introduced. Afterwards a novel small-signal stability analysis for island microgrids is proposed. Finally, an optimization algorithm based on particle swarm optimization (PSO) is proposed to design the virtual impedances. The optimization algorithm analyzes all possible operating points and aims at maximizing the microgrid stability index while keeping the reactive power mismatches at minimum level. The fractional objective function facilitates reaching at these objectives simultaneously. The proposed optimization algorithm is implemented in two separate case-studies and the corresponding virtual impedances are drawn in any microgrid. On the other hand, The voltage drops are checked as a condition in the optimization process. The results drawn from two separate case-studies verify that the proposed algorithm effectively maximizes the microgrid stability index and minimizes the reactive power mismatches.

In this study, a Lyapunov energy function based control method with output voltage feedback loops is proposed for three-phase uninterruptible power supply (UPS) inverters. The presented paper demonstrates that the traditional Lyapunov-energy-function-based control method not only leads to considerable steady-state error in the output voltage, but also distorts the output voltage waveforms. Therefore, a modification has been performed on the traditional Lyapunov-energy-function-based control by incorporating the output voltage feedback loops in the control variables. The robustness of the proposed control method has been studied analytically through transfer functions which are expressed as the ratio of the output voltage to its reference. These analytical results are validated experimentally. In addition, the steady-state and dynamic performances of the proposed control method are also tested experimentally on a three-phase UPS inverter operating with linear (resistive) and nonlinear (diode-bridge rectifier) loads. As a consequence of incorporating output voltage feedback loops into the control variables, the proposed control method offers strong robustness against variations in LC filter parameters, high-quality sinusoidal output voltage along with acceptable total harmonic distortion (THD) values under linear and nonlinear loads, fast dynamic response under abrupt load changes, and negligibly small steady-state error in the output voltage.INDEX TERMS Uninterruptible power supply inverter, Lyapunov energy function, output voltage feedback, steady-state error, robustness.

Three-level converters have drawn extensive attention due to their ability to deliver high-quality power. High semiconductor count is the main drawback of three-level converters. As a solution to this, a 10-switch converter is presented, that has advantages over both two- and three-level converters, simultaneously, plus it is applicable to a variety of power ranges. However, the switching pattern of 10-switch converter is not as simple as standard three-level converter due to lack of medium vectors. This paper presents a novel space vector modulation (SVM) for a 10-switch converter to reduce total harmonic distortion (THD) and common mode voltage (CMV) of this converter in comparison to prior carrier-based modulation methods. A simplified, low-cost modulation algorithm for the converter is proposed. The designed switching sequence has aimed at a low output THD and enhancement of DC bus voltage utilization. The performance of the proposed SVM is then compared to upgraded sinusoidal PWM. AC power quality and CMV of a 10-switch converter based on two modulation methods are investigated via simulation models. It was validated via simulation and experimental models that the proposed SVM utilized DC bus voltage more efficiently, generated remarkably less THD compared to other methods, and had a lower peak and rms CMV.

Integration of renewable energy sources (RESs) into power systems is growing due to eco-friendly concerns and ever-increasing electricity demand. Voltage source converters (VSCs) are the main interface between RESs and power grids, which have neither rotational inertia nor damping characteristics. Lack of these metrics make the power grid sensitive to frequency disturbances and thereby under frequency, to load shedding activation or even large-scale collapse. In this regard, the contribution of this paper is to develop a new control technique for VSCs that can provide virtual inertia and damping properties with the DC-link capacitors inhered in the DC-side of grid-tied VSCs. The applied VSC is controlled in the current controlled model, with the capability of injecting extra active power with the aim of frequency support during perturbations. The dynamics assessment of the proposed platform is derived in detail. It is revealed that the control scheme performs in a stable region even under weak-grid conditions. Finally, simulations are conducted in MATLAB to depict the efficacy and feasibility of the proposed method. The results show that frequency deviation is mitigated under step up/down changes in the demand, and the rate of change of frequency is improved by 47.37% compared to the case in which the synthetic inertia loop is canceled out.

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