Grid faults are one of the most severe perturbations in power systems. During these extreme disturbances, the reliability of the grid is compromised and the risk of a power outage is increased. To prevent this issue, distributed generation inverters can help the grid by supporting the grid voltages. Voltage support mainly depends on two constraints: the amount of injected current and the grid impedance. This paper proposes a voltage support control scheme that joins these two features. Hence, the control strategy injects the maximum rated current of the inverter. Thus, the inverter takes advantage of the distributed capacities and operates safely during voltage sags. Also, the controller selects the appropriate power references depending on the resistive-inductive grid impedance. Therefore, the grid can be better supported since the voltage at the point of common coupling is improved. Several voltage objectives, which cannot be achieved together, are developed and discussed in detail. These objectives are threefold: to maximize the positive sequence voltage; to minimize the negative sequence voltage; and to maximize the difference between positive and negative sequence voltages. A mathematical optimal solution is obtained for each objective function. This solution is characterized by a safe peak current injection, and by the optimization of the voltage profile in any type of grid connection. Therefore, the proposed control scheme includes advanced features for voltage support during voltage sags, which are applicable to different power facilities in different types of networks. Due to system limitations, a suboptimal solution is also considered, analyzed, and discussed for each of the optimization problems. Experimental results are presented to validate the theoretical solutions.

Inverter-based distributed generation plays a vital role in the stability and reliability of new power systems. Under voltage sags, these systems must remain connected to the electrical network according to the stringent requirements of grid codes (GCs). Low-voltage ride-through (LVRT) control strategies are becoming a common trend in power electronics research. However, previous studies of these control strategies have not dealt with the different possible scenarios presented by new GCs, and many of them focus on a very limited number of control objectives. In this study, an algorithm to maximize the converter capabilities was developed and subjected to experimental tests during different voltage sags. In this research, based on unbalanced voltage drops of several severity levels, six different cases of current injection are identified while taking into consideration the restrictions imposed by GCs. The research results represent a further step toward the development of flexible controllers adaptable to the environments of intelligent electricity grids with high integration of distributed generation.

Flexibility and reliability of ac microgrids are enhanced by reducing the dependence on communications in multilayer hierarchical control. In this sense, this paper presents a secondary control with no communications for islanded microgrids. It is based on a control scheme that switches between two configurations according to a time-dependent protocol. With this proposal, an excellent performance in terms of transient response and frequency restoration in steady state is achieved. This paper also presents design considerations for the control parameters. In addition, the theoretical predictions are validated by experimental results from a laboratory microgrid.

Traditionally, the vector operation technique (VOT) has been used to control three-phase converters using one cycle control. In this paper, a three-phase active power filter large-signal model with a VOT in a new coordinate system is presented. By using the VOT, only two phase-legs are switching at high frequency, thus reducing the switching losses. This paper not only covers the literature gap about the modeling of three-phase converters using the vector operation, but also presents a sliding mode control for this converter. The control scheme consists of a nonlinear matrix transformation in order to obtain the voltages and currents in a new two-dimensional frame, γθ-frame, a sliding mode controller designed in these coordinates, and a modulator to obtain the control signals in a natural frame. The sliding mode control is designed with the help of the presented large-signal model assuring sinusoidal grid currents in phase with the grid voltages. This controller provides a fast transient response against sudden load changes with a good current tracking capability and a reduction of the switching losses. A stability analysis is performed in order to validate the control parameters. Experimental results are provided using a fully digital control system in order to validate the performances of the proposed controller.

Voltage sags are considered one of the worst perturbations in power systems. Distributed generation power facilities are allowed to disconnect from the grid during grid faults whenever the voltage is below a certain threshold. During these severe contingencies, a cascade disconnection could start, yielding to a blackout. To minimize the risk of a power outage, inverter-based distributed-generation systems can help to support the grid by appropriately selecting the control objective. Which control strategy performs better when supporting the grid voltage is a complex decision that depends on many variables. This paper presents a control scheme that implements a smart and simple strategy to support the fault: the maximum voltage support for the lowest phase voltage. Therefore, the faulted phase that is more affected by the sag can be better supported since this phase voltage increases as much as possible, reducing the risk of undervoltage disconnection. The proposed controller has the following features: 1) maximizes the voltage in the lowest phase, 2) injects the maximum rated current of the inverter, and 3) balances the active and reactive power references to deal with resistive and inductive grids. The control proposal is validated by means of experimental results in a laboratory prototype.