This paper deals with the improvement of the transient response and harmonic, subharmonic, and dc-offset voltage rejection capability of a grid voltage sequence detection scheme based on a second-order generalized integrator (SOGI). To perform that, the SOGI structure is first analyzed in deep, emphasizing both its tradeoff limits between settling time and harmonic attenuation and the sensitivity to grid subharmonics and dc-offset voltage. Then, a study of the effect of grid voltage harmonics and subharmonics in SOGI and in the SOGI-FLL and MSOGI-FLL structures is introduced. Hence, to overcome these problems, a new structure based on the use of the SOGI filter as prefilter for the previous structures is proposed to achieve a faster time response and higher harmonic rejection. This structure is used in a sequence detection scheme for the detection of the grid voltage components in the αβ-frame and it is applied in a three-phase PV system. Experimental and comparative results are shown to validate this proposal.

Grid faults are one of the most severe problems for network operation. Distributed generation power plants can help to mitigate the adverse effects of these perturbations by injecting the reactive power during the sag and the postfault operation. Thus, the risk of cascade disconnection and voltage collapse can be reduced. The proposed reactive power control is intended to regulate the maximum and minimum phase voltages at the point of common coupling within the limits established in grid codes for continuous operation. In balanced three-phase voltage sags, the control increases the voltage in each phase above the lower regulated limit by injecting the positive sequence reactive power. In unbalanced voltage sags, positive and negative sequence reactive powers are combined to flexibly raise and equalize the phase voltages; the maximum phase voltage is regulated below the upper limit and the minimum phase voltage just above the lower limit. The proposed control strategy is tested by considering a distant grid fault and a large grid impedance. Selected experimental results are reported in order to validate the behavior of the control scheme.

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.

This paper presents an improved variable hysteresis-band current-control in natural frame for a three-phase unity power rectifier. The proposed control algorithm is based on three decoupled sliding-mode controllers combined with three independent Kalman filters. The use of Kalman filters instead of a nonadaptive state observer improves the quality of the estimated signals in presence of noise, increasing the immunity of the control loop in noisy environments. To reduce drastically the computational load in the Kalman algorithm, a reduced bilinear model is derived which allows to use a Kalman filter algorithm instead of an extended Kalman filter. A fast output-voltage control is also presented which avoids output-voltage variations when a sudden change in the load or a voltage sag appears. Moreover, a fixed switching frequency algorithm is proposed which uses a variable hysteresis-band in combination with a switching decision algorithm, ensuring a switching spectrum concentrated around the desired switching frequency. The overall control proposal has been fully integrated into a digital signal processor. Selected experimental results are introduced to validate the theoretical contributions of this paper.