Ancillary services for distributed generation (DG) systems become a challenging issue to smartly integrate renewable-energy sources into the grid. Voltage control is one of these ancillary services which can ride through and support the voltage under grid faults. Grid codes from the transmission system operators describe the behavior of the energy source, regulating voltage limits and reactive power injection to remain connected and support the grid under fault. On the basis that different kinds of voltage sags require different voltage support strategies, a flexible control scheme for three-phase grid-connected inverters is proposed. In three-phase balanced voltage sags, the inverter should inject reactive power in order to raise the voltage in all phases. In one- or two-phase faults, the main concern of the DG inverter is to equalize voltages by reducing the negative symmetric sequence and clear the phase jump. Due to system limitations, a balance between these two extreme policies is mandatory. Thus, over- and undervoltage can be avoided, and the proposed control scheme prevents disconnection while achieving the desired voltage support service. The main contribution of this work is the introduction of a control algorithm for reference current generation that provides flexible voltage support under grid faults. Two different voltage sags have been experimentally tested to illustrate the behavior of the proposed voltage support control scheme.

Distributed generation inverters have become a key element to improve grid efficiency and reliability, particularly during grid faults. Under these severe perturbations, inverter-based power sources should accomplish low-voltage ride-through requirements in order to keep feeding the grid and support the grid voltage. Also, rated current can be required to better utilize reactive power provisions. This paper presents a reference generator capable to accomplish these two objectives: to keep the injected currents within safety values and to compute the power references for a better utilization of the inverter power capacity. The reference generator is fully flexible since positive and negative active and reactive powers can be simultaneously injected to improve ride-through services. Selected experimental results are reported to evaluate the performance of the proposed reference generator under different control strategies.

Nowadays, the majority of the photovoltaic (PV) power sources are connected to the public grid. One of the main connection problems occurs when voltage sags appear in the grid due to short circuits, lightning, etc. International standards regulate the grid connection of PV systems, forcing the source to remain connected during short-time grid-voltage faults. As a consequence, during the voltage sag, the source should operate with increasing converter currents to maintain the injection of the generated power. This abnormal operation may result in nondesired system disconnections due to overcurrents. This paper proposes a controller for a PV three-phase inverter that ensures minimum peak values in the grid-injected currents, as compared with conventional controllers. From the system analysis, a design method is presented in order to set the parameters of the control scheme. Selected experimental results are reported in order to validate the effectiveness of the proposed control.

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.

The resonant current control has been extensively employed to reduce the current harmonic distortion in a wide range of grid-connected distributed generation applications, including photovoltaic (PV) inverters, wind and water turbines, and fuel-cell inverters. However, the performance of these systems is deteriorated when the utility grid voltage experiences abnormal conditions such as voltage harmonics and imbalances. Several advanced control solutions have been recently introduced to cope with this problem but at the cost of a significant increase in the control computational load. This paper first analyzes the limitations of the standard resonant current control operating under abnormal grid conditions and then introduces a control scheme that improves the current harmonic distortion in such adverse conditions without increasing the computational load of the standard current control. This theoretical contribution is validated by means of selected experimental results from a three-phase PV inverter.

Voltage sags are one of the main problems in transmission and distribution grids with high penetration of distributed generation. This paper proposes a voltage support control scheme for grid-connected power sources under voltage sags. The control is based on the injection of reactive current with a variable ratio between positive and negative sequences. The controller determines, also, the amount of reactive power needed to restore the dropped voltage magnitudes to new reference values confined within the continuous operation limits required in grid codes. These reference values are chosen in order to guarantee low current injection when fulfilling the voltage support objective. Selected experimental results are reported in order to validate the effectiveness of the proposed control.