IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS 1 Active and Reactive Power Strategies with Peak Current Limitation for Distributed Generation Inverters During Unbalanced Grid Faults Antonio Camacho, Miguel Castilla, Jaume Miret, Member, IEEE, Angel Borrell and Luis Garc´ ıa de Vicu˜ na Abstract—Distributed generation inverters have become a key element to improve grid efficiency and reliability, especially 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. Index Terms—Power control, low-voltage ride-through, voltage sag, grid fault, peak current, reference generator. I. I NTRODUCTION T HE high penetration level of renewable energy sources and distributed generation (DG) plants has led to a change in the requirements for ancillary services, particularly during grid faults. Among these new services, low-voltage ride-through in wind plants, photovoltaic parks and other grid connected power devices is gaining an increasing attention due to their capability to improve grid efficiency, safety and reliability. Low-voltage ride-through was first required to withstand voltage sags. Therefore, power plants could remain connected and avoid sudden tripping and loss of power generation. As DG penetration was increased [1], network operators intro- duced reactive power injection to support the grid voltage and to reduce the possibility of voltage collapse. Next generation of grid codes could demmand negative sequence current injection when needed [2], and rated current to better exploit reactive Manuscript received January 17, 2014; revised April 19, 2014 and July 18, 2014; accepted July 25, 2014. Copyright © 2014 IEEE. Personal use of this material is permitted. However, permission to use this material for any other purposes must be obtained from the IEEE by sending a request to pubs-permissions@ieee.org. This work was supported by the Spanish Ministry of Economy and Com- petitiveness under Grant ENE2012-37667-C02-02 and CYTED programme under Grant 713RT0475. A. Camacho, M. Castilla, J. Miret and L. G. de Vicu˜ na are with the De- partment of Electronic Engineering, Technical University of Catalonia, 08800 Vilanova i la Geltr´ u, Spain (e-mail: antonio.camacho.santiago@upc.edu; miquel.castilla@upc.edu; jmiret@eel.upc.edu; vicuna@eel.upc.edu) A. Borrell is with the Department of Electrical Engineering, Escola Uni- versit` aria Salesiana de Sarri` a, Autonomous University of Barcelona, 08017 Barcelona, Spain (e-mail: aborrell@euss.es) power provisions [3]. Under these requirements, improved services could be attended, and new control schemes may be developed. The new distributed nature of these power systems allows for a wide range of control strategies since DG is required to operate flexibly and to enhance the services provided to the grid. To avoid the disconnection of power suppliers, the injected phase currents should be safely controlled at any instant. The problem of controlling the amplitude of the phase currents becomes more complex during unbalanced grid faults, mainly because the trend of simultaneously inject active and reactive power via positive and negative sequences. In such cases, the injection of positive and negative sequence power inherently induce different amplitudes in the injected phase currents. However, some interesting benefits can be obtained with these flexible strategies as will be shown later. It should be mentioned that a different approach to control the amplitude of the injected currents could be to find the current references instead of the power references. However, the power approach is mostly used in power converters control. Thus, a fully flexible reference generator with peak current limitation, which is the main contribution of this paper, can be useful to develop novel strategies and to avoid overcur- rent tripping. This new requirements are emerging from grid operators due to the fact that DG participation in the total power production is constantly increasing. Therefore, inverter- based DG are one of the key components to flexibilize the operation during voltage sags, and to define the behaviour of grid connected power systems under such contingencies. From the source’s point of view, the main problem during grid faults is the loss of power capacity to inject the active power production. As a result, dc-link voltage begin to increase and can lead to inverter tripping. To avoid this problem, some safety mechanisms must be activated, as for example crowbars to elliminate the excess of active power production, or to develop ride-through strategies to minimize the problem. Advanced control algorithms for low-voltage ride-through are mainly based on symmetric sequences [4]–[19]. Some of these studies have been proposed to achieve particular control objectives related to the current loop, power oscillations, power quality, dc-link ripple or voltage support during grid faults. However, few works have been developed for peak current limitation during unbalanced voltage sags [13]–[19], and they are not fully flexible since only particular cases are investigated to simplify the study. In [13], only active power is injected and no limitation is considered. In [14]–[17], only