Introduction

Power electronics is a key for the integration of renewable energy systems [1-9]. With an increasing adoption of gridconnected Photovoltaic (PV) systems, a strong emphasis is placed on their dynamic behaviors and impacts on the public grid [10-17]. Accordingly, most countries have revised their grid codes to utilize this huge capacity to improve the grid stability during grid faults [18, 19]. These new requirements enforce distributed generation systems to remain connected to the grid and inject reactive power to the grid under fault incidents [20-24]. Up to now, a vast array of technical literature has been presented on the performance of wind turbines under grid faults [25, 26]. These issues are now gaining more considerations in PV systems [27-32], as the power capacity of an individual PV system is also increasing. Detection of voltage sags, current limitation, current reference calculation, active and reactive power oscillation, and dc-link voltage oscillation are among the important issues. In addition, they are the key issues to a proper operation of grid-connected PV converters under faults [33]. Among them, the Current Reference Calculation (CRC) plays the most impressive role to satisfy the grid requirements, especially under unbalanced grid faults.

Different CRC methods can be found in the literature. In [34], the Instantaneous Active Reactive Control (IARC) has been proposed. The IARC controls three-phase voltages and does not use the Positive Sequence (PS) and Negative Sequence (NS) voltages. Despite of good performance under balanced faults, in case of unbalanced voltage sags, the IARC control strategy leads to non-sinusoidal output currents with a high Total Harmonic Distortion (THD). In [27], the Average ActiveReactive Control (AARC) has then been proposed to mitigate high order harmonics from the IARC method. However, if both active and reactive current are injected, active power suffers from oscillations of twice the fundamental frequency. Hence, the Positive and Negative Sequence Compensation (PNSC) has been introduced to inject sinusoidal PS and NS currents to the grid [34]. However, in case of injecting both active and reactive power, double the grid fundamental frequency oscillations during unbalanced grid faults will appear. Furthermore, the Balanced Positive Sequence Control (BPSC) method is presented in [35] to inject a set of balanced and sinusoidal currents with PS components. Unfortunately, the active and reactive power also have oscillatory components. In [36], a control scheme has been presented which injects both PS and NS proportional to a certain adjustable parameter according to the grid fault severity. As observed from the above, the priorart control methods suffer from either active power and dc-link voltage oscillations or injection of non-sinusoidal currents into the grid under unbalanced grid faults.