Malfunction Operation of LVRT Capability of Wind Turbines under Islanding Conditions Bakhtyar Hoseinzadeh, Filipe Faria da Silva, Claus Leth Bak Department of Energy Technology Aalborg University Aalborg, Denmark {bho, ffs, clb}@et.aau.dk Mozhgan Mirehbaygi Graduated from Dept. of Electrical Eng. University of Kurdistan Sanandaj, Iran shadiamirbaygi@yahoo.com Abstract—The Low Voltage Ride Through (LVRT) capability of Wind Turbines (WTs) determines the connectivity of WT to the grid based on both voltage sag magnitude and duration locally measured at Point of Common Coupling (PCC), in order to protect the WT against overloading out of its tolerable apparent power. If the WT is still inside its safe and secure operating condition and there is still some standby capacity available in it to support the grid, the WT should remain connected to the power system regardless of voltage sag magnitude and duration until the WT apparent power exceeds its nominal value. In case of islanding, which often is accompanied with a low voltage drop, the WT may be improperly disconnected while operates with less than half of its nominal apparent power. This situation necessitates investigation or perhaps a revision of LVRT grid code to be efficient for all possible incidents, not only short circuit faults, but also cascading events and islanding. Index Terms—Wind turbine, low voltage ride through (LVRT), grid code, voltage sag, reactive power support, permanent magnet synchronous machine (PMSG). I. I NTRODUCTION The fast-growing and widespread concerns about global warming leads the energy provision toward Renewable Energy Sources (RESs), especially the Wind Power (WP), which recently has attracted a great attention in terms of development [1], [2]. The WP is the most promising source of energy that is relatively more affordable rather than the others. As the penetration level of RESs in power system is remarkably augmented, their impacts on the power system operation and control can no longer be overlooked. The intermittent behavior and stochastic nature of RESs imposes the uncertainty to the energy sources, which challenges the reliability of grid [3], [4]. Massive deployment of RESs necessitates the system op- erators to legislate technical standards known as grid codes in which RESs have to meet them while they are connected to the grid. The grid codes provide a full range of ancillary services same as conventional Synchronous Machines (SMs) to support the grid during the fault/s [5], [6]. The grid code standards for the next generation of RESs are continuously assessed and revised to be efficient under various operating conditions of RESs [7], [8]. The technical specification of grid code covers both static and dynamic requirements. The static part addresses the per- formance of the WT in its steady state e.g. power flow at PCC, whereas the dynamic part i.e. the most important part, deals with behavior of WT under fault conditions. The dynamic requirements basically encompass many features and ancillary services such as power factor, frequency and voltage regulation and Fault Ride Through (FRT). The FRT capability is a more general feature that includes LVRT and other fault types such as over speed ride through in WTs. The LVRT is the most important part of the FRT and is the ability of a generating unit to maintain its output voltage given short-term power dips. The WT may undergo decreased wind speed leading to a voltage dip at its PCC. In other words, the LVRT is the capability to respond to a major decrease in the energy input for RESs, e.g. wind and solar plants. The LVRT capability requires the WTs to remain connected to the grid for a specific period of time in presence of grid voltage sag/plunges. The Permanent Magnet Synchronous Generator (PMSG) type WT is gradually become dominant among the other available WT types especially in the offshore applications due to the following merits: Full range of wind speed, Low noise, High efficiency, Self excitation and Gearboxless. As depicted in Fig. 2, the generator in PMSG is indirectly connected to the grid via a full scale back-to-back voltage source converter. The LVRT behavior of PMSG in case of disturbance chiefly depends on the Gid Side Converter (GSC), since the generator mechanical/electrical dynamics are completely decoupled from the grid [9], [10]. Most of the internal control loops of GSC are implemented based on conventional PI controllers, which are tuned ac- cording to the steady state operation point. Therefore, the PI controllers are unable to handle all possible situations such as severe transient dynamics following the voltage sags. The inrush current of GSC even with the DC link voltage inside the normal range, may not be properly limited by them, which may damage the GSC of PMSG and Rotor Side Converter (RSC) of Doubly Fed Induction Generator (DFIG) [9]. II. LVRT GRID CODE REQUIREMENTS In order to achieve a reliable integration of high WP penetration into the power system, the WTs are required to be equipped with LVRT capability. The LVRT requires the WTs to remain connected to the grid and contribute to the