Modern renewable energy systems (RES) require fault ride through (FRT) operation in order to provide voltage and frequency support to the power grid when faults occur. The grid synchronization of a RES could be ensured by the appropriate operation of a phase-locked loop (PLL). A new hybrid PLL (dαβPLL) has recently been suggested by the authors and could operate accurately and faster than the other existing PLLs. The dαβPLL could be a very useful tool in the FRT operation of a RES, since the faster performance of the new PLL could boost the time performance of the FRT algorithm when a disturbance occurs. This paper investigates the performance of the flexible positive and negative sequence control (FPNSC) when the new dαβPLL is used. The paper also deals with the improvement on the FRT operation of the RES that could be obtained from the outstanding performance of the dαβPLL. 1. Introduction The ever increasing penetration of renewable energy sources (RES) in the power grid requires the improvement of control techniques for the grid side converter (GSC) in order to ensure the synchronization and the appropriate operation of the RES during grid faults. Control methods in modern RES should be enhanced with fault ride through (FRT) capability in order to fulfill the grid codes for interconnected RES [1–3]. Especially in the case of small autonomous power grids, FRT operation of RES is particularly necessary in order to avoid cascading and catastrophic events. The most important aspect for a RES when a fault occurs, is to first keep the synchronization with the power system and then to operate in a way to support the power grid. The grid synchronization is ensured in RES by estimating the phase angle and the amplitude of the grid voltage at the point of common coupling (PCC) through phase-locked loop (PLL) [4] as shown in Figure 1. Conventional PLL algorithms [5–7] are not able to operate successfully under unbalanced grid faults. A new hybrid PLL [8], named d PLL, has been suggested by the authors and could operate faster and with lower frequency overshoot than the ddsrfPLL [9] (which also operates accurately under unbalanced conditions) when a fault occurs. Figure 1: An interconnected RES. The enhanced PLLs (d PLL and ddsrfPLL) estimate the phase angle and the amplitude of the positive and negative sequence of the grid voltage. These quantities are provided to the FRT control algorithm in order to calculate the appropriate positive and negative reference currents, as described in [10–14]. FRT algorithms usually require accurate injection of positive and
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