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Stringent grid codes require wind turbines (WTs) to remain connected even during severe faults. This can result in PLL instability and Loss of Synchronisation (LoS), which is the main subject of this paper. For a better understanding, the phenomenon is analysed in the dq equivalence. The presented analysis enlightens the root cause of the associated instability and clarifies the key role of PLL in that. Based on this analysis, a hybrid solution combining an Adaptive-PLL with impedance estimation is proposed. In this regard, the PLL-gains during the fault are selected in accordance with accuracy of the impedance estimation: the higher the accuracy of the estimation, the lesser the required changes in the gains of PLL. The work also proposes a logical circuit for a more reliable detection of LoS, allowing the controller to only act when necessary. Simulation results and discussions support the proposals.INDEX TERMS Adaptive-PLL, estimated impedance value, grid following wind turbines, loss of synchronisation, PLL instability, severe faults.

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The popularity of converter‐based generations, such as wind and solar, suggests that future power systems will be dominated by power‐electronic converters, reducing the share of conventional generation, and, inevitably decreasing inertia and short circuit contribution of the system. When this happens in power systems, they become more vulnerable to instabilities. A promising solution for counteracting this vulnerability could be equipping converter‐based generation units with grid forming control schemes. A study of the fault‐ride through capability of wind turbines with different grid forming control schemes is presented in this paper. In this context, Virtual Synchronous Machines, Power Synchronisation Control, and Distributed PLLs based Control are considered. The performance of these control schemes is evaluated and compared during three‐phase symmetrical faults using a case study that consists of a wind turbine connected to an AC grid. The simulated results indicate the strengths and drawbacks of each control scheme during faults.

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