PLL Instability of Wind Turbines During Severe Faults

Abstract

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.

Fault ride through capability of grid forming wind turbines: A comparison of three control schemes

“…A solution for maintaining synchronisation with the ac grid during faults is to switch to conventional PLL [9]. However, this has also some disadvantages, such as the additional transients and potential instability of PLL that may result [23,24]. Since switching to PLL during fault is not an intrinsic characteristic of PSC (it can continue without switching), for a more complete picture, the result of PSC without switching to the backup PLL is also presented in Section 4.…”

“…Particularly, dPLL and PSC experience frequency drift when the fault impedance is zero. In these GFCSs, frequency drift occurs due to PLL instabilities during severe faults [23,24].…”

“…Again, that is because PSC2 does not embed any PLL and has a stiff voltage controller accompanied by a first-order synchronisation loop. PSC and dPLL are experiencing frequency drift during the fault; due to the instability which PLL experiences during severe faults [23,24]. Despite the different response of PSC and PSC2 during fault, their post fault transient is identical since in both cases the synchronisation with the main ac grid is lost during fault and the controller needs to re-gain its synchronisation with the grid.…”