Grid-Synchronization Stability Improvement of Large Scale Wind Farm During Severe Grid Fault

Abstract

Abstract-Loss of synchronization between wind farm and power grid during severe grid faults would cause wind farm tripping. In this paper, the mechanism of grid-synchronization is uncovered, described as motion of an autonomous nonlinear differential equation with specific initial states. The revealed mechanism indicates that even though steady state working point exists, improper initial states and poor system dynamic properties could lead to synchronization instability. In order to keep wind farm synchronous with the power grid during severe grid faults, special requirements on system dynamic properties are stated. Moreover, to satisfy all the requirements, a current injecting method is proposed. By adjusting active and reactive output currents of the wind farm, the proposed method could ensure system synchronization stability during severe grid faults. Implementation of the proposed method on PMSG and DFIG based wind farm is illustrated. Simulation results validate the analysis and the control method.

An Overview of Assessment Methods for Synchronization Stability of Grid-Connected Converters Under Severe Symmetrical Grid Faults

“…Compared to the just described steady-state approach, this method, as developed in [25], [27], [49], has the advantage of including the effect that the PLL has on the injected current, which is coupled to the PCC voltage. The inner current loop can be assumed to behave as a controlled current source provided the bandwidth, in general, is much higher than that of the PLL.…”

“…Nevertheless, some work have addressed PLL instability during low-voltage situations using different approaches of modeling and analysis methods [21], [24]- [30]. Alongside modeling, several studies propose controller mitigation techniques to avoid LOS during low and zero-voltage grid events including freezing/blocking the PLL [31], [32], zero or limited current injection [33], voltage-dependent active current injection [24], current injection based on X/R characteristics of the network impedance [27], and active current injection based on the PLL frequency error [21], [30].…”

“…In [21], [24], the current injection limit resulting in LOS is derived for steady-state network conditions. Including the PLL dynamics, a quasi-static large-signal model is developed in [25]- [27] which identifies a destabilizing positive feedback term to the PLL model as a result of the coupling between the injected current and grid voltage. Another assessment tool is developed in [28]- [30] where the Equal Area Criterion (EAC) normally used for rotor-angle transient stability assessment of synchronous machines is utilized to analyze the LOS mechanism.…”

A novel VSG model suitable for the analyses of static/transient stability and rated power/energy capacity of energy storage system

“…θ m is the phase angle of the VSG and calculated by integrating ω m . ω g is obtained by the phase locked loop (PLL) [13]. The difference between P m and P out is absorbed by the ESS.…”

Impedance-Based Analysis of Interconnected Power Electronics Systems: Impedance Network Modeling and Comparative Studies of Stability Criteria

“…Recently, extensive efforts have been dedicated to the PLL in particular in the context of weak AC grids. Its effects on either VSC's small signal [23] or large signal [24] stability is discussed in depth. Among them, the most evident effect of PLL in view of impedances is the dq asymmetry property [25].…”