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.

Grid codes require wind farm to remain on-grid and inject specific reactive current when grid fault occurs. To satisfy the requirements, reactive power devices, such as the static synchronous compensator (STATCOM), are usually used in modern wind farms. In order to produce reactive currents, the wind energy generation system (WECS) and the STATCOM are normally controlled with the phase-locked loop (PLL) oriented vector control methods. Due to the active power imbalance between the generation and consumption, the wind farm has the risk of losing synchronization with the grid under severe fault conditions. This paper analyzes the dynamic synchronization mechanism and stability criteria of the wind farm and proposes a coordinated current control scheme for the WECS and the STATCOM during severe grid-fault period. The synchronization stability of both the WECS and the STATCOM is remained by the active power balancing control of the wind farm. The control objectives of the generator- and grid-side converters for the WECS are swapped to avoid the interaction between the dc-link voltage control loop and the synchronization loop. The synchronized STATCOM produces additional reactive currents to help the wind farm meet the requirements of the grid code. Effectiveness of the theoretical analyses and the proposed control method is verified by simulations.

Grid-connected renewable energy conversion systems (RECSs) are usually required by grid codes to possess the low voltage ride through (LVRT) and reactive power support capabilities so as to cope with grid voltage sags. During LVRT, RECS's terminal voltage becomes sensitive and changeable with its output current, which brings a great challenge for the RECS to resynchronize with the grid by means of phase-locked loops (PLLs). This paper indicates that loss of synchronism (LOS) of PLLs is responsible for the transient instability of grid-connected RECSs during LVRT, and the LOS is essentially due to the transient interaction between the PLL and the weak terminal voltage. For achieving a quantitative analysis, an equivalent swing equation model is developed to describe the transient interaction. Based on the model, the transient instability mechanism of RECSs during LVRT is clarified. Furthermore, a transient stability enhancement method is proposed to avoid the possibility of transient instability. Simulations performed on the New England 39-bus test system verify the effectiveness of the method.

Power electronic-interfaced renewable energy sources (RES) exhibit lower inertia compared to traditional synchronous generators. The large-scale integration of RES has led to a significant reduction in system inertia, posing significant challenges for maintaining frequency stability in future power systems. This issue has garnered considerable attention in recent years. However, the existing research has not yet achieved a comprehensive understanding of system inertia and frequency stability in the context of low-inertia systems. To this end, this paper provides a comprehensive review of the definition, modeling, analysis, evaluation, and control for frequency stability. It commences with an exploration of inertia and frequency characteristics in low-inertia systems, followed by a novel definition of frequency stability. A summary of frequency stability modeling, analysis, and evaluation methods is then provided, along with their respective applicability in various scenarios. Additionally, the two critical factors of frequency control—energy sources at the system level and control strategies at the device level—are examined. Finally, an outlook on future research in low-inertia power systems is discussed.

Grid codes stipulate that grid-connected voltage source converter (VSC) interfaced generation units should possess fault ride-through capability during grid faults. Resynchronization with the post-fault grid is crucial for this purpose. However, it is not always easy for VSCs connected to a high-impedance weak grid to achieve the resynchronization during severe grid faults. This paper studies loss of synchronism (LOS) of VSCs during grid faults. A nonlinear model describing the dynamics of phaselocked loop (PLL) is developed and a modified equal area criterion is utilized to identify the crucial factor affecting the resynchronization. The findings show that PLL’s integral regulator is unfavorable for the resynchronization, since it probably causes the system operating point to enter negative damping zones and even reverse regulation zones, consequently resulting in LOS. A variable structure PLL method with great simplicity is proposed to improve the resynchronization capability. The method removes PLL’s integral regulator during grid faults to eliminate its unfavorable effect. Experimental and simulation comparisons with existing methods verify the performance of the method.