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.

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.

Phase-locked loop (PLL) synchronization instability of grid-connected converters under grid faults is a serious concern, in particular for multiconverter plants/stations connected to a weak grid. The multiconverter interaction can lead a large number of converters to lose stability successively. The PLL synchronization instability of a single-converter system has been well studied previously, but it remains unclear how the multiconverter interaction affects the synchronization stability. This article is aimed to fill this gap, in which multiconverter modeling and interaction analysis, stability level assessment, and stability improvement are addressed. It is found that the interaction is detrimental to the near-end converters, but it is beneficial to the far-end ones. From the most prone-to-instability converter, a simple method is proposed to efficiently assess the synchronization stability level of the entire system. In the aspect of improving synchronization stability, the existing methods are evaluated comparatively, and a simple feedforward-compensated PLL is proposed to address the disadvantage of the existing methods. Among them, recommendations are provided by considering multiple aspects of performance and different application requirements. The performance of the methods is verified by hardware-in-the-loop experiments. The results of this study provide a simple engineering method for grid-connected multiconverter synchronization stability assessment and practical guidelines for the selection of stability improvement methods.

This paper investigates the transient stability of power systems co-dominated by different types of grid-forming (GFM) devices. Synchronous generators (SGs and VSGs) and droop-controlled inverters are typical GFM devices in modern power systems. SGs/VSGs are able to provide inertia while droop-controlled inverters are generally inertialess. The transient stability of power systems dominated by homogeneous GFM devices has been extensively studied. Regarding the hybrid system jointly dominated by heterogeneous GFM devices, the transient stability is rarely reported. This paper aims to fill this gap. It is found that the synchronization behavior of the hybrid system can be described by a second-order motion equation, resembling the swing equation of SGs. Moreover, two significant differences from conventional power systems are discovered. The first is that the droop control dramatically enhances the damping effect, greatly affecting the transient stability region. The second is that the frequency state variable exhibits a jump at the moment of fault disturbances, thus impacting the post-fault initial-state location and stability assessment. The underlying mechanism behind the two new characteristics is clarified and the impact on the transient stability performance is analyzed and verified. The findings provide new insights into transient stability of power systems hosting heterogeneous devices.

This study examines the transient stability of low-inertia power systems with inverter-based generation (IBG) and proposes a sufficient stability criterion. In low-inertia grids, transient interactions are induced between the electromagnetic dynamics of the IBG and the electromechanical dynamics of the synchronous generator (SG) under a fault. For this, a hybrid IBG-SG system is established and a delta-power-frequency model is developed. Based on this model, new mechanisms of transient instability different from those of conventional power systems from the energy perspective are discovered. First, two loss-of-synchronization (LOS) types are identified based on the relative power imbalance owing to the mismatch between the inertia of the IBG and SG under a fault. Second, the relative angle and frequency will jump at the moment of a fault, thus affecting the system energy. Third, the cosine damping coefficient induces a positive energy dissipation, thereby contributing to the system stability. A unified criterion for identifying the two LOS types is proposed using the energy function method. This criterion is proved to be a sufficient stability condition for addressing the effects of the jumps and cosine damping coefficient on the system stability. The new mechanisms and effectiveness of the criterion are verified based on simulation results.

Despite many intensive researches regarding frequency-locked loops (FLLs) in recent years, the basic problems of principles, models, and structures of FLL algorithms remained open. Conventional FLL comprises a notch filter and a frequency adaptive loop. The notch filter can be categorized into two types: real coefficient filter (RCF) and complex coefficient filter (CCF). However, the principles, models, and structures of RCF and CCF are generally explained by various theories, and thus, a unified understanding and further developments of FLLs are restricted. This paper proposes a generalized design framework of FLLs based on the Popov adaptive theory. Grounded on the proposed framework, three main contributions can be achieved: 1) the mathematical equivalence of RCF-based FLLs and CCF-based FLLs are revealed; 2) not only the wellknown FLLs (e.g., the DSOGI-FLL and the AVF-FLL) can be derived, but also the other FLLs with novel features can be generated, and thus, the existing case designs of FLLs are developed into a family of designs; 3) the performance differences in terms of stability and convergence speed of the FLLs are quantitatively analyzed, and thereby an enhanced FLL (referred to as EFLL) is recommended. The designs and analyzes have been verified by both simulations and experiments.