Due to various controls in voltage-source converters, their impacts and mutual influences on transient stability of new-generation power systems are still obscure. Different from the mature different-order transient equations for synchronous generator (SG), fully accepted transient equations of converters still lack. In this article, a novel methodology is proposed to uniformly evaluate the controller effects on the transient stability and then establish the hierarchical transient equations of converters based on the properties of controlling-unstable-equilibrium point (CUEP). The participation factors on unstable eigenvalue of CUEP are applied to measure the contribution of state variables in transient dynamics, and dominant variables and key loops of grid-following and grid-forming converters are determined and compared. It is found that the synchronous loop, e.g., phase-locked loop in grid-following converters and virtual-synchronous loop in grid-forming converters, plays a primary role, and the power balance loop on the DC capacitor, including DC-voltage control and DC capacitor dynamics, plays a secondary role. Additionally, the impacts of synchronization and power balance are compared with the rotor swing of the SG. Analytical results are demonstrated by time-domain simulations and transient stability assessments. The dominant transient models and the CUEP-based participation factor analysis provide an improved physical insight on our understanding of transient dynamics in the new-generation power systems dominated by not only grid-following but also grid-forming converters.

Synchronization is one of the key issues in three-phase AC power systems. Its characteristics have been dramatically changed with the large-scale integration of power-electronic-based renewable energy, mainly including a permanent magnetic synchronous generator (PMSG) and a double-fed induction generator (DFIG) for wind energy and a photovoltaic (PV) generator for solar energy. In this paper, we review recent progresses on the synchronization stability and multi-timescale properties of the renewable-dominated power system (RDPS), from nodes and network perspectives. All PMSG, DFIG, and PV are studied. In the traditional synchronous generator (SG) dominated power system, its dynamics can be described by the differential–algebraic equations (DAEs), where the dynamic apparatuses are modeled by differential equations and the stationary networks are described by algebraic equations. Unlike the single electromechanical timescale and DAE description for the SG-dominated power system, the RDPS dynamics should be described by the multiscale dynamics of both nodes and networks. For three different timescales, including the AC current control, DC voltage control, and rotor electromechanical timescales, their corresponding models are well established. In addition, for the multiscale network dynamics, the dynamical network within the AC current control timescale, which should be described by differential equations, can also be simplified as algebraic equations. Thus, the RDPS dynamics can be put into a similar DAE diagram for each timescale to the traditional power system dynamics, with which most of power electrical engineers are familiar. It is also found that the phase-locked loop for synchronization plays a crucial role in the whole system dynamics. The differences in the synchronization and multiscale characteristics between the traditional power system and the RDPS are well uncovered and summarized. Therefore, the merit of this paper is to establish a basic physical picture for the stability mechanism in the RDPS, which still lacks systematic studies and is controversial in the field of electrical power engineering.

Synchronization stability is one of central problems in power systems, and it is becoming much more complicated with the high penetration of renewable energy and power electronics devices. In this paper, we review recent work by several nonlinear models for renewable-dominated power systems in terms of multiple timescales, in particular, grid-tied converters within the DC voltage timescale. For the simplest model, a second-order differential equations called the generalized swing equation by considering only the phase-locked loop (PLL) is obtained, which shows a similar form with the well-known swing equation for a synchronous generator in the traditional power systems. With more outer controllers included, fourth-order and fifth-order models can be obtained. The fourth-order model is called the extended generalized swing equation, exhibiting the combined function of grid synchronization and active power balance on the DC capacitor. In addition, a nonlinear model for a two coupled converter system is given. Based on these studies, we find that the PLL plays a key role in synchronization stability. In summary, the value of this paper is to clarify the key concept of the synchronization stability in renewable-dominated power systems based on different nonlinear models, which still lacks systematic studies and is controversial in the field of electrical power engineering. Meanwhile, it clearly uncovers that the synchronization stability of converters has its root in the phase synchronization concept in nonlinear sciences.

Synchronous stability in power systems is of essential importance for system safety and operation. For the phase-locked loop (PLL)-based synchronous stability in power electronic-based power systems, which has recently stimulated interest in researchers in the field of electrical power engineering, but is still controversial, this paper divides the topic into two aspects, including the PLL device stability and the system stability. It is found that the PLL device is always stable and the error between the PLL output angle ${\theta }_{pll}$ and the terminal voltage angle ${\theta }_t$ is always finite. Therefore, the synchronization of power electronic-based power systems should be understood as the output synchronization between the electrical rotation vectors (${\theta }_t$ or ${\theta }_{pll}$) from each item of grid-tied equipment, rather than the synchronization of the PLL device itself. In addition, it is found that ${\theta }_{pll}$ plays an active role in the system synchronization dynamics not only in electromagnetic timescales but also electromechanical timescales and it could be selected as a dominant observable. In this paper, the concept of synchronous stability is well clarified. These findings are well supported by theoretical analyses and MATLAB/Simulink simulations, and thus could provide insights on the synchronous stability mechanism.

Transient switching control (TSC) of renewable equipment under severe faults makes transient characteristics of new-type power systems much more complicated. Although most of recent studies focused on the during-fault stage under low voltage ride through of grid-connected converters or detailed electromagnetic transient simulations, a unified understanding of entire transient process of permanent magnet synchronous generator (PMSG) grid-connected system is still lacking. Therefore, based on the TSC schemes of PMSG, this article considers the switching dynamical effects in detail, divides the transient process into four distinctive stages: prefault, during-fault, early postfault, and late postfault, and establishes the corresponding mechanism models with the associated machine–network interface relations for each stage. Furthermore, the influence of major parameters on the transient stability is studied, including the infinite-bus voltage dip depth and grid-side converter active current on the during-fault stage, and the active current climbing rate on the early postfault stage. It is also found that the large-disturbance stability on the sole during-fault stage is only a sufficient condition, but not a necessary condition, and actually the system after all four sequential stages can be stable under wider parameter regions. These findings are supported by wide numerical and experimental verification, and could provide a theoretical basis for transient synchronous stability analysis of PMSG grid-connected system.

Synchronous stability is crucial for the safety and operation of AC power systems. However, most of the current researches focused on the stability of grid‐connected converters, and that of renewable equipment still lacked. In this article, the impact of the additional inertia control (AIC) on the permanent magnet synchronous generator (PMSG) is studied. It is found that with the AIC, the machine‐side converter dynamics of the PMSG cannot be ignored, and the system dominant dynamics shifts from the electromagnetic to electromechanical timescales. This article develops a simplified model for the single‐PMSG infinite‐bus system with the AIC within the electromechanical timescale, and reveals the transient synchronization stability mechanism from three aspects: the machine‐network interface, transient dominant variable, and interaction between the synchronization loop and the power imbalance loop. Finally, this article analyzes the swing characteristics of the PMSG system, and uncovers the relationship between the energy transmission and synchronization. These findings are supported by wide experimental verification and can provide the deeper physical insight and theoretical basis for the transient synchronous stability analysis of renewable‐dominated new‐type power systems.