In power systems, voltage source converters (VSCs) are gradually replacing traditional devices and dramatically changing power system dynamic behavior, as the VSC’s intrinsic high-order nonlinearity and multiple time-scale characteristics. As a simple case of transient stability for power-electronic-based power systems, in this work we study a VSC system with a fifth-order nonlinear model, with the aid of nonlinear dynamics analysis. For the transient behavior, we find that both the amplitude and phase of the terminal voltage of VSC show discontinuity when the fault occurs or is cleared. The behavior of time-scale separation is also generally observable, namely, the fast time-scale dynamics damps and disappears quickly, and the system bulk behavior is dominated by the slow time-scale dynamics. In addition, we find that the system transient stability is uniquely determined by whether the fault-clearing state is within or out of the basin of attraction of the post-fault equilibrium point. Finally, simulations and hardware-in-the-loop experiments verify the analytical results. We expect all these interesting findings could provide an improved physical insight on our understanding of transient stability of power-electronic-based power systems.

The traditional power system dominated by synchronous generators (SGs) is now evolving into a new-generation power system dominated by voltage source converter (VSC)-based renewable energy, causing substantial changes in the dynamical behavior. This paper investigates the small-signal synchronous stability of a new-generation power system composed solely of grid-following and/or grid-forming power electronics devices in the absence of an infinitely strong bus. The phase-lock-loop (PLL)-based VSC is taken as a representative of grid-following devices and treated as a controlled-current source whose phase is driven by the power factor angle. In sharp contrast, the virtual synchronous generator (VSG)-based VSC is taken as a representative of grid-forming devices and treated as a controlled-voltage source whose phase is driven by the active power. Small-signal synchronization stability models are established for multiconverter power systems within the framework of the classical Phillips–Heffron model for traditional power systems. Explicit expressions for equivalent inertia, damping, and synchronization coefficients are obtained. We find that both PLL-based and VSG-based VSCs show similar synchronization principles and contribute to the inertia and damping of the system. All these findings are well supported and verified by our modal analysis and time-domain simulations.

Recently power electronics devices have been widely used in power system, and the dynamical behavior of power system has been changing with their increasing penetration. Now we are confronting some emergent serious risks of oscillation instability, which are quite different with those in the traditional power system dynamics. Some small-signal stability methods have been developed, including impedance analysis and amplitude-phase analysis, and it is highly desirable to study their relations. The impedance model and amplitude-phase model are based on the same primitive linearized system, but from the perspective of different input-output relations: voltage versus current for the impedance model, and (imbalanced) active and reactive powers versus voltage phase and amplitude for the amplitude-phase model. In this paper, we deduce, in a rigorous mathematical manner, that the amplitude-phase model and the impedance model are equivalent, in terms of the stability of the whole close-loop system. Furthermore, a voltage source converter model within the DC-link voltage control timescale (around the frequency range of 1Hz to 10Hz) is studied to numerically verify the validity. Clearly this paper establishes a solid bridge between the two methods, which is expected helpful for researchers to understand complicated dynamical behavior of power-electronics-based power system.

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.

The emergence of various power electronic devices, including voltage source converters (VSCs), has greatly changed the dynamics of power systems. It is a great challenging to put forward new theories for modeling, analysis, and understanding of power-electronic-based power systems. In this paper, we first establish a detailed nonlinear model of multiple VSC systems, and then, by using the singular perturbation technique, obtain a reduced-order nonlinear model within the DC-link timescale. The latter model consists of differential algebraic equations (DAEs) and keeps slow dynamics in transient processes, showing a balance between computational accuracy and system complexity. Furthermore, the model is compared with the traditional electromechanical dynamical model of power systems, which are dominated by synchronous generators (SGs). Our model of multiconverter systems can be put into a similar diagram to the traditional power system dynamics, with which most of power electrical engineers are familiar. Finally, wide simulation results verify the efficiency of the proposed model.