The large-scale integration of power electronicbased systems poses new challenges to the stability and power quality of modern power grids. The wide timescale and frequency-coupling dynamics of electronic power converters tend to bring in harmonic instability in the form of resonances or abnormal harmonics in a wide frequency range. This paper provides a systematic analysis of harmonic stability in the future power-electronic-based power systems. The basic concept and phenomena of harmonic stability are elaborated first. It is pointed out that the harmonic stability is a breed of small-signal stability problems, featuring the waveform distortions at the frequencies above and below the fundamental frequency of the system. The linearized models of converters and system analysis methods are then discussed. It reveals that the linearized models of ac-dc converters can be generalized to the harmonic transfer function, which is mathematically derived from linear time-periodic system theory. Lastly, future challenges on the system modeling and analysis of harmonic stability in large-scale power electronic based power grids are summarized.

This paper proposes a unified impedance model of grid-connected voltage-source converters for analyzing dynamic influences of the phase-locked loop (PLL) and current control. The mathematical relations between the impedance models in the different domains are first explicitly revealed by means of complex transfer functions and complex space vectors. A stationary (αβ-) frame impedance model is then proposed, which not only predicts the stability impact of the PLL, but also reveals its frequency coupling effect. Furthermore, the impedance shaping effects of the PLL on the current control in the rotating (dq-) frame and the stationary (αβ-) frame are structurally comapred. The frequency-domain case studies on a three-phase grid-connected converter are next presented, and subsequently validated in timedomain simulations and experimental tests. The close correlations between the measured results and theoretical analysis confirm the effectiveness of the stationary-frame impedance model. Index Terms-Grid, impedance model, phase-locked loop (PLL), stability, voltage-source converters (VSCs). 0885-8993

This paper addresses the harmonic stability caused by the interactions among the wideband control of power converters and passive components in an ac power-electronics-based power system. The impedance-based analytical approach is employed and expanded to a meshed and balanced three-phase network which is dominated by multiple current-and voltage-controlled inverters with LCL-and LC-filters. A method of deriving the impedance ratios for the different inverters is proposed by means of the nodal admittance matrix. Thus, the contribution of each inverter to the harmonic stability of the power system can be readily predicted through Nyquist diagrams. Time-domain simulations and experimental tests on a three-inverter-based power system are presented. The results validate the effectiveness of the theoretical approach.

The interconnection stability of a grid-connected voltage-source converter (VSC) can be assessed by the passivity properties of the VSC input admittance. If critical grid resonances fall within regions where the input admittance acts passively, i.e., has nonnegative real part, then their destabilization is generally prevented. This paper presents an overview of passivity-based stability assessment, including techniques for space-vector modeling of VSCs whereby expressions for the input admittance can be derived. Design recommendations for minimizing the negative-real-part region are given as well.

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As the proportion of converter-interfaced renewable energy resources in the power system is increasing, the strength of the power grid at the connection point of wind turbine generators (WTGs) is gradually weakening. Existing research has shown that when connected with the weak grid, the dynamic characteristics of the traditional grid-following controlled converters will deteriorate, and unstable phenomena such as oscillation are prone to arise. Due to the limitations of linear analysis that can not sufficiently capture the stability phenomena, transient stability must also be investigated. So far, standalone timedomain simulations or analytical Lyapunov stability criteria have been used to investigate transient stability. However, time-domain simulations have proven to be computationally too heavy, while analytical methods are more complex to formulate, require many assumptions, and are conservative. This paper demonstrates an innovative approach to estimating the system boundaries via hybrid -linearised Lyapunov function-based approach and the time-reversal technique. The proposed methodology enables compensation for both time-consuming simulations and the conservative nature of Lyapunov functions. This work brings out the clear distinction between the system boundaries with different post-fault active current ramp rate controls. At the same time providing a new perspective on critical clearing times for wind turbine systems. Finally, the stability boundary is verified using

This paper presents an overview of the synchronization stability of converter-based resources under a wide range of grid conditions. The general grid-synchronization principles for grid-following and grid-forming modes are reviewed first. Then, the small-signal and transient stability of these two operating modes are discussed, and the design-oriented analyses are performed to illustrate the control impact. Lastly, perspectives on the prospects and challenges are shared.

Distributed power-generation systems (DPGS) contribute significantly to the power generation in modern power systems. Wind and solar photovoltaics (PVs), as representative renewable energy sources, are two major resources for DPGSs. However, owing to their inherent characteristics, the large-scale adoption of DPGSs poses challenges. To resolve these issues and leverage renewable-energy DPGSs, this paper presents DPGS technologies based on wind and solar PVs, as well as their impacts on the distributed grid. Moreover, schemes for enhancing the integration and connection of DPGSs are introduced, and protection issues are discussed in order to increase the robustness of the connection.ABSTRACT | Continuously expanding deployments of distributed power-generation systems (DPGSs) are transforming the conventional centralized power grid into a mixed distributed electrical network. The modern power grid requires flexible energy utilization but presents challenges in the case of a high penetration degree of renewable energy, among which wind and solar photovoltaics are typical sources. The integration level of the DPGS into the grid plays a critical role in developing sustainable and resilient power systems, especially with highly intermittent renewable energy resources. To address the challenging issues and, more importantly, to leverage the energy generation, stringent demands from both utility operators and consumers have been imposed on the DPGS. Furthermore, as the core of energy conversion, numerous power electronic converters employing advanced control techniques have been developed for the DPGS to consolidate the integration. In light of the above, this paper reviews the power-conversion and control technologies used for DPGSs. The impacts of the DPGS on the distributed grid are also examined, and more importantly, strategies for enhancing the connection and protection of the DPGS are discussed.