This paper concerns with the emerging power system stability issues, classification, and research prospects under a high share of renewables and power electronics. The decades-old traditional power system is undergoing a fast transition with two most prominent features: 1) high-penetration of renewable power generators, utilizing intermittent renewable sources such as wind and solar, and 2) high-penetration of power electronic devices in the generation e.g., wind turbine converters and solar power inverters, transmission e.g., flexible ac or dc transmission system converters, and distribution/utilization systems e.g., electric vehicle and microgrid. The development of modern power systems with dual high-penetrations, i.e., high-penetrations of renewables and power electronic devices, influences the power system dynamics significantly and causes new stability issues. This paper first overviews equipment-level features and system-level stability challenges introduced under the dual high-penetration scenario of the modern power system. Next, the impacts of emerging stability challenges on various aspects of the classical stability issues and classifications are highlighted. Under this context, this paper discusses the validity and limitations of the existing classical and extended power system stability classifications proposed by different IEEE/Cigre Working Groups. Furthermore, a new power system stability classification framework is proposed, which not only maintains the inherent logic of the classical classification but also provides wide coverage and future adaptability of the emerging stability issues. Finally, various classification-oriented research prospects in the power system stability domain are highlighted.

Power system stability issues, classifications and research prospects in the context of high-penetration of renewables and power electronics

When investigating voltage and current stresses in critical main circuit components during faults of the converter, a detailed equivalent model capable of representing the balancing control strategies of the capacitor voltages on the submodule level, along with blocking and delocking, is always necessary. Among previously proposed equivalent models of the modular multilevel converter (MMC), only submodule averaged models (SAMs) can capture interested inner dynamics inside each arm. However, the simulation accuracy of SAMs is not always satisfactory, especially when the time step is larger than 10 $\mu$ s. In order to further improve the simulation accuracy with guaranteed simulation efficiency, the shifted frequency modeling of the half- and full-bridge hybrid MMC is proposed in this paper. Therein, each submodule is represented by Thevenin equivalents derived by submodule dynamic phasors. The arm of the MMC is represented by Norton equivalents to guarantee the efficiency, considering both normal and dc-blocking conditions. The effectiveness of the proposed model in terms of accuracy and efficiency is validated by simulating an MMC-based HVdc transmission.

Shifted Frequency Modeling of Hybrid Modular Multilevel Converters for Simulation of MTDC Grid

To accurately capture the dynamics of a large-scale ac/dc system as a whole and the interactions between its individual components, a simulation method with high precision and efficiency is in great demand. For this purpose, we develop a multi-domain co-simulation method, in which the target system is partitioned into multiple dc and ac subsystems, represented by our proposed shifted-frequency phasor (SFP) models and the traditional EMT models, respectively. SFP models can be simulated with a much larger time step, leading to a significant improvement in simulation efficiency under a given requirement of precision. Further, a new interface model, namely, hybrid multi-domain transmission-line model (HMD-TLM), is developed to reflect the interactions between SFP models and EMT models, with the additional benefit of producing instantaneous and phasor waveforms simultaneously. Thus, the multi-domain co-simulation is implemented based on the efficient SFP models, the HMD-TLM, and a designed time sequences of simulation. The performance (efficiency and accuracy) of the proposed method has been validated on the well-known CIGRE ac/dc system as well as a practical system integrating large ac grids and modular-multilevel-converter-based multi-terminal dc grids.

A Multi-Domain Co-Simulation Method for Comprehensive Shifted-Frequency Phasor DC-Grid Models and EMT AC-Grid Models

In the year 2015-2018, there are many sub- and super-synchronous interaction (S $^2$ SI) events, happened in China. However, traditional transient stability models, electro-magnetic transient (EMT) models and hybrid TS and EMT simulation methods fail to capture the desired wide frequency band interactions between large-scale AC grids and wind farms. To accurately and efficiently capture wide frequency band interactions between AC grids and wind farms, a simulation method which can extend the time-step to 500  $\mu$ s and further adopt the multi-rate structure is highly required. For this objective, we propose a multi-rate co-simulation method, in which the target system is partitioned into electro-magnetic transient (EMT) and shifted frequency phasor (SFP) subsystems, represented by our proposed transformation based SFP models and the traditional EMT models, respectively. The simulation efficiency of simulating large-scale AC grids is significantly improved by adopting a much larger time-step. Further, the multi-rate multi-domain transmission-line model (MD-TLM) and the multi-rate frequency dependent MD-TLM are respectively proposed to reflect wide-band interactions between AC grids and wind farms. Eventually, the multi-rate co-simulation is implemented based on the efficient SFP models, network partitioning, and so-called the multi-rate (FD)-MD-TLM. The performance (efficiency and accuracy) of the proposed method has been fully validated on a practical system integrating large AC grids and wind farms.

A Multi-Rate Co-Simulation of Combined Phasor-Domain and Time-Domain Models for Large-Scale Wind Farms

A multi-area Thevenin equivalent based multi-rate co-simulation for control design of practical LCC HVDC system

This paper proposes a novel multirate co-simulation method to improve simulation efficiency of large-scale AC plus multiterminal DC (MTDC) grids. In this method, the whole system is partitioned into different AC and MTDC subsystems. They each are simulated with different time steps according to their requirements of accuracy. Unlike the existing methods where simplified models are adopted, the proposed approach fully reserves both the detailed behavior of converters and the nonlinear dynamics of large-scale AC systems. To realize the coordination between different subsystems, updating and discretization of the interface models are significant to guarantee the overall numerical accuracy and stability of the co-simulation method. Accordingly, the interface models of the partitioned AC and DC networks are represented by time-varying Thevenin and Norton equivalents. To eliminate the aliasing or time-delay errors, their parameters are derived using moving-window prediction, stepwise correction, and averaging techniques. A numerical oscillation suppression algorithm is developed for the discretization of the interface model. Thus, numerical stability is well guaranteed. The variables inside each subsystem as well as the interfaces, which are derived by augmented network equations (ANEs), are calculated simultaneously. The rate ratio of large AC and MTDC systems is determined based on our proposed criterion of electromagnetic transient simulation accuracy. The effectiveness of the cosimulation method is validated on a practical system integrating large AC and modular-multilevel-converter-based MTDC grids.

A Multirate EMT Co-Simulation of Large AC and MMC-Based MTDC Systems

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A Multirate EMT Co-Simulation of Large AC and MMC-Based MTDC Systems