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Recently, a high-frequency resonance (HFR) incident has been captured in an actual HVDC project of China Southern Power Grid, which is regarded as a new stability issue caused by the interaction between the modular multilevel converter (MMC) and the weak ac grid. To investigate this event, this paper proposes a stability analysis method based on the frequency-coupling impedance model (FCIM). First, the FCIMs of the MMC and the ac grid are established in abc-frame individually with the frequency coupling effect of the MMC taken into account. Next, they are aggregated into a two-by-two impedance matrix by numerical calculation. Then, based on the aggregated FCIM, a criterion is developed to locate the HFR modes and to evaluate their stability. Finally, the results obtained from FCIM-based analysis are verified by electromagnetic transient (EMT) simulations as well as the field measurements. [ABSTRACT FROM AUTHOR]

With the increasing integration of power electronic converters into the power system, the interactions between converters and their adjacent transmissions bring emerging oscillation issues. A new type of sub- and super-synchronous interactions (S $^2$ SI) between STATCOMs and the weak AC/DC grid were detected in China Southern Grid. As an oscillation incident never reported before, as far as the authors know, its characteristics were quite different from the previous ones. Field measurements as well as simulation studies have revealed that the various factors contributing to the S $^2$ SI include the number of online STATCOMs, the strength of the AC system (in terms of effective short-circuit ratio), the status of series compensation, and the loading level of the HVDC. The mechanism of such oscillation has been clarified and it is caused by the S $^2$ SI between STATCOMs and weak AC grids, where sub- and super-synchronous components are involved in a coupled way. To fully represent such a coupling effect, the concept of sub- and super-synchronous coupled impedance model (IM), or S $^2$ SC-IM, is proposed and developed in order to establish a stability criterion for the S $^2$ SI. Based on the IM-based method as well as the time-domain simulations, the impacts on S $^2$ SI from the various factors are quantitatively analyzed. The results show good consistence among theoretical analysis, electromagnetic simulations, and field measurements. Therefore, the effectiveness of the developed models and the stability criterion has been well verified. [ABSTRACT FROM AUTHOR]

Recent years have witnessed emerging oscillatory stability issues in power systems with high-penetration renewables. These issues are generally caused by the dynamic interaction between the AC/DC grid and power electronic converters used for the integration of renewables. Some previous studies have explored the issues with simplified system models containing only a few converters and idealized networks. However, the interactions between many converters and traditional generators/HVDCs have not been fully considered. To fill this gap, this paper first presents the concept of a unified dq frame impedance network model (INM), with which different converters as well as traditional generators/HVDCs can be incorporated to form an integrated s-domain model of a practical system. As the INM is aggregated into a lumped impedance matrix, a new criterion is then proposed to quantify the oscillatory stability, just by analyzing the frequency characteristics of the determinant of the matrix. Finally, the modeling method and the stability criterion are applied to a real-world system with a very high share of renewables and a realistic risk of oscillatory instability. Their effectiveness has been validated by both field measurements and electromagnetic transient simulations. [ABSTRACT FROM PUBLISHER]

Recently, an emerging subsynchronous resonance (SSR) issue was observed in series-compensated systems comprising multiple wind farms. However, only simplified single-machine-infinite-bus system models were adopted to investigate the characteristics of this issue in the previous work. As a result, it is unable to reflect the impact on SSR from some critical factors, such as the network topology, the spatial distribution of wind farms, the diversity of wind turbine generators, and the distributed wind speed. To fill in this gap, this paper proposes an impedance network model (INM) based SSR stability analysis method. The impedance models of each wind farm and transmission line are established, and then interconnected according to system topology to form the whole INM. Such INM is further aggregated into lumped impedance. By analyzing the impedance-frequency features of the latter, a new stability criterion is developed to quantify the stability of SSR. The application of the criterion is demonstrated on a practical power system containing multiple wind farms. Both field measurements and time-domain simulations have been presented to validate its effectiveness and accuracy. The proposed method is of great potential in analyzing the SSR issue for very large-scale wind power systems. [ABSTRACT FROM PUBLISHER]

Supplementary excitation damping controller (SEDC) has been widely acknowledged as one of the effective measures to stabilize subsynchronous resonance (SSR). However, how to quantify its stabilizing ability is still an open issue raising widespread concerns. There are few in-depth studies or conclusions in this aspect yet. This paper presents a quantitative measure of SEDC's ability to damp SSR, namely stabilizing ability index (SAI). It is defined as the percentage margin of the effective gain relative to the critical stability gain (CSG). The CSG is the minimum gain of SEDC to guarantee SSR stability, while the effective gain is the control gain of SEDC actually functioning following system disturbances. A practical system is used to investigate the various factors determining the effective gain, the CSG and thus the SAI, based on eigenvalue analysis as well as time-domain simulation. The dominant factors identified include: the total equivalent series-compensation degree of system, the parameters and operating status of the generator, the response time and ceiling voltage of the exciter, and the severity of the disturbance. Why and how these factors affect the control capability of SEDC is examined and thus a practical method is offered for the assessment of SEDC's ability to stabilize SSR. [ABSTRACT FROM AUTHOR]

Saturation nonlinearities caused by control limiters will be activated during large disturbances, complicating the stability of grid-tied voltage source converters (VSCs). This letter investigates the large-disturbance synchronization stability of the grid-tied VSC considering two typical control limiters, i.e., the hard limiter of the reference current and the circular limiter used to avoid overmodulation. A unified differential equation (UDE) model is proposed to quantify the transient responses and stability boundaries of the system under different saturation conditions. It is revealed that the VSC behaves as a current source when the hard limiter is activated; while the interaction between the two limiters drives the VSC to work as a voltage source, making it immune to synchronization instability. [ABSTRACT FROM AUTHOR]

Most of the existing stability analysis methods are capable of determining the oscillatory stability of black-box converter interfaced generators (CIGs) under given operating conditions only but can hardly identify the critical operating conditions. In this letter, an oscillatory stability region (OSR) is defined with the short-circuit ratio and output power as variables, and a method to obtain the OSR for the black-box CIG is proposed. First, the CIG is modeled as a variable-operating-point impedance model (VOPIM) through perturbation tests. Then, the critical operating points, for which the CIG system has a zero-damping oscillation mode, are determined by VOPIM-based oscillatory stability analysis so as to obtain the OSR. Finally, the accuracy of the OSR is verified through control hardware-in-the-loop tests. The OSR is of significant meaning for the control strategy and operation plan optimization of the CIGs. [ABSTRACT FROM AUTHOR]

The interactions between permanent magnet synchronous generators (PMSGs) and weak grids may cause negatively damped or growing subsynchronous oscillation (SSO). The growing-SSO would evolve into a sustained-SSO when it is predominantly governed by the nonlinearities in the PMSG's converter control, such as current limiter. Most of the existing frequency-domain modeling and stability analysis methods are in the small-signal sense and thus unable to consider the impact of such nonlinearities. Besides, these methods can hardly give vital information to characterize the sustained-SSO, for instance, the magnitude. This paper presents a detailed characteristic analysis of the sustained-SSO while accounting for the effect of current limiter nonlinearity. First, a single-input-single-output (SISO) frequency-domain model of the PMSG is developed. This SISO model comprises the transfer functions of outer-/inner-loop controls and phase-locked loop (PLL). The nonlinear part, i.e., current limiter, is incorporated into the model using the describing function (DF) method. Then, the equivalent transformation on the system's closed-loop characteristic equation is used to separate the SISO model into independent linear and nonlinear parts. This separation enables the use of the DF-based Nyquist criterion for evaluating the system stability and determining oscillation attributes, such as the magnitude and frequency. Both the analytic results and time-domain simulations reveal that the current limiter has a significant impact on the characteristics of the sustained-SSO. [ABSTRACT FROM AUTHOR]