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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]

The complex interactions between renewables and AC/DC networks have brought new oscillation issues. To effectively monitor and suppress these oscillations, the observability and controllability of the oscillatory modes at different locations of the system must be investigated. To achieve this, the paper first defines the concept of modal observability and controllability using the responses of unit-pulse perturbations. Then an eigenvalue-decomposition based method is proposed to calculate these two indexes with the impedance network model (INM) as the input. The obtained modal observability(controllability) has many important applications and one of them is to facilitate the aggregation of the INM. The paper investigates this application by selecting an optimal aggregation port to ensure that the aggregated impedance can reflect the complete system dynamics. Finally, the proposed method is verified on a simple passive circuit and a practical wind power system that experienced actual subsynchronous resonance incidents. Theoretical results as well as EMT simulations demonstrate the effectiveness of the method. [ABSTRACT FROM AUTHOR]

The characteristics of subsynchronous oscillation (SSO) in series‐compensated wind power systems are significantly affected by the system's operating condition. Besides the variation in the magnitude and frequency of the SSO during an SSO event, the fundamental frequency may also deviate from its nominal value. This study aims at capturing the dynamics of both subsynchronous and fundamental frequency components simultaneously. This work first explores the strengths and weaknesses of various Kalman filtering based frequency tracking algorithms for joint tracking of the fundamental and SSO components. Then, it proposes a novel adaptive extended Kalman filtering (AEKF) algorithm, in which the process noise covariance is updated online by maximising the probability density function of the predicted error. The process noise covariance factor changes to positive non‐zero value whenever the frequency of the fundamental component deviates. Thus, the proposed AEKF extracts the time‐varying subsynchronous component while also tracking the small variations in the fundamental frequency. The tracking performance of the AEKF is validated on computer‐generated test signals as well as on the electromagnetic transient simulation model of an actual wind power system facing SSO. The captured fundamental and subsynchronous dynamics can be used for designing monitoring, protection, and control schemes for the SSO. [ABSTRACT FROM AUTHOR]

The mainstream researches focus on the linearised impedance/admittance model and its related coupling effects or stability problems. However, since various non‐linear components exist in control loops, their non‐linear dynamics shall be described to improve the modelling accuracy. To fill in this gap, this study develops a large‐signal impedance/admittance model (LSIM/LSAM) to describe the dominant non‐linearity of modulation saturation in a grid‐connected voltage‐source converter (VSC). The LSAM represents the non‐linearity with the equivalent saturation gains based on the describing function theory and then incorporates the derived gains into conventional small‐signal AM (SSAM) to obtain a partially non‐linear admittance. The proposed LSAM distinguishes itself from SSAM by reflecting the improvement in the magnitude‐ and phase‐response of admittance during non‐linear oscillations. Case studies on a grid‐connected VSC containing saturation non‐linearity are conducted to validate the model's effectiveness in stability analysis. It is found that the frequency deviation and sustained oscillation caused by non‐linearity, which can hardly be assessed using SSAM, can be well captured by LSAM. The results of parameter analysis verify that the modulation saturation dominates the system non‐linearity. [ABSTRACT FROM AUTHOR]

This paper analytically examines how the condition of weak grid connection affects the torsional sub-synchronous oscillations (SSOs) induced by a grid-connected DFIG in a power system. The examination focuses on the SSO mode of grid-connected DFIG, which is dominated by the phase locked loop (PLL) or DC voltage control. Theoretical analysis is carried out to prove that the condition of weak grid connection amplifies the impact of modal resonance between the DFIG and torsional dynamics of synchronous generators. The amplifying effect of weak grid connection exhibits as the decrease of damping of SSO mode of DFIG and increase of the associated residue when the grid connection weakens. Consequently, the DFIG is more likely to induce torsional SSOs under the condition of weak grid connection, leading to system instability in the worst case. Analytical examination presented in the paper reveals the mechanism about how the condition of weak grid connection plays a key role for the DFIG to induce the torsional SSOs. An example multi-machine power system with the grid-connected DFIG is presented. Results of modal analysis and non-linear simulation using detailed model of DFIG validate the analysis and conclusions made in the paper. [ABSTRACT FROM AUTHOR]

Due to the wide frequency range of sub-synchronous oscillations (SSOs) in converter-tied grids, existing SSO parameter estimators generally use a long window to pre-identify the SSO frequency, and thus generally have slow responses in SSO parameter estimation. To deal with this problem, this paper proposes an online SSO stability analysis-based SSO parameter estimator. The operating condition of the grid is updated online by measuring the fundamental voltage/current phasor. Then the impedance of the converter is updated based on the identified operating condition, and the SSO stability is analyzed online. If an unstable SSO mode is detected, the identified SSO frequency is used to build the imaginary exponential function-based signal model, and the SSO parameters are estimated based on the least square method. Case studies show that the proposed method has a much faster response than the methods recently published in the literature, and can obtain accurate SSO parameter estimates. • A new SSO estimator is shown • SSO analysis results are used • It is fast and accurate [ABSTRACT FROM AUTHOR]

With the increasing penetration of renewables, the modern power system is facing emerging stability issues. A prime example of such issues is the subsynchronous oscillation (SSO) phenomenon observed in large‐scale wind farms. It has been studied that hydrogen production equipment (HPE) has the potential to smooth the fluctuation in generated power. However, it is still unclear whether the HPE can be used to mitigate dynamic stability problems induced by wind generators. This study is aimed at filling the gap by exploring the capability of HPE to damp the emerging SSO. Firstly, the overall configuration of an MW‐scale HPE is presented. Then its electromagnetic transient model is established for system dynamics studies. Next, a supplementary subsynchronous damping control (SSDC) of HPE is proposed to modulate its power absorption at the subsynchronous frequency, which can improve system damping and stabilise SSO. Finally, the effectiveness of the proposed SSO mitigation strategy is verified on the simulation model of a practical wind power system that suffered actual SSO incidents. The results demonstrated that the HPE with properly designed SSDC, along with suitable capacity and location, can efficiently mitigate the SSO, thus offers a new option to improve the dynamic stability of renewable power systems. [ABSTRACT FROM AUTHOR]

Owing to the fast control response and distinct characteristics of the voltage source converter (VSC), various types of oscillation incidents related to the VSC-HVDC systems have been observed around the world. However, the causes of such oscillations are complex due to the interactions among the components of the transmission system and the various control methods of the converter station, and yet not systematically comprehended and summarised. Therefore, this study reviews several typical VSC-HVDC oscillation incidents and focuses on the root causes of the oscillation phenomenon. The analysis indicates that the oscillations in the VSC-HVDC system can occur either in subsynchronous frequency or in high-frequency range, up to 1.2 kHz. The modelling methods, including the conventional detailed model, equivalent detailed model, and average equivalent model, are also affecting the accuracy of describing the oscillations. The control methods, including adding active control scheme, passive filter at oscillation frequency, and temporary power plant disconnections, are also discussed. The advantages and drawbacks of these modelling methods, analysis methods, and control methods are summarised based on the various system conditions. In the end, the future research perspectives and key challenges are provided for further research references. [ABSTRACT FROM AUTHOR]