Super-synchronous Oscillation Research Articles

Wideband oscillation analysis of VSC-HVDC connected DFIG wind farm

This paper investigates the wideband oscillations of a doubly fed induction generator (DFIG)-based wind farm connected via a voltage source converter-based HVDC (VSC-HVDC) system across the full frequency band. A small signal model of the system is developed to facilitate this analysis. Oscillation mode characteristics are then examined through eigenvalue analysis and participation factors. The study further explores the impact of strongly correlated factors on typical wideband oscillation modes and their inter-mode interactions. Findings reveal that interactions between the DFIG wind farms and the VSC-HVDC system can induce new super-synchronous oscillations. Additionally, a damping coupling is observed between the medium-frequency oscillation interactive transformation mode and the stator-line mode.

A data-driven method for SSCI identification using synchrophasor measurements

In recent years, subsynchronous control interaction (SSCI) frequently occurred due to the rapid development of inverter based resources. Online monitoring and identifying SSCI are thus of great importance for the safe operation of power systems. This paper proposes a data-driven method for SSCI identification using synchrophasors generated by phasor measurement units (PMUs). The challenges of this work mainly result from two factors: 1) SSCI may simultaneously involve a sub-synchronous oscillation (Sub-SO) mode and a super-synchronous oscillation (Sup-SO) mode. The spectra of these two modes would be aliased due to the limited reporting rate of PMUs. 2) The reporting filter in PMUs significantly weakens the oscillation signal as the frequency of SSCI is generally far away from the nominal frequency. To tackle these challenges, the influence of Sub-SO and Sup-SO on spectra of synchrophasors is analyzed first. It is revealed that the positive and negative spectra are linearly correlated with Sub-SO and Sup-SO modes. Several features are then extracted via the analysis of the spectra which can reflect the SSCI features and a data-driven identification method is further proposed through a fast reduced kernel extreme learning machine (RKELM) model. Compared with the traditional classification techniques, the RKELM algorithm based on kernel function has stronger generalization performance and higher accuracy while ensuring shorter training time. This significantly improves the accuracy and efficiency of SSCI identification, especially under noise conditions. Both simulations and field tests demonstrate the effectiveness and usefulness of the proposed method.

Wind-farm-level sub-/super-synchronous oscillation suppression strategy based on generalized reactance sensitivity

In recent years, several sub-/super-synchronous oscillation accidents caused by the grid integration of the direct-drive permanent magnet synchronous generator (D-PMSG) have occurred, which seriously threatens the safe and stable operation of the power system. The existing research on the mechanism and influencing factors of sub-/super-synchronous Oscillation (SSO) caused by D-PMSG and the unit-level suppression method of SSO have been deeply studied, but there is still a lack of research on the suppression strategy of wind-farm-level SSO. To address the problem, this paper proposes a wind-farm-level SSO suppression strategy based on generalized reactance sensitivity. Firstly, the variation trend of the external reactance of the D-PMSG and the system’s pole with parameter changing is analyzed, which provides support for characterizing the system’s stability only through the external reactance. Inspired by the idea that impedance sensitivity is used to find key parameters, the concept of generalized reactance sensitivity is proposed to find the dominant units that have a great influence on the system’s stability. Based on this, corresponding SSO suppression strategies are proposed respectively with and without dominant units. Finally, the effectiveness of the proposed strategy is verified by simulation.

Research on the Mechanism of Simultaneous Instability of dq Frame Sub- and Super-Synchronous Oscillations in Grid-Tied Converter

Research on the Mechanism of Simultaneous Instability of <i>dq</i> Frame Sub- and Super-Synchronous Oscillations in Grid-Tied Converter

Observer-Based Single-Sensor Control Schemes for LCL-Filtered Grid-Following Inverters

In the single-sensor grid-following control of <i>LCL</i>-type grid-connected inverters, extended-observers, which behave as &#x201C;virtual sensors,&#x201D; are typically employed for active damping, disturbance rejection, current tracking, and grid synchronization. However, most of the existing observer-based single-sensor control schemes choose grid current as the system measurement and output, and the performance of other combinations is still unclear. Hence, in this article, with the help of impedance modeling and Nyquist stability criterion, a systematic comparative assessment for selecting different measurements and controlled outputs is introduced. In this comparison, the sub- and super-synchronous oscillation issues induced by grid faults, lower current reference, high phase-locked loop (PLL) bandwidth and large grid inductance, are investigated. In addition, two easily overlooked interval stability corollaries are proved, facilitating the system stability analysis. Finally, the correctness of the theoretical analysis is verified by a series of steady-state and transient experimental tests.

PLL phase margin design and analysis for mitigating sub/super-synchronous oscillation of grid-connected inverter under weak grid

Under weak grid, the grid-connected inverter can easily cause sub/super-synchronous oscillations, which are determined by the oscillation modes of system. Firstly, based on the eigenvalue analysis, the sub/super-synchronous oscillation modes of the grid-connected inverter are revealed with considering the phase-locked loop (PLL) and control delay. With the decline of grid stiffness, the typical roots of sub-synchronous oscillation (SSO) mode gradually shift from the left-half plane to the right-half plane, which means that the grid-connected inverter will oscillate under weak grid. By analyzing the participation factor of SSO mode, it finds that PLL is the dominant factor. Based on the root locus analysis, the stable regions of PLL parameter are obtained. Then, according to the stable regions of PLL parameter, the phase margin of PLL can be calculated, which can make the grid-connected inverter operate stably. It finds that PLL phase margin should be increased for suppressing this SSO. However, when the PLL phase margin is greater than a certain value, the system will enter a supersynchronous oscillation mode, which can cause a supersynchronous oscillation. Thus, to suppress the sub/super-synchronous oscillations of the grid-connected inverter under weak grid, the phase margin of PLL should neither be too small nor too large. Finally, the proposed suppression strategy of the sub/super-synchronous oscillations for the inverter integrated into a weak grid is validated with the simulation and experimental studies.

Parameter identification of Sub-synchronous/Super-synchronous oscillations based on synchrophasor rotation and spectral shift

Benefiting from the advantage of a unified time scale of measurement data from different sites, the sub-synchronous oscillation (sub-SO) and super-synchronous oscillation (super-SO) detection and mode parameter identification based on a wide-area measurement system can be applied to analyze the development process of oscillation. The phasor measurement units report rate constrains the existing methods, indicating that they can only identify the sub-SO parameters and cannot be extended to super-SO. This study proposes a novel method for sub-SO and super-SO detection and parameter identification based on synchrophasor rotation characteristics and spectral shift. Analyzing the spectral superposition of sub-SO and super-SO components in the real part of the synchrophasor is crucial for realizing sub-SO and super-SO detection. Subsequently, the fundamental, sub-SO, and super-SO components in the synchrophasor are decoupled using the rotational characteristics of the synchrophasor to identify parameters accurately. Compared with the existing methods, the proposed method can realize the expansion of the parameter identification range to super-SO. The effectiveness of the proposed method is verified by simulation and practical engineering cases.

Mitigation of Oscillations in Three Phase LCL-Filtered Grid Converters Based on Proportional Resonance and Improved Model Predictive Control

For three phase <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> -filtered gird converters, this paper designs a robust control strategy to reduce high frequency and subsynchronous or supersynchronous oscillations. Two components, namely the grid side inductor component and the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> filter component, constitute a three phase <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> -filtered grid converter. Model predictive control (MPC) with a disturbance observer is used to control the interconnection voltage of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> filter. Proportional resonance (PR) control regulates the grid side current. It is possible to combine MPC with PR's advantages. The dynamic performance is enhanced by MPC's intrinsic ability to achieve active damping without extra control and reduce modulation latency. In addition to achieving zero steady state error, PR control greatly simplifies the control process when compared to the overall MPC of the entire grid converter. By analyzing the frequency response of the transfer function and output impedance, it is possible to determine that the proposed control has a sufficient phase margin and that, even when the system and control parameters change, the grid converters’ output impedance is always resistive or inductive at the entire frequency, suppressing subsynchronous and high frequency oscillations. To further reduce the oscillations and harmonics, an improved MPC control framework and a feedback compensation mechanism are proposed. The effectiveness and reliability of the proposed control in current tracking, harmonic suppression, and response to grid impedance variations are verified by comparative analysis of simulation results.

An Improved Interpolated DFT-Based Parameter Identification for Sub-/Super-Synchronous Oscillations With Synchrophasors

The sub-synchronous resonance oscillations (SSO) caused by considerable renewable energy generations and power electronic devices seriously affect power system stability. Synchrophasor-based SSO parameter identification can effectively monitor SSOs. Aiming at the fundamental and frequency-coupled sub-/super-synchronous components that appear simultaneously in SSOs, an improved parameter identification algorithm is proposed in this paper based on the original interpolation Discrete Fourier Transform (DFT) and Hann-window. As the complement to the original algorithm, the improvements proposed in this paper additionally focus on the influence between the positive-/negative-frequency parts of the sub-/super-synchronous and fundamental components. Thus, the frequency, amplitude, and phase of the fundamental, sub-synchronous, and super-synchronous components can be accurately obtained by the proposed algorithm. This algorithm is compared to the original interpolated DFT and Prony analysis with both the simulated Phasor Measurement Unit (PMU) data and PMU data recorded during an actual SSO incident to verify its correctness and effectiveness. In the improved algorithm, the additional consideration for the super-synchronous component and the additional identified parameters significantly enhance the practical value of the interpolated DFT algorithm.

A grid-side multi-modal adaptive damping control of super-/sub-synchronous oscillations in type-4 wind farms connected to weak AC grid

• A multi-modal damping control (MADC) is proposed to damp multiple oscillation modes independently. • A general design framework for damping multiple oscillation modes is given. • MADC is validated to damp sub-synchronous and super-synchronous oscillations in Type-4 wind farms. Type-4 wind farms connected to weak AC grids have the risk of sub-synchronous oscillation/interaction (SSO/SSI) with multiple oscillation modes, which can lead to unplanned outages and system destabilization. This paper first presents the magnitude and frequency characteristics of the oscillations followed by the control design requirements. Then, it gives a general design of grid-side multi-modal adaptive damping control (MADC) to stabilize multiple oscillation modes. The MADC utilizes an FFT-based inter-harmonic phasor measurement algorithm, which can detect and track multiple oscillation modes simultaneously. The MADC uses bus voltages and line currents as inputs to extract and control each oscillation mode without considering the coupling of the modes due to the frequency coupling effect. The unstable modes are stabilized by injecting phase-shifted currents with multiple frequencies such that the impedance response of the whole system around the concerned frequencies is reshaped. Notable features of the proposed damping control include automatic and independent detection, tracking and suppression of multiple oscillation modes. The proposed MADC's damping performance is validated through extensive electromagnetic transient (EMT) simulations of type-4 wind farms connected to a weak ac grid causing super-/sub-synchronous oscillations. The system model and the proposed damping control are designed and tested in commercial EMT software.

Parameter Identification With Synchrophasor Trajectory Fitting Technique for Sub-/Super-Synchronous Oscillations

To enhance the real-time monitoring of sub-/super-synchronous oscillation dynamics by utilizing the advantages of wide-area time-synchronization and dynamic measurement of synchrophasor measurement data, the idea of a parameter identification algorithm with synchrophasor trajectory fitting (PTF) is proposed in this letter to identify the parameters of each component of the instantaneous signal during sub-/super-synchronous oscillations. Simulated PMU data are used to demonstrate the proposed idea and verify the performance of PTF compared with the interpolated DFT algorithm. Discussions are also provided for further improvements.

Stability analysis of sub‐/super‐synchronous oscillation based on closed‐loop transfer function poles of D‐PMSG wind power systems

In recent years, the sub‐/super‐synchronous oscillation (SSO) caused by grid integration of direct‐drive permanent magnet synchronous generators (D‐PMSG) has attracted wide attention. However, most of the existing stability analysis methods are either complex or cannot quantitatively analyze the influence of parameter changes on system stability. To address the problems, this paper proposes a novel stability analysis method for SSO in the D‐PMSG integrated power system based on closed‐loop transfer function poles. Firstly, the impedance modeling method was introduced to analyze the stability of the D‐PMSG power system. Then, a stability criterion was proposed based on the poles of the closed‐loop function. The proposed criterion is simple and has a clear physical background. The real part of the poles represents the damping while the imaginary part of the pole reflects the oscillation frequency. Finally, simulation results verify the accuracy and effectiveness of the proposed method. Moreover, its application in influence factors for quantitative analysis was also validated. © 2022 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Impedance Modeling and Stability Analysis of All-DC Delivered Offshore Wind Farm

The all-DC delivered offshore wind farm has great potential in power engineering application due to its advantages in large capacity and long distance delivery. Considering the cost and reliability, the all-DC wind farm can be connected to the onshore grid via the Line-Commuted Converter High Voltage Direct Current (LCC-HVDC) but more interaction may also occur in the form of sub-/super-synchronous oscillations. In order to analyze the stability issue of such offshore wind farm system, the DC-port impedance are firstly established for the aggregated wind farm based on permanent magnet synchronous generator (PMSG) and LCC-inverter by using the harmonic linearization method, where the power outer loop cascaded by the current inner loop of PMSG wind farm and the phase-lock loop (PLL) of the LCC inverter are taken into consideration. Furthermore, the interaction between these two subsystems is analyzed based on the stability criteria through their impedance characteristics. Finally, simulation results are conducted by reproducing the time-domain oscillation in order to evaluate the effectiveness of impedance model for the all-DC delivered offshore wind farm system.

Impedance Modeling and Stability Factor Assessment of Grid-connected Converters Based on Linear Active Disturbance Rejection Control

With the increase of converter-based renewable energy generation connected into the power grid, the interaction between renewable energy and grid impedance has introduced lots of new issues, among which the sub- and super-synchronous oscillation phenomenon makes a big concern. The linear active disturbance rejection control (LADRC) is a potential way to improve the damping characteristics of the grid-connected system, but the key factors and influencing mechanism on system stability are unknown. This paper establishes the equivalent impedance and coupling admittance models of a typical three-phase grid-connected converter. Then, the influence of the key factors such as the bandwidth of the LADRC and grid impedance on the stability and frequency coupling effect is assessed in detail. Finally, the theoretical analysis results are verified by simulations and experiments.

Admittance Modeling and Stability Analysis of Grid-Connected Inverter With LADRC-PLL

It tends to cause system oscillation when the inverter with a phase-locked loop based on proportional integral controller (PI-PLL) is connected to the weak grid. To improve the oscillation suppression ability of the grid-connected inverter, a linear active disturbance rejection controller is applied to PLL (LADRC-PLL). Considering the influence of linear extended state observer, voltage outer-loop, current inner-loop, and frequency coupling, the admittance model of the grid-connected inverter with LADRC-PLL is established. Based on the established admittance model and the generalized Nyquist criterion, the system stabilities of the grid-connected inverters with LADRC-PLL and PI-PLL are compared. The comparison results indicate that the grid-connected inverter with LADRC-PLL has better adaptability to the weak grid and shows a certain ability to suppress the sub and supersynchronous oscillation. Then, the influence of the control parameters of LADRC-PLL on the system stability is studied. It is found that the system keeps stable when the control parameter of LADRC-PLL is changed in a relatively wide range, which indicates LADRC-PLL has good robustness. Furthermore, the dynamic performances of the grid-connected inverters with LADRC-PLL and PI-PLL are analyzed. It is revealed that the grid-connected inverter with LADRC-PLL has better dynamic performance. Finally, the correctness of the analysis is verified by experiments.

A Harmonic Phasor Domain Cosimulation Method and New Insight for Harmonic Analysis of Large-Scale VSC-MMC Based AC/DC Grids

Recently, frequency coupling oscillation events, such as harmonic oscillations, subsynchronous and supersynchronous oscillations, driven by multiple converters with different switching frequencies have brought new challenges of ac/dc grid modeling, despite the traditional one of improving the accuracy and efficiency. There is an urgent requirement of practical engineering projects to produce a modeling method that can produce instantaneous and harmonic phasor waveforms simultaneously, dedicated to harmonic interaction analysis of large-scale ac/dc grids. To realize the above-mentioned targets, the harmonic phasor domain (HPD) modeling of power electronic based dc grids is proposed. The HPD modeling is then combined and coordinated with electromagnetic transient (EMT) based ac-grid models based on the proposed HPD transmission line model (HPD-TLM). The overall HPD cosimulation is realized by the EMT-based ac-grid models, HPD-based dc-grid models, and HPD-TLM, which is governed by the decoupled and coordinated time sequences. The performance (efficiency and accuracy of instantaneous and harmonic phasor waveforms) for the HPD cosimulations is validated on practical large-scale voltage source converter modular multilevel converter based ac/dc grids in China.

Assessment of subsynchronous oscillations in AC grid‐connected VSC systems with type‐4 wind turbines

Converters are the key for increasing the development of renewable energy generation but their dynamic interaction with the grid has an important impact on stability. Oscillatory instabilities in different grid-connected converter systems at several frequency ranges are reported. In particular, sub and supersynchronous oscillatory instabilities in AC power systems with type-3 and type-4 wind turbine generators (WTGs) were recently registered at several wind farm areas. A number of works based on eigenvalue analysis and frequency domain approaches are carried out to analyse this new stability issue and more research is going on to analyse in detail the electric and control parameters that may affect this phenomenon. This study contributes analysing in detail the influence of system parameters on the subsynchronous oscillations (SSOs) in AC power systems with type-4 WTGs. This study is based on a new approach for subsynchronous stability assessment which could also be used for analysing other types of SSOs (e.g. subsynchronous control interactions between doubly-fed induction generators and series-compensated networks) as well as supersynchronous oscillations. A representative example of weak AC grids with type-4 WTGs is used to illustrate the contributions of the study. Results are validated with PSCAD/EMTDC time-domain simulations.

Frequency-Coupling Admittance Modeling of Converter-Based Wind Turbine Generators and the Control-Hardware-in-the-Loop Validation

Recently, sub- and super-synchronous oscillation (SSO) associated with converter-based wind turbine generators (WTGs) has been attracting wide attentions. To investigate such SSO incidents, impedance or admittance-based modeling and analysis were widely used. It was discovered that coupling effects exist between impedances or admittances at complementary (i.e., sub and super-synchronous) frequencies. However, there is a lack of experimental validation on the coupling effects of different WTGs. In this paper, the frequency-coupling admittance models (FCAMs) of two types of widely adopted WTGs, i.e., direct-drive and doubly-fed WTGs, are presented at first. Secondly, a perturbation based identification method is proposed to obtain the FCAMs. Next, the method is implemented in a control-hardware-in-the-loop (CHIL) platform based on RTDS. Then with extensive CHIL tests, the FCAMs of both types of WTGs are obtained, and the impacts of frequency-coupling effects on SSO are validated. Finally, the characteristics of frequency-coupling effects and the root causes are further examined. This study is of important reference value to select appropriate models for oscillatory stability analysis.

Guest Editorial Special Section on Asynchronous Interconnect and Generation

With the rapid growth of modern power systems and the wide application of high-capacity HVDC systems, the stability, security, and reliability of AC-DC synchronous system may become compromised by various complex faults in the system including the possibility of multi-HVDC blocking faults. Interconnecting two or more large AC power subsystems or transmitting large amounts of energy from power plants directly to load centers through HVDC links is deemed as an effective pathway for expanding the grid and enabling new solutions paradigms. However, a wider adaptation of this new technology also brings many technical challenges, including lack of standardized equipment, the design of versatile control to ensure stability, mismatch of generation and load, interactions between various equipment and their controllers, harmonics, resonances, sub- and super-synchronous oscillations in weak AC systems, etc. Research and development efforts are currently underway in many countries to address these challenges at the system and equipment levels. The areas of focus include the planning, design, construction, modeling, analysis, operation, control, protection, and stability of asynchronous interconnection of modern and future power systems. New high-efficiency topologies and effective measurements are also among the key nontrivial issues when developing practical asynchronous interconnected power systems.

Impedance Network Modeling and Quantitative Stability Analysis of Sub-/Super-Synchronous Oscillations for Large-Scale Wind Power Systems

Recently, emerging sub-/super-synchronous oscillation (SSO) issues occurred in large-scale wind power systems, which are caused by the interactions between the power electronic converter based-wind turbine generators (WTGs) and the ac/dc grids. In previous studies, the practical system is usually simplified to equivalent system models, which consist of a (several) large-capacity WTGs connected to a radial equivalent line. However, such equivalent models can hardly represent the dynamics of the practical large-scale wind systems. Besides, the impacts on SSO characteristics from such important factors as network topology, steam turbine generators, and high-voltage direct currents have not been clarified yet. To deal with these challenges, this paper proposes a systematic impedance network modeling and quantitative stability analysis method. All system components of the large-scale wind systems are represented with impedance models (IMs) in a unified reference frame, and then, they are interconnected based on the system topology to form the impedance network model (INM) of the whole system. With the INM reduced to an aggregated IM, a stability criterion is developed to assess SSO stability just by analyzing the frequency characteristics of the determinant of the aggregated IM, and a quantitative analysis method is proposed to quantify the damping, frequency, and sensitivity of the SSO mode. The electromagnetic transient simulations on a practical wind system, which suffered actual SSO issues, have validated the effectiveness and accuracy of the proposed method. This paper also conducts comparative analysis on four types of popular stability analysis methods to show the advantages of the proposed method.