Subsynchronous Oscillation Research Articles

Characteristics of sub-synchronous oscillation in grid-connected wind farm system

With the rapid development of wind power, wind turbines are accompanied by a large quantity of power electronic converters connected to the grid, causing changes in the characteristics of the power system and leading to increasingly serious sub-synchronous oscillation (SSO) problems, which urgently require the generalized classification and characterization of the emerging oscillation problems. This paper classifies and characterizes the emerging types of SSO caused by grid-connected wind turbines to address these issues. Finally, the impact of the typical system parameters changes on the oscillation pattern is analyzed in depth to provide effective support for the subsequent suppression and prevention of SSO.

Identification of dominant propagation paths based on sub-synchronous oscillation using branch oscillation energy distribution coefficient

The large-scale integration of wind power into the power grid can cause a new type of sub-synchronous power oscillation, different from traditional thermal power generation. The oscillation energy will spread extensively in the grid, causing power oscillation and even grid-cascading events. To address this issue, this article proposes a method for quantitatively analyzing the propagation characteristics of oscillation energy based on branch oscillation energy. Firstly, analyzing the oscillation energy shared by different branches in the network based on transient energy function. Next, a method is proposed to identify the dominant propagation path of sub-synchronous oscillation by defining the oscillation energy of branches under the dominant oscillation mode and the oscillation energy distribution coefficient of each branch. The oscillation partition set formed by the dominant propagation path can be used to locate the high-risk oscillation area of the system. Finally, the effectiveness of the method proposed in this paper for studying the wide-area propagation characteristics of sub-synchronous oscillations was verified through time-domain simulation analysis.

Fast and accurate method for sub‐synchronous oscillation detection

AbstractDetection of sub‐synchronous oscillation (SSO) accurately within a short time poses a big challenge to avoiding the potential risks of the SSO phenomenon. This paper proposes a novel technique for detecting the SSO phenomenon based on identifying the SSO frequency and magnitude rather than applying fast Fourier transform. The voltage magnitude signal is used as an input signal. The SSO magnitude is identified based on estimating the max value of the voltage magnitude signal, whereas the time corresponding to the zero crossings is defined for calculating the SSO frequency. In order to confirm its effectiveness, the proposed technique is tested with different SSO types (induction generator effect, torsional interaction, and sub‐synchronous control interaction). Moreover, the impact of the series compensation level change on the proposed method performance is investigated. The results prove the effectiveness and the speed of the proposed method in all studied cases. Compared to the traditional methods within a wide range of compensation levels, the proposed technique is fast and accurate. The IEEE first benchmark model is adapted with a doubly‐fed induction generator in the MATLAB program to analyze the performance of the proposed method for detecting the SSO phenomenon.

QFS-CNN for sub-synchronous oscillation source location based on dissipating energy flow

Sub-synchronous oscillation (SSO) caused by large-scale renewable energy generations can threaten the safe and reliable operation of power systems. In this paper, a dissipating energy-based quaternion feature set convolutional neural network (QFS-CNN) method is proposed to locate the SSO sources quickly and accurately, providing the foundation for isolation of the SSO sources from power systems in time. Firstly, aiming at the difficulty in feature extraction of SSO sources due to the poor observability of power system, a temporal and spatial feature extraction method based on oscillation energy is proposed to characterize the SSO sources information in the form of images with limited measurement data. The temporal and spatial feature images are extracted based on the variation of oscillation energy in time and the direction of energy flow power in space, respectively. Then, the correlation between the oscillation energy and the SSO sources location can be revealed by the feature images with less calculation and storage space. Secondly, a QFS-CNN method based on temporal and spatial feature images for SSO sources location is proposed to address the insufficient labeled data of SSO. The QFS data augmentation technique increases the feature set via meaningful recombination of the existing labeled feature images. Then, the augmentative feature set is used to ensure the model training accuracy of QFS-CNN to improve the location accuracy of SSO sources. Finally, the proposed method is demonstrated and evaluated by a modified IEEE 39-bus wind power generation system. Simulation results show that the method has high accuracy and stronger anti-noise ability in the case of poor system observability and few sample data.

Locating Sources of Sub-Synchronous Oscillations in Wind Farms Based on Instantaneous Energy Supply on Port and Bicoherence

The sub-synchronous oscillation (SSO) which occurred in direct-driven wind generators (DDWG) based wind farms became a research hotspot. The SSO excited the shaft torsional vibration of thermal generators and caused trip failures, leading to economic losses and stabilization problems. However, the existing phasor-based oscillation source locating methods cannot be applied to the SSO since the power system quasi-steady-state assumption was not satisfied in the SSO scenarios. Thus, a novel locating method for SSO sources was urgently required. To fill this gap, this paper proposed an improved method for locating nonlinear sub-synchronous oscillation sources based on Energy Supply on Port (ESP) and bicoherence. The ESP was calculated with the instantaneous values of measured voltages and currents at some predefined network interfaces, i.e., the ports. The tendency of the ESP indicated the transient energy injection from a certain subsystem into the rest of the network during a designated period, which can help figure out the oscillation source of the SSO. After oscillation source localization, the nonlinear index and its threshold were proposed by defining the bicoherence coefficient in consideration of the nonlinear oscillation characteristics. According to the power-quality standard and the comparison of the bicoherence with the threshold, the nonlinearity of the oscillation source was examined. The feasibility and effectiveness of the proposed method were proved in the case study which simulated the oscillation scene of the accident in Hami.

Analysis of Subsynchronous Oscillation Caused by Multiple VSCs With Different Dynamics Under Strong Grid Connections

It is known that multiple voltage source converters (VSCs) with similar dynamics may induce subsynchronous oscillations (SSOs), particularly under weak grid connections. This study investigated the small-signal stability of multiple VSCs with different dynamics, utilizing both the model with default parameters and the model that accounts for deviations in VSC dynamics. The results indicates that the DC voltage control loops of the VSCs may cause growing SSOs even under strong grid connections. The direction and amplitude of the deviations are affected by the differences between the actual and default parameters. The impact of deviations is the largest when all the parameters are larger or smaller than the default parameters. However, the impact is reduced when some of the parameters are larger and others are smaller than the default parameters. On this basis, two reduced-order stability analysis methods were proposed to quickly and accurately assess the SSO risk in the power system. A sample power system with multiple VSCs is used to demonstrate and evaluate the conclusions and proposed methods.

Ambient data-driven SSO online monitoring of type-3 wind turbine generator integrated power systems based on MMPF-KF method

Series compensation grids connected with type-3 wind turbine generator (WTG)-based wind farms have suffered numerous subsynchronous oscillation (SSO) events worldwide. For early alerting of SSO and effective development of protection and control strategies, it is critical to monitor and identify SSO accurately and quickly. Ambient data is continuously available, which is useful for online monitoring. This paper proposes an ambient data-driven SSO online monitoring method based on the Kalman filter (KF) combined with the multi-model partitioning filter (MMPF). The KF is utilized to fit the measured ambient data with an auto regressive (AR) model. Then, the damping factor (or damping ratio) and frequency in the SSO mode can be acquired by solving the roots of the characteristic polynomial corresponding to the AR model. Moreover, the MMPF is an effective model order selection method applied to the KF for better identification. The performance of the MMPF-KF method is demonstrated by simulations and real-time experiments. The results of case studies validate the effectiveness of the proposed method under various conditions.

Impedance network-based oscillatory stability analyzer (INOSA) – A frequency domain tool for investigating IBR-grid interactions

Unstable subsynchronous oscillations (SSOs) may be excited due to adverse interactions between inverter-based resources (IBRs) and series-compensated or weak ac networks. The frequency-sweep impedance modeling and analysis method is widely used for scanning grid-tied IBRs against the risk of SSO. The stability of such a system is heavily influenced by system-wide parameters such as IBR dispatching conditions (input resource, and different dispatch scenarios), and network status (short-circuit and/or series-compensation levels). The variation in the operating conditions must be taken into account when conducting frequency-scanned impedance-based modeling and analysis. This paper proposes a tool, named impedance network-based oscillatory stability analyzer (INOSA), which integrates decades of research on impedance-based analysis to investigate the risk of SSO under varying operating conditions. The INOSA tool is developed as a MATLAB application package. It takes the available system parameters and frequency-scanned impedance data as input and processes it to carry out impedance network-based stability analysis under all operating conditions. The developed tool outputs key stability information such as damping and frequency of each oscillation mode, and a multi-dimensional security boundary/region plot that can be used to assess the risk of SSO and take appropriate measures. A case study on an actual power system is presented to demonstrate the features and capability of the analysis tool.

Suppression Strategy of Subsynchronous Oscillation Based on Dynamic Voltage Compensation Control for Grid-Forming D-PMSG in Weak Grid

The virtual impedance can suppress the low-frequency oscillation effectively caused by the power coupling effect for the grid-forming direct-drive permanent magnet synchronous generator (D-PMSG). However, there are few studies on the influence of virtual impedance on the subsynchronous oscillation caused by the interaction between the grid-forming D-PMSG and the weak grid. In this paper, the mechanism of virtual impedance improving the stability of virtual synchronous generator (VSG) control for the converter is revealed from the perspective of power coupling, and the influence of virtual impedance on the system stability in subsynchronous frequency range was analysed based on the sequence impedance model. The analysis results show that the virtual inductance cannot effectively improve the oscillation suppression effect, but the virtual resistance can greatly improve the damping characteristics of the system which has a good oscillation suppression effect. However, excessive increase in virtual resistance will cause power coupling oscillation, even subsynchronous oscillation, which shows limitations in subsynchronous oscillation suppression. To solve the problem, a dynamic voltage compensation control method based on active disturbance rejection control (ADRC) was proposed, and the parameters are designed by bandwidth method. The method can suppress the subsynchronous oscillation effectively in a certain frequency range under different grid strength and disturbance conditions. And it shows strong adaptability and good system robustness in weak grid. Finally, the effectiveness of the proposed oscillation suppression strategy was verified by time domain simulation.

Impact of Active Frequency Support Control of Photovoltaic on PLL-Based Photovoltaic of Wind-Photovoltaic-Thermal Coupling System

The coupling systems formed by wind power, photovoltaics, and thermal generators possess broad application prospects. To build grid-friendly renewable energy stations, active frequency support control (AFSC) is adopted. There are potential risks on subsynchronous oscillation (SSO) dominated by the phase-locked loop (PLL) based photovoltaic considering AFSC of photovoltaic. The impact of AFSC on PLL dominant SSO mode (PLL-SSOM) of the photovoltaic is still not clear. To address that, this paper firstly observes the SSO characteristics of the wind-photovoltaic-thermal coupling system (WPTCS) with AFSC. Considering the second-order rotor motion equation of the thermal generator and ignoring the dynamics of DFIG, a simplified model of WPTCS with AFSC is established. Compared with the detailed model, the correctness of the simplification is verified. Based on that, the transfer function of additional AFSC is derived. The study reveals that the root cause of the impact of AFSC on PLL-SSOM is that AFSC will make the synchronous torque and damping torque corresponding to the PLL-SSOM transform each other. Finally, the frequency-domain and time-domain simulation results verify the correctness of the model simplification and theoretical analysis.

Theoretical Investigation on the Connections Between Loop Impedance-Based Criterion and State-Space-Based Method

The loop impedance-based criterion (LIC) is a practical approach for studying the sub-synchronous oscillation (SSO) instability induced by converter-interfaced wind turbine generators (WTGs) via checking whether the resistance is positive or not. However, few existing works have discussed the background mathematical theory of such criterion. In this paper, a theoretical impedance solving method is raised based on a forced oscillation analysis method, which is efficient to obtain accurate impedance and can connects the state-space function, eigenvector and impedance inherently. Therefore, the applicability of LIC can be revealed in different stability regions from the viewpoint of the strict eigen-analysis based method (EBM). The general LIC stability criteria is then derived, and the conditions where LIC may cause misjudgment are discussed. Cases considering different operating scenarios are conducted to verify the proposed impedance calculation method and illustrate the connections and differences between LIC and EBM.

Online SSO stability analysis-based oscillation parameter estimation in converter-tied grids

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.

Mitigation of power system oscillations in weak grids with battery energy storage systems: A real-world case study

Sub-synchronous oscillations are becoming commonplace in weak areas of power systems with high levels of renewable generation, affecting their operation. Moreover, there is a lack of methods and techniques that facilitate the understanding of such phenomena and the analysis of mitigation alternatives from a practical-knowledge point of view. In this manuscript, the combination of static and dynamic techniques is utilized and consolidated to derive general conclusions when mitigating sub-synchronous oscillations by means of grid-forming battery energy storage systems (GFM BESSs). The grid impedance shaping effect of GFM BESSs, which allows the mitigation of undesired oscillations, is demonstrated through small-signal (static) and frequency scanning (dynamic) analyses when studying an actual and extremely weak grid, the Australian West Murray Zone. These findings are further validated when performing detailed real-time EMT digital simulations of the grid. Furthermore, additional dynamic simulations demonstrate the ability of GFM BESSs to provide system strength and increment the hosting capacity of renewables in weak areas. The methods and techniques presented in this work can be applied to any power system, further supporting existing industry practices.

Battery energy storage-based system damping controller for alleviating sub-synchronous oscillations in a DFIG-based wind power plant

This paper presents the issue of the Sub-synchronous resonance (SSR) phenomenon in a series compensated DFIG-based wind power plant and its alleviation using a Battery Energy Storage-based Damping Controller (BESSDCL). A supplementary damping signal is developed considering the angular speed deviation and is incorporated into the BESS control system. Wide-area Measurement System data is used to determine the angular speed deviation. A linearized system model is developed to perform eigenvalue analysis, and to detect and examine unstable SSR modes. The variation of wind speed and three-phase fault are also taken into consideration to validate the robustness of the controller. To further verify the efficacy of the proposed damping controller, time-domain simulations are performed using MATLAB/Simulink. The application of the proposed BESSDCL stabilizes all the unstable system modes effectively at wind speeds of 7 m/s, 9 m/s, and 11 m/s, and at 40%, 50%, and 60% series compensation levels, as well three-phase fault conditions.

Damping sub-synchronous resonance in DFIG wind farms: An innovative controller utilizing wide-area measurement systems

The use of series shunt compensators along with wind energy in power systems has increased significantly. One of the main disadvantages of wind energy and series compensation is the phenomenon of sub-synchronous amplification. In this paper, a control method with fuzzy system based on wide area measurement systems (WAMS) and considering time delay in order to increase the damping of sub-synchronous oscillations in compensated networks with capacitive compensators and wind turbines. The DFIG type is provided in the GSC converter’s voltage control loop for the system’s stable operation. The simulation results using MATLAB/Simulink software on Modified IEEE First Bench Mark test networks show the effectiveness of the proposed method in damping sub-synchronous oscillations in power systems compensated with series capacitors.

Analysis and mitigation of SSCI when integrating wind power to series compensated lines

As more power electronics are introduced into the power system, its stability is impacted, e.g., through undesired interactions. One such interaction is called sub-synchronous control interaction (SSCI), an example being an interaction between a DFIG wind farm and a series compensated line. In this paper, two methods are used to assess the risk of SSCI: the reactance crossover method, and the Nyquist criterion. The analysis is performed on three case studies: one system based on the IEEE First Benchmark System, and the other system is modelled as a typical Swedish transmission system with two different degrees of series compensation. Both methods predict SSCI in all three case studies, with the Nyquist criterion being able to predict the oscillation frequency more accurately. To mitigate the sub-synchronous oscillations, a PV farm is implemented and placed in parallel to the wind farm. The performed simulations show that it is able to damp the oscillations successfully in all case studies.

Re-examination of small-signal instability in weak grid-connected voltage source converters

Weak grid sub-synchronous oscillation (SSO) is an important research topic. Although some analysis and mitigation schemes have been conducted, misperceptions still exist. This paper first aims to update the understanding of weak grid instability of voltage source converters (VSCs) and then points out that instability risks originate in the super-synchronous frequency range rather than in the sub-synchronous frequency range. Moreover, the resonance frequency can be even larger than the double fundamental frequency under certain parameter conditions. This paper also refines the impacts of VSC control parameters on weak grid instability mechanism and resonance frequency. Based on the results, the classification of this phenomenon should be revised.

Protection against sub-synchronous oscillations, a relay model

With increased integration of renewable energy resources, FACTs devices and series compensation, sub-synchronous oscillations (SSO) have become more common in electrical power systems in recent years. Specially designed relaying devices are often employed to detect and isolate harmful SSO conditions as when unconstrained, they can lead to widespread equipment damage and system instability. This paper presents design and implementation of a SSO relay model that can effectively extract sub-synchronous components in system measurements to quickly detect SSO conditions. Dependability and security of the developed relay model are validated using electro-magnetic transient (EMT) type simulations. In addition, performance of the developed relay model is compared against a commercial (physical) SSO relay. Obtained results demonstrate the effectiveness of the implemented relay model in detecting SSO and protecting electrical power systems against SSO conditions.

A Sub-Synchronous Oscillation Suppression Strategy Based on Active Disturbance Rejection Control for Renewable Energy Integration System via MMC-HVDC

To realize the consumption of renewable energy such as wind power and photovoltaics in the power system, renewable energy integration system via modular multilevel converter (MMC)-based high voltage direct current (MMC-HVDC) has been widely applied. However, with the large-scale grid connection of renewable energy units, sub-synchronous oscillation (SSO) is prone to occur. Aiming at the problem, this paper proposes an SSO suppression strategy for renewable energy integration system via MMC-HVDC based on active disturbance rejection control (ADRC) theory. Using the direct drive permanent magnet synchronous generators (PMSG)-based wind farm integration system via MMC-HVDC as an example, firstly the topology and control system principles of the system are described, and a simulation model is built in PSCAD/EMTDC. Moreover, the SSO mechanism of the system is revealed by Nyquist stability criterion, and the major factors affecting the SSO of the system are simulated and analyzed. Subsequently, an additional sub-synchronous damping controller (ASSDC) is proposed based on ADRC theory. Compared to traditional additional damping controllers, the proposed controller considers disturbances of the system during the designing process and has stronger robustness. In addition, when faults happen, the speed of the system with ASSDC reaching a steady-state operating point rises by 33.7% as compared to the system without ASSDC. Finally, the effectiveness of the proposed suppression strategy is verified through simulation analysis.

Propagation path criterion of sub synchronous oscillation power in the large-scale wind farm integrated system

Sub-synchronous oscillations (SSO) caused by large-scale wind farm integrated systems have become an important issue threatening the security and stability of power systems. Some practical cases have shown that SSO can propagate through multiple voltage levels to more than hundreds kilometers. Many studies have focused on this problem, but there is a lack of research on the power propagation characteristics of SSO. In this paper, a measurement-based method for sub synchronous oscillation (SSO) propagation path criterion in SSO events is proposed. The method is based on the concept of sub synchronous power (SSP). By monitoring the sign of SSP generated by different wind farms or turbines, the propagation path can be identified. The method gives an effective way to quantitatively analyze the power propagation path of sub synchronous oscillations from the power perspective. Experimental cases demonstrate the practicality of the method.