Subsynchronous Interaction Research Articles

Unstable Subsynchronous Interaction Induced by Blocking Filters in a Wind-Thermal Combined Power System

Blocking filters (BFs) are applied to reduce the risk of shaft related subsynchronous resonance (SSR) in thermal power plants connected to series compensated transmission lines. However, when doubly-fed induction generator (DFIG)-based wind farms are connected in parallel with the existing thermal power plants, the same BFs may cause unstable subsynchronous interaction (SSI) with the DFIGs. This paper investigates the potential risk of this new type of SSI caused by BFs, which are originally designed to reduce the SSR risk. First, a representative system model of thermal power units with BFs and DFIGs integrated into grid is established. Then, impedance-based analyses and time-domain simulations are carried out to analyze the mechanism and characteristics of the SSI. The results indicate that the interaction between BFs and DFIGs may induce unstable oscillation in the system. In particular cases where series compensation in the transmission line is bypassed, the risk of SSI instability still exists. With series compensation operating normally, the interaction among BFs, DFIGs, and series compensated lines becomes even more complex. The paper also investigates the effects of system parameters such as the number of online DFIGs, wind speed, and series compensation level on the SSI characteristics.

Multivariable-Controlled Shunt Compensator for Damping Subsynchronous Interactions in Electrical Distribution Networks

In recent years, the massive integration of converter-interfaced devices to electrical distribution networks has lead to appearance of poorly-damped subsynchronous interactions in the grid. These oscillation modes amplify electrical disturbances originated from the grid and may cause an interaction between grid elements threatening the system stability. Shunt compensators at the distribution level have already been used to improve the damping of these networks. However, their control system strongly depends on the distribution network dynamics, that is difficult to know at all times. In this paper, a multivariable controller for a shunt compensator is proposed. First, the dynamics of a distribution network with high penetration of electronic interfaces is explored in order to understand the interactions. Then, the controller of the compensator is designed based on an estimated model of the plant so that no previous information of the network is required. This controller is based on a state-feedback controller and a state observer, both designed by using the linear quadratic regulator (LQR) theory. Analytical results and computer simulations are used to verify the effectiveness of the controller. The main findings are validated in a laboratory environment comprising two 15 kW distributed generators, a 15 kW compensator and a 75 kW grid emulator.

A new stationary frame multi-input multi-output EMT-level frequency scanning method for inverter based resources

Impedance-based stability analysis (IBSA) is an effective method to identify subsynchronous interaction (SSI) problem between the inverter-based resources (IBRs) and series compensated or weakly tied AC grids. The electromagnetic transient (EMT) level positive sequence and dq-frame frequency scanning methods (p-scan and dq-scan, respectively) are used to obtain the sequence single-input single-output (SISO) and dq multi-input multi-output (MIMO) impedance of IBRs, respectively. The dq MIMO impedance usage in IBSA provides more accurate results compared to the sequence SISO impedance. This paper proposes an EMT-level αβ-frame frequency scanning method (αβ-scan) to obtain the αβ MIMO impedance and its usage in IBSA. The αβ-scan requires significantly less time compared to dq-scan for the convergence of MIMO IBR impedance representation and offers similar accuracy with dq-scan in IBSA. The accuracy of the proposed αβ MIMO IBSA is validated by comparing with dq MIMO IBSA and EMT simulations on series capacitor SSI cases with different type IBRs (Doubly-fed induction generator (DFIG)-based wind park (WP) and full-scale converter (FSC)-based WP). This paper also uses a multivariable structure function(MSF)-based method for the first time in IBSA of SSI to achieve Bode plot-based analysis for providing better visualized presentation of resonance frequency and stability margins compared to generalized Nyquist criterion.

Design of a robust and practicable SSI damping controller using H[formula omitted] technique for series compensated DFIG-based wind farms

This paper designs a robust and practicable subsynchronous interaction (SSI) damping controller using H∞ technique for the safe operation of series capacitor compensated wind farms (WFs) with doubly-fed induction generator (DFIG) wind turbines (WTs). Mixed sensitivity control design together with pole placement are formulated into a set of linear matrix inequalities (LMIs) to obtain the controller parameters. The LMI technique allows to include both desirable frequency and time domain specifications. The proposed damping controller is integrated into the WT controller (WTC) and receives the DFIG converter currents as inputs. The implementation of the proposed controller does not require any communication links between the WTs and WF secondary control layer. The controller output signals are applied to the inner control loops of DFIG converters and are dynamically limited for the desired fault-ride-through (FRT) performance. The effectiveness of the damping controller is verified through detailed electromagnetic transient (EMT) simulations. In these simulations, the complete medium-voltage (MV) collector grid is modeled with all details, and it is assumed that the wind speed at the location of each turbine follows a Gaussian distribution. The collected results confirm the accuracy of the modeling of the entire WF as an aggregated WT with the average wind speed.

Evaluation of damping of subsynchronous interaction between direct-drive wind farm and LCC-HVDC based on the damping path analysis

The interaction between direct-drive wind farm (DDWF) and LCC-HVDC may make the power system more prone to subsynchronous oscillation (SSO). This paper presents a damping path analysis method to analyze and evaluate the subsynchronous interaction between them from the perspective of the internal disturbance transfer of the system. First, damping torque analysis (DTA) is extended to the SSO mode dominated by phase-locked loop (PLL), and the stability of the DDWF with the LCC-HVDC (DDWFL) system is analyzed. Then based on the closed-loop transfer function block diagram of the DDWFL system, the disturbance transfer process between DDWF and LCC-HVDC is revealed to analyze the interaction mechanism in the SSO mode, and the closed-loop disturbance transfer paths that reflect the system damping are called the damping paths. Further, damping path of the subsynchronous interaction between DDWF and LCC-HVDC is separated, and its effect on the SSO mode damping is evaluated by damping coefficient. It is concluded that the SSO mode dominated by PLL has instability risk in the DDWFL system, and the subsynchronous interaction between DDWF and LCC-HVDC provides negative damping to the system. Finally, the time domain simulation results demonstrate the validation of damping evaluation based on damping path analysis.

Secure Sampled-Data Observer-Based Control for Wind Turbine Oscillation Under Cyber Attacks

Inspired by the recent surge of interest in security analysis of renewable energies, this article proposes a cyber resilient control framework for large-scale wind parks (WPs) against denial of service (DoS) and deception attacks. A detailed cyber-physical model for a doubly-fed induction generator (DFIG)-based WP is first developed. It is shown that the WP is subject to a stability issue known as the sub-synchronous interaction (SSI). Additionally, two cyber threats in the forms of DoS and deception attack attempt to destabilize the WP from the subsynchronous perspective. To stabilize the WP under the SSI phenomenon and these cyber attacks, an observer-based fuzzy control scheme is proposed which requires only periodic samples of the output. Using a looped-Lyapunov functional (LLF) method for stability analysis, the control and observer gains are computed from the solution of some linear matrix inequality (LMI) conditions. The proposed design framework allows including the desired level of cyber resilience to DoS and deception attacks for computing proper control and observer gains. The effectiveness of the proposed controller against the considered threats is demonstrated via electromagnetic transient (EMT) simulations performed using the EMTP-RV software and a co-simulation platform.

Sub‐synchronous interaction damping controller for a series‐compensated DFIG‐based wind farm

Sub-synchronous interaction (SSI) phenomenon is one of the dynamic system problems that have an adverse influence on the safety and stability of the system.The use of supplementary damping controllers (SDCs) in the double-fed induction generator (DFIG) converter controllers is quite promising due to their simplicity,low costs, effectiveness, and easiness of tuning. This paper presents a new effective input control signal, rotor voltage, to the SDC for damping SSI in a series-compensated DFIG-based wind farm. The proposed SDC is embedded into both the q-axis and d-axis of the rotor-side converter inner current loops. Particle swarm optimization algorithm is used to identify the optimum parameters of the SDC which maintain the system stability at various operation conditions. Both eigenvalue analysis and time-domain simulations have been carried out to demonstrate the capability of the proposed SDC for enhancing the system stability and damping SSI. Compared to the conventional SDC, the proposed SDC has the best performance where it can quickly and robustly damp the SSI at different compensation levels, different wind speeds, and sub-synchronous control interaction.

Investigation of Subsynchronous Oscillation in a DFIG-based Wind Power Plant Connected to MTDC Grid

This paper investigates a new type of subsynchronous oscillation (SSO) event observed in a real-world doubly fed induction generator (DFIG)-based wind power plant (WPP) connected to a multiterminal dc (MTDC) grid in Northern China. The system under study is unique and, to date, is the world's largest four-terminal DC transmission network, which connects far-away wind and photovoltaic power plants to regional load centers. For the first time, this paper utilized field-measured data to analyze the characteristics and reveal the underlying mechanism of the SSO in doubly fed induction generators (DFIGs) connected to HVDC grids. The analysis is carried out using the impedance network model established from the impedances obtained from field recorded data. The investigations on the field data concluded that the SSO is essentially caused by the subsynchronous interaction between the DFIGs and the MTDC grid without the presence of a physical series/shunt compensation or weak AC grid. Instead, it is found that the SSO is triggered due to the converter-grid resonance where the MTDC grid presented itself as a virtual capacitor and the DFIGs showed negative resistance at the subsynchronous frequency range.

Evaluation of Damping of Subsynchronous Interaction between Direct-Drive Wind Farm and Lcc-Hvdc Based on the Damping Path Analysis

Evaluation of Damping of Subsynchronous Interaction between Direct-Drive Wind Farm and Lcc-Hvdc Based on the Damping Path Analysis

An embedded VFAF in DFIG-based wind farms for SSCI mitigation

Due to the increasing penetration of wind generations, a new type of sub-synchronous interactions, the so called sub-synchronous control interaction (SSCI), has emerged in power systems in the past decade. Incidents of this type have caused severe consequences in real-life power systems, including equipment damage and loss of power supply. To overcome the impacts and minimize the adverse influences of such phenomenon, this paper proposes a novel varying-frequency adaptive filter (VFAF) in doubly fed induction generators-based wind farms for SSCI mitigation. The proposed VFAF utilizes a linear prediction model and coefficient-based polynomials to analyse the spectral components of the controller signals. In addition, singular value decomposition is used to identify the number of signal components in the controllers. Then, based on the analysis, the VFAF produces compensation signals to modulate the distorted controller signals and thus mitigates the oscillations during SSCI. Based on this mechanism, the VFAF is auto-adaptive to various oscillation frequencies, including those that are time-varying. The above feature is of particularly importance in modern power systems. Testing cases of the VFAF have been conducted in the Power Factory software, and the results verify the effectiveness of the proposed VFAF under relevant system conditions.

A Cyber Attack Mitigation Scheme for Series Compensated DFIG-Based Wind Parks

Subsynchronous Interaction (SSI) phenomenon is known to be one of the most frequent and severe stability issues of a Wind Park (WP), and can potentially lead to a significant loss of power generation. The broad impacts of this phenomenon on a power grid have made WPs interesting targets for cyber attacks. To initiate the SSI, an adversary can target either the power grid (external attacks) or the cyber system of WPs (internal attacks). This paper proposes a mitigation scheme for attacks that initiate the SSI phenomenon in series compensated doubly-fed induction generator (DFIG)-based WPs. External attacks are addressed by employing a robust static-output-feedback Subsynchronous Damping Controller (SSDC), which is designed based on the insensitive strip region and Linear Matrix Inequality (LMI) techniques. Internal attacks, however, are detected by comparing the estimated and measured converters’ currents. Once the compromised measurements are detected, the designed SSDC is restructured to mitigate the attacks. The effectiveness of the proposed method is demonstrated using detailed Electromagnetic Transient (EMT) simulations for both internal and external cyber attacks. Additionally, the performance of the proposed method is corroborated using a real-time co-simulation framework.

Energy-shaping L2-gain controller for PMSG wind turbine to mitigate subsynchronous interaction

As the serious stability issue of wind power systems, subsynchronous interaction (SSI) problem has resulted in the significant loss of generation. Much effort is put into investigating the risk for SSI in wind farms by manufacturers and research community. To mitigate SSI for permanent magnetic synchronous generator (PMSG), this paper proposes energy-shaping L2-gain controller (ESLGC) for machine side converter (MSC) and grid side converter (GSC). ESLGC not only assures the global asymptotical stability but also enhances system robustness against disturbances. An aggregated series-compensated PMSG-based wind farm and a multi-PMSG wind power system are adopted to evaluate the effectiveness of ESLGC. The results of eigenvalue analysis and time-domain simulation show better robustness and damping performance of the proposed ESLGC as compared to conventional PI controller and filter-based subsynchronous damping controller (SSDC) under various operating conditions.

Modal Analysis on Control Impact of Fully Rated Converters-Based PMSG-WECS Connected to Turbine-Generator

The power converter-based wind energy conversion systems (WECS) connected to weak AC networks may cause subsynchronous interactions that can potentially lead to instability. The interaction between the WECS converters and a nearby turbine-generator (TG) may lead to severe torsional vibrations. This article investigates the impact of a fully rated converters-based permanent magnet synchronous generator WECS (PMSG-WECS) on the subsynchronous torsional interactions (SSTI) of a nearby multimass TG connected to series compensated transmission line. A simple but effective systematic approach of modal analysis using open-loop systems is proposed. The proposed approach is used to evaluate the impact of PMSG-WECS on SSTI. Further, the impact of PMSG-WECS is analyzed through the damping torque, eigenvalue analysis and transient simulation under various control modes of PMSG-WECS converters. The results demonstrate that the inclusion of PMSG-WECS increases the subsynchronous resonance frequency of the network, thereby reduces the damping of the torsional modes in the high-frequency range of subsynchronous resonance frequency. The negative damping effect of PMSG-WECS significantly reduces the damping of subsynchronous network mode, thereby can destabilize the system. Also, the negative damping impact of PMSG-WECS significantly reduces the damping of high-frequency torsional modes. As a result, the torsional interactions aggravate at lower compensation levels.

A novel adaptive linear prediction-based parameter estimation method for monitoring sub-/inter-harmonics during SSI events

Sub-Synchronous Interaction (SSI), caused by the increasing penetration of inverter connected nonsynchronous generation, is posing a serious threat on power system stability. As the sub-/inter-harmonics emerging during SSI are featured by time-varying frequencies, concerns regarding the accuracy and the robustness of existing methods for monitoring oscillations of such phenomenon have been raised. This paper proposes a novel two-stage numerical method for monitoring the sub-harmonics, harmonics and inter-harmonics during SSI events. In the first stage, the model order of the estimation is selected using the proposed Adaptive Model Order Selection (AMOS) scheme, which is achieved through singular value decomposition. In the second stage, a linear prediction-based parameter estimation approach with high accuracy and strong robustness is used to estimate the signal parameters of the sub-harmonics, harmonics and inter-harmonics. By merging the above two stages, a novel and effective numerical method is obtained, namely the Adaptive Linear Prediction-based Parameter Estimation (ALPPE) method. In addition, an estimation calibration is implemented to increase the estimation accuracy. The proposed ALPPE method has then been tested using computer simulated signals and field measured data from an actual SSI event. The results of the tests confirmed the ALPPE method's high accuracy, adaptability, processing speed and immunity to noise.

Comparison of dynamic phasor, discrete-time and frequency scanning based SSR models of a TCSC

Sub-synchronous Control/Torsional Interactions occur in a power system due to adverse interactions between the electrical network, turbine-generators and power-electronic systems. The extraction of time-invariant models of the system for analysis and design is often an onerous task due to the complex dynamics associated with power-electronic systems. The modelling techniques proposed in the literature include the use of discrete-time mapping, dynamic phasors and frequency scanning. Frequency scanning is a black-box approach, wherein a time-invariant model is extracted from the simulated response of the system to a probing signal. The first two techniques derive the model directly from the analytical equations (with several simplifying assumptions). This paper compares the accuracy of these models using the example of a SSR-vulnerable system which includes a TCSC. The presence of several poorly damped torsional modes and the use of a TCSC to damp them make this an excellent test case for modelling accuracy. It is seen that the frequency scanning based state-space model is quite accurate and its extraction using simulation and the vector fitting technique presents no significant difficulties, irrespective of the extent of modelling detail. The results demonstrate that this approach can be both convenient and accurate for sub-synchronous interaction studies.

Analysis and Mitigation of the Communication Delay Impacts on Wind Farm Central SSI Damping Controller

The use of supplementary controllers for mitigating subsynchronous interaction (SSI) in series compensated DFIG-based wind farms is quite promising due to high effectiveness and low cost. Implementation of such a controller requires effective communications between individual turbines and the wind farm controller, where the control performance is very much affected by the communication delays involved. This paper delivers the first detailed analysis on the impact of communication delays on SSI damping controller performance. A novel algorithm is proposed to calculate the stability delay margin (SDM) of the closed-loop system based on Rekasius Substitution and Guardian Map Theorem with the advantage of reduced computational burden particularly for high-order systems. Based on the proposed algorithm, the impacts of wind farm operating conditions and turbine control parameters on the SDM are investigated. To strengthen the SSI damping controller performance against communication delays, a Smith predictor scheme is also developed. The effectiveness of the proposed delay analysis framework and Smith predictor scheme based delay immune controller is validated through Electromagnetic Transient (EMT) simulations on a realistic test system with multiple series capacitor compensated lines considering various operation conditions.

Mitigation of Subsynchronous Interactions in Hybrid AC/DC Grid With Renewable Energy Using Faster-Than-Real-Time Dynamic Simulation

Mitigation of Subsynchronous Interactions in Hybrid AC/DC Grid With Renewable Energy Using Faster-Than-Real-Time Dynamic Simulation

Nonlinear Subsynchronous Oscillation Damping Controller for Direct-Drive Wind Farms With VSC-HVDC Systems

Due to the subsynchronous interaction between the grid-side converter (GSC) of wind farms and the rectifier (REC) of voltage source converter-based high-voltage direct current (VSC-HVDC) transmission system, subsynchronous oscillations (SSOs) may occur in direct-drive wind farms with VSC-HVDC systems. Considering the nonlinearities and uncertainties of the system, a nonlinear SSO mitigation strategy is proposed in this article based on the feedback linearization theory and sliding mode control (SMC). The feedback linearization theory is used to eliminate the nonlinearities, and the SMC is adopted to improve the robustness against uncertainties and disturbances. The proposed feedback linearization SMC (FLSMC) takes the advantages of feedback linearization control (FLC) and SMC. The FLC transforms the nonlinear forms of the GSC and REC into the linear forms through the coordinate transformation and feedback. Considering that the FLC is sensitive to parameter uncertainties and external disturbances, the SMC is combined with the FLC to improve the system robustness. An eigenvalue analysis and time-domain simulations are carried out, which demonstrates that the FLC outperforms over the traditional proportional–integral control for the SSO mitigation and decoupling. Meanwhile, the FLSMC shows better robustness against parameter uncertainties and external disturbances over the FLC and traditional damping control.

Mitigation of Subsynchronous Interactions in Hybrid AC/DC Grid With Renewable Energy Using Faster-Than-Real-Time Dynamic Simulation

Transmission line capacity enhancement by series compensation is commonly used in power systems, which consequently faces potential subsynchronous interaction (SSI). In this work, faster-than-real-time (FTRT) simulation based on the field-programmable gate arrays is proposed to mitigate the disastrous SSI in a hybrid AC/DC grid integrated with wind farms. Dynamic simulation is applied to the AC system to gain a high speedup over real-time, and a detailed multi-mass model is specifically introduced to the synchronous generator to show the electrical-mechanical interaction. Meanwhile, the DC grid undergoes electromagnetic transient simulation to reflect the impact of power converters' control on the overall grid, and consequently, the EMT-dynamic co-simulation running concurrently due to FPGA's hardware parallelism is formed. As the two simulations are inherently distinct, a power-voltage interface is adopted to separate them which enables their coexistence in one program. It shows that following the detection of a contingency, the FTRT hardware platform can generate an optimum solution with precisely quantified power flow changes in advance to keep the hybrid AC/DC grid stable. The FTRT efficacy is proven by a number of cases where the accuracy is validated by offline simulation tool Matlab/Simulink.

Robust subsynchronous interaction damping controller for DFIG-based wind farms

This paper proposes a robust controller to improve power system stability and mitigate subsynchronous interaction (SSI) between doubly-fed induction generator (DFIG)-based wind farms and series compensated transmission lines. A robust stability analysis is first carried out to show the impact of uncertainties on the SSI phenomenon. The uncertainties are mainly due to the changes in the power system impedance (e.g., transmission line outages) and the variations of wind farm operating conditions. Then, using the µ-synthesis technique, a robust SSI damping controller is designed and augmented to the DFIG control system to effectively damp the SSI oscillations. The output signals of the supplementary controller are dynamically limited to avoid saturating the converters and to provide DFIG with the desired fault-ride-through (FRT) operation during power system faults. The proposed controller is designed for a realistic test system with multiple series capacitor compensated lines. The frequency of the unstable SSI mode varies over a wide range due to the changes in power system topologies and wind farm operating conditions. The performance of the proposed controller is verified through electromagnetic transient (EMT) simulations using a detailed wind farm model. Simulation results also confirm the grid compliant operation of the DFIG.