Subsynchronous Control Interaction Research Articles

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.

Mechanism and Mitigation Strategy of Sub-synchronous Oscillation Caused by Grid-connected DFIG with Series Compensation

Power systems are at risk of sub-synchronous oscillation (SSO) when connecting doubly-fed induction generator (DFIG)-based wind turbines to transmission lines with series compensation. This paper focuses on the mechanism and mitigation strategy of SSO. Firstly, the mechanism of SSO is studied from two aspects: induction generator effect (IGE) and sub-synchronous control interaction (SSCI). An equivalent circuit is developed to illustrate the negative resistance effect. Based on the result of the analysis, the SSO mitigation strategy using notch filter and virtual resistance control is proposed. In order to verify the result of analysis and the effectiveness of mitigation strategy, time-domain simulations are carried out based on PowerFactory DIgSILENT.

Subsynchronous Control Interaction: Real-World Events and Practical Impedance Reshaping Controls

Subsynchronous Control Interaction: Real-World Events and Practical Impedance Reshaping Controls

PI Parameters Tuning for Sub-Synchronous Interactions in DFIG Based on GSSA Algorithm

The interaction between the control of power electronic devices and the series compensating capacitors is called sub-synchronous control interaction (SSCI). Because SSCI seriously affects the stability of the power grid, it has attracted much attention. In this paper, the initial population of salp swarm algorithm (SSA) is optimized based on good point theory and an improved SSA algorithm (GSSA) is proposed. Then, based on the improved GSSA algorithm, the PI parameters are adjusted to minimize the oscillation. Finally, the validity of the method is verified by simulation analysis. In addition, the robustness of the optimized parameters under different wind speeds and series compensation levels is studied. The results show that the optimized parameters can not only effectively suppress SSCI, but also have strong robustness in different working conditions.

Toward carbon-neutral electricity and mobility: Is the grid infrastructure ready?

Toward carbon-neutral electricity and mobility: Is the grid infrastructure ready?

Frequency Scan–Based Mitigation Approach of Subsynchronous Control Interaction in Type-3 Wind Turbines

Integration of wind energy resources into the grid creates several challenges for power system dynamics. More specifically, Type-3 wind turbines are susceptible to subsynchronous control interactions (SSCIs) when they become radially connected to a series-compensated transmission line. SSCIs can cause disruptions in power generation and can result in significant damage to wind farm (WF) components and equipment. This paper proposes an approach to mitigate SSCIs using an online frequency scan, with optimized phase angles of voltage harmonic injection to maintain steady-state operation, to modify the controllers or the operating conditions of the wind turbine. The proposed strategy is simulated in PSCAD/EMTDC software on the IEEE second benchmark model for subsynchronous resonance. Simulation results demonstrate the effectiveness of this strategy by ensuring oscillations do not grow.

Mitigation of subsynchronous control interaction in DFIGs using active disturbance rejection control

Recently, a new type of subsynchronous oscillation (SSO) has occurred in doubly-fed induction generator (DFIG)-based wind farms. This oscillation is also called the subsynchronous control interaction (SSCI), which is mainly caused by the interaction between the transmission series compensation and the DFIG controller. Therefore, the oscillation frequency of the SSCI is susceptibly impacted by DFIG operating conditions, which presents major challenges to conventional mitigation strategies. To solve this problem, a grid-connected series-compensated DFIG-based wind farm is established, and the formation process of the SSCI is analysed. The strategy of active disturbance rejection control (ADRC), which can automatically estimate and compensate for the total disturbance of the system in real-time, is introduced into the rotor-side converter to mitigate the SSCI in the DFIG. ADRC is designed based on dynamic parameter tuning and chaos optimization. Finally, a detailed model-based time-domain simulation is conducted to evaluate the performance of the proposed optimized ADRC (O-ADRC) compared to conventional virtual impedance and ADRC. The simulation results show that O-ADRC can effectively mitigate the SSCI under different operating conditions and provide sufficient damping for the DFIG-based wind farms.

Variable-Gain Super-Twisting Sliding Mode Damping Control of Series-Compensated DFIG-Based Wind Power System for SSCI Mitigation

Subsynchronous oscillation, caused by the interaction between the rotor side converter (RSC) control of the doubly fed induction generator (DFIG) and series-compensated transmission line, is an alleged subsynchronous control interaction (SSCI). SSCI can cause DFIGs to go offline and crowbar circuit breakdown, and then deteriorate power system stability. This paper proposes a novel adaptive super-twisting sliding mode SSCI mitigation method for series-compensated DFIG-based wind power systems. Rotor currents were constrained to track the reference values which are determined by maximum power point tracking (MPPT) and reactive power demand. Super-twisting control laws were designed to generate RSC control signals. True adaptive and non-overestimated control gains were conceived with the aid of barrier function, without need of upper bound of uncertainty derivatives. Stability proof of the studied closed-loop power system was demonstrated in detail with the help of the Lyapunov method. Time-domain simulation for 100 MW aggregated DFIG wind farm was executed on MATLAB/Simulink platform. Some comparative simulation results with conventional PI control, partial feedback linearization control, and first-order sliding mode were also obtained, which verify the validity, robustness, and superiority of the proposed control strategy.

Damping of Subsynchronous Control Interactions in Large-Scale PV Installations Through Faster-Than-Real-Time Dynamic Emulation

Large-scale photovoltaic (PV) power plant has witnessed a dramatic increase in the integration into transmission and distribution network, manifesting subsynchronous control interaction (SSCI) when the host grid is weak. In this work, the oscillation modes of a typical PV network are analyzed, and a faster-than-real-time (FTRT) emulation is proposed for predicting the SSCI and consequently mitigating its impacts on AC grid by taking the effective active/reactive power control action. The electromagnetic transient (EMT) simulation is utilized to model the PV panels and converter stations to reflect the actual dynamic process. Meanwhile, the AC grid undergoes transient stability (TS) simulation to obtain a high speed up over real-time, and consequently, a power-voltage interface is adopted for the coexistence of different simulation methods. The reconfigurability and parallelism of the field-programmable gate arrays (FPGAs) enable the EMT-TS co-emulation strategy to run concurrently. With a remarkable 122 FTRT ratio, the proposed hardware emulation can provide an effective solution before the SSCI causes serious disruption following its detection. The hardware emulation results are validated by the off-line simulation tool Matlab/Simulink® and TSAT® in the DSATools™ suite.

Energy-Shaping Controller for DFIG-Based Wind Farm to Mitigate Subsynchronous Control Interaction

Increasing use of series compensation in doubly-fed induction generator (DFIG)-based wind farms has led to subsynchronous control interaction (SSCI) incidents. The irregular exchange of energy between the network side and generator side is considered responsible for SSCI. Therefore, an energy-shaping controller (ESC) is proposed in this paper to mitigate SSCI from the energy perspective. Firstly, based on energy relationship and physical nature, Hamiltonian model is established to describe the system structure. Secondly, the controller is obtained by solving the energy equation. Damping is then injected into the controller dynamics and it is back-propagated to the system, ensuring the global asymptotical stability. Thirdly, the controller is further improved to make gamma-dissipation inequality hold, thereby enhancing the robustness. Time-domain simulation and impedance model (IM)-based stability analysis are conducted to evaluate the damping performance of ESC. Simulation results demonstrate the effectiveness of ESC under a wide range of operating conditions. Moreover, ESC also improves system robustness against external disturbance and parameter uncertainty.

Vector‐fitting‐based quantitative SSCI analysis for series‐compensated wind power systems

The occurrence of unstable subsynchronous control interaction (SSCI) between wind farms and series-compensated networks poses a considerable threat to the safety of power system. Different from the conventional eigenvalue analysis and Nyquist method, the black-box-model based RLC fitting technique becomes more attractive for the assessment of the SSCI risk. However, the method has merely been verified by radial networks whose SSCI mode can be well explained by an RLC circuit. A practical wind power system, however, can be meshed networks comprising multiple geographically distributed wind farms and series capacitors. Thus, it is questionable whether the application of the RLC fitting technique to such systems can still yield a sufficient accuracy. To clarify this aspect, in this work, the performance of RLC fitting in various test systems was examined. The results indicated that the aggregation technique considerably influences the accuracy of the assessment, and the RLC fitting cannot be applied when multiple series capacitors are involved in the SSCI mode. To address this problem, a more advanced frequency-domain fitting technique known as vector fitting was adopted, and its robustness and effectiveness were verified considering a modified The Electric Reliability Council of Texas (ERCOT) system.

Adaptive Higher-Order Sliding Mode Control of Series-Compensated DFIG-Based Wind Farm for Sub-Synchronous Control Interaction Mitigation

Sub-synchronous control interaction (SSCI) is an oscillation phenomenon caused by the interaction of converter control and series-compensated transmission line. This paper proposes a novel adaptive higher-order sliding mode (AHOSM) control strategy for damping the SSCI of a series-compensated DFIG-based wind power system. On the basis of system modeling and oscillation mechanism analysis, SSCI suppression is converted to the current tracking control problem. Firstly, an auxiliary feedback control is employed for the nonlinear series-compensated system, then integral sliding mode functions are defined to design a second-order sliding mode control law for the equivalent system. Adaptive laws for the control gains are then conceived based on the Lyapunov function considering unknown upper bounds of uncertainty derivatives. System stability is also analyzed in detail along with adaptive laws’ design. The effectiveness of the proposed control scheme is verified under different series-compensated level, different wind speed, symmetric and asymmetric short circuit fault, and internal and external disturbances. The PI control and conventional first-order sliding mode control scheme are also executed to compare the damping effect.

Subsynchronous oscillation mitigation strategy based on adaptive band-stop filter in DFIG-based wind farms

Subsynchronous Oscillation (SSO) may exist in doubly-fed induction generator (DFIG)-based wind farms connected with a series compensated transmission network, also called subsynchronous control interaction (SSCI). Different from thermal power units, the SSO frequency of wind farm varies with the change of system operating conditions, and has the characteristic of time-varying. When the operating conditions change, the existing mitigation strategies may lose the effect. To address this issue, a SSO mitigation strategy based on adaptive band-stop filter (ABSF) is proposed in this paper: When SSO occurs in the system, the ABSF parameter is adjusted according to the oscillation frequency measured by PASTd algorithm to filter out the subsynchronous component in the RSC loop effectively, so as to break the generation path of SSO and suppress SSO. Compared with the existing mitigation strategies, the proposed strategy doesn’t need the mathematical model of the system, and can ensure strong robustness under various operating conditions. Finally, the effectiveness and robustness of the proposed strategy are verified on the Matlab/Simulink.

Mitigating subsynchronous control interaction using fractional sliding mode control of wind farm

This paper proposes a fractional sliding mode control (FSMC) based on direct torque control to mitigate subsynchronous control interaction (SSCI) in doubly-fed induction generator (DFIG)-based wind farm. The proposed FSMC realizes chattering attenuation and SSCI mitigation simultaneously. Moreover, FSMC improves the system robustness and convergence property, which make contributions to achieving effective damping of SSCI under different operating points. Given the high number of control parameters of FSMC, genetic algorithm (GA) is adopted to optimize control parameters. Impedance-based analysis, time-domain simulation, and experimental test are conducted to test the performance of FSMC as compared to subsynchronous damping control (SSDC) and terminal sliding mode control (TSMC). Results confirm the better damping capability of FSMC under different compensations and wind speeds. FSMC also shows strong robustness against parametric uncertainty and disturbance.

Subsynchronous control interaction studies in DFIG-based wind farms using selective modal analysis

In this paper the Selective Modal Analysis (SMA) method is used for reproducing the eigenvalues involved in subsynchronous control interactions (SSCI) between DFIG-based wind farms and series compensated transmission lines. The SMA method provides a reduced system matrix containing a desired part of the eigenstructure of a system without computing the eigenvalues and eigenvectors of the full system. The key concept for obtaining such reduced matrix resides in an appropriate identification of a subset of state variables and eigenvalues of the system. The intrinsic problems in identifying the corresponding subset for analyzing the SSCI phenomenon are explored and illustrated using a simplified radial system. In addition, a systematic approach for obtaining a subset with minimum dimension is proposed. The methodology consists in a two-phase iterative procedure that reduces the computational load and promotes the convergence of the SMA method to the desired eigenstructure. The proposed algorithm is validated on a relative large power system derived from a practical network.

Power System Stability with Power-Electronic Converter Interfaced Renewable Power Generation: Present Issues and Future Trends

The energy sector is currently undergoing a rapid transformation with the integration of power electronic converter (PEC)-interfaced renewable energy sources (RES), such as wind and solar photovoltaic (PV) systems, at both the transmission and distribution networks. Power system stability has been significantly influenced by this power grid transformation. This paper comprehensively reviews major power system stability issues affected due to large-scale integration of PEC-interfaced RES in power grids, with some example case studies relevant for each stability category. According to the review, stability issues are mainly originating from reduction in synchronous inertia, reduction in reactive power reserve, low short-circuit strength of the power network, and fault ride-through (FRT) strategy/capability of the PEC-interfaced RES. Decrease in synchronous inertia could affect both the rotor angle stability and the frequency stability, while decrease in short-circuit strength and reactive power reserve could cause voltage stability and rotor angle stability issues in power networks. Sub-synchronous control interactions are also receiving a lot of attention by the power industry due to increasing oscillatory stability incidents reported in power networks with PEC-interfaced RES. FRT capabilities/strategies of PEC-interfaced RES are also playing a pivotal role in power grid stability due to its influence on active and reactive power, hence more emphasis should be placed on FRT schemes of PEC-interfaced RES, since future power grids are expected to operate with 100% PEC-interfaced generation sources. Stability improvement strategies could be implemented to address multiple stability issues in PEC-interfaced power networks; however, rigorous stability studies are required to identify the optimal conditions to implement these improvement strategies. Furthermore, ongoing structural changes in power grids to accommodate remotely sited PEC-interfaced RES are also influencing the stability of power grids. Therefore, all these factors must be carefully considered by system operators when planning and operating power grids in a secure and stable manner with high penetration levels of PEC-interfaced RES.

Mitigation of Subsynchronous Control Interactions Using Multi-Terminal DC Systems

This article presents a control strategy to mitigate subsynchronous control interactions in doubly-fed induction generator (DFIG)-based wind farms. The strategy makes use of a multi-terminal dc (MTDC) system to damp subsynchronous oscillations (SSOs) by adding a supplementary damping control (SDC) to MTDC grid converters. The SDC is focused on stations with modular multilevel converters. This converter topology currently allows the integration of several high-voltage and high-power MTDC applications into modern power systems. Additionally, the case in which SDCs are added to both MTDC grid converters and DFIG converters is analyzed. The SDC is designed using an identified model of the system. The identification method injects probing signals from the converter terminals and obtains an identified model by measuring the response of the system. This method facilitates the control tuning and implementation of the SDC in practical systems in which either the equations and parameters of some equipment can be difficult to know precisely or the model of commercial equipment is not available.

Analysis and Mitigation of SSCI in DFIG Systems With Experimental Validation

Sub-synchronous oscillations (SSOs) in doubly-fed induction generator (DFIG)-based series compensated power systems are mainly caused by sub-synchronous control interaction (SSCI). SSCI is the most recently found type of sub-synchronous resonances. In this article, SSCI is elaborated and investigated by performing eigenvalue analysis on a mathematically modeled DFIG system. The occurrence of SSCI is observed and the results of eigenvalue analysis are validated through a down-scaled 7.5-kW experimental setup of a grid-connected DFIG. Based on the analysis, the proportional control parameters of the rotor-side converter (RSC) are found to be very sensitive towards the sub-synchronous modes of the system. The results obtained from both the simulation and the experimental analysis show that if the sensitive proportional parameters of the RSC are tuned properly, then the DFIG system can become immune to the SSCI for any level of series compensation.

Hardware-in-the-Loop and Field Validation of a Rotor-Side Subsynchronous Damping Controller for a Series Compensated DFIG System

In the past decade, several subsynchronous control interaction (SSCI) incidents have been reported in various large-scale grid-interfaced doubly-fed induction generator (DFIG) based wind farms. The subsynchronous oscillation caused by the SSCI endangers the safe and reliable operation of wind power systems. The DFIG's converter controls play a crucial role in such interactions; therefore, several SSCI mitigation schemes have been reported, which modify the DFIG's converter controls to stabilize the subsynchronous oscillation caused by the SSCI. However, the effectiveness of such converter control modifications has not been verified through laboratory or field experiments. This paper considers a practical rotor-side subsynchronous damping controller (RSDC) and fills the gap by first validating the RSDC's performance through controller hardware-in-the-loop (CHIL) simulations, and then implementing it in a real-world DFIG connected to a series compensated line. The RSDC fundamentally reshapes the effective impedance of the DFIG to stabilize the subsynchronous oscillation. The CHIL simulations, as well as the field tests, successfully validated the RSDC's ability to suppress the SSCI. The results enhanced the practical confidence of the SSCI mitigation strategy based on RSC's converter control modification.

SSCI mitigation of grid‐connected DFIG wind turbines with fractional‐order sliding mode controller

AbstractTo mitigate subsynchronous control interaction (SSCI) in doubly fed induction generator (DFIG)‐based wind farm, this paper proposes a robust controller for rotor‐side converter (RSC) using fractional‐order sliding mode controller (FOSMC). The proposed FOSMC can improve robustness and convergence properties of the controlled system, thus achieving SSCI damping under various operating conditions. Impedance‐based analysis and time‐domain simulation are performed to check the capability of the designed FOSMC as compared with conventional sliding mode control (SMC) and subsynchronous damping control (SSDC). Simulation results demonstrate that FOSMC can mitigate SSCI within shorter time and effectively reduce the fluctuation range of system transient responses under various operating conditions of wind speeds and compensation levels. Moreover, FOSMC also improves system robustness against parameter uncertainties and external disturbances, which is important for safe operation of realistic wind farms.