DFIG-based Offshore Wind Farm Research Articles

Damping of Subsynchronous Resonance in a Hybrid System With a Steam-Turbine Generator and an Offshore Wind Farm Using a Unified Power-Flow Controller

This article proposes a unified power-flow controller (UPFC) to suppress subsynchronous resonance occurred in a hybrid power-generation system connected to an infinite bus through a series-capacitor compensated line. The hybrid power-generation system contains a steam-turbine generator (STG) and an offshore wind farm (OWF) based on doubly-fed induction generator (DFIG). The d – q axis equivalent-circuit model under three-phase balanced loading conditions is derived to establish the complete system model including the STG set, the DFIG-based OWF, the series-capacitor compensated line, the UPFC, etc. A damping controller of the proposed UPFC is designed by using pole-assignment approach based on modal control theory to effectively damp out unstable STG modes of the studied system. Both small-signal stability and transient simulation results of the studied system are systematically performed. The simulation results show that the proposed UPFC joined with the designed damping controller can effectively suppress unstable STG modes of the studied power system.

Study of resonance issues between DFIG-based offshore wind farm and HVDC transmission

Wide-frequency range of resonances can occur in HVDC-connected DFIG-based offshore wind farm (OWF), thus induce large harmonic distortion or endanger system stability. Previous studies usually treat OWF as a single-machine system without considering the switching states of individual wind turbines and collecting cables, or ignore the converter responses to wind speed variation and generation control order. In this paper, such factors are included in the modelling of OWF. Their impacts on resonance frequency, amplification level and the occurrence of negative damping are analyzed applying the resonance mode analysis method. Harmonic stability and power quality issues are demonstrated for the frequency range from several Hz to a few kHz. Simulations in MATLAB/Simulink validate the analytical results. Additionally, resonance damping strategies are discussed regarding the identified resonances in widely spread frequencies.

Frequency-Coupled Impedance Modeling and Resonance Analysis of DFIG-Based Offshore Wind Farm With HVDC Connection

Motivated by recent years' resonance incidents in wind-integrated power systems, this paper investigates the resonance-induced harmonic distortion and stability issues in doubly fed induction generator (DFIG)-based offshore wind farm (OWF) with high-voltage direct current (HVDC) grid connection. To accurately capture the dynamic characteristics of DFIG-based wind farm, a comprehensive impedance modelling considering the detailed PI control loop and DC dynamics of wind turbine as well as the cable connections of the medium-voltage (MV) collector system is conducted. Through stepwise simulation verifications, aggregated modelling of MV collector system is proved to be suitable for wideband resonance analysis. On this basis, the resonance analysis regarding grid topology change and controller parameter variation is conducted and the impact of frequency coupling on subsynchronous resonance (SSR), middle- and high-frequency resonances is analyzed using the aggregated models derived from a practical HVDC connected DFIG-based OWF. The strength or degree of the frequency coupling between the sequence impedances of wind farm, which is induced by the asymmetrical converter control of wind turbines, is found to be dependent on the impedances of all the components of the system. Moreover, case studies are conducted to demonstrate the importance of including the frequency coupling in SSR stability assessment. Simulations in MATLAB / Simulink validate the modelling and resonance analysis.

Analysis of High-Frequency Resonance in DFIG-Based Offshore Wind Farm via Long Transmission Cable

During the past two decades, the doubly fed induction generator (DFIG) based wind farm has been under rapid growth, and the increasing wind power penetration has been seen. Practically, these wind farms are connected to the three-phase ac grid through long transmission cable which can be modeled as several Π units. The impedance of this cable cannot be neglected and requires careful investigation due to its long distance. As a result, the impedance interaction between the DFIG-based wind farm and the long cable is inevitable, and may produce high-frequency resonance (HFR) in the wind farm. This paper discusses the HFR of the large-scale DFIG-based wind farm connected to the long cable. Several influencing factors, including 1) the length of the cable, 2) the output active power, and 3) the rotor speed, are investigated. The transformer leakage inductances in the transmission system are taken into consideration when investigating the HFR. Simulation validations using MATLAB/Simulink have been conducted to verify the theoretical analysis.

Transient Stability Enhancement of DFIG based Offshore Wind Farm Connected to a Power System Network using STATCOM

This paper presents the transient stability enhance of DFIG-based offshore wind farm connected power system network using STATCOM. The studied system is simulated in MATLAB/Simulink platform for study the effect of transient disturbances like, three phases to ground fault, sudden load change, voltage sag & swell. A fully aggregate model of wind DFIG is used for simulation study of system. The DFIG based wind generator, more sensitive to the grid faults than other wind generators. During transient disturbances DFIG terminal voltage reduced below the critical value and hence DFIG trips. The external reactive power support is required for stabilization of wind farm during the transient disturbances. In studied system STATCOM is used to fulfil the reactive power requirement to stabilize the wind farm. In presence of STATCOM, DFIG does not trip during transient disturbances of power system and all bus voltages & DFIG terminal voltage are also improved.

Nonlinear Controller Design for Series-Compensated DFIG-Based Wind Farms to Mitigate Subsynchronous Control Interaction

This paper proposes a nonlinear controller to mitigate subsynchronous control interaction (SSCI) in series-compensated doubly fed induction generator (DFIG)-based wind farms. The controller is designed based on partial feedback linearization (PFL) and the proposed design approach involves scrutinizing the partial feedback linearizability of the system. The stability of the internal dynamics, which is not transformed into linear autonomous subsystems by PFL, is also analyzed in the process of deriving the control laws. The frequency scanning method is used to evaluate the performance of the proposed PFL controller, and the performance is compared to that of a finely tuned conventional proportional integral controller. A grid-connected series-compensated 100-MW DFIG-based offshore wind farm is used to demonstrate the performance of the proposed scheme through the identification and mitigation of subsynchronous resonance. An analysis of the power system reveals that the resistance is negative across the entire subsynchronous frequency range, while the reactance becomes negative around 42 Hz. The proposed controller effectively mitigates SSCI, and it can be observed that it results in positive resistance and reactance values across the entire subsynchronous frequency range. Results from the eigenvalue (modal) analysis and electromagnetic transient simulation also confirm the results obtained from frequency scanning.

Stability Enhancement of a Power System With a PMSG-Based and a DFIG-Based Offshore Wind Farm Using a SVC With an Adaptive-Network-Based Fuzzy Inference System

This paper presents the stability-improvement results of a synchronous generator (SG)-based one-machine infinite-bus system with a permanent-magnet SG (PMSG)-based offshore wind farm (OWF) and a doubly fed induction generator (DFIG)-based OWF using a static VAR compensator (SVC). The operating characteristics of the studied two OWFs are simulated by an equivalent aggregated PMSG driven by an equivalent wind turbine (WT) and an equivalent aggregated DFIG driven by an equivalent WT through an equivalent gearbox, respectively. A damping controller of the SVC is designed by using adaptive-network-based fuzzy inference system (ANFIS) to contribute adequate damping characteristics to the dominant modes of the studied SG under various operating conditions. A frequency-domain approach based on a linearized system model using root-loci technique and a time-domain scheme based on a nonlinear system model subject to various disturbances are both utilized to examine the effectiveness of the proposed control scheme. It can be concluded from the simulation results that the proposed SVC joined with the ANFIS damping controller is capable of improving the stability of the studied SG system subject to different disturbances.

Power Flow Control and Stability Improvement of Connecting an Offshore Wind Farm to a One-Machine Infinite-Bus System Using a Static Synchronous Series Compensator

This paper presents the stability improvement and power-flow control results of a DFIG-based offshore wind farm (OWF) connected to a one-machine infinite-bus (OMIB) system using a static synchronous series compensator (SSSC). An oscillation damping controller (ODC) of the proposed SSSC is designed by using modal control theory to render proper damping to the dominant mode of the studied synchronous generator (SG). A frequency-domain approach based on a linearized system model using eigenvalue analysis is accomplished. A time-domain scheme based on a nonlinear system model subject to a disturbance is also performed. It can be concluded from the simulation results that the proposed SSSC joined with the designed ODC can effectively improve the stability of the studied OMIB system with an OWF under various disturbances. The inherent low-frequency oscillations of the OMIB system can also be effectively suppressed by the proposed control scheme.

Stability Improvement of a Hybrid DFIG-Based and PMSG-Based Offshore Wind Farm fed to a SG-Based Power System Using a STATCOM

This paper presents the stability-improvement results of a synchronous generator (SG)-based one-machine infinite-bus (OMIB) system with a hybrid doubly-fed induction generator (DFIG)-based and permanent-magnet synchronous generator (PMSG)-based offshore wind farm (OWF) using a static synchronous compensator (STATCOM). The operating performance of the studied PMSG-based OWF is simulated by an equivalent PMSG driven by an equivalent wind turbine (WT) while an equivalent DFIG driven by an equivalent WT through an equivalent gearbox is used to simulate the operating characteristics of the DFIG-based OWF. The hybrid OWF is connected to the point of common coupling (PCC) of the SG-based OMIB system through an undersea cable and an underground cable. The proposed STATCOM is connected to the PCC to supply the required reactive power to the SG-based power system. A frequency-domain approach based on a linearized system model using eigenvalue technique and a time-domain scheme based on a nonlinear system model subject to various disturbance conditions are both utilized to examine the effectiveness of the proposed control scheme. It can be concluded from the simulation results that the proposed STATCOM is capable of offering adequate damping characteristics to stabilize the studied OMIB system under various operating conditions.

Comparative Stability Analysis of a Combined DFIG-based and PMSG-based Offshore Wind Farm Connected to a Large Power Grid through a LCC-HVDC Link and an HVAC Line

This paper presents the comparative dynamic-stability analysis of an integration of a doubly-fed induction generator (DFIG)-based offshore wind farm (OWF) and a permanent-magnet synchronous generator (PMSG)-based OWF fed to a large power grid through a high-voltage AC (HVAC) line and a high-voltage direct-current (HVDC) link based on line-commutated converter (LCC). The operating performance of the studied 60-MW PMSG-based OWF is simulated by an equivalent PMSG driven by an equivalent wind turbine (WT) while an equivalent DFIG driven by an equivalent WT through an equivalent gearbox is used to simulate the operating characteristics of the 80MW DFIG-based OWF. A frequency-domain approach based on a linearized system model using eigenvalue technique and a time-domain scheme based on a nonlinear system model subject to various disturbance conditions are both utilized to compare the damping characteristics contributed by the HVAC line and the proposed LCC-HVDC control scheme. It can be concluded from the simulation results that the proposed LCC-HVDC link is capable of rendering better damping characteristics to stabilize the studied integrated two OWFs fed to power grid subject to a severe fault than the HVAC line.

Dynamic Stability Analysis of a DFIG-Based Offshore Wind Farm Connected to a Power Grid Through an HVDC Link

This paper presents the dynamic-stability analyzed results of an 80-MW offshore wind farm (OWF) connected to a power grid through a line-commutated high-voltage direct-current (HVDC) link. The studied OWF is simulated by an equivalent 80-MW doubly-fed induction generator (DFIG) driven by an equivalent wind turbine through an equivalent gearbox using an aggregation method. The damping controller of the rectifier current regulator (RCR) of the proposed HVDC link is designed by using modal control theory to contribute adequate damping to the studied OWF under various wind speeds and different disturbance conditions. A systematic analysis using a frequency-domain approach based on eigenvalue calculations and a time-domain scheme based on nonlinear model simulations is performed. The eigenvalue analysis is performed to validate the effectiveness of the designed damping controller under different wind speeds. The nonlinear model simulations are carried out to demonstrate the effectiveness of the designed damping controller under various disturbance conditions. It can be concluded from the simulation results that the proposed line-commutated HVDC link joined with the designed damping controller not only can render adequate damping characteristics to the studied DFIG-based OWF under various wind speeds but also effectively mitigate the fluctuations of the OWF under disturbance conditions.