Doubly Fed Induction Generator Converters Research Articles

Mitigating SSO in an Actual DFIG-MTDC System: Field Implementation and Tests

Recently, a new type of subsynchronous oscillation (SSO) event was observed in the doubly fed induction generator (DFIG)-based wind farm connected to a multi-terminal dc (MTDC) grid. It has been established that converter-based devices can interact with each other and other grid components. Generally, it is difficult to replicate an SSO event and devise a mitigation scheme due to the complex and black-box nature of converter-based devices. This paper demonstrates the field implementation of a model-independent method to mitigate the SSO using additional band-rejection filters in the DFIG's converter controller. The proposed method is unique in the sense that the complete design and implementation were carried out without detailed models or prior EMT studies. Field results illustrate that the risk of SSO can be significantly reduced by supplementing the DFIGs with properly tuned band-rejection filters, which reshape the DFIGs' impedance characteristics at the desired frequency. The test results obtained under various operating conditions further confirm the effectiveness of the mitigation scheme. The implementation and field tests of the filter-based mitigation scheme in an actual system can provide a practical reference for solving similar SSO issues in other large-scale systems without going through detailed EMT studies.

Improved Transient Performance of a DFIG-Based Wind-Power System Using the Combined Control of Active Crowbars

A significant electromotive force is induced in the rotor circuit of a doubly fed induction generator (DFIG) due to its high vulnerability to grid faults. Therefore, the system performance must be increased with appropriate control actions that can successfully offset such abnormalities in order to provide consistent and stable operations during grid disturbances. In this regard, this paper presents a solution based on a combination of an energy storage-based crowbar and a rotor-side crowbar that makes the effective transient current and voltage suppression for wind-driven DFIG possible. The core of the solution is its ability to restrict the transient rotor and stator overcurrents and DC-link overvoltages within their prescribed limits, thereby protecting the DFIG and power converters and improving the system’s ability to ride through faults. Further, the capacity of an energy storage device for transient suppression is estimated. The results confirmed that the proposed approach not only kept the transient rotor and stator currents within ±50% of their respective rated values in severe system faults but also limited the DC-link voltage variations under ±15% of its rated value, achieving transient control objectives precisely and maintaining a stable grid connection during the faults.

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.

Performance Enhancement of Doubly Fed Induction Generator–Based Wind Farms With STATCOM in Faulty HVDC Grids

In this study, an investigation of different faults for a wind turbine–based doubly fed induction generator (DFIG) system is studied and the performance using a static compensator (STATCOM) is observed. The DFIG network is connected to a voltage source converter high-voltage dc link with a fault occurring near the wind generator network. The ride through capability of DFIG is promising with STATCOM using the proposed control strategy. The ac and dc voltage and torque oscillations are damped effectively, and improved power flow is observed. The low voltage AC grid fault occurs for an HVDC transmission, and the DFIG performance without and with STATCOM is compared, where the DFIG converter control schemes are developed using the proposed improved field-oriented control (IFOC) method. In this, the reference rotor flux value alters to a new synchronous speed value or a slighter value or a standstill depending on the stator voltage dip due to grid disturbance. This speed variation leads to introducing rotor current at that new rotor slip frequency as there is a change in the rotor speed because of the fault, which further decreases the stator flux dc component. Hence, this dc-offset constituent in the stator flux is alleviated and decays rapidly in scheming the divergence of the speed of the rotor to a new orientation speed with decay in the rotor flux. This operation is done in the inner control scheme of the rotor converter, which is quicker in response to the faults. Apart from this, the stator’s real and reactive power also changes accordingly based on the lookup table mechanism–based closed-loop control action of the pulse generator, and this power change is done in the outer loop. The analysis for DFIG and HVDC operation is verified under different faults without and with STATCOM.

Identification of Control Parameters for Converters of Doubly Fed Wind Turbines Based on Hybrid Genetic Algorithm

The accuracy of doubly fed induction generator (DFIG) models and parameters plays an important role in power system operation. This paper proposes a parameter identification method based on the hybrid genetic algorithm for the control system of DFIG converters. In the improved genetic algorithm, the generation gap value and immune strategy are adopted, and a strategy of “individual identification, elite retention, and overall identification” is proposed. The DFIG operation data information used for parameter identification considers the loss of rotor current, stator current, grid-side voltage, stator voltage, and rotor voltage. The operating data of a wind farm in Zhangjiakou, North China, were used as a test case to verify the effectiveness of the proposed parameter identification method for the Maximum Power Point Tracking (MPPT), constant speed, and constant power operation conditions of the wind turbine.

Module Lifetime Evaluation Method for the Power Converter of the DFIG Based on the Analysis of the Field Wind Speed Probability and Ambient Temperature

The junction temperature fluctuation (ΔTj) of the doubly fed induction generator (DFIG) around the synchronous speed point, deteriorates the reliability of the power converter considerably. Therefore, a lifetime evaluation method for the power converter of DFIG is proposed in this paper. Combining the speed-power curve of the generator with the grid side converter (GSC) and the rotor side converter (RSC), and using the equivalent thermo-electric network of IGBT module, the junction temperature (Tj) model of the power converter is established to calculate the average junction temperature and fundamental frequency temperature fluctuation. Based on the field measured data of the wind speed probability and ambient temperature of three wind power plants in different latitude region, the operation life curves of GSC and RSC are obtained by curve fitting, and the life spans of five type of power modules are evaluated and compared. It can be seen that the lifetime of the converter could be reduced significantly due to the ambient temperature-induced low frequency temperature fluctuations in the thermal cycling, while, the fundamental frequency thermal cycle caused by the running frequency of the converter has slight effect on the overall life.

Coordinated Control of DFIG Converters to Comply with Reactive Current Requirements in Emerging Grid Codes

The doubly-fed induction generator (DFIG) is considered to provide a low-reactance path in the negative-sequence system and naturally comply with requirements on the negative-sequence reactive current in emerging grid codes. This paper shows otherwise and how the control strategy of converters plays a key role in the formation of the active and reactive current components. After investigating the existing control strategies from the perspective of grid code compliance and showing how they fail in addressing emerging requirements on the negative-sequence reactive current, we propose a new coordinated control strategy that complies with reactive current requirements in grid codes in the positive- and negative-sequence systems. The proposed method fully takes advantage of the current and voltage capacities of both the rotor-side converter (RSC) and grid-side converter (GSC), which enables the grid code compliance of the DFIG under unbalanced three-phase voltages due to asymmetrical faults. The mathematical investigations and proposed strategy are validated with detailed simulation models using the Electric Power Research Institute (EPRI) benchmark system. The derived mathematical expressions provide analytical clarifications on the response of the DFIG in the negative-sequence system from the grid perspective.

Recurrence of Sub-Synchronous Oscillation Accident of Hornsea Wind Farm in UK and Its Suppression Strategy

A large-scale power system breakdown in the United Kingdom caused blackouts in several important cities, losing about 3.2 percent of the load and affecting nearly 1 million power users on 9 August 2019. On the basis of the accident investigation report provided by the UK National Grid, the specific reasons for the sub-synchronous oscillation of Hornsea wind farm were analyzed. The Hornsea wind power system model was established by MATLAB simulation software to reproduce the accident. To solve this problem, based on the positive and negative sequence decomposition, the control strategy of grid-side converter of doubly-fed induction generator is improved to control the positive sequence voltage of the generator terminal, which can quickly recover the voltage by compensating the reactive power at the grid side. Consequently, the influence of the fault is weakened on the Hornsea wind farm system, and the sub-synchronous oscillation of the system is suppressed. The simulation results verify the effectiveness of the proposed control strategy in suppressing the sub-synchronous oscillation of weak AC wind power system after being applied to doubly-fed induction generator, which serves as a reference for studying similar problems of offshore wind power.

Fixed switching frequency scheme for current predictive control of DFIG

This paper suggests a new current predictive control algorithm for the wind-driven doubly-fed induction generator (DFIG) based on space vector modulation (SVM) with fixed switching frequency. Combining the current predictive controller and SVM has the benefits of a predictive controller, and fixing the switching frequency improves the output waveform quality. However, if classical predictive control is used, the computational burden will still be high. The proposed algorithm uses SVM to fix the switching frequency of the rotor side converter. Additionally, by using an incremental algorithm, the proposed technique prevents examining all inputs over the prediction horizon. As a result, in the proposed current predictive algorithm the computational time is significantly reduced. The detailed state-space model of DFIG and rotor side converter are used to execute the predictive algorithm. The proposed algorithm uses the rotor current to predict the next behavior of the DFIG, and then, by using SVM, the duty cycles of 2-level voltage source inverter are obtained. Finally, the proposed controller's responses are compared to those of the SVM-based field-oriented control strategy. Simulation results of the proposed controller on a two-level voltage source inverter under a balanced three-phase power system illustrate the satisfactory active and reactive power tracking and improved quality of the inverter outputs.

Improved Short-Circuit Current Calculation of Doubly Fed Wind Turbines With Uninterrupted Excitation

With the enlarging scale of a doubly fed induction generator (DFIG) connected to a power system, the influence of short-circuit current on the system relay protection could not be ignored. Setting and configuring relay protection would be affected by an imprecise short-circuit current calculation. However, some existing studies only consider the condition that the input is the Crowbar and the rotor excitation is blocked. China's new network standard requires the output reactive support current of a DFIG and will change the characteristics of short-circuit current. To solve this problem, on the basis of analyzing the characters of the transient equivalent potential of a DFIG, the transient model of a DFIG with uninterrupted excitation is provided. Based on the characteristics of a non-abrupt change of flux linkage and the requirement of a new grid standard reactive support current, the short-circuit current calculation method of a DFIG with uninterrupted excitation is put forward. Based on the real-time digital simulator (RTDS), a digital-analog experimental platform containing the actual control unit of the DFIG converter is founded, the proposed short-circuit current root mean square (RMS) value calculating method is validated.

Harmonics Reduction using a Modified Controller for Doubly Fed Induction Generator

This paper discusses a control system for the converter of doubly fed induction generator (DFIG) driven by wind energy. The converter of DFIG consists of two main parts; rotor side converter (RSC) and grid side converter (GSC). Two advanced control methods for GSC are presented here in this paper; vector control strategy based on pulse width modulation (PWM) and direct power control (DPC). DPC is based on switching table for voltage vector selection. In addition to grid voltage position, direct and quadrature current components are used here as inputs to the switching table instead of active and reactive powers respectively. This paper proposes a modified switching table for DPC to reduce ripples and total harmonic distortion (THD) of current produced by GSC. A Simulink model for 9 MW wind system based on DFIG is given in this paper. Simulation results are given for PWM, DPC and Proposed system for GSC. In view of THD calculations and simulation results a comparative study and conclusion are given in the end of this paper.

A Common Capacitor Based Three Level STATCOM and Design of DFIG Converter for a Zero-Voltage Fault Ride-Through Capability

To meet the augmented load power demand, the doubly-fed induction generator (DFIG) based wind electrical power conversion system (WECS) is a better alternative. Further, to enhance the power flow capability and raise security margin in the power system, the STATCOM type FACTS devices can be adopted as an external reactive power source. In this paper, a three-level STATCOM coordinates the system with its dc terminal voltage is connected to the common back-to-back converters. Hence, a lookup table-based control scheme in the outer control loops is adopted in the Rotor Side Converter (RSC) and the grid side converter (GSC) of DFIG to improve power flow transfer and better dynamic as well as transient stability. Moreover, the DC capacitor bank of the STATCOM and DFIG converters connected to a common dc point. The main objectives of the work are to improve voltage mitigation, operation of DFIG during symmetrical and asymmetrical faults, and limit surge currents. The DFIG parameters like winding currents, torque, rotor speed are examined at 50%, 80% and 100% comparing with earlier works. Further, we studied the DFIG system performance at 30%, 60%, and 80% symmetrical voltage dip. Zero-voltage fault ride through is investigated with proposed technique under symmetrical and asymmetrical LG fault for super-synchronous (1.2 p.u.) speed and sub-synchronous (0.8 p.u.) rotor speed. Finally, the DFIG system performance is studied with different phases to ground faults with and without a three-level STATCOM.

Simulation of reactive power regulation of DFIG

Doubly fed induction generator (DFIG) wind power generation system is widely used in wind farm all over the world. Reactive power can be generated both in grid-side converter and generator-side converter of DFIG. In this paper, working principle and control method of DFIG are introduced, and the reactive power limit of DFIG is derived, finally reactive power regulation is simulated in Simulink.

Damping Oscillation Techniques for Wind Farm DFIG Integrated into Inter-Connected Power System

According to the new international grid codes, ancillary services from renewable energy sources are essential, especially when these sources are harnessed as distributed generators. Power oscillation damping (POD) is a supplemented feature needed from wind power plants. This paper investigates different control techniques that can support wind farms based on a doubly fed induction generator (DFIG) with the proper POD. A damping control loop (DCL) is inserted into the control circuit of the back-to-back converter of DFIG to enhance system oscillation damping. The main function of DCL is similar to that of the conventional power system stabilizer. However, under congestion situations, external regulation devices such as STATCOM are required to support system performance and maintain wind farms tracking grid code requirements. A 2-area 4-machine system which consists of three thermal power plants and one wind power plant is examined. The dynamic performance of the system is investigated using power system stabilizer (PSS) as an embedded feature with the control circuit of the thermal power plants. A comparison between the PSS and STATCOM based on system dynamic performance is performed using MATLAB/Simulink. Additionally, the particle swarm optimization technique is used to enhance the performance of the proposed control techniques, taking the voltage stability margins into consideration.

A Fault-Tolerant Control Framework for DFIG-Based Wind Energy Conversion Systems in a Hybrid Wind/PV Microgrid

This article proposes a fault-tolerant control framework for a doubly fed induction generator (DFIG)-based wind energy conversion system (WECS) in a hybrid wind/photovoltaic system (PV) microgrid structure. It implements a fractional-order sliding mode control (SMC) for the DFIG converters to mitigate the grid faults and ensure robustness against mismatched uncertainties. It also includes a shared reactive power support strategy where both the WECS and PV system participate in providing the necessary reactive current by utilizing their converters as STATCOM. The proposed approach is validated using a wind/PV system installed in a feeder of a test microgrid system subject to short-term unbalanced grid voltage faults and mismatched disturbances. Its performance is further compared to that of a standard SMC-based approach. The obtained results show that the proposed framework improved the dynamic stability of the DC voltage and enabled grid support during both symmetrical and asymmetrical grid faults. Providing fast and robust control of the converters, ensuring compliance with the new grid codes and avoiding the activations of the crowbar system are among the positive features of the proposed framework.

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.

Modified rotor-side converter control design for improving the LVRT capability of a DFIG-based WECS

The rapid integration of wind-power generation with existing power grids has caused reliability and stability concerns owing to the negative impact on the dynamic behaviors of power systems. During a fault, large electromotive force is induced in the rotor circuit of a doubly fed induction generator (DFIG) as the circuit is highly vulnerable to it. Such circumstances increase the importance of the low-voltage ride-through (LVRT) capability of a DFIG to ensure stability of the electric grid during transient conditions. Considering these factors, this study focuses on the mitigation of rotor overcurrents and DC-link voltage variations by modifying the control structure of the DFIG converter, thereby enhancing its LVRT capability. Additional voltage terms are injected into the rotor-voltage references to improve the transient behavior of the DFIG control system. In the proposed design, transient rotor currents and DC-link voltage variations are effectively suppressed. Because the voltage terms are introduced outside the current loops, there is no impact on their stability. Furthermore, electromagnetic torque oscillations during the faults are considerably suppressed. Finally, the validity of the proposed design during abnormal grid conditions is demonstrated via MATLAB/Simulink. The results confirm the feasibility and effectiveness of the improved converter control design to enhance the LVRT capability of a DFIG.

DFIG Model Suitable for SSR Research

The simulation model of doubly fed induction generator (DFIG) is the basis of the research on subsynchronous resonance (SSR) of DFIG. In this paper, the conditions that the model of DFIG is suitable for the research of subsynchronous oscillation (SSO) are analyzed firstly. Then, the drive system, doubly fed induction motor, AC / DC / AC power converter and control strategy of DFIG are modeled, and these models are unified as electromechanical model. At last, the validity of the model is verified by the example of SSR when the DFIG is sent out through series compensated transmission line.

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.

A simple approach to modelling and control of DFIG-based WECS in network reference frame

The control design of grid interfaced Doubly-Fed Induction Generator (DFIG)-based Wind Energy Conversion Systems (WECS) requires the linear time-invariant model of DFIG-WECS in a common reference frame called a network reference frame. The transient analysis of DFIG-WECS requires the detailed modelling of both mechanical and electrical systems. In this paper, we present a simple approach to develop the mathematical modelling of DFIG-WECS, which consists of the equations to carry out the initial value calculation and transient analysis of grid-connected DFIG-WECS in a network reference frame. We use the Vector Control (VC) method to control the back-to-back converters of DFIG. The stator voltage-oriented control (SVOC) is used to accomplish the decoupled vector control of DFIG. The performance analysis of grid connected DFIG-WECS is evaluated through the transient simulation under various disturbances. The results show that the transient performance of DFIG-WECS with proposed converter controllers is satisfactory under various disturbances.