Control Strategies of DFIG Technology-based Variable-Speed Wind Turbines-A Review

This review paper examines the advancements and limitations of wind energy technology, while concentrating on the utilization of Doubly Fed Induction Generators (DFIG) to capture maximum power in variable speed winds. The paper evaluates the efficacy of several control strategies for DFIG relying on WT (wind turbines), on the basis of their simulation results, key features, and control objectives. The paper highlights the potential areas for investigation to improvise the performance as well as efficiency of wind energy generation.


Introduction
The urgent need to give maintaining India's power generating resources top priority has been brought home by the rising demand for electricity.The thermal power plant is the main generator of electricity out of all of these resources [1][2][3].It is crucial to guarantee the thermal power plant's continuous operation.Unfortunately, equipment breakdowns cannot be prevented, but their frequency can be decreased by following an effective maintenance approach [4].An essential indicator of system performance under predetermined operating circumstances is availability.Spontaneous failures of thermal power plants (TPP) are caused by wrong maintenance, inappropriate operation, and inadequate design [5].Modern engineering challenges are frequently solved using optimization approaches.Making the best or most efficient use of a circumstance or resources is what it means to do.Each firm must use an optimization approach tailored to their needs in order to thrive in the fiercely competitive marketplace [6].For the accessibility assessment of the TPP, the study done in [7] illustrates the application of a Markov-based probabilistic technique.The maintenance record of the TPP is where servicing data or breakdown data are gathered.
The most potential renewable energy source in light of the worldwide movement toward sustainable energy sources is wind power generation [8].In order to avoid using fossil fuels to generate electricity, substantial attention is drawn for harnessing renewable energy like wind power [9][10][11][12][13].When wind blows through wind turbines, it harnesses its energy to generate electricity mechanically.There are two categories of generators used in wind turbines: fixed speed as well as adjustable speed generators [14].
There is a growth in the use of non-conventional resources, like wind turbines, hydropower, solar panels, and biomass, to meet energy requirement of the society which leads issue of global-warming and the terrible environmental pollution.The best possible use of all these non-conventional energy sources is being pursued by a large number of engineers and institutions.One of the alternative energy sources that has been substantial to the development of civilization is wind energy.Due to a lack of technology in prior generations, large-scale production of non-conventional energy sources was not possible.However, in recent years, electricity output has increased exponentially because to advances in wind turbine technology.Due to many developments and enhanced performance in numerous Wind Energy Conversion Systems (WECS), wind power has gained its own significance.Modern wind power systems are being used to generate the majority of the world's electricity.One of the greatest markets for wind energy is Germany.The industrialization and population expansion that have occurred during the past few decades, especially in developing nations, have significantly increased the world's need for electrical energy.Additionally, the price of energy is rising since there are less non-renewable resources available to feed power plants.The significant energy demands of the planet, which might increase by 50% or more by 2030 [15], are best met by renewable energies.
The study emphasizes how important renewable energy is as a source for generation energy and how the wind energy has significant possibilities in achieving the requirements.It describes several generator types and describes the DFIG as a key component of wind turbine technology.The study additionally elaborates on DFIGbased system control methodologies, providing helpful information for maximizing wind energy conversion.Overall, this research offers in-depth knowledge about improving the efficacy and efficiency of wind energy generation, which helps to produce sustainable energy for future generations.
DFIG-based units are extensively used in a variety of wind turbine (WT) technologies due to the benefits related to requirement of modest converters (where, generator rating=1/3rd).Since active and reactive power may still be controlled separately, this lowers their cost in compared to other variable speed WTs [16].In the literature, a DFIG is suggested as a viable solution for installations of wind turbine with varying speeds.The adoption of this DFIG was prompted by a variety of factors.The foremost is that these machines are renowned for their longevity along with minimal need for mechanical component maintenance.Their potential for both proactive and preventative control power is the second factor.The generator rotor can, however, only transmit a portion of the system's total power when grid is connected to it, through back-to-back converters.The losses along with the price of the power electronics converters' components are subsequently decreased [17].

Types of Generators
There are various types of Generators.They are briefly explained below.

Synchronous generator (SG).
The synchronous generator are the type of generators used in variable speed turbine applications, that provides electricity at grid frequency because of its slow rotating synchronous speed; as such, it may be a viable alternative for variable speed operation.A pitch control system is not required since it is possible to reduce the cost of the turbine without endangering the turbine along with the generator.SG [19] will generate power with variable voltage and frequency.

Induction generator (IG).
The induction generator (IG) uses the same technology as an induction motor to generate electricity, while synchronous generators use an alternating-current design.To rotate their rotor faster than synchronous speed, IG uses mechanical means.While the rotor flux still generates currents in the stator, the opposing rotor flux now splits the stator coils, resulting in production of active current in the stator coils.When a generator is running, the rotor rotates much faster than synchronous speed (negative slip).Due to this, the motor may once again generate electricity, recharging the electricity grid [18].1285 (2024) 012007 IOP Publishing doi:10.1088/1755-1315/1285/1/0120073 2.1.3.DFIG.The DFIG is a well-liked series of this kind of generator which absorbs the maximum energy from variable speed winds and employs an electronic power interface to control the rotor currents.A connected gearbox should be used to match the speed ranges of the rotor and generator.Less than one-fourth of the total output power is captured by the rotor's operation since it is controlled by power electronics switches.For variable frequency drives, the power switching converter converts electricity from AC to DC to AC. IGBT converters and other soft switches with high stability are frequently constructed using the following materials: The grid side converter along with rotor side converter was attached together via a DC-link.By properly separating the grid frequency from the rotor mechanical frequency, the power switching converter also advances variable speed operation.Rotor receives the power from the power switching converter architecture, and the power converter located at the side of rotor regulates active as well as reactive powers at the generator with regulated harmonics, ensuring an adequate power factor on the grid [18].
Figure 2. Basic structure of a wind power generation network [12].
Two distinct operating technologies for wind farms are there, such as variable-speed as well as fixed-speed, respectively.When using a fixed-speed WEC system, the wind farm's output was previously constrained to a specific rotating speed.Variable-speed WEC sy Stems eventually become accessible.In comparison to fixed-speed WEC systems, these systems have a number of advantages, one of which being the capacity to adjust speed in response to wind speed variations.The tower, gearbox, and other drive train parts endure less wear and tear as a result of this increase in efficacy.Additionally, by reducing power fluctuations supplied to the grid, these devices increase energy productivity [19].
The following are some DFIG wind turbine advantages. 1) The DFIG system's generator/converter cost is reduced by 25-30% thanks to the rated power converter.2) The design of DFIG wind turbines is straightforward and easy to understand.3) The DFIG system requires a smaller filter since it uses a partial converter rather than a full-scale converter.4) DFIG is capable of operating at both sub-as well as super-synchronous speeds.5) The DFIG system's partial power converter causes low harmonics to be introduced into the grid.

Control Strategies of DFIG.
A wind based turbine with variable speed might harness the wind for more energy production than previous models due to the fact that DFIG-based variable-speed wind turbines comprised of back-to-back converters (AC-DC-AC converters) in the rotor circuit for operating at wind speeds which are either higher or lower as compared to the frequency of the grid (i.e; 50 Hz or 60 Hz).The standard vector controller is also utilised to provide decoupled actual and reactive power regulation in the control loops of the grid-side converter (GSC) along with rotor-side converter (RSC) [21].The DFIG is an extensively utilised wind turbines (WTs) in wind farms, and has grown in popularity in consequence of the benefits of the generator's variable speed, the small converter capacity, and the separate management of the reactive power along with the active power [22].There are various advantages as well as disadvantages of different control methods for wind turbines, few of them are shown in table 1  More number of PI controllers [22] Sequence domain control (SDC), modified DPC Eliminates current harmonics and is able to sustain voltage disturbances SDC requires higher switching frequencies since there are more PI controllers, making SDC more complex. [23]

Related Work
For wind based turbines, FLC tunned PI which is an interval type-2 for the adjustment of optimal torque have been utilised by a control strategy driven through DFIG has been developed by Hamdan, I., et al. [31] and published in Nature Communications.The proposed control reduces mechanical rotor speed error which leads to boost up the improve output torque and performance of power, which raises the output of the entire research system.The recommended control integrates the strengths of the two methods: the adaptive features of interval type-2 FLC and the quick reaction of conventional PI control.The focus of the study is nine megawatt wind farm, which comprises of six 1.5 megawatt wind turbines.A variety of wind speed functionalities, such as change in extreme, step, and have high-wind at constant speed, are being studied.The findings show that the type-2 FLC tuned PI reacts more fast for power and optimal torque as contrasted to type-1 fuzzy tuned PI and conventional PI control.The simulation results suggest that, while comparing FLC (type -1) and conventional PI, FLC (interval type-2) may more effectively increase the wind energy based system in terms of reliability and stability.
For the world to have a sustainable future, it is necessary to generate power with minimal environmental pollution, and generation of electricity through wind is one of those sources, according to Vanitha, V. et al. ( 2022) [32].Because of its benefits over both permanent magnet synchronous generators (PMSG) as well as DFIG, such as the absence of brushes, expensive permanent magnets, slip rings, the BDFIG, or brushless DFIG, which had been developed with the use of the cascaded induction machine technology, is becoming more and more popular as a wind electric generator.Research into wind energy is substantial if a nation is to develop economically and survive throughout time.A laboratory experiment set-up is needed to simulate the wind turbine's steady-state behavior.It is crucial that the wind electric generator be able to function at a wider speed range than the DFIG for maximizing the amount of power which can be pull out from the wind turbine.The outcomes of both hardware and simulation provide as evidence for this.
Since they offer more dependability than DFIGs in wind turbines, brushless doubly fed induction machines (BDFMs) are attracting more research attention.Given the abundance of alternative ways to developing BDFMs in the literature, BDFMs currently lack a clear framework and design process.In this essay, the design of BDFMs is thoroughly reviewed by Olubamiwa, O. I., & Gule, N. (2022) [33] in context of the literature that is currently in existence.First, an illustration of how BDFMs developed from cascade induction machine systems.Modern BDFM designs have been influenced by groundbreaking research from the evolution, which is highlighted.Then, pertinent studies on various BDFM design elements of today are presented.In the literature that is currently accessible, BDFM design and optimization approaches are being investigated.
Alam, S. et al. (2022) [34] discussed a device which is used for conversion of wind energy relying on DFIG on the grounds of modeling and control approach.On the reactive power rotor, Or active power stator, and reactive power, the effects of the directly rotor current and quadrature based rotor current are being studied using the vector-oriented stator flux technique.The outcomes of the simulation for the three megawatts wind based turbine are displayed in cases of the MATLAB/Simulink environment.The DFIG rotor circuit specifies control algorithms for rotor side converters (RSC) and grid side converters (GSC), wherein the setup is statistically modelled.This approach enables decoupled regulation of electromagnetic torque (active) and reactive power.The main DFIG converter chain component has been modelled in addition to the MPPT control technique.
To accomplish voltage control, P. Khatri et al. (2022) [35] suggests maintaining a constant voltage over all buses in a power system.In this study, multiple voltage regulation control strategies for DFIGs adopted in wind power plants (WPPs) have been examined.These regulation systems are particularly well suited for injecting reactive power for maintaining the voltage at the point of interconnection (POI) during an interruption in the absence of a requirement for external components like, flexible AC transmission system (FACTS) devices in a WPP.The WPP controller utilises the reactive power, voltage control, as well as reactive current control modes, in contrast, the DFIG controllers use the reactive power, voltage control, and reactive current control modes.This study describes each control scheme's advantages and disadvantages, contrasts them, and outlines a broad range of potential areas for future research based on the categorization.The comparison results are probably going to be beneficial for scientists and engineers that study the voltage regulation of DFIG-based WPP.
Several different circumstances include the use of metaheuristic optimization methods (MOT).Reviews of MOT were written by Kumeshan, R., & Saha, A. K. (2022) [36].The simplicity and stochastic character of MOT 1285 (2024) 012007 IOP Publishing doi:10.1088/1755-1315/1285/1/0120076 are well recognized, and they have been effectively used to handle challenging engineering problems.There are other MOT types, but the focus of this research is on swarm intelligence techniques.The swarm-based MOT control of the DFIG is also covered in their work.Control of the DFIG is a particular focus for applications involving wind energy.PI controllers are typically adopted to implement control over the DFIG.The Ziegler-Nichols and Cohen-Coon methods are two well-known PI controller tuning techniques, although despite their established status, both techniques frequently fall short of the current standards for control.Additionally, it had been acknowledged that the MOT based on swarm had the drawback of effortlessly slipping into the local optimum; nevertheless, a number of alternatives have been put up to address this issue.Based on the results of using comparable strategies to other engineering problems, it is probable that doing so for the DFIG will result in exceptional results.
Ali, M. et al. ( 2022) [37] focused on the subject of DFIG in their work.In this paper, the DFIG's rotor side vector control uses a non-singular fast terminal sliding mode control (NSFTSMC) in the speed loop.Traditional control techniques, such PI controllers, frequently have slow response times, and oscillations in the transient state are quite prevalent.It takes skill to choose the gain values for the PI control since it exhibits less invariance behavior against system uncertainties.Contrarily, the SMC is popular for its quicker convergence, improved transient, steady-state performance, along with robustness.Less speed fluctuation with changes in wind speed has been produced via torque component of the current (i_q^*), which is maintained by managing the proposed NSFTSMC technique.The power converters, DFIG, and associated control methods are all modelled in detail in this article as well.Additionally, a stability study of the suggested methodology ensures that the overall system is finite time stable in practise.Matlab/Simulink is used for the comparative controller performance as well as validation.The proposed control technique yields significantly superior outcomes as compared to conventional PI-based control.
The wind turbine (WT) based on variable speeds integrated with a DFIG has drawn a lot of interest because of its numerous benefits over other concepts related to wind turbines, opines Bouzem et al. (2022) [38], who claim that this is because of the industry technologies' advancement in the wind turbine industry.The WT's effectiveness can be boosted by utilizing as much wind energy as is practical.Numerous wind system control strategies are evolving to become more intelligent in an effort to assist this quickly expanding increase in order to manage the WT around its optimal performing with high security and dependability.The sophisticated ANNcontrollers at the IFOC optimize the active and reactive powers of the generator.The simulation results produced with the aid of the Simulink/MATLAB software provides the effective performance of the system which is suggested for control design; the findings are acceptable as well as authenticate the potential of the projected strategy for maintaining the system's operation at the anticipated response.
According to Shuaibu, M. et al. (2021) [39], the electrical grid is incorporating renewable energy sources (RES) as a supplement to conventional sources and to meet the rising demand for electrical energy globally.Grid connected DFIG-based WECS are subject to network interruptions since the windings of the stator and grid are directly interconnected.The Fault Ride-Through (FRT) capabilities of a WT refers to its capacity to maintain connectivity during grid disturbances.Due to their improved dynamic performance, higher power quality, flexible regulation of active as well as reactive power, four quadrant converter operation, as well as variable speed operation, System for Conversion of Wind Energy Conversion (WECS) with DFIG are more competitive in the global electricity market.The use of compensating devices during a grid outage enables the optimal FRT compliance, protecting the converters and DFIG from overcurrents and voltage fluctuations.The DFIG-based WTs must continue to operate as well as back the stability of grid throughout disruptions in grid of about1500 ms in order to be integrated into power networks in accordance with the grid code's criteria.An overview of the methods used to ensure FRT compliance is included in the study.Modern methods for minimizing fault shortcircuit current and mitigating voltage sag/swell are also suggested in the article.
An enhanced control approach for a wind turbine based on DFIG suggested by Ali and Ouassaid (2020) [40].Particle Swarm Optimization (PSO) as well as Artificial Neural Networks (ANN) are the foundations of the devised approach.It is advised to use PSO coupled ANN to monitor MPPT when there is any variation in wind speed.PSO is used for enhancement in the DFIG's dynamic performace through maximisation of its Proportional Integral (PI) controller gains.With the use of MTLAB/Simulink environment, the suggestd control technique has been validated while utilising a 2 MW DFFIG-WT.The collected findings support the suggested PSO-PI as a useful tool to enhance the DFIGWT's dynamic behaviour; they show that overshoots are 50% less frequent than with the conventional PI.The proposed control technique also results in a faster transient response.
A BFIG-based sensorless decoupled P-Q control for a wind energy conversion system (WECS) was researched by Andreas Giannakis, Karlis, et al. (2018) [41] utilizing intelligent controllers.During the mechanical sensors' absence, the speed and location of the rotor can be estimated taking the utility of a modified phase locked loop (MPLL) method.The speed algorithm suggested under steady state conditions was implemented and experimentally validated.Additionally, the management of active power, rotor speed, reactive power, along with dc-link voltage is a key emphasis of this study.It makes use of both traditional PI (Proportional-Integral) and fuzzy logic controllers.By combining fuzzy logic with PI, which takes into consideration both the significance as well as difficulty to gain determination, the usage of the first as the second's gain tuner is addressed.
In contrast to applications that are connected to the grid, stand-alone systems require the DFIG to produce both the frequency as well as the voltage.A unique control of a variable speed wind turbine with a DFIG for standalone applications is designed and implemented in the study of Arnaltes, S., et. al. (2017) [42].The pitch actuator then regulates the rotating speed of the wind turbine for the power balancing.If the load power surpasses the amount of wind power that can be produced, a load shedding device is required.Thorough simulation results are exhibited and explained, highlighting the benefits and contributions of the suggested control strategy.
Two nonlinear methods based on DFIG model analysis are utilised to independently regulate the active as well as reactive powers supplied by the DFIG's stator side to the grid when DFIG model is fed through a direct AC-AC converter.Examples of these methods include sliding mode and fuzzy adaptive logic.These results are compared to those of the traditional PI controller in terms of reference tracking, resilience to changes in machine parameters, reaction to unexpected speed variations, according to Benkahla, M. et al. (2016) [43].
In order to satisfy the growing energy demand, it is crucial to produce energy effectively, with high efficiency and low production costs.Instead of installing new power plants or expanding their capacity, power suppliers today are searching for ways to improve the efficiency of their current systems.By choosing the best conversion and control systems, maximum power source efficiency can be attained.One of the best options for applications requiring variable speed, i.e., the DFIG has a complex control system.Field Oriented Control (FOC) for DFIG was introduced by E. Aydin et al. (2016) [44].The use of FOC simulation was used to model the DFIG in the MATLAB/Simulink application.The control algorithm is effective at controlling DC link voltage, along with active and reactive power according to the simulation results.

CONCLUSION
The paper provides valuable insights into the latest research and development in wind energy technology, which can be used to enhance the efficiency as well as effectiveness of wind energy generation and contribute towards sustainable energy production for future generations.For the DFIG found in variable-speed wind turbines, this study offers a variety of control alternatives.These strategies include PI control, pitch control, vector control, voltage control mode, bandwidth-based repetitive control, super-twisting sliding mode control, and DFIG coordination control strategy.Every control strategy has a different set of critical components, simulation outcomes, as well as control objectives.These control strategies are assessed for their efficacy based on their potential to maximise energy capture, improve robustness against grid frequency deviation, produce the best aerodynamic torque, maintain the stator output voltage along with frequency constant, keep the DFIG running perpetually during fault isolation in addition to power supply recovery, and keep the capacity of the DFIG functioning during fault isolation as well as power supply recovery.These control strategies can enhance the performance, efficiency, along with stability of the wind turbines that are based on DFIG, and their effectiveness is validated through practical applications along with simulation models.

TABLE 1 .
below BENEFITS AND DRAWBACKS OF VARIOUS CONTROL METHODS FOR WIND TURBINES

TABLE 2 .
CONTROL STRATEGIES WITH SIMULATION RESULTS OF DFIG-BASED WIND ENERGY CONVERSION SYSTEM (WECS)