Heliyon 8 (2022) e11263

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Heliyon
journal homepage: www.cell.com/heliyon

Review article

Wind energy conversion technologies and engineering approaches to

enhancing wind power generation: A review
Belachew Desalegn a, b, *, Desta Gebeyehu c, Bimrew Tamrat a
a
Bahir Dar Energy Center, Bahir Dar Institute of Technology, Bahir Dar University, P. O. Box 26, Bahir Dar, Ethiopia
b
Department of Physics, College of Natural and Computational Science, Wolaita Sodo University, P. O. Box 138, Wolaita Sodo, Ethiopia
c
Department of Physics, Addis Ababa University, P. O. Box 1176, Addis Ababa, Ethiopia

A R T I C L E I N F O A B S T R A C T

Keywords: Nowadays, engineers are toiling away to achieve the maximum possible wind energy harvesting with low costs
WECS technologies through enhancing the performances of WECSs in efforts to realize the wind power future forecasts. In fact,
PECs achieving this is basically not an easy task due to the intricacies that partly stem from the stochastic nature of
ESS devices
wind energy. Further, the efforts in this regard can also be impacted by the ongoing trends in various wind energy
Automated control strategies
Hybrid control algorithm
conversion-related technologies, and engineering approaches. Hence, the wind power optimization is determined
MBPC algorithm depending on the types of WECS technologies, output power smoothing, and design development approaches that
MBD approach be employed. Currently, the variable speed operations-based WECS technologies are generally opted in wind farm
applications. Meanwhile, power management system is the heart of a WECS, where smoothing output power with
reducing costs could be implemented. On the other hand, the automated control strategies were reported in
literatures to better optimize WECSs’ performances particularly in terms of costs compared to ESS devices. On this
basis, MBPC and hybrid control algorithms were commonly presented as the current state-of-the-art for systems
modeling, whereas MBD was preferred to be an efficient and cost-saving approach for advanced development of
automated control systems. This study aims to conduct comparative analyses on WECS technologies (with
different generators, and PECs) based on their energy harvesting capability, cost-effectiveness, and advances in
designs. Assessments of the approaches and strategies for smoothing power production are also presented. Finally,
the study concludes that trends in PECs, automated control strategies and MBD are the most compelling.

1. Introduction criteria, and their performances differ accordingly. For instance, based on
their alignment to the ground [5], WECSs generally depend on either
Wind resource is ubiquitous, and it has been rapidly emerging as the HAWTs or VAWTs, where HAWTs are extensively opted in wind power
efficient source of nonpolluting and inexhaustible energy for generating industry for their better wind energy harvesting performance. Moreover,
electric power across the globe. Indeed, electric power generation from depending on wind generator operating speed with reference to the
wind resources has been undergoing varying levels of incremental im- fluctuating wind speeds [6], WECS technologies are usually classified as
provements over the course of the last several decades in different re- the constant-speed and variable-speed technologies. Based on this clas-
gions of the world [1]. Nowadays, wind energy is second only to hydro sification criterion, various types and topologies of wind generator
(water) energy as the most powerful tributary of renewable and sus- technologies have been introduced for generating electricity from wind
tainable power in contributing to global electrification [2]. More resources. The constant-speed-based SCIG; and variable-speed-based
importantly, wind power generation has also been predicted to sustain generator technologies such as DFIG, PMSG, and EES are among the
the remarkable growths in the future, in accordance with the emission most prominent in the modern wind farm industry.
goals that were set by UNCCC [3, 4]. Perhaps, different wind energy The most recent WECSs generally depend on variable-speed generator
conversion technologies were developed and contributed for the technologies because of their outstanding efficiencies, and wider possi-
achievement of the past and recent milestones in wind power generation. bility for future enhancement. In the recent days, DFIG- and PMSG-based
These technologies can be classified into different types based on some variable-speed WECS technologies are closely competing in the global

* Corresponding author.
E-mail addresses: belachewdesalegn76@gmail.com (B. Desalegn), dgebeyeh68@gmail.com (D. Gebeyehu).

https://doi.org/10.1016/j.heliyon.2022.e11263
Received 25 February 2022; Received in revised form 26 April 2022; Accepted 20 October 2022
Available online xxxx
2405-8440/© 2022 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
B. Desalegn et al. Heliyon 8 (2022) e11263

wind energy commercial market [7]. Furthermore, the performance of technologies. This deems that power maximization demands should be
WECSs also relies on the type of mechanical linkage between wind tur- considered in association with the amount of costs and time needed to
bine and generator shaft: gearbox, and direct-drive technologies. For develop and use WECS technologies in addition to the efficiency and
instance, among the leading variable-speed technologies, multiple- and reliability of the approach employed [30]. In this sense, enlarging wind
single-gearbox systems with DFIGs are usually characterized to have low turbine blades and reinstalling grid infrastructures are related to the
dynamic performance and high energy harvesting efficiency per cost physical prototyping-based engineering approach of enhancing wind
whereas single-gearbox and direct-drive systems with PMSGs have high energy harvesting technologies for harnessing maximum wind power
dynamic capability and superior power efficiency but the PMSGs-based from wind resources. But, this approach is not feasible for enhancing
WECSs are generally costly [8, 9, 10]. Yet, even though DFIG WECS wind energy harvesting due to the fact that developing wind turbine
has been recently reported to have better cumulative advantages, future physical systems in general, and enlarging the radius of the swept area of
trends of research studies indicated that PMSG WECS could become the turbine blades in particular require much time, and high material costs.
leading choice for wind farm application as its operation is smoothly Besides, installation of wind farm infrastructure require large land re-
compatible with the extended voltage and power scales of its electrical sources, which bring another challenge to the process of power genera-
conversion components [11, 12]. Hence, the optimization of the elec- tion. Moreover, the physical prototyping-based design approach of
trical components of PMSG-based WECS is one of the major themes of the developing WECSs has several serious drawbacks. In its stage of design
recent and future research studies in the field of wind power engineering development, it was reported that this approach [31] relies on the textual
[13]. In the same time, EESG-based WECS is currently under continuous specification, which is ambiguous to analyze, and testing and validation
research studies for the better enhancement of its design efficiency in processes could lead to erroneous results that not be reversible. It also
terms of cost, size, and weight though it is relatively less popular due to pose complications to designing, and implementing the robust power
its cumulatively compromised performance in wind energy harvesting management systems for WECSs.
[9, 14]. Nowadays, the fundamental goal of enhancing WECSs is to broad-
Moreover, PECs have huge impact on the overall performance of the ening the scales of wind energy extraction from varying wind speed
grid-connected WECS technologies. Among these technologies, the two- ranges for significantly maximizing electricity generation with remark-
level (2L) – current source converter (CSC) [15, 16], and voltage source ably decreasing costs. This principle of enhancing wind energy conver-
converter (VSC) [11, 17] topologies in back-to-back (BTB) configurations sion should be met by ensuring the safety and integration of WECS
were conventionally being employed in small- and medium-scale wind technologies such as generators, power electronics converters, and grids.
farms for the last several decades; and they were usually compatible with According to research reports [32, 33], WECS technologies have prom-
DFIG-based WECS technology. Here, one of the main drawbacks of isingly improved recently, and this has enabled to maximize wind power
DFIG-based WECS is that it does not maintain operational compatibility generation at fewer costs. In addition, researchers and engineers are still
with power converters of increasing power and voltage capacities [18]. working to further improve the efficiency of WECSs in order to get
Nevertheless, besides its considerable cost advantages, this technology is further optimized output power with lower costs. Yet, power efficiency
largely suitable for the vast application in small- and medium-scale enhancement is obviously a demanding research problem as there are
onshore wind generation particularly with BTB 2L-VSC [19, 20]. On already several factors that contribute to influencing wind energy cap-
the other hand, modular multi-cell converter (MMC) [21, 22, 23] has tures, which include wind generators, PECs, control systems, environ-
been under continuous physical design development with different mental conditions, etc. In the enhanced theories and practical operations,
voltage capacities, and is recently being considered as the state-of-the-art the electricity management systems [34, 35] were proven to be the heart
particularly for application in PMSG-based wind farm industry with of the variable-speed WECS technologies; and therefore, the imple-
large-scale electricity production. The main attractive feature of mentation of efficient management systems for electric generators and
PMSG-based WECS design is that the capacity of its power electronics PECs can have a significant impact in increasing wind energy harvesting,
converter can be scalable to increasing voltage levels, which makes the and hence enhancing WECS efficiencies. On account of the intermittence
application of this technology highly desirable for multi-mega scale characteristic of wind speed, the wind power generation during the
offshore wind energy deployment though its higher cost is still the major operation of WECSs (generators, PECs, etc.) is fluctuating constantly,
impediment. Several additional designs of converter technologies were dramatically and rapidly. Due to this, a number of profound challenges
also proposed in the multiple studies for applications in the wind power were reported to be resulted from the power systems disturbances, which
industries. These include DCC [20, 24], NPC [25, 26], ANPC [27, 28], include [36, 37, 38, 39]: the grid frequency variation, the real power
etc., and they were introduced to be employed in the wind farms of disturbance, and the voltage flicker at the buses of the power grid. In
large-scale power capacities that are mainly based on PMSG systems. In other words, this creates the degraded output power quality and insta-
addition, similar studies were indicating that these converters are yet to bility in the WECSs.
be sufficiently matured for the smooth practical applications in the recent In general, wind energy has a considerable influence on the dynamic
wind farm trends. Hence, significant improvements were suggested to be behavior of power systems during regular operations and abnormal
achieved subsequently in several aspects of the named converters’ limi- conditions with increasing penetration into the grid system. Particularly,
tations that are associated with operation and maintenance costs, weight, the study of the influence of wind power on WECS transient stability has
size, and power conversion capability. become a crucial research issue nowadays [40]. This implies the main
On the other hand, enhancing wind power generation with WECS challenge in the operation of the WECSs is the robust performance under
technologies has relied for many years now on the common trend of transient fault conditions. On one hand, different power smoothing op-
maximizing electricity generation whereby continual installations of tions were reported to tackle the outlined problems such that a number of
wind power grid infrastructures are mandatory. For instance, this trend options would apply the ESS devices that include batteries, flywheel,
involves deploying of many wind farms across vast areas with the compressed air storage, and so forth [41, 42]. However, implementing
intention to capture wind resources over broader geographic ranges. ESS devices is generally not a desirable option due to the fact that these
Furthermore, various methods of design engineering have been imple- devices add high extra costs to WECSs even though they were proven to
mented to enhance WECS technologies. Accordingly, increasing the show good performance in smoothing output power. Consequently, the
radius of the swept area of wind turbine blades for extracting energy from economical and robust power smoothing system should be developed in
a larger volume of air was one of the methods revealed to enhance WECSs place of the applications of ESS devices. Accordingly, a virtual system
design at component level [29]. However, increasing electric power was widely favored by multiple engineering studies [43, 44, 45, 46], and
generation should be realized with meeting important requirements in it can be built through implementation of various automated control
the processes of developing, and installing wind energy conversion strategies, which would ultimately regulate different operating

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parameters of WECSs. Recently, a dual objectives‒control technique was characteristics of the WECSs, such as those that are based on: SCIG, DFIG,
presented to reduce the torque ripples of the turbine shaft by imple- PMSG, and EESG by identifying the most advanced designs for wind farm
menting the frequency separation principle [47, 48]. In addition, the real applications. Here, the basic comparison metrics including energy har-
current control method [49], the generator torque control strategy [50], vesting efficiency, capital cost, power reliability, and FRT capability are
real and reactive power control [51], and independent pitch control [52] being considered to demonstrate the recent trends in these systems. As
were employed to streamline the generator output power. Moreover, the the core technologies and component parts of the WECSs, this study also
kinetic energy optimization-based inertial control strategies [46, 53] discusses the development trends of various PECs for enhancing the
were demonstrated through simulation to be identified as outstandingly performances of the recent and future wind farms. Furthermore, as the
outperforming virtual power smoothing approach. keys to smoothing WECSs operations for the enhanced electricity gen-
Ultimately, reliable, efficient and effective control strategies are eration, two power engineering approaches that are based on the ESS
required to be designed in WECSs to maintain systems’ comprehensive devices, and the automated control strategies/virtual systems are
performance. In this sense, different control design strategies can be examined based on various standards including recent and future
implemented to enhance WECS technologies for the reduced costs and research perspectives. Conclusively, MBPC strategy is earned a special
smoothed output power. More importantly, hybrid [54], and consideration; and as the emerging approach for developing and evalu-
model-based predictive [55] control design strategies were highly rec- ating a design of a WECS's core component (control system), MBD
ommended in recent studies due to their robust performances that would methodology is demonstrated.
enable them to circumvent the nonlinear and unpredictable character-
istics of WECSs operations. Hybrid control strategies were demonstrated 2. WECS technologies
in [56] as being designed by combining hard control that includes pro-
portional integral derivative (PID), sliding mode control (SMC), adaptive Wind energy harvesting technologies [8, 71, 72] are configured to
control, etc., and soft control that involves fuzzy logic control (FLC), harness the energy of wind movement for generating electric power by
neural network control (NNC), genetic algorithm (GA), etc. so as to make employing various mechanical and electrical subsystems such as wind
use of the cumulative advantages of hard and soft control strategies by turbine rotors, generators, control systems, and the interconnection ap-
reducing the control complexity of the systems in improving efficiency paratuses such as possible PECs and transformers. The principal com-
and dynamic stability. In practical applications, hybrid design strategies ponents of the present-day wind turbines are the tower, the rotor, and the
could optimize the systems by alleviating the respective limitations of nacelle, which accommodate the transmission mechanisms and the
PID, SMC, FLC, NNC, etc. and by fusing their respective advantages. generator. The wind turbine harnesses the kinetic energy of wind in the
Furthermore, the fusions could also possibly be made between soft and rotor composed of two or more blades systematically tied to an electrical
soft controls, whereas the hard and soft combinations were characterized machine or generator. The main module of the mechanical design is the
in some studies [57, 58] as more efficient strategies. Similarly, gearbox, which transfigures the inadequate spinning speeds of the wind
model-based predictive control (MBPC) was prevalently demonstrated by turbine to considerable spinning speeds on the electrical machine side.
the recent research works [59, 60, 61] as the advanced strategy having The spinning of the electrical machine's shaft run by the wind turbine
appealing features, which can be utilized to develop efficient and produces electric power, whose output is preserved according to speci-
cost-effective power smoothing system. In general, the ultimate goal of fications, by making use of desirable control and supervision strategies.
implementing these strategies (including hybrid and MBPC) are to In addition to managing the electrical outputs, these control units also
establish the stringent power control systems that eventually meet involve protection schemes to protect the overall system from the
advanced operation requirements (power reliability, FRT capability, possible damage that could be caused by the sudden electrical circuit
maximum power production, overall cost optimization) for WECS tech- faults. The general structure of WECS is illustrated in Figure 1. In general,
nologies. Moreover, these control design strategies can enhance WECS the energy transforming network can be structured as four main units
technologies by reducing their overall design complexities, and thereby [73]: aerodynamic unit, comprising primarily the turbine rotor, which is
achieving rapid dynamic and transient responses. made up of blades, and turbine hub that is the bearer for blades; drive
In the end, the WECSs control design strategies can be developed and train, usually consisting of: slow-speed shaft – tied to the turbine hub,
evaluated by employing several different approaches. Basically, the speed enhancer and maximum-speed shaft – running the electrical
model-based design (MBD) approach was introduced against that of generator; electromagnetic unit, comprising basically of the electric
physical prototyping to smooth the design development and optimization generator; and electric component, involving the devices for grid inte-
processes in the particular case of complex systems including WECSs. In a gration (power electronics converter, transformer, etc.), and local grid.
number of recent studies [62, 63, 64, 65], MBD was reported to be
methodologically effective and efficient particularly for modeling and 2.1. Classifications of WECSs
evaluating WECSs control system designs based on the proposed control
design strategies. In the typical case, a WECS control system model can be WECS technologies can be divided into various classifications on the
simulated, tested, and preliminarily validated based on a model predic- basis of different criteria or factors. According to [6, 8, 10, 74], the most
tive algorithm and by using MATLAB/SIMULINK software platform, popular classification factors include: (i) WECS electric output power
external target computer, and controller. On the other hand, a complete scale (small, moderate, and large power), (ii) aerodynamic power control
confirmation of a WECS design's real performance would be challenging strategy for strong wind-speed characteristics (stall pitch control), (iii)
according to advanced research reports [66, 67, 68]. Yet, regardless of its configuration of wind generator shaft with reference to the installation
limitations, the MBD methodology was generally considered by large ground (HAWT and VAWT), (iv) type of system to deliver the electric
study projects [69, 70] as a compelling approach as it could ultimately output power (autonomous and grid-tied), (v) wind generator applicable
enable to achieve the efficient and reliable wind energy conversions, speed with reference to the changing wind speeds, (vi) site for installa-
which will result in more energy transferred to the electrical power tion of WECS (onshore and offshore), (vii) type of mechanical integration
systems without needing to build complex infrastructures, and for the across the turbine and generator shaft (with gearbox and direct-drive),
similar scales of energy extracted from the wind resources. and (viii) wind speed velocities (slow, medium, and maximum) impact-
The main objective of this study is conducting a comprehensive ing the WECS. The overall quality of wind energy conversion is generally
assessment on the most recent wind power generation-based – technol- not satisfactory with VAWTs, and hence, the modern commercial WECSs
ogy systems (turbine generators and PECs) and engineering approaches implement HAWTs with three rotor blades operation. Besides, depending
in a manner that it will have a potential contribution in helping to inspire on wind generator applicable speed with reference to the changing wind
further studies in the future. Accordingly, it aims to explore the operating speeds, WECSs can be designed for either a constant (fixed) speed

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Figure 1. Typical wind energy conversion networks and power transformation phases for enhanced electricity generation.

Table 1. Advantages (✓) and limitations (⨯) of WECSs based on: the alignment of Table 2. Qualitative comparisons of various variable-speed technologies: TG-
wind generator shaft with reference to the ground, and wind generator appli- DFIG, DD-EESG, DD-PMSG, SG-PMSG and SG-DFIG WECSs.
cable speed with reference to the changing wind speeds. Metrics TG-DFIG DD-EESG DD- SG-PMSG SG-DFIG
WECS Advantages Limitations PMSG
types
System cost Relatively Highest of Lower Lower Relatively
HAWT [75, ✓ Robust in converting wind ⨯ Increased probability of system [9, 106] lower than all than DD- than DD- lower than
76, 77] energy to electrical output failure and maintenance due the rests EESG but EESG and TG-DFIG
power design complexity higher DD-PMSG
✓ Preferable for accessing to ⨯ The wind direction adjustment is than the and
reliable wind energy extraction mandatory rests higher
DFIGs
VAWT [78, ✓ Suitable for installation and ⨯ Insufficient capability of wind
79, 80, 81] maintenance due to simplicity energy harvesting
Power Relatively Slightly Higher Slightly Relatively
of its configuration ⨯ Responsible for increased torque yield [5, 9, lower than higher than all higher Higher
✓ Not reliant on the direction of ripples and susceptible to 107] the rests than TG- the rests than TG- than TG-
wind for effective operation mechanical disturbances DFIG and DFIG and DFIG
SG-DFIG SG-DFIG
FSWT [6, ✓ No complexity in structure, ⨯ Comparatively low energy
and nearly but lower
82, 83] sually not prone to failures, harvesting capability
equal to than DD-
reliable ⨯ Maximum fatigue loads
SG-PMSG PMSG
✓ Reduced installation and ⨯ Inferior power quality to the grid
maintenance costs
Power Slightly Lower Higher Higher Higher
yield/cost lower than than the than DD- than DD- than the
VSWT [84, ✓ Superior wind energy ⨯ Extra cost due to making use of
[8, 19] SG-DFIG but rests EESG and EESG and rests
85, 86] harvesting efficiency converters, which result in
higher than slightly DD-PMSG
✓ Enhanced power quality and electrical losses
DD-EESG lower and
stability ⨯ Highly sophisticated control
and DD- than the slightly
✓ Minimized mechanical fatigue system, which adds complexity to
PMSG rests lower
loads design process
than SG-
DFIG

Reliability Low In the Higher Higher In the
application (FSWT), or for the variable-speed operation (VSWT), which [9, 87, middle than the next to middle
has cumulatively superior energy conversion performance. Further 108] rests DD-PMSG
comparisons are made among HAWT vs. VAWT, and FSWT vs. VSWT in
FRT Weak Strong Strong Strong Weak
Table 1. For instance, variable-speed WECS has outstanding energy capability
harvesting quality with minimized mechanical stress and lessened noise. [19, 74]
Moreover, variable-speed WECSs generate higher power than

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fixed-speed one, in comparison, but it necessitates advanced power 2.2.2. Doubly-fed induction generators with single-stage and three-stage
converters, control devices to offer constant frequency and fixed power gearbox (SG-DFIG and TG-DFIG)
factor, and this raises the system complexity. This paper primarily deals The general structure of WECS incorporating a DFIGs and power
with variable-speed-based HAWT (WECS) technologies. electronic converter is illustrated in Figure 3. As its naming hints, energy
harvested by the DFIG is delivered to the grid via stator and rotor
2.2. Configurations and features of prominent wind power generation windings. The converter in the rotor circuit is designed to manage
systems entirely the slip power; hence, the conversion efficiency of this system is
limited to 30% of the electric generator real power. Employing a partial-
The wind electric power generation network comprises electromag- scale (30%) converter has the benefits of minimizing cost, weight, and
netic and electrical subsystems inseparably. In addition to the electrical nacelle space necessity. The power converter generally comprises two-
generator and power electronics converter it usually includes an elec- stage voltage source converters (VSCs) tied in a BTB configuration. A
trical transformer to establish the grid voltage compliance. Nevertheless, rotor-side converter (RSC), regulates the generator torque/speed or real/
the design structure of power generation system relies on the type of reactive power, while the GSC handles the net DC-bus voltage.
WECS and on its grid interface. The WECSs can have various configura- Furthermore, since the rated power of the converter for both DFIGs
tions. Accordingly, in [5, 87], the general configurations of WECSs were with single- and triple-stage gearbox is only 30% of the systems, this
named based on a blending of two criteria: (1) the electric generator presents the special advantages in terms of start-up investment and en-
applicable speed with reference to the changing speed and (2) the type of ergy harvesting performance as opposed to the technologies with the full-
mechanical integration across turbine and generator shaft. These con- range power converters, particularly EESGs (Section 2.2.3). On the other
figurations include: hand, because of the only one-level of speed maximizing, the generator
speed is appreciably low, whereas the torque is appreciably high, and
A. Constant Speed WECS with triple-stage Gearbox. therefore, the SG-DFIG needs to be designed with an increasing diameter
B. Partial-scale variable speed WECS with triple-stage gearbox. and air gap. This sequentially results in the generation of substantial
C. Full-scale variable speed gearless WECSs. magnetizing current and considerable power losses. The main benefits
D. Partial-scale variable Speed WECS with a Single-stage Gearbox. and drawbacks of DFIG-based wind energy harvesting technologies are
E. Full-scale variable Speed WECS with a Single-stage Gearbox. summed up below [7, 12, 91, 92]:

Based on the stated criteria and under the general configurations ✓ The power converters enables two-way power transport in the rotor
listed above (from A to E), there are some specific WECSs that were circuit. The generator speed can be synchronized 30% greater than or
considered as popular in [9]. The structures, advantages and drawbacks less than the synchronous speed. Hence, the energy harvesting
of these WECSs are briefly discussed under subsections to follow. The capability is outstanding and fatigue loads on the mechanical sub-
comparisons between advanced wind energy conversion technologies are systems is insignificant.
also demonstrated based on the main requirements of electricity gener- ✓ The power converter works as a smoothing solution for grid inte-
ation in Table 2. gration and grid-side reactive power reserve. Hence, soft starters and
capacitor banks are not required.
2.2.1. Squirrel cage induction generator with triple-stage gearbox (TG-SCIG) ✓ The power converter additionally offers superior dynamic capability
The TG-SCIG system is depicted with exclusion of a power converter and reliability by alleviating power system instabilities in contrast
component in Figure 2, where the generator is tied to the grid via a soft with TG-SCIG.
starter and coupling transformer. This system is the traditional and it was ⨯ Increase in the system installation investment and its design
employed in wind energy industry since the very beginning of starting to complexity due to incorporation of the power electronics converter.
harness wind resources. The main benefits and limitations of this system ⨯ Incompatible for offshore wind industries due to the consistent
design are [88, 89, 90]: maintenance requirement by the slip rings and brushes in DFIG with
the triple-stage gearbox.
✓ Low complexity of energy harvesting structure. ⨯ FRT tractability is challenging due to the straight grid-coupled DFIG
✓ Reduced start-up and operation costs due to its cheap component, stator terminals and partial (30%) load power converter.
low-cost soft starter.
✓ Stable operation since power converter is not required. 2.2.3. The direct-drive WECS with an electrically excited synchronous
⨯ Insufficient wind energy harvesting capability due to limited (1%) generator (DD-EESG)
speed scale. The gearless variable-speed WECS with the EESG and full-load con-
⨯ Intermittence characteristics of wind speed result in grid frequency verter is illustrated in Figure 4. The design of DD-EESG is developed with
fluctuations. a rotor accompanying the field system equipped with a DC activation.
⨯ Grid disturbances generate high stress on its mechanical subsystems. The generator should be configured with an increasing number of poles

Figure 2. Structure of a constant-speed WECS with triple-stage gearbox SCIG.

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Figure 3. Configuration of wind energy harvesting system with DFIG and partial-scale power converter.

Figure 4. Structure of a gearless wind energy harvesting system with EESG and full-load converter.

Figure 5. Configuration of wind energy harvesting system with PMSG and full-scale power converter.

to enhance the ungeared system. As a result, the volume and weight of Furthermore, the cons and pros of DD-EESG are briefly summarized as
this slow-speed generator is highly larger compared to those of the triple- follows [9, 14, 93]:
stage gearbox-generators, namely TG-SCIG and TG-DFIG. The slip rings
and brushes are essential in the DD-EESG for activating windings which ✓ Full-scale PEC allows it to completely handle the frequency and
raise the necessity for system maintenance. Besides, the field winding amplitude of the voltage on the machine side.
result in power losses, and, thus deteriorating the system performance. ✓ Relatively generates high electrical power compared DFIG WECSs.

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Table 3. Summaries of recent research problems aiming at improving the limitations of each WECS outlined in Table 2.

Proposed WECSs General research problems Description of specific problems Ref.

Based on DFIG Enhancing system's FRT Enhancing power quality of grid connected system through the employment of low voltage ride through (LVRT) [95]
technology capability and protection scheme that was designed as capacitor-inductor series connection and capacitor/inductor-resistor parallel
connection.
Estimating the impact of rotor current attenuation process on the power generation stability and devices based on [96]
the real-time data service (RTDS) and physical controller of converter in helping to enhance power quality by
maintaining the safety protection for power devices.
Enhancing power quality by employing non-superconducting fault current limiter based on bridge-type flux [97]
coupling method.
Ensure to maintain system's continuous operation during voltage dips (low voltage ride through enhancement) by [98]
employing external retrofit and internal control techniques.
Compensating voltage swell by limiting the fault short circuit current through the application of dynamic voltage [99]
resistor (DVR)-FLC technique in ensuring to develop robust system of enhanced power quality.
Improving power reliability Reducing the model complexity of different DFIG-turbine systems by making use of the novel model reduction [100]
margin (MRM) as optimization strategy and the New England test system (NETS) as evaluation model; while
evaluating the damping torque contribution to stability margin from the dynamic model components of these
systems.
Employing an enhanced primary frequency response (PFR) strategy so as to reduce the pitch angle to slowly feed the [101]
active power to the grid system in improving the frequency stability of the system.
Enhancing the capability of frequency regulation by considering the interdependences among the variables [109]
including rotor speed, rotor current frequency, and power system frequency by using a novel control strategy
applied to maximum energy harvesting.
Based on PMSG Optimizing cost and reliability Implementing swap control scheme that facilitates to use the turbine-generator rotor inertia for storing surplus [102]
technology power during grid voltage dips, which ultimately helps to achieve the removal of extra hardware devices; and ensure
operation compatibility, lowering size, cost and switching losses of the system.
Reducing chattering problem, enhancing system's operation reliability, increasing its lifespan, and thus optimizing [103]
its cost by regulating the generator and grid-side converter through implementation of an enhanced power
smoothing strategy based on continuous switching control.
Solving the intricacies associated with the transient power stability by considering the insulated gate bipolar [104]
transistors' (IGBTs') excitation parameters, and employing a severe three-line-to-ground fault scheme.
Realizing a fast transient response to smooth operation of the system by employing different optimization strategies [105]
with the evaluation model based on braking chopper (BC).
Based on EESG Optimizing design Developing the robust control system design under the consideration of the wind turbine mechanical resonance, and [14]
technology by implementation the resonant damping control strategy for rotor speed and torque.
Enhancing low voltage ride through (LVRT) based on the provision of: active power in proportion to the voltage [93]
retained during voltage dip, and maximum reactive current until the voltage starts recovering.

✓ Produces reduced noise due to the reason that it is gearless. activation and slip rings are not required in gearless PMSG system, thus
⨯ Considerable system cost at installation level because of the use of its energy harvesting capability and dynamic performance is better
costly electronic components. compared to EESG. Besides, in comparison with single-stage and triple-
⨯ Requires the application of a DC source with brushes and slip rings for stage geared WECSs, advantage of DD-PMSG is that turbine noise is
the excitation of rotor winding. minimized since it is the gearless technology with independent acti-
⨯ Bigger size of geometric shapes and massive generator weight. vation system. However, until the recent moment, it was not feasible for
the wind industry to design wind generators with increased external
2.2.4. The single-stage gearbox and the direct-drive with permanent magnet diameter due to the logistics and construction technology complica-
synchronous generators (SG-PMSG and DD-PMSG) tions, which restrict the advancement of the ungeared WECSs with high
The configuration of grid-connected PMSG (single-stage geared and MW power scale.
gearless) with a wind generator and a full-range power converter The step-up transformer can be avoided in PMSG WECSs by adjusting
comprised the electric machine-side converter (MSC), DC-link capacitor, the power converters at a MV scale. In general, the main advantages and
and GSC is depicted in Figure 5. As opposed to DFIG-based WECSs for disadvantages of the full-scale power converter PMSG technologies are
which the power converter is tied in the rotor circuit to generate slip the following [8, 9, 59, 60, 94]:
power, PMSG-based WECSs use a power converter across the wind
generator stator terminals and power grid to run all the electric power ✓ Outperforming energy harvesting efficiency and no fatigue load on
generated. Hence, the efficiency of the power converter is raised from mechanical subsystems due to implementation of full-load (0–100%)
30% to 100%. The commercial price of a power converter in DFIGs and application.
WECSs with full-scale variable-speed power converter including those ✓ Autonomous real and reactive power regulation help to maintain
based on PMSGs and EESGs is nearly 5% and 7–12% (based on the outstanding FRT capability.
version of converter technology) of the entire each WECS price, respec- ✓ The electric machine is entirely detached from the grid. The power
tively [8]. A full-load (100%) power converter results in a full-variable- converters additionally ensure smooth grid integration.
speed scale (0%–100%) and the power generated by PMSG WECSs is ⨯ Due to the full-scale power converter, the start-up cost and nacelle
exceedingly high. Compared to the groups of wind generators, PMSG is space necessity as well as the whole system sophistication rise.
the most prominent in variable-speed WECSs with full-scale power ⨯ Increasing power losses in the converter deteriorate the whole power
converters. system performance.
Moreover, the gearless PMSG WECS is the most promising tech- ⨯ The sophistication of digital control system design for power con-
nology to date. Unlike direct-drive EESG technology, the external verters escalates.

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Figure 6. 2-L BTB VSC configuration for DFIG-based WECS.

2.3. Variable-speed power generation systems (DFIGs, PMSGs and EESG model reduction margin (MRM) & New England test system (NETS)
WECSs): operational characteristics and research problems [100]; an enhanced primary frequency response (PFR) [101]; etc. as it
can be seen from Table 3.
According to the discussions that have been made so far (under Various modeling strategies were also proposed by different studies
Sections 2.2.1 to 2.2.4), the variable-speed WECSs, which are based on to enhance power reliability and optimize the overall cost for PMSG-
DFIGs, PMSGs and EESG generally seem to have better overall perfor- based WECS as indicated in Table 3. For instance, swap control strat-
mance compared to traditional SCIG. On the other hand, the comparisons egy was proposed to be implemented to facilitate the application of
among triple-stage gearbox (TG)-DFIG, direct-drive (DD)-EESG, direct- turbine generator rotor inertia for storing maximum power during the
derive (DD)-PMSG, single-stage gearbox (SG)-PMSG and single-stage occurrence of grid voltage dips [102]. The most interesting part of this
gearbox (SG)-DFIG are briefly summarized in Table 2 by considering strategy is that it helps to realize the elimination of extra hardware
the fundamental operational characteristics of machines such as cost, devices by ensuring system's operation compatibility along with
power yield, power yield/cost, reliability, and FRT capability as metrics. reducing its size, cost, and switching losses. Further, chattering prob-
It can generally be interpreted that the performance of DD-EESG is lems associated with PMSG system was reported to be minimized so as
moderately good, whereas DFIGs and PMSGs are cumulatively high- to enhance power reliability, increase system's lifespan, and thus opti-
performing according to the implemented metrics or criteria, and thus mize its cost by implementing continuous switching control strategy
based on the main objectives of this study. In addition, the summaries of [103]. A severe three-line-to-ground fault [104]; and braking chopper
research problems aiming at studying the outlined limitations of these (BC) [105] schemes were also implemented to smooth the operations of
WECSs are presented in Table 3 in terms various layouts of power gen- PMSG-based WECS. On the other hand, only few studies were recently
eration systems that are generally rely on DFIG, PMSG, and EESG introduced based on the design optimization of EESG wind power
technologies. generation system, and two of them are similarly presented based on
Multiple recent research studies were largely focused on introducing [14], and [93].
different methods that could be pursued to enhance the power perfor-
mances of particularly the DFIG- and PMSG-based WECSs, as it can also 2.4. Power electronics converters advances for wind farm applications
be observed from the outlines presented in Table 3. Accordingly, DFIG-
based WECS with varying power capacities was proposed to be Power electronics converters (PECs) have become the crucial com-
enhanced by improving its FRT capability along with maintaining its ponents of the WECSs particularly that which rely on the variable-speed
operation safety based on different methods and protection schemes. For and grid operations. In general, PECs play a prime role in the wind farm
instance, the power quality of the grid connected DFIG system was applications such that their overall performances can be further
considered to be enhanced by employing low voltage ride through enhanced in helping to achieve the most important and immediate ob-
(LVRT) strategy that was developed as capacitor-inductor series jectives of wind power production. Accordingly, the objectives of
connection and capacitor/inductor-resistor parallel connection [95]; the developing PECs should be ultimately aiming at minimizing the costs of
impact of rotor current attenuation process on the DFIG system compo- wind power, ensuring the energy harvesting on the broader wind speed
nents and power stability was studied based on the real-time data service ranges, improving the power reliability, creating the fault-resilient
(RTDS) and physical controller in enabling the enhancement of power WECSs, reducing the weight and footprint of the WECSs, attaining
quality and power devices protection [96]. In addition, excellent output power quality, and enhancing the grid integration with
non-superconducting fault current limiter that was based on bridge-type the stringent grid codes. The uses of power converter technologies in
flux coupling method [97]; external retrofit and internal control tech- WECSs are stated as below:
niques [98]; and dynamic voltage resistor (DVR)-FLC technique [99]
were also proposed to enhance FRT capability based on the modeling of
A soft starter is used in TG-SCIG WECS for smoothing grid integration
DFIG system components. Moreover, the power reliability with DFIG by alleviating startup in-rush currents [6].
system operations was reported to be improved based on the imple-
A partial-scale converter is used in DFIG-based WECSs for managing
mentation of different research modeling strategies including: a novel slip power and raising speed range in the application [110].

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B. Desalegn et al. Heliyon 8 (2022) e11263

Figure 7. Multi-cell 2L-BTB VSC for PMSG-based WECS.

The full-scale converters are used in PMSG-based WECSs for decou- large-MW-based converters such as active-neutral-point-clamped
pling the generators from the grid and offering a full-speed scale (ANPC), modular multilevel converter (MMC), etc. are yet at very
operations [111]. recent development stages and they are promising to be fully matured by

A full-scale BTB converter is applicable to regulate the DC-excitation being broadly compatible with wind power industries in the future.
in EESG WECS so that the output voltage and frequency of machine Furthermore, the converter topologies whose operation characteris-
comply with the grid characteristics [112]. tics and applications are briefly stated in the preceding paragraphs, and
which generally include CSC, VSC, DCC, NPC and ANPC are broadly
In addition, PEC technologies have gone through significant im- summarized in Table 4 on the basis of their purpose of development, and
provements, and the most advanced technologies are based on the full- application and advancement trends that are pertinent to the onshore and
scale large-power operations. Nowadays, the wind industries widely offshore wind farm systems.
implement the applications of PEC technologies in power generation
systems and wind farms for achieving the maximum possible wind en- 3. Output power smoothing methods for variable-speed WECSs
ergy harvesting along with enhanced grid integration. The function of
PECs is to advance variable-speed applications prominently in DFIG and Due to the total nonlinearity of wind speed, the wind energy har-
PMSG WECSs while avoiding the necessity of a soft starter and reactive vested by WECSs is significantly alternating. A number of considerable
power balance. To ensure the grid integration of the stated WECSs, the issues are yet created by the power ripples including: the grid frequency
unsteady voltage/frequency of the wind generator must be transformed variation, the real power instability and the voltage flicker at the buses of
into a steady voltage/frequency. Hence, a broad option of energy trans- the power grid. Ultimately, this creates the inferior power quality and
forming levels can be implemented by different converter topologies. disturbances in the WECSs. Furthermore, wind energy has a profound
Larger number of these energy transforming levels have earned com- influence on the dynamic performance of WECSs in the course of regular
mercial applications, whereas others have been recommended in studies operations and transient faults particularly with the increasing deploy-
with interesting features for future advancement, while still the rests ment of grid-connected systems. This further complicates the systems
have been introduced from the variable-speed electric drives industry. failures and, hence, the study of the impact of wind energy on the power
Power converters are mainly grouped as direct and indirect: direct con- grid transient stability has become a highly compelling problem since
version employs single-stage AC/AC power converters, while indirect recently. Accordingly, one of the most important objectives in the
conversion employs two-level (AC/DC þ DC/AC) or three-level (AC/DC application of the WECS technologies is to maintain resilient operation
þ DC/DC þ DC/AC) power converters. during fault experiences.
Direct AC/AC (matrix) power converters battle with two-level (AC/ In response to the outlined challenges that can severely impact the
DC þ DC/AC) VSCs in the electric drives industry due to the removal of efficiency and competitiveness of wind power systems, different power
DC-link devices and exceeding robustness. Indirect two-level (AC/DC þ smoothing approaches have been introduced in many recent studies in
DC/AC) power converters are largely incorporated in a back-to-back aiming to achieve the various objectives of wind electricity generation by
(BTB) tied configuration. An entirely regulated AC/DC power con- enhancing the performances (power efficiency and cost-effectiveness/
verter, DC-link devices that include capacitors and inductors, and a DC/ competitiveness) of WECSs. That is, based on the diverse power
AC converter make up the BTB configuration. BTB converters can be smoothing options, the power smoothing approaches of the WECSs be
implemented as either VSCs or CSCs. For the wind energy generation categorized into two groups: one that can be implemented through the
application, BTB VSCs are efficient, economically advantageous, and applications of the Energy Storage Systems (ESSs), external hardware
robust. Another important feature of BTB VSCs topology is that they are devices-based power smoothing systems; and another that based on the
compatible for both DFIG and PMSG WECSs at small and medium-power computational control algorithms, which can be employed to develop
scales. The conventional two-level (2L) BTB VSC configuration is virtual power smoothing systems for regular WECSs that do not rely on
compatible for DFIG-based small/medium-power scale WECS (Figure 6), the external hardware storage devices (the general comparison between
whereas the multi-cell 2L-BTB VSC parallel configuration is the state-of- these power smoothing approaches is demonstrated in Table 5 based on
the-art technology for PMSG-based medium-power scale WECS various parameters). In this regard, various research studies were con-
(Figure 7). These converters smooth (characteristically) a four-quadrant ducted to enhance the power performances of WECSs based on the
application with a comparatively uncomplicated design layout. consideration of ESS devices, and with the implementation virtual power
For multi MW-power scale wind farm industries, high-voltage con- smoothing strategies (summaries of the research perspectives based on
verters are preferable according to literature. For instance, the three- both approaches are outlined in Table 6). On the other hand, several
stage diode-clamped converter (DCC) or neural-point-clamped (NPC) comparative studies and wind power-related reports unveiled the overall
converters are in large-power scale category but they are only compatible preference of the power smoothing approach without external hardware
with PMSGs, and other synchronous machine-based WECSs. Moreover, devices against the ESS devices-based approach on the basis of important

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Table 4. Popular PEC designs in modern wind farm applications: Advanced summaries based on literatures.

PEC designs Operational statuses Recent trends and advances in PEC designs
CSC [15, 16, Generally compatible with both onshore and offshore wind farm systems, whereas CSC-based several various topologies of wind PEC were continuously developed for
17, 113] the most recent developments were largely recommended by studies for offshore applications in wind farms of different layouts. On this basis, line communicated
applications CSC topology was proven to demonstrate adequate power conversion capability
particularly for onshore wind farm application during the last decades; whereas it is
recently incompatible for application in offshore wind power generation. On the
other hand, the modified topologies based on various multilevel CSC designs were
recently proposed as better candidate to enhance electricity production by
improving power reliability and quality. Moreover, PWM-CSCs were most recently
reported in literatures as the advanced designs for large-wind power applications
particularly in offshore farms.
VSC [10, 20, Comparatively received the wider acceptances in the onshore wind power Here, the conventional two-level VSCs in back-to-back configurations were
114] applications than the offshore ones, as the onshore-based VSC designs generally developed to meet compatible operation with DFIG-based WECSs of small and
operate with optimized costs medium rated power outputs; and the novel multi-cell two-level VSC topologies
were particularly designed to achieve flexible and scalable power rating with PMSG-
based WECSs so as to produce electricity at relatively higher-MW levels. However,
even though the two-level VSCs based on multi-cell designs were practically proven
to show robust wind energy conversion capability, the high material costs
associated with the construction of multi-cell converters is still recognized as a main
barrier to power production with PMSG-based systems.
MMC [23, Largely compatible with offshore-based wind farm operating systems, and less This converter technology is generally considered as a recent state-of-the-art in the
115, 116] common in the applications for onshore energy harvesting due to increasing wind farm industries. Wind farms have been utilizing this technology at varying
capacities in its power and voltage levels power scales, and voltage capacities. For instance, MMC topology with power rating
of 864 MW, and voltage capacity of 320 kV is presently being implemented at
wind farms. Besides, additional topology with more advanced power generating
capability was recently unveiled to be under construction for further enhanced
application in the future.
DCC [23, Mostly suitable for offshore wind energy harvesting application, and less applicable This converter system was introduced for multi-megawatt scale power generating
24] for onshore-based wind farms wind farm application. As it was claimed in one of the recent system modeling-based
studies, power generation through the implementation of this converter technology
can be significantly enhanced by resulting in increased electricity production and
reduced weight of the converter itself.
NPC [26, More common for offshore wind power generation application than for onshore The application of this power converter technology was limited for DFIG-based
117, 118] one wind farms though it was proven to show promising candidacy for PMSG-based
systems according to literature. Hence, critical consideration is required in this
regard to possibly work on design advancement of this converter for its broader
application in wind farms.
ANPC [27, Largely applicable for offshore-based wind farms than for onshore-based ones Three partial converters that are based on half-bridge modules make up the
28] advanced topology of this converter. Its operation can be severely affected by a
short-circuit fault, and some studies proposed methods to deal with this challenge.
One of these studies implemented a method based on separation of partial
converters so as to limit the impact of short-circuit in a single partial converter. More
works are still underway to modify the operation of this converter for enhanced
application in the future.

Table 5. ESS devices-based WECSs vs. automated systems (without external technologies in wind farms raise safety concerns as the surrounding en-
storage devices)-based WECSs. vironments could be susceptible to the potential explosion hazards from
Parameters ESS-based Automated system-based these technologies.
WECSs WECSs Consequently, the cost-competitive and high-performing power
Overall cost [46, 119, 120] High Reduced smoothing approach that does not involve ESS devices, and that which
Efficiency [45, 121] High Moderate to high can be implemented without requiring the external hardware devices
Structural load [122, 123, 124] Complex Simplified other than (for example, MPPT algorithms and internal controllers) was
Reliability [125, 126] Affected High recently opted to develop automated systems for various WECSs. Based
Safety hazards [46, 127] Probable improbable on the specific methods of virtual power smoothing approach, different
Lifespan [44, 128] Affected Improved
algorithms can be designed to regulate various operational parameters of
Level of development [66, 129] Relatively Emerging
generators and PECs in WECSs so as to achieve the power smoothing
matured objectives. Recently, a dual objectives-control method has been intro-
System's capacity (in MW) [119, Large Medium to large duced to lessen the torque fluctuations of the turbine shaft that depends
121, 130] on the frequency separation principle. The real current regulation, the
generator torque regulation, real and reactive power regulation, and
independent pitch regulation strategies have been employed to settle the
criteria. Accordingly, the capital costs of ESS devices-based wind farms generator output power disturbances. In the case of PECs, the special
are very high compared to the total power production that can be uti- features of rotor tied back-to-back voltage source PWM converter control,
lized. Moreover, the ESS devices-based WECSs usually lead to the comprising minimized flicker, variable speed fixed frequency applica-
frequent systems' components failures due to the increasing complexities tion, self-standing control capabilities for real and reactive powers, and
in whole systems’ configurations compared to the WECSs without ESS comparatively reduced converter cost and power losses have drawn the
devices. This can generally cause the systems to fatigue severely and end researchers' and manufacturers’ attentions across the world. Further-
up in the lower lifespans in addition to the possible reliability degrada- more, robust and high-performing controllers are needed to be built in
tions. Besides, the application of ESS devices particularly lithium battery WECSs to ensure reliability, enhance efficiency, and eliminate costs.

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Table 6. Summaries of research perspectives with ESS- and non-ESS-based (automated) power smoothing approaches.

Smoothing approaches Objectives of the studies Proposed power systems Employed methods Ref.
ESS devices Studying the feasibility of a wind power PMSG-based power plant Battery energy storage device [131]
installation with application of energy
storage technology
Aiming at optimizing the investment costs Large-scale wind farm Battery energy storage device [132]
of energy storage device so as to maximize
its benefits
Aiming at sizing a large-scale energy Large-scale wind farms Compressed air energy storage device [133]
storage system based on a parametric
analysis in the application to smooth power
supply based on high-scale grid integration
Investigating the use of energy storage Wind energy converter Compressed air energy storage device [134]
technology for decoupling a power
converter from electricity and smoothing
its power output
Utilizing a real world data to simulate a Wind farm Flywheel energy storage device [135]
power system operating with energy
storage device
Reducing stochastic fluctuations of wind Wind turbine Flywheel energy storage device [136]
energy
Mitigating wind energy fluctuations and Large-scale MW wind farm Hybrid energy storage device (compressed air and flywheel) [137]
augmenting power production
Automated control Enhancing power quality; Increasing power PMSG-based WECS MPPT algorithm; Continuous switching control that applies [103]
strategies production sliding mode controller
Improving power quality and reliability by MW-level PMSG-based wind Swap control scheme [102]
avoiding the necessity for extra hardware power plant
storage devices
Maximizing power yield; maintaining the PMSG-based WECS MPPT algorithm combined with pitch control strategy [138]
frequency and amplitude of the system's
output voltage
Sustaining the dynamic system frequency DFIG-based wind farm A two-phase short-term frequency response (STFR) scheme [139]
so as to maintaining the MPPT operation
Regulating the power transmission 9 MW wind farm Pitch angle control based on fuzzy logic [43]
between the wind energy harvesting
system and the load by developing a static
transfer switch
Developing the system with strong DFIG-based wind power Auto-disturbance rejection control (ADRC) [140]
robustness and adaptability by controlling generation
rotor-side PWM converter – realizing
power decoupling control objective
Tracking the MPP by controlling the rotor DFIG-based wind turbine Novel intelligent control (NIC) scheme [141]
side VSC – allowing independent control of
the generated active and reactive power
along with the rotor speed

Further, section 4 broadly focusses on the computational algorithms- (WECSs) is depicted in Figure 8. The discussion provided under this
based output power smoothing approach (automated control strate- subtitle is pertinent to variable-speed WECSs that include DFIG, and
gies) that can be particularly applicable for variable-speed WECSs with PMSG. The stator and rotor couplings of DFIG WECS are represented by
DFIG and PMSG operations. the dotted lines. The energy harvesting system in DFIG and PMSG is
transformed by RSC þ GSC and MSC þ GSC, in their respective order. The
4. Automated control for prominent variable-speed WECSs WECSs mostly comprise six control levels, in which the Level I control
loop includes rapidly changing parameters and the Level VI control loop
Control strategies empower WECS to meet the required operation consists gradually changing parameters. The stringent regulation of pa-
standard by enhancing wind energy harvesting capability, minimizing rameters in the Level I loop is crucial to attain the real and reactive power
energy costs, increasing the lifespan of WECS subsystems, simplifying demands enforced by the supervisory control in the Level VI control loop.
structural loading, decreasing turbine downtimes, and proving an The control loops additionally inspect standard and anomalous perfor-
outstanding dynamic and steady-state capabilities. Yet, the most promi- mance of WECSs. Under this subtitle, the control strategies for mechan-
nent variable speed WECSs such as those based on DFIG, and PMSG are a ical and electrical energy harvesting subsystems are analyzed
blend of aerodynamic, mechanical, electromagnetic, and electronic sys- descriptively in the subsequent paragraphs.
tems, and consequently, management of the numerous subsystems under As it is depicted in Figure 8, throughout grid irregularities, the FRT
combination of steady and transient states is challenging. In particular, control in the Level IV loop accommodates a fault enable signal sf . The
the growing interest in the grid-connected wind power development has mechanical and electrical control units in the Level I to IV loops integrate
led to further rigorous grid codes, which determine that the WECS must for superior control capability throughout grid irregularities. For
stay linked to the system even under a fault experiences and, hence, it instance, at times of grid faults, the GSC holds on to delivering real power
should offer reactive currents to compensate the grid voltages. and generates reactive power to the grid, the pitch control unit begins
The schematic illustration of the all-inclusive power systems regula- operating to slowdown energy harvesting process, and the DC chopper
tion strategy for the advanced wind energy harvesting technologies begins responding to halt the DC-bus voltage from surpassing the

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Figure 8. Schematic illustration of all-inclusive control strategy for variable-speed WECS.

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Table 7. Comparison of typical control design strategies for modern variable-

speed WECSs.
Control strategies Control design Efficiency Reliability
PID [146, 147, 148] Simple Poor High
SMC [149, 150, 151] Complex Excellent Low
FLC [72, 152, 153] Simple Moderate High
Hybrid (SMC þ FLC) [154, 155, 156] Moderate Excellent Moderate
MBPC [20, 59, 157] Moderate Excellent High

first wind farm embraces P*WF;1 and QWF;1

*
, and the nth wind farm em-
braces P*WF;n and QWF;n
*
demands from supervisory control. The power
demands from the Level VI control is embraced from the wind farm
centralized control (Level V). The wind turbines are linked to the wind
Figure 9. CP versus λT curve with different β values. farm centralized control by coupling networks to divide the real and
reactive power production levels. The wind farm centralized control
stands cautiously ready to supervise the wind turbines so that the P and Q
P(W)
references enforced by the top Level VI control loop are satisfied every
time. The aerodynamic interaction of the wind turbines is also surpassed
by the wind farm centralized control.
As illustrated in Figure 9, the wind turbine centralized control (Level
IV) comprises systems of mechanical and electrical controls. The pitch
control and yaw control are totally incorporated in mechanical control,
while RPG and FRT match electrical control. By integrating different
mechanical and electrical control units, the wind turbine centralized
control supplies real power reference P*s for wind turbine MSC (P*s and
reactive power reference Qs* for RSC), together with P*g and Qg* to the GSC.
Under regular grid operations, Qg* is adjusted to zero to preserve unity
grid PF in PMSG WECS. In DFIG WECS, grid PF is regulated via Qs*
Figure 10. Typical representation of the wind turbine in the plane (power, command while adjusting Qg* to zero [8].
rotational turbine speed) for modern WECSs. Current variable-speed WECSs employ a pitch strategy to adjust the
spinning of blades in their longitudinal axis. Like it is depicted in
Figure 8, when pitch angle β magnifies, CP subsides together with the
maximum specification range. The signals response from the WECSs that harvested wind power, and the generator power returns to the actual
include grid voltages vg , grid currents ig , generator voltages vs , generator value.
currents is , DC-link voltage vdc , generator rotational speed ωm , rotor The Level III control loop embraces maximum energy harvesting,
position angle θm , and wind speed vw are employed by different control commonly known as MPPT, grid integration, and synchronization. The
loops. For DFIG WECS, the rotor currents are regulated besides others. control system for a GSC performs to synchronize and integrate grid by
The regulation standards are attained by producing ideal gating signals making use of a phase-locked loop (PLL). The product of the grid
sr , si , and sch for the MSC/RSC, GSC, and DC chopper, sequentially. interconnection system is the input DC-bus voltage v*dc and input grid
The high-level supervisory control (Level VI) dispatches real and reactive power Qg* . For a specified grid voltage size v*dc is conven-
reactive power demands to individual wind farm linked to the grid. The

Figure 11. Ideal power versus wind speed characteristics.

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B. Desalegn et al. Heliyon 8 (2022) e11263

Figure 12. Hybrid control strategies.

Figure 13. Model-based predictive control strategy.

tionally set to be fixed as per the desired specification index of a GSC where: CP opt: Optimum Power Coefficient; λopt: Optimum Tip Speed
[142]. Ratio of turbine blades; ρ: Density of the air; R: Length of turbine blades;
The typical of the ideal power of a wind turbine is entirely hard to V: Wind Speed.
predict and “bell-shaped”. The WECSs should track the possible peak The WECS demands an intelligent tracking of the ideal power curve
powers for all wind speeds, which is corresponding to tracking the ideal such that to perform an advanced operation. To achieve this, MPP should
rotational speed. Figure 10 depicts the typical curves of the wind turbine be employed. The mechanism of MPPT control involves in regulating the
in the plane (power, rotational turbine speed). Each curve correlates to a electromagnetic torque so as to transform the mechanical speed in a
wind speed Vv . The peaks of these properties introduce the desired ideal manner that results in increasing the electrical power production.
points, which can be represented by a curve called the ideal/optimal There are four performance regions of the variable-speed WECSs and
power curve and is mathematically defined as [92]: these can be demonstrated by Figure 11. At Region I, wind speeds are too
low and inadequate to run the WECSs and generate power whereas at

ρ  π  R2  V 3 Region II, the wedge angle is remained fixed, and the regulation of the
Popt ¼ CP opt λopt 
2

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Table 8. Summaries of recent research reports on wind power maximization by Table 9. Conventional design development Vs. Model-based design
using MBPC strategy. development.
Control Proposed power Control inputs Power Ref. Conventional Development Model-Based Design (MBD)
strategy system model maximization Requirement Documents [171]: Executable Specification [176]:
Model-based Wind farm with Blade pitch angle 0.4–1.4% [61]
Hard to analyze
Not difficult – simple to comprehend
predictive unspecified number and Tip speed
Arduous to control as they change
Systems optimization– modeling overall
control of turbines ratio Physical Prototypes [172]: system environment
DFIG-based offshore Rotor angular 2% [161]
Deficient and costly
Sharing of models to enhance learning
wind farm speed
Do not allow rapid iteration and collaboration

System-level testing is ineffective
Safe validation and test configuration
PMSG-based turbine Rotor angular 20% [60]
Manual Coding [173]: Automatic Code Generation [177]:
speed and Axial

Inefficient
Avoid deficiencies from manual-coding
induction factor

Causes errors and discrepancy
Regenerate smoothly for various
A 2  3 wind farm Axial induction 2–8% [162]
Hard to reapply objectives
layout factor Traditional Testing [174]: Continuous Test and Verification [64]:
Wind farm Yaw angle and 30.4–33.2% [163]
Design and integration problems
Sense defects fully in design process
composed of 100 Blade pitch angle detected late
Minimize relying on physical prototypes
turbines
Ambiguous to feed insights back into
Reapply tests throughout the design
the design process buildout process
Two wind farms Tip speed ratio Up to 38% [164]

Ascribable Platform independence [63]:
composed of 2 and Blade pitch
Experimental Arrangement [175]:
be code implementation time has to be
turbines; and 9 angle

Conspicuous optimized
turbines

Makes good understanding of
Device-specific modulations need to be
Wind farm layout Thrust coefficient 8–21% [165] physical reality included
composed of 12 6 Real-time Simulation [169, 178]:
turbines
Not fully represent real physical systems
Wind farm layout Thrust coefficient Nearly 30% [166]
composed of 4  4 and Yaw angle
turbines rate to the DC-link voltage vdc does not transcend the specified maximum
Wind farm Yaw angle 7–11% [167] range vmax
dc .
composed of 9
turbines
4.1. Power control design strategies for wind farms

Most often, the principles and applications of conventional hard

electromagnetic torque will be enacted in a way to harness the high
control and standard (hard and soft) control designs have been proposed
possible energy for individual WECS (by MPPT strategy). In this region,
in multiple research works for: achieving the maximum power extraction
the generator power curve retains a swift progression. At Region III, the
from wind, alleviating fatigue loads on WECSs, and maintaining output
generator speed is remained fixed at its peak in contrast to an acceptable
power dynamic stability according to power quality standards. However,
torque. The rise in the wind speed results in a reduction in the coefficient
each control method has its own capabilities and limitations (compari-
CP and a gradual rise in the revived power. When the peak of the power
sons of different control strategies are given in Table 7). For example, the
generator is attained, the angle of the blades (pitch) is adjusted (Passage
conventional control design that is based on PID is not intricate and offers
from β1 to β2 ) so as to deteriorate the coefficient CP . In Region IV, when
reliable operation, but it functions this robustly only for linear models of
the wind speed sharply goes up VM , an automatic apparatus is employed
WECSs. Hence, since WECSs have mostly nonlinear characteristics,
to shut the WECS (No electricity generation) so that to avert damage.
conventional control design not be applicable well for a broad range of
More importantly, to realize maximum-energy harvesting and supply
operations. On the other hand, nonlinear control designs such as standard
the electricity to the grid, the control strategies listed below should be
hard and soft control methods generally perform better than linear or
implemented particularly in large scale-power WECSs:
conventional (PID) control method but still both hard and soft control
designs have their own specific favorable operational characteristics and

MPPT for each wind-speed range [143].
drawbacks. Hard control design includes PID (conventional), SMC

Net DC-bus voltage regulation to meet acceptable operation standards
(standard), adaptive control (standard), etc.; and soft control design
for the GSC [144].
consists of FLC, NNC, GA, etc. As it has been already indicated, standard

RPG to match the grid codes [145].
hard and soft control design strategies have proven to be efficacious over
a broad spectrums of WECSs operating regions, but there is no well-
A veracious regulation of wind electric machines and power con-
defined offset between two contradicting control objectives (i.e.
verters is indispensable to attain the outlined control strategies above.
maximum power conversion and minimum fatigue damage) when indi-
The MPPT strategy is executed by the MSC/RSC, while the GSC moni-
vidual strategies are to be implemented. For instance, SMC design
tors the remaining two strategies. Level II control generates the input
strategy is entirely efficient at modeling errors and instabilities in man-
machine/generator and grid currents (i*s and i*g ), whereas the Level I
aging the energy conversion operation, but it usually causes to introduce
control generates reference signals (sr and si ), so as to the output ma- the chattering problem in WECSs, which could eventually deteriorate the
chine and grid currents (is and ig ) track their inputs (i*s and i*g ) strictly. In lifespans of the systems’ mechanical components and degrade output
addition, the power transport across the electric machine and utility power qualities. On the other hand, FLC, NNC, GA, etc. design strategies
grid is strictly monitored by Level I control at times of both standard can smooth the complex conditions whose characteristics are unpre-
and irregular operations. Under the grid irregular operations, the excess dictable, inaccurate or have maximum degree of nonlinearity; and yet,
power across the machine and utility grid is transferred to the resistive these strategies usually demonstrate only moderate efficiencies in man-
load via a DC chopper, hence transforming the rotational power of the aging the power conversion processes.
turbine system into heat. The control unit of the DC chopper interac- More interestingly, hybrid or fusion control, and model-based pre-
tively determines the portion of power to be transferred to the resistor. dictive control (MBPC) design strategies are quite appealing due to their
The DC chopper control component detects the disturbance signal sf unique features that can circumvent the limitations posed by the char-
magnitude and supplies the reference signal sch to the DC chopper so as acteristics of individual strategies including PID, SMC, FLC, etc. Hybrid

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B. Desalegn et al. Heliyon 8 (2022) e11263

Figure 14. Rapid Control Prototyping (RCP) development process of WECS.

control design can be developed by the combination of hard and soft maximization were independently studied in [161] and [60] with
computing strategies as illustrated in Figure 12. This control design different layout models, where power production of 2% and 20% were
strategy can improve the dynamic efficiency of the WECSs by minimizing claimed to be maximized respectively. Furthermore, axial induction
the systems’ complexities along with enhancing output power stability; factor [162], and yaw angle –and– blade pitch angle [163] were
and yet, this strategy was reported to require additional high-cost to considered to be optimized as the control inputs in aiming to maximize
develop control system for the wind farm application [158]. On the other the power production of two wind farms with different layout models,
hand, MBPC design strategy was reported to have a number of attractive and varying scales and ranges of increments indicating 2–8% and
features that make it more desirable for the advanced control of the 30.4–33.2% were respectively reported to be achieved. The analyses
PECs-based power generation systems; for instance [20, 56, 159]: it is conducted so far can also apply to the rests of the studies presented in
able to handle well multivariable systems, constraints imposed on the Table 8. Based on these study results, it can be generalized that appli-
systems are also dealt with satisfactorily, online optimization can be cation of MBPC strategy for the real-world wind farm optimization lead
achieved, it is not too intricate to develop control systems, and cost op- to maximization of power production.
timizations can be better achieved during operation. Block diagram for
MBPC design is illustrated in Figure 13. To achieve the objectives of 5. WECS design development approaches
control design, the MPC strategies execute the following operations
[160]: measure the current state of the WECSs to control; predict the For the technologies as sophisticated as the WECSs, the capability to
trajectories of the WECSs to control from the current state and for a group mimic the real-world systems (mechanical, electrical, hydraulic, etc.) and
of specified reference (control input) signals; select reference (control control systems under a unified framework is indispensable to the design
input) signals that reduce the MBPC algorithm, which potentially relies development process. In line with this consideration, MBD has been
on the predicted trajectories and the references (control inputs); and recently introduced by researchers as the effective and efficient approach
apply the PEC signals for the finite amount of time. for modeling structurally sophisticated energy conversion technologies
Moreover, recent studies largely proposed MBPC design strategy particularly that of wind [168]. It allows engineers to incorporate spec-
along with various power systems control inputs, and optimization ifications into the design development process, to develop design at the
models in aiming to increase power production of different wind farms, system level, and to predict and enhance overall system performance
and WECSs/individual turbine technologies across the world. As it can be with no need to necessarily relying on hardware/physical prototypes
seen from the summaries presented in Table 8, these studies claimed [169]. In addition, it accelerates design development process, enhance
varying levels of increments in the power production by the imple- systems, and enables to minimize design development costs. For instance,
mentation of MBPC strategy with different models of wind farms and the wind turbine technology developers that employ MBD methodology
turbine technologies, and under consideration of various control inputs. achieve substantial savings when compared to traditional methods as
These increments generally seem to depend on the models of the pro- reported by recent studies. MBD has further several advantages against
posed wind farms or turbine technologies, and the considered control conventional design (the comparison between conventional design and
inputs for optimizations. For instance, a wind farm model with unspec- MBD methods is detailed in Table 9.) The major savings can be attained
ified number of turbines was studied to maximize its power production from healthier requirements analysis combined with early and contin-
based on the optimization of blade pitch angle and tip speed ratio by uous testing and verification. As requirements and designs are developed
researcher in [61], and power maximization of 0.4–1.4% was reported. applying models, defections are recognized particularly at advantageous
Similarly, DFIG, and PMSG turbine technologies-based power time, while they are at stages of development costing less to handle.

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B. Desalegn et al. Heliyon 8 (2022) e11263

Furthermore, MBD relies on the overall simulation model of the machine electricity with outstanding power capacity, particularly in the case of
and the associated control algorithm under development, i.e. the control offshore applications. On the other hand, extensive research studies are
algorithm is developed in a simulation platform that offers early vali- undergoing in enhancing the complex structural design, and optimizing
dation capabilities by simulation experiments. WECS models have a the high commercial cost of EESG-based wind energy harvesting tech-
distinguished role in MBD, and they are mainly designed in MATLAB/- nology. Yet, the recent trends in wind power-related engineering studies,
Simulink, which is widely a desirable platform particularly for simulation and technology deployments indicate the prevalence of DFIG- and PMSG-
WECSs’ control designs [170]. based systems.
As the main electrical components of WECSs, PECs occupy consid-
5.1. Key elements and methodology of MBD erable space in impacting the overall performance of wind power pro-
duction. Yet, the wind energy conversion performance of PECs depends
The improvement of WECSs is mainly determined by their efficacious on their different types of topologies and configurations. For the last
operation, which can be profoundly enhanced by self-regulating several decades, 2LVSC converter topologies in BTB configurations were
approach, which is model-based control. Study on this approach has dominantly opted for applications in wind farm industries due to the
been carried out in recent years and it is now highly introduced in in the reason that they are well proven, efficient and reliable in addition to their
wind energy industry as a way to meet challenges in energy harvesting compatibility with both DFIG- and PMSG-based WECSs. To be more
and power networks. The various stages of MBD are illustrated in specific, the conventional 2LVSC (with reduced voltage capacity) is
Figure 14. In this regard, RCP [179] becomes largely an important employed for DFIG-based system; and multi-cell 2LVSC (with extended
technology in the MBD workflow for feeding control algorithms into a voltage capacity) in BTB configuration is compatible with operation of
real physical systems. Furthermore, RCP comprises a devoted PMSG-based system. Here, the general limitation of DFIG-based system is
high-performing real-time target computer and the associated software caused by the incompatibility of its design structure with the extended
environments with which the control algorithms could be validated converter capacity, and thus it needs to be possibly advanced for appli-
effectively in the real physical system. The MBD/RCP development cations in the wind farms of increasing power capacities. On the other
process begins with modeling a WECS completely in the software envi- hand, huge consideration should be intended to optimize the commercial
ronment, namely MATLAB and Simulink. Different block models mimic cost of PMSG-based WECS that is particularly associated with its power
the real-world WECS comprises of mechanical, electrical, and hydraulic converter design.
components. These WECS components are accompanied by models of For the multi-megawatt scale wind power generation, PECs with
aerodynamic loads and are simulated by various input parameters that high-voltage capacities (including MMC, DCC, NPC, ANPC, etc.) were
include wind speed, wind direction, etc. being introduced as the viable solutions for wind power industries that
In the rapid prototyping process, the WECS simulation model pro- primarily employ PMSG systems. Some of these converter technologies
duces C code. This means that the C code is produced from the control were reported to be still at their early stages of developments, and the
algorithms or software-in-the-loop (SIL) that is developed in the model requirements for further improvements in terms of their sizes, weights,
for the supervisory control unit, and this generated c code can be power efficiency, and material/design costs have been indicated to be
employed for two objectives. First, it can be fed into high-performance met in the future. Thus, despite the fact that important milestones were
controller device system. To test this control code and the controller achieved in the WECS technologies in improving the wind-energy
device, hardware-in-the-loop (HIL) tests be implemented in place of the extraction efficiencies so far, more significant advances are still
whole hardware frameworks of WECSs. HIL requires employing the required to be made to meet highly optimized electricity generation in
WECS models of the hardware systems (mechanical, electrical, and hy- the future, which would match with UNCCC's goals (large increase in
draulic) to produce C code and feed it to a real-time target computer. power production, and significant reduction in costs by 2050 in tran-
Second, the HIL real-time target computer links to the hardware sitioning from recent trends). To be clear, these [UNCCC] goals were set
controller and mimics the characteristics of the real-world WECSs. primarily based on the anticipation that rapid and sequential improve-
Consequently, developers can test the control unit over a broader scale of ments would take place in enabling technologies. Furthermore, the cur-
operations than would be effective with the whole hardware frameworks. rent trends of wind power generation indicate that more advanced and
Eventually, by applying the similar WECS models of the real-world sys- rapid progresses are required to be made in wind energy conversion-
tems as applied during the initial processes of design development, the related engineering methods and technologies to smooth transition to-
engineers can also verify and confirm that the produced code operates wards the goals. For instance, immediate consideration should be to-
accurately like it run through desktop simulation. wards the full developments (design, power efficiency, and cost
optimization) of PECs that were recently recognized by studies as the
6. Conclusion and future prospects current state-of-the-art solution (MMC), and promising technologies
(DCC, NPC, ANPC, etc.) for future applications.
In this paper, the topologies and features of various WECSs along with In addition, it can be generally understood that power management
their power output smoothing methods, control strategies, and design system is a core component of a WECS operation as it could be imple-
approaches have been sequentially considered, and the important mented to ensure enhanced (more reliable, efficient, and safe) wind
conclusion can be drawn here. Modern WECSs generally operate as the power production along with decreasing electricity costs. As it was
variable-speed technologies so as to maximize wind power generation by indicated through this work, two different approaches be independently
ensuring the production of electricity below the rated power along with employed to achieve some objectives of electricity management: one
minimized loads on the drivetrains. Moreover, in the last few decades, approach could be built as the external hardware systems (commonly
relentless efforts were made by researchers and manufacturers in intro- referred to as ESSs) to WECSs, and another could be designed as virtual
ducing various enhanced WECS technologies that have already contrib- systems by being embedded in WECSs configurations. The ESSs are
uted to the globally maximized power production, improved power usually characterized to have reliably high capacities of storing wind
reliability and quality along with reduced costs of wind energy. In this energy that could be used during power peak times; whereas the virtual
regard, the DFIG-based wind energy conversion technology is the systems could be internally implemented to further enhance the gener-
dominant system largely in onshore wind energy industries, and its high ators and PECs so that their operations are smoothed stringently, and
power production per cost performance makes it exceedingly desirable; power demands are automated. Here, the main recent challenge in
whereas PMSG-based system has recently become to challenge DFIG commissioning the wind farms with application of ESS devices-based
system's future global power generation share due its increasingly WECSs is the high associated capital and operating costs, which need
emerging electrical components that can better smooth the production of to be significantly optimized in the future through the possible

17
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