Offshore Wind Farm-Grid Integration: A Review on Infrastructure, Challenges, and Grid Solutions

Recently, the penetration of renewable energy sources (RESs) into electrical power systems is witnessing a large attention due to their inexhaustibility, environmental benefits, storage capabilities, lower maintenance and stronger economy, etc. Among these RESs, offshore wind power plants (OWPP) are ones of the most widespread power plants that have emerged with regard to being competitive with other energy technologies. However, the application of power electronic converters (PECs), offshore transmission lines and large substation transformers result in considerable power quality (PQ) issues in grid connected OWPP. Moreover, due to the installation of filters for each OWPP, some other challenges such as voltage and frequency stability arise. In this regard, various customs power devices along with integration control methodologies have been implemented to deal with stated issues. Furthermore, for a smooth and reliable operation of the system, each country established various grid codes. Although various mitigation schemes and related standards for OWPP are documented separately, a comprehensive review covering these aspects has not yet addressed in the literature. The objective of this study is to compare and relate prior as well as latest developments on PQ and stability challenges and their solutions. Low voltage ride through (LVRT) schemes and associated grid codes prevalent for the interconnection of OWPP based power grid have been deliberated. In addition, various PQ issues and mitigation options such as FACTS based filters, DFIG based adaptive and conventional control algorithms, ESS based methods and LVRT requirements have been summarized and compared. Finally, recommendations and future trends for PQ improvement are highlighted at the end.


I. INTRODUCTION
Wind energy generation is increasingly becoming one of the key sources of electric energy for industrial as well as domestic use, due to its several advantages over fossil fuel counterparts [1]. This growth in wind energy can be seen from its installed capacity figures, where China is leading with a 237 GW of wind power available followed by US at around The associate editor coordinating the review of this manuscript and approving it for publication was Feng Wu. 105 GW [2]. Wind energy growth across Europe shows a remarkable improvement with installed capacity of 1.48 GW in 2008 to 25 GW in 2020. The UK and Germany having the largest installed capacity with a total of thirteen countries contributed in wind energy to achieve these numbers [3]- [5].
When it comes to offshore wind energy, the main advantage over its onshore counterpart is the consistency in generating power at a steady rate due to stronger and more consistent wind gusts. Offshore wind farms are distributed in various countries around the world, mostly in Europe, VOLUME 9, 2021 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ . It is noteworthy that offshore wind industry in the world has succeeded to add 6.1 GW of new capacity in 2019, which is considered the best year in history so far. An average annual increase of 18.6% until 2024 and 8.2% till the decade ends, whereas new annual offshore installations are expected to cruise up to achieve a milestone of around 20 GW in 2025 and 30 GW by 2030 [7]. Furthermore, the US government had announced a shared goal to deploy 30 GW of offshore wind energy in the America by 2030. The total installed offshore wind capacity in the most ambitious scenario by the European countries, including UK and Norway, will reach 450 GW and supply about 1/3 of the electricity demand. Wind energy being a zero CO 2 emitting source, has a remarkable energy balance. According to a report delivered by The Global Wind Energy Council [8], with over 140,000 wind turbines operating in nearly 70 countries, an annual CO 2 saving of 1.5 billion tons has been achieved in 2020. This figure would ramp up to 3.2 billion tons/annum of emission reduction by 2030. Although deployment of offshore wind farms managed to solve the issues of climate change by bringing about a significant reduction, the penetration of such large scale OWPP in power system results in increased PQ and system stability issues. These system challenges originate due to operational mode (standalone or grid connected) as well as structural and performance constraints by OWPP. Some of these PQ issues include flickers, harmonics, voltage fluctuations, voltage sag and swells [9]- [11]. In several works, it is reported to address the issues of PQ in OWPP using simulation studies [12], [13] as well as through hardware approaches [14], [15]. With regards to stability of OWPP connected to the grid, different aspects that are reported in studies include; steady state frequency control [16], damping control [17], voltage control [18] as well as transient stability control [19].
Stability of HVDC based OWPPs is compromised due to oscillatory interactions of power electronic converters with AC grid compensation equipment and dynamic power controllers [20], [21]. Over charging of large subsea cables and inrush currents from the connecting transformers are some of the sources of harmonic resonance oscillations in OWPP. This issue of resonance instability in OWPP is addressed in several studies, where its impact is analyzed by locating peaks (parallel resonance) and valleys (series resonance) obtained from a frequency domain scans, using different methodologies such as impedance-based approach [22] and modal analysis [23].
The grid codes usually state connection requisites of the offshore wind farms with the grid during and after a fault occurs. Several works in the literature discuss grid codes for wind power integration to the gird. For instance, authors in [24], [25] have thoroughly listed these codes for European countries, the grid code requirements of LVRT as well as active and reactive power for a wind farm connected to the grid. Mehrdad et al presented a comprehensive review on fault ride through (FRT) guidelines and corresponding grid codes in various countries [26]. Similarly, a comparative study of US and China standards on integration of wind energy with power grid is discussed in [27], while [28] investigates the technical requirements for Turkish grid.
The advances in flexible AC transmission systems (FACTS) devices used in the electric power network to overcome the PQ issues has also excelled the reliable use of OWPP in the electric grid [29], [30]. These are power electronics based high frequency switching devices combined with passive elements to mitigate PQ issues by controlling the network parameters [31]. Among those devices, static synchronous compensator (STATCOM), static VAR compensators (SVC), dynamic voltage restorer (DVR), and unified power flow controller (UPFC) are effective in resolving PQ issues like harmonics, load unbalancing and to retain voltage regulation [32]- [34]. The DVR, when employed with a proper control scheme, is an efficient device for rectifying power fluctuations occur due to intermittent wind energy. It also improves LVRT ability of a wind turbine generator in case of symmetrical and asymmetrical faults occur on grid [35], [36]. STATCOM is known for its reactive power injection and absorption ability to achieve voltage control and improved stability with fast response time [37]. It enhances the dynamic stability in line commutated HVDC offshore wind systems where conventional passive filtering fails to provide adequate reactive power compensation [29], [38].
Although preceding literature discusses methods on PQ issues and their mitigation in wind energy systems, a comprehensive review encompassing the new challenges of integration, transmission, PQ and stability with regards to offshore wind energy is not presented in an inclusive manner yet. Several studies have reported methods of PQ issue mitigation in conventional wind farms [39], [40]. Furthermore, authors reviewed aspects of stability [41], LVRT standards [42], grid codes [43] in relation to wind energy systems. The aim of this paper is to thoroughly discuss and summarize the contemporary PQ challenges faced due to the penetration of OWPPs in the power grid. Advances in stability aspects of OWPP connected grid systems have been meticulously analyzed. Updated grid code regulations and standards concerning OWPP are reviewed along with in depth breakdown of reported LVRT schemes applied to achieve a robust power output.
This paper is organized as follows: Section I introduces some latest globally installed capacity figures in wind based energy systems along with literature being published on PQ problems. The OWPP overview and different wind turbine type's attributes with regard to PQ are addressed in section II. Section III presents various PQ as well as stability challenges occurring in OWPP. Grid code requirements for interconnection of wind energy systems and LVRT requirements are summarized in section IV. Section V presents various mitigation techniques for PQ and stability issues along with LVRT methodologies in OWPP. Section VI concludes the article by pondering upon some recommendations and future trends with regards to offshore wind farm development.

II. OVERVIEW OF OFFSHORE WIND POWER PLANT (OWPP)
In the course of the past few years, various schemes for OWPP-grid interconnection have experienced enormous progress. This is further enhanced by different PECs topologies used in wind turbines for better flexibility of operation. This section reports the OWPPs integration methodologies as well as available wind turbine topologies that are prerequisite to maintain the stability and quality of the energy being supplied.

A. OWPP-GRID TRANSMISSION INTEGRATION
A generic schematic of grid connected wind power system is depicted in Fig. 1. The mechanical system includes tower, drive train, rotor blades, wind sensors and brakes [44]. Components of electrical system include the generator, electronic converters, harmonic filters on grid side, transformers, cables and common coupling point [45]. The control system components usually consist of intelligent devices responsible to maintain efficient and robust operation between electrical as well as mechanical systems [18], [46]- [49].
Nowadays, the existing transmission system technologies for long distance OWPP include, a) AC collection-AC transmission and b) AC collection-DC transmission as shown in Fig. 2. The former high voltage alternating current (HVAC) transmission ( Fig. 2(a)) is used extensively in offshore-based farms due to its simple design and mature technology. The generated voltage is usually around 700V, which is then stepped-up to 33 kV (66KV in some cases [50]) medium voltage level [51]. The transmitted power is compensated with suitable shunt reactor to counter over-voltage level and charging current issues of the subsea cable, increasing the overall transmission capacity. To further increase the transmission distance as well as capacity, low frequency AC transmission methodology is reported in literature [52], [53]. In this scheme, power is transmitted at one third of the system frequency to achieve reduced charging power, skin effect and improved real power capacity.
The AC collection -DC transmission makes use of high voltage direct current (HVDC) where turbine is linked through a cable, to a DC converter facility. Transmission is either line commutated converter (LCC) based or voltage source converter (VSC) based [54]. The active and the reactive power flow in VSC-HVDC system depend on the voltages magnitude, the phase angle δ between these voltages and the reactance of the transformer ( Fig. 2(b)). The sending end active power and the reactive power can be expressed as Similarly, for the receiving end active and reactive power are calculated by where V sA and V sB are source voltages, V cA and V cB are converter output voltages, while X TA and X TB are transformer reactances at both ends. HVDC interconnection scheme is still in development stages as it involves placement of HV switches and filter bank. This results a high initial cost particularly for large-scale interconnection of offshore and onshore stations [55]. Its advantages include independent control of active and reactive power control without the need of external voltage source. Besides, it can achieve current control by altering the triggering angles of the converters [56], [57].

B. TURBINE TOPOLOGIES
Based on speed control, there are four types of wind turbines used in OWPP. Type I turbine is a conventional squirrel cage induction generator (SCIG) based structures with fixed-ratio gearbox connected to grid through a step up transformer. The soft starter here limits the inrush current, while reactive power compensation is provided by capacitor banks as depicted in Fig. 3(a). The type II, as presented in Fig. 3(b), is another structure of the wind turbine, where it generally comprises a wound rotor induction generator (WRIG), having variable resistors in rotor circuit. Through this variable resistance, rapid rotor current control can be achieved, which helps maintain constant power during adverse wind conditions [58]. Type III turbines make use of doubly fed induction generators (DFIGs) as presented in Fig. 4(a). The possibility of having a constant-frequency AC from a DFIG when operated by a variable-speed rotor enhances the energy efficacy of the system [59]. As this type of turbine applies the power electronics converters to control the active and reactive power, it results in generating the 3 rd , 5 th and 7 th harmonic components [60]. Related pros and cons of DFIG based turbine system are listed in Fig. 4 Type IV or permanent magnetic synchronous generator (PMSG) based turbine are directly driven systems with full-scale AC-DC-AC converters ( Fig. 5(a)). The possibility to be directly connected to a wind turbine without using the gearbox is crucial in harsh weather conditions [61], [62]. PMSG based turbine are suitable for offshore installations due to advantages they offer such as high energy density, simple control schemes, no rotor winding, bidirectional power flow and self-excitation structure. Fig. 5(b) compares some aspects of the PMSG based wind turbine. The voltage and frequency from grid side can be controlled with small influence from the wind speed. Various control schemes like Pulse Width Modulation (PWM) for converter control and adaptive system learning for maximum power tracking are described in [63]- [65].

III. CHALLENGES IN OWPP
The aspect of PQ in OWPP cannot be undermined, as a single wind turbine can contribute up to 10 MW of power into the grid and it is also used as standalone DG [66]. In OWPP, due to intermittent nature of wind as well as presence of nonlinear power circuitry, bring about various PQ issues fluctuations in voltage and frequency, and circulation of harmonics that ultimately effect the system stability [67]. This section addresses these challenges, which need to be taken into consideration for continuous supply of stable energy to the customer.

A. POWER QUALITY ISSUES
PQ disturbances at OWPP are regularly monitored to check whether, output complies with the permitted grid regulations. It is worth mentioning that PQ issues include flickers, voltage sags/swell, harmonics, transients as well as voltage and frequency fluctuations. Table 2 shows some PQ issues with respect to severity to the grid [68].

1) FLICKERS
Flickers can be defined as fast variation in supplied voltage that may last for a certain period so that the variation in electric light can be recognized visually [69]- [71]. Voltage fluctuation value k can be estimated by where Un is the nominal voltage while U max and U min show voltage peak value of the half period RMS value. IEC standard 61400-21 establishes codes for defining PQ requirements of wind generation [72]. A portion of the mentioned standard makes use of the time domain readings of current and voltage taken at terminals of the wind turbine to measure flickers due to the switching action. A wind turbine produces more flickers in weak grids because of inverse relationship of flickers emission and short-circuit  capacity [73]. Another important factor to affect flickers level is grid impedance angle. Flickers emission reduction can be achieved by maintaining 90 • angle difference between grid impedance angle and turbine power factor. As variable speed turbines have the ability to manage reactive power, flickers can be alleviated by regulating this reactive power [74].

2) HARMONICS
In an OWPP penetrated grid system, the voltage source inverter stimulates some harmonics while the others harmonics are from grid background, which are reflected back to the turbine farm terminals, and therefore at the point of common coupling (PCC). In addition, the large capacitance in the form of subsea cables and compensating capacitor banks resonant frequencies influences the resonant frequencies of the system. These resonances result in the amplification of the harmonic emission in two ways. Firstly from individual turbines into the power grid i.e. primary emission. Secondly, flow of harmonic currents from the grid into the wind park termed as secondary emission [77]. The harmonic analysis of OWPP connected to the electric-grid can be conducted by an equivalent circuit depicted in Fig. 6 [82].
where I PCC and V PCC represent the current and voltage at the PCC, Z p and Z s respectively show the harmonic impedance of the wind farm side and utility grid, I h,p and V h,s are the wind farm harmonic current source and grid harmonic voltage source, respectively. Using these parameters, harmonic contribution from either side can be assessed by means of various methods like active power direction method, IEC voltage phasor method and voltage-current ratio approach [75]. In addition to harmonics, OWPP could also cause supraharmonics (SP), i.e. distortion in the frequency range above 2 kHz. SP discharges in OWF are closely associated with PWM strategy used for conversion in power inverter of wind turbine. The magnitude of SP at the PCC is usually lower than regular harmonics (3 rd , 5 th , 7th) [76]. However, lack of standard limits to quantify the effects of these high frequency emissions in OWPPs is the area that needs to be investigated. Ammar et al in [77] propose that in order to measure SP up to 9 kHz, the chosen signal-sampling rate should be in accordance with the rules of signal analysis. Nonetheless, FFT samples need to be fetched by power of two, such that the total sampling becomes 20.48 kHz, according to IEC  61000-4-7. Lastly, raw components of 5 Hz FFT resolution are distributed into 200 Hz bands, as shown in Fig. 7.

3) VOLTAGE SAG AND SWELL
Temporary reduction of the RMS voltage between 0.9 and 0.1 pu of nominal value from half-cycle to few seconds is termed as a voltage sag, while the voltage swell or over voltage is an increase in the RMS (effective) voltage to between 1.1 and 1.8 pu of normal value from half-cycle to one minute. It is in accordance with European Standard EN 50160 -IEEE 1159 [78]- [80]. A voltage sag due to wind generator penetration may result in disconnection, if voltage drops under a certain value. This voltage dip can also result in a high current through IGBT inverter which connects a PMSG to the grid [81]. Fast removal of sag and swell in such cases events is crucial. Hence, voltage and time duration limits allowed in various countries grid code and regulations are shown in Table 3 [77].

B. STABILITY ISSUES
As wind power penetration level increases in electric grid, researchers focus more towards the problem of power system stability. Stability issue in OWPP is mostly caused, either by any uncertainty occurring at wind turbine side, or a fault/ disturbance at the grid side. With an increasing number of wind farms connected, controlling dynamic characteristics of overall power systems is becoming more challenging [82]. A related phenomenon of sub synchronous resonance (SSR) has gained attention in recent years. It has been reported to cause oscillations below system's rates frequency in grid connected wind farm system in vicinity of series compensated transmission line [83], [84]. To investigate this issue, an impedance-based stability analysis (SA) is usually adopted, which makes use of ratios of analytically derived impedances of OWPP and line converters looking from the PCC side. Nyquist plots of above-mentioned impedances are mostly used to identify potential low frequencies causing SSR [85].

1) SMALL SIGNAL STABILITY
The small signal stability refers as to how a system behaves whenever a minor change occurs in its state variables [86]. This stability issue generally happens due to lack of damping torque, which leads to increase the amplitude of rotor oscillations. Any variation in the operating conditions of the power system can be analyzed through eigenvalues of the system state matrix [87], [88]. The small signal stability studies of the power system variability due to the OWPP have been reported in literature by using different schemes like singular value decomposition based least squares analysis and Prony analysis [89], [90]. Such schemes help in understanding the effects of load change on grid side as well as wind speed inconsistency on small-signal angular stability. Authors in [91] discuss the extension of small-signal stability region to a more robust small-signal stability region via hyperplane approximation, within which the power system retains stability even with perturbations due to uncertain nodal variations from OWPP. Similarly, a DFIG based direct drive control scheme is given in [92] to study small signal stability and dynamic response of the system. Comparative results proved the superiority of the proposed controller under system disturbances while linked with a single as well as fourmachine infinite bus system. Small signal stability analysis of a large variable speed wind turbines is presented in [93]. This method used bifurcation analysis to compute the boundaries of small signal stability, when system is subjected to line trip and load variations.

2) TRANSIENT STABILITY
Transient stability is the capability of the power grid to sustain synchronism when experiencing large disturbances [94]. Severe disturbances may contain component outages, reduced inertia, load variations or faults that cause large excursions of generator's rotor angle. The response of the system is affected by the nonlinear power-angle relationship. The interval of interest while observing transient stability is generally 3 to 5 sec following the disturbance. This time period can extend up to 10-20 sec in case of large system with prevailing inter-area fluctuations [99]. A study presented in [95] applies efficient feedback control scheme by utilizing mechanical power as well as DC channel power of the system as control objective to enhance the transient stability of the interconnected wind farm system. Another scheme in [96], uses an advanced real-time technique relating to corrected kinetic energy to calculate critical clearing time (CCT), to assess the transient stability of an interconnected grid with wind turbines. Schemes related to adaptive control have proved to be effective for augmenting transient stability in DFIG based grids. These methods make use of neural networks based on dynamic programming to control reactive power and improves the closed-loop performance of the wind connected power grids [97], [98]. Table 4 summarizes different SA and their solution methodologies in OWPP.

C. VOLTAGE AND FREQUENCY ISSUES
In an electrical power system, the reactive power flow is related to the voltage variation, while the frequency change corresponds to a change in the active power in the system. In an OWPP, electromechanical devices that are usually used to generate the electricity are the induction generators, which require reactive power for excitation. Therefore, DFIG based wind farm are of lesser help for supporting the grid with reactive power when compared to synchronous machines [42].

1) VOLTAGE CONTROL
With increasing level of offshore wind energy penetrations into the grid, voltage stability is becoming a challenge. PMSG based wind turbines are able to manage reactive power in case of voltage deviation at the PCC. Nevertheless, for induction based wind generators it is not the case as they accelerate during disturbances/faults, which require reactive power consumption [100]. Voltage control of a wind farm need to be designed in such a way that demand of reactive power output to be met is in accordance with the dispatch instructions form grid side to support the voltage of PCC. In this regard, voltage and reactive power control must meet the requirements below [101]: • Voltage at PCC should be maintained ranging from 97% to 105% of the nominal voltage at grid side • Its speed of regulation and control accuracy should meet the demand of voltage control of grid operation. The reactive power issue in OWPP is addressed by Rather et al. [102], in which voltage stability performance of the system is examined using mixed integer dynamic optimization approach. It is concluded that better system efficiency and reliability can be achieved by optimal allocation of dynamic reactive power sources. Wang et al. [103] show through simulations, the influence of the voltage variation at the PCC by various capacities and underwater cable lengths for large-scale offshore wind farm connected to power grid in Taiwan. In large OWPP systems, the use of hierarchal control to regulate PCC voltage is a viable option. This strategy have two levels viz. centralized control (CC) and local control (LC), as shown in the Fig. 8. The CC manages the voltage at PCC, whereas the LC supervises the power injection from GSI to trail the expected set point from the CC [104].

2) IMPACTS OF FREQUENCY VARIATION
OWPP penetration in the grid proportionally increases the unpredictability and frequency variation of the power output. The operating conditions change due to these deviations in frequency, because it alters the frequency related parameters like reactance and the slip of the wind turbines. Furthermore, modern variable speed wind turbines such as type III and IV are connected to the grid with end-to-end power converters. The intermediate DC voltage in this AC-DC-AC interface creates electrical decoupling between OWPP and the grid. Consequently, the system inertia reduces, as the generator  rotor does not see the changes in frequency [105]. Additionally the interconnected conventional power units are overburdened to regulate the frequency variations. This phenomenon deteriorates frequency and inertia regulation, especially in islanded systems with no grid support [106].
In European power grids, frequency regulation is maintained within limits using three-way control strategy as depicted in Fig. 9. In case of any deviation, primary control keeps the balance between power generation and consumption. Secondary control triggers automatically when there is active power deficit on the line. This stage lasts for about 15 minutes, and if the frequency deviation of frequency persists, tertiary control comes inline. The tertiary control mitigates the persistent frequency deviations after the production outages or long-lasting load variations. Time control makes sure that discrepancy between synchronous time and universal coordinated time of the system must be avoided, which happens due to frequency deviation [107].

3) RAMP RATE REQUIREMENTS
Wind power changes continuously and as a result, an important issue related to grid integration is upholding the economic cost of wind energy and the system resiliency. This intermittency of wind energy is countered by calling fast ramping or peak-load units, so that balance of supply and demand must be kept. Such events threaten the benefits of wind energy-low fuel cost and reduced CO 2 footprint [108]. An OWPP need to have the ability to limit increased and decreased power rates. By and large, the ramp-rate limit (RRL) for a short duration of 1-minute addresses system regulation capability. A longer duration of 10-minute typically is applied on grid load-following capability [43].
In this regard, active RRL by Denmark specifies the commencement of constraint to be accomplished with in 2 to 10 seconds, whenever an order to change the parameter is received. The maximum RRL constraint is 100 kW/sec [109]. The grid code standards of China stipulates the power dispatch facility to deliver the power RRL within 1 to 10 minutes [110]. According to ISO-NE department in US, response rate to a peak or valley in power output OWPP should not overdo 10 % /sec.

IV. GRID CODE REQUIREMENTS
Grid codes mainly contain standards and guidelines for active and reactive power control of the system. The interruption of OWPP under PQ issues is undesirable when a substantial share of the total network generation comes from wind power. Hence, grid codes need OWPP to continue their uninterrupted operation under several fault situations. This ability of wind turbine to stay in connection with the power grid in case of any fault or voltage imbalance for specified amount of time is termed as LVRT. LVRT capability is instantly needed as it takes into the account fault percentage and interval during which wind farms need to be able to withstand the fault [111]. Electrical network operators have grid code requirements that vary from one country to another depending on the organization and characteristic features of the system. Although that needs to comply with international standards like IEC and IEEE. In Table 5, a comparative description of different IEEE and IEC standards is provided related to LVRT techniques as well as harmonic limits for wind generation systems.
In the context of OWPP, IEC has formulated standards for measurement and assessment of PQ characteristics of grid connected wind turbines i.e. IEC 61400-21 [120]. As a result of connecting wind farms to a transmission system instead of a distribution system [72], modern wind turbines acquire the ability to control both active and reactive power in steady as well as in transient conditions. A detailed study of grid code requirements to enhance the participation of OWPP in frequency stability and virtual inertia is presented in [121]. Frequency bounds adopted by the different countries as per international grid codes requirements is provided in Table 6 [122]. Frequency variation between the OWPP and power grid can cause synchronization imbalance as well as disturbance in active power output. Unlike conventional generators, wind turbine generators usually do not have the ability of giving frequency regulation to the network due to the absence of inertia [123]. This results in decreased frequency nadir in DFIG based OWPPs with lacking inertia [124].
Active and reactive power requirements should be met in all grid codes to maintain load voltage and power factor levels at the PCC. Under nominal power generation, recommended practice is to operate the wind turbines in a way to retain 0.95 power factor leading or lagging. In UK grid codes, gradual reduction in reactive power is required, as the plant achieves 20% to 50% of rated power. However, the German grid codes demand a continuous reactive power at different power plant generations. With the Danish grid codes, it is requisite to reach a power factor set point with a tenacity of 0.01. Table 7 compares operating limits of power factor in global grid codes for which OWPP are required to remain connected to grid [125].

V. POWER QUALITY IMPROVEMENTS TECHNIQUES IN OWPP
Mitigation of PQ and stability issues attracted the researchers or operators attention due to the increased penetration of OWPP in the grid. Various devices and control techniques have been developed recently to improve the PQ of the interconnected power grid in general.

A. DFIG CONVERTERS BASED CONTROL TECHNIQUES
PQ improvement and harmonic current mitigation have been reported in the literature to be achieved by applying control algorithms on DFIG converters [39]. In [126], the grid side  control (GSC) of the wind energy system has been adjusted to regulate the dc link voltage by using d-q transformation method. In addition to DC link voltage modification through active filtering, mitigation of harmonic currents is achieved. A DFIG-based wind energy system with nonlinear loads at PCC is shown in Fig. 10 [127]. This DFIG based system works as an active filter as well as source of real power generation. The harmonic current caused by the nonlinear loads is mitigated by GSC control. The rotor side control (RSC) is based on voltage-oriented reference frame for achieving unity power factor at the stator side as well as MPPT.
A sensorless stator field oriented technique is used to control the RSC, which eventually reduces the harmonics levels at the grid [128]. In [127], the control of GSC is implemented such that the signal fed to power switches is bounded by a hysteresis current method. The experimental results show significant improvement of total harmonic distortion (THD) at the grid after filtering. Nian and Pang [129] proposed an impedance reshaping control method for a 2MW DFIG system. This control strategy enables the system to suppress the high frequency resonance thereby avoiding the unstable condition. Furthermore an active damping strategy that can compensate for the matrix converter LC filter under different operating conditions is presented in [130]. Here, a small signal stability analysis has been performed and it is shown that the input filter resonance issue becomes severe as the DFIG operates at sub-synchronous speed with unity power factor.

B. FACTS BASED ACTIVE POWER FILTER TECHNIQUES
Flexible Alternating Current Transmission Systems or FACTS are power electronic based devices that can control VOLUME 9, 2021  and regulate the active and reactive power in a power network. These compensating devices can be coupled in series, or in combined topology, contingent on specific requirements when connecting to the power system grid [131]. FACTS based on series connection include thyristor controlled series capacitors (TCSC) as well as static synchronous series compensator (SSSC) [132], while Shunt based FACTS comprise of static VAR compensator (SVC) and static synchronous compensator (STATCOM). Unified power flow controller (UPFC) are hybrid FACTS i.e. series-shunt devices are capable to control voltage and power flow simultaneously [133]. Table 8 specifies comparison of various FACTS effects on PQ parameters in electric power grid [134].
The principle of the STATCOM is to either consume or deliver the reactive power based on the estimated RMS value at the PCC. Consequently, it mitigates voltage drops and improves the active and reactive power flow in the transmission line. A scheme of modified STATCOM controller with synchronous frame reference controller for minimizing some of the PQ issues of a hybrid wind and PV based system is proposed in [135]. The proposed controller manages to achieve 25.35% improvement in system's THD. Voltage variations have been reduced and active power flow significantly improves using the proposed scheme. Sayahi et al. [37] studied two cases for improving the PQ in OWPP connected to power grid. That include sharing of the STATCOM reactive power with the electric grid as well as regulating the voltage at the PCC, as shown in Fig. 11.
Another STATCOM compensation based PQ improvement study on a OWPP and PV hybrid power system has been presented in [136]. The simulation results show that load bus voltage is regulated at about 1.0 p.u. while THD in voltages and currents are maintained at 1%. The UPFC consists of series as well as shunt converters, coupled to a dc link capacitor as shown in Fig. 12. The series part provides the regulation of active and reactive powers while shunt part ensures the voltage support at PCC by regulating voltage at DC link of UPFC [137]. Dynamic performance of a DFIG based OWPP  under wind gusts and disturbances is proposed in [138] using an adaptive model-free controller for UPFC. Uncertainties like voltage sag, voltage swell, wind gust and LVRT capability of the system significantly improved when suggested UPFC based adaptive controller is compared to conventional PI controller. The UPFC show promising results, while damping the SSR oscillations occur mostly in systems with seriescapacitor compensated lines. One such hybrid steam turbine generator (STG) -DFIG based OWPP uses UPFC with its damping controller to mitigate SSR oscillations by improving transient stability of the system [139].

C. ENERGY STORAGE SYSTEM BASED SOLUTIONS
The energy storage systems (ESS) play a significant role in PQ improvement of the generated power by regulating output voltage of the OWPP. The electrical power produced by the wind turbines is dependent upon the wind speed and affects both the PQ as well as system planning [140]. The ESS generally includes battery, flywheel, superconducting magnetic energy storage system SMES, super capacitor, compressed air system, pump hydro and hydrogen production. Usually the ESS connected to the intermediate DC link capacitor of back to back power electronic converters for power conversation as illustrated in Fig. 13. The battery energy storage system (BESS) in addition to storing the power, also regulates the active and reactive power at PCC, and maintains system stability [141].
The control and stabilization scheme for grid connected wind farm with reactive power compensation by SMES is proposed in [142]. This multilevel control method enhances the transient stability by fast controllability of the SMES four-quadrant operation. Similarly, a fuzzy logic based controlled SMES in [143] shows an improved transients response and voltage regulation. In [144], authors presented a system of DFIG with fuel cell connected to the power grid. The proposed scheme used vector control on two back-to-back power electronic converters to regulate the power flow to the grid, fuel cell, and generator. Beside SMES, the flywheel energy storage (FES) systems are also effective in alleviating power surges occur during faults in OWPP connected to grid [145]. Another study reported in [146], exploits the ability of FES system to smooth out power fluctuation, where it is interfaced with 80 MW DFIG based OWPP. Super capacitor is another ESS based solution for voltage flicker issue in weak grid having wind energy integration. With appropriate filtering control algorithm, these systems achieve superior flicker mitigation as compared to the reactive power control techniques [147].

D. METHODOLOGIES FOR LVRT IN OWPP
Various schemes have been reported in literature for efficient fault ride through in offshore wind based generations [26]. The commonly used methods include simple protective circuits such as crowbar, stator damping resistor (SDR) and rotor current limiter (RCL). These schemes are mostly used in DFIG based systems. Other methods include VARs injection through FACTS based devices and the implementation of robust control strategies to enable system to ride through during fault conditions. Organization of different LVRT strategies in OWPP is illustrated in Fig. 14.

1) LVRT PROTECTIVE CIRCUITS
Most common LVRT method for type III wind turbine is using crowbar (CB) protective circuit. Crowbar is a resistive circuit which is connected across rotor winding terminals [148]. In case a fault occurs, a large rotor current generates due to DFIG rotor and stator magnetic coupling.
To counter this high current, resistor crowbar bank is installed to disable the rotor side converter during the fault, so that power flow can be controlled independently. For optimal value of crowbar resistance, R C,opt , the following relation is applied [149]: where ω s is the synchronous speed, V is the stator voltage; V max is the maximum rotor voltage while Ls is the stator inductance. An upgraded scheme make use of bidirectional switches connected to stator or rotor resistors is discussed in [42]. Whenever a fault occurs, switch turned OFF, connecting the resistors of RCL and SDR in series with rotor and stator as shown in Fig. 15. The advantage of these (RCL, SDR) circuits over crowbar implementation is the ability to limit high transient current while retaining the connection with GSC.

2) LVRT BY VARS INJECTION
The dynamic voltage restorer (DVR) is a series connected device usually linked with DFIG based wind generation systems at PCC to counter voltage sag or swell and PQ issues. In addition, it enhances the LVRT ability by injecting voltage and provides power compensation to the grid to clear faults [150], [151]. An onsite FRT testing methodology for OWPP prior to grid integration is presented in [152] combining with DVR control to maintain voltage swells for testing requirements. A flexible AC transmission system (FACTS) based device i.e. STATCOM is a shunt-connected, reactive power compensation scheme. It can mitigate harmonic resonance issues and provides load compensation to the grid [31]. Heydari-Doostabad et al. [153] suggested an updated STAT-COM based control method to improve LVRT ability of wind turbine in case of voltage fault. There proposed STATCOM compensator model enables the system to clear the maximum fault duration swiftly thereby improving the stability margin.

3) CONTROL SCHEMES FOR LVRT
Control techniques are crucial to the robust operation of OWPP in case, PQ issues happen. Proper tuned controllers stabilize the system by improving its LVRT capability. A fuzzy control technique is implemented in DFIG based OWPP to limit fault current by improving LVRT ability of the system [154]. A similar scheme with integrated terminal sliding mode control is proposed in [155] to enable DFIG to ride through deep voltage sag under different operating conditions. Direct-model predictive control (DMPC) technique developed in [156] optimize the LVRT ability of DFIG energy conversion systems by optimizing the switching states of the embedded power converter.

VI. CONCLUSION AND RECOMMENDATIONS
The unprecedented growth of offshore wind generation has paved an accelerated path towards a global decarbonized VOLUME 9, 2021 future with clean renewable energy. Nevertheless, the intermittent nature of wind power poses various challenges related to the integration and transmission systems of the wind turbine such as PQ and system stability challenges. Although AC transmission is most commonly used in OWPPs nowadays, the transmission voltage level constraints will encourage the use of HVDC transmission in near future. This paper reviewed PQ issues of OWPP integration into electric gird and discussed various solution methodologies. Statistics regarding OWPP installed capacity and future growth have been listed. Different types of wind turbine topologies with their distinguishing features, control algorithms and variability have been discussed. The mentioned challenges focused on PQ issues like flickers, harmonics, voltage sag and swells as well as voltage and frequency stability problems and recent control schemes with regard to OWPP. This article also sheds light on crucial aspects like LVRT methods and grid code requirements in different parts of the world. These grid codes are essentially intended for grid support in case of steady-state as well as grid faults conditions. In addition, this paper reviewed available solution approaches including DFIG controllers systems, filters, energy storage systems and customs power devices to deal with the challenges of PQ problems in OWPP. A performance comparison of these FACTS devices shows UPFC and STATCOM to be better options for solving PQ issues, followed by SVC and TCSC. A continuous renewal of grid standards in different countries depending upon wind variability, generation portfolio and LVRT requirements and schemes have been discussed thoroughly.
For the review topic under consideration, some recommendations with regard to PQ challenges, standards and solution methods for future improvements in the prevailing system are: 1. The need of a fast communication system to exchange and manage data between far-flung offshore and onshore stations. Latest protocols like internet of things (IoT) can help in this regard to mitigate any communication delays that may affect system stability. 2. Online impedance assessment analysis for OWPPs networks can be achieved by installing advanced synchronized waveform monitoring devices. 3. PQ issues like harmonics and flickers need to have precise grid codes for measurements and contribution of different stakeholders involved in the radial OWPPs. 4. SH mitigation with regards to OWPP is the area which still need researcher's attention with well-established grid regulations are required to solve this issue. 5. LVRT capability requirements need to contain information that is more detailed, because the FRT ability vary with wind farm rating as well as type of faults. 6. Detailed grid codes need to be implemented for integration of various ESS technologies, which can provide smooth output and system flexibility from wind generation for a specific duration in case of any fault.

7.
A standardized and regulated policy from various countries to enhance OWPP development, while at the same time providing economic, environmental as well as social growth to those nations. The above recommendations may be helpful in achieving improved PQ with regards to OWPP, which is fast growing power provider market and will dominate its peers in the near future. This review could be extended to further studies to enhance PQ and stability in OWPP by overcoming various system drawbacks.
SYED WAJAHAT ALI received the bachelor's degree in electronics engineering from the University of Engineering and Technology Peshawar, Pakistan, in 2008, and the master's degree in electrical engineering from COMSATS University Islamabad, Pakistan, in 2012. He is currently pursuing the Ph.D. degree in electrical engineering with the National Kaohsiung University of Science and Technology (NKUST), Taiwan. In 2013, he joined COMSATS University Islamabad as a Lecturer. His research interests include power quality, power system analysis and control, offshore wind energy systems, and their energy management.
MUHAMMAD SADIQ received the B.S. degree in textile engineering from Balochistan University of Information Technology, Engineering and Management Sciences, Quetta, Pakistan, in 2013, and the M.S. degree in industrial engineering and management from the University of the Punjab, Lahore, Pakistan, in 2019. He is currently pursuing the Ph.D. degree in electrical engineering with the National Kaohsiung University of Science and Technology, Taiwan. He joined Nishat Mills as the Deputy Manager for two years. His research interests include harbor microgrids, energy management, and modeling of DC EV charging station for harbor applications.
YACINE TERRICHE received the B.S. degree in electrical engineering from the University of Science and Technology, Constantine, Algeria, in 2011, the M.S. degree in electrical engineering from the University of Constantine 1, Constantine, and the Ph.D. degree in electrical engineering from Aalborg University, Aalborg, Denmark, in 2020. During his Ph.D. degree, he has been working as a Research Assistant at Kaohsiung University of Science of Technology, Kaohsiung City, Taiwan, from December 2019 to September 2020. Since November 2020, he has been working as a Research Assistant at Aalborg University. His research interests include power electronics modeling and control, signal processing, power quality issues, microgrids, cyber security, active and passive power filters, static VAR compensators, and maritime microgrids. LE QUANG NHAT HOANG received the bachelor's degree in mechanical engineering from Ho Chi Minh City University of Industry, Vietnam, in 2011. He is currently pursuing the master's degree with the Department of Microelectronics Engineering, National Kaohsiung University of Science and Technology, Taiwan. He is working as a Lecturer and a Teaching Assistant at Dong Nai Technology University, Vietnam. His research interests include machine-learning techniques, shipboard power systems, and control algorithm.