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Arabian Journal for Science and Engineering

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Open Access 06-07-2023 | Research Article-Electrical Engineering

Technical Assessment of the Key LVRT Techniques for Grid-Connected DFIG Wind Turbines

Authors: Ali H. Kasem Alaboudy, Heba A. Mahmoud, Adel A. Elbaset, Montaser Abdelsattar

Published in: Arabian Journal for Science and Engineering | Issue 11/2023

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Abstract

The article delves into the technical assessment of key LVRT techniques for grid-connected DFIG wind turbines, which are crucial for maintaining grid stability during faults. It begins with an introduction to the growing importance of renewable energy sources, particularly wind energy, and the challenges posed by grid voltage deviations. The paper then classifies LVRT techniques into exterior (protection-based, FACTS-based, and hybrid) and interior (control strategies-based) methods, evaluating their effectiveness and drawbacks. A systematic taxonomy of LVRT optimization techniques is presented, followed by a detailed discussion of various protective and control strategies. The paper also includes simulations using MATLAB/SIMULINK to demonstrate the impact of these techniques on DFIG performance during grid faults. The conclusions highlight the importance of LVRT capabilities for wind turbines and provide guidelines for selecting the most effective techniques based on grid code requirements and cost considerations.

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RESs

Renewable energy sources

WTs

Wind turbines

LVRT

Low voltage ride-through

DFIG

Doubly fed induction generator

DFIG-WTs

DFIG-based wind turbines

GWEC

Global wind energy council

WTGs

Wind turbine generators

PMSG

Permanent magnet synchronous generator

Direct current

SFCL

Superconducting fault current limiter

SDBR

Series dynamic brake resistor

FACTS

Flexible AC transmission system

STATCOM

Static synchronous compensator

DVR

Dynamic voltage restorer

SSSC

Static synchronous series compensator

TCSC

Thyristor controlled series compensator

RSC

Rotor side converter

GSC

Grid side converter

Series resistor

GCSC

Gate-controlled series capacitor

HTSFCL

High-temperature superconducting fault current limiter

ESS

Energy storage system

Superconducting coil

VSC

Voltage source converter

Inductors capacitors

Vector control

BPA

Blade pitch angle

Current compensation

SMC

Sliding mode control

DNC

Decentralized nonlinear control

ISS

Input-to-state stability

SDR

Series dynamic resistor

RNIO

Real-time neural inverse optimal

1 Introduction

In the last few years, the utilization of renewable energy sources (RESs) has grown rapidly in order to meet increasing energy requirements [1]. This is due to a number of advantages, such as affordable cost, high efficiency, minimal environmental impact, and inexhaustible energy sources. Among them, wind energy plays a vital role and has grown at the quickest pace [2, 3]. Where in 2019, 60 GW of installed wind energy capacity was added and 92 GW of installed capacity was added in 2020, despite the coronavirus. By 2025, the global wind energy council (GWEC) predicts even a higher yearly growth of about 94 GW [4, 5]. Wind turbines (WTs) are becoming more common in electrical power networks as wind energy develops. The four fundamental kinds of WTs are: fixed-speed WTs restricted variable-speed-controlled WTs, DFIG-WTs, and complete variable-speed-controlled WTs. In practical applications, DFIG-WTs are the most popular among the four kinds of wind turbines [6, 7].

1.1 Literature Review

The emergence of efficient wind turbine generators (WTGs) such as the permanent magnet synchronous generator (PMSG) and DFIG has led to a tremendous rise in power generation. Out of all the WTGs, DFIG has several advantages, such as variable speed and constant frequency operation, active/reactive powers control, and reduced power converter ratings [8, 9]. DFIG is the most widely used WTG and undoubtedly a market bestselling due to its exceptional performance. Although benefits of DFIG-WTs, they are extremely susceptible to grid voltage deviations for various grid failures [10‐13]. When a DFIG encounters a rapid reduction in the grid voltage, the DFIG stator currents dramatically increase beyond the rated levels [14]. This is due to the magnetic connection between the stator and the rotor circuits. These currents have the potential to harm the power converter's power electronic equipment.

Furthermore, the electromagnetic torque of the DFIG begins to fluctuate with significant amplitudes; even a minor variation in the grid has an impact on the DFIG's performance. This aspect of DFIG has given rise to the LVRT capability feature. The capacity of a wind turbine generator to stay connected to the grid, and in certain circumstances support it, when the grid is experiencing a fault is known as LVRT capability [15]. To put it in another way, it is WTs' capability to ride through grid fault problems.

During grid voltage failure situations, the focus had previously been on the protection of DFIG-WTs by tripping the turbine. However, the power system is not permitted to disconnect the majority of WTs from the grid. The cause has a significant impact on the power system's ability to operate in a steady and secure manner [16]. As a result, in order to ensure the stable operation of the power system, in certain countries (such as Germany), network operators have revised the grid code by incorporating new standards [17, 18].Turbines should remain linked to the grid during grid failures, according to the new requirements. In order to maintain electrical systems running safely and reliably. Furthermore, WTs must produce reactive current during voltage drops in order to maintain grid voltage. This voltage control must be turned on within 20 ms after detecting voltage sag. The voltage dip determines the quantity of reactive current required. WTs must continue to provide active power that is at least 20% higher than the rated power per second after the fault has been addressed [19].

In order to comply with such grid requirements, it is major for the generators to have an improved LVRT capability. As a result, several researchers provided several LVRT-optimized approaches in Refs. [20‐25]. To safeguard generators, the solutions employ extra circuitry. A crowbar circuit, direct current (DC) chopper, superconducting fault current limiter (SFCL), series dynamic brake resistor (SDBR), and other circuits may be fitted. Overcurrent suppression, machine torque oscillations, and DC link overvoltage are all included in these approaches. The most common method is to use a crowbar. During a fault, this method draws all of the needed reactive power from the grid, which is extremely undesirable. Furthermore, the controls are separated from the rotor, rendering the system inoperable. Because of the crowbar circuit's flaws, researchers have been looking for other protective mechanisms to improve LVRT capabilities. Various topologies have been proposed over the years.

In Refs. [26‐28], LVRT techniques based on power injecting devices are introduced. In such techniques, devices like static synchronous compensators (STATCOM), dynamic voltage restorers (DVR), static synchronous series compensators (SSSC), and thyristor-controlled series compensator (TCSC), are utilised to regulate/optimise grid voltages. These devices can be tuned to push/set active and reactive powers in four quadrants, which will consequently improve its LVRT capabilities of wind generators. Using these devices adds to the system extra equipment/size, which has a negative impact on the compactness feature of the system. Control topologies-based solutions have been shown to be more successful in overcoming these difficulties.

In Refs. [29‐32], it has been stated that, a combined method of virtual resistance and demagnetizing control was used. In these topologies, various system controllers are developed to reject perturbations in different parameters like rotor current, electromagnetic torque, and DC-link voltage under fault circumstances in these ways. Because of their potential to efficiently address nonlinear problems, artificial intelligence (AI) approaches have also been investigated. The researchers also gave several categories of LVRT approaches in Refs. [21, 33‐35]. However, it is critical to examine thoroughly various LVRT strategies in order to select the optimum option for meeting network needs.

1.2 Contributions of Paper

As explored in the preceding subsection, several published papers reviewed the various types of LVRT techniques with an in-depth description. Techniques are categorized into numerous classes, which are backed by simulation findings. Other articles follow the evolution of LVRT technology. In this paper, the authors intended to concisely review and analyse the recently emerged LVRT techniques for DFIG-WTs. Furthermore, a thorough examination of LVRT techniques is provided with emphasis on their benefits and drawbacks. The primary contributions to the paper can be summarized as follows:

Present an overview of LVRT techniques for DFIG-WTs that have recently appeared with critical analysis.
A thorough examination of the numerous characteristics of the techniques used.
Discuss the effects of LVRT techniques in line with grid codes.
Using MATLAB/SIMULINK to discuss the influence of the most promising LVRT techniques on DFIG performance.

1.3 Paper Layout

The following is a breakdown of the paper structure: Modelling of a grid-connected DFIG-WT is shown in Sect. 2. Section 3 discusses the primary contributions of LVRT approaches, including classifications, merits, and demerits, as well as assessments based on grid codes. In Sect. 4, we explore how to increase LVRT capabilities during grid failures by employing better protective strategies using MATLAB/Simulink. The simulation results for LVRT techniques are provided in Sect. 5. The guidelines and recommended technical designs for the LVRT techniques for DFIG-WTs to cope with local grid codes are highlighted in Sect. 6.

2 Grid-Connected DFIG-WTs Modelling

In DFIG-WTs, the DFIG stator winding and the grid are directly linked [36]. The turbine is controlled by back-to-back power converters, which comprise the rotor side converter (RSC), grid side converter (GSC), and a DC-link capacitor, as shown in Fig. 1.

Fig. 1

The graphic diagram of the grid-connected DFIG-WTs

The modelling equations are as follows [37]:

$${V}_{\mathrm{s}}^{\to }={R}_{\mathrm{s} }{i}_{\mathrm{s}}^{\to }+d{\psi }_{\mathrm{s}}^{\to } /{d}_{t }+{j\omega }_{\mathrm{s}} {\psi }_{\mathrm{s}}^{\to }$$

(1)

$${V}_{\mathrm{r}}^{\to }={R}_{\mathrm{r} }{i}_{\mathrm{r}}^{\to }+d{\psi }_{\mathrm{r}}^{\to } /{d}_{t }+{j(\omega }_{\mathrm{s}}-{\omega }_{\mathrm{r}} ){\psi }_{\mathrm{r}}^{\to }$$

(2)

$${\psi }_{\mathrm{s}}^{\to }={L}_{\mathrm{s} }{i}_{\mathrm{s}}^{\to }+{L}_{m }{i}_{\mathrm{r}}^{\to }$$

(3)

$${\psi }_{\mathrm{r}}^{\to }={L}_{m }{i}_{\mathrm{s}}^{\to }+ {L}_{\mathrm{r} }{i}_{\mathrm{r}}^{\to }$$

(4)

The current, voltage, flux, resistance, and inductance are represented as, $\overrightarrow{\mathrm{i}}$, $\overrightarrow{\mathrm{V}}$, $\overrightarrow{\uppsi }$, R, L, respectively. The stator and rotor are denoted by the letters s and r, respectively. From Eqs. (3) and (4), the stator and rotor currents are indicated as follows:

$${i}_{\mathrm{s}}^{\to }={\psi }_{\mathrm{s}}^{\to }/\dot{{L}_{\mathrm{s}}}-{K}_{\mathrm{r}}{\psi }_{\mathrm{r}}^{\to }/\dot{{L}_{\mathrm{s}}}$$

(5)

$${i}_{\mathrm{r}}^{\to }={-K}_{\mathrm{s} }{\psi }_{\mathrm{s}}^{\to }/\dot{{L}_{\mathrm{r}}} - {\psi }_{\mathrm{r}}^{\to } / \dot{{L}_{\mathrm{r}}}$$

(6)

3 LVRT Techniques

Systematic taxonomy of LVRT optimization techniques is presented as seen in Fig. 2 which includes (1) Exterior LVRT techniques. There are three parts of the exterior techniques that are protection-based techniques, FACTS-based techniques, and hybrid techniques. (2) Interior LVRT techniques include control strategies-based techniques. The goal of this classification is to offer a more detailed classification, evaluate the various techniques based on their effectiveness and provide a tabular breakdown of the various methods. The next subsections will go over the various techniques.

Fig. 2

Systematic taxonomy of LVRT optimization techniques

3.1 Exterior LVRT Techniques

Crowbar, series resistor (SR), chopper circuit, and other approaches are examples of these techniques. They' re divided into rotor and stator side protection circuits based on how they're connected. Initially, the rotor side was home to nearly all of the protective circuits. This configuration, however, resulted in the machine's uncontrolled operation and absorbed a lot of reactive power during breakdowns. To solve these concerns, efforts were made on the stator side to link protective circuits, with promising results [29, 38‐45]. All the designs described in this category attempt to meet the requirements of grid codes. Furthermore, consideration is made to have provisions in place to limit excessive reactive power absorption. An overview of various methods is given below.

3.1.1 Protection Based Techniques

These techniques include the following:

3.1.1.1 The Crowbar Protection Technique

Figure 3 depicts the conventional diagram for crowbar protection [46, 47]. The crowbar purpose is to keep the rotor current low during a fault so that, the power converter can be protected [48, 49]. In addition, as described in [50, 51], there are several techniques based on crowbar protection. Reference [52] proposes two novel single-phase crowbar protection topologies. During asymmetric faults, the proposed single-phase dc crowbar shows great performance, which can replace the classic three-phase dc crowbar. The prominent advantage of the single-phase.

This method has the following advantage: It lowers overcurrent's in the rotor and stator.
The following are some of the disadvantages of this solution:
DFIG-WTs cannot be controlled during grid disturbances.
The generator absorbs reactive power, causing grid voltage to deteriorate further, and the possibility of disconnecting the turbine from the grid. As a result, the likelihood of other turbines going offline increases, causing grid instability [52, 53].

Fig. 3

The crowbar protection technique [47]

Despite the crowbar disadvantages, many studies have been done to improve the crowbar's operation. In order to improve the performance of DFIG-WTs by ensuring active/reactive power control [54]. For example, to enable the RSC, an active crowbar proposed for disconnection at the correct time. The grid code requirements for reactive power injection could be met by disconnecting the crowbar after a few milliseconds [55‐57].

3.1.1.2 The SR Protection Technique

The SR is made up of a series of resistors linked in series with the stator or rotor winding [28], as illustrated in Fig. 4. The SR's priority is to keep the rotor current under control. It also protects the DC-link from over-voltage and torque oscillations by dissipating active power. In various experiments, SR proposed that, the DFIG be allowed to ride through extreme voltage dips, both symmetrical and asymmetrical, such as:

Ref. [28] recommended a rotor current controller and dynamic resistors in series with the stator for unbalanced voltage dips.
Ref. [58] proposed the use of SR in addition to correct rotor current reference values.
On the stator side, a series passive-resistive network also explored in [59]. This approach had been proved to have higher and more effective performance.

Fig. 4

DFIG-WT with some protective techniques

3.1.1.3 DC Link Chopper Protection Technique

The chopper circuit is illustrated in Fig. 4. A DC-chopper is a circuit that consists of a switch and a resistor connected in parallel to the DC-link. When a grid malfunction is detected, the switch is closed and the resistor is connected in series with the DC-link capacitor, preventing overcharging at low grid voltage. The resistor will absorb the extra active power as shown in Ref. [53]. In Refs. [60, 61] control-delay approach for DC-chopper is explored. In this approach, a present rotor current threshold releases the chopper's application, and feedback control restores shorter rectification duration. However, the results of this sophisticated technique were unsatisfactory. It is also demonstrated that, the DC link chopper performs worse in terms of LVRT than a normal crowbar.

Moreover, there are many protection-based techniques introduced in the literature. So, a critical examination and comparison for these techniques have been presented in Tables 1 and 2.

Table 1

Synopsis of studies based on the LVRT capability of rotor side protection techniques

Author	Technique	Features	Defects	Contribution
Niu et al.[22]	DC brake chopper with DC link capacitor	Prevents the traces of stray inductance	Rise in the rating of the anti-parallel diodes in RSC	It was discovered that a simple delayed control mechanism was more efficient
Pannell et al. [23]	Single phase crowbar circuits	Possibility of current state of zero Prevents significant overvoltage	Huge circuit Expensive Not controlled operation	Reduced uncontrolled operation time and better outcomes than the crowbar circuit
Zou et al. [62]	Super-capacitor and modified DC-link based on polypropylene	Compared to DVR, it is more cost effective Simple Sagging is eliminated	RSC that is more bulky	The supercapacitor-based system outperformed both the DC chopper and the polypropylene-based modified DC link in both faults
Mendes et al.[63]	GCSC in series with rotor	Suppression of RSC inrush currents The operation is under control	This method makes system larger	For single-phase faults, the DC link voltage profile is inferior than the crowbar-based approach
Yang et al.[64]	Inductor-type superconducting coil (SC)	In the steady state, power fluctuations are dampened Fault mitigation near the DFIG	Costly	The usage of optimal SC outperformed other auxiliary devices like batteries, STATCOM, and non-optimal SC

Table 2

Synopsis of studies based on the LVRT capability of stator side protection techniques

Author	Technique	Features	Defects	Contribution
Haidar et al. [39]	Switch mode operation of DFIG	The stator is completely isolated from the grid	Reactive power absorption Power transmission is limited	MSDFIG provides greater results than a system that is not protected
Wei et al. [40]	R-type SFCL	Hybrid Simple to operate	Reactive power requirement	Limits quantities to suitable levels with success
Zheng et al. [42]	Switch type SFCL	No needs for an overvoltage bypass circuit. Improved control ability	Costs have increased The size has grown	The switch type SFCL was found to be superior in every way
Guo et al. [65]	R-type HTSFCL	Better voltage and angle stability Less complicated	Performance degrades when sag reaches 100%	Although HTSFCL allowed for LVRT the DVR, system provided more elasticity
Alaraifi et al. [29]	Low-rated SDBR on stator side	Less expensive than most other stator side protection High-speed synchronization	The price fluctuates a lot Determining parameters is boring	The SDBR methodology produced best results than the SRC method at various SDBR levels

3.1.2 FACTS Based Techniques

FACTS devices have the ability to regulate the parameters of transmission systems, allowing them to operate more efficiently. The static synchronous compensator (STATCOM) and the SSSC are two FACTS controllers that are rapidly gaining popularity in power systems. They employed to improve LVRT capability by compensating deep and shallow symmetric and asymmetric effort sags [26‐28].

3.1.2.1 The Static Synchronous Compensator Technique

As illustrated in Figure 5, the STATCOM is a shunt-connected device that employs power electronics to manage voltage at its terminals by controlling the amount of reactive power introduced into or absorbed from the power system. As a result, in both steady state and transient-state conditions, high-performance voltage control is possible [66].

Despite the fact that the STATCOM or STATCOM/ESS is a common solution for improving LVRT capabilities, installing and maintaining this FACTS device in a wind farm will raise the total cost of the system.

Fig. 5

DFIG-WT with STATCOM

3.1.2.2 The Dynamic Voltage Restorer (DVR)

As illustrated in Fig. 6, DVR is made up of a voltage source converter (VSC), inductors capacitors (LC) filters, and coupling transformers that are coupled in series with the grid to correct for voltage dips in the gridline during disturbances. Indeed, the DVR topologies have allowed DFIG to effectively ride through significant voltage dips. It is suggested that, it was connected to the generator in series to boost the stator voltage. As a result, the rotor current can be kept below the maximum allowable value. The DVR voltage can be controlled to the point where it absorbs all or almost all of the active power provided by the generator in order to optimize the generator injected reactive power [67]. In the case of asymmetric fault, Reference [27] proposes the coordinated control of the feedforward and feedback voltage of the DVR. The feedforward control adjusts the output power of the DVR according to the voltage drop depth. The feedback control is mainly to reduce the fluctuation of the PCC voltage of the wind farm. However, it has numerous drawbacks [67]:

The DVR's rating should match the WT's rated output;
DVRs are quite expensive due to many ancillary components;
Increased of system size.

Fig. 6

DFIG-WT with DVR

3.1.2.3 Energy Storage System (ESS)

As shown in Fig. 7, this method employs a bidirectional dc/dc converter in conjunction with a dc bus. In this configuration, the dc bus voltage is controlled by either GSC or storage converter. Some energy is conserved in this situation, while the remaining is exported to the grid via the GSC. In order for fault currents to pass through the DFIG rotor circuit, the RSC must be sized appropriately [68].

Fig. 7

DFIG-WT equipped with ESS

Although an ESS can smooth the output power while stabilising the DC link voltage. Overcurrent and electromagnetic torque oscillations are notoriously difficult to avoid. However, the system's cost and complexity will increase, as more energy storage devices are required [69‐71].

3.1.3 Hybrid Techniques

Methods that rely entirely on a single methodology were suggested in the previous sections' references. Every technique has its own set of challenges. As a result, the proposed methods based on these methodologies are not without flaws. However, it had been seen that, a feature that cannot be offered by one approach is contributed by a different method. As a result, by combining two methods, the disadvantages of one can be eliminated. As a result, hybrid techniques are now on the table. The following are the categories of hybrid techniques, which are made up of a blend of traditional methods:

3.1.3.1 The Crowbar Integrated with the Series R–L Technique

In Refs. [52, 72‐74], crowbar in conjunction with a DC-link chopper and/or a series dynamic resistor, respectively. The authors presented a mechanism for redirecting transient rotor current while regaining generator power control in 45 ms, allowing DFIG-WT to fulfil grid-code requirements.

3.1.3.2 The Crowbar Integrated with the SDR

This is supported by Ref. [50]. The RSC is connected to the rotor winding by the Series Dynamic Resistor (SDR), which keeps generator power control until there are significant grid disruptions, in which case the crowbar is activated. Using a crowbar, on the other hand, results in the same conflicts, potentially destroying the converter and the DC-link capacitor.

3.1.3.3 The Crowbar, S R and the DC-Link Chopper

The DFIG-WTs' reflexes during both symmetrical and asymmetrical effort sags are considerably improved by this technology [51].

All of these technologies aim to reduce crowbar uptime and increase LVRT capacity. In addition to the aforementioned, Table 3 lists the benefits and drawbacks of other hybrid techniques.

Table 3

Hybrid technique evaluation [32, 75‐77]

Technique	Fuzzy controller with crowbar	RSC control with SDR	Active compensator with RCL
Features	Swell mitigation may be possible Capable of dealing with a wide range of grid disturbances More rapid convergence	Simpler Cost-efficient Negative sequence oscillations are effectively dampened	Deep sags are minimized Enhancement performance
Defects	Sagging can be reduced by up to 50%	Losses in conduction during normal operation	Including RCL could cause disturbances

3.2 Interior LVRT Techniques

3.2.1 Control Strategies Based Techniques

The power generated from the DFIG-WTs is regulated by the vector control (VC). On the other hand, the VCs are not able to obtain LVRT capabilities since they are primarily designed for DFIG-WT steady-state operation.As a result, the updated VC must be used to meet grid code requirements. Many changes were recommended by the researchers. Consider the following:

In Ref. [78] control technique, is suggested, which combined transient compensators into a standard RSC current regulator to increase the DFIG's LVRT capabilities. The control approach matched the RSC ac-side output voltage with the transient-induced voltage when a fault occurs, minimizing crowbar interruptions to a minimum.
According to Ref. [79], during grid faults, which might produce stator and rotor overcurrent, the stator currents as the reference rotor current must be reduced. The gains of the RSC's PI current controllers were ideally set to reduce rotor over-currents.
Also in [60, 80] the controllers for both the RSC and the GSC are built using a linear quadratic output feedback decentralized control approach to limit the oscillations and peak value of the rotor current and the DC-link voltage.
In addition, to improve the DFIG's LVRT capacity, an adaptive internal model controller with a variable gain adjustment method is presented in Ref. [81].
Refs. [82‐84] increased the WT's LVRT capabilities during voltage dips without affecting system stability; advanced control-based techniques have been presented and adopted. The majority of them explain several methods for achieving the aim. The DFIG system's transient responsiveness was also improved using a robust control technique and a hysteresis-based current regulator.
Refs. [85, 86] improved the DFIG-LVRT WT's capabilities, model predictive control is employed to improve system stability and flux tracking control utilizing an upgraded vector control technique. Internal model control was also proposed to improve the DFIG-LVRT WT's capabilities. The fundamental concept is to algebraically transform the nonlinear system dynamics into an equivalent (completely or partially) linear one using an appropriate coordinate transformation and a nonlinear control input before employing linear control techniques.
However, a few of these strategies were just too complicated to be employed in practical applications, and they rely largely on the correct modelling and control parameters or the estimation of certain parameters, which may also jeopardize their resiliency [73].
In addition to the foregoing, Table 4 summarizes the benefits and drawbacks of other control strategies based techniques, as well as inferences made from references.
In Refs. [87, 88] some strategies designed to operate under asymmetric fault conditions have been proposed.These strategies allow to damp the torque oscillation or to balance the DFIG’s currents by controlling the positive- and negative-sequence rotor-current components. Since these strategies need positive- and negative-sequence detectors and a greater number of controllers with respect to the standard strategies, their implementation results are complex and the dynamic behaviour and stability can be degraded.
Some proposals allow controlling the DC-Link voltage and the active and reactive power injected into the grid when it has asymmetric voltages [89, 90]. Usually, they are focused on maintaining constant the active and reactive power injected by the DFIG-based system to the PCC, and not injecting sinusoidal currents with only positive-sequence components. Also, their implementation results are complex due to the requirement of positive- and negative-sequence detectors and additional PI regulators for each sequence component.
An integral control strategy for DFIG-based systems is presented in this [87]. The proposal allows minimizing the torque oscillations when the grid has asymmetric voltages by implementing an SFO-VC at the same time that it allows injecting sinusoidal currents to the grid by the control of the GSC. Therefore, mechanical efforts on the generator are minimized while the impact on the power quality of the grid is reduced by injecting sinusoidal currents into the PCC.

Table 4

Comparison of control strategies-based techniques

Sr	Author	Technique	Features	Defects	Contribution
1	Alaraifi et al. [29]	SRC control	A less expensive option Simplicity	Relatively low results	This scheme is inferior to the SDBR scheme
2	Vrionis et al. [30]	Current magnetizing controller	Quicker response No need for sag detection	High turbine speed has a negative impact	A successful ride through
3	Wessels et al. [31]	RSC and GSC's control actions have been altered	Cost-efficient There is no need for overvoltage/overcurrent protection	Mechanical stress has increased	The proposed plan was found to be more effective
4	Falehi et al. [32]	Fuzzy controller with GA tuning	No additional hardware is required Even for greater dips, this is a cost-effective solution	Disturbances can make it difficult to perform Complexity has risen	Effective improvement of LVRT capability

As stated previously, to emphasize different aspects of the LVRT techniques outlined above. To address this problem, researchers looked into the causes for DFIG's failure to stay connected to a malfunctioning grid, as well as potential solutions to assist it. Grid codes were created to serve as guidelines for the management of wind farms. According to these codes, Tables 5, 6 give a detailed evaluation of them in terms of the fundamental elements of LVRT improvement techniques [6, 87]. In these tables “√”denotes that, the utilized LVRT technology effectively controls the network fault impact, “x” denotes that, the utilized LVRT technology has no impact on the fault, and “*” denotes that, the utilized LVRT technology may exacerbate the network fault impact.

Table 5

Impacts of exterior LVRT techniques on low voltage faults

LVRT Techniques	Rotor			Stator		Support power		Increase DC-link voltage
LVRT Techniques	Increase current	Increase voltage	Current oscillations	Increase current	Current oscillations	Active	Reactive	Increase DC-link voltage
Crowbar Protection	√	X	√	X	X	X	*	√
DC link chopper	*	X	X	X	X	X	X	√
SDR	√	√	X	X	X	X	√	√
ESS	*	X	*	√	√	√	√	√
STATCOM	X	X	X	*	*	X	√	√
Fault Current Limiter	X	X	X	√	X	X	X	X
Series GSC	X	X	X	X	X	√	X	√

Table 6

Impacts of Interior VRT techniques on low voltage faults

LVRT techniques	Rotor			Stator		Support Power		Increase DC-link Voltage
LVRT techniques	Increase current	Increase voltage	Current oscillations	Increase current	Current oscillations	Active	Reactive	Increase DC-link Voltage
BPA control	√	√	√	√	√	*	*	√
Vector control	√	X	√	X	√	X	X	√
CC control	X	√	X	X	√	X	X	√
Feed forward control	√	X	√	X	√	√	√	X
SMC control	√	√	√	X	X	X	X	X
DNC control	X	X	√	X	√	√	√	X
ISS control	X	√	√	X	√	√	√	X
SDRE control	√	√	√	X	√	√	√	X
Fuzzy control	√	X	√	√	√	X	X	√
PI-R control	X	X	√	√	√	√	X	X
PI-DFR control	X	X	√	√	√	√	X	√
RNIO coordinated	X	X	X	X	X	X	X	√

However, it has been seen from Tables 5 and 6 that, some interior LVRT techniques may satisfy the requirements of grid codes but some of these strategies are too complex to be implemented and used in industrial applications. In fact, the schemes sorted under exterior LVRT techniques attempt to satisfactorily meet the requirements of grid codes; some are also inexpensive and simple to install. As a result, the following section will demonstrate various exterior LVRT techniques for improving DFIG-WTS performance. The LVRT techniques ensures the safety of energy converters, rotor circuits, and DC-link capacitors while limiting rotor currents, DC-link voltages, and torque changes. The effectiveness of the LVRT techniques wares also assessed for asymmetrical and symmetrical fault situations. MATLAB/SIMULINK is used here to materlise and quatify the concepts of LVRT methods.

4 Improving LVRT Capability of DFIG-WTS

In this section, the exterior technologies that were utilized are discussed as follows:

4.1 SR Protection Technique

The SR protection method was employed in the rotor side of the DFIG. The SR made up of a series of resistors linked to the rotor winding as illustrated in Fig. 8. It uses power electronic switches to regulate the connect of resistance into the rotor circuit [91]. During faults, the resistor is to be inserted in series with the rotor winding circuit. When the SR inserted into the rotor circuit during a fault, the power from the induced voltages in the rotor that occur during the fault dissipated, limiting the rotor over current. The charging current of the DC-link capacitor reduced by limiting the rotor currents. This reduces the risk of DC-link overvoltage. The high voltage will be shared by the resistance due to SR. Converter control will not be lost due to the series topology. As a result, the SR not only prevents rotor overvoltage from causing the RSC to lose control, but it also prevents the RSC from losing control. During the fault, however, the RSC does not need to be stopped.

Fig. 8

Topological structure of the SR protection technique

4.2 The Crowbar Integrated with the DC-Chopper

As illustrated in Fig. 9, the protective technique includes two circuits: a DC-chopper and a crowbar. A crowbar is a set of resistors linked to the rotor winding. During a fault, the crowbar action started by raising the rotor current value by blocking the converter. As a result, current flows into the DC-link via the freewheeling diodes, causing a fast voltage increase. For restricted overvoltage's, the DC-chopper is turned on during symmetrical and asymmetrical grid failures to safeguard the power electronic converter. The protective circuits have been improved to make use of resistors to reduce rotor current and DC-link overvoltage.

Fig. 9

Topological structure of the crowbar integrated with the DC-chopper

Additionally, Fig. 10 shows the detailed DFIG –WT simulation setup. The simulation was performed for a wind farm, which is linked to a 25 kV distribution system and outputs power to a 120 kV grid through 30 kms of 25 kV feeders. In MATLAB/SIMULINK, the wind farm is represented by a single DFIG machine model (6 * 1.5 MW) using the 1.5 MW DFIG's parameters shown in Table 7.

Fig. 10

A single-line schematic of the system under investigation

Table 7

Base and rated values for the system under investigation

Quantum	Amount
Base power	10 MVA
Generator terminals' base voltage	575 V
Base frequency	60 Hz
The generator's rotational speed base	1200 rpm
Rotational speed of the generator	1.2 pu
Rated wind speed	11 m/sec
Rated dc voltage	1200

5 Simulation Results

In this section, we will give the above-mentioned simulation findings in the various scenarios of grid faults, including symmetric and asymmetric faults.

5.1 The Symmetrical Fault: A Case Study

5.1.1 SR Protection Technique

As shown in Fig. 11, a three-phase fault is simulated at the grid connection point, commencing at the time (t = 0.7 s) until clearing at the time (t = 0.9 s). This diagram depicts the system's response to a 0.95pu voltage drop lasting 0.2 s. The power from the induced voltages in the rotor that generated during the failure was dissipated in this simulation due to the inclusion of SR in the rotor winding. The increase in stator and rotor currents is minimized consequently. The stator currents are lowered from 2.66 to 2.02 pu for the most crucial phase. The rotor currents have been lowered from 2.5 to 1.21 pulsations per second. As a result, the DC-link voltage and electrical torque oscillations are decreased. In terms of raising the damping currents at the generator terminals, it can be shown that SR protection is more effective.

Fig. 11

A three-phase 0.95pu voltage drop for 0.2 s using the SR protection technique

5.1.2 The Crowbar Integrated with the DC-Chopper

Figure 12 depicts the system's response to a 0.95pu voltage decrease lasting 0.2 s. The rotor currents that rise owing to the initial rotor current peak initiate the crowbar firing. The safeguard normally disables the converter's electronic switches. However, through the freewheeling diodes, current and energy continue to flow into the DC-link, resulting in a very rapid voltage rise. As a result, the DC-chopper is turned on to prevent overvoltage. The stator currents were reduced from 2.66 to 1.81 pu during the most dangerous phase by including parallel resistance in the rotor circuit and by connecting resistance in parallel with the DC-link. The rotor currents have been reduced from 2.5 to 1.42 pu. When using the technique, the enhanced state-transient response characteristic may be shown significantly.

Fig. 12

A three-phase 0.95pu voltage drop for 0.2 s using the crowbar integrated with the DC-chopper

5.2 The Asymmetrical Fault: A Case Study

5.2.1 SR Protection Technique

A double-phase (phase-b and phase-c) fault is simulated at t = 0.7 s, and Fig. 13 depicts the system reactions under asymmetrical fault circumstances. The SR is efficient in dissipating the induced voltage in the rotor when phase's b and c are short-circuited together. The stator currents dropped from 2.09 to 1.04 pu. In addition, at the most dangerous phase, the rotor currents are reduced from 1.91 to 1.104 pu. As a result, SR minimizes DC-link voltage and electrical torque variations greatly.

Fig. 13

A Short circuit from phase b to phase c for 0.2 s using SR protection technique

5.2.2 The Crowbar Integrated with the DC-Chopper

The system reactions during asymmetrical fault circumstances are depicted in Fig. 14. Short-circuiting occurs in phase's b and c. The stator currents are lowered from 2.09 to 1.38 pu at the most dangerous phase when the DC chopper and crowbar switches are triggered simultaneously. For the most serious phase, the rotor currents are reduced from 1.91 to 1.06 pu, resulting in a considerable reduction in DC-link voltage and electrical torque variations. As indicated in the figure, the crowbar and DC-chopper systems are fairly adequate.

Fig. 14

A short circuit between phases B&C for 0.2 s using the crowbar integrated with the DC-chopper

The simulation results demonstrate that, the SR is helpful under symmetrical and asymmetrical grid failures because it reduces rotor over current, torque oscillations, and DC-link over-voltage compared to the other scheme. Thus, it supports the system's stability under grid failures. Also, the SR is reducing rotor currents more effectively than a crowbar or DC-chopper protection. During the fault, both techniques have reactive power and electrical torque changes. They are substantially bigger for crowbar protection. When compared to crowbar protection, the electrical torque ripple is smaller with SR protection.

6 Conclusion

This paper discussed the recently developed techniques for Low Voltage Ride Through (LVRT) of Doubly-Fed Induction Generator Wind Turbines (DFIG-WTs). Because of the expanding use of wind energy in the grid, improving the LVRT capability is crucial. Different LVRT techniques were described in detail, and the advantages and disadvantages of each were examined in order to meet grid code requirements. LVRT techniques can be divided into two categories: exterior (protection-based, FACTS-based, and hybrid) and interior (control strategies-based).

For simplicity, exterior LVRT techniques are extensively employed in previously installed WTs. However, cost considerations continue to be a key impediment to the adoption of these technologies. Hybrid approaches can help improve LVRT by overcoming the shortcomings of one method by inserting the benefits brought by the other. In fact, interior LVRT techniques adjustments can provide greater control effects during low grid voltage. As the LVRT capability of DFIG-WTs can be enforced without any external supplementary circuit, interior techniques have better economic benefits over exterior LVRT techniques. Thus, interior techniques are preferable in freshly erected WTs, considering the cost factor advantages. The simulation performances of the exterior LVRT techniques are analysed in this paper. The SR protection technique and the crowbar integrated with the DC-chopper are considered here. Both strategies enforce great LVRT performances in fault situations. However, the SR protection technique has been shown to be more effective than the crowbar integrated with the DC-chopper.

In brief, this paper has presented the formation of guidelines and recommended technical designs for LVRT technologies for DFIG-WTs to meet the requirements of local grid codes; key points follow. Recent grid regulations demand that wind turbines stay grid connected while providing reactive power assistance, necessitating the need for them to be outfitted with LVRT capability. While some LVRT techniques are only able to handle symmetric faults, asymmetric faults are more common and thus the techniques should be able to handle both types of flaws. Additionally, the methods used should be simple and cost-effective, and should be designed to work with weak power grids, as the infrastructure connecting wind farms is often not robust.

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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Title: Technical Assessment of the Key LVRT Techniques for Grid-Connected DFIG Wind Turbines
Authors: Ali H. Kasem Alaboudy
Heba A. Mahmoud
Adel A. Elbaset
Montaser Abdelsattar
Publication date: 06-07-2023
Publisher: Springer Berlin Heidelberg
Published in: Arabian Journal for Science and Engineering / Issue 11/2023
Print ISSN: 2193-567X
Electronic ISSN: 2191-4281
DOI: https://doi.org/10.1007/s13369-023-07975-7

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