# Fuzzy-Based EV Charging Station and DVR-Fed Voltage Compensation for a DFIG-Fed Wind Energy System during Grid Faults

Authors: R. Uthra; D. Suchitra; Thanikanti Sudhakar Babu; Belqasem Aljafari
Journal: International Transactions on Electrical Energy Systems
 (2022)

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## Abstract

In recent years, it can be seen that more and more wind energy systems are integrated to focus on developing a more reliable energy system. A doubly-fed induction generator is the most employed machine in wind energy systems having the advantages of variable speed operation, improved power quality, and high energy capture. In a wind energy conversion system (WECS), the generator’s capability to remain connected during short electric faults resulting in voltage sag is known as fault ride through (FRT). Over the last few years, electric vehicles have been providing a remarkable solution for many sustainability issues such as global warming, depletion of fossil fuel reserves, and emission of greenhouse gas that needs attention to detail. A voltage compensation using Dynamic Voltage Restorer and Electric Vehicle charging station both employing a fuzzy controller is proposed in this paper for sustaining FRT capability. The variation in the stator voltage is tracked and utilized to inject the necessary shortfall of voltage in the system via DVR or EV charging station for the intensity of the created voltage sag. Vehicle-to-grid unit of the electric vehicle charging station comes into action when voltage sag intensity is 0.9 p.u. to 0.51 p.u. Value of the nominal voltage and the DVR takes over when voltage sag falls between 0.5 p.u. and 0.2 p.u. Consequently, this voltage compensation regulates the other relative parameters like DC link voltage and active power and retains them within the permissible limits during the fault.


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## Body

## 1. Introduction

With increasing awareness with respect to the depletion of natural energy resources, the shift toward renewable energy is the main focus in recent years. In this regard, the changes in the transportation industry have become inevitable, enabling the transition to electric vehicles. Till 2019, approximately 4.8 million of battery electric vehicles (BEVs) were operative globally, and about 1.5 million of new BEVs were introduced throughout the world [1]. Electric vehicles play a very imperative role in sustaining a zero-emission, healthy environment. Thereupon, more and more EV charging stations are being developed to cater to the arising needs for electric vehicles globally. As of March 2021, there are 1800 EV charging stations in India [2]. With this increasing penetration of electric vehicles into the global market, many EV charging stations are being set up to cater to the needs of the EV. An EV charging station can also be utilized as an adaptable load connected to the grid to normalize the fluctuation of voltage during power generation. In this regard, the main idea of this paper is to fuse an EV charging station with a grid integrated wind energy system.The development of modern wind energy conversion systems has been one of the primary sources of energy since 1970 and in recent years WECS has seen a vast penetration in the energy market. In governing the WECS, an important requisite called grid codes was amended in the last decades to measure up to the technicalities needed for a wind farm. These grid codes are divided into two types namely static and dynamic stipulations. Steady-state performance and power flow analysis fall under static stipulations, and the response of the generator during faults like operating voltage range, frequency, and FRT capability comes under dynamic stipulations. Among the grid codes, FRT capability is the most challenging one. FRT identifies the capability of a WECS to continue being connected to the grid, for a short duration of time when a fault occurs at the grid. FRT basically warrants any fault which can be usually rectified, and evidently there is absolutely no loss in wind power being generated. Figure1 indicates the FRT characteristics based on per grid codes followed in India [3]. In the characteristics, Vf denotes 15% of the nominal value of the voltage, and Vpf denotes 80% of the nominal value of the voltage. Hence, the wind turbine stays connected with the system within the duration of “T” seconds. This necessitates FRT protection and voltage compensation to recover the voltage within this specified time limit. The wind turbine generators will continue to provide active power proportional to the grid voltage instantly after the fault is cleared.Figure 1
FRT characteristics.Variable speed generators (VSGs) are being increasingly used in WECS as they provide excellent frequency support and active and reactive power supply. Among various types of VSGs available, the Doubly Fed Induction Generator proves to be a major point of interest due to its several advantages. Some DFIGs have independent control concerning active and reactive power, have relatively less mechanical stress, and can be directly connected to the grid. The main issue about DFIG is that it is directly connected to the grid and hence any fault occurring at the PCC makes it very sensitive creating voltage sags [4–7]. Therefore, as per the grid codes, the FRT capability should be effectively carried out, thereby providing voltage compensation depending on the intensity of the voltage sag. Thus, DFIG-based WECS needs auxiliary support for voltage compensation during fault.In grid integrated system, fault diagnosis has attracted growing attention to minimize system downtime and avert severe damage to the power system. As a result, Yang et al. provided a current state-of-the-art assessment on wind converter fault diagnosis of wind energy systems including both model-based and pattern-based techniques [8]. The primary requirement in fault diagnosis is to design two separate control loops, one on the rotor side and the other on the grid side. Many researchers in the literature have explored various control strategies in this regard. Two independent controls, namely rotor side control (RSC) and grid side control (GSC), have been analyzed in detail [9–12]. In all the above papers concerning fault diagnosis, various control circuits involving the control of rotor and grid parameters have been discussed. Out of all these methods, vector or field-oriented control is used largely as it provides competent regulation of rotor and grid parameters.Sardar Kamil et al. [13] reviewed the several control strategies for maintaining the low voltage ride through (LVRT) capability of grid-feeding converters, with their benefits and drawbacks, thereby presenting a comparative listing of various current control loops where vector control is chosen as the primary control with various rotor parameters as reference. In [14], stator flux orientation and Lyapunov theory are proposed as an LVRT solution with an enhanced control technique of stator powers exchanged between the DFIG and the grid. Geng et al. developed control strategies with grid side converters (GSCs) for unsymmetrical faults in grid integrated wind systems [15, 16]. In all the above-said literature, vector control was taken as the fundamental control scheme with different variables as reference. Hence, considering all this survey, a vector control scheme with reactive power and DC link voltage as references have been adopted for the grid, along with rotor control in this paper. Even with an appropriate control strategy, the voltage sag developed due to fault is not compensated. Hence, it becomes necessary to identify enhancement solutions to achieve FRT.Appropriate solutions for FRT should be provided by protecting, compensating, and regulating the voltage during fault at the grid. These solutions can be provided as an external retrofit or internal control circuitry. External retrofit refers to auxiliary power electronic devices to protect the system and compensate for the voltage in the event of a fault. Flexible AC Transmission System (FACTS) devices have been used to provide supplementary support for a long time. FACTS devices based on power injection are used to achieve FRT and also help to uphold the transient stability of the system. But these devices are added additionally to the grid integrated system with their own control unit which increases the total complexity of the system. The main classification of auxiliary devices for FRT capability is depicted in Figure2. Amongst all the protection methods, the crowbar protection proves to be the simplest and most productive circuit [17]. Noureldeen et al. discussed intelligent protection techniques with a crowbar to attain FRT in DFIG-based WECS [18, 19]. As the main focus of this paper is voltage compensation during the fault, therefore the crowbar protection which has already been established as an effective technique is included in the proposed system with a fundamental crowbar circuit.Figure 2
FRT enhancement methods.The voltage imbalance caused by the fault can be evened out using auxiliary support devices. Some of the largely used support devices are shown in Figure2. The compensation using Static Synchronous Compensator (STATCOM) supercapacitor system at the PCC in DFIG fed wind system is detailed in [20]. STATCOM is primarily a voltage source inverter which is fed from a DC capacitor. Hence, during the event of fault voltage recovery, STATCOM produces a decelerating torque in the generator. Hiremath et al. [21] presented the latest techniques of LVRT for different wind generators using different converter controllers. Many control schemes were discussed in detail and the merits and demerits of different schemes were highlighted. Many of the reviewed control schemes had taken vector control as the basis with different controlling parameters to regulate active and reactive power. FRT capability using Static Var Compensator (SVC) is addressed in [22]. SVC is a shunt-connected FACTS device that improves voltage stability and regulation. The shunt-connected devices are capable of providing smooth and swift steady-state as well as transient voltage control at PCC. But unstable voltage oscillations occur in shunt compensation. Also, this type of compensation is not very effective in high voltage fault conditions, unlike series connections.DVR which is a device connected in series is found to give the best performance and stabilizes the wind generator system very effectively. The biggest advantage of DVR voltage compensation is that during steady-state conditions, the DVR stays idle and only during the duration of fault, depending on the value of voltage, sag appropriate voltage is injected. DVR has been implemented with dual voltage controllers in [23]. This makes the circuit sluggish and thus it takes a longer time for the restoration of voltage. DVR with a conventional PI controller proved to give a better response than the dual voltage controllers [24, 25]. But this paper does not include error tracking in voltage for various faults. He et al. [26] proposed an LVRT scheme for a grid integrated wind/PV hybrid system. This paper has presented an active output control by taking DC link voltage as reference for vector control to provide stable active and reactive power during faults thereby improving the dynamic response of the system. In [27], DVR is operated with a fuzzy controller to regulate the injecting value of voltage during a fault, based on the voltage sag. This yielded a fairly good control of DVR, but it was not integrated with any power system but intercepted individually. Hence, in the proposed technique, a fuzzy controlled DVR for voltage compensation as a secondary compensation device during fault is integrated with a grid-connected WECS.The number of EV charging stations that are allotted incentives in various states in India, as of January 2020, is shown in Table1 [28]. With the current and upcoming trend of EV charging stations, it can be used as a compensation device in larger rating generation systems rather than introducing an additional supplement device. Based on the demand from the grid and the site of charging stations, EVs collaborate and discharge their energy to the grid for a certain duration of time, thereby adjusting the voltage and frequency of the electric power in the grid. The batteries themselves in EV charging stations are powered using independent renewable energy sources and thus completely making the system sustainable in all aspects. In this regard, an idea of an EV charging station available nearest to the renewable power generation system has been slated as a primary compensation in this paper. On reviewing the literature concerning this type of compensation, various papers support the credibility of this idea.Table 1
EV chargers sanctioned under FAME.
S. No.StateAllotment for EV charging stations1)Maharashtra3172)Andhra Pradesh2663)Tamil Nadu2564)Gujarat2285)Uttar Pradesh2076)Rajasthan2057)Karnataka1728)Madhya Pradesh1599)West Bengal14110)Telangana13811)Kerala13112)Delhi7213)Chandigarh7014)Haryana5015)Meghalaya4016)Bihar3717)Sikkim2918)Jammu and Kashmir2519)Chhattisgarh2520)Assam2021)Odisha1822)Uttarakhand1023)Puducherry1024)Himachal Pradesh10In a hybrid AC/DC system, a coordinative control in charging hybrid electric vehicles has been introduced [29]. A vehicle-to-grid (V2G) unit is explored, where a grid-connected electric vehicle charging station supplies a distributed system based on the deviation in the frequency at the plug-in terminal, depending on the imbalance in the power grid [30]. Hence, this paper mainly contemplates frequency deviation at PCC rather than voltage variation during faults. Mohammad et al. [31] articulated that EVs can operate as a supplementary unit, to minimize the challenges related to renewable energy systems and thus provide ancillary services to the grid, in regulating voltage and frequency. But the DFIG-based wind system is more proactive concerning energy management. Hence, applying the above-said compensation to a DFIG-based system will prove to be more effective in managing power during faults.Literature [32, 33] focused on the contemporary modeling of grid-connected PV systems tied to EV charging stations. This idea can be extended to wind energy systems integrated with EV charging stations for voltage compensation. In all the above-discussed V2G units, the battery charging units are regulated with a PI controller.Taking all the literature review into consideration, it can be seen that the voltage compensation during fault has been carried out with only one primary device. Table2 summarizes the benefits of various sectors on integrating electric vehicles into the grid [25]. This paper proposes a technique that involves a primary as well as a secondary compensation using a V2G unit and a DVR. The EV charging station is becoming a needful one with more and more electric vehicles being introduced as a form of green energy unit. Hence utilizing the battery unit available in the charging station as a support device in addition to a secondary unit of DVR is the main novelty presented in the proposed system. In the proposed WECS, a V2G unit with a fuzzy controlled battery unit acts as a primary source of compensation device providing precise control during charging and discharging of the batteries, and a fuzzy controlled DVR is used as a secondary compensating device. A fuzzy-based control provides clear compensation and control even in the vague input conditions as well as transient conditions.Table 2
Electric vehicle grid integration benefits.
S. No.SectorsBenefits1.Distribution system(i) Delay of distribution capacity upgrades(ii) Better assimilation of renewables and reduction in backflow on the distribution system in high solar penetration areas(iii) Voltage support and resiliency services (reliability services)2.Transmission system(i) Delay of transmission capacity expansion(ii) Better integration of renewables(iii) Primary frequency response and voltage support(iv) Operational efficiency and black-start services3.Wholesale market(i) Primary and secondary regulation of frequency (response)(ii) Reactive power support and management of energy imbalances4.Compliance(i) Utilities/consumers: achievement of regulatory requirements and/or renewable energy targets(ii) Cities/states: achievement of renewable energy targets5.Societal and environment(i) Reduction of power system/transport sector’s carbon footprint(ii) Reduction of criteria air pollutants and air toxics(iii) Creation of economic opportunities(iv) Generation of employment and upskilling opportunitiesThe significant contribution of this paper is that the two support devices will operate based on the voltage sag intensity.(i)
The feasibility of using an electric vehicle charging station as an auxiliary support device during fault to achieve FRT capability has been analyzed and successfully achieved.(ii)
A venture has been made to devise a strategy in using DVR and EV chargers as primary and secondary compensation devices depending on the intensity of the fault in grid integrated wind systems.(iii)
The EV charging station has to charge electric vehicles; hence, the proposed system has been designed in such a way that when the intensity of voltage sag is less (0.9 p.u. to 0.51 p.u.), the EV charging station supplies voltage to the grid to restore the original voltage.(iv)
As for the fault with higher intensity voltage sag (0.5 p.u. to 0.2 p.u.), the secondary device DVR takes over to provide voltage compensation by tracking the deficit voltage.(v)
Both the compensating units of the EV charging station and DVR are regulated with a fuzzy controller and can provide accurate compensation with a wide range of operating conditions, unlike PI controllers.The course of this paper is organized as follows: Section2 outlines the modeling and control of WECS. The design and control of DVR and EV charging stations are discussed in Sections 3 and 4 respectively. Section 5 illustrates the different modes of operation of the complete system which is followed by the discussion of simulation results in Section 6. The conclusion of the paper is proffered in Section 7.


## 2. Wind Energy Conversion System

Wind turbines basically convert kinetic energy to electrical energy as indicated in Figure3. Maximum power is extracted from the wind turbine using an MPPT controller. The DFIG generates electrical output. Operating a wind system, there involves two controllers namely RSC and GSC.Figure 3
Wind energy conversion system with RSC and GSC.RSC and GSC have connected with a capacitor called a DC link capacitor and the voltage that appears across this capacitor is denoted asVDC. The primary role of RSC is to control and regulate the rotor parameters. Similarly, GSC does the job of sustaining the DC link voltage to a constant value. The output from the grid side inverter is fed to the grid which in turn feeds the stator of the DFIG.


### 2.1. Modeling of DFIG

DFIG parameters are defined in dq reference frame for both stator and rotor [34, 35]. The grid voltage is linked with the DFIG stator. The DFIG equivalent circuit is depicted in Figure 4 with all the voltage and current parameters represented in dq reference frame.Figure 4
Doubly fed induction generator equivalent circuit.Descriptive equations of DFIG are [19](1)Vds=Rsids+dλdsdt−ωeλqs,Vqs=Rsiqs+dλqsdt–ωeλds,Vdr=Rridr+dλdrdt−ωe−ωrλqs,Vqr=Rriqr+dλqrdt−ωe−ωrλqs,where Vds, Vqs  = dq axes stator voltages, Vdr, Vqr  = dq axes rotor voltages, ids, iqs  = dq axes stator currents, idr, iqr  = dq axes rotor currents, λds,λqs  = stator flux linkages of dq axes, λdr,λqr= rotor flux linkages of dq axes, and Rs,Rr= resistances of stator and rotor.Active and reactive powers are(2)Ps=32Vqsiqs+Vdsids,Qs=32Vqsiqs−Vdsids.


### 2.2. Rotor Side Converter

RSC as shown in Figure5 is made up of power converters with their own control unit. The active and reactive powers are controlled by establishing minimum power loss during power conversion and are achieved through a vector control scheme. In this scheme, the measured three-phase rotor currents are converted to their corresponding dq axes equivalents. The d-axis parameter controls the reactive power and the q-axis the active power. The current in d-axis (id) and the current in q-axis (iq) are obtained and are then fed to the PI controllers to obtain the voltage reference values which are fed to a pulse width modulated converter to generate converter pulses.Figure 5
Rotor side converter.


### 2.3. Grid Side Converter

The GSC in Figure6 usually governs the voltage at the DC link capacitor in turn controlling the active and reactive power. A vector control scheme is adopted here which maneuvers the grid parameters such that the voltage at DC link between the two converters remains constant. The grid parameters are observed and translated to their equivalent dq values, which are then aligned with that of the grid values. The three-phase grid voltages serve as the reference for this alignment, which is accomplished with the use of a phase-locked loop. Once this is accomplished, the PI controller is fed with the d-axis and q-axis grid currents to obtain the reference value for the PWM controller for the generation of pulses to the converter. Since the rotor’s active power must flow through the DC link, it helps in regulating the active power precisely.Figure 6
Grid side converter.


## 2.1. Modeling of DFIG

DFIG parameters are defined in dq reference frame for both stator and rotor [34, 35]. The grid voltage is linked with the DFIG stator. The DFIG equivalent circuit is depicted in Figure 4 with all the voltage and current parameters represented in dq reference frame.Figure 4
Doubly fed induction generator equivalent circuit.Descriptive equations of DFIG are [19](1)Vds=Rsids+dλdsdt−ωeλqs,Vqs=Rsiqs+dλqsdt–ωeλds,Vdr=Rridr+dλdrdt−ωe−ωrλqs,Vqr=Rriqr+dλqrdt−ωe−ωrλqs,where Vds, Vqs  = dq axes stator voltages, Vdr, Vqr  = dq axes rotor voltages, ids, iqs  = dq axes stator currents, idr, iqr  = dq axes rotor currents, λds,λqs  = stator flux linkages of dq axes, λdr,λqr= rotor flux linkages of dq axes, and Rs,Rr= resistances of stator and rotor.Active and reactive powers are(2)Ps=32Vqsiqs+Vdsids,Qs=32Vqsiqs−Vdsids.


## 2.2. Rotor Side Converter

RSC as shown in Figure5 is made up of power converters with their own control unit. The active and reactive powers are controlled by establishing minimum power loss during power conversion and are achieved through a vector control scheme. In this scheme, the measured three-phase rotor currents are converted to their corresponding dq axes equivalents. The d-axis parameter controls the reactive power and the q-axis the active power. The current in d-axis (id) and the current in q-axis (iq) are obtained and are then fed to the PI controllers to obtain the voltage reference values which are fed to a pulse width modulated converter to generate converter pulses.Figure 5
Rotor side converter.


## 2.3. Grid Side Converter

The GSC in Figure6 usually governs the voltage at the DC link capacitor in turn controlling the active and reactive power. A vector control scheme is adopted here which maneuvers the grid parameters such that the voltage at DC link between the two converters remains constant. The grid parameters are observed and translated to their equivalent dq values, which are then aligned with that of the grid values. The three-phase grid voltages serve as the reference for this alignment, which is accomplished with the use of a phase-locked loop. Once this is accomplished, the PI controller is fed with the d-axis and q-axis grid currents to obtain the reference value for the PWM controller for the generation of pulses to the converter. Since the rotor’s active power must flow through the DC link, it helps in regulating the active power precisely.Figure 6
Grid side converter.


## 3. Modeling of DVR

DVR is connected in series with the grid and injects a three-phase voltage at PCC to recover the deficit voltage during faults. DVR is a unit comprising a DC power source and an inverter with an injection transformer. The switching signals for the inverter are generated using the pulse width modulation technique. Thus, DVR compensates for the sag arising due to fault thus securing the grid as well the wind turbine from any potential damage. The governing equations of a DVR model are represented in (3) to (4) [23].The total power delivered is(3)SDVR=Vinj∗IL,(4)Vinj=2∗VL,where IL  = load current, Vinj  = injected voltage, and VL  = load voltage.


### 3.1. Control Strategy of DVR

In the proposed method, two FLCs, each for direct and quadrature axis of voltage, are designated to control DVR with two input and one output function as shown in Figure7. The voltage at the load varies when there occurs a fault at the grid. Hence, the load voltage when compared with the reference grid voltage will yield an error value. These error values in all the phases are taken as the inputs for the fuzzy controller. The primary input is the error voltage (eVdL, eVqL), which is derived as the difference between the rated voltage and the varying voltage at load during fault. The other input is the change in error voltage (∆eVdL, ∆eVqL). The output is the control voltage (Vd, Vq) given as the reference voltage to the PWM controller of the inverter.Figure 7
The fuzzy control diagram of DVR.


### 3.2. Fuzzy Logic Controller of DVR

Even though PI is a traditional controller, it does not work well in changing and noisy environments. Also, there are many cases where superfluous proportional action may lead to unsteady output and an excessive integral action may lead to overshoots. To overcome this, a fuzzy operated DVR is explored in this research paper in the wind energy system. Fuzzy logic control (FLC) is a control system in which the net output depends on the state of the input and the corresponding change in the state of the input. Hence, fuzzy control yields more precise voltage restoration as close to that of nominal values, unlike a PI controller. As per the proposed method in this paper, two FLCs are used. One FLC is for the d-axis of the stator voltage and the other FLC is for the q-axis of the stator voltage each with two numbers of input membership functions (MFs) which are the error voltage and change in error voltage.


#### 3.2.1. Membership Functions

In both FLCs, eight variables are assigned for the MF error and three variables are assigned for MF change in error respectively. The output MF has thirteen linguistic variables. Figures8–10 illustrate the MFs of the input and output which are normalized between the values 1 and −1. The rules followed by the MFs are furnished in Table 3.Figure 8
eVdL and eVqL—input MF.Figure 9
ΔeVdL and ΔeVqL—input MF.Figure 10
Vd and Vq—output MF.Table 3
Rule base for FLC of DVR.
∆eVLeVLVLPLPLMPMPSMPVSPZNPVLP3VLP2LP2MP2SMP2VSP2ZNZVLP3VLP1LP1MP1SMP1VSP1ZNNVLP3VLP1LP1MP1SMP1VSP1ZNThe rule viewer for FLC is shown in Figures11 and 12 where error voltages are denoted by Vd and Vq respectively and changes in error voltages are denoted by DelVd and DelVq respectively. Vd∗ and Vq∗ are the outputs of the two FLCs each for the direct axis and the quadrature axis.Figure 11
Rule viewer forVd.Figure 12
Rule viewer forVq.


## 3.1. Control Strategy of DVR

In the proposed method, two FLCs, each for direct and quadrature axis of voltage, are designated to control DVR with two input and one output function as shown in Figure7. The voltage at the load varies when there occurs a fault at the grid. Hence, the load voltage when compared with the reference grid voltage will yield an error value. These error values in all the phases are taken as the inputs for the fuzzy controller. The primary input is the error voltage (eVdL, eVqL), which is derived as the difference between the rated voltage and the varying voltage at load during fault. The other input is the change in error voltage (∆eVdL, ∆eVqL). The output is the control voltage (Vd, Vq) given as the reference voltage to the PWM controller of the inverter.Figure 7
The fuzzy control diagram of DVR.


## 3.2. Fuzzy Logic Controller of DVR

Even though PI is a traditional controller, it does not work well in changing and noisy environments. Also, there are many cases where superfluous proportional action may lead to unsteady output and an excessive integral action may lead to overshoots. To overcome this, a fuzzy operated DVR is explored in this research paper in the wind energy system. Fuzzy logic control (FLC) is a control system in which the net output depends on the state of the input and the corresponding change in the state of the input. Hence, fuzzy control yields more precise voltage restoration as close to that of nominal values, unlike a PI controller. As per the proposed method in this paper, two FLCs are used. One FLC is for the d-axis of the stator voltage and the other FLC is for the q-axis of the stator voltage each with two numbers of input membership functions (MFs) which are the error voltage and change in error voltage.


### 3.2.1. Membership Functions

In both FLCs, eight variables are assigned for the MF error and three variables are assigned for MF change in error respectively. The output MF has thirteen linguistic variables. Figures8–10 illustrate the MFs of the input and output which are normalized between the values 1 and −1. The rules followed by the MFs are furnished in Table 3.Figure 8
eVdL and eVqL—input MF.Figure 9
ΔeVdL and ΔeVqL—input MF.Figure 10
Vd and Vq—output MF.Table 3
Rule base for FLC of DVR.
∆eVLeVLVLPLPLMPMPSMPVSPZNPVLP3VLP2LP2MP2SMP2VSP2ZNZVLP3VLP1LP1MP1SMP1VSP1ZNNVLP3VLP1LP1MP1SMP1VSP1ZNThe rule viewer for FLC is shown in Figures11 and 12 where error voltages are denoted by Vd and Vq respectively and changes in error voltages are denoted by DelVd and DelVq respectively. Vd∗ and Vq∗ are the outputs of the two FLCs each for the direct axis and the quadrature axis.Figure 11
Rule viewer forVd.Figure 12
Rule viewer forVq.


## 3.2.1. Membership Functions

In both FLCs, eight variables are assigned for the MF error and three variables are assigned for MF change in error respectively. The output MF has thirteen linguistic variables. Figures8–10 illustrate the MFs of the input and output which are normalized between the values 1 and −1. The rules followed by the MFs are furnished in Table 3.Figure 8
eVdL and eVqL—input MF.Figure 9
ΔeVdL and ΔeVqL—input MF.Figure 10
Vd and Vq—output MF.Table 3
Rule base for FLC of DVR.
∆eVLeVLVLPLPLMPMPSMPVSPZNPVLP3VLP2LP2MP2SMP2VSP2ZNZVLP3VLP1LP1MP1SMP1VSP1ZNNVLP3VLP1LP1MP1SMP1VSP1ZNThe rule viewer for FLC is shown in Figures11 and 12 where error voltages are denoted by Vd and Vq respectively and changes in error voltages are denoted by DelVd and DelVq respectively. Vd∗ and Vq∗ are the outputs of the two FLCs each for the direct axis and the quadrature axis.Figure 11
Rule viewer forVd.Figure 12
Rule viewer forVq.


## 4. Electric Vehicle Charging Station

A vehicle-to-grid unit as shown in Figure13 integrates the EV charging station to that of the grid. During the event of a fault, the wind system should stay connected to the grid as per the grid codes. Within this short duration, the voltage sag due to fault should be compensated instantly to avoid the disconnection of the wind source. Hence, the battery of the EV charging station provides compensation about the voltage sag intensity. The DC voltage of the battery is fed to an inverter of appropriate ratings. The output of the inverter is connected to the grid so that the voltage compensation takes place effectively and rapidly when a fault occurs.Figure 13
Voltage compensation with V2G unit.


### 4.1. EV Charging Station Structure

The EV charging station structure shown in Figure14 is mainly powered by a PV source and includes bidirectional converters in which the energy can flow in both directions of the converter. A bidirectional converter regulates the voltage by regulating the duty cycle of the PWM controller with a certain frequency and duty ratio. By the variation of firing pulses, the buck and boost mode of the bidirectional converter charges and discharges the battery in the EV charging station depending on the necessity. The voltage during the battery discharge is conditioned and tied to the grid to supplement the voltage sag due to fault. The control unit of the charging station uses a P&O MPPT controller to retrieve maximum power from the PV source and generates a duty cycle for the PWM generator of the DC-DC converter. The pulses for the bidirectional converter are controlled and regulated by a fuzzy controller. The EV charging station is energized by a PV source whose parameters are listed in Table 4. An MPPT controller extracts maximum power from the source and it is fed to a boost converter as indicated in Figure 15. The output from the boost converter is fed to a bidirectional inverter for charging the battery. The bidirectional converter charges or discharges alternately depending on its pulses. These pulses are controlled using a fuzzy controller. The fuzzy controller has two input membership (Vd, SoC) functions and one output membership function (I). The output of the fuzzy controller along with its reference voltage variables is fed to a PI controller which generates pulses for the bidirectional converter based on (5). The complement of (5) yields pulses for discharging conditions.(5)V=Vd+kpIbatt−Ibatt∗+ki∫Ibatt−Ibatt∗dt,where V = reference voltage for PWM controller for charging condition, Vd = DC-DC converter voltage, V, I∗batt = reference current, Ibatt = battery current, kp = proportional constant of the PI controller, and ki = integral constant of the PI controller.Figure 14
EV charging station.Table 4
Electric vehicle design parameters.
ParametersValueDC-DC converterL5 mHC100µFBidirectional converterL0.5 mHC1 mFFigure 15
(a)Vd—input MF. (b) SoC—input MF. (c) I—output MF.


#### 4.1.1. Membership Functions

The MFs have five linguistic variables as shown in Figure15. The rules for the FLC are depicted in Table 5. The voltage from the DC/DC converter (Vd) and State of Charge (SoC) of the battery are intercepted as the inputs to the FLC and the current is intercepted as the output of the FLC.Table 5
Rules for FLC of EV charging station.
VdSoCPVBPBPMPSPVSPBPBNBNBPBPBPSPBNSNSPSPSZOPBPSZOZOZONSPBPSPSPBPBNBPBPBPBPBPB


## 4.1. EV Charging Station Structure

The EV charging station structure shown in Figure14 is mainly powered by a PV source and includes bidirectional converters in which the energy can flow in both directions of the converter. A bidirectional converter regulates the voltage by regulating the duty cycle of the PWM controller with a certain frequency and duty ratio. By the variation of firing pulses, the buck and boost mode of the bidirectional converter charges and discharges the battery in the EV charging station depending on the necessity. The voltage during the battery discharge is conditioned and tied to the grid to supplement the voltage sag due to fault. The control unit of the charging station uses a P&O MPPT controller to retrieve maximum power from the PV source and generates a duty cycle for the PWM generator of the DC-DC converter. The pulses for the bidirectional converter are controlled and regulated by a fuzzy controller. The EV charging station is energized by a PV source whose parameters are listed in Table 4. An MPPT controller extracts maximum power from the source and it is fed to a boost converter as indicated in Figure 15. The output from the boost converter is fed to a bidirectional inverter for charging the battery. The bidirectional converter charges or discharges alternately depending on its pulses. These pulses are controlled using a fuzzy controller. The fuzzy controller has two input membership (Vd, SoC) functions and one output membership function (I). The output of the fuzzy controller along with its reference voltage variables is fed to a PI controller which generates pulses for the bidirectional converter based on (5). The complement of (5) yields pulses for discharging conditions.(5)V=Vd+kpIbatt−Ibatt∗+ki∫Ibatt−Ibatt∗dt,where V = reference voltage for PWM controller for charging condition, Vd = DC-DC converter voltage, V, I∗batt = reference current, Ibatt = battery current, kp = proportional constant of the PI controller, and ki = integral constant of the PI controller.Figure 14
EV charging station.Table 4
Electric vehicle design parameters.
ParametersValueDC-DC converterL5 mHC100µFBidirectional converterL0.5 mHC1 mFFigure 15
(a)Vd—input MF. (b) SoC—input MF. (c) I—output MF.


### 4.1.1. Membership Functions

The MFs have five linguistic variables as shown in Figure15. The rules for the FLC are depicted in Table 5. The voltage from the DC/DC converter (Vd) and State of Charge (SoC) of the battery are intercepted as the inputs to the FLC and the current is intercepted as the output of the FLC.Table 5
Rules for FLC of EV charging station.
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## 4.1.1. Membership Functions

The MFs have five linguistic variables as shown in Figure15. The rules for the FLC are depicted in Table 5. The voltage from the DC/DC converter (Vd) and State of Charge (SoC) of the battery are intercepted as the inputs to the FLC and the current is intercepted as the output of the FLC.Table 5
Rules for FLC of EV charging station.
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## 5. Modes of Operation

### 5.1. Normal Mode

In this mode of operation as indicated in Figure16, the power produced from the WECS is integrated with the grid. The DFIG-based wind system generates AC voltage which is converted to DC using RSC. The converted voltage is stored in the DC link capacitor. The DC link voltage is converted to AC via an inverter. The inverted AC output is integrated with the grid. In an EV charging station, the battery is charged by a separate PV source and continues to charge electric vehicles and the DVR stays idle in this mode of operation. Even though the output from the EV charging station and DVR is bound to the grid, in this operating mode the charging station or the DVR does not impact the grid integrated hybrid system. When the wind system encounters the fault, the charging station or the DVR takes over immediately pertaining to the intensity of the voltage sag thereby supplementing the shortfall voltage that is developed due to the fault.Figure 16
Normal mode of operation.


### 5.2. Fault-Ride-Through Mode

This mode of operation is called FRT as a fault inculcated at PCC. During the event of a fault, the wind system should stay connected to the grid as per the grid codes indicated in Figure1. Within this short duration, the voltage sag due to fault should be compensated instantly to avoid the disconnection of the wind source. Hence, in this mode either the battery of the EV charging station discharges or the voltage is supplied by DVR, thereby compensating the deficit voltage about the intensity of the sag. The power flow for this mode of operation is shown in Figure 17. The V2G unit and the DVR are controlled using a switch about the intensity of the voltage sag. If voltage sag falls between 0.9 p.u. and 0.51 p.u., the EV charging station provides voltage from the battery which is fed to an inverter of appropriate ratings. The output of the inverter is connected to the grid so that the voltage compensation takes place effectively and rapidly when a fault occurs. For voltage sag falling between 0.5 p.u. and 0.2 p.u., the DVR dominates in providing voltage compensation.Figure 17
Fault ride Through Mode.


## 5.1. Normal Mode

In this mode of operation as indicated in Figure16, the power produced from the WECS is integrated with the grid. The DFIG-based wind system generates AC voltage which is converted to DC using RSC. The converted voltage is stored in the DC link capacitor. The DC link voltage is converted to AC via an inverter. The inverted AC output is integrated with the grid. In an EV charging station, the battery is charged by a separate PV source and continues to charge electric vehicles and the DVR stays idle in this mode of operation. Even though the output from the EV charging station and DVR is bound to the grid, in this operating mode the charging station or the DVR does not impact the grid integrated hybrid system. When the wind system encounters the fault, the charging station or the DVR takes over immediately pertaining to the intensity of the voltage sag thereby supplementing the shortfall voltage that is developed due to the fault.Figure 16
Normal mode of operation.


## 5.2. Fault-Ride-Through Mode

This mode of operation is called FRT as a fault inculcated at PCC. During the event of a fault, the wind system should stay connected to the grid as per the grid codes indicated in Figure1. Within this short duration, the voltage sag due to fault should be compensated instantly to avoid the disconnection of the wind source. Hence, in this mode either the battery of the EV charging station discharges or the voltage is supplied by DVR, thereby compensating the deficit voltage about the intensity of the sag. The power flow for this mode of operation is shown in Figure 17. The V2G unit and the DVR are controlled using a switch about the intensity of the voltage sag. If voltage sag falls between 0.9 p.u. and 0.51 p.u., the EV charging station provides voltage from the battery which is fed to an inverter of appropriate ratings. The output of the inverter is connected to the grid so that the voltage compensation takes place effectively and rapidly when a fault occurs. For voltage sag falling between 0.5 p.u. and 0.2 p.u., the DVR dominates in providing voltage compensation.Figure 17
Fault ride Through Mode.


## 6. Discussion of Simulation Results

A2.5 MW capacity WECS connected to the grid with DVR was taken as a test system and was simulated. The system data of the test system are furnished in Tables6 and 7, respectively. Figures 18(a) and 18(b) portray the wind curve of the wind energy system and PV curves of the EV charger system. The design specifications of the EV charging station are listed in Tables 4 and 8.Table 6
DFIG machine ratings.
DFIG characteristicsValuePower2.5 MWStator voltage690 VStator current1760 AFrequency50 HzStator connectionStarRotor connectionStarPoles2Rs2.6e − 3 ΩLs0.087e − 3 HLm2.5e − 3 HRr2.9e − 3 ΩTable 7
DVR ratings.
DVR parametersValuePower2.5 MVAL0.1 mHC1μFSwitching frequency1000 HzFigure 18
(a) Wind power curve and (b) PV curve.Table 8
PV parameters of EV charging station.
PV parametersValuePower1 MWVoc37.3 VIsc8.66 AVmp30.7 VImp8.15 ACells per module60Parallel strings88Series strings45


### 6.1. Low Voltage Ride Through

LVRT has been tested for a voltage sag occurring due to various faults such as three-, two-, and single-phase faults. Figure19(a) indicates a three-phase fault at the grid and the voltage compensation using DVR and EV charging stations depending on voltage sag intensity is indicated in Figures 19(b)–19(d), respectively. In all three cases of fault as it can be observed, the voltage compensation is provided by the DVR or V2G unit depending on the intensity of voltage sag.Figure 19
Three-phase fault. (a) Fault voltage. (b) Injected voltage using DVR. (c) Injected voltage using EV. (d) Voltage compensation.Two-phase fault (Phase A and Phase B) is indicated in Figure20(a) with its voltage compensation using DVR and EV charging station shown in Figures 20(b)–20(d). Phases A and B are exclusively provided with the compensation. Figure 20(b) depicts the injected voltage using DVR and Figure 20(c) indicates EV compensated voltage.Figure 20
Two-phase fault. (a) Fault voltage in Phases B and C. (b) Injected voltage using DVR. (c) Injected voltage using EV. (d) Voltage compensation.Likewise, the single-phase fault and its compensation are illustrated in Figure21. Figures 22(a)–22(c) show single-phase fault (Phase A) along with its compensation using DVR and EV charging station. Figure 22(d) shows the net compensated voltage together with the DVR and EV charging station.Figure 21
(a) Three-phase fault current. (b) Three-phase compensated current. (c) Two-phase fault current. (d) Two-phase compensated current. (e) Single-phase fault current. (f) Single-phase compensated current.
(a)(b)(c)(d)(e)(f)Figure 22
(a) Voltage during single-phase fault (Phase A). (b) Injected voltage using DVR. (c) Injected voltage using EV. (d) Voltage compensation for single-phase fault.The stator current output for different faults is shown in Figure21. When a fault occurs, due to the resulting voltage sag in the faulty phases, there is a sharp rise in current to a very high value which is shown in Figures 21(a), 21(c), and 21(e) for three-, two-, and single-phase faults successively. When DVR and EV charging stations offer voltage compensation, the restoration should include the current to fall back to its nominal range as well. This is achieved as shown in Figures 21(b), 21(d), and 21(f), respectively, for three-, two-, and single-phase fault.The DC link voltage is an important parameter in providing a constant voltage to the grid. Whenever a fault occurs, owing to its accompanying voltage sag, there is an immediate drop in DC link voltage. One of the other jobs of any voltage compensation network is to retain a steady DC link voltage and active power. DVR and V2G compensation as can be seen in Figure23 shifts the sudden drop in the DC link voltage and brings it to the nominal value. The regulation of DC voltage concerning both PI and fuzzy controller for V2G and DVR compensation is indicated in Figure 24. The fuzzy controller provides better stabilization of DC link voltage as compared to the PI controller in maintaining the constant DC link voltage during the different intensities of faults at 2 and 2.5 seconds respectively.Figure 23
DC link voltage.Figure 24
Active power for a wind speed of 9 m/s.The power curve in Figure24 highlights the DVR compensation and V2G compensation both wielding fuzzy control, thereby providing excellent compensation in maintaining constant active power at PCC.


### 6.2. DVR Output Voltage

Table9 encapsulates the DVR voltage compensation between 2 and 2.2 seconds for different faults in p.u. values for PI and fuzzy controller. With three-phase faults, there is a uniform dip in all the three-phase voltages. It can be seen that with fuzzy controlled DVR, the voltage recovered is close to the nominal value for all the faults when compared to PI control.Table 9
DVR compensated voltage with PI and fuzzy control.
DVR controllerFault voltage in p.u.Compensated voltage in p.u.Phase APhase BPhase CPhase APhase BPhase CThree-phase faultPI0.36020.36130.35040.95641.1840.9793Fuzzy0.36010.36140.35051.0551.0151.021Two-phase faultPI1.020.38620.32651.021.1691.088Fuzzy1.020.38640.32661.021.0281.023Single-phase faultPI1.0271.0120.31051.0291.0120.8846Fuzzy1.0271.0120.31031.0211.011.044


### 6.3. EV Battery Charging and Discharging

The battery in the EV charging station will continue to charge from its PV source during the normal operation of the hybrid system. When a fault occurs, the battery automatically discharges to the grid to supplement the deficit of voltage occurring due to voltage sag. Figure25 indicates the battery SoC when a fault occurs between the instant 2.5 and 2.7 seconds with PI and FLC. It can be seen that the battery begins to discharge at exactly the instant of 2.5 seconds. At exactly 2.7 seconds, the battery with fuzzy control again resumes charging as the fault period with less intensity is only between 2.5 and 2.7 seconds. But, with the PI controller, the charging resumes only after 2.8 seconds as indicated in Figure 25. Hence, fuzzy control provides more precise control in balancing the charging and discharging period than PI control.Figure 25
V2G compensation voltage for 3 phases.


## 6.1. Low Voltage Ride Through

LVRT has been tested for a voltage sag occurring due to various faults such as three-, two-, and single-phase faults. Figure19(a) indicates a three-phase fault at the grid and the voltage compensation using DVR and EV charging stations depending on voltage sag intensity is indicated in Figures 19(b)–19(d), respectively. In all three cases of fault as it can be observed, the voltage compensation is provided by the DVR or V2G unit depending on the intensity of voltage sag.Figure 19
Three-phase fault. (a) Fault voltage. (b) Injected voltage using DVR. (c) Injected voltage using EV. (d) Voltage compensation.Two-phase fault (Phase A and Phase B) is indicated in Figure20(a) with its voltage compensation using DVR and EV charging station shown in Figures 20(b)–20(d). Phases A and B are exclusively provided with the compensation. Figure 20(b) depicts the injected voltage using DVR and Figure 20(c) indicates EV compensated voltage.Figure 20
Two-phase fault. (a) Fault voltage in Phases B and C. (b) Injected voltage using DVR. (c) Injected voltage using EV. (d) Voltage compensation.Likewise, the single-phase fault and its compensation are illustrated in Figure21. Figures 22(a)–22(c) show single-phase fault (Phase A) along with its compensation using DVR and EV charging station. Figure 22(d) shows the net compensated voltage together with the DVR and EV charging station.Figure 21
(a) Three-phase fault current. (b) Three-phase compensated current. (c) Two-phase fault current. (d) Two-phase compensated current. (e) Single-phase fault current. (f) Single-phase compensated current.
(a)(b)(c)(d)(e)(f)Figure 22
(a) Voltage during single-phase fault (Phase A). (b) Injected voltage using DVR. (c) Injected voltage using EV. (d) Voltage compensation for single-phase fault.The stator current output for different faults is shown in Figure21. When a fault occurs, due to the resulting voltage sag in the faulty phases, there is a sharp rise in current to a very high value which is shown in Figures 21(a), 21(c), and 21(e) for three-, two-, and single-phase faults successively. When DVR and EV charging stations offer voltage compensation, the restoration should include the current to fall back to its nominal range as well. This is achieved as shown in Figures 21(b), 21(d), and 21(f), respectively, for three-, two-, and single-phase fault.The DC link voltage is an important parameter in providing a constant voltage to the grid. Whenever a fault occurs, owing to its accompanying voltage sag, there is an immediate drop in DC link voltage. One of the other jobs of any voltage compensation network is to retain a steady DC link voltage and active power. DVR and V2G compensation as can be seen in Figure23 shifts the sudden drop in the DC link voltage and brings it to the nominal value. The regulation of DC voltage concerning both PI and fuzzy controller for V2G and DVR compensation is indicated in Figure 24. The fuzzy controller provides better stabilization of DC link voltage as compared to the PI controller in maintaining the constant DC link voltage during the different intensities of faults at 2 and 2.5 seconds respectively.Figure 23
DC link voltage.Figure 24
Active power for a wind speed of 9 m/s.The power curve in Figure24 highlights the DVR compensation and V2G compensation both wielding fuzzy control, thereby providing excellent compensation in maintaining constant active power at PCC.


## 6.2. DVR Output Voltage

Table9 encapsulates the DVR voltage compensation between 2 and 2.2 seconds for different faults in p.u. values for PI and fuzzy controller. With three-phase faults, there is a uniform dip in all the three-phase voltages. It can be seen that with fuzzy controlled DVR, the voltage recovered is close to the nominal value for all the faults when compared to PI control.Table 9
DVR compensated voltage with PI and fuzzy control.
DVR controllerFault voltage in p.u.Compensated voltage in p.u.Phase APhase BPhase CPhase APhase BPhase CThree-phase faultPI0.36020.36130.35040.95641.1840.9793Fuzzy0.36010.36140.35051.0551.0151.021Two-phase faultPI1.020.38620.32651.021.1691.088Fuzzy1.020.38640.32661.021.0281.023Single-phase faultPI1.0271.0120.31051.0291.0120.8846Fuzzy1.0271.0120.31031.0211.011.044


## 6.3. EV Battery Charging and Discharging

The battery in the EV charging station will continue to charge from its PV source during the normal operation of the hybrid system. When a fault occurs, the battery automatically discharges to the grid to supplement the deficit of voltage occurring due to voltage sag. Figure25 indicates the battery SoC when a fault occurs between the instant 2.5 and 2.7 seconds with PI and FLC. It can be seen that the battery begins to discharge at exactly the instant of 2.5 seconds. At exactly 2.7 seconds, the battery with fuzzy control again resumes charging as the fault period with less intensity is only between 2.5 and 2.7 seconds. But, with the PI controller, the charging resumes only after 2.8 seconds as indicated in Figure 25. Hence, fuzzy control provides more precise control in balancing the charging and discharging period than PI control.Figure 25
V2G compensation voltage for 3 phases.


## 7. Conclusion

This paper explores the effectiveness of DVR and V2G integrated system-based voltage compensation for the FRT capability of DFIG fed wind energy systems. DVR connected in series with the stator voltage is considered to be a more effective compensation technique for the stator voltage when the voltage sag intensity falls between 0.5 p.u. and 0.2 p.u. during various faults such as three-, two-, and single-phase faults. Similarly, the V2G system comes into action when the voltage sag intensity falls between 0.9 p.u. and 0.51 p.u. The voltage restored reaches the original stator voltage as soon as DVR or the V2G system is switched on. The voltage across the DC link capacitor falls to a low value without any compensation which is also brought to its original value with the compensation. The results showed clearly that employing fuzzy control for both supplementary devices yields a better control than a PI controller. Similarly, the change inactive power due to voltage sag is found to be balanced out with both DVR and V2G units clearly for all the faulty conditions. Thus, the DVR/V2G system is found to give reliable compensation and FRT is achieved during the period of fault.



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*Source: 1860266-2022-07-15.xml*