Control of DFIG-based microgrid and seamless transition from stator connected and disconnected modes

Abstract

This study aims to ameliorate the contribution capability of doubly-fed induction generator (DFIG) to participate in standalone microgrid operation. The islanded microgrid consists of a solar photovoltaic array for solar energy conversion and battery energy storage in addition to DFIG-based wind energy conversion system. Using a simplified control approach, the study describes multi-mode operation of a DFIG-based AC/DC microgrid using a stator-side solid-state transition switch (SSTS). Using SSTS operation, the DFIG stator can be seamlessly disconnected and reconnected from the point of common coupling without interrupting power to the loads in the microgrid. Additionally, non-ideal AC loads can be handled efficiently without the need for computationally exhaustive approaches to enhance stator voltages and currents.

Keywords

Doubly-fed induction generator microgrid power quality solar photovoltaic array wind energy conversion system

Introduction

Access to electricity has become a top concern in rural regions owing to challenges like geography, long distances from the centralized electric power grid, the high cost of power transmission lines, technical limitations in remote locations, and reliability concerns of the electric power supply (Kumar et al., 2018). Renewable energy sources (RES), especially solar and wind, can be a viable way to generate electricity in rural areas (Barik and Pota, 2012; Strunz et al., 2014). This framework can provide rural areas a dependable power supply and can actually be more profitable than the conventional electrification solutions, particularly in the telecommunications, hazardous chemical factories in remote locations, hilly areas, small islands, etc. The introduction of the microgrid concept offers new solutions to incorporate renewable energy and enhance system dependability (Deng et al., 2008; Katiraei and Iravani, 2006). A microgrid based on hybrid renewable sources can supply electricity to a remote location and can either operate completely independently of the grid or reduce the negative impact of RES on the grid. The installed capacity of the microgrid can depend on the specific application, such as for buildings, a neighborhood, a small town, the forecasted availability of renewable energy in that locale, storage capacity and load demand. AC microgrids are the preferred configuration for integrating various distributed energy sources with local demand. However, microgrid configurations are changing to DC nature with the advent of DC power supplies, DC interface loads, and energy storage devices in new power systems. Hybrid AC-DC microgrids reduce multiple power conversions in a single AC or DC microgrid, combining the main advantages of AC and DC microgrid configurations to provide variable AC and DC power supplies and their respective loads simultaneously (Ahmed et al., 2023). As the local energy demand rises, there is an increasing demand for wind turbines to operate in microgrids. Wind power has witnessed the greatest growth among the RESs, primarily as a result of dropping manufacturing cost. For wind energy conversion systems in microgrids, especially in remote areas, the DFIG has been prominently utilized (Shahabi et al., 2009). The ability to operate at varying wind speeds by employing the maximum power extraction techniques in DFIG enables the microgrid to maximize utilization. With the addition of adequate control strategies, DFIGs are capable of providing the required competencies without incurring high costs on the power electronics hardware.

The DFIG based wind energy conversion system WECS, both in standalone and grid connected modes, is the topic of substantial literature. A DFIG-based wind energy system for supplying local loads in standalone mode, with and without battery, has been presented in Swami Naidu and Singh (2016). An attractive feature of a DFIG-based wind power unit is the ability to integrate energy storage components due to the converter’s intermediate DC link and quick response time (Abbey and Joos, 2007; Cardenas et al., 2004). The BES can be incorporated into the DFIG based microgrid as a portable storage option in the form of a plug-in vehicle as well as a stationary storage option. The BES can also be utilized as a power backup and to smoothen the microgrid output power variations caused by changing wind speed and solar irradiation (Roy et al., 2022). DFIG-based microgrids have exceptional control flexibility but its control is a challenging task considering its inherent nonlinearity, fast dynamics, and unpredictable disturbances acting on the system. Increased efficiency, smooth integration, and improved power quality depend on the effectiveness of DFIG-based microgrid control structures. The significant proportion of the DFIG control methods are designed to operate in grid-connected mode and is mainly related to active and reactive power exchange with the grid. In contrast, the control goal of DFIG in islanded operation is related to the generation of stable voltage and frequency on the stator side. Various operational challenges put forth by DFIG based microgrids have been addressed in literature. The sensorless voltage and frequency control of the standalone DFIG system uses a new technique called a sliding mode observer (SMO) to estimate the rotor velocity and angle (Mondal et al., 2022). It has been noted that the estimation algorithm and voltage controller both perform well despite variations in load at the generator terminals. An improved steady-state and transient performance model-based predictive current control approach for an unbalanced stand-alone DFIG system is presented in Phan and Lee (2011). Other important concerns of DFIG based microgrid related to fault-ride through voltage sags (Morshed and Fekih, 2018), frequency support (Han and Ha, 2019; Zhao et al., 2016), harmonic compensation (Bhattacharyya and Singh, 2022; Wei et al., 2012) etc have also been explored. Research on disruption-free powers under different operating conditions and smooth transition features has been focused recently. With this pretext many control algorithms have been designed for operating DFIG-based microgrid systems in various scenarios. In Tiwari et al. (2018), a control strategy is devised to feed the local load requirement and run the DFIG-SPV system in standalone mode. In Meng et al. (2020), a droop-based approach with cascading control loops is suggested for a smooth transition between various modes. An enhanced resynchronizing mode control is developed in Fatu et al. (2014) for wind energy conversion system that takes into account the impacts of both non-linear AC and DC loads. Since the stator terminals of DFIG are directly interfaced with the grid in DFIG-based microgrid, any uncontrolled mode transfer could cause a significant transient disruption. The DFIG-based system’s functional sensitivity to erratic grid conditions like imbalance and distorted voltages is another problem at remote locations. One such control framework for the DFIG–based system while taking these factors into account has been reported in Das and Singh (2023). When it comes to islanding modes, the operational scheme guarantees uninterrupted load power, while the control technique facilitates smooth transitions without causing over-currents or voltage spikes during mode changeover. A microgrid consisting of a solar photovoltaic (SPV) array, a battery, and a wind turbine-driven synchronous reluctance generator is demonstrated by the authors in Deepu Vijay et al., 2021). Additionally, based on the grid’s availability, smooth transition controls have been offered for seamless interaction. A variety of control algorithms have been designed for synchronizing process of DFIG-based microgrids in grid connected and islanded modes, and literature in this regard can be found in other references as Sekhar and Kumaresan (2022), Tapia et al. (2009), and Thakallapelli et al. (2019). The authors of Chen et al. (2011) have presented a microgrid that smoothly integrates with the grid and is powered by a wind turbine-driven synchronous reluctance generator and battery. The controls for both off-grid and on-grid modes have been covered. In all the available literature, various operating modes have been designed for operating system in on-grid and off-grid conditions and mostly resynchronization with grid is focused. An adaptive fuzzy logic control-based power management strategy for DFIG-based microgrids is presented by the authors in Fekry et al. (2020). However, the DFIG is coupled to the point of common coupling without a static switch in its configuration. The stator terminals are connected to the AC load using a concise breaker in the majority of the literature on islanded microgrids, which integrate multiple power sources and loads with a DFIG system. The issue with such a connection manifests when the generation from wind energy conversion system is stopped abruptly, either for maintenance purposes or when the wind speed is below cut-in regions. In order to prevent power outages caused by sudden gusts of wind, the microgrid must be disconnected from sources of power like SPV systems and energy storage devices while the circuit breaker is opened. The power to the AC loads is disintegrated until the load voltage, which has been preliminarily imposed by the DFIG stator, is restored by the renewable sources and energy storage. This disruption can be disastrous for critical loads, like medical emergencies, data centers, telecom towers etc., fed by the islanded microgrid. The change of operation from the DFIG stator connected mode to the DFIG stator disconnected mode in a DFIG based based hybrid AC/DC microgrid, without any interruption of power to the loads, presents an intriguing challenge for investigation. The stator also needs to be reconnected to the AC loads as the possibility of generation from the DFIG-based wind energy system is reinstated. Researchers have discussed resynchronizing the stator terminals with the AC terminals. However, most of the research has been devoted to resynchronizing the stator terminals with the grid. In addition, in order to ensure that the system is not subjected to transient over-currents or over-voltages, the DFIG stator must be seamlessly connected to the AC loads once wind energy generation is restored.

The following major contributions are made in this study based on the identified research gaps:

A control algorithm that ensures uninterrupted supply to the system of loads is designed for an hybrid DFIG-based AC/DC microgrid, supported by the SPV array and battery energy storage system (BES).

The proposed co-ordinated control scheme operates the microgrid in DFIG stator connected (SCM) and disconnected (SDM) modes owing to the insufficient generation from the wind energy source.

Along with improving power quality in stator currents and PCC voltages and preventing unnecessary distortions in rotor currents, the control system also helps eliminate the impacts of unbalance and non-linearity in AC loads.

Throughout microgrid operation, the control technique is aimed at maintaining renewable energy sources at their maximum power levels. For optimal use of the generator rating, the DFIG stator is operated at unity power factor.

System configuration

The schematic diagram of an hybrid DFIG-based microgrid is illustrated in Figure 1. The proposed hybrid AC-DC microgrid integrates DFIG-based wind energy conversion system, SPV system and battery energy storage (BES). The stator terminals of the DFIG are connected to the point of common coupling through a solid state transfer switch (SSTS) while as the rotor terminals are connected to the rotor side converter (RSC). The SSTS has bidirectional power transfer capacity and enables the smooth stator mode shift of DFIG in various modes. As the RSC regulates the voltage and frequency of the DFIG, it also controls its power. RSC is coupled to the dc link capacitor which is further connected to the common DC bus. The load side converter (LSC) is integrated to the DC bus on one side and on the other side it is connected to the SSTS through an interfacing inductor $L_{f a b c}$ and a transformer. LSC manages the power quality functions imposed on by imbalance, harmonic distortions and the reactive power demand incurred by non-linear AC loads. The LSC used for power conversion and control generates current harmonics/ripples on the AC side due to converter switching, therefore interfacing inductors are used to smoothen the converter output current. The function of the transformer is to step up the LSC voltage to the AC grid/load level and meet the combined kVA requirement of load and filters. A ripple filter is assigned to each end of the SSTS so that the switching harmonics in both modes of operation can be eliminated. The microgrid is fed with a variety of loads consisting of non-linear AC loads and low voltage and medium voltage DC loads. The dc-dc buck converter is used to generate a low voltage (LV) dc interface while the medium voltage (MV) loads are integrated directly at a common dc bus. SPV system is integrated to the dc bus through a DC-DC boost converter that is in particular controlled to perform maximum power point tracking operation. An energy storage in the form of BES is incorporated in the proposed microgrid whose control is established by bidirectional DC-DC converter. Using BES control, the converter regulates the DC link voltage to the predefined value.

Figure 1.

DFIG-based hybrid AC/DC microgrid.

The control algorithm at first estimates the generation from the DFIG wind energy system and generates an appropriate control signal $S_{signal}$ . In the event of insufficient wind energy generation, the microgrid switches to DFIG stator disconnected mode. In this mode, RSC is turned off and as such there is no voltage at the stator terminals. During this mode, RSC is turned off, and the stator terminals do not have voltage. In this scenario, LSC provides the voltage requirements for AC loads at the PCC. The amount of solar irradiation determines how much power is effectively extracted from the BES to feed loads in this mode. Depending upon the solar irradiation intensity, an appropriate signal $P_{signal}$ is generated that imposes the dc-dc boost (BC) converter control into ON/OFF mode. If the DFIG generates appreciable power, RSC is turned ON and the voltage is induced at the stator terminals. In this phase, the RSC is controlled such that the stator voltage amplitude, phase angle and frequency are comparable to the voltage at PCC established by the LSC. The control technique generates the signal to activate SSTS as the corresponding errors between the stator and load voltage amplitude, phase, and frequency reduce below the predefined threshold. The microgrid then switches to stator connected mode, where the DFIG supplies active power to the microgrid and stator voltage is established at the PCC terminals. Bidirectional converters are controlled to exchange power effectively by BES, which depends on PV generation (BD). The flowchart describing the different modes is shown in Figure 2. The comprehensive control enables the mode transition with minimized current and voltage transients and ensures uninterrupted supply to loads in both cases. The modeling of some of the important components of the proposed system is discussed in the following sections.

Figure 2.

Description of AC/DC microgrid control operation.

Wind Turbine modeling

The power captured by the wind turbine $P_{t}$ is calculated by the following equation.

P_{t} = 0.5 ρ π R^{2} V_{w}^{3} C_{p} (λ, β)

(1)

where $ρ$ is the air density in $k g / m^{3}$ , R is the rotor radius in $m$ , $V_{w}$ is the wind speed in $m / s$ and $C_{p}$ is the power coefficient, which is the function of the tip speed ratio, $λ$ and pitch angle $β$ . $C_{p}$ is expressed by the following set of equations

C_{p} (λ, β) = c_{1} {(\frac{c_{2}}{λ_{i}}) - c_{3} β - c_{4}} e^{\frac{c_{5}}{λ_{i}}} + c_{6} λ

(2)

where,

\frac{1}{λ_{i}} = \frac{1}{λ + 0.08 β} - \frac{0.035}{β^{3} + 1}

(3)

$λ$ expresses the relation between wind speed and tip speed of blades and is expressed as.

λ = \frac{Ω_{t} R}{V_{w}}

(4)

$ω_{t}$ is the wind turbine rotor shaft speed. Considering step up gear mechanism with gear-box ratio $η_{g}$ , the generator rotor speed is given by following relation.

ω_{r} = η_{g} \times Ω_{r}

(5)

The power flow in DFIG occurs through the stator and the rotor depending upon the mode of operation of the machine, as such the total power output $P_{t}$ is described in terms of rated stator $P_{s}$ and rotor power $P_{r}$ as shown.

P_{t} = P_{s} + P_{r}

(6)

Expressing the rotor power delivered by the DFIG by slip times the stator power, above equation can be put as,

P_{t} = P_{s} + s_{max} P_{s}

(7)

Considering the generator rotor speed range from 110 to 198 rad/s equivalent to the slip range of $s_{m i n} = 0.3 %$ to $s_{m a x} = - 0.26 %$ , the machine rating of 14.3 kW DFIG turbine is deduced to be 11 kW.

SPV modeling

PV array is modeled by single diode equivalent circuit model that is explained in detail in Villalva et al. (2009). The solar PV array is modeled for a maximum power capacity of approximately 10 kW at the proposed system’s DC bus. The entire ratings for the solar PV array is given in Table 1:

Table 1.

PV module parameters for design of solar PV array.

PV array specification
Parameter	Value
Short circuit current $(I_{s c})$	7.84 A
Open circuit voltage $(V_{o c})$	36.3 V
PV voltage at mpp, $(V_{m p p})$	29 V
PV power at mpp, $(P_{m p p})$	10 kW
PV current at mpp, $(I_{m p p})$	7.35 A
Series modules, $(N_{s})$	9
Parallel modules, $(N_{p})$	5

PV modules are required to be connected in series $(N_{s})$ and parallel $(N_{p})$ connections as per the following equations to design (1) PV system.

N_{s} = \frac{V_{dc}}{V_{m p p}}

(8)

N_{p} = \frac{P_{mpp}}{V_{m p p} \times I_{m p p}}

(9)

Hysteresis current control

The switching pulses for the RSC and LSC are generated by comparing the respective reference and actual currents and passing the resultant through a hysteresis current controller (HCC) as shown in Figure 3. The working principle of hysteresis current control is explained in Khan et al. (2019). The control generates the upper and lower bands for the rotor $i_{rabc}$ and load side reference currents $i_{s a b c}$ by adding and subtracting hysteresis band h to the references. The switching logic to generate pulses for top switch of first leg of the converter, to control various currents, is as follows.

If sensed (reference + h/2): Top switch is OFF and bottom switch is on.

else if sensed (reference − h/2): Top switch is ON and bottom switch is off.

else if (reference − h/2) ¡sensed¡ (reference + h/2): Switches current status is retained.

Figure 3.

Pulse generation through hysteresis current control.

Disturbed loads

Non-linear loads which are commonly used generate harmonics and reactive power, which are serious problems in power systems. When a device draws harmonic currents, the component of the current must be created at the source, which implies that these harmonic components go through all of the power system components between the source and the load. Harmonics have a variety of detrimental impacts on grids, including higher losses, interference with the functioning of sensitive equipment like as measuring devices, a reduction in total equipment service life due to accelerated aging, and a decrease in system dependability. Furthermore, there has been a significant increase in renewable penetration at the residential level, and the harmonic effects that domestic users have on the grid are not paid much attention. The installation of a single-phase load in a three-phase system causes the system to produce unbalanced current and neutral current, which further causes poor voltage regulation, a high level of neutral current, too much reactive power and load unbalancing. The disturbed loads and the induced harmonics are handled by the control of converters used in renewable integration.

Control scheme for DFIG-based AC/DC microgrid

The co-ordinated control scheme for all the converters required for developing operational strategy of the DFIG-based AC/DC microgrid is described in this section.

RSC control scheme (SCM and SDM)

During the SCM of operation, the RSC control algorithm generates stator voltages at the rated amplitude and frequency for the AC loads at PCC. RSC also ensures maximum power capture from PV array during this mode and reduces errors in stator and load voltage magnitudes, frequency and phase angle. The illustration of the RSC control during SCM and SDM is shown in Figure 4. The DFIG stator-field oriented reference frame is utilized to implement the RSC control. The phase voltages $(v_{s a b c}, v_{l a b c})$ and amplitudes $(V_{s t p}, V_{l p})$ are computed from the sensed stator and load voltages $(v_{s a b c}, v_{l a b c})$ .

V_{s t p} = \sqrt{2 (v_{s a}^{2} + v_{s b}^{2} + v_{s c}^{2}) / 3}

(10)

V_{l p} = \sqrt{2 (v_{l a}^{2} + v_{l b}^{2} + v_{l c}^{2}) / 3} .

(11)

Figure 4.

RSC control in SCM and SDM for hybrid AC/DC microgrid.

Phase locked loops (PLLs) are used to compute the instantaneous stator $(θ_{s t})$ and load voltage angular locations $(θ_{l})$ , and the slip angle $(θ_{r})$ is calculated from the stator flux angle $(θ_{s})$ as illustrated.

θ_{r} = θ_{s} - θ_{m} = (θ_{e} - π / 2) - θ_{m}

(12)

$θ_{m}$ denotes the machine rotor angle position and $θ_{e}$ is the fixed electrical angle obtained from reference angular frequency. As shown in the flowchart, the control signals $S_{signal}$ and $S_{SSTS}$ are actuated during SCM. Stator peak voltage $V_{sp}$ is regulated by the d-axis element $I_{r d}^{*}$ after the decoupled control is accomplished in synchronous reference frame (dq), while active power is regulated by the q-axis element $I_{r q}^{*}$ as shown in Figure 3.

The resulting dq references are obtained by regulating the respective errors using PI controllers as shown.

I_{r d}^{*} (k) = I_{r d}^{*} (k - 1) + k_{p v s} [ε_{v s} (k) - ε_{v s} (k - 1)] + k_{i v s} ε_{v s} (k)

(13)

I_{r q}^{*} (k) = I_{r q}^{*} (k - 1) + k_{p ω} [ε_{ω} (k) - ε_{ω} (k - 1)] + k_{i ω} ε_{ω} (k)

(14)

$ε_{v s}$ , $ε_{ω}$ is the error generated from the d- and q-axis PI controller respectively. The dq rotor current references are then converted into three phase rotor currents $i_{rabc}^{*}$ by using $θ_{s}$ .

During the SDM operation, if $S_{signal}$ is deactivated, the RSC remains OFF and there is no voltage across the stator terminals. If the incident wind speed $(V_{w})$ is stable and the possibility of wind generation unfurls, then $S_{signal}$ is enabled and the RSC is switched ON. In this instance, the microgrid is operating in the stator reconnection mode, and the load voltage $v_{l a b c}$ set by the LSC is used to induce the stator voltage $v_{s a b c}$ . To control the RSC in this phase, the d-axis loop reference $I_{r d}^{*}$ is generated by processing the error between $v_{sp}$ and $v_{l p}$ through PI controller as shown:

I_{r d}^{*} (k) = I_{r d}^{*} (k - 1) + k_{p v} [ε_{v} (k) - ε_{v} (k - 1)] + k_{iv} ε_{v} (k)

(15)

$ε_{v}$ is the error generated from the PI controller. The phase compensator is used to evaluate the error between $θ_{s t}$ and $θ_{l}$ to obtain the q-axis loop reference $I_{r q}^{*}$ . The controller generates gating pulses for $S_{SSTS}$ to reconnect the stator after the $v_{s a b c}$ and $v_{l a b c}$ have been effectively matched, and the RSC control automatically changes to SCM. Thus, without any transients or interruptions in power to loads in the microgrid, the stator and PCC are seamlessly reconnected. The dq current references are transformed to three phase rotor currents using slip angle, $θ_{r}$ . A hysteresis pulse width modulation (PWM) strategy is utilized comparing $i_{rabc}^{*}$ and $i_{rabc}$ to generate pulses for the RSC in SCM and SDM.

LSC control scheme (SCM and DCM)

The LSC control provides optimal performance of DFIG by maintaining unity power factor operation during SCM and hence load reactive power requirement is met through the LSC. Moreover, LSC compensates harmonic and unbalanced load current components during SCM, resulting in sinusoidally balanced stator currents and PCC load voltages. In the SDM, LSC maintains sinusoidally balanced voltages at the PCC and continuously powers the loads (AC and DC) regardless of the availability of wind or solar energy.

An indirect control of stator currents is used for the LSC in the SCM. The control signals are generated through DFIG stator field-oriented reference frame. The stator d-axis component $I_{s d}^{*}$ controls the reactive power requirement of the LSC and is set as:

I_{s d}^{*} = 0

(16)

The q-axis component $I_{s q}$ from the evaluation of the field orientation principle is expressed as:

I_{s q}^{*} = - (L_{m} / L_{s}) I_{r q}

(17)

$I_{r q}$ is the d-axis rotor current and $L_{m}$ and $L_{s}$ represent the magnetizing and stator inductance of DFIG. The d-q stator components are then transformed to three phase reference stator currents $i_{s a b c}$ using $θ_{s}$ as shown in Figure 5.

Figure 5.

LSC control in SCM and SDM for hybrid AC/DC microgrid.

The obtained stator current references $i_{s a b c}$ are compared with actual stator currents $i_{s a b c}$ and the resultant error is fixed by hysteresis PWM control to generate gating pulses for the LSC. In the SDM, a direct control approach is adopted where the reference compensating currents $i_{c a b c}^{*}$ for the LSC control. The reference PCC load voltages $v_{l a b c}^{*}$ are computed as follows:

\begin{matrix} v_{l a}^{*} = V_{pk}^{*} \cos (θ_{e}) \\ v_{l b}^{*} = V_{pk}^{*} \cos (θ_{e} - 2 π / 3) \\ v_{l c}^{*} = V_{pk}^{*} \cos (θ_{e} - 4 π / 3) \end{matrix}

(18)

The $v_{l a b c}^{*}$ are compared with sensed load voltages $v_{l a b c}$ and the steady-state errors between the two is minimized by PR regulators. The output of the PR regulators generate $i_{c a b c}^{*}$ that are further compared with sensed compensating currents $i_{c a b c}$ . The resultant error signals are processed by hysteresis PWM generating pulses for the LSC in SDM operation. Thus, this control enables the uninterrupted load power supply from the microgrid. Furthermore, in the absence of renewable generation, the loads are fed with the BES support to ensure continuous power supply to the loads.

BC control for PV array

Depending on the intensity of irradiation $I_{rr}$ and hence availability of PV generation, the DFIG microgrid turns the dc-dc boost converter ON/OFF in both modes of operation. The converter control strategy is designed using the perturb and observe (P and O) algorithm, such that peak power is extracted from the PV array at varying solar irradiation. The output of the PV array $P_{p v}$ is controlled by the PV voltage $V_{p v}$ adjusting the duty ratio $d_{btc}$ of the converter as shown in Figure 6.

Figure 6.

BD control for BES of hybrid AC/DC microgrid.

BD control for BES

The bidirectional converter regulates the dc bus voltage at the reference value and consequently compensates for the power deficit to the microgrid through BES support. The control scheme for the operation of BD converter is shown in Figure 7

Figure 7.

BK control for LV loads of hybrid AC/DC MG.

Control for voltage regulation of DC loads

The proposed DFIG-based microgrid feeds DC loads operating at MV and LV level. The MV loads are connected directly at the dc bus of the microgrid and LV loads are interfaced through a buck converter. The control scheme implementation for the BK converter is illustrated in Figure 8.

Figure 8.

BK control for LV loads of hybrid AC/DC microgrid.

Simulation results and discussion

Based on MATLAB/Simulink simulations, the simulation results illustrate microgrid performance based on DFIG among various scenarios.

PQ consideration with regard to non-linear loads

The compensational capability of the hybrid AC/DC microgrid has been tested by creating sudden unbalance of the non-linear AC load.

Phase “b” of three phase AC load $i_{l a b c}$ has been disabled from the time instant 0.4 to 0.45 seconds as shown in Figure 9. With the implementation of microgrid control, the converter generates $i_{c a b c}$ currents to compensate for the load unbalance. The source (stator) voltage $v_{s a b c}$ and source (stator) currents $i_{s a b c}$ are perfectly sinusoidal and in balanced state throughout the operation. The microgrid also ensures that the reactive power necessary for the machine magnetization is provided through RSC. This is evident from the response of stator reactive power $Q_{s}$ which is negligible in magnitude $(\approx 0)$ . Also, to sustain unity power factor at the stator end, the reactive power demand of the load is met from the LSC, as a result no reactive power is drawn from the source. The behavior of rotor currents $i_{rabc}$ also illustrates the smooth functioning of the wind turbine despite load disturbance.

Figure 9.

Performance analysis of microgrid with load disturbance.

Performance analysis of microgrid with varying PV and DC loads

In Figure 10 are shown the operations of DC loads and PV arrays. In the event of a sudden step-down in solar irradiation, the system is affected. MPPT is subsequently utilized to alter the PV voltage $(V_{p v})$ and current $(I_{p v})$ , as shown in the figure. A signal is generated by the microgrid control when solar irradiation falls below a certain threshold, turning OFF the boost converter signaling proper operation of the associated control system. Also shown in the figure are sudden increments in medium voltage and low voltage DC loads. Microgrid generates a well-regulated low voltage (lv) supply through coordinated control strategies ensuring that DC loads are met through the microgrid properly.

Figure 10.

Performance analysis of microgrid with varying PV and DC loads.

Performance analysis of microgrid transitioning from SCM to SDM as shown in Figure 11, the hybrid microgrid seamlessly transitions from DFIG stator connected to DFIG stator disconnected mode of operation. A microgrid control monitors the incident wind speed and generates a signal $S_{signal}$ (high/low) to indicate whether wind generation is possible. Following the microgrid operational strategy, the $S_{SSTS}$ signal turns low when $S_{signal}$ the reaches a low level. Whenever this mode transition occurs, the load voltage (vlabc) is generated by the LSC instead of the DFIG stator. The load current $(i_{l a b c})$ for AC loads remains undisturbed during this transition, demonstrating the uninterruptible supply to the load. This transition leads to the RSC being turned off and the $v_{s a b c}$ , $i_{s a b c}$ , and stator power $(P_{s})$ disappearing.

Figure 11.

Performance analysis of microgrid from SCM to SDM.

Performance analysis of microgrid transitioning from SDM to SCM

Figure 12 depicts the seamless transition of the hybrid microgrid from DFIG stator detached to DFIG stator connected mode of operation. The $S_{sgnl}$ is set to “high” based on the appreciable sensed incident wind speed. At this point, the stator current reconnection phase begins, and the RSC is turned ON. The RSC begins inducing sinusoidal voltages at the stator terminals, and the accumulation of $v_{s a b c}$ and $i_{rabc}$ begins. The RSC modifies the stator voltage amplitude, phase angle, and frequency to match the corresponding values of the AC load voltage. The microgrid control turns the $SSTS$ signal “high” when the error between these signals falls below a certain threshold. The DFIG stator terminals are then reconnected to the PCC. The $i_{s a b c}$ and the LSC current $(i_{c a b c})$ achieve an acceptable transitional response with negligible overshoot. The load voltages and load currents are observed to be uninterrupted, as in the previous transition case.

Figure 12.

Performance analysis of microgrid from SDM to reconnected phase.

Conclusion

The paper discusses the operation of DFIG-based MG supported by the SPV and battery energy storage. The control strategy guarantees that power is delivered continuously to a wide range of loads supported by the microgrid, including AC linear/non-linear loads, medium voltage, and low voltage DC loads. To compensate for insufficient wind energy generation, a co-ordinated control scheme is designed to operate the microgrid in DFIG stator connected (SCM) and disconnected (SDM) modes. For this seamless transition between the DFIG stator modes, a bidirectional solid state transfer switch (SSTS) is utilized for disconnection and reconnection of the stator terminals from AC load terminals of the microgrid. The non-linear load disturbances and the induced harmonic distortions are also compensated by the co-ordinated control through LSC. The maximum power is extracted and generator is operated at unity power factor for optimal utilization through RSC control.

Footnotes

Appendix

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Bisma Hamid

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