Investigation of LVRT capability of wind driven dual excited synchronous generator

Abstract

This paper proposes an effective control technique for low voltage ride through (LVRT) capability in dual excited synchronous generator (DESG) wind turbines. The proposed control technique is dependent on controlling the field circuit parameters. Where the active power is controlled by the field-current space phasor magnitude and the reactive power is controlled by the field-voltage space phasor phase. With the proposed control strategy, the DESG can generate additional reactive power to support grid voltage recovery under grid faults. The DC-link voltage is kept within an acceptable limit since the excess power, due to the power mismatch between the mechanical and armature power is stored in the generator inertia. Using the proposed control strategy, the DESG can enhance the LVRT capability efficiently without using extra protection circuits or any additional control techniques during fault conditions. To test the proposed control method, simulation, and experimental results for a 1.1 kW DESG wind turbine system were obtained.

Keywords

Low voltage ride through DESG-based wind turbine reactive power control maximum power point tracking

Introduction

Wind energy has become one of the most rapidly developing renewable forms of electrical energy owing to its many advantages, such as minimal pollution, high production capacity, and sustainability (Gualtieri, 2019). There are various types of wind energy conversion systems (WECSs) such as fixed, limited, and variable speeds depending on the speed of the wind generator (Cheng and Zhu, 2014). Because of the rapid development of power electronic converters in recent decades, variable-speed WECS has become the most extensively utilized technique. Variable speed technology has the benefits of generating more energy, reducing mechanical stress, allowing reactive power management, and reducing voltage fluctuations (Yaramasu et al., 2015). Variable-speed wind turbines based on doubly-fed induction generators (DFIG) and permanent magnet synchronous generators (PMSG) are commonly utilized in electrical power systems due to their numerous advantages (Arani and Mohamed, 2016; Ngamroo, 2017). Due to the significant attention in WECSs, many developed generators, such as brushless doubly-fed generators (BDFG) (Cheng et al., 2020), and dual excited synchronous generators (DESG) (Abdel-Wahab et al., 2020; Yonah et al., 1993) have been proposed for WECS.

In various previous publications, DESGs have been analyzed as an important strategy for improving dynamic and steady-state stability, and increasing system reliability (Reddy et al., 2017; Xu et al., 2021). Russia in 1980s, has successively manufactured DESG with capacity from 110 up to 800 MW, and utilized them into the capital power grid with high reliability (Xu et al., 2022). The DESG can operate in low-speed wind applications with a low gearbox ratio because of its capacity to be designed with a high number of poles (D’Arco et al., 2011). In Garner et al. (2022) the finite element analyses-optimization of two variants of the non-overlap wound-field machines; wound-field flux switching machine (WF-FSM) and DESG; for wind generator drives, are compared. From the overall optimization of both machines, the torque per mass of the DESG is found to be double of the WF-FSM and direct grid-connected DESG generator gives better efficiency performance compared to the WF-FSM.

Using a proper control technique for the DESG field parameters, the DESG can operate as a variable-speed constant-frequency (VSCF) generation system with controlling the output active power or as a constant-speed constant-frequency (CSCF) generation system with the advantage of controlling reactive power similar to the DFIG with the advantages of easier control, and higher efficiency (Piegari et al., 2007; Yassin et al., 2022).

To sustain reliable and stable operation in a power generation system, wind turbines must comply with grid code requirements (GCRs). Grid codes were developed by power system operators to guarantee that grid technical and operational requirements are met, and they serve as basic rules for wind turbine manufacturers and wind farm planners (Tohidi and Behnam, 2016). The most typical code requirements for a grid-connected wind farm, according to the international grid code, are active power and frequency control, reactive power and voltage control, voltage–frequency operating ranges, and low voltage ride through (LVRT) capability (Mohseni and Islam, 2012). During normal operation, the wind turbine should be controlled to harvest maximum power from the wind, which is identified as maximum power point tracking (MPPT) (Kumar and Chatterjee, 2016; Wei et al., 2016). On the other hand, during grid faults, the provided active power to the grid be rapidly reduced in proportional to the reduction in grid voltage, even though the electrical system’s response is faster than the mechanical system’s (Hu et al., 2017). As a result of the imbalance between the mechanical power and the output active power supplied to the grid, there is an excess of power. Consequently, the generator and power converters may be damaged due to the increased generator currents and increased DC-link voltage, as well as mechanical parts being damaged due to over-speeding of the turbine beyond its acceptable limits (Wu et al., 2019). So, during grid failures, the generated mechanical power must be reduced, or the extra power must be wasted.

Several hardware solutions and software methodologies for the LVRT capability of DFIG and PMSG under grid faults have been presented in many previous literatures.

On the first hand, in literature, various hardware solutions have been introduced for the LVRT capability of DFIG. A crowbar circuit has been introduced as one of the most commonly used ways to protect the rotor side converter switches as well as the DC-link capacitor from the influence of transient-state by completely disconnecting the converter from the rotor terminals (Din et al., 2019; Qu et al., 2019). In Hu et al. (2015), a DC chopper protection circuit is installed in parallel with the DC-link capacitor, which is used to keep the DC-link voltage within an acceptable range during grid faults to protect the converter’s switches from overvoltage, since the surplus power is dissipated in a DC-link resistor. It is possible to combine the effectiveness of DC chopper circuits and the crowbar since the DC chopper maintains the peak value of DC-Link voltage while the crowbar is used to control the generator’s current (Haidar et al., 2017). Also, a series dynamic braking resistance (SDBR) protection circuit is connected in series with the stator terminals (Huang et al., 2015). The effectiveness of SDBR circuits and the DC chopper could be combined (Abed et al., 2013). The hybrid combination provides for a reduction in stator current peak values and ensures that the DC-Link voltage stays within safe limits. Using reactive power compensators, like dynamic voltage restorers, static VAR compensators, and STATCOM, are other options to enhance the LVRT capability of WECSs (Cheng and Nian, 2012; Döşoğlu et al., 2017; Enuka and Angarao, 2014). By equipping the DFIG with fault current limiter devices, the LVRT capacity of the DFIG can be improved (Firouzi and Gharehpetian, 2018; Zheng et al., 2019). Instead of dissipating the surplus power, it could be stored in energy storage systems (ESSs) during grid faults (Kim et al., 2019; Zhao et al., 2015). The stored energy is returned to the grid after the faults have been cleared (Liu et al., 2018). However, these technologies frequently increase the system’s overall cost and complexity, and some of them cannot supply reactive power to the grid. Therefore, several software methodologies were provided as alternative preferred LVRT methods. Modifying the pitch angle controller is one of these approaches, in which the generated power can be reduced during a grid failure by regulating the blade pitch angle (Howlader et al., 2013). In Martinez et al. (2011), the sliding-mode algorithm was introduced to control the rotor-side power converters of the DFIG. The control algorithm makes the rotor-side converter capable of removing the fluctuations of the reactive power and the electromagnetic torque during faults. In Wei et al. (2017), the sensorless vector control scheme’s performance is proved in the presence of power grid voltage dips. The sliding-mode observer (SMO) is employed in this situation to estimate rotor position and the speed information for DFIG vector control, with the benefits of parameter insensitivity and strong robustness. In Zhu et al. (2015), the stator flux-based control technique was introduced to satisfy the LVRT. The proposed control approach was employed to decrease torque oscillations by reducing over-currents on both the rotor and stator sides. In Jayanthi and Devaraj (2022), a new adaptive hysteresis controller for improving the DFIG based WECS LVRT capacity has been proposed. In this scenario, a modified PQ controller manages active and reactive power, while fuzzy logic control controls the DC-link voltage.

On the other hand, the LVRT methods for PMSG-based wind turbines could be classified into using external devices methods as well as modified controller methods. By using external devices, the surplus power could be dissipated in active elements such as crowbars (Camacho et al., 2015) or stored in the different energy storage systems (ESSs) (Nguyen and Lee, 2010). Also, the FACTS devices are used as a source of reactive power to support the grid voltage (Nguyen and Lee, 2013). The crowbar circuit is used to protect the DC-link circuit since it consumes the surplus power during grid faults (Camacho et al., 2015). The main disadvantage of this technology is that it necessitates a high rating resistance which increases the overall cost. While, the ESS can store the surplus power during grid faults and hence improve the LVRT capability of the system by avoiding the increase in the DC-link voltage (Kim and Kim, 2020). However, this technology necessitates additional control circuits that increases the system complexity, in addition to the high cost of the ESS devices (Xu et al., 2013). FACTS devices are another preferable option to improve the LVRT. The static compensator (STATCOM) and the Static VAR Compensator (SVC) are the most important FACTS devices used to improve the system performance during transient and steady-state operations (Geng et al., 2018). These devices could inject controllable reactive power to support the grid voltage. In Mahmoud et al.(2019), a comparison between the active crowbar protection and the thyristor controlled series capacitor (TCS) has been introduced. The obtained results showed that both methods can enhance the LVRT capability for grid connected PMSG. Regarding control solutions, different control modification solutions are provided as the most preferable LVRT solution methods (Jahanpour-Dehkordi et al., 2019). The pitch angle control is one of the simple and relatively cheap techniques to control the turbine output power. By controlling the pitch angle of the turbine blades, the generated turbine power can be controlled during normal as well as abnormal operating conditions (Howlader et al., 2013; Liu et al., 2014). In some previous studies, the roles of the machine side converter (MSC) and the grid side converter (GSC) during grid fault are exchanged (Yassin et al., 2015). Since the MSC is used to control DC-link voltage and the GSC is used to achieve MPPT. In this case, the surplus power is stored in the rotor inertia and returned to the grid after the fault is cleared to prevent the DC-link overvoltage (Alepuz et al., 2013; Arani and Mohamed, 2016). The use of de-loading droop to improve the LVRT capability for PMSG was discussed in Nasiri et al. (2020). By using this method, the surplus power is stored in the generator inertia and the GSC can inject reactive current into the grid. In Basak et al. (2020), the system’s LVRT capability is achieved utilizing four separate control methods: de-loading, crow-bar protection, modulation index control, and swapping the roles of the two converters to prevent the DC link voltage from rising. Based on the results presented, the authors’ advice for obtaining LVRT for PMSG is to swap the MSC and GSC functions.

To meet the grid code criteria, this paper presents a novel control method for DESG in both healthy and faulty conditions. During healthy conditions, the proposed control method is used to harvest maximum mechanical power, as well as control the generated reactive power. Besides that, the LVRT criteria are met with the same suggested control technique as the proposed control technique controls the injected reactive power during the grid faults, to support grid-voltage recovery and keep the DC-link voltage within a suitable range. In this case, the surplus power is stored in the system’s inertia. These objectives, active and reactive power control and LVRT capability enhancement can be achieved by selecting an appropriate control technique of the DESG field currents. In this control scheme, the magnitude of the field current space phasor is utilized to regulate the electromechanical torque and hence the generated active power, while the phase angle of the field voltage space phasor is used to manage the armature reactive power.

The main features of the introduced control technique can be briefed as follows:

The maximum generated power could be extracted during wind speed variations.

The injected reactive power into the grid is easily controlled.

The LVRT criteria can be efficiently achieved.

Eliminating the need to use extra protection circuits or use any additional control techniques during fault conditions.

It is simple to apply to the real system because it is dependent just on the measurement data of the wind turbine.

To confirm the effectiveness of the introduced technique, different studies on grid healthy and faulty operations are considered in MATLAB/Simulink for a 1.1 kW DESG wind turbine system. Also, experimental results are executed to validate the simulation results. Based on the results, the DESG wind turbine system with the presented control technique achieves MPPT and controls the injected reactive power during normal operation. Also, it improves the LVRT capability without using any additional hardware elements or modifications on the control technique.

This paper is organized as follows: The modeling of the DESG-based wind turbine system is deduced in section 2. The essential principles of the implemented control approach are presented in section 3. Section 4 presents the simulation results and their comments. The experimental results are presented in Section 5. Finally, in Section 6, the conclusion is offered.

Modeling of DESG wind energy conversion system

Figure 1 describes the basic arrangement of the DESG-based WECS. A gearbox couples the DESG to the turbine, while the armature terminals are connected directly to the grid, and the field windings are fed through a back-to-back converter.

Figure 1.

Schematic of DESG wind turbine system.

Modeling of wind turbine

The mechanical power generation model of wind turbines is described using wind turbine parameters. The tip speed ratio ( $λ$ ), rotor radius (R), pitch angle ( $β$ ), and power coefficient (C_p) are the turbine parameters. The tip speed ratio can be computed using wind speed (V_w) and the rotational angular rotor speed of the turbine ( $ω_{t}$ ), as given below.

λ = \frac{ω_{t} R}{V_{w}}

(1)

The major control variables are the tip speed ratio and pitch angle, which are used to determine the power coefficient. The maximum available wind power may be achieved by appropriately managing the rotor speed with the wind speed. That is known as maximum power point tracking. The mechanical power (P_t) is described by the following equation:

P_{t} = \frac{1}{2} ρ π R^{2} C_{p} (λ, β) V_{w}^{3}

(2)

Where ρ is the air density (kg/m³) and β is the blade pitch angle. The turbine’s rotational angular speed (ω_t) is related to the generator’s angular speed (ω_m) and the gearbox ratio (GR), as given below.

ω_{m} = ω_{t} GR

(3)

Mathematical model of DESG

The existence of two field windings rather than one distinguishes the DESG from the conventional synchronous generators. Figure 2 depicts a DESG with symmetrical three-phase windings in the armature and two nonidentical orthogonal field windings.

Figure 2.

Schematic of DESG.

It is possible to derive the analytical model of DESG in rotor reference frame using the following hypotheses: ignore core and mechanical losses, assume linear magnetic circuit, assume a round rotor construction, and assume sinusoidal the air gap MMF as follows:

{\vec{V}}_{ds} = - R_{s} {\vec{I}}_{ds} - L_{s} \frac{d {\vec{I}}_{ds}}{dt} + ω_{m} L_{s} {\vec{I}}_{qs} + L_{m} \frac{d {\vec{I}}_{f 1}}{dt} - ω_{m} L_{m} {\vec{I}}_{f 2}

(4)

{\vec{V}}_{qs} = - R_{s} {\vec{I}}_{qs} - L_{s} \frac{d {\vec{I}}_{qs}}{dt} - ω_{m} L_{s} {\vec{I}}_{ds} + ω_{m} L_{m} {\vec{I}}_{f 1} + L_{m} \frac{d {\vec{I}}_{f 2}}{dt}

(5)

{\vec{V}}_{f 1} = R_{f 1} {\vec{I}}_{f 1} + L_{f 1} \frac{d {\vec{I}}_{f 1}}{dt} - L_{m} \frac{d {\vec{I}}_{ds}}{dt}

(6)

{\vec{V}}_{f 2} = R_{f 2} {\vec{I}}_{f 2} + L_{f 2} \frac{d {\vec{I}}_{f 2}}{dt} - L_{m} \frac{d {\vec{I}}_{qs}}{dt}

(7)

θ_{m} = \int ω_{m} dt

(8)

The voltage and current could be stated in space phasor form as follows:

\vec{V_{s}} = V_{s} e^{j (ω_{s} t - θ_{m})} = {\vec{V}}_{ds} + j {\vec{V}}_{qs}

(9)

\vec{I_{s}} = I_{s} e^{j (ω_{s} t - θ_{m} - θ_{1})} = {\vec{I}}_{d s} + j {\vec{I}}_{qs}

(10)

\vec{V_{f}} = V_{f} e^{j (ω_{s} t - θ_{m} + γ)} = {\vec{V}}_{f 1} + j {\vec{V}}_{f 2}

(11)

\vec{I_{f}} = I_{f} e^{j (ω_{s} t - θ_{m} + γ - θ_{2})} = {\vec{I}}_{f 1} + j {\vec{I}}_{f 2}

(12)

The armature and field voltages space vectors in the rotor frame are $\bar{V_{s}}$ and $\bar{V_{f}}$ , respectively. The armature and field current space vectors are $\bar{I_{s}}$ and $\bar{I_{f}}$ respectively. R_s and L_s are the phase resistance and self-inductance of the armature winding. R_f1, R_f2, L_f1, and L_f2 are the field winding resistances and self-inductances of the field winding, and L_m is the mutual inductance of the machine. The angular frequency of the armature voltage is ω_s, and the angular position of the rotor frame relative to the armature frame is θ_m. The armature and field impedance angles are θ₁ and θ₂, respectively, and (γ) is the angle between the field voltage space vector and the armature voltage space vector.

As a function of the space phasors of the voltage and currents, the electromagnetic torque, as well as the active and reactive power of the armature, could be described as follows.

T_{em} = {pL}_{m} Im (\overset{⇀}{I_{s}} {\overset{⇀}{I_{f}}}^{*})

(13)

P_{s} = R (\overset{⇀}{V_{s}} {\overset{⇀}{I_{s}}}^{*})

(14)

Q_{s} = Im (\overset{⇀}{V_{s}} {\overset{⇀}{I_{s}}}^{*})

(15)

Where p is the number of pole pairs, R (x) and Im (x) are the complex number’s real and imaginary portions, and * is the conject operator.

Then, the system mechanical equation is given as:

T_{t} - T_{em} = J \frac{d ω_{m}}{dt} + B_{m} ω_{m}

(16)

Where T_t denotes the turbine torque, T_em is the electromechanical torque, J denotes the moment of inertia, and B_m denotes the viscous friction constant.

Control technique

The ability to capture maximum mechanical power regardless of the wind speed variations and controlling the generating reactive power are the most important advantages of using the DESG in wind generating systems. By selecting an appropriate excitation current control approach, these functions could be performed efficiently. Current with a frequency corresponding to the slip speed must be fed to the field windings. To handle the injection of active and reactive power into the grid, two single-phase half bridge inverters supply the two field windings as shown in Figure 1. In the suggested control technique, the GSC is used to keep the DC-link voltage at the desired level, and the MSC is used to control the generated active and reactive power to meet the grid code requirements.

Machine side converters control

By analyzing the DESG mathematical model in steady-state operation, the electromechanical torque and the armature reactive power could be characterized as a function of field current parameters as given in (17) and (18) (Yassin et al., 2022). The field current space phasor amplitude and the field voltage space phasor phase angle ( $γ$ ) affect both the electromechanical torque and the armature reactive power at the same time. In the suggested control strategy to optimize the captured power from the wind, the magnitude of the field current space phasor is employed to control the electromechanical torque, while the phase angle of the field voltage space phasor controls the armature reactive power. Figure 3 shows a diagram of the proposed control method for the MSC.

T_{em} = \frac{p R_{s} V_{s} L_{m}}{R_{s}^{2} + ω_{s}^{2} L_{s}^{2}} I_{f} \sin (γ - φ_{r}) + \frac{p ω_{s} V_{s} L_{s} L_{m}}{R_{s}^{2} + ω_{s}^{2} L_{s}^{2}} I_{f} \cos (γ - θ_{2}) + \frac{pRs ω_{s} L_{m}^{2}}{R_{s}^{2} + ω_{s}^{2} L_{s}^{2}} I_{f}^{2}

(17)

Q_{s} = \frac{ω_{s}^{2} V_{s} L_{s} L_{m}}{R_{s}^{2} + ω_{s}^{2} L_{s}^{2}} I_{f} \sin (γ - φ_{r}) + \frac{R_{s} ω_{s} V_{s} L_{m}}{R_{s}^{2} + ω_{s}^{2} L_{s}^{2}} I_{f} \cos (γ - θ_{2}) + \frac{ω_{s} L_{s} V_{s}^{2}}{R_{s}^{2} + ω_{s}^{2} L_{s}^{2}}

(18)

Figure 3.

The proposed MSC control technique.

Grid side converter control

The GSC’s main objective is to keep the DC-link voltage within safe limits. The field-oriented control algorithm is used to accomplish this purpose. Figure 4 shows the arrangement for this technology. This technique employs the d-axis component of the grid current (i_dg) to maintain the DC-link voltage constant. To ensure that the rotor reactive power is zero, the q-axis current (i_qg) is set to zero. As a result, any reactive power exchange takes place only on the armature side, limiting the rated capacity of the power converter.

Figure 4.

The proposed GSC control technique.

Simulation results

To prove the efficiency of the suggested control approach, the previous control technique was modeled using MATLAB/SIMULINK on a 1.1 kW DESG wind turbine. The DESG-based WECS parameters used in the simulation are given in Yassin et al. (2022). To test the DESG’s ability to meet the grid code requirements, the suggested control technique is tested under two different scenarios: healthy grid operation and faulty grid operation. In the aforementioned scenarios, the voltage on the DC-link is controlled at 120 V.

Case 1: Operation under healthy conditions

With a stochastic wind speed profile, the proposed control technique has been evaluated for a step-change in armature reactive power as shown in Figure 5. Figure 5a shows the applied wind speed profile to generate the simulation results.

Figure 5.

Simulation results for healthy conditions: (a) wind speed profile (b) generator rotational speed, (c) armature reactive power, (d) mechanical and armature active powers, (e) DC-link voltage, and (f) field currents.

According to the proposed MPPT technique, the generator’s rotational speed achieves its reference value, as shown in Figure 5b. As shown in Figure 5c, the suggested controller has been successful in controlling the step changes reactive power to the desired reference value. As seen in Figure 5d, the mechanical and the active armature powers have been adjusted according to the operating wind speed. The DC-link voltage is controlled at its reference value shown in Figure 5e. Figure 5f depicts the time response of field currents. The controller changes the phase sequence and the amplitude of the field currents to optimize power extraction at each wind speed. Due to the non-identical orthogonal field windings, these currents have a 90° phase shift, but their amplitudes differ. As can be seen, the field currents have a slip frequency corresponding to the operating speed. When the generator speed is changed from sub-synchronous to super-synchronous, the sequence of the two filed currents is reversed. The field currents have a DC value in the synchronous speed operating zones.

Case 2: Operation under faulty conditions

The performance of the DESG wind turbine has been tested during the grid fault to demonstrate its ability to handle LVRT standards. Two different scenarios are studied under a three-phase symmetrical grid fault with a 60% voltage dip

The first scenario involves running the generator at 3000 rpm (the synchronous speed) and controlling the output reactive power (Q_s) at 50 VAR. A symmetrical fault with a 60% voltage dip occurred at t = 1 second and cleared at t = 1.165 second as shown in Figure 6a. The suggested control strategy, as shown in Figure 6b, controls the generator to deliver additional reactive power to support the grid voltage during the fault occurrence. The mechanical power, as well as the armature active power, is shown in Figure 6c. During the fault period, the armature power reduced in proportion to the reduction in grid voltage, even though the mechanical power appears to remain unchanged. As a result of the power imbalance between the turbine and generator output, the turbine’s speed increases as displayed in Figure 6d. The DC-link voltage pattern is shown in Figure 6e. Using the suggested control technique, the DC-link voltage is kept within a tolerable range. The field currents are shown in Figure 6f.

Figure 6.

Simulation results for a symmetrical three-phase fault with a 60% voltage dip at 3000 rpm and 50 VAR: (a) grid voltages, (b) armature reactive power response, (c) mechanical and armature active power, (d) generator speed, (e) DC-link voltage response, and (f) field current response.

The second scenario is investigated at an operating speed of 3150 rpm (super-synchronous speed), and the generated reactive power (Q_s) is controlled at 200 VAR. In this case, a symmetrical fault with a 60% depth of voltage dip lasts 7.5 cycles between 1 and 1.15 second as shown in Figure 7a. To support the grid voltage, the generator injects more reactive power into the grid during the fault period as shown in Figure 7b. Figure 7c reveals the captured mechanical power as well as the armature active power. As a result of the power imbalance between the turbine and grid sides, the speed of the turbine increases, as shown in Figure 7d. Figure 7e depicts the DC-link voltage behavior during the voltage dip. The DC-link voltage is controlled within its acceptable range using the proposed control technique. Figure 7f shows the field currents.

Figure 7.

Simulation results for a symmetrical three-phase fault with a 60% voltage dip at 3150 rpm and 200 VAR: (a) grid voltages, (b) armature reactive power response, (c) mechanical and armature active power, (d) generator speed, (e) DC-link voltage response, and (f) field current response.

To evaluate the LVRT capability of the DESG with the proposed control technique, different simulation verifications were performed under different grid voltage dips with different fault durations. Table 1 introduces examples of fault conditions results containing the percentage increase in the rotor speed (Δn), reactive power (ΔQ_s), DC link voltage (ΔV_dc), and the reduction in mechanical power (ΔP_m), and armature active power (ΔP_s).

Table 1.

Samples of fault conditions results.

Fault type	Voltage dip (%)	Fault duration (s)	Δn (%)	ΔQ_s (%)	ΔV_dc (%)	ΔP_m (%)	ΔP_s (%)
Operating at 3000 rpm and 50 VAR	40	0.165	3	62	1.7	2.6	13.2
	40	0.4	4.7	84	2.4	4.6	19.9
	50	0.165	3.9	68	2	3.4	18.2
	50	0.4	6.9	96	4.2	5.8	27.5
	60	0.165	4.8	72	2.2	4.3	23.7
	60	0.4	8.3	100	6.7	7.6	33.9
Operating at 3315 rpm and 200 VAR	40	0.15	2.6	4	3.4	1.9	14.5
	40	0.4	5.6	14	5	4.4	20
	50	0.165	3.3	5	4.8	2.8	18.7
	50	0.4	7.7	16	6	6	27.4
	60	0.15	4.8	6.5	6.7	3.7	22.9
	60	0.4	9.9	18	9.2	8.2	34.2

Based on the simulation results, it could be concluded the following points:

With the suggested control technique, the DESG wind turbine can extract the maximum generated power with variations of wind speed.

During the fault period, the suggested control strategy controls the generator to inject additional reactive power to support the grid voltage.

During the fault period, the armature active power is decreased even though the mechanical power appears to remain unchanged; hence the turbine’s rotor speed is increased due to the power imbalance.

After the fault is cleared, the DESG’s generated active power progressively increases as the rotor speed gradually decreases to its initial state.

The DC-link voltage does not vary seriously during the fault occurrence.

The GCRs have been efficiently achieved in both healthy and faulty conditions without using extra protection circuits or using any additional control techniques during fault conditions.

Experimental results

Experiment studies have been employed to verify the proposed control strategy. Figure 8 shows the photo of experimental setup, which comprises a wind turbine emulator, a 1.1 kW DESG, power converters, and host PCs. Since a 1.3 kW DC motor is employed as a turbine emulator. DESG’s armature terminals are directly connected to the grid while the field windings are connected to the grid via two single-phase power converters, with a DC-link linking the two converters. The DC-link is made up of two series 6800F capacitors. One of the power converters is known as GSC and is controlled via a data acquisition card, while the other is known as MSC and is controlled by the dSPACE DS1104 R&D controller. The DESG and DC motor parameters are given in (13). The system is tested experimentally with two different operating conditions, just as it was in the simulation portion.

Figure 8.

The experimental setup in the laboratory.

Case 1: Operation under healthy conditions

Figure 9 depicts the experimental results of variable speed operation with a constant reactive power value (at 100 VAR). Figure 9a displays the wind speed profile, while Figure 9b demonstrates that the actual generator speed tracks its reference value to capture maximum generated power at each wind speed. Figure 9c shows the injection of reactive power into the grid, which is aligned with its reference. Figure 9d depicts the active power of the turbine and armature. Figure 9e shows the DC-link voltage response, which is constant at its reference value of 120 V. The response of the injected field current is shown in Figure 9f. The controller changes the injected field currents’ amplitude, phase shift, and frequency in response to the operational wind speed. As a result, the field currents in the synchronous speed operating zone are DC.

Figure 9.

Experimental results healthy conditions: (a) wind speed profile (b) generator rotational speed, (c) armature reactive power, (d) active power, (e) DC-link voltage, and (f) field current response.

Case 2: Operation under faulty conditions

Figure 10 shows the DESG wind turbine experiment results utilizing the proposed control approach for three-phase fault at operating conditions of 50 VAR and 3000 rpm. As shown in Figure 10a, a symmetrical fault with a 60% depth of voltage dip is occurred at t = 1 second and cleared at 1.165 second. To support the grid voltage, the generator injects more reactive power into the grid during the fault period as shown in Figure 10b. The DESG’s active power and mechanical power are shown in Figure 10c. The imbalance in power raises the rotor speed during the fault as shown in Figure 10d. The electrical power of the DESG steadily rises and the rotor speed of the wind turbine returns to its initial state when the grid voltage returns to normal operation. The DC-link voltage is controlled within its acceptable range using the proposed control technique as shown in Figure 10e. Figure 10f shows the field currents.

Figure 10.

Experimental results for a symmetrical three-phase fault with a 60% voltage dip at 3000 rpm and 50 VAR: (a) grid voltages, (b) armature reactive power response, (c) mechanical and armature active power, (d) generator speed, (e) DC-link voltage response, and (f) field currents response.

Figure 11 shows the experimental results for three-phase fault with operating conditions of 200 VAR and 3150 rpm. As shown in Figure 11a, a symmetrical fault with a 60% depth of voltage dip is occurred at t = 1 second and cleared at 1.15 second. The generator injects extra reactive power into the grid to support the grid voltage during the fault period, as shown in Figure 11b. Figure 11c depicts the mechanical power as well as the armature active power. Although the mechanical power appears to be constant during the fault time, the armature power is reduced in proportion to the reduction in grid voltage. As seen in Figure 11d, the turbine’s speed increases as a result of the power imbalance. The DC-link voltage is controlled within a reasonable range using the suggested control technique. Figure 11e depicts the DC-link voltage pattern during the decrease. Figure 11f shows the field currents.

Figure 11.

Experimental results for a symmetrical three-phase fault with a 60% voltage dip at 3150 rpm and 200 VAR: (a) grid voltages, (b) armature reactive power response, (c) mechanical and armature active power, (d) generator speed, (e) DC-link voltage response, and (f) field currents response.

According to the experimental results, it could be proved that the same conclusions were reached in the simulation results. This demonstrated the DESG’s ability to meet GCRs in both healthy and faulty conditions when used with the proposed control approach.

Conclusion

This paper offers an innovative control technique to enhance the LVRT capability of the DESG-based WECS. To investigate the system’s performance, a full mathematical model of the overall system with its proposed control method has been derived in the rotor reference frame. The proposed control strategy’s major goal is to control the field circuit parameters to adjust the DESG active and reactive powers to support the grid voltage recovery under grid faults. In the suggested control technique, the magnitude of the field current space phasor has been used to regulate the generated active power, while the phase angle of the field voltage space phasor has been used to control the injected reactive power to the grid. To test the suggested control technique, simulation results for a 1.1 kW DESG based WECS as well as experimental results are obtained. With the suggested control technique, the obtained results indicate that:

The DESG wind turbine can capture the maximum mechanical power during the variations of wind speed simultaneously with controlling the injected reactive power,

During the grid fault, the control technique responses to the unbalance between the mechanical power and the generated active power by changing the field current space phasor magnitude and field voltage space phasor angle. Due to the dynamic changes in field current space phasor magnitude and field voltage space phasor angle, the generator naturally injects additional reactive power to the grid, supporting its voltage without needing any changes in the pre-fault set point of the reactive power

The injected reactive power value is dependent on the voltage dips as well as the fault duration (Direct proportional).

The values of the injected reactive power during fault are mainly dependent on the pre-fault values (reverse proportional),

After the fault is cleared, the DESG’s generated active power progressively increases as the rotor speed gradually decreases to its initial state.

The DC-link voltage does not vary seriously during the fault occurrence as a result of the surplus power is stored in the system’s inertia. The DC-link voltage was changed within a safe range by a maximum value of 9.2% (where the recommended value, less than 20%).

The increasing in the values of the rotor speed and the DC-link voltage are dependent on the voltage dips as well as the fault duration (Direct proportional).

The obtained results revealed a good correlation with some differences between the experimental and simulation results. These differences are occurred due to the assumptions made during the simulation works such as neglecting the core losses and the effects of saturation.

Finally, it is clear that the DESG can achieve the GCRs efficiently when the DESG field variables are controlled using the proposed control strategy in both healthy and faulty states without needing any additional protection circuits or additional control strategies during fault conditions. Hence it is recommended to be used in WECSs.

Footnotes

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

Hanafy Hassan Hanafy

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