Advanced control strategy of DFIG during symmetrical grid fault

Received Dec 22, 2020 Revised Apr 5, 2021 Accepted Jul 7, 2021 This article proposes a novel scheme to improve the doubly-fed induction generator (DFIG) behavior during grid fault. The DFIG’s are sensitives to voltage variations when abrupt variations of the wind velocity arrive. For enhancing DFIG behavior, protecting the converters, and smoothing the fluctuations power output of the DFIG under sag voltage; a novel hybrid energy storage system scheme and its controller are proposed. The main advantages of our approach are a faster response and suppressing overvoltage on DC bus and globally less stress in the storage system. The control structure decreases the tiredness on the battery and restores the DC bus voltage rapidly, globally the battery system operating time increases. The results obtained by simulations in MATLAB validate the benefits of the suggested control.


INTRODUCTION
During the last decade the wind turbine (WT) integration in the grid has intensified, especially the DFIGs which is most used in WT farms. The DFIG is one of the WT systems most used in the wind farm, this due to several advantages; variable speed operation, sizing of the converters power represents only 25%-30% DFIG power and the decoupled reactive and active power control [1], [2]. In system of the DFIG, a stator winding is connected to the grid, but the winding rotor is interfaced with the electrical network by two power converters; converter associated with the rotor side (RSC), converter associated with the electrical grid (GSC) connected to network and the DC link, which connects these two converters. The RSC is using for controlling the decoupled powers (field orientation control). The GSC oversees the reactive power injected, and to stabilize the DC voltage. Several classical strategies controls were employed to control DFIGs systems; control by the direct power (DPC), vector controls (VC) and control by the direct torque (DTC) [3], [4].
During the fault in the electrical grid, the power system requires that DFIG must remain connected to network and contributed in the stability of the power systems. A sudden reduction voltage stator, a stator flux linkage could contain DC component. During voltage drop, DFIG is found exposed of two problems. First problem is: the transient current appearing in the coils of the rotor; Second problem is the DC bus overvoltage, that can cause performance degradation in DFIG system perturbing the global system behavior it could damage the power converters [5], [6]. With the increased DFIG penetration in electrical grid, performance of the DFIG and output power fluctuations injected under fault become research subjects [7].
The solutions presented to deal the low-voltage-ride-through divided in two groups: additional  [8], the improved control strategies are used like stator current feedback control [9], demagnetization control [10], inductance emulating control (IEC) [11].
The auxiliary system solutions such as static synchronous compensators (STATCOM) [12], crowbar protection circuit using a resistance series [13], could solve a part of the LVRT capability problems [13]. Series grid side converter (SGSC) that it is detailed in [14], the dynamic voltage restorer (DVR) is studied in [15], superconducting magnetic energy storage and storage energy systems both are characterizing by highly efficient energy storage and power controllability under faults [16], [17] having a good result for enhancing the LVRT capacity. The strategies mentioned, act during the electrical grid fault during the sudden variation speed of the wind and remain inactive in the normal operation.
In this article, a new structure is suggested for enhancing behavior WT system based DFIG, reducing power fluctuation and improving LVRT capability. The model of DFIG WT and the hybrid energy storage system (HESS) are developed in MATLAB/Simulink; the simulation tests are performing to validate our proposed control. The paper is structured as behavior DFIG under fault with the overall system architecture of the HESS are discussed in second section. Stability study and controller of the HESS system are is studied in third section. Discussion with the simulations is given in fourth section, after you find the conclusion.

DFIG SYSTEM UNDER VOLTAGE SAG 2.1. System model
According to the Park model [18], the rotor variables is referred in the stator circuit are expressed by: While i, v, R, ψ and ω represents current, voltage, resistance, magnetic flux, and rotor electrical angular velocity, s and r designates the variables of the stator and the rotor.
In DFIG system, the coils stator has linked to the grid directly. The electrical grid determines the stator voltage v s . The RSC regulates the rotor voltage v r . The rotor and stator flux has Expressed with: While a magnetizing inductance is denoted by L m . L r and L s are the of the stator inductance and rotor inductance. Vr represents the rotor voltage; this is an important variable of the RSC. The rotor flux variation induces a rotor voltage, with (3), (4) is given by: The leakage factor is designed by σ and the transient inductance of the rotor is represented by σLr. With the formulas (5) and (2), we can obtain (6).
The Vr formula represented in (6) shown two parts; first is an electromagnetic field (EMF), the second items are the voltage sag.

Grid fault model of the DFIG
During normal conditions, we can consider the stator resistance equal to zero, the flux of the stator can be presented by [19].
While  s, Vs are the stator angular velocity and the stator voltage terminal. Then, using (6) the EMF is (8).
While  sr , s are slip angular frequency and slip. The slip s is proportional The EMF amplitude sVsLm/Ls to the. Under symmetrical voltage drop, the flux of the stator is given by [19], [20]: While p,  s voltage sag depth and the stator flux time constant. A positive sequence component is shown by the first item, and the second part of the expression is the DC component. Using (6), the EMF can be expressed by (10):

HESS CONTROLLER APPROACHES
The main circuit of DFIG and the additional HESS are showing in Figure 2. The DC link system contains RSC, GSC, DC chopper and HESS. The HESS energy system allows a barter of the real power in a short period [20], [21]. The HESS controller is used to enhance power flow regulation of a DFIG WT [22], [23]. The HESS system is based on the battery who takes care of fluctuations, and on Supercapacitor (SC) that deals with fast transitions. The proposed system and the control also improve also the battery lifetime. These advantages of this system approach are: i) faster response of the restoration DC link voltage; ii) less stress in storage system; iii) suppress overvoltage in DC link voltage. The HESS configuration represented in Figure 3, which include two energy storage systems: super-capacitor and battery. The HESS is linked to DFIG by its DC bus voltage.
While v sc and v batt are the super capacitor voltage and battery voltage respectively, L sc and L batt are inductor parameters of the SC and battery. C sc and C batt are the filter capacitance of SC and battery.

Proposed control
The controller adopted for the HESS system is shown in Figure 4. The main objectives are decreasing the tiredness on the battery and restoring the DC bus voltage rapidly, the operating time of the battery system increases [24]. During charge / discharge variation, the necessary power to balance and to smooth the flow of power in the DC bus is classified in two terms, (i) transient power (Ptr), and (ii) average power (Pm). The power balance can be expressed: Where, P sc (t), P b (t), P rsc (t) and P gsc (t) are SC power, battery power, rsc power a gsc power respectively. For regulating the voltage of the DC link, the delivered HESS power is: The total current HESS system is: The voltage loop can be used to determine the total current:   (15) give the average current: Where i bref (s) and ω c are the reference current of the battery and cut-off frequency [25]. The battery system determines the average value of the current. The uncompensated power P b_uncomp (s) is expressed by: For raising performance of the super capacitor, the uncompensated power is used. The current controller for SC (i scref (s)) for SC is: The error between real current and the generated current of the battery with the supercapacitor reference currents is feeding to corrector (PI), and then the duty ration is generated. The method purpose is to regulate the DC voltage during the disturbed regime, thus to guarantee the power balance. The behavior of the HESS depends to the SC, the battery and RSC currents during the variation of the intermediate circuit voltage.
The small DC link voltage variation of the can be given as: While ∆V dc represent the DC voltage variation. G b, G sc and G rsc are respectively the impedance signals of SC, battery and RSC. ∆i' sc , ∆i' batt and ∆i' rsc represents the currents variations of SC system, battery system and RSC system respectively.

SC current loop
The SC current function is expressed by: While G cl_sc and G ol_sc represents closed loop and the open loop respectively. K i_sc and K p_sc presents the integral and the proportional coefficient current controller of the SC. H sc is the super capacitor current gain of the control loop feedback. The SC current bode plot on open loop is presented in Figure 5 (a). The SC current controller parameters are determined by SISO toolbox: K p_sc = 0.25 and K i_sc = 180.

Battery current loop
The battery open loop (G ol_b ) is given by: In Table 1 K i_b , K p_b represents the battery current integral and the battery current proportional coefficient respectively of the corrector. H b is the gain feedback of the control loop current of the battery designed in (21). The battery current bode plot is presented in Figure 5 (b). The parameters controller is determined by SISO toolbox: K p_b = 0.53 and K i_b = 120.

SIMULATIONS AND DISCUSSIONS
For validate the effectiveness the suggested system, the DFIG system with the HESS design is established in MATLAB/Simulink, for checking the effectiveness of the suggested scheme, a wind farm is used, it is consisting of six DFIG system, Table 2 shows the elements of each DFIG. The wind speed variation and the sag voltage are shown in Figure 6 and Figure 7. The Figure 8 show the rotor winding current with the traditional scheme under sag voltage. With the suggested system, the peak of the rotor current has been decreased show Figure 9. With HESS into DFIG system, the active power injected into the network becomes more smoothen with Figure 10, precisely between the two moments t 1 =0.8s and t 2 =1.4s.  The curve of a reactive power obtained by the suggested is kept around zero with the HESS system Figure 11. The DC voltage overshoot is shown in Figure 12, the voltage drop causes an overvoltage peak, which reaches 1425 with the traditional system as it is shown, and this voltage value can cause a material 1429 damage, with the proposed system this peak has disappeared. During a symmetrical voltage dip, the performances of the DFIG with the traditional system and the proposed system are evaluated. The simulations carried out with or without the HESS system show that the effectiveness of the results, which made it possible to improve the behavior of the DFIG while protecting it. Figure 11. Reactive power Figure 12. DC-link voltage

CONCLUSION
A new control scheme for wind turbine based DFIG systems, which a HESS and it control are employed to improve the DFIG system behavior, has been evalued. The proposed system is beneficial to improve the behavior of the DFIG system, and offer a best transient performance under a disturbed regime.
The results of the simulation shows that the Hybrid Energy Storage System can smooth rapidly the active power fluctuations, keep reactive power around zero and the same time reducing the voltage variation of the intermediate circuit.
To have a broad view, our future work will deal with the proposed system during an asymmetric fault.