Secures and maintains non-linear control of DFIG-wind turbine by implementing the appropriate protection configuration against overcurrent in the rotor circuit under grid fault

. One significant disadvantage of the doubly fed induction generator (DFIG) is its high vulnerability to grid faults, which can be attributed to the direct linkage of its stator windings to the electrical network. The electromagnetic pairing of the stator and rotor in the DFIG implies that a voltage dip causing excessive stator current results in high currents in the sensitive back-to-back inverters and an overcharge in the DC-link capacitor. A comparative study of two protection configurations with a sliding mode non-linear control (SMC) for the rotor transient current for the better operation of the DFIG under network faults is presented in this document. One conventional configuration consists of a crowbar with DC-chopper and another is changed by adding to the crowbar an RL series device known as an appropriate protection configuration (APC). Both are placed within the rotor windings and ride-side converter (RSC) to achieve secures and maintains SMC of DFIG. The comparison of the results reached with the MATLAB/ SIMULINK application is evidenced by the success of these two configuration reduce the high rotor current and DC-link voltage. Additionally, the conventional configuration, in conjunction with the APC, diminishes the current RSC to levels below 0.2kA and exactly 2.25kA, while also being capable of absorbing up to 2.52kA and 1.3kA in the event of a grid fault. So the main difference is that with the APC, the decoupling of the RSC to the rotor in the presence of a fault can be averted, thereby assuring the control of all stator power via the RSC converte.


Introduction
The recent trend to consume more electricity from fossil fuels or nuclear power, which has a negative impact on the environment, has encouraged scientists to come up with environmentally friendly and innovative solutions.Now, wind energy comes as one of the biggest and most challenging sources of green energy in the global economy [1].
Today, the most popular type of installed generation system for wind power is the DFIGbased wind plant, as presented in Figure 2, which is also used in wind energy production units over 1 MW (Mega Watt) [2].On the other hand, The DFIG is an asynchronous generator equipped with a coil rotor, and its stator is linked to the electrical grid without the necessity of an additional power source, and the rotor is connected to the electrical grid via a pair of inverters, which are reversible static power frequency converters [3].Using a control system for the converters can continuously optimize the power output by finding the point where the maximum power is produced [4].
However, because the DFIG is being coupled to the network, it is affected by network faults due to its integrated power electronics and especially at network voltage dips.Consequently, this causes high pics in the stator current, surges in the active and reactive power of the stator, and oscillations in electromagnetic torque [5].Furthermore, the high electromagnetic linkage of the stator and rotor can be used to transfer the rotor peak current easily to fragile power inverters.Should this situation arise, the DC-link capacitor will experience overcharging, and the rotor converters may operate outside their safety limits.As a result, converters suffer from degradation and the DFIG's quality of power control is not available to the user once the fault has been corrected.The original concept, in order to prevent the converter from being damaged during a network fault, involved disconnecting the DFIG from the grid.The present large penetration of the network by wind turbines makes it more difficult to restore voltage to the network.This leads that network experts developing strict network codes that demand that wind turbines remain connected in case of a network fault and, in some instances, provide reactive energy to the network to help restore its level of voltage.Figure 1 illustrates the LVRT curve considered in this document, which requires turbines to remain connected to the grid in case of a short-circuit fault.To stay attached to the network in the event of a disturbance, the system must incorporate techniques to overcome the fault using the fault ride through technique .
Several fault ride-through strategies have been presented in the literature, and they can be categorized into two main groups: the inclusion of an additional protective device, as outlined in [5][6][7], and the utilization of a reactive power supply device, as detailed in [8][9][10][11][12].The protection device most usually used is the conventional configuration consists of a crowbar with DC-chopper.It involves an exclusive setup of resistor banks that are serially connected via an Insulated Gate Bipolar Transistor (IGBT) switch alongside the rotor windings.During a fault event, the RSC is disengaged from the rotor, and the collector rings are redirected towards the crowbar resistors.As a result, the DFIG resembles a generator with a squirrel cage, requiring reactive energy from the grid, causing a voltage drop at the park's connection point.Moreover, the disconnection of the RSC from the rotor results in a loss of control over both active and reactive power in the stator.For the present work, a appropriate protection configuration (APC) of the DFIGs system is suggested to improve its LVRT capability and then compared with the traditional configuration in the course of voltage dip.The APC incorporates one crowbar assembly with a series of RL devices with not using a DC-chopper circuit for limited DC-link tension as in the [6], collected from the rotor windings to the RSC under fault grid conditions.In addition, we avoided using the DC-chopper circuit with a better choice of RL values in several simulation results.Finally, the reduction of the high rotor current, the limited intermediate circuit voltage, and the sustainment of the RSC connection to the rotor are successfully achieved.Furthermore, the generator is partially made as DFIG, so it has minimized the demand of reactive energy to the network and keeps the regulation of the stator's active and reactive power, Compared to the conventional configuration based on a crowbar with a DCchopper, this is advantageous.
The following sections of this document are listed below: modeling and control of DFIGs are given in section 2. In Section 3, we present our suggested protection configuration to mitigate the high rotor current to secure and to control the DFIG, then the results of the scheme are analyzed and compared to the conventional configuration using a DFIG-2MW model in a MATLAB/Simulink based simulation during a three-phase short circuit.In section 4, the paper presents its final conclusion.

Modeling and control of DFIG
This system combines the use of windings rotor induction generators with a 3-phase power system.These generators have stator windings that are linked directly to the electrical system.The variable frequency AC/DC/AC power inverters, which include the RSC, DC-link, GSC, and RfLf, is used to connect these rotor windings to the electrical networks (Figure 2).

Model of DFIG and RSC
In order to achieve autonomous control over active and reactive power, the technique outlined in [13,14] , known as Stator Voltage Orientation, is implemented using a Phase-Locked Loop (PLL) to align the reference frame's d-axis with the stator voltage position.In equations ( 1) and ( 2), the derivatives of the dq components of the rotor current are written as a function of the DFIG generator parameters.

Controller DFIG-RSC
The rotor currents, which are correlated with active and reactive stator powers through equation system (2), need to align with the designated current references.Consequently, a sliding mode command approach based on the aforementioned Park reference frame is utilized.The sliding surfaces illustrate the difference between the measured rotor currents and the reference currents, are given in the system of equations (3).
In order to simulate equation system (4) And for quadrature-rotor current.
We choose   =    +    .And, in sliding mode and steady state, we find.
For achieving favorable dynamic performance, the control vector is enforced in the following manner.

( )
With   is the controller gain (constant positive).
In Figure 3

Model of DFIG in case of a voltage dip
A sudden drop in stator tension to a lower level causes a surge in rotor current and intermediate circuit tension.These surges, while lasting just a brief time, can destroy both the RSC and the linking DC condenser.this is primarily due to the low capacity of the converter, which is not able to generate the necessary voltage to command the generator [14].Also, in equation ( 11), the relation of both stator and rotor voltages is shown.
Equation (11) simplifies to equation ( 12) at steady state, when When the fault appears, an abrupt change in the AC voltage ∆Vs is observed.However, in the moment of appearance of the default, the stator and rotor fluxes will not vary abruptly because   = ∫    and   =∫   .Regarding equation (11), the change in this second mandate will not be abrupt, and, consequently, at the moment when a failure appears, the variation of the stator tension may be written in the following manner: To maintain    = 0 during the default, there needs to be a great change in step in the rotor voltage ∆Vr to keep up with the change in the stator voltage.Nevertheless, the RSC is unable to generate the requested ∆Vr because the RSC's maximum voltage capacity is only around 30%.As a result, a significant overcurrent will be caused in the winding of the rotor, which requires protection.

Protection against rotor overcurrent
The following section outlines the appropriate protection configuration suggested for rotor current in DFIG.The most usually used DFIG security device is a crowbar and DC-chopper which is deployed to operate in a timing sequence (Figure 4a).It is one of many preferred strategies for the reduction of surge currents in the rotor due to its high reliability, reduced cost, and easy integration.However, enabling the crowbar in case of a grid fault stops the RSC converter from functioning, causing a complete disruption of generator command.In this situation, the machine is similar to a SCIG with high-resistance working with a very strong slip, which leads to a high demand for reactive energy.This elevated level of reactive energy request removes the voltage at the connection point and vibrations of the generator that can damage mechanical parts of the production unit.

Appropriate protection configuration (APC)
A principle scheme of a suggested current rotor protection scheme used to enhance the secure and control of DFIG against grid disturbances is shown in Figure 4b.This consists of the combination of a crowbar and a device from the RL series.This appropriate protection configuration is placed on the windings of the rotor at C, and terminal RSC is located to S if the current of the rotor passes its maximum limit.At this point, the upper rotor current (   ) is fractional to the crowbar current (Icrw) that is determined by the size of the crowbar resistor    (Resistor of APC) and by the RSC current (Irsc) that is defined in size to the values of the RL device (Zseries).By not considering the switching maneuvers during the trip and elimination of faults, the RSC stays linked to the rotor windings at all times during the function of the DFIG.In addition, with the APC activated, the windings of the rotor of the generator become partially shunted via the crowbar of APC and partially wired to the RSC.An automatic command of the generator output through the RSC controller is kept, which is a benefit compared to the conventional configuration of protection.Furthermore, this type of protection with rotor windings partially shorted through the crowbar and partially collected to the RSC through the RL is making a temporal mode of the generator.This is an intermediate state in which the machine operates partly as a wound-rotor induction generator and partly as a squirrel-cage induction generator when the APC is in action.The APC can become a typical crowbar if a big serial impedance is selected with    =   (Resistor of conventional configuration) used, However, this varies according to the values of RL parameters and the crowbar resistance (   ).Also, if a low Zseries is used and a high crowbar resistor is chosen, the overcurrent of the rotor circulates in the RL circuit to the converter that might be destroyed.So, it is required to preserve the ideal settings of both    , R and L to achieve the APC goals.

Simulation results
In this section, we investigate the recommended APC by conducting comprehensive studies within a MATLAB/Simulink environment.A single wind turbine with a DFIG, featuring both RSC and GSC, physically separated by the DC-link, is represented as a unified twomass unit, with the parameter values for the DFIG sourced from [5].A 3-phase fault of a short circuit more than 30 km from the connection point was started at t = 7.5s and removed at t = 7.62s by isolating the fault section (Figure 1).We showcase the confirmed response outcomes for a 2MW-DFIG unit by contrasting two situations: one with the conventional configuration and the other employing the APC.Thus, these two different responses are compared without any protection.In order to guarantee the activation of all protection configurations during critical failure situations, the rotor's limiting current is intentionally set to Ir-thr =1.5p.u. Figure 5 illustrates the switching diagram for both protection configurations.Through many compilations of simulation tests and the use of the test and error procedure, the optimum numbers for the parameters of    and    , R and L are found (   = 0.18Ω ,R=12Ω & L=30H).As depicted in Figure 6a, the RMS rotor current without any protective measures is around 5.2 kA, but after implementing both the traditional configuration and the APC, it decreases to 2.8 kA.Additionally, when the APC is applied, the RSC current is reduced to less than 2.25 kA, as opposed to the 0.2 kA achieved with a conventional configuration according to references [6].This illustrates the preservation of the RSC's connection to the rotor, as depicted in Figure 6b.Moreover, when the APC is activated, its crowbar absorbs 1.3kA of rotor current, compared to the 2.52kA seen with the conventional configuration, as demonstrated in Figure 6c.It's crucial to emphasize that the main purpose of implementing the conventional DFIG protection configuration is to maintain the secure connection of the RSC to the rotor and efficiently disperse excess rotor current through the grounded crowbar resistance.The objective of the APC design is to maintain the linkage between the RSC and the rotor.The built-in crowbar within the APC reroutes excess current to the ground through its resistor.This process mitigates part of the excess current from the rotors and ensures that the RSC current remains within acceptable limits.In power electronics designs, these safety tolerances are used to cover any transient conditions.For instance, if the RSC limit current were doubled (2×1.775kA), the DFIG would operate without any grid disconnection.But the main disadvantage of the conventional configuration is that the generator cannot control whether the crowbar is in action.With APC, the RSC does not need to be deactivated, and DFIG operation is consequently improved.
The DC-link voltage is also preserved in its safe range (Vdc <1.45 p.u).In Figure 7 we can see that the DC-link tension reaches 1.96 kV with no protection system, while with the use of the APC,the voltage is decreased to a maximum value of 1.35 kV in comparison with the traditional configuration used in [6,7] its value reduces to 1.3 kV.So the APC can limit the Vdc without the use of a DC-chopper. Figure 8a shows the dynamic response of the stator's active power under the SMC.First, with conventional configuration, its value reaches 2MW when the fault occurs , and decreases rapidly to less than zero for the rest of the fault duration, then it starts to increase again with a small oscillation after the fault is eliminated.On the other hand, with the added APC, active power dynamics are improved especially during the fault duration , and reach 1.5MW instead of -0.1MW with a conventional configuration [6]. Figure 8b shows the dynamic response of the reactive power with the SMC.In the first place, with conventional configuration [6], at the moment when a fault occurs, the system requires a substantial amount of reactive power from the grid.However, it quickly drops to a level of 5.1 MVAr for the remainder of the fault duration, and then a strong oscillation after the fault occurrence.The second aspect to note is that with APC protection, the dynamic response of reactive power is enhanced, particularly in the event of a fault.This

Conclusion
In this work, an improved secure and command strategy for wind turbines based on DFIG is developed and evaluated in comparison with the conventional configuration method.The suggested APC is installed directly behind the rotor coils and the RSC converter.It includes a classic crowbar and RL circuit in series.In a similar manner to the conventional configuration, the APC works to both protect the RSC and the capacitor of the DC-link and keep the RSC linked to the rotor circuit, as a result, this positioning prevents any possible loss of generator control.By configuring the APC settings optimally for the RL series device equipped with an integrated crowbar, the DFIG transforms into a semi-SCIG while still retaining its semi-DFIG characteristics.Therefore, it is crucial to take precautionary measures when selecting settings to meet the demands of protective features while ensuring the uninterrupted operation of the RSC.This comparative analysis demonstrated that by using APC, we can enhance the DFIG's capability to withstand network faults and boost the stator efficiency of the DFIG in a wind energy system when grid faults occur.
, it is necessary to incorporate the following fixed conditions.