The evolution of renewable energy is the best solution to protect the environment and reduces the pollution caused by nuclear energy and fossil resources, renewable energy sources come from: wind energy, sun, geothermal, biomass, marine energy and hydroelectric. Among these sources of energy, the wind energy is the cleanest, effective and promising resource; it’s a sustainable way to produce electricity without warming emissions [1, 2], that leads to an increasing insertion in the electrical networks.
The most installed machine in wind farms is the doubly-fed induction generator (DFIG) and the permanent magnet synchronous generator (PMSG). Actually, the DFIG is the widely generator employed because it presents several advantages [3,4,5] the power electronics converters connecting between the network and the rotor use a part of the total rated power, typically about (30%), which reduces the costs and loss, and it has the flexibility for controlling the reactive and active powers independently, in addition, the operating on the variable speed makes possible to maximize the efficiency of the energy produced. The structure of a doubly-fed induction aero-generator is presented in Fig. 1. In this topology, the stator windings are directly linked to the electrical grid, and the rotor windings are linked the network via a back-to-back power electronic converter, which is placed a DC bus voltage, where the derived name of ‘doubly fed’ [2].
DFIG is a highly nonlinear system, so several control strategies have been examined to improve the DFIG operation under disturbances [6,7,8,9,10,11]. Among these control techniques, the most used one is the classical control based on the proportional-integral (PI) correctors, it’s very used for it reliability and simplicity of implementation, but its performance degrades when the internal generator parameters varied [12, 13]. In work [6], a nonlinear MPPT control of a DFIG based on feedback linearization control (FLC) was designed, and the stability of the system was achieved using Lyapunov theory. In reference [7], to enhance the power stability of a DFIG using in wind energy, a sliding mode controller based perturbation observer was designed. Paper [8], propose a novel perturbation estimation based robust state feedback control in order to achieve MPPT of the DFIG, the results obtained was verified by simulation and compared whit other classical methods.
The voltage dip is a temporary and sudden drop of the amplitude from 10% to 90% of the RMS voltage affecting one or more phases at a point in the electrical network; it is characterized by two parameters: the amplitude and duration [13]. Previously, when faults grid occurs, the system protection turns-off and disconnects the generator from the network to protect the aero-generators, which implies a loss of production for several minutes. In this context, the new grid codes require the generator to continue the production of electricity and remain connected to the grid during the faults [14, 15]. Figure 2 presents the limits supported by the wind system during voltage dips in some countries, above the critical line limited and determined by the grid code, the wind turbines must remain connected and should contribute to the return of voltage to it rated value, the capability of the WECS to stay connected to the network during the period of the voltage drops is called Low Voltage Ride-Through (LVRT) capability.
As the stator windings of the DFIG are linked directly to the network, when severe voltage drop occurs, high peak currents appear on the stator windings, due to the coupling between the stator and rotor circuit, over currents appear on the rotor winding then the rotor side converter (RSC), which leads to increases the voltage in the DC bus capacitor [16] and the oscillations on the torque, consequently, if no protection measurements were installed, the high currents transient can destroy the rotor side converter and the generator [17]. So to protect the system and keep the DFIG connected to the grid during the period of the fault, several control strategies have been proposed and studied in the literature [6,7,8,9,10,11, 14,15,16,17,18,19,20,21,22,23,24,25,26,27], these control strategies propose using a hardware protective devices or improving control methods.
The crowbar circuits protection are the most popular solution, the principle of this technology, is to connect directly a resistance bank to the rotor of the DFIG when voltage sag is detected, and the doubly-fed induction generator becomes likes a squirrel cage machine [3, 18, 19], that make the generator consumes more reactive power, which leads to voltage instability [20]. This solution limits the high current transient and protects the power converter, but the controllability of the generator becomes impossible [17], so the control of reactive power is lost under the crowbar operation. Another alternative was proposed to compensate the reactive power in [21,22,23] is the use of the Static Synchronous Compensator (STATCOM) in the WECS, this solution works well but it is very expensive; consequently, it increases the costs.
Alternatively, many control methods have been developed and proposed; in [16, 18] and [24] a DPC was developed to control the stator powers of the DFIG during the voltage sags, the proposed control strategy was verified by laboratory experiments. In [15] a stator flux control method based on the Active Disturbance Rejection Control (ADRC) corrector was applied to maintain the magnetization of the DFIG during the voltage drop associated whit the crowbar devices, this method work well, but when the crowbars were activated the DFIG control is lost. The stator current control loop using the Proportional and Integral (PI) correctors was proposed in [25], this control strategy was verified and validated by experiments. The conventional and modified vector control approach based on PI correctors was investigated in [26], to ensure the continuity of service of the WECS during the voltage sags, this method was verified by simulation, but the robustness of PI corrector degrades when the internal parameters of the machine are subject to variation. Paper [27] proposes a direct power control (DPC) method based on the sliding mode control (SMC) during a Low Voltage Ride-Through (LVRT), a satisfactory simulation results were achieved.
In this work, we develop a new Direct Power Control (DPC) strategy using the nonlinear Backstepping technique to help the operation of the system, ensure the continuity of service, and the connection to the network during and after the symmetrical voltage dips. The direct powers control objectives: control the active and reactive stator powers of the generator during the faults: the active stator power is controlled to suppress the high peak currents on the stator and rotor circuit, and the reactive power is controlled to contribute to the return of the voltage at the nominal value. The DPC was developed using the doubly-fed induction generator mathematical model, The Backstepping controller was designed from the rotor voltage equations, and the references voltages values are expressed as a function of the stator powers and its errors, during the voltages dips the stator flux is considering variants. In order to validate and analyze our proposed strategy, the aero-generator dynamic model of a rated power of 1.5 kW and DPC-based Backstepping approach were implemented, simulated, and compared whit VC control under symmetrical voltage dips using Simulink® environment.
This research paper is presented as the following sections; section 2, the doubly fed induction generator mathematical model and its behavior under symmetrical fault grid were presented. The direct power control based-Backstepping technique is developed in section 3. In section 4 the simulations results of the system are discussed and analyzed. Finally, some conclusions are given in section 5.