Optimal Control of a Doubly Fed Induction Generator of a Wind Turbine in Island Grid Operation

Great significance is given to the use of energy from renewable sources, especially in industrial and municipal applications. The present article is devoted to the optimal control of a DFIG generator with the help of a rotor-side converter (RSC). Its aim is to ensure the delivery of the voltage of a three-phase network with appropriate parameters while operating in an islanded grid. Such a grid is usually characterized by an uneven loading of each phase. Additionally, the load of these phases changes randomly in time. In order to ensure the assumed parameters of line voltages, the optimal control is applied with a square cost function. This ensures the shape of voltage that is in accordance with the referential voltage. Moreover, higher harmonics with a given number are detected and reduced. The simulations that were executed confirm compliance with the conditions of the parameters of the output voltage in the islanded grid. Attention was paid to oscillations in the power flowing through the rotor-side converter (RSC). The methods to accelerate the suppression of these oscillations are presented.

Approach Control of DFIG Rotor Side Converter Based on Grid Frequency Coordinated Control

This paper presents an approach frequency coordinated control of Doubly Fed Induction Generator (DFIG) applying in the Rotor Side Converter (RSC) using an order reference active power (Pref ). Pref is obtained from the frequency deviation, speed regulation and kinetic energy stored in the DIFG. The Pref is employed as a main controller parameter of the dq-axis currents in the RSC under two case studies. In case1 study, the Pref is used to regulate the q-axis reference current and the grid reactive power controlled d- axis reference current. Whilst in case2 study, the d- axis reference current is produced by Pref , and the rotormechanical speed responsible to generate the q-axis reference current. The modified vector control method is used to control the Grid Side Converter (GSC) in two case studies. The transient performance of two case studies is simulated by PSCAD/EMDTC program under constant, step and variable wind speeds. A comparative result between two case studies shown that the frequency coordinated control has an ability to control both rotor dq-axis currents, and it enhancing the system frequency as well as improved DFIG voltage stability.However, case2 study has a better response than case1 study during system operated under random wind speed.

Starting of induction motor fed with stand-alone DFIG

This paper presents dynamic simulation and control of stand-alone doubly fed induction generator (DFIG) loaded with 3-phase induction motors (IMs). The study reveals that direct on-line starting of large IMs causes a large voltage sag across the generator terminals as the starting current drawn reaches up to 8-9 times the rated load current. Traditionally, this problem has tackled by oversizing of the generator or employment of special starters, under the pretext of mitigating voltage sag. This work explores ways that the starting current can be reduced economically by applying constant V/f control side by side with indirect field-oriented control (FOC) applied on the rotor side converter of the DFIG. This methodology enables starting of larger IMs and mitigation of voltage sag that occurs during the start-up period. Two different rating of IMs loaded with speed-squared mechanical torque are mainly considered. Simulation results of the system behavior under study confirm the capability of the proposed control.

Solve Coupled Axes Problem Without FOANR Based on Substitution Method to Control DFIG Used in Wind Application

In order to control output powers generated by doubly fed induction generator (DFIG) used in wind application (WA) many previous studies, mainly based on flux orientation control (FOC) and neglecting resistance to get a simple model of DFIG with decoupled axis. However, this control strategy requires several hypotheses: low and stability of grid voltage in order to orientated the statoric flux, high power of generator to neglecting statoric resistance. As a result that may not be present in realty due to direct connection between stator and the grid In addition to the presence of resistance, whatever the power of the generator, therefore the DFIG represents a complex model and required a nonlinear control without previous approaches closer to reality to respond highly against DFIG nonlinear model, this is the first paper presents a novel strategy to control nonlinear model of DFIG based on substitution method to solving (d,q) coupled axes without flux orientation and neglecting resistance (FOANR) and also does not take into account stability of grid voltage, for produce required reference active and reactive power by controlling the voltage of rotor side converter (RSC), using classical proportional-integral (PI) controller in a non-linear synthesis form by three methods :direct control (D) and indirect open loop (IOL) and indirect with power loop (IWPL),we compared three controls and check their performance towards the real model of DFIG to verify our control and proving its effectiveness without previous approaches. Finally, the simulation results of the studied controls are presented, analyzed and compared.in terms of power reference tracking, robustness to the parametric variation and the ability to respond to sudden wind speed variation.

Dynamic Low Voltage Ride through Detection and Mitigation in Brushless Doubly Fed Induction Generators

Brushless doubly-fed induction generators have higher reliability, making them an attractive choice for not only offshore applications but also for remote locations. These machines are composed of two back-to-back voltage source converters: the grid side converter and the rotor side converter. The rotor side converter is typically used for reactive current control of the power winding using the control winding current. A low voltage ride through (LVRT) fault is detected using a hysterisis comparison of the power winding voltage. This approach leads to two problems, firstly, the use of only voltage to detect faults results in erroneous or slow response, and secondly, sub-optimal control of voltage drop because of static reference values for reactive current compensation. This paper solves these problems by using an analytical model of the voltage drop caused by a short circuit. Moreover, using a fuzzy logic controller, the proposed technique employs the voltage frequency in addition to the power winding voltage magnitude to detect LVRT conditions. The analytical model helps in reducing the power winding voltage drop while the fuzzy logic controller leads to better and faster detection of faults, leading to an overall faster response of the system. Simulations in Matlab/Simulink show that the proposed technique can reduce the voltage drop by up to 0.12 p.u. and result in significantly lower transients in the power winding voltage as compared to existing techniques.

Research on Mechanism and Damping Control Strategy of DFIG-Based Wind Farm Grid-Connected System SSR Based on the Complex Torque Method

The subsynchronous resonance (SSR) of a double-fed induction generator (DFIG) and its suppression method are studied in this paper. The SSR may be aroused by the interaction between the double-fed induction generator and the series-compensated transmission lines. This paper proposes an expression of the electrical damping for assessing the SSR stability based on the complex torque method. The expression is derived by linearizing the DFIG model at the operating point. When the mechanical damping is neglected, the expression can be used to calculate whether the electrical damping is positive or negative to judge the SSR stability. The expression can quantitatively analyze the impact of the wind speed, the compensation degree, and the parameters of the rotor speed controller and the rotor-side converter controller on the SSR stability. Furthermore, a subsynchronous damping control (SDC) strategy is designed to suppress the SSR. The parameters of the SDC are optimized by particle swarm optimization (PSO) based on the electrical damping. Finally, the above research is verified by the PSCAD/EMTDC time-domain simulations. The results show that the stability of SSR decreases with the decrease of wind speed, the increase of series compensation degree, the increase of proportional coefficient, and the decrease of integral coefficient in rotor speed controller and rotor-side converter. The designed subsynchronous oscillation controller can suppress the SSR of a DFIG.