Design and Analysis of Grid-Interactive DFIG Based WECS for Regulated Power Flow

This paper focuses on the design, analysis and control of grid interfaced doubly fed induction generator (DFIG)-based wind energy conversion system (WECS) with maximum power point tracking (MPPT) capability. The modified control of grid side converter (GSC) is adapted to supply regulated power to the grid by maintaining DC link voltage. The control of rotor side converter (RSC) is modified for achieving MPPT and unity power factor (UPF) operation at the stator terminals. The dynamic modeling of a wind turbine driven DFIG is simulated in this study employing MATLAB/SIMULINK, as well as the outcomes are studied for given wind speeds.

To investigate the fault features of doubly-fed induction generator (DFIG) with rotor side converter (RSC) control, an analytic method of calculating the DFIG short-circuit current is proposed in this paper. Based on the space vector model of DFIG with RSC control, the dynamic equation of rotor current reflecting both the machine transient process and the impact of RSC control is derived, and analytic expressions of rotor and stator fault current in time-domain are obtained. The calculation result is certified by being compared with the PSCAD simulation result. According to the derived analytic expressions, the components of the DFIG short-circuit current are analyzed. It indicates that the machine terminal voltage dip of DFIG, the current reference value, the orientation angle, and the proportional-integral value of RSC controller are main factors influencing the short-circuit current characteristics.

This paper deals with reduced sensor based control algorithm for grid-interfaced doubly fed induction generator (DFIG), exhibiting twin functionalities of wind generation along with power quality enhancement capabilities. The grid side converter (GSC) control is modified to supply harmonics currents caused by nonlinear loads prevalent in the power network. Additionally, it helps in supplying load reactive power and maintaining unity power factor at the supply side. The GSC and rotor side converter control (RSC) are based on a stator current estimation scheme that requires a single current sensor as opposed to two in conventional control algorithm. A generalized integrator based stator flux estimation is used in reduced current sensor algorithm to avoid any possible DC offset and distortion in the estimated flux. An experimental prototype is developed to validate the proposed system.

A new control method for a grid-connected doubly fed induction generator (DFIG) is proposed in this paper, which is robust against parametric uncertainty and measurement noise. In general, the DFIG controllers can be divided into two main groups: the rotor side converter (RSC) and the grid side converter (GSC) controllers. The parameters of a DFIG may deviate from their rated values due to the operating conditions. For this parametric uncertainty, a robust H∞ vector control (VC) is employed using the complex sensitivity approach. The design of the RSC controller has been carried out using the vector control strategy, and instead of proportional-integral (PI) controllers, a designed robust controller is used. One of the steps in vector control is the extraction of the measured currents to be used in the control equations. If the currents are polluted with noise, the system control will be impaired. Thus, using a Kalman filter is suggested to solve this problem. The effectiveness of the proposed method is then investigated using extensive simulations under various conditions. The obtained results confirm the efficient performance and robustness of the presented controller with model and measurement uncertainties.

Doubly fed induction generator (DFIG) needs to get adapted to situations like rapid change in wind speed and sudden requirement for grid disturbances to meet modern grid rules. The paper explains the design of controllers for MPPT algorithm for the turbine, DFIG converters, and sensorless rotor speed estimation. These intelligent controllers are used to maintain equilibrium in rotor speed, generator torque, and stator and rotor voltages. They are also designed to meet desired reference real and reactive power during the turbulences like sudden change in reactive power or voltage with concurrently changing wind speed. The turbine blade angle changes with variations in wind speed and direction of wind flow and improves the coefficient of power extracted from turbine using MPPT algorithm. Rotor side converter (RSC) helps to achieve optimal real and reactive power from generator, which keeps rotor to rotate at optimal speed and to vary current flow from rotor and stator terminals. For tracking reactive power demand from grid and in maintaining synchronism when grid voltage changes due to fault at a very faster and stable way Grid side converter (GSC) is helpful. Rotor speed is estimated using stator and rotor flux estimation algorithm. Parameters like stator and rotor voltage, current, real, reactive power, rotor speed, and electromagnetic torque are studied using MATLAB simulation. The performance of DFIG is compared when there is in wind speed change only; alter in reactive power and variation in grid voltage individually along with variation in wind speed.

© 2017 IEEE. In this paper, a harmonic mitigation technique is introduced to the doubly fed induction generator (DFIG) based wind energy conversion system (WECS) by modifying its grid side converter (GSC) classical control. When this technique implemented, the DFIG's GSC acts like an active power filter, which compensates for current harmonics generated by a nonlinear load connected to the point of common coupling (PCC), in addition to its primary purposes which are maintaining the dc-link voltage constant and ensuring unity power factor operation with the grid. This technique is simple, easy to be integrated with the existing system, and has the ability to compensate for the current harmonics even if the generator is in brake mode. Moreover, a robust control method is applied to DFIG's rotor side converter (RSC) to verify that the proposed technique does not affect the DFIG system nominal operation. Experimental results show that the proposed DFIG technique reduces the current total harmonic distortion (THD) at PCC.

This paper deals with the modeling, analysis, control and simulation of a doubly-fed induction generator (DFIG) driven by a wind turbine. This grid connected wind energy conversion system (WECS) is composed of DFIG and two back-to-back PWM voltage-source converters in the rotor circuit. A mathematical model of the machine, derived in an appropriate dq reference frame is established. The grid voltage oriented vector control is used for the grid side converter (GSC) in order to maintain a constant DC bus voltage and to compensate for reactive power at the power network. The stator voltage orientated vector control is adopted in the rotor side converter (RSC) control strategy, providing efficient handling of active and reactive power at the stator, as well as a maximum power point tracking (MPPT) method for the DFIG-based wind turbine. The proposed system is simulated for different operating conditions to illustrate the reliability of the control technique. Corresponding system simulation results under nonlinear load variations and wind speed transients are presented to demonstrate the significance of MPPT in WECS, and the effectiveness of adopted control technique.

This paper examines the impact of power grid strength, power level, rotor side converter (RSC) controller parameters, and shaft parameters on the small-signal stability of a doubly fed induction generator (DFIG)-integrated power system. In each of the parametric variations, both sub-and super-synchronous operating conditions of DFIG are considered. Using the eigenvalue technique the effects of parameter are analyzed and these eigen-inferences are further validated using time-domain simulations. From the analysis it is observed that the in sub-synchronous operating point, weak-grid conditions and RSC controller parameters may result in small-signal instability.

The paper analyzes the capability of a wind power plant, based on doubly-fed induction generators (DFIGs) with an improved control system, to fulfill low voltage ride-through (LVRT) capability of the wind turbines providing the reactive power support. According to grid code requirements, tripping of wind generators under grid faults is not allowed. The sensitivity of DFIGs to grid faults requires the use of crowbar protection for not damaging the electronic equipment. In this paper, the model of a wind power plant based on DFIG using active crowbar protection is developed in Matlab/Simulink. Nevertheless, the results indicate that DFIGs equipped with crowbar protection can protect the electronic equipment, but the rotor side converter is blocked due to crowbar activation and the reactive power control is lost. Therefore, for improving the LVRT, a coordinated control system of the grid side converter, active crowbar protection, and rotor side converter is implemented and proposed, The grid side converter of DFIG is controlled similar to a STATCOM, considering also the control parameters of the crowbar protection. The simulation results show that the proposed coordinated control grid side converter'active crowbar-rotor side converter can supply the reactive power in order to answer the grid codes requirement for LVRT. Therefore, this control system can be useful to improve the LVRT capability of wind power plants based on DFIG.

Hundreds of megawatt level variable speed Doubly Fed Induction Generator (DFIG) pumped storage turbine will play an important role in voltage and frequency regulation, system stability, active and reactive power control etc.. However, similar but different from the DFIG wind turbines, such power rating and reversible pump-turbine with small capacity converter to control the whole variable speed system is challenging. In this paper, mathematical analysis and modeling of variable speed DFIG pumped storage turbine including reversible pumped turbine, DFIG and back to back converter is build first. On this basis, the control method is presented. The grid side converter stabilizes the dc-side voltage and control the grid-connected current sinusoidal. The rotor-side converter can control and decouple the active power and reactive power independently. Finally, the real time simulation results of 300MW pumped storage turbine based on Real Time Digital Simulator platform indicate the proposed control strategy can achieve good performance in different working conditions of pumping and generation. The system has fast response and good stability.

One of the problems with the doubly-fed induction generator (DFIG) is its high vulnerability to network perturbations, notably voltage dips, because of its stator windings being coupled directly to the network. As the DFIG’s stator and rotor are electromagnetically mated, the stator current peak occurs during a voltage dip causing an inrush current to the critical converter back-to-back and an overload of the DC-link capacitor. For this purpose, a series of researchers have achieved a linear and non-linear controller with a crowbar-based protection scheme. With this type of protection, the Rotor Side Converter (RSC) is disconnected momentarily, and consequently, its control of both the active and reactive output power of the stator is totally lost, leading to incorrect power quality at the point of common coupling (PCC). In this document, a robust nonlinear controller by Advanced Backstepping with Integral Action Control (ABIAC) is initially employed to monitor the rotor and the network side converters under normal network operations. In the presence of a network fault, an improved protection scheme (IPS) is tacked on to the robust nonlinear control to help enforce the behavior of the DFIG system to be able to overcome the fault. The IPS, which is formed by a crowbar and an RL series circuit, is typically located in the space between the rotor coils and the RSC converter. Compared to a standard crowbar, the developed scheme is successful to limit the rotor transient current and DC-link voltage, also an RSC disengagement to rotor windings can be prevented during the fault. Furthermore, the controllers of both the RSC and the Network Side Converter (NSC) are modified to boost the supply voltage at the PCC. A comparative study is also performed between the IPS without and with modification of the reactive power control loops. The simulation results mean that with the modified controllers during the fault, the amount of reactive power sustainment with ABIAC at the PCC is optimized to 17.5 MVAr instead of 15 MVAr with proportional-integral control (PIC). Therefore, the voltage at the PCC is fort increased in order to comply with the voltage requirements of the farm and absolutely to maintain the connection to the network in case of voltage dip.

Wind farm (WF) grid codes require wind generators to have low voltage ride through (LVRT) capability, which means that normal power production should be resumed quickly once the nominal grid voltage has been recovered. However, WFs with fixed-speed wind turbines with squirrel cage induction generators (FSWT-SCIGs) have failed to fulfill the LVRT requirement, which has a significant impact on power system stability. On the other hand, variable-speed wind turbines with doubly fed induction generators (VSWT-DFIGs) have sufficient LVRT augmentation capability and can control the active and reactive power delivered to the grid. However, the DFIG is more expensive than the SCIG due to its AC/DC/AC converter. Therefore, the combined use of SCIGs and DFIGs in a WF could be an effective solution. The design of the rotor-side converter (RSC) controller is crucial because the RSC controller contributes to the system stability. The cascaded control strategy based on four conventional PI controllers is widely used to control the RSC of the DFIG, which can inject only a small amount of reactive power during fault conditions. Therefore, the conventional strategy can stabilize the lower rating of the SCIG. In the present paper, a new control strategy based on fuzzy logic is proposed in the RSC controller of the DFIG in order to enhance the LVRT capability of the SCIG in a WF. The proposed fuzzy logic controller (FLC) is used to control the reactive power delivered to the grid during fault conditions. Moreover, reactive power injection can be increased in the proposed control strategy. Extensive simulations executed in the PSCAD/EMTDC environment for both the proposed and conventional PI controllers of the RSC of the DFIG reveal that the proposed control strategy can stabilize the higher rating of the SCIG.

Many wind turbine generators (WTGs) are installed in remote, rural areas, where the power grids are usually weak, characterized by unbalanced voltage conditions. If the voltage unbalance is not taken into account in the control system, it will cause poor power quality and poor operating performance of the WTG systems. This paper proposes a novel control scheme to improve the operating performance of a wind turbine equipped with a doubly fed induction generator (DFIG) under unbalanced network voltage conditions. The rotor-side converter (RSC) and grid-side converter (GSC) of the DFIG are controlled in a positive (dq)+ reference frame as well as in a negative (dq)- reference frame. Control of the RSC and GSC in the (dq)+ reference frame is the same as the regular control under balanced network conditions. The supplementary control of the RSC in the (dq)- reference frame minimizes the electromagnetic torque pulsations of the DFIG caused by the unbalanced network voltage; the supplementary control of the GSC in the (dq)- reference frame contributes to balance the total output currents of the DFIG. The proposed control scheme is implemented in PSCAD/EMTDC on a 3.6 MW DFIG wind turbine connected to a power network with unbalanced voltage. Results show that it improves the operating performance of the DFIG wind turbine system.

An enhanced low voltage ride through (LVRT) control strategy for a doubly fed induction generator based wind energy conversion system (WECS) is presented in this paper. The control for the rotor side and grid side converter is simple compared to the other methods. And pitch control will enable when the rotor speed increase beyond the rated value to protect WECS. Grid side converter (GSC) control reduce the DC link fluctuation and rotor side converter (RSC) control decreases the transient current during the period of fault. The advantages of energy storage to improve the LVRT is also utilized. The Switch type fault current limiter (STFCL) effectively reduce the over current in the rotor side during the fault. Fault current limiting inductors are placed in series with the stator to limit the rotor over current during the fault. SFCL absorbs the excessive energy stored in the stator during the LVRT, for the protection of converters. The feasibility of the proposed method is validated by MATLAB/SIMULINK studies on a 1.5 MW DFIG system.

The modern grid requirement has caused that the wind power system behaves more like conventional rotating generators, and it is able to support certain amount of the reactive power. For a typical doubly fed induction generator (DFIG) wind turbine system, the reactive power can be supported either through the rotor-side converter or the grid-side converter. This paper first compares the current ripples and supportive reactive power ranges between the conventional L and optimized LCL filter, if the reactive power is injected from the grid-side converter. Then, the loss distribution is evaluated both for the generator and the wind power converter in terms of the reactive power done by the rotor-side converter or the grid-side converter with various grid filters. Afterward, the annual energy loss is also estimated based on yearly wind profile. Finally, experimental results of the loss distribution are performed in a down-scaled DFIG system. It is concluded that overexcited reactive power injected from the grid-side converter has lower energy loss per year compared to the overexcited reactive power covered by the rotor-side converter. Furthermore, it is also found that the annual energy loss could even become lower with the optimized filter and thereby more energy production for the wind turbine.