Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> 5) OTHER LVRT METHODS <s> In this paper, a solution is described that makes it possible for wind turbines using doubly-fed induction generators to stay connected to the grid during grid faults. The key of the solution is to limit the high current in the rotor in order to protect the converter and to provide a bypass for this current via a set of resistors that are connected to the rotor windings. With these resistors, it is possible to ride through grid faults without disconnecting the turbine from the grid. Because the generator and converter stay connected, the synchronism of operation remains established during and after the fault and normal operation can be continued immediately after the fault has been cleared. An additional feature is that reactive power can be supplied to the grid during long dips in order to facilitate voltage restoration. A control strategy has been developed that takes care of the transition back to normal operation. Without special control action, large transients would occur. <s> BIB001 </s> Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> 5) OTHER LVRT METHODS <s> Summary form only given. This paper analyzes the ability of a doubly fed induction generator (DFIG) in a wind turbine to ride through a grid fault and the limitations to its performance. The fundamental difficulty for the DFIG in ride-through is the e.m.f. induced in the machine rotor during the fault, which depends upon the d.c. and negative sequence components in the stator flux linkage and the rotor speed. The investigation develops a control method to increase the probability of successful grid fault ride-through, given the current and voltage capabilities of the rotor side converter. A time domain computer simulation model is developed and laboratory experiments are made to verify the model and the control method proposed. Case studies are then performed on a representatively sized system to define the feasibility regions of successful ride-through for different types of grid fault. <s> BIB002 </s> Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> 5) OTHER LVRT METHODS <s> A cascaded nonlinear controller is designed for a variable speed wind turbine equipped with a Doubly Fed Induction Generator (DFIG). The main objective of the controller is wind energy capture optimization while avoiding strong transients in the turbine components and specially in the drive train. The inner loop controller ensures an efficient tracking of both generator torque and stator flux, while the outer loop controller achieves a close tracking of the optimal blade rotor speed to optimize wind energy capture. It is combined to a wind speed estimator that provides an estimation of the wind speed and the aerodynamic torque involved in the controller. The global controller is firstly tested with a simplified mathematical model of the aeroturbine and DFIG for a high-turbulence wind speed profile. Secondly, the aeroturbine controller is validated upon a flexible wind turbine simulator. These new control strategies are compared to other existing controllers based on tests upon an aeroelastic wind turbine simulator. The obtained results show better performance in comparison with the existing controllers. <s> BIB003 </s> Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> 5) OTHER LVRT METHODS <s> This paper proposes a decoupled fault ride-through strategy for a doubly fed induction generator (DFIG) to enhance network stability during grid disturbances. The decoupled operation proposes that a DFIG operates as an induction generator (IG) with the converter unit acting as a reactive power source during a fault condition. The transition power characteristics of the DFIG have been analyzed to derive the capability of the proposed strategy under various system conditions. The optimal crowbar resistance is obtained to exploit the maximum power capability from the DFIG during decoupled operation. The methods have been established to ensure proper coordination between the IG mode and reactive power compensation from the grid-side converter during decoupled operation. The viability and benefits of the proposed strategy are demonstrated using different test network structures and different wind penetration levels. Control performance has been benchmarked against existing grid code standards and commercial wind generator systems, based on the optimal network support required (i.e., voltage or frequency) by the system operator from a wind farm installed at a particular location. <s> BIB004 </s> Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> 5) OTHER LVRT METHODS <s> The application of a dynamic voltage restorer (DVR) connected to a wind-turbine-driven doubly fed induction generator (DFIG) is investigated. The setup allows the wind turbine system an uninterruptible fault ride-through of voltage dips. The DVR can compensate the faulty line voltage, while the DFIG wind turbine can continue its nominal operation as demanded in actual grid codes. Simulation results for a 2 MW wind turbine and measurement results on a 22 kW laboratory setup are presented, especially for asymmetrical grid faults. They show the effectiveness of the DVR in comparison to the low-voltage ride-through of the DFIG using a crowbar that does not allow continuous reactive power production. <s> BIB005 </s> Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> 5) OTHER LVRT METHODS <s> This paper proposes a control strategy of doubly fed induction generators (DFIGs) with new protection schemes for enhancing fault ride through capability of wind farms composed of DFIGs and induction generators (IGs). Since the DFIGs will be stressed or overloaded in the process of stabilizing the wind farm during a grid fault, it is paramount to consider a protection scheme for the DFIG, in order to protect its power converters. Two schemes, the dc-link chopper-controlled braking resistor with the supplementary rotor current (SRC) control of the rotor side converter of the DFIG and series dynamic braking resistor (SDBR) connected to the stator of the DFIG, are proposed and compared. Merits and drawbacks of both schemes are highlighted as well. The simulation results in power system computer aided design and electromagnetic transient including DC (PSCAD/EMTDC) show that the two proposed schemes can eliminate the need for an expensive crowbar switch in the rotor circuit, because both could limit the rotor current of the DFIG within its nominal value during a grid fault. Finally, considering the overall system performance, the latter is recommended. <s> BIB006 </s> Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> 5) OTHER LVRT METHODS <s> Abstract The doubly fed induction generator based wind turbine (DFIG-WT) has strong nonlinearities originated from the aerodynamics of the wind turbine and the coupled dynamic of the DFIG, and can operate under a time-varying and wide operation region. This paper investigates a feedback linearisation controller based on the detailed model of the DFIG-WT, the control objective is to maximize energy conversion for this system. The original nonlinear system is partially linearized to a third-order linear system and a remained second-order internal nonlinear system. Fully decoupled control of the external dynamics is achieved, and the stability of the remained internal dynamics is analyzed via Lyapunov stability method. Moreover, modal analysis is applied for the nonlinear system controlled by the proposed nonlinear controller to verify its global optimal performance and low-voltage ride-through (LVRT) capability over various wind operation range. Simulation studies verify that more accurate tracking and better LVRT capability can be achieved in comparison with conventional vector control (VC). <s> BIB007 </s> Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> 5) OTHER LVRT METHODS <s> Nowadays, most double fed induction generators (DFIGs)-based wind turbines are equipped with a rotor crowbar connected in parallel with the rotor side converter (RSC). The parallel rotor side crowbar (PRSC) is used to protect the RSC and dc-link capacitor by dissipating the rotor energy during grid fault condition. In this paper, two types of crowbar protections are used, one in the rotor winding and the second in the dc link. During the fault condition, the rotor winding crowbar connects in series with the rotor winding and RSC to decrease the RSC current and dissipate the rotor energy. The general PRSC does not have the ability to significantly decrease the over-current. To protect the semiconductor switches of RSC, DFIG should not be kept connected with the utility grids under severe faults. The dc-link capacitor crowbar (DCCC) operates only if the dc capacitor voltage exceeds a threshold level. Both the series rotor side crowbar (SRSC) and the DCCC operate in coordination with each other to protect RSC and dc link during fault condition, and improve the fault ride through of the DFIG. Using the proposed SRSC, RSC continues its operation to control the DFIG during fault condition. Thereby, the reactive power can be injected to support the voltage at the point of common coupling. The behavior of the DFIG is investigated when the combined crowbars are operating with the proposed coordinated control approach and results are presented. <s> BIB008 </s> Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> 5) OTHER LVRT METHODS <s> Doubly fed induction generator (DFIG) based wind turbines are sensitive to grid faults due to utilising small-scale rotor side converter (RSC). The application of crowbar protection to improve the fault ride-through (FRT) capability of the DFIG converts it to a squirrel cage induction generator, which makes it difficult to comply with grid codes. This study proposes an innovative DC-link controllable fault current limiter (C-FCL) based FRT scheme for the RSC to improve the FRT capability of the DFIG. The proposed scheme replaces the AC crowbar protection and eliminates its disadvantages. The C-FCL does not affect the normal operation of the DFIG. By means of the proposed scheme, rotor over-currents are successfully limited during balanced and unbalanced grid faults, even at zero grid voltage. Also, the C-FCL prevents rotor acceleration and high torque oscillations. In this study, an analysis of the proposed approach is presented in detail. The performance of the proposed scheme is compared with the conventional crowbar protection scheme through simulation studies carried out in power system computer-aided design/electromagnetic transients, including dc software (PSCAD/EMTDC). Moreover, the main concept of the proposed approach is validated with an experimental setup and test results are presented. <s> BIB009 </s> Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> 5) OTHER LVRT METHODS <s> Fault-ride-through (FRT) is an imperative capability in wind turbines (WTs) to ensure grid security and transient stability. However, doubly fed induction generator-based WTs (DFIG-WTs) are susceptible to disturbances in grid voltage, and therefore require supplementary protection to ensure nominal operation. The recent amendments in grid code requirements to ensure FRT capability has compelled this study of various FRT solutions. Therefore, for improving FRT capability in pre-installed WTs, re-configuration using external retrofit-based solutions is more suitable and generally adapted. The most relevant external solutions based on retrofitting available are classified as (a) protection circuit and storage-based methods and (b) flexible alternating current transmission system-based reactive power injection methods. However, for new DFIG-WT installations, internal control modification of rotor-side converter (RSC) and grid-side converter (GSC) controls are generally preferred. The solutions based on modifications in RSC and GSC control of DFIG-WT are classified as (a) traditional control techniques and (b) advanced control techniques. This study ensures to curate and compare the FRT solutions available based on external retrofitting-based solutions and internal control modifications. Also, the future trends in FRT augmentation of DFIG-WTs are discussed in this study. <s> BIB010
A consistent control method for parallel rotor side crowbar (PRSC) and series rotor side crowbar (SRSC) with the dclink capacitor crowbar (DCCC) protections to the DFIG converters is presented in BIB008 . Another strategy that suggests connecting a set of resistors to the DFIG rotor in order to limit faults high current is presented in BIB001 . Reference BIB002 introduces improvement in the LVRT control by observing the stator-flux linkage components. The research article BIB005 investigates the application of a dynamic voltage restorer (DVR) which can be easily integrated with existing DFIG based WECSs. Reference BIB004 proposes that the DFIG should operate in a decoupled operation like an induction generator (IG) and as a reactive power source in the case of sever grid disturbances. Series dynamic braking resistor (SDBR) connected to the stator and dc-link chopper-controlled braking resistor with supplementary rotor current (SRC) control of RSC are discussed in BIB006 . Another interesting LVRT strategy which is DC-link controllable fault current limiter (CFCL) is presented in BIB009 . Another recent nonlinear scheme which aims to achieve a globally consistent power generation BIB003 and enhance LVRT is presented in BIB007 . More LVRT strategies for DFIG based WECS are available in BIB010 .