Doubly Fed Induction Generator Open Stator Synchronized Control during Unbalanced Grid Voltage Condition

This paper presents a new configuration of doubly fed induction generator (DFIG)-based wind farm with multilevel cascaded H-bridge (CHB) converter and its control system, which is suitable not only for unbalanced grid voltage conditions, but also for small voltage sags. The proposed configuration of DFIG can balance the point of common coupling (PCC) voltage without needing additional DC link capacitor or energy storage unlike other configurations and methods. Consequently, the stator voltage and the other loads voltage connected to the PCC are kept constant at the nominal value. The new configuration improves all the DFIG signals under unbalanced grid voltage and small voltage sag conditions unlike other methods. Some of the DFIG units in wind farm require to connect the CHB converter, therefore, it is cost-effective. Matlab/Simulink is used for simulation of a 13.5-MW wind farm consisting of nine 1.5-MW DFIG units and the results demonstrate the good efficiency of the proposed scheme.

Doubly fed induction generator (DFIG) still shares a large part in today's wind power market. It provides the benefits of variable speed operation cost-effectively, and can control its active and reactive power independently. Crowbar protection is often adopted to protect the rotor-side voltage source converter (VSC) from transient overcurrent during grid voltage dip. But under unbalanced grid voltage condition, the severe problems are not the transient overcurrent, but the electric torque pulsation and dc voltage ripple in the back-to-back VSCs. This paper develops dynamic models in MATLAB/Simulink, validates it through experiments, investigates the behavior of DFIG during unbalanced grid voltage condition, and proposes new controllers in separated positive and negative sequence. Methods to separate positive and negative sequence components in real time are also developed, and their responses to unsymmetrical voltage dip are compared. Simulation results prove that the separated positive and negative sequence controllers limit the torque pulsation and dc voltage ripple effectively.

This paper presents an independent operation of the rotor-side converter (RSC) and grid-side converter (GSC) for a doubly fed induction generator (DFIG)-based wind energy conversion system under unbalanced grid voltage conditions. In this paper, the RSC is controlled to achieve four different control targets, including balanced stator current, sinusoidal rotor current, smooth stator active and reactive powers, and constant DFIG electromagnetic torque. The GSC is commanded to keep the dc voltage at a constant value. Additional feedback compensators using resonant regulators for the RSC are employed, and the decompositions of the positive and negative sequence components and calculations of the rotor negative current references can be avoided. Another similar compensator is used in the GSC to suppress the dc voltage fluctuates and remove the GSC reactive power oscillations without the stator or rotor power information. The proposed method can make the RSC and GSC available to an independent operation with a simple implementation for higher reliability. The experimental results demonstrate the effectiveness of the proposed control strategy for both the RSC and GSC under unbalanced grid voltage conditions.

A DFIG-based Wind Turbine Operation under Balanced and Unbalanced Grid Voltage Conditions

A novel control scheme for DFIG-based wind energy systems under unbalanced grid conditions

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.

This paper presents a PIR regulator using direct torque control strategy of grid connected wind turbine driven doubly fed induction generators (DFIGs) when the grid voltage is unbalanced. Under the unbalanced grid voltage condition, the stator voltage and current quality is strongly affected due to the negative and distorted components. It will be reducing the performance of other normal loads connected to the DFIG. That control scheme consisting of indirect matrix converter using DTC. To verify the value of the proposed control strategy, simulation results with 500 MVA DFIG topology are presented and discussed in the paper. Finally,the simulation studies are carried out on a 500 MVA wind-turbine driven DFIG system under unbalanced grid voltage conditions. All the results are validated by using PSCAD simulation.

In this paper, a dual-converter three-phase pulse width modulation (PWM) rectifier based on unbalanced one-cycle control (OCC) strategy is proposed. The proposed rectifier is used to eliminate the second harmonic waves of DC voltage and distortion of line currents under unbalanced input grid voltage conditions. The dual-converter PWM rectifier employs two converters, which are called positive-sequence converter and negative-sequence converter. The unbalanced OCC system compensates feedback currents of positive-sequence converter via grid negative-sequence voltages, as well as compensates feedback currents of negative-sequence converter via grid positive-sequence voltages. The AC currents of positive- and negative-sequence converter are controlled to be symmetrical. Thus, the workload of every switching device of converter is balanced. Only one conventional PI controller is adopted to achieve invariant power control. Then, the parameter tuning is simplified, and the extraction for positive- and negative-sequence currents is not needed anymore. The effectiveness and the viability of the control strategy are demonstrated through detailed experimental verification.

This paper presents a mathematical model for Brushless Doubly-Fed Machine (BDFM) based on Stator Flux Orientation (SFO) in positive and negative reference frames under unbalanced grid voltage conditions. A negative controller is designed to eliminate the effects of unbalanced conditions on electrical torque. The model and controller performance during unbalanced conditions is validated by time domain simulation in MATLAB/Simulink.

This study proposes a model predictive stator current control (MPSCC) strategy of doubly fed induction generator (DFIG) under unbalanced grid voltage conditions. Sinusoidal and balanced stator currents injected into the power grid can be ensured due to the direct control of stator currents rather than rotor currents. Model predictive control instead of traditional vector control is adopted, which can increase the current loop bandwidth and obtain faster dynamic responses. Conventional resonant regulators such as second‐order generalised integrators or second‐order vector integrators that are usually used to eliminate unbalanced components in stator currents can also be avoided. Moreover, both extractions of negative sequence components in rotor or stator currents and calculation of commanded rotor current are avoided, which simplifies the control scheme. The proposed MPSCC strategy can also implement the grid connection by introducing the virtual stator currents without any changes in the control scheme. Finally, experimental results based on a 1 kW lab DFIG system are provided to validate the effectiveness of the proposed control strategy.

The direct connectivity of doubly fed induction generator (DFIG)-based wind energy conversion systems to the power grid makes them highly vulnerable to grid faults and network disturbances. This paper proposes a double integral sliding mode controller (DISMC) for a DFIG-based wind energy conversion system operating under unbalanced grid voltage conditions. The proposed control framework is derived using a double integral sliding manifold, specifically designed to mitigate adverse dynamics caused by grid faults and frequency deviations. The stability of the system dynamics and convergence in finite time are demonstrated through Lyapunov theory. The controller's effectiveness is assessed under various faulty scenarios, including a single line-to-ground fault, a three-phase symmetrical fault, and frequency excursion. Simulation results validate the proposed method's robustness, efficiency, and rapid transient response.

Under unbalanced grid voltage conditions, negative-sequence will generate the double fundamental frequency in the dq synchronous rotating reference frame. In order to detect the positive-sequence component of the grid voltages fast, this paper proposes a new phase-locked loop method, called double fundamental frequency phase-locked loop (DFF-PLL). The operation principle is analyzed, giving the extract formulas of the positive- and negative-sequence with double fundamental frequency, the process of DFF-PLL and the detection of positive-sequence component of grid voltages. The paper improves the second order generalized integrator quadrature-signals generation (SOGI-QSG), in order to remove the DC bias voltage and guarantee the orthogonality of SOGI-QSG. The performance of DFF-PLL is verified by using simulation and experimental results. Compared with DSOGI-PLL, this method increases the detection speed and accuracy under unbalanced grid voltage conditions.

Wind energy is often installed in rural, remote areas characterized by weak, unbalanced power transmission grids. In induction wind generators, unbalanced three phase stator voltages cause a number of problems, including overheating and stress on the mechanical components from torque pulsations. Therefore, beyond a certain amount of unbalance (for example 6%), induction wind generators are switched out of the network. In doubly-fed induction generators, control of rotor currents allows for adjustable speed operation and reactive power control. In addition, it is possible to control the rotor currents to correct for the problems caused by unbalanced stator voltages. This paper presents a novel controller design for a doubly-fed induction generator that provides adjustable speed and reactive power control while greatly reducing torque pulsations.

Due to the latest grid codes, wind energy conversion systems (WECSs) are required to remain connected to grid under grid voltage sags and supply reactive power into the grid. So, this paper proposes an enhanced scheme to improve low-voltage ride through (LVRT) capability of doubly fed induction generator (DFIG)-based WECSs under both balanced and unbalanced grid voltage sags. The proposed scheme is composed of active and passive LVRT compensators. The active compensator is performed by controlling the rotor- and grid-side converters of the DFIG to decrease the stator flux oscillations and inject reactive power into the grid. The passive compensator is based on a three-phase stator damping resistor (SDR) located in series with the stator windings. The proposed scheme decreases the negative effects of grid voltage sags in the DFIG system including the rotor over-currents, electromagnetic torque oscillations, and DC-link over-voltage and also injects reactive power into grid to support the grid voltage. So, the LVRT capability of DFIG is enhanced and new grid code requirements are addressed. Simulation results on a 1.5-MW DFIG-based WECS using MATLAB/Simulink demonstrate the effectiveness of the proposed LVRT scheme under both balanced and unbalanced grid voltage sags.

This study presents an optimisation control strategy for the power control of voltage source converter‐based high‐voltage direct current transmission under unbalanced grid voltage conditions. To facilitate simple implementation and flexible operation, the current references are derived and synthesised into one generalised equation that introduces two individually adaptable parameters. The two parameters can be adapted to eliminate the oscillations of instantaneous active and reactive power, as well as the currents at the grid side under unbalanced grid voltage conditions. The optimisation method based on particle swarm optimisation algorithm is presented to determine the two parameters. The simulation is conducted with PSCAD/EMTDC. The results show that the proposed strategy is effective and feasible.