PIR Regulator Using DTC for DFIG during Unbalanced Grid Voltage Conditions

It is significant importance to develop sustainable renewable energy sources due to fast depletion of conventional sources and the environmental pollution caused by them. Wind energy conversion system have been given much attention for generation of electrical power. This paper presents the wind driven doubly fed induction generator(DFIG) based variable speed operation in detail. The control strategy and power electronic converter structure for variable speed constant frequency mode of operation for sub synchronous and super synchronous speed is analyzed and presented. Further, the detailed steady state performance analysis of the DFIG system has been carried out and presented in the manuscript. The presented steady state analysis can be used for grid connected and stand alone applications. Moreover, a simplified analysis has also been proposed and developed along with conventional method. To validate the proposed simplified method, MATLAB program has been formulated for the predetermination. The predetermined results are compared with conventional method for verification. Finally, the experimental results are obtained by using a laboratory prototype machine and compared with predetermined values for validate the proposed method of steady state analysis of DFIG system.

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.

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.

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 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.

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.

An augmented state observer-based sensorless control of grid-connected inverters under grid faults

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.

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.

In this paper, a systematic synchronization procedure is proposed for a doubly fed induction generator (DFIG) during unbalanced grid voltage conditions. The initial induced voltage at the open stator terminal is required to synchronize with the grid voltage in magnitude, frequency and phase. An open stator negative sequence rotor current controller is implemented with the conventional DFIG vector controller, which allows the induced stator voltage to become as unbalanced as the grid voltage, hence enabling a smooth connection. A brief comparison is provided for practical issues such as controller structure variation between DFIG open stator and normal operating conditions, and initial encoder rotor angle measurement offset. The procedure is validated experimentally on a 2.2 kW laboratory-scaled DFIG test bench.

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

This paper proposes a novel finite control set model predictive control (FCS-MPC) strategy with merely grid-injected current sensors for an inductance-capacitance-inductance (LCL)-filtered grid-tied inverter, which can obtain a sinusoidal grid-injected current whether three-phase grid voltages are balanced or not. Compared with the conventional FCS-MPC method, four compositions are added in the proposed FCS-MPC algorithm, where the grid voltage observer (GVO) and Luenberger observer are combined together to achieve full status estimations (including grid voltage, capacitor voltage, inverter-side current, and grid-injected current), while the sequence extractor and the reference generator are applied to eliminate the double frequency ripples of the active or reactive power, or the negative sequence component (NSC) of the grid-injected current caused by the unbalanced grid voltage. Simulation model and experimental platform are established to verify the effectiveness of the proposed FCS-MPC strategy, with full status estimations under both balanced and unbalanced grid voltage conditions.

The grid-connected inverter works as a controlled current source in grid-connected mode, while operates as a controlled voltage source in island mode. The grid-connected inverter should continue to operate normally under unbalanced grid voltage. In case of utility serious faults or intentional islanding, the inverter should be able to seamlessly transfer from grid-connected mode to island mode. In order to achieve these aims, this paper proposed a new feedback multi-loop control strategy with capacitor current, capacitor voltage and grid-side current in αβ reference frame, in which a proportional resonant (PR) regulator is adopted. Compared with the dual-PI control scheme in dq reference frame, the PR controller does not require decomposition of positive-and negative-sequence grid side currents, the transient performance will be improved and low voltage ride through (LVRT) capability of the grid-connected inverters during unbalanced grid voltage condition. The proposed scheme can realize the adjustable power factor, and produce symmetrical and balanced currents in case of unbalanced voltage dips. With this control method, seamless transfer can be achieved between island and grid-connected mode, and load voltage quality during the transition is improved with the proposed control method. Simulations are carried out to verify the proposed controllers and algorithms.

A photo-voltaic (PV) system based grid-connected current source inverter has been investigated in this paper. In this paper, a photovoltaic (PV) system based grid-connected current source inverter has been investigated. For obtaining the maximum power from the PV system, we have developed a Lyapunov energy function-based control scheme. Operation of the system under unbalanced grid voltage condition has been reported and a methodology to extract maximum power from the PV system is articulated. In addition, to achieve optimal performance of the Lyapunov energy function based power controller, an existing parameter update scheme is employed and an optimal performance of the overall system is achieved. Detailed analysis of the proposed power controller with the feature to operate successfully under unbalanced grid voltage condition has been substantiated and discussed along with the effect of parameter update on the overall control architecture. The proposed system is verified via software simulations in MATLAB/Simulink and PLECS domain and various case study results are presented which corroborate the efficacy of the proposed framework.

To deal with the control problems of the three-phase PWM rectifier under the unbalanced grid voltage conditions, this paper proposes a modified direct power control strategy using adaptive sliding mode control algorithm with model of the PWM rectifier in stationary αβ frame. The active and reactive power dynamics of the PWM rectifier are linearized and decoupled based on the inverse system algorithm. Then a sliding mode control scheme is designed to improve the robustness and reduce the influences caused by the system parameters variations and load disturbances. In order to satisfy the state requirements, an adaptive law is introduced to the control strategy to reduce the sliding mode control gain on the premise that the system is stable. Simulation results confirm the feasibility of the proposed control strategy.