Stator Stationary Reference Frame Research Articles

An Improved Direct Power Control for Doubly Fed Induction Generator

A novel voltage-modulated direct power control (VM-DPC) is first designed for a back-to-back (BTB) converter in the doubly fed induction generator (DFIG) system. The proposed VM-DPC is built in the stator stationary reference frame (αβ). The proposed method uses a simple feed-forward and feedback structure without a phase-locked loop and the Park transformation. Therefore, it can be easily implemented in the BTB converter. Another essential advantage of the proposed VM-DPC is that it can transform the closed-loop DFIG system into a linear-time-invariant one, which can be analyzed and designed through multiple linear control techniques. The proposed method guarantees exponential stability in the stiff grid as well as in weak-grid integration, which is proved based on eigenvalue analysis. Simulation results demonstrate that the proposed VM-DPC has a faster transient response than conventional vector-oriented control (VOC). Also, it maintains a satisfactory steady-state performance at the same level as the VOC. The robustness of proposed VM-DPC against distorted voltage conditions and parameter mismatch is also tested. Finally, the proposed VM-DPC control strategy is validated in an experimental hardware prototype of a 7.5-kW DFIG system operating in real time.

Load Capacity Analysis of Self-Excited Induction Generators Based on Routh Criterion

In this paper, the Routh criterion has been used to analyze the stability of a self-excited induction generator-based isolated system which is regarded as an autonomous system. Special focus has been given to the load capacity of the self-excited induction generator. The state matrix of self-excited induction generators with resistor-inductor load has been established based on transient equivalent circuits in the stator stationary reference-frame. The recursive Routh table of self-excited induction generators is established by the characteristic polynomial coefficients of the state matrix. According to the Routh stability criterion, the necessary and sufficient condition to predict the critical loads of self-excited induction generators is deduced, from which the critical load impedance can be calculated. A simple self-excited induction generator-based isolated power system has been built up with a 2.2 kW self-excited induction generator. The theoretical analysis and experiments were all carried out based on this platform. In the range determined by the minimum excitation capacitance (Cmin) and the maximum excitation capacitance (Cmax), the critical loads under various power factors have been calculated. The agreement of the calculated theoretical results and experimental results demonstrate the effectiveness and accuracy of the proposed analysis method. The conclusions achieved lay a foundation for further application of Routh stability criterion in self-excited induction generator-based power systems analysis.

Adaptive Torque and Flux Control of Sensorless IPMSM Drive in the Stator Flux Field Oriented Reference Frame

In this paper, direct torque and flux control of interior permanent magnet synchronous motor has been described in the stator flux field-oriented reference frame based on adaptive input-output feedback linearization (AIOFL) control method. The proposed control method does not need to know the motor two-axis inductances, the rotor permanent magnetic flux linkages, and the rotor position. However, the knowledge of the stator resistance is mandatory and that is estimated by AIOFL method. In practice, the rotor speed is approximately estimated by using the derivation of the stator flux vector angle in the stator stationary reference frame. The experimental and simulation results presented in this paper show the effectiveness and capability of the proposed control approach.

Stator current vector control strategy of doubly fed induction generator using proportional–resonant regulators

This study proposes the stator current vector control (VC) strategy of the doubly fed induction generator. It is implemented in the stator stationary reference frame without the phase-lock loop. As a result, the phase-locked loop is removed with a simplified structure. Then, in the stator stationary reference frame, the commanded stator current values are calculated on the basis of the instantaneous power theory. By this means, the calculations of the stator current references are irrelevant to the generator parameters. It can greatly reduce the parameter dependency of the control system. Since the currents, the voltages, and the flux linkages are ac signals, the control system would operate on the ac quantities. Then, for zero steady errors, proportional–resonant controllers, tuned at the nominal frequency, are employed to regulate the stator currents to track their commanded values. Finally, the simulation results are presented to validate the effectiveness of the proposed stator current VC strategy.

Predictive Direct Power Control of Doubly Fed Induction Generators Under Unbalanced Grid Voltage Conditions for Power Quality Improvement

This paper proposes a new control strategy of doubly fed induction generators (DFIGs) under unbalanced grid voltage conditions. The proposed controller includes a model predictive direct power control (MPDPC) method and a power compensation scheme. In MPDPC, the appropriate voltage vector is selected according to an optimization cost function, hence the instantaneous active and reactive powers are regulated directly in the stator stationary reference frame without the requirement of coordinate transformation, PI regulators, switching table, or PWM modulators. In addition, the behavior of the DFIG under unbalanced grid voltage is investigated. Next, a power compensation scheme without the need of extracting negative stator current sequence is developed. By combining the proposed MPDPC strategy and the power compensation scheme, distorted currents injected into the power grid by the DFIGs can be eliminated effectively.

Sensorless Control Strategy by Square-Waveform High-Frequency Pulsating Signal Injection Into Stationary Reference Frame

In this paper, a new sensorless control strategy based on the square-waveform high-frequency pulsating voltage signal injection into the stationary reference frame is proposed. Similar to the sinusoidal-waveform injection method, by injecting the high-frequency square-waveform pulsating voltage carrier voltage into the α- (or β-) axis of the stationary reference frame, the response carrier current will fluctuate with the position-dependent saliency, and then the rotor position information can be retrieved without any filtering. Furthermore, with higher frequency, the bandwidth of the position estimation can be significantly improved compared with all the sinusoidal-waveform injection strategies, and the influence of the winding resistance can also be fully eliminated. Meanwhile, the proposed new strategy injects a pulsating high-frequency carrier voltage into the stator stationary reference frame as stable as the rotating carrier signal injection method. Then, the rotor position information can be retrieved from the carrier current response, which is amplitude modulated by the machine saliency as simple as the pulsating carrier signal injection method. The experimental results validate that this strategy can achieve accurate rotor position estimation with good steady-state and dynamic performances by considering the cross-saturation effect even when there exhibits some noise in the estimated rotor position.

Discrete sliding mode control strategy for direct real and reactive power regulation of wind driven DFIG

This paper investigates a discrete sliding mode control (DSMC) strategy for direct real and reactive power regulation for wind driven doubly fed induction generator (DFIG). The real and reactive power errors are eliminated by directly calculating the rotor control voltages through DSMC. In the stator stationary reference frame, direct real and reactive power control strategy is implemented. Therefore, it does not require the angular information of the stator and rotor currents or voltages. It does not involve any extra current loops which results into simple design. The use of constant converter switching frequency via space vector pulse width modulation eases the AC harmonic filter design and improves the power quality. The use of fast and flexible discrete controller makes the system competent with modern digital world. Detailed simulations have been carried out to validate the method. The simulation results reveal that the real and reactive power references are followed smoothly even in the presence of speed perturbations and performance of the system is robust against parameter variations and system disturbances.

Sliding-Mode-Based Direct Power Control of Grid-Connected Wind-Turbine-Driven Doubly Fed Induction Generators Under Unbalanced Grid Voltage Conditions

This paper proposes an improved direct power control (DPC) strategy of grid-connected wind-turbine-driven doubly fed induction generators (DFIGs) when the grid voltage is unbalanced. The DPC scheme is based on the sliding mode control (SMC) approach, which directly regulates the instantaneous active and reactive powers in the stator stationary reference frame without the requirement of either synchronous coordinate transformation or phase angle tracking of grid voltage. The behavior of DFIGs by the conventional SMC-DPC, which takes no negative-sequence voltage into consideration, is analyzed under unbalanced grid voltage conditions. A novel power compensation method is proposed for the SMC-based DPC during network unbalance to achieve three selective control targets, i.e., obtaining sinusoidal and symmetrical stator current, removing stator interchanging reactive power ripples and canceling stator output active power oscillations, respectively. The active and reactive power compensation components are calculated via a simple method and the proposed three control targets can be achieved, respectively, without the need of extracting negative-sequence stator current components. Experimental results on a 2 kW DFIG prototype are presented to verify the correctness and validity of the proposed control strategy and power compensation method.

Enhanced Proportional-Resonant Current Controller for Unbalanced Stand-alone DFIG-based Wind Turbines

An enhanced control strategy for variable-speed unbalanced stand-alone doubly-fed induction generator-based wind energy conversion systems is proposed in this paper. The control scheme is applied to the rotor-side converter to eliminate stator voltage imbalance. The proposed current controller is developed based on the proportional-resonant regulator, which is implemented in the stator stationary reference frame. The resonant controller is tuned at the stator synchronous frequency to achieve zero steady-state errors in rotor currents without decomposing the positive and negative sequence components. The computational complexity of the proposed control algorithm is greatly simplified, and control performance is significantly improved. Finally, simulations and experimental results are presented to verify the feasibility and the robustness of the proposed control scheme.

Reinforced Control and Operation of DFIG-Based Wind-Power-Generation System Under Unbalanced Grid Voltage Conditions

This paper proposes an enhanced control and operation of a doubly fed induction generator (DFIG) based wind power generation system under unbalanced grid voltage conditions. System behaviors of grid-side and rotor-side converters (GSCs and RSCs) are described. The RSC is controlled to eliminate the electromagnetic torque oscillation at double the grid frequency. Meanwhile, three selective control targets for the GSC, i.e., reducing the pulsations in the total active or reactive power, or unbalanced current outputs from the overall system, are identified and analyzed during voltage imbalance. A new current-control scheme is presented for the GSC and RSC without involving the decomposing of positive and negative sequence currents. The controller consists of a proportional (P) regulator and a resonant (R) one tuned at the grid frequency, which is implemented in the stator stationary reference frame. Finally, simulation studies are carried out on a 1.5-MW wind-turbine-driven DFIG system. The validity of the presented current controller and the feasibility of the proposed control targets are all confirmed by the simulated results.