Proposed Direct Power Control Strategy Research Articles

Direct Power Control of a Bipolar Output Active Rectifier for More Electric Aircraft Based on an Optimized Sector Division

This paper presents a novel direct power control (DPC) strategy based on an optimized sector division for a three-phase coupled inductor-based bipolar output active rectifier (TCIBAR) applied in more electric aircraft (MEA). First, based on the instantaneous power theory, the power model of the TCIBAR is built in the synchronous rotating coordinate system. Second, to implement the hysteresis power control of TCIBAR without causing the runaway of the zero-sequence current in the three-phase coupled inductor (TCI), a set of new voltage vectors that have the same zero-sequence voltage (ZSV) component are synthesized and adopted in the proposed DPC strategy. Third, by quantitatively analyzing the effect of the new synthesized voltage vectors on the power variation of TCIBAR, an optimized sector division method is proposed to improve the accuracy of power control and reduce the phase current harmonics in TCIBAR. Finally, to maintain the voltage balance of the bipolar dc ports in TCIBAR, voltage balance control is studied in the proposed DPC strategy. The proposed DPC strategy is researched on an experimental platform of TCIBAR, and the results show that the proposed DPC strategy is feasible and has good static and dynamic performance.

A Novel Dynamic Switching Table Based Direct Power Control Strategy for Grid Connected Converters

Direct power control (DPC) has gained much attention for medium-voltage applications due to its fast power control capability. However, a conventional static switching table based DPC can lead to unsatisfactory response when the converter is connected to an electric grid because it is commonly designed assuming ideal and constant voltages of the grid and the dc link. Therefore, the actual grid voltage and the actual converter dc-link voltage should be considered while designing the switching table. This paper proposes a novel dynamic switching table for the DPC, considering actual grid and dc-link voltages variation. The proposed dynamic switching table incorporates a new definition of power influence crossover angles of each converter space vector. The proposed scheme dynamically adapts the switching table by feeding forward the actual grid and dc-link voltages, and hence the converter's power is precisely controlled. The proposed technique is verified for the three-level neutral point clamped converters connected to a standard IEEE 9-bus test system. Moreover, the real-time simulations are performed in a real-time digital simulator, OPAL-RT. The obtained results verified the effectiveness of the proposed DPC strategy under different grid fault scenarios.

Direct Power Control of Doubly Fed Induction Generator Without Phase-Locked Loop Under Harmonically Distorted Voltage Conditions

This paper proposes an improved direct power control (DPC) strategy of the doubly fed induction generator without phase-locked loop (PLL) under harmonically distorted voltage conditions. The proposed DPC scheme is implemented in a virtual synchronous reference frame, so that the potential instability problems caused by PLL can be eliminated. A second-order vector integrator is integrated with DPC to suppress the harmonic components directly, so that three different control targets of smooth stator active and reactive powers, sinusoidal stator current and constant electromagnetic torque, can be achieved. The sequential separations and the complex calculations of the power compensating items can also be eliminated. Meanwhile, a simple grid frequency estimation scheme is proposed, which can make the control strategy self-adaptive to the frequency variation in the practical situation. Finally, the experimental results validate the availability of the proposed DPC strategy.

Direct Power Control of DFIG System Without Phase-Locked Loop Under Unbalanced and Harmonically Distorted Voltage

This paper proposes an improved direct power control (DPC) strategy for the rotor-side converter (RSC) and grid-side converter (GSC) of the doubly fed induction generator under unbalanced and distorted voltage conditions. The proposed scheme is implemented in a synchronous reference frame with the fixed angular speed, so that the phase-locked loop can be removed and relevant potential instability issues can be avoided. Besides, the sequential separations and complex calculations of the power compensating items can be eliminated with the direct-resonant method based on a dual-frequency frequency adaptive vector PI. Thus, the negative and harmonic components can be directly eliminated, achieving balanced and sinusoidal current injected into the power grid. Meanwhile, the control strategy can achieve self-adaptive to the grid frequency variation in the practical situation with the proposed frequency estimation scheme. Finally, the experimental results validate the availability of the proposed DPC strategy.

Coordinated Direct Power Control of DFIG System Without Phase-Locked Loop Under Unbalanced Grid Voltage Conditions

This paper proposes a coordinated direct power control (DPC) scheme for the rotor-side converter (RSC) and the grid-side converter (GSC) of the doubly fed induction generator (DFIG) under unbalanced grid voltage conditions. In order to eliminate the coupling interactions between the phase-locked loop (PLL) and the local unbalanced network, a virtual phase angle is used to replace the actual one acquired by the PLL. Thus, the PLL is removed out of RSC and GSC in the proposed DPC scheme. During network unbalance, the RSC is controlled to reduce torque ripples, while three selectable control targets are identified for the GSC, i.e., constant total active power, constant total reactive power, and balanced currents. A single-side resonant controller with the frequency discrimination between the positive- and negative-sequence signals of the same frequency is employed in the coordinated DPC scheme to avoid the complex calculations of the power compensating components. Meanwhile, the sequential separations of the voltages and currents are also eliminated. Then, the control performance, including the limits of the dc-link voltage, the dc-capacitor power oscillations, the impacts of the frequency deviations, and the grid synchronization of the proposed DPC strategy, is discussed. Finally, the experimental results of DFIG system verify the effectiveness of the proposed DPC strategy under unbalanced grid voltage conditions.

Multiple target implementation for a doubly fed induction generator based on direct power control under unbalanced and distorted grid voltage

This paper presents a multiple target implementation technique for a doubly fed induction generator (DFIG) under unbalanced and distorted grid voltage based on direct power control (DPC). Based on the mathematical model of DFIG under unbalanced and distorted voltage, the proportional and integral (PI) regulator is adopted to regulate the DFIG average active and reactive powers, while the vector PI (VPI) resonant regulator is used to achieve three alternative control targets: (1) balanced and sinusoidal stator current; (2) smooth instantaneous stator active and reactive powers; (3) smooth electromagnetic torque and instantaneous stator reactive power. The major advantage of the proposed control strategy over the conventional method is that neither negative and harmonic sequence decomposition of grid voltage nor complicated control reference calculation is required. The insensitivity of the proposed control strategy to DFIG parameter deviation is analyzed. Finally, the DFIG experimental system is developed to validate the availability of the proposed DPC strategy under unbalanced and distorted grid voltage.

Improved DPC Strategy of Grid-connected Inverters under Unbalanced and Harmonic Grid Conditions

This paper presents an improved direct power control (DPC) strategy for grid-connected voltage source inverter (VSI) under unbalanced and harmonic grid voltage conditions. Based on the mathematic model of VSI with the negative sequence, 5th and 7th harmonic voltage components consideration, a PI controller is used in the proposed DPC strategy to achieve the average output power regulation. Furthermore, vector PI controller with the resonant frequency tuned at the two times and six times grid fundamental frequency is adopted to regulate both negative and harmonic components, and then two alternative targets of the balanced/sinusoidal current and smooth active/reactive output power can be achieved. Finally, simulation results based on MATLAB validate the availability of the proposed DPC strategy.

Direct Power Control of Doubly Fed Induction Generator Under Distorted Grid Voltage

This paper presents a direct power control (DPC) strategy for a doubly fed induction generator (DFIG)-based wind power generation system under distorted grid voltage. By analyzing the six times grid frequency power pulsation produced by the fifth and seventh grid voltage harmonic components, a novel DPC strategy with vector proportional integrated (VPI) regulator has been proposed to implement the smooth active and reactive power output of DFIG. The performance analysis of the proposed DPC strategy, including the steady and dynamic state performance, closed-loop operation stability, and rejection capability for the grid voltage distorted component and back EMF compensation item has been investigated. The availability of the proposed DPC strategy with a VPI regulator is verified by experiment results of DFIG system under harmonically distorted grid condition.

Direct Power Control Strategy of Three-phase PWM Rectifier

For traditional Direct Power Control (DPC), the switching frequency is not fixed and the characteristics in steady state are not well. In order to solve these problems, a proposed DPC strategy based on virtual flux is designed. Firstly, the mathematical model of three-phase PWM rectifier is presented. Then, the virtual flux estimator is established to calculate the active and reactive power. Finally, the system model is built and simulated in Matlab/Simulink. The electrical waveforms show that the control effect is excellent. The proposed DPC system has simple structure and algorithm, rapid dynamic response, high power factor, fixed switching frequency and low harmonic distortion, and so on.

Improved Direct Power Control of a Wind Turbine Driven Doubly Fed Induction Generator During Transient Grid Voltage Unbalance

This paper proposes an improved direct power control (DPC) strategy for a doubly fed induction generator (DFIG)-based wind power generation system under unbalanced grid voltage dips. The fundamental and double grid frequency power pulsations, which are produced by the transient unbalanced grid faults, are mathematically analyzed and accurately regulated. Five selectable control targets, with proper power references given, are designed for different applications during network unbalance. In order to provide enhanced control performance, two resonant controllers, which are tuned to have large gain at the power pulsation frequencies, are applied together with the proportional-integral controller to achieve full control of the DFIG output power. The effectiveness of the proposed DPC strategy is verified by the experimental results of a 5-kW DFIG system under different unbalanced voltage dips, which are generated by a specially designed voltage dip generator.

Improved Direct Power Control of a DFIG-Based Wind Turbine During Network Unbalance

This paper proposes an improved coordinate direct power control (DPC) strategy for the doubly fed induction generator (DFIG) and the grid side converter (GSC) of a wind power generation system under unbalanced network conditions. Two improved DPC schemes for the DFIG and the GSC are presented, respectively. The DPC for DFIG is to eliminate the torque and stator reactive power pulsations, while the DPC for GSC is to compensate for the pulsations of stator active power. In order to provide enhanced control performance of the overall system, the resonant controllers tuned at the fundamental and double grid frequency are applied in the improved DPC to eliminate the torque and power pulsations produced by the transient unbalanced grid faults, so that the output power of DFIG and GSC can be directly regulated without any necessity of the positive and negative sequence decomposition. The effectiveness of the proposed DPC strategy is verified by the simulation results of a 2-MW DFIG system under unbalanced grid voltage conditions.