This paper presents an analysis and control design of a doubly-fed induction generator (DFIG)-based wind generation system operating under unbalanced network conditions. A DFIG system model in the positive and negative synchronous reference frames is presented. Variations of stator active and reactive powers and generator torque are fully defined in the presence of negative sequence voltage and current. Alternative DFIG control targets during network unbalance, such as reducing stator current unbalance, torque, and power pulsations minimization, are identified. A rotor current control strategy based on positive and negative (dq) reference frames is used to provide precise control of the rotor positive and negative sequence currents. Simulation results using EMTDC/PSCAD are presented for a 2-MW DFIG wind generation system. It shows that conventional vector control of DFIG without considering network unbalance results in excessive oscillations on the stator active/reactive power, electromagnetic torque, and stator/rotor currents even with a small stator voltage unbalance. In contrast, with the proposed control strategy, enhanced system control and operation such as minimizing oscillations in either active power, or electromagnetic torque, or stator or rotor currents can be achieved

Dynamic Modeling and Control of DFIG-Based Wind Turbines Under Unbalanced Network Conditions

The reliability of the microinverter is a very important feature that will determine the reliability of the ac-module photovoltaic (PV) system. Recently, many topologies and techniques have been proposed to improve its reliability. This paper presents a thorough study for different power decoupling techniques in single-phase microinverters for grid-tie PV applications. These power decoupling techniques are categorized into three groups in terms of the decoupling capacitor locations: 1) PV-side decoupling; 2) dc-link decoupling; and 3) ac-side decoupling. Various techniques and topologies are presented, compared, and scrutinized in scope of the size of decoupling capacitor, efficiency, and control complexity. Also, a systematic performance comparison is presented for potential power decoupling topologies and techniques.

A Review of Power Decoupling Techniques for Microinverters With Three Different Decoupling Capacitor Locations in PV Systems

This paper presents an advanced control strategy for the rotor and grid side converters of the doubly fed induction generator (DFIG) based wind turbine (WT) to enhance the low-voltage ride-through (LVRT) capability according to the grid connection requirement. Within the new control strategy, the rotor side controller can convert the imbalanced power into the kinetic energy of the WT by increasing its rotor speed, when a low voltage due to a grid fault occurs at, e.g., the point of common coupling (PCC). The proposed grid side control scheme introduces a compensation term reflecting the instantaneous DC-link current of the rotor side converter in order to smooth the DC-link voltage fluctuations during the grid fault. A major difference from other methods is that the proposed control strategy can absorb the additional kinetic energy during the fault conditions, and significantly reduce the oscillations in the stator and rotor currents and the DC bus voltage. The effectiveness of the proposed control strategy has been demonstrated through various simulation cases. Compared with conventional crowbar protection, the proposed control method can not only improve the LVRT capability of the DFIG WT, but also help maintaining continuous active and reactive power control of the DFIG during the grid faults.

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Advanced Control Strategy of DFIG Wind Turbines for Power System Fault Ride Through

Doubly fed induction generator (DFIG) technology is the dominant technology in the growing global market for wind power generation, due to the combination of variable-speed operation and a cost-effective partially rated power converter. However, the DFIG is sensitive to dips in supply voltage and without specific protection to “ride-through” grid faults, a DFIG risks damage to its power converter due to overcurrent and/or overvoltage. Conventional converter protection via a sustained period of rotor-crowbar closed circuit leads to poor power output and sustained suppression of the stator voltages. A new minimum-threshold rotor-crowbar method is presented in this paper, improving fault response by reducing crowbar application periods to 11-16 ms, successfully diverting transient overcurrents, and restoring good power control within 45 ms of both fault initiation and clearance, thus enabling the DFIG to meet grid-code fault-ride-through requirements. The new method is experimentally verified and evaluated using a 7.5-kW test facility.

Minimum-Threshold Crowbar for a Fault-Ride-Through Grid-Code-Compliant DFIG Wind Turbine

Incentives provided by European governments have resulted in the rapid growth of the photovoltaic (PV) market. Many PV modules are now commercially available, and there are a number of power electronic systems for processing the electrical power produced by PV systems, especially for grid-connected applications. Filling a gap in the literature, Power Electronics and Control Techniques for Maximum Energy Harvesting in Photovoltaic Systems brings together research on control circuits, systems, and techniques dedicated to the maximization of the electrical power produced by a photovoltaic (PV) source.
Tools to Help You Improve the Efficiency of Photovoltaic Systems
The book supplies an overview of recent improvements in connecting PV systems to the grid and highlights various solutions that can be used as a starting point for further research and development. It begins with a review of methods for modeling a PV array working in uniform and mismatched conditions. The book then discusses several ways to achieve the best maximum power point tracking (MPPT) performance. A chapter focuses on MPPT efficiency, examining the design of the parameters that affect algorithm performance. The authors also address the maximization of the energy harvested in mismatched conditions, in terms of both power architecture and control algorithms, and discuss the distributed MPPT approach. The final chapter details the design of DC/DC converters, which usually perform the MPPT function, with special emphasis on their energy efficiency. 
Get Insights from the Experts on How to Effectively Implement MPPT
Written by well-known researchers in the field of photovoltaic systems, this book tackles state-of-the-art issues related to how to extract the maximum electrical power from photovoltaic arrays under any weather condition. Featuring a wealth of examples and illustrations, it offers practical guidance for researchers and industry professionals who want to implement MPPT in photovoltaic systems.

Power Electronics and Control Techniques for Maximum Energy Harvesting in Photovoltaic Systems

Wind turbines are very susceptible to any disturbance, faults and low grid voltages. They are disconnected from the grid whenever these conditions arise and reconnected once the grid is healthy. Maintaining the wind turbine connected to grid is real challenge. Future grid codes will require wind turbines connected to grid even during the fault condition and have low voltage ride through (LVRT) feature installed. LVRT feature is not available in most of the present wind turbine and few do with addition circuitry and control increasing the cost of the system. In this paper, a novel control strategy and LVRT for wind turbines with doubly fed induction generators is presented to maintain the turbine connected to grid during fault and post fault condition without addition circuitry. The controller is designed using H/sub /spl infin// technique and /spl mu/-analysis based robust control technique to take into account various adverse conditions. The controller is tested on low power hardware prototype and the results are presented.

A novel robust low voltage and fault ride through for wind turbine application operating in weak grids

This paper presents a power converter for coupling photovoltaic arrays to the utility grid. The converter draws a programmable, ripple-free DC current from the photovoltaic array and injects power into the grid at unity power factor. The programmable input current feature makes this converter ideal for use with maximum power point tracking technology. The proposed converter has an additional unique feature in that the internal dc link carries a large (approximately 25%) ripple voltage. Allowing a large ripple on the DC link reduces the required size of the link capacitor. This paper includes basic system information, analysis of filter requirements, controller design and preliminary hardware results.

Utility-connected power converter for maximizing power transfer from a photovoltaic source while drawing ripple-free current

Doubly-fed induction generator (DFIG) based wind turbines are highly susceptible to grid voltage imbalances Normally the turbines are disconnected from the grid in the event of even moderate imbalances which contribute to poor utilization of the turbine In this paper, a control strategy to use the grid-side converter in the rotor circuit of the DFIG to compensate for the grid voltage imbalance is presented A robust Hinfin controller is synthesized to minimize the adverse effects of the imbalances even while providing reactive power support to the grid The control scheme is tested on a small-scale DFIG prototype and the results are presented

A novel H/sub /spl infin// based controller for wind turbine applications operating under unbalanced voltage conditions

This paper analyzes dual voltage controllers for single-phase Power Factor Correction(PFC) Circuits using boost topology. The proposed scheme uses dual voltage controllers, one of the voltage controller having low bandwidth operates only during steady condition, other voltage controller with high bandwidth operates during the transient condition. At any given instant only one voltage controller is in circuit. Lower bandwidth controller, operating during steady state condition, maintains the THD within the required limits and the dc-bus voltage at its reference value. High bandwidth controller improves the transient performance by having faster response to the disturbance. Faster response to disturbances necessitates less energy storage in the dc-bus capacitor, hence a smaller capacitance value is required which resulting in a compact and a light-weight circuit. The comparative study between the proposed scheme and the conventional PFC circuit is carried out. The experimental results for the proposed scheme are also presented.

Dual voltage controller based Power Factor Correction circuit for faster dynamics and zero steady state error

Doubly-fed induction generator (DFIG) based wind turbines are highly susceptible to disturbances and faults Normally these turbines are disconnected from the grid whenever a fault occurs and then reconnected once the grid is restored to normal Thus keeping turbines connected to grid during faults and post fault conditions is a real challenge In this paper, a control strategy for the grid-side converter in the rotor circuit of the DFIG is implemented to keep the turbine connected to the grid A robust H00 controller is synthesized to avoid disconnection of turbine The control scheme is tested on a small-scale DFIG hardware prototype and the results are presented

M.R. Rathi

Papers

A novel robust low voltage and fault ride through for wind turbine application operating in weak grids

Utility-connected power converter for maximizing power transfer from a photovoltaic source while drawing ripple-free current

A novel H/sub /spl infin// based controller for wind turbine applications operating under unbalanced voltage conditions

Dual voltage controller based Power Factor Correction circuit for faster dynamics and zero steady state error

A Novel H/spl infin/ Controller for Wind Turbine Application Operating in Weak Grids