This paper presents a comprehensive study on the state-of-the-art and emerging wind energy technologies from the electrical engineering perspective. In an attempt to decrease cost of energy, increase the wind energy conversion efficiency, reliability, power density, and comply with the stringent grid codes, the electric generators and power electronic converters have emerged in a rigorous manner. From the market based survey, the most successful generator-converter configurations are addressed along with few promising topologies available in the literature. The back-to-back connected converters, passive generator-side converters, converters for multiphase generators, and converters without intermediate dc-link are investigated for high-power wind energy conversion systems (WECS), and presented in low and medium voltage category. The onshore and offshore wind farm configurations are analyzed with respect to the series/parallel connection of wind turbine ac/dc output terminals, and high voltage ac/dc transmission. The fault-ride through compliance methods used in the induction and synchronous generator based WECS are also discussed. The past, present and future trends in megawatt WECS are reviewed in terms of mechanical and electrical technologies, integration to power systems, and control theory. The important survey results, and technical merits and demerits of various WECS electrical systems are summarized by tables. The list of current and future wind turbines are also provided along with technical details.

High-Power Wind Energy Conversion Systems: State-of-the-Art and Emerging Technologies

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

This paper presents a comprehensive study on the latest grid code regulations enforced by transmission system operators on large wind power plants (WPPs). First, the most common requirements included in the majority of international grid codes are compared; namely, low and high voltage ride-through capabilities, active and reactive power responses during and after faults, extended range of voltage–frequency variations, active power (frequency) control facility, and reactive power (voltage) regulation support. The paper also presents a discussion on the global harmonization of international grid codes as well as future trends expected in the regulations. Finally, the evolution of different wind generator technologies to fulfill various grid code requirements is investigated. The presented study will assist system operators to establish their connection requirements for the first time or to compare their existing regulations with other operators. It also enables wind turbine manufacturers and wind farm developers to obtain a more precise understanding from the latest international requirements imposed on modern wind farms.

Review of international grid codes for wind power integration: Diversity, technology and a case for global standard

Nonlinear maximum power point tracking control and modal analysis of DFIG based wind turbine

The application of Photovoltaic (PV) in the distributed generation system is acquiring more consideration with the developments in power electronics technology and global environmental concerns. Solar PV is playing a key role in consuming the solar energy for the generation of electric power. The use of solar PV is growing exponentially due to its clean, pollution-free, abundant, and inexhaustible nature. In grid-connected PV systems, significant attention is required in the design and operation of the inverter to achieve high efficiency for diverse power structures. The requirements for the grid-connected inverter include; low total harmonic distortion of the currents injected into the grid, maximum power point tracking, high efficiency, and controlled power injected into the grid. The performance of the inverters connected to the grid depends mainly on the control scheme applied. In this review, the global status of the PV market, classification of the PV system, configurations of the grid-connected PV inverter, classification of various inverter types, and topologies are discussed, described and presented in a schematic manner. A concise summary of the control methods for single- and three-phase inverters has also been presented. In addition, various controllers applied to grid-tied inverter are thoroughly reviewed and compared. Finally, the criteria for the selection of inverters and the future trends are comprehensively presented.

A comprehensive review on inverter topologies and control strategies for grid connected photovoltaic system

As the penetration of wind power in electrical power systems increases, it is required that wind turbines remain connected to the grid and actively contribute to the system stability during and after grid faults. This study proposes an efficient control strategy to improve the low-voltage ride-through (LVRT) capability in doubly fed induction generators (DFIGs). The proposed scheme consists of passive and active LVRT compensators. The passive compensator is based on a new crowbar arrangement located in series with stator windings. It considerably reduces the rotor inrush current at the instants of occurring and clearing the fault. The active LVRT compensator is realised through rotor voltage control and reduces the oscillations of electrical torque, rotor instantaneous power and DFIG transient response during the voltage dip. The proposed LVRT control not only reduces the peak rotor fault current, but also minimises the rotor instantaneous power oscillations, and consequently limits the dc-link voltage fluctuations. At the end, results of theoretical analyses are verified by time-domain simulations.

Efficient control scheme of wind turbines with doubly fed induction generators for low-voltage ride-through capability enhancement

Grid-fault ride-through analysis and control of wind turbines with doubly fed induction generators

This paper deals with the coordinated control of rotor- and grid-side converters in wind turbines with doubly fed induction generators (DFIGs) to improve the low-voltage ride-through capability. The rotor-side converter control and additional equipment, called stator damping resistor, are used to limit the rotor inrush current and to reduce the oscillations and settling time of DFIG transient response during the voltage dip. Also, the grid-side converter is controlled to limit the dc-link overvoltage during the voltage drop. It is found that the dynamics of the grid-side converter and dc-link voltage exhibit nonminimum phase behavior, and thus there is an inherent limitation on the achievable dynamic response during the fault. Since the dc-link dynamics is nonlinear, the linear control scheme cannot properly limit the dc-link voltage under large voltage dips. Thus, a nonlinear control scheme applied to the grid-side converter is proposed, which stabilizes the internal dynamics and limits the dc-link voltage fluctuations during the fault. The proposed ride-through approaches limit the peak values of rotor current and dc-link voltage at the instants of occurring and clearing the fault. They also limit the oscillations of electromagnetic torque, and consequently, improve the DFIG voltage dip behavior.

Coordinated Control Approaches for Low-Voltage Ride-Through Enhancement in Wind Turbines With Doubly Fed Induction Generators

This paper first discusses dynamic characteristics of wind turbines with doubly fed induction generator (DFIG). Rotor back electromotive force (EMF) voltages in DFIG reflect the effects of stator dynamics on rotor current dynamics, and have an important role on rotor inrush current during the generator voltage dip. Compensation of these voltages can improve DFIG ride-through capability and limit the rotor current transients. It is found that the electrical dynamics of the DFIG are in nonminimum phase for certain operating conditions. Also, it is shown that the dynamics of DFIG, under compensation of rotor back EMF and grid voltages, behave as a partially linearizable system containing internal and external dynamics. The internal and external dynamics of DFIG include stator and rotor dynamics, respectively. It is found that under certain operating conditions, the internal dynamics, and thus, the entire DFIG system becomes unstable. This phenomenon deteriorates the DFIG postfault behavior. Since the DFIG electrical dynamics are nonlinear; the linear control scheme cannot properly work under large voltage dips. We address this problem by means of a nonlinear controller. The proposed approach stabilizes the internal dynamics through rotor voltage control, and improves the dynamic behavior of the DFIG after clearing the fault.

Mohsen Rahimi

Papers

Efficient control scheme of wind turbines with doubly fed induction generators for low-voltage ride-through capability enhancement

Grid-fault ride-through analysis and control of wind turbines with doubly fed induction generators

Coordinated Control Approaches for Low-Voltage Ride-Through Enhancement in Wind Turbines With Doubly Fed Induction Generators

Transient Performance Improvement of Wind Turbines With Doubly Fed Induction Generators Using Nonlinear Control Strategy

Dynamic behavior analysis of doubly-fed induction generator wind turbines ― The influence of rotor and speed controller parameters