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Voltage Regulation of Wind Farms Equipped with Variable-Speed Doubly-Fed Induction Generators Wind Turbines
Abstract
This paper presents a new technique for regulating grid-connected wind farms terminal voltage. The wind farm under investigation is assumed equipped with doubly-fed induction generators (DFIGs). The proposed technique is based on using the grid-side converter to regulate the farm's terminal voltage rather than using the rotor-side converter to achieve this task. Description for the studied system with the used dynamic modeling equations and detailed illustrations for the control loops block diagrams are presented. Simulation results for different operating conditions are also presented to reveal the accuracy of the proposed technique. Finally, Corresponding simulation results under unity power factor operation conditions are presented to demonstrate the effectiveness of the control technique.
... Grid supplies the reactive VAR required for the induction machine while it is connected to grid. Another type of induction generator is wound rotor involves double fed induction generator, provides power conversion involving both stator and rotor [5]. DFIG has been become popular after the advancement in power electronics converters which makes possible to operate in variable speed operation. ...
... For what concerns the integration of PV systems into a residential area, the papers [26] and [27] offer some interesting proposals, while the mix of RES and electrical storage is conversely described in [21,[28][29][30]. Extending the concept of RES away from the PV power plants, several voltage controls have been proposed in literature [31][32][33][34][35] for ...
The increasing presence of nonprogrammable renewable energy sources (RES) forces towards the development of new methods for voltage control. In the case of centralized generation, the hierarchical regulation or secondary voltage regulation (SVR) is guaranteed by coordinated voltage and reactive power controls in transmission systems. This type of regulation loses effectiveness when the generation becomes distributed and based on small and medium sized generators. To overcome this problem, it is important that also distributed generators, typically based on RES, participate in the voltage regulation. By starting from the methodologies already applied, this work wants to present a new method for involving distributed generators in SVR. The novelty is given by the application of an existing methodology to the new configuration of electrical grids characterized by a relevant distributed generation. The aim is to control the distributed generators (DGs) as coordinated sources of reactive power for conveniently supporting the voltage regulation. In this paper, a real large photovoltaic (PV) plant is considered. The power plant is composed of several PV generators connected through a distribution network. With the algorithm proposed, the set of generators can be treated as a single traditional power plant that can participate in the hierarchical voltage regulation. The reactive power of each single generator is coordinated in a way similar to the SVR used in several national systems.
... Most projects require financial support from international and organizations, as there is only limited regional support. The air density is a function of air pressure and air temperature, which both are functions of the height above sea level [2]. The theoretical optimum for utilizing the power in the wind by reducing its velocity was first discovered by Betz in 1926, so the theoretical maximum power that can be extracted from wind is; ...
This paper proposes a study of Zafarana Egypt wind farm performance and verification of its equivalent by studying the load power flow and short circuit for both the detailed model and the equivalent model; also it discusses some possible layouts for offshore wind farms and comparison of method of connections. It was noticed that Zafarana wind farm consists of two types of induction machines, fixed speed and variable speed induction machine. Finally optimal capacitor placement is proposed using the simulation program (ETAP).
... The most recent voltage regulation-based techniques involve controlling the rotor side converter in such a manner as to capture the maximum available active power and to regulate the stator terminal voltage [113 -116]. In [117], another technique is established by controlling the rotor side converter for compensating the DFIG magnetizing reactive power, whereas the grid-side converter is controlled to regulate the terminal voltage. Current practices for voltage regulation mode involve deactivating the grid-side converter from exchanging any reactive power with the grid terminals, until the rotor-side converter reaches its rated value. ...
Wind energy resources, unlike dispatchable central station generation, produce power dependable on external irregular source and that is the incident wind speed which does not always blow when electricity is needed. This results in the variability, unpredictability, and uncertainty of wind resources. Therefore, the integration of wind facilities to utility electrical grid presents a major challenge to power system operator. Such integration has significant impact on the optimum power flow, transmission congestion, power quality issues, system stability, load dispatch, and economic analysis. Due to the irregular nature of wind power production, accurate prediction represents the major challenge to power system operators. Therefore, in this thesis two novel models are proposed for wind speed and wind power prediction. One proposed model is dedicated to short-term prediction (one-hour ahead) and the other involves medium term prediction (one-day ahead). The accuracy of the proposed models is revealed by comparing their results with the corresponding values of a reference prediction model referred to as the persistent model. Utility grid operation is not only impacted by the uncertainty of the future production of wind farms, but also by the variability of their current production and how the active and reactive power exchange with the grid is controlled. To address this particular task, a control technique for wind turbines, driven by doubly-fed induction generators (DFIGs), is developed to regulate the terminal voltage by equally sharing the generated/absorbed reactive power between the rotor-side and the gridside converters. To highlight the impact of the new developed technique in reducing the power loss in the generator set, an economic analysis is carried out. Moreover, a new aggregated model for wind farms is proposed that accounts for the irregularity of the incident wind distribution throughout the farm layout. Specifically, this model includes the wake effect and the time delay of the incident wind speed of the different turbines on the farm, and to simulate the fluctuation in the generated power more accurately and more closer to real-time operation. Recently, wind farms with considerable output power ratings have been installed. Their integrating into the utility grid will substantially affect the electricity markets. This thesis investigates the possible impact of wind power variability, wind farm control strategy, wind energy penetration level, wind farm location, and wind power prediction accuracy on the total generation costs and close to real time electricity market prices. These issues are addressed by developing a single auction market model for determining the real-time electricity market prices.
The reactive power demand of the induction generator is significantly increased when the generator runs close to the rated operating point. The losses are increased due to the increased reactive power flow. Due to the limitation of converter ratings in the doubly fed induction generator (DFIG) system, power clipping may occur at turbine-rated conditions. In this paper, an optimum strategy for reactive power flow in the DFIG scheme is proposed to capture maximum wind power with minimum total losses. The proposed strategy considers the machine-copper loss, converters' loss, and filter loss. Reasonable values for converter ratings are investigated and compared. The modeling of the DFIG-based wind turbine along with its control scheme is designed in Matlab/Simulink environment. Simulation results are presented to ensure the validity and feasibility of the proposed strategy for reactive power management.
In recent years, there has been a strong trend towards wind energy production and therefore, new problems related to the efficient production of renewable energy, to the management and operation of energy transfer and to the energy distribution are arising. Hence, the optimisation of wind energy production and consequently the economic benefits derived from its connection to the grid becomes one of the most important problems to be solved in the very near future. Two control strategies are developed in this paper for the active power regulation of wind farms comprising doubly fed induction generators, to obtain the maximum active power from the wind hitting the blades of a mill.
It is now recognized that many large wind farms will employ doubly fed induction generator (DFIG) variable speed wind turbines. A number of such wind farms are already in operation and more are planned or under construction. With the rising penetration of wind power into electricity networks, increasingly comprehensive studies are required to identify the interaction between the wind farm(s) and the power system. These require accurate models of doubly fed induction generator wind turbines and their associated control and protection circuits. A dynamic model has been derived, which can be used to simulate the DFIG wind turbine using a single-cage and double-cage representation of the generator rotor, as well as a representation of its control and protection circuits. The model is suitable for use in transient stability programs that can be used to investigate large power systems. The behavior of a wind farm and the network under various system disturbances was studied using this dynamic model. The influence of the DFIG control on the stability of the wind farm was also investigated by considering different control gains and by applying network voltage control through both stator side and rotor side converters.
With increasing concern over climate change, a number of countries have implemented new renewable energy targets, which require significant amounts of wind generation. It is now recognized that much of this new wind generation plant will be variable speed type using doubly fed induction generators (DFIG). In order to investigate the impacts of these DFIG installations on the operation and control of the power system, accurate models are required. A fifth order and reduced order (3rd) machine models are described and the control of the wind turbine discussed. The capability of the DFIG for voltage control (VC) and its performance during a network fault is also addressed.
Induction generators are generally simulated by means of a well-known model described by Brereton et al. [1], based on the induction motor equations derived by Stanley [2]. In this model the possibility of opening the rotor circuit in order to inject a voltage source is not taken into account, although there are other models where it is dealt with [3]. This paper presents an alternative way of obtaining the mentioned model and introduces the possibility of modeling voltage sources in the rotor circuit, which can be very useful when simulating some generating schemes, such as variable speed asynchronous wind turbines.
In the last few years, there is a strong trend towards decentralised production and supply, leading to a situation where a growing number of small and medium size producers will be connected to energy networks. But at the same time, the power quality of the generation must be ensured and this means that the electrical parameters of the distribution network have to be maintained within their upper and lower limits. Therefore, new problems related to the management and operation of energy transfer and distribution and to the efficient distribution of renewable energy in the grids are actually arising. Hence, it is reasonable to think that dispersed generation (wind energy generation in this paper) should start to take part in the control of electric variables, and in particular, in reactive power control which is directly related to the voltage level control of distribution networks. This paper presents a control strategy developed for the reactive power regulation of wind farms made up with double fed induction generators, in order to contribute to the voltage regulation of the electrical grid to which farms are connected.
The fast development of wind power generation brings new requirements for wind turbine integration to the network. After the clearance of an external short-circuit fault, the voltage at the wind turbine terminal should be reestablished with minimized power losses. This paper concentrates on voltage recovery of variable speed wind turbines with doubly fed induction generators (DFIG). A simulation model of a MW-level variable speed wind turbine with a DFIG developed in PSCAD/EMTDC is presented, and the control and protection schemes are described. A new control strategy is proposed to reestablish the wind turbine terminal voltage after the clearance of an external short-circuit fault, and then restore the normal operation of the variable speed wind turbine with DFIG, which has been demonstrated by simulation results.
Automatic control is crucial to the efficiency and reliability of wind power generation systems. Predictive functional control for doubly fed induction machine is proposed. The doubly fed induction machine is used in generating mode for large-scale grid-connected variable speed wind turbines. In order to control the active power and reactive power exchanged between the stator and the grid, the rotor is fed by a bi-directional power converter. A dynamic model of doubly fed induction generator (DFIG) written in a d-q reference frame synchronized with the stator flux is established to synthesize controllers and to investigate simulations. The DFIG is controlled by a predictive functional controller whose performances are compared with a traditional proportional integral controller in terms of reference tracking, sensibility to perturbations and robustness. Simulation results verify the effectiveness of the proposed methods.
The prospect of using the doubly fed induction machine (DFIM) drive for renewable energy sources like wind and wave energy has been well established. The state-of-the-art technology uses two back-to-back IGBT converters for controlling the machine and line side interface. However, IGBTs are costly compared to thyristors, especially at high power. In this paper, a novel machine side converter (MSC) and associated control strategy for the DFIM is proposed which will replace one of the IGBT converters. The proposed converter consists of a thyristor bridge and a boost/buck-boost DC-to-DC interface. This converter is capable of bidirectional power flow and thus the machine can be controlled in both the super-synchronous and subsynchronous speed ranges. The proposed converter has the ability for successful commutation of the thyristors for all speeds of operation. Through the suitable control of the IGBT based front end converter (FEC), the composite system interfaces with the grid at nearly unity power factor. The FEC supports the power flow between the rotor circuit and the utility and in addition, it is made to work as an active filter to compensate for any additional harmonics injected by the stator of the machine to the utility. With the proposed power converter combination, the potential cost of the system is reduced without any major sacrifice in the system performance.
A grid-connected wind-power generation scheme using a doubly-fed
induction generator (DFIG) in conjunction with a direct AC-AC matrix
converter is proposed. The analysis employs a stator-flux vector-control
algorithm and a space vector modulated matrix converter to control the
generator rotor current. The system enables optimal speed tracking for
maximum energy capture from the wind and high performance active and
reactive power regulation. The paper describes the operating principles
of this power generation scheme. The matrix converter-based rotor
current control scheme is highlighted. Simulation studies of the
proposed power generation system were carried out. Results obtained are
presented illustrating the good control performance of the system
The dynamic modelling of large (MW) capacity fixed and variable speed induction generator wind turbines is discussed. A reduced order dynamic machine model is derived suitable for modelling both fixed speed and doubly-fed asynchronous generator wind turbines. Control schemes for variable speed turbines, using doubly-fed induction generators (DFIG), are described and simulated. Speed control characteristics and converter protection of the DFIG are implemented in the model. The operation of the models during power system disturbances such as network voltage sags and three-phase faults, as well as the possibility of network voltage instability, are investigated. Simulation results are presented using typical turbine and network data for wind farm installations.
The paper describes the engineering and design of a doubly fed
induction generator (DFIG), using back-to-back PWM voltage-source
converters in the rotor circuit. A vector-control scheme for the
supply-side PWM converter results in independent control of active and
reactive power drawn the supply, while ensuring sinusoidal supply
currents. Vector control of the rotor-connected converter provides for
wide speed-range operation; the vector scheme is embedded in control
loops which enable optimal speed tracking for maximum energy capture
from the wind. An experimental rig, which represents a 7.5 kW variable
speed wind-energy generation system is described, and experimental
results are given that illustrate the excellent performance
characteristics of the system. The paper considers a grid-connected
system; a further paper will describe a stand-alone system
This work demonstrates that the integration of variable-speed wind systems with doubly fed induction generators (DFIG) and a four-quadrant AC-to-AC converter connected to the rotor windings increases the transient stability margin of the electrical grids, when compared with the case where the fixed speed wind systems with cage generators are used. It is due to the influence of the two dedicated rotor current regulators of the DFIG on the dynamic behavior of the other generators in the system. Besides, adequate models to represent the behavior of the DFIG in transient stability studies are presented. From the simulation results, some important conclusions can be extracted to guide the integration of the wind farms on weak or strong grids.
This work presents a control strategy applied to a doubly fed induction machine (DFIM) for wind generation in a medium power system. In order to control the active power and reactive power exchange between the machine and the grid, the rotor is fed by a link of AC-DC-AC converters modeled as voltage and controlled current sources, respectively. The control of the rotor-side converter is realized by fuzzy controllers whose performances are compared with that of conventional Proportional-Integral controllers. The control of the grid-side converter is carried out by a control block based on the instantaneous power theory and it is controlled to maintain the DC link voltage constant and to improve the power factor of the system. Some design considerations of the control schemes are discussed. A multimachine model is used in digital simulations to assess the dynamic behavior of the power system control scheme proposed in this work.