Abstract
In discussing the DFIG as a wind park based, this article illustrates on the functioning of the rotor and grid side converter controller, which constitutes the efficiency. These form the basis for the mechanism of the DFIG. The article also entails the discussion about the fault ride through (FRT), which helps in maintaining the control of the DFIG- WT while ensuring that the system is able to support itself without the consideration of disconnection during any fault on the grid. The Double fed induction generator (DFIG) is an application normally used to complement on the mechanism of the wind turbines. Due to its efficiency, the DFIG now acts as the option an individual can go for when using the multi-MW wind turbines. The DFIG works efficiently in the variable-speed systems, which always contains small requirements for the speed range.
Introduction
Most of the large companies making use of the wind turbines to produce energy have now turned to the use of the double fed induction generator. The DFIG operates in increasing the efficiency and the speed of the wind turbines through the principles of induction. The discussion of the doubly fed induction generator as a wind park will heavily rely on the determination of the major components of the system. The term doubly fed originates from the nature of the DFIG whereby the rotor and the stator have connection to electrical sources. The major components of the DFIG system include the rotor, grid side convertor controls and the slip rings. The identification of the roles of each of these components will helps in describing the mechanism under which the DFIG operates. The DFIG also contains the rotor windings, which are capable of increasing the efficiency together with the slip rings. The presence of windings of multiphase rotor and slip rings, under the induction generator allows for the rotor windings access. The rotor always contains three phase windings that only work in the presence of the three-phase currents. These components of the rotor will also contribute to explaining the functioning of the rotor. The main function of the rotor is to produce torque, in the presence of the presence of the stator fields. The toque is the value for the efficiency of the turbines. The size of the toque produced will depend on the strength of the field produced by the rotor and the stator. Consequently, the action of the rotor forms the basis for the operation of DFIG system.
The rotor and grid side converter controllers
The rotor acts as a major component of the doubly fed induction generator. It always has a connection to an electrical source, which contributes to the induction nature of the system. The rotor always works in close association with the stator in the DFIG system. It contains a wound consisting of three phases that, in turn, get their energy from a current also occurring in three phases. The presence of these rotor currents is what establishes the critical magnetic field for the rotor that would help in the induction process. The formation of the rotor magnetic field triggers for the interaction with the magnetic fields for the stator leading to the formation of torque. The magnetic fields of the rotor and the stator will need to be strong in order to achieve a higher magnitude torque. The size of the torque always depends on the magnitude of the two fields (stator and the rotor magnetic fields). Further, there is need for the consideration of the angular displacement between the rotor and the stator magnetic fields to ensure that there is a stronger torque. The angle between the stator and the rotor should always be perpendicular to obtain the optimum torque. In this case, the rotor magnetic fields act as a magnetic pole which is opposite to that of the stator magnetic poles (acting as magnetic pole). The following figure shows the interaction between the stator magnetic fields and the rotor magnetic fields:
The rotor circuit also contains parasitic elements, the rotor leakage reactance, Lr, and the rotor resistance, Rr. There is also the extra rotor resistance, Rr(1–s)/s, which helps in manipulating the produced mechanical power. This leads to the definition of the slip through the following equation:
S = (ns – nr)/ns whereby ns is the synchronous speed and the nr is the mechanical speed in the rotor. Additionally, the synchronous speed is
Ns = 60 fc / p.
León-Martínez and Montañana-Romeu asserts that the equations above contributes to explanation of the mechanism of the rotor such that when there is faster turning of the rotor in the sub-synchronous mode, the frequency related to the output terminal will decrease while the rotor accelerates towards the synchronous speed [1149]. The reach of the synchronous speed means there is zero frequency while when it exceeds the super-synchronous mode, there will be another increase in the frequency value even though on opposite phase to that of the sub-synchronous mode.
The grid-side converter is also another important element of the DFIG. It is crucial for supporting the grind when in unbalanced load state while also correcting power factor. The converter perfoms these functions without the disturbance of the DFIG wind turbines. Its functioning bases on the symmetrical constituents together with the vector control scheme that contributes to its stability. The presence of the grid side converter also allows for the support of power compensation at a certain place, which is at a distance from the turbine plant. This contributes to its usefulness in promoting efficiency of using the doubly fed induction generator (DFIG). Hodder et al also states that its usefulness also arises from its independency on the rotor-side control besides operating at zero output power from the DFIG-WT [1478].
Project description and aim of the project
The project involves the study of a wind park based on Doubly Fed Induction Generator in order to determine its mechanism in increasing the efficiency off the wind turbines. The project involves the study of the major components of the DFIG, which may have an upper hand in contributing to the much preference for the DFIG. It involves the study of rotor and stator, which forms a crucial part when it comes to the efficiency of the system. The determination of the relationship between the two helps in gauge of the magnitude of any toque produced. The projects also involve the study on how to increase the performance of the fault ride-through (FRT). The FRT are independent and able to support itself without the impacts of the grid faults.
Fault ride-through in DFIG.
The fault ride-through (FRT) helps in maintaining the control of the DFIG- WT while ensuring that the system is able to support itself without the consideration of disconnection during any fault on the grid. This calls for the need to apply techniques that would improve the quality of the fault ride through. This has called for the implementation of the fault current limiter (FCLs). The implementation of the FCLs will call for the need of electromagnetic transient program simulation program (EMTDC). The FCL consists of the superconducting coil that always acts as non-inductive and at high temperature. Wang el asserts that the functioning of the superconducting coil depends on the major components like the operation control algorithm, sequence of events and fault detection techniques [368]. The improvement of the FRT will also rely on the control algorithm, associated with the novel damping voltage, for the STATCOM. Further, the novel damping voltage aids in damping the system oscillation of the whole DFIG thereby increasing the efficiency. Sharma et al asserts that the installation of the FCL is in the close association with the high voltage, on the transformer’s side of the windpark, because of the series arrangement between the two [190]. The FCL generally helps in reducing the fault, which may arise due to the large amount of fault currents besides increasing the efficiency of the FRT.
Conclusion
Analysing the components of DFIG is important in the discussion of the system as significant element in the windmill plants. The rotor and the grid side controller form the main elements, which contributes to the mechanism of the DFIG. The nearing values between the grid frequency and the natural frequency contribute to the functioning of the double fed induction generator. The nature of the frequencies triggers the occurrence of the natural oscillations witnessed on the stator flux and the other related dependable, associated with the rotor. The production of the electromotive forces is what makes the double fed induction generator to be suitable in accelerating the rotation of the windmill. The grid side converter is crucial for supporting the grind when in unbalanced load state while also correcting power factor. The DFIG system also has the AC-DC-AC converter that helps in reducing the power los from the electronic converter. The fault ride-through (FRT) is also crucial when there is need to maintain the control of the DFIG- WT while ensuring that the system is able to support itself without the consideration of disconnection during any fault on the grid.
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