21-01-2013, 01:16 PM
DFIG using its FACTS Features through the Grid Side Converter in Grid-Connected Wind Power Application
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Abstract
This paper proposes a new control strategy to the grid side converter (GSC) of a DFIG-based wind turbine enabling the DFIG’s FACTS features. This control strategy is based on the pq Theory which take advantage of this converter to improve the energy quality at the point of common coupling (PCC) acting as an active filter in the specific case adopted in this article. The control strategy of the rotor side converter (RSC) adopted was the classical field-oriented control which determines the active and reactive power that will be injected in the electrical system. Simulation results in PSCAD/EMTDC and some discussions are also shown throughout the paper.
Introduction
Wind power is an economically competitive means of electricity and has experienced a tremendous growth during last decade [1]. The wind power systems are highly dependent on the wind. It is necessary to link them with the power grid for these systems can continuously provide electric power to customers, which is a big incentive for both customers and utilities companies.
Moreover it is very easy to connect a wind power resource to the grid [2]. The increasing capacity of the installed wind power generation facilities linked to the electrical network made it necessary to redesign the existing grid code (GC) requirements [3].
In this context the wind turbines based on Doubly-Fed Induction Generator (DFIG) are widely used and they are already responsible for a significant part of the global wind power.
DFIG Model
Assuming that DFIG stator has a low resistance, a low leakage inductance, a vector model with fast synchronization system and also considering that the magnetic circuit of DFIG is linear, it’s possible to simplified the 5th order model normally adopted [4].
The steady-state stator currents after the simplification in dq synchronous reference frame are shown in (1) and (2) [3]-[4].
DFIG Model
Assuming that DFIG stator has a low resistance, a low leakage inductance, a vector model with fast synchronization system and also considering that the magnetic circuit of DFIG is linear, it’s possible to simplified the 5th order model normally adopted [4].
The steady-state stator currents after the simplification in dq synchronous reference frame are shown in (1) and (2) [3]-[4].
Simulation Results
The system was simulated for a strong grid with short circuit ratio (SCR) equal to 100. At t = 2.0s, a 350kVA nonlinear load was connected to the AC bus. The wind turbine based on DFIG provides 1.0MW to the power. This study included variation in wind speed in order to make the simulation closer to a real case. Initially, it will be analyzed cases for the proposed control strategy.
Figure 6(a) shows that just at the moment of activation of the nonlinear load, the rotor speed suffers a small disturbance.
The active and reactive power generated from the wind turbine are shown in Figure 6(b). These powers are controlled by the rotor side converter. It is possible to note that the waveforms of the active and reactive power are in steady-state.
Figure 6© can be noted that occur a disturbance in the DC-link voltage at the time that the load was turned on.
From now on some comparisons between the classical and proposed control strategy will be performed.
Conclusion
In this paper was presented a wind turbine based on DFIG and its mathematical modeling. It was proposed a control strategy for the grid side converter (GSC) based on the instantaneous active and reactive power theory. This control strategy allowed exploring the FACTS features from the GSC of the DFIG.
In order to show the robustness of the proposed control strategy a simulation was performed in PSCAD/EMTDC, in which a nonlinear load is connected to the system. Thus, this scenario was simulated using both the classical and proposed control strategy.
The simulation results showed that if there are no significant nonlinear loads connected to the system, both control strategies work properly and fairly similar. However, in the presence of significant nonlinear loads the proposed control strategy which compensates the current drained by the nonlinear load, seems to be more effective than the classic solution. As a consequence, the current injected into the power grid, for the last case, presents harmonic distortion less than when is used the classical control strategy.