ABSTRACT:
Grid
codes require wind turbines to have capability to withstand a certain grid
voltage unbalance without tripping. However, existing controls for brushless
doubly-fed induction generator (BDFIG) based wind turbine under grid unbalance
have many problems such as difficulty in realizing decoupling control,
involvement with flux or current estimations, and complex control structure.
Moreover, the existing studies only focused on the control of machine side
converter (MSC), but the coordinated control between MSC and grid side
converter (GSC) and the control objectives of overall BDFIG wind turbine system
have not yet been addressed so far. To overcome these problems and improve the
control capability, this paper proposes a coordinated control strategy by
considering MSC and GSC together. First, the enhanced control objectives for
overall BDFIG wind turbine system are determined. Second, the simple single
current closed-loop controllers without involving with any flux or current
estimations are designed for MSC and
GSC,
respectively. Meanwhile, in current loops, all the disturbances and
cross-coupling terms on dq axes are derived and used for feedforward
control so as to achieve decoupling control and improve system dynamic response.
Further, a fast sequence decomposition approach is employed to enhance the
control characteristics of the whole system. Finally, the effectiveness of
proposed control is validated through case studies for a 2 MW BDFIG based wind
generation system. The results demonstrate that the proposed control can effectively
achieve the control objectives of overall wind turbine system under grid
voltage unbalance and provide excellent dynamic and stable performance.
KEYWORDS:
1. Brushless
doubly-fed induction generator (BDFIG)
2. Voltage
unbalance
3. Decoupling control
Wind turbine
Sequence decomposition
SOFTWARE: MATLAB/SIMULINK
BLOCK DIAGRAM:
Fig.1.
configuration of BDFIG-based wind turbine system
EXPECTED SIMULATION RESULTS:
Figure 2. Sequence Decomposition
Results Of Grid Voltage With Notch Filter And Fast Decomposition Algorithm. (A)
Grid Voltage Vgabc (P.U.). (B) Positive Sequence Component In __ Reference Frame
(P.U.) (C) Negative Sequence Component In Reference Frame (P.U.). (D) Positive Sequence
Component In (Dq)C Reference Frame (P.U.). (E) Negative Sequence Component In
(Dq)
Reference Frame (P.U.). (A) Decomposition Method With Notch Filter. (B) Fast Sequence
Decomposition Algorithm
Figure 3. Waveforms With
Elimination Of Torque Oscillations And Three Selectable Control Objectives
Under 10% Grid Voltage Unbalance (!R D 0:7 P.U., " D 10%). (A) Total
Output Current (P.U.). (B) Gsc D- Axis And Pw Q- Axis Currents (P.U.). (C) Gsc
Q- Axis And Pw D-Axis Currents (P.U.). (D) Gsc Dc Axis And Qc Axis Currents
(P.U.). (E)Total Output Active Power (P.U.). (F) Pw And Gsc Active Power
(P.U.). (G) Total Output Reactive Power (P.U.). (H) Pw And Gsc Reactive Power
(P.U.). (I) Electromagnetic Torque (P.U.). (J) Dc Link Voltage (P.U.).
Figure 4. Waveforms With Two Control Modes Under " D 10% Grid Voltage Unbalance (!R D 1:2 P.U.). (A) Total Output Current (P.U.). (B) Pw Voltage (P.U.). (C) Gsc Current (P.U.). (D) Cw Current (P.U.). (E) Total Output Active Power (P.U.). (F) Pw And Gsc Active Power (P.U.). (G) Total Output Reactive Power (P.U.). (H) Pw And Gsc Reactive Power (P.U.). (I) Bdfig Electromagnetic Torque (P.U.). (J) Dc Link Voltage (P.U.). (A) Control Mode 1. (B) Control Mode 2.
Figure 5. Waveforms With
Variation Of Rotating Speed Under " D 10% Grid Voltage Unbalance. (A)
Total Output Current (P.U.). (B) Cw Current (P.U.). (C) Total Output Active
Power (P.U.). (D) Pw And Gsc Active Power (P.U.) (E) Total Output Reactive
Power (P.U.). (F) Pw And Gsc Reactive Power (P.U.). (G) Mechanical Torque And
Electromagnetic Torque (P.U.). (H) Rotor Rotating Speed (P.U.). (I) Dc Link
Voltage (P.U.).
CONCLUSION:
In
this paper, the mathematical model of BDFIG based wind turbine system under
grid voltage unbalance is derived in detail. Based on such model, a coordinated
control strategy by considering MSC and GSC together is proposed. Compared to
existing controls, proposed control for MSC is greatly simplified and more
applicable and has much better parameter robustness due to adopting single current
loop control structure without involving with PW flux, CW flux, and rotor
current estimations. Meanwhile, in cur- rent loop, all the cross-coupling terms
and disturbances are derived and used for feedforward control, thus decoupling controls
for the d-axis and q-axis currents as well as the average PW
active and reactive power can be achieved. On the other hand, GSC is used to
realize coordinated control with MSC so as to achieve three selectable enhanced
control objectives, i.e., eliminating unbalanced total output current,
oscillations of the total output active or reactive power. Further, a fast
sequence decomposition approach instead of notch filers and enhanced PLL for
MSC and GSC are employed to improve the control characteristics of the whole
system. The effectiveness of proposed control is verified by means of theoretical
analysis and case studies. The results demonstrated that the proposed control
can improve the capability of withstanding grid voltage unbalance significantly
and provide excellent dynamic and stable performance.
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