Magnitude and frequency control of grid connected doubly fed IG based synchronised model for wind power generation
- 2. Magnitude and frequency (MFC) strategy is proposed for the doubly fed induction generator. The proposed MFC makes the DFIG equivalent to a synchronous generator in power system. Many wind farms are adopting doubly fed induction generator (DFIG) technology. 2
- 3. 1.The conventional control strategy of the DFIG is based on rotor current vector control with the d-q frame. 2. flux magnitude and control (FMAC)adjust the magnitude of the rotor voltage for the control of the stator voltage and phase angle of the rotor voltage for the control of the electrical power. FMAC can also add auxiliary control loops. However the conventional and FMAC involves relatively complex transform between the rotor and synchronous reference frame 3
- 4. 3. Another useful control strategy is based on the direct power control (DPC) With appropriate rotor voltage vectors. DPC can also achieve decoupled active and reactive power control. However, the switching frequency is not constant with the variation of operating conditions. This makes the design of the harmonic filter of the rotor side power converter difficult. Based on the synchronized model, a new control scheme has been proposed. This control strategy relies on adjusting the magnitude and frequency of the rotor voltage to control the stator voltage and active power. 4
- 5. DFIG-based wind turbine connected to an infinite bus. A back-to-back converter is connected to the rotor brushes to implement bi-directional transfer of slip power. 5
- 6. By neglecting the stator transient, the voltage equations of the DFIG in the arbitrary d-q reference frame can be expressed as follows Ud1 = -r1 id1 – Ψq1 ω1 Uq1 = -r1 iq1 + Ψd1 ω1 Ud2 = r2 id2 + p Ψd2 – Ψq2 ω2 Uq2 = r2 iq2 + p Ψq2 + Ψd2 ω2 …….(1) 6
- 7. The corresponding flux linkage can be expressed as ψd1 = -L1id1 + Lm id2 Ψq1 = -L1 iq1 + Lm iq2 Ψd2 = L2 id2 – Lm id1 Ψq2 = L2 iq2 – Lm iq1 ……..(2) Thus, the relations between the stator and rotor currents can be expressed as id2 = ψ+Lm id1 ÷ L2 Iq2 = Lm ÷ L2 iq1 ……..(3) 7
- 8. In order to eliminate the rotor variables in stator equations, define Èq = ω Lm / L2 * ψ2 ……(4) X´ = ω1L1 ……(5) By substituting (5) into (1)–(3), the stator voltage equations can be written as follows Ud1 = -r1 id1 + X΄1 iq1 Uq1 = -r1 iq1 - X΄1 id1 + E΄q …….(6) 8
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- 10. Stator currents be calculated as id1 = E΄q – U1 cos δ / X΄1 iq1 = U1 sin δ / X΄1 ……(7) Then the active and reactive powers of the DFIG stator can be written as P1 = U1 I1 cos φ = E΄q U1/X΄ * sin δ Q1 = U1 I1 sin φ = E΄q U1/X΄1 cos δ – U1^2/X΄1 ……..(8) 10
- 11. The power angle in synchronous generator is relatively small in normal operation which is often below 30 ,This condition can be also met in DFIG. Classic synchronous generator theory indicates that the active power transfer depends mainly on the power angle δ and the reactive power transfer depends mainly on the voltage magnitude of E΄q, By analogy of synchronous generator, the control of the stator active power and reactive power of the DFIG can be seen as the control of phase and magnitude of E΄q. The DFIG has an advantage in that the power angle δ (and therefore the active power) is controllable by the rotor converter whereas δ in the synchronous generator is determined by the axis of the field winding. 11
- 12. The active power of the DFIG rotor can be expressed as P2 = ud2 id2 + uq2 iq2 …….(12) By substituting (1),(3),(4) and (8) into (12) ,The active power of the DFIG rotor can be expressed as P2 = Pr2 + ω2/ω1 * p1 12
- 13. Depending on the rotor speed ωr, the rotor current frequency, ω2 = ω1 – ωr, can be positive and negative and therefore the rotor power changes direction The active power of the rotor is positive when the DFIG operates at the sub-synchronous mode (ω1>ωr) negative when the DFIG operates at the super synchronous mode (ω1<ωr) 13
- 14. The magnitude and frequency control (MFC) method only controls the magnitude and frequency of the rotor voltage. By employing this control strategy, the DFIG has characteristics similar to the synchronous generator. It has two feedback loops, first loop regulates the active power and the second loop regulates the grid voltage magnitude. 14
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- 17. When independent active and reactive power control is needed in DFIG-based wind turbines, an auxiliary outer loop is added to the MFC control block in order to realize reactive power control. Simulation results of MFC with the auxiliary reactive power loop are shown which shows the dynamic response of the DFIG system when the active power reference steps from 2 to 6 kW followed by a step decrease back to 2kW. 17
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- 22. A MFC strategy has been proposed. Simulation and experiment results have shown that the proposed MFC is effective for the DFIG system. This new method controls active and reactive powers of the Stator by controlling the magnitude and frequency of the rotor current. The proposed MFC enables the DFIG to have similar characteristic to the synchronous generator. Future work : The parameters of the PI control can be optimized or advanced control methods can be used in future to improve the system performance. 22
- 23. EKANAYAKE J.B., HOLDSWORTH L., XUEGUANG W., JENKINS N:‘Dynamic modeling of doubly fed induction generator wind turbines’, IEEE Trans. Power Syst., 2003, 18, pp. 803–809 LEDESMA P., USAOLA J.: ‘Doubly fed induction generator model for transient stability analysis’, IEEE Trans. Energy Convers., 2005, 20, pp. 388–397 VICATOS M.S., TEGOPOULOS J.A.: ‘Steady state analysis of a doubly fed induction generator under synchronous operation’, IEEE Trans. Energy Convers., 1989, 4, (3), pp. 495–501 SHI L., XU Z., HAO J., NI Y.: ‘Modelling analysis of transient stability simulation with high penetration of grid connected wind farms of DFIG type’, Wind Energy, 2007,10, (4), pp. 303– 320 FEIJOO A., CIDRAS J., CARRILLO C.: ‘A third order model for the doubly-fed induction machine’, Electric Power Syst. Res., 2000, 56, (2,1), pp. 121–127 23