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Magnitude and frequency control of grid connected doubly fed induction generator based on synchronized model for wind power generation

By-Neha KardamM.Tech(Power system)11/pps/010

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Introduction 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.

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Control strategies of DFIG1.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

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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.

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DFIG based wind turbine 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.

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Modeling of the DFIG stator

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)

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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)

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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)

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Vector diagram of the DFIG

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Equivalent circuit of the DFIG

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)

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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.

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Modeling the DFIG based wind turbine

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

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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)

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Magnitude and frequency control based on synchronized model of DFIG

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.

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MFC controller diagram

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Simulation results of MFC

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Auxiliary loop

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.

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Auxiliary loop to provide reactive power control

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Simulation results

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Experimental rotor current and voltage

Dynamic behavior of MFC without reactive power loop

Experimental stator active and reactive powers

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Experimental rotor current and voltage with reactive power control loop

Dynamic behavior of MFC with reactive power loop

Experimental stator active and reactive power with reactive power control loop

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Conclusion 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.

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References EKANAYAKE J.B., HOLDSWORTH L., XUEGUANG W., JENKINS

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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