Magnitude and frequency control of grid connected doubly fed IG based synchronised model for wind power generation

1. By-
Neha Kardam
M.Tech(Power system)
11/pps/010
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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.
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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
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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.
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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.
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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)
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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)
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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)
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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)
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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.
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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
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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)
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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.
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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.
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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.
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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
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