wind turbine doubly-fed induction generator (phasor type)

7/31/2019 Wind Turbine Doubly-Fed Induction Generator (Phasor Type)

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Turbine Doubly-Fed Induction Generator (Phasor Type) :: Blocks ... jar:file:///D:/Program%20Files/MATLAB/R2006b/help/toolbox/

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SimPowerSystems

Wind Turbine Doubly-Fed Induction Generator (Phasor Type)Implement phasor model of variable speed doubly-fed induction generator driven by wind turbine

Library

Distributed Resources/Wind Generation

Description

The wind turbine and the doubly-fed induction generator (WTDFIG) are shown in the figure called The Wind Turbine andthe Doubly-Fed Induction Generator System. The AC/DC/AC converter is divided into two components: the rotor-sideconverter (Crotor) and the grid-side converter (Cgrid). Crotorand Cgrid are Voltage-Sourced Converters that use

forced-commutated power electronic devices (IGBTs) to synthesize an AC voltage from a DC voltage source. A capacitorconnected on the DC side acts as the DC voltage source. A coupling inductor L is used to connect C grid to the grid. The

three-phase rotor winding is connected to Crotorby slip rings and brushes and the three-phase stator winding is directlyconnected to the grid. The power captured by the wind turbine is converted into electrical power by the inductiongenerator and it is transmitted to the grid by the stator and the rotor windings. The control system generates the pitchangle command and the voltage command signals V rand Vgc for Crotorand Cgrid respectively in order to control the power

of the wind turbine, the DC bus voltage and the reactive power or the voltage at the grid terminals.

The Wind Turbine and the Doubly-Fed Induction Generator System

Operating Principle of the Wind Turbine Doubly-Fed Induction Generator

The power flow, illustrated in the figure called The Power Flow, is used to describe the operating principle. In this figurethe followings parameters are used:


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Pm Mechanical power captured by the wind turbine and transmitted to the rotor

Ps Stator electrical power output

Pr Rotor electrical power output

Pgc Cgrid electrical power output

Qs Stator reactive power output

Qr Rotor reactive power output

Qgc Cgrid reactive power output

Tm Mechanical torque applied to rotor

Tem Electromagnetic torque applied to the rotor by the generator

r Rotational speed of rotor

sRotational speed of the magnetic flux in the air-gap of the generator, this speed is named synchronousspeed. It is proportional to the frequency of the grid voltage and to the number of generator poles.

J Combined rotor and wind turbine inertia coefficient

The mechanical power and the stator electric power output are computed as follows:

For a loss less generator the mechanical equation is:

In steady-state at fixed speed for a loss less generator and

It follows that:

where s is defined as the slip of the generator:

The Power Flow


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Generally the absolute value of slip is much lower than 1 and, consequently, Pris only a fraction ofPs. Since Tm is

positive for power generation and since s is positive and constant for a constant frequency grid voltage, the sign ofPr is

a function of the slip sign. Pr is positive for negative slip (speed greater than synchronous speed) and it is negative for

positive slip (speed lower than synchronous speed). For super-synchronous speed operation, Pris transmitted to DC bus

capacitor and tends to rise the DC voltage. For sub-synchronous speed operation, Pr is taken out of DC bus capacitor and

tends to decrease the DC voltage. Cgrid is used to generate or absorb the powerPgc in order to keep the DC voltage

constant. In steady-state for a loss less AC/DC/AC converterPgc is equal to Prand the speed of the wind turbine is

determined by the powerPrabsorbed or generated by Crotor. The power control will be explained below.

The phase-sequence of the AC voltage generated by C rotor is positive for sub-synchronous speed and negative for

super-synchronous speed. The frequency of this voltage is equal to the product of the grid frequency and the absolutevalue of the slip.

Crotorand Cgrid have the capability of generating or absorbing reactive power and could be used to control the reactive

power or the voltage at the grid terminals.

C_rotor Control SystemThe rotor-side converter is used to control the wind turbine output power and the voltage (or reactive power) measured atthe grid terminals.

Power Control

The power is controlled in order to follow a pre-defined power-speed characteristic, named tracking characteristic. Anexample of such a characteristic is illustrated in the figure called Turbine Characteristics and Tracking Characteristic, bythe ABCD curve superimposed to the mechanical power characteristics of the turbine obtained at different wind speeds.The actual speed of the turbine r is measured and the corresponding mechanical power of the tracking characteristic is

used as the reference power for the power control loop. The tracking characteristic is defined by four points: A, B, C andD. From zero speed to speed of point A the reference power is zero. Between point A and point B the trackingcharacteristic is a straight line, the speed of point B must be greater than the speed of point A. Between point B and pointC the tracking characteristic is the locus of the maximum power of the turbine (maxima of the turbine power vs turbine

speed curves). The tracking characteristic is a straight line from point C and point D. The power at point D is one per unit(1 pu) and the speed of the point D must be greater than the speed of point C. Beyond point D the reference power is aconstant equal to one per unit (1 pu).

Turbine Characteristics and Tracking Characteristic


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The generic power control loop is illustrated in the figure called Rotor-Side Converter Control System. The actualelectrical output power, measured at the grid terminals of the wind turbine, is added to the total power losses (mechanicaland electrical) and is compared with the reference power obtained from the tracking characteristic. A Proportional-Integral

(PI) regulator is used to reduce the power error to zero. The output of this regulator is the reference rotor current Iqr_refthat must be injected in the rotor by converter Crotor. This is the current component that produce the electromagnetic

torque Tem. The actual Iqr component of positive-sequence current is compared to Iqr_ref and the error is reduced to zero

by a current regulator (PI). The output of this current controller is the voltage Vqr generated by C rotor. The current

regulator is assisted by feed forward terms which predict Vqr.

Rotor-Side Converter Control System

Voltage Control and Reactive Power Control

The voltage or the reactive power at grid terminals is controlled by the reactive current flowing in the converter Crotor. The

generic control loop is illustrated in the figure called Rotor-Side Converter Control System.

When the wind turbine is operated in voltage regulation mode, it implements the following V-I characteristic.

Wind Turbine V-I Characteristic


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phase of the voltage generated by converter Cgrid (Vgc) from the Idgc_ref produced by the DC voltage regulator

and specified Iq_ref reference. The current regulator is assisted by feed forward terms which predict the Cgridoutput voltage.

The magnitude of the reference grid converter current Igc_ref is equal to . The maximum value of

this current is limited to a value defined by the converter maximum power at nominal voltage. When Idgc_ref and Iq_refare such that the magnitude is higher than this maximum value the Iq_ref component is reduced in order to bring back themagnitude to its maximum value.

Grid-Side Converter Control System

Pitch Angle Control SystemThe pitch angle is kept constant at zero degree until the speed reaches point D speed of the tracking characteristic.Beyond point D the pitch angle is proportional to the speed deviation from point D speed. The control system is illustratedin the following figure.

Pitch Control System

Turbine Model

The turbine model uses the Wind Turbine bloc of the Distributed Resources/Wind Generation library. See documentationof this model for more details.

Induction Generator

The doubly-fed induction generator phasor model is the same as the wound rotor asynchronous machine (see theMachines library) with the following two points of difference:

Only the positive-sequence is taken into account, the negative-sequence has been eliminated.1.

A trip input has been added. When this input is high the induction generator is disconnected from the grid and fromCrotor.

2.

Dialog Box and Parameters

The WTDFIG parameters are grouped in four categories: Generator data, Converters data, Turbine data, and Controlparameters. Use the Display listbox to select which group of parameters you want to visualize.

Generator Data Parameters


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WTDFIG modeled using positive-sequence only

The WTDFIG is modeled by a three-wire system using two current sources. The WTDFIG does not generate anyzero-sequence current, but it can generate negative-sequence currents during unbalanced system operation.

Nominal power, line-to-line voltage and frequencyThe nominal power in VA, the nominal line-to-line voltage in Vrms and the nominal system frequency in hertz.

Stator [Rs, Lls]The stator resistance Rs and leakage inductance Lls in pu based on the generator rating.

Rotor [Rr', Llr']The rotor resistance Rr' and leakage inductance Llr', both referred to the stator, in pu based on the generatorrating.

Magnetizing inductance LmThe magnetizing inductance Lm in pu based on the generator rating.

Inertia constant, friction factor and pairs of polesCombined generator and turbine inertia constant H in seconds, combined viscous friction factor F in pu based onthe generator rating and number of pole pairs p.

You may need to use your own turbine model, in order for example, to implement different power characteristics orto implement the shaft stiffness. Your model must then output the mechanical torque applied to the generator shaft.If the inertia and the friction factor of the turbine are implemented inside the turbine model you specify only thegenerator inertia constant H and the generator friction factor F.

Initial conditionsThe initial slip s, electrical angle in degrees, stator phasor current magnitude in pu, stator phasor current phaseangle in degrees, rotor phasor current magnitude in pu and rotor phasor current phase angle in degrees.

Converters Data Parameters


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Converter maximum powerThe maximum power of both Cgrid and Crotor in pu of the nominal power. This parameter is used to compute the

maximum current at 1 pu of voltage for Cgrid. The maximum current for Crotor is 1 pu.

Grid-side coupling inductor [L R]The coupling inductance L and its resistance R in pu based on the generator rating.

Coupling inductor initial currentsThe coupling inductor initial phasor current in positive-sequence. Enter magnitude IL in pu and phase ph_IL indegrees. If you know the initial value of the current corresponding to the WTDFIG operating point you may specifyit in order to start simulation in steady state. If you don't know this value, you can leave [0 0]. The system will reachsteady-state after a short transient.

Nominal DC bus voltageThe nominal DC bus voltage in volts.

DC bus capacitorThe total capacitance of the DC link in farads. This capacitance value is related to the WTDFIG rating and to theDC link nominal voltage. The energy stored in the capacitance (in joules) divided by the WTDFIG rating (in VA) is atime duration which is usually a fraction of a cycle at nominal frequency. For example, for the default parameters,

(C=10000 F, Vdc=1200 V, Pn=1.67 MVA) this ratio is 4.3 ms, which represents 0.26cycle for a 60 Hz frequency. If you change the default values of the nominal power rating and DC voltage, you

should change the capacitance value accordingly.

Turbine Data Parameters


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External mechanical torqueIf this parameter is checked, a Simulink input named Tm appears on the block, allowing to use an external signalfor the generator input mechanical torque. This external torque must be in pu based on the nominal electric powerand synchronous speed. For example, the external torque may come from a user defined turbine model. Followingthe convention used in the induction machine, the torque must be negative for power generation.

Electric Power-Speed characteristicThis parameter is visible only when the External mechanical torque parameter is checked. It is used to specify aseries of speed-power pairs for the tracking characteristic. The speed is in pu based on synchronous speed andthe power is in pu based on nominal generator power.

Display wind turbine power characteristicsIf this parameter is checked, the turbine power characteristics at zero degree of pitch angle are displayed for

different wind speeds. The tracking characteristic is also displayed on the same figure.

This parameter is not visible when the External mechanical torque parameter is checked.

Nominal wind turbine mechanical output powerThis parameter is not visible when the External mechanical torque parameter is checked.

The nominal turbine mechanical output power in watts.

Tracking characteristic speedsThis parameter is not visible when the External mechanical torque parameter is checked.

Specify the speeds of point A to point D of the tracking characteristic in pu of the synchronous speed. speed_Bmust be greater than speed_A and speed_D must be greater than speed_C.

Power at point CThis parameter is not visible when the External mechanical torque parameter is checked.

Specify the power of point C of the tracking characteristic in pu of the Nominal wind turbine mechanical outputpower.

Wind speed at point CThis parameter is not visible when the External mechanical torque parameter is checked.

Specify wind speed in m/s for point C. The power at point C is the maximum turbine output power for the specifiedwind speed.

Pitch angle controller gain [Kp]

This parameter is not visible when the External mechanical torque parameter is checked.Proportional gain Kp of the pitch controller. Specify Kp in degrees/(speed deviation pu). The speed deviation is thedifference between actual speed and speed of point D in pu of synchronous speed.

Maximum pitch angleThis parameter is not visible when the External mechanical torque parameter is checked.

The maximum pitch angle in degrees.

Maximum rate of change of pitch angleThis parameter is not visible when the External mechanical torque parameter is checked.

The maximum rate of change of the pitch angle in degrees/s.


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Control Parameters

Mode of operation

Specifies the WTDFIG mode of operation. Select eitherVoltage regulation orVar regulation.

External grid voltage referenceThis parameter is not visible when the Mode of operation parameter is set to Var regulation.

If this parameter is checked, a Simulink input named Vref appears on the block, allowing to control the referencevoltage from an external signal in pu. Otherwise a fixed reference voltage is used, as specified by the parameterbelow.

Reference grid voltage VrefThis parameter is not visible when the Mode of operation parameter is set to Var regulation or when theExternal grid voltage reference parameter is checked.

Reference voltage, in pu, used by the voltage regulator.


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External generated reactive power referenceThis parameter is not visible when the Mode of operation parameter is set to Voltage regulation.

If this parameter is checked, a Simulink input named Qref appears on the block, allowing to control the referencereactive power, at grid terminals, from an external signal in pu. Otherwise a fixed reference reactive power is used,as specified by the parameter below.

Generated reactive power QrefThis parameter is not visible when the Mode of operation parameter is set to Voltage regulation or whenthe External generated reactive power reference parameter is checked.

Reference generated reactive power at grid terminals, in pu, used by the var regulator.

External reactive current Iq_ref for grid-side converter referenceIf this parameter is checked, a Simulink input named Iq_ref appears on the block, allowing to control the grid-sideconverter reactive current from an external signal in pu. Specify a positive value for Iq_ref for generated reactivepower. Otherwise a fixed reactive current is used, as specified by the parameter below.

Grid-side converter generated reactive current reference (Iq_ref)This parameter is not visible when the External reactive current Iq_ref for grid-side converter referenceparameter is checked.

Reference grid-side converter reactive current, in pu, used by the current regulator. Specify a positive value ofIq_ref for generated reactive power.

Grid voltage regulator gains [Kp Ki]This parameter is not visible when the Mode of operation parameter is set to Var regulation.

Gains of the AC voltage regulator. Specify proportional gain Kp in (pu of I)/(pu of V), and integral gain Ki, in (pu ofI)/(pu of V)/s, where V is the AC voltage error and I is the output of the voltage regulator.

Droop XsThis parameter is not visible when the Mode of operation parameter is set to Var regulation.

Droop reactance, in pu/nominal power, defining the slope of the V-I characteristic.

Reactive power regulator gains [Kp Ki]This parameter is not visible when the Mode of operation parameter is set to Voltage regulation.

Gains of the var regulator. Specify proportional gain Kp in (pu of I)/(pu of Q), and integral gain Ki, in (pu of I)/(pu ofQ)/s, where Q is the reactive power error and I is the output of the var regulator.

Power regulator gains [Kp Ki]Gains of the power regulator. Specify proportional gain Kp in (pu of I)/(pu of P), and integral gain Ki, in (pu of I)/(puof P)/s, where P is the power error and I is the output of the power regulator.

DC bus voltage regulator gains [Kp Ki]Gains of the DC voltage regulator which controls the voltage across the DC bus capacitor. Specify proportionalgain Kp in (pu of I)/(Vdc), and integral gain Ki, in (pu of I)/(Vdc)/s, where Vdc is the DC voltage error and I is theoutput of the voltage regulator.

Grid-side converter current regulator gains [Kp Ki]Gains of the grid-side converter current regulator.

Specify proportional gain Kp in (pu of V)/(pu of I) and integral gain Ki, in (pu of V)/(pu of I)/s, where V is the outputVgc of the current regulator and I is the current error.

Rotor-side converter current regulator gains [Kp Ki]Gains of the rotor-side converter current regulator.

Specify proportional gain Kp in (pu of V)/(pu of I) and integral gain Ki, in (pu of V)/(pu of I)/s, where V is the outputVr of the current regulator and I is the current error.

Maximum rate of change of reference grid voltageThis parameter is not visible when the Mode of operation parameter is set to Var regulation.

Maximum rate of change of the reference voltage, in pu/s, when an external reference voltage is used.


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Maximum rate of change of reference reactive powerThis parameter is not visible when the Mode of operation parameter is set to Voltage regulation.

Maximum rate of change of the reference reactive power, in pu/s, when an external reference reactive power isused.

Maximum rate of change of reference powerMaximum rate of change of the reference power in pu/s.

Maximum rate of change of converters reference current

Maximum rate of change of the reference current in pu/s for both the rotor-side and the grid-side converters.

Inputs and Outputs

A B C

The three terminals of the WTDFIG.

Trip

Apply a simulink logical signal (0 or 1) to this input. When this input is high the WTDFIG is disconnected and itscontrol system is disabled. Use this input to implement a simplified version of the protection system.

Wind (m/s)

This input is not visible when the External mechanical torque parameter is checked.

Simulink input of the wind speed in m/s.

Tm

This input is visible only when the External mechanical torque parameter is checked.

Simulink input of the mechanical torque. Tm must be negative for power generation. Use this input when using anexternal turbine model.

Vref

This input is visible only when the Mode of operation parameter is set to Voltage regulation and theExternal grid voltage reference parameter is checked.

Simulink input of the external reference voltage signal.

Qref

This input is visible only when the Mode of operation parameter is set to Var regulation and the Externalgenerated reactive power reference parameter is checked.

Simulink input of the external reference generated reactive power signal at grid terminals.

Iq_ref

This input is visible only when the External reactive current Iq_ref for grid-side converter parameter is checked.

Simulink input of the external reference grid-side converter reactive current signal.

m

Simulink output vector containing 29 WTDFIG internal signals. These signals can be individually accessed byusing the Bus Selector block. They are, in order:


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Signal Signal Group Signal Names Definition

1-3 Iabc (cmplx)(pu)

Ia (pu)Ib (pu)Ic (pu)

Phasor currents Ia, Ib, Ic flowing into theWTDFIG terminals in pu based on thegenerator rating.

4-6 Vabc (cmplx)(pu)

Va (pu)Vb (pu)Vc (pu)

Phasor voltages (phase to ground) Va, Vb,Vc at the WTDFIG terminals in pu basedon the generator rating.

7-8 Vdq_stator (pu)

Vd_stator (pu)Vq_stator (pu)

Direct-axis and quadrature-axiscomponent of stator voltage in pu basedon the generator rating. Vd_stator andVq_stator are respectively the real andimaginary parts of the positive-sequencestator phasor voltage.

9-11 Iabc_stator (cmplx)(pu)

Ia_stator (pu)Ib_stator (pu)Ic_stator (pu)

Phasor currents Ia, Ib, Ic flowing into thestator in pu based on the generator rating.

12-13 Idq_stator (pu)

Id_stator (pu)Iq_stator (pu)

Direct-axis and quadrature-axiscomponent of stator current in pu basedon the generator rating. Id_stator andIq_stator are respectively the real andimaginary parts of the positive-sequencestator phasor current.

14-15 Vdq_rotor (pu)

Vd_rotor (pu)Vq_rotor (pu)

Direct-axis and quadrature-axiscomponent of rotor voltage in pu based onthe generator rating. Vd_rotor andVq_rotor are respectively the real andimaginary parts of the positive-sequence

rotor phasor voltage.

16-17 Idq_rotor (pu)

Id_rotor (pu)Iq_rotor (pu)

Direct-axis and quadrature-axiscomponent of currents flowing into therotor in pu based on the generator rating.Id_rotor and Iq_rotor are respectively thereal and imaginary parts of thepositive-sequence rotor phasor current.

18 wr (pu) Generator rotor speed (pu)

19 Tm (pu) Mechanical torque applied to the

generator (pu)

20 Te (pu) Electromagnetic torque in pu based on thegenerator rating.

21-22 Vdq_grid_conv(pu)

Vd_grid_conv (pu)Vq_grid_conv (pu)

Direct-axis and quadrature-axiscomponent of grid-side converter voltagein pu based on the generator rating.Vd_grid_conv and Vq_grid_conv arerespectively the real and imaginary partsof the grid-side converter phasor voltage.


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Signal Signal Group Signal Names Definition

23-25 Iabc_grid_conv(cmplx)(pu)

Ia_grid_conv (pu)Ib_grid_conv (pu)Ic_grid_conv (pu)

Phasor currents Ia, Ib, Ic flowing into thegrid-side converter in pu based on thegenerator rating.

26 P (pu) WTDFIG output power. A positive valueindicates power generation.

27 Q (pu) WTDFIG output reactive power. A positivevalue indicates reactive power generation.

28 Vdc (V) DC voltage (V).

29 Pitch_angle (deg) Blade pitch angle in degrees.

Example

See the power_wind_dfig demo which illustrates the steady-state and dynamic performance of the WTDFIG in a 9 MW

Wind Farm connected on a 25 kV, 60 Hz, system.

References

[1] R. Pena, J.C. Clare, G.M. Asher, "Doubly fed induction generator using back-to-back PWM converters and itsapplication to variable-speed wind-energy generation," IEE Proc.-Electr. Power Appl., Vol. 143, No. 3, May 1996

[2] Vladislav Akhmatov, "Variable-Speed Wind Turbines with Doubly-Fed Induction Generators, Part I: Modelling inDynamic Simulation Tools," Wind Engineering Volume 26, No. 2, 2002

[3] Nicholas W. Miller, Juan J. Sanchez-Gasca, William W. Price, Robert W. Delmerico, "DYNAMIC MODELING OF GE1.5 AND 3.6 MW WIND TURBINE-GENERATORS FOR STABILITY SIMULATIONS," GE Power Systems EnergyConsulting, IEEE WTG Modeling Panel, Session July 2003

See Also

Wind Turbine, Wind Turbine Induction Generator (Phasor Type)

Wind Turbine Wind Turbine Induction Generator (Phasor Type)

1984-2006 The MathWorks, Inc. Terms of UsePatentsTrademarksAcknowledgments

wind turbine doubly-fed induction generator (phasor type)

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