Wind Farm Simulation with Doubly-Fed Induction Generator (WFS-DFIG)

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					       2ND INTERNATIONAL CONFERENCE ON MODERN POWER SYSTEMS MPS 2008, 12-14 NOVEMBER 2008, CLUJ-NAPOCA, ROMANIA




   Wind Farm Simulation with Doubly-Fed Induction
              Generator (WFS-DFIG)
                                                          Silviu Darie#1
                             #
                                 EDSA Micro Corporation, USA, Technical University of Cluj Napoca
                                  16870 West Bernardo Dr., Suite 330, San Diego, CA 92127, USA
                                       15 C. Daicoviciu Str., 400020 Cluj Napoca, Romania
                                                       !
                                                        sdarie@edsa.com

   Abstract—EDSA Micro Corporation - USA has incorporated               • pitch angle for controlling frequency and power;
in the Paladin DesignBase programs state of the art models for          • reactive power for controlling the WTG bus voltage;
Doubly Fed Induction Generator in both DesignBase’s Advanced
                                                                        • protection for over/under voltage and frequency trip.
Power Flow and Transient Stability Simulation programs. The
Doubly-Fed Induction Generator (DFIG) dynamic model is                Figure 1 represents the DFIG Model – Controls Block
comprehensive with highly flexible control system. All individual   Diagrams developed and implemented in DesignBase
wind turbine generators (WTGs) in a large wind farm are             environment, [1].
represented by an equivalent single WTG machine, which
behaves in an integrated system for realistic approximation.               III. MECHANICAL WIND POWER CHARACTERISTICS
DFIG injects active and reactive power to the bulk power               The mathematical equations of both the wind turbine
network system. The electromagnetic power output                    generators (WTGs) and DFIG are presented in details in [1].
instantaneously follows the power output order from the control
                                                                       Fixed-speed wind generation systems (systems without
systems. The paper describes the implementation of the DFIG
model in the Paladin DesignBase Transient Stability Program.
                                                                    active speed control) typically utilize an induction generator
                                                                    directly connected to the network. Variable-speed systems
                       I. INTRODUCTION                              make use of either induction generators or synchronous
   With the increased use of wind power, particularly in wind-      generators. Both of these systems require a power electronic
farms, the voltage and frequency behaviors of the power             converter to obtain torque and speed control. In this case,
system networks are likely to be affected significantly.            induction generators with a wound rotor are mainly used. The
Doubly-Fed Induction Generators (DFIG) are mainly used for          use of a wound rotor allows a power electronic converter to be
wind energy conversion in MW power plants. DFIG has a               connected to the rotor circuit via slip rings. The potential of
rotor inverter and a front-end converter while the stator is        using variable rotor speed for adjusting aerodynamic power is
linked directly to the power system.                                important, however, this is not the main reason for using
   EDSA Micro Corporation - USA has incorporated state of           turbine speed control. Instead, it is the fact that the variable
the art models for Doubly Fed Induction Generator in both           speed operation gives the potential to reduce mechanical
DesignBase ‘s Advanced Power Flow and Transient Stability           stresses on drive-train components by means of shaft torque
simulation programs. The DFIG dynamic model is                      control. Incoming power variations are absorbed by changes
comprehensive with highly flexible control system.                  in the rotor speed and the shaft torque is smoother, which also
   All individual wind turbine generators (WTGs) in a large         gives smoother electric output power.
wind farm are represented by an equivalent single WTG                  The mechanical power output of a wind turbine depends on
machine, which behaves in an integrated system for realistic        the wind speed and the pitch angle. As the wind speed varies
approximation. The model neglects the dynamics of stator and        stochastically, the pitch angle is the only means by which the
rotor windings fluxes. Therefore, the DFIG model behaves in         power output of a wind unit can be controlled continuously.
an algebraic, controlled source by the electrical and               The control approach is fairly straightforward. The pitch angle
mechanical control systems of DFIGs. DFIG injects active            is normally adjusted for maximum output except under
and reactive power to the bulk power network system. The            conditions of wind over-speed during which the output power
electromagnetic power output instantaneously follows the            is limited to the rated value by the pitch angle control. The
power output order from the control systems.                        pitch angle in a stall-controlled turbine is fixed. The rotor is
                                                                    designed in such a way that it stalls at wind over-speed
                     II. MODEL OVERVIEW                             thereby protecting the turbine from mechanical damage.
                                                                       The rotor speed in the variable-speed area is controlled in
   The wind turbine DFIG is especially emphasized in its fast
                                                                    order to keep the optimal tip speed ratio, i.e., Cp is kept at
and dominant control function, and the rotor aerodynamic
                                                                    maximum as long as the power or rotor speed is below its
mechanical power characteristics associated with the wind and
                                                                    rated values. As mentioned before, the pitch angle is at higher
the turbine rotor’s mechanical movement.
                                                                    wind speeds controlled in order to limit the input power to the
However the DFIG model involves:
                                                                    wind turbine, when the turbine has reached the rated power.
     • rotor rotation (swing) equation;
                                                                    The calculation of Cp requires knowledge of aerodynamics on



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       2ND INTERNATIONAL CONFERENCE ON MODERN POWER SYSTEMS MPS 2008, 12-14 NOVEMBER 2008, CLUJ-NAPOCA, ROMANIA




blade element theory and it is quite complicated. Therefore,                                                power requirements, i.e. maintain constant power factor at the
numerical approximations have been adopted. A general                                                       generator output at any load. They can also be used for
mathematical formula of Cp has accordingly been developed                                                   voltage level control in the power system like a static voltage
[6] and has been implemented in [1].                                                                        regulator of synchronous generators. It is worth mentioning
   When the rotor speed is within the normal operation range                                                that with appropriate control of reactive power, the voltage
(generally 0.75-1.2 p.u.), it follows an optimal power output                                               variations (active power generation changes due to wind speed
scheme, which allows the rotor speed follow the wind                                                        variations) can be counteracted hence reducing flicker in the
fluctuation in order to obtain the maximum efficiency from                                                  power system. The reactive power control implemented in the
the DFIG. When the rotor speed reaches the rated value ωopt,                                                DesignBase’s model for DFIG is shown in Figure 3.
it tries to keep the speed until the output of corresponding
active power reaches its maximum power limit Pmax. When
the rotor speed is above its maximum value ωmax or below its
minimum value ωmin, it will send a trip signal to trip out the
DFIG from the network. Once the trip flag is set, it will not be
reset, even if the frequency recovers.

         IV. ACTIVE POWER AND CURRENT CONTROL                                                                                                                        Fig. 3
   Figure 2 represents the Active Power and Current Control
block diagram developed by EDSA Micro Corporation, [1].                                                                       VI. PITCH ANGLE CONTROL
                                                                                                               The blade pitch angle regulates the active power delivered
                                                                                                            by the turbine. As the pitch angle increases smaller power
                                                                                                            coefficient Cp is obtained. This system is used to limit the
                                                                                                            power delivered to the grid under high wind speed conditions.
                                                                                                            The output, also bounded, is the reference pitch angle. Under
                                                                                                            strong wind, the optimal speed could exceed the upper
                                                                                                            mechanical speed limit and therefore it should be limited, for
                                                                                                            instance to 110 % of the synchronous value. Rotor current
                                       Fig. 2                                                               control is tuned to obtain the fastest possible response with a
                                                                                                            reasonable damping. When the generator reaches rated power,
                V. REACTIVE POWER CONTROL                                                                   the pitch control starts acting, limiting the power produced by
    The reactive power control of a wind turbine is determined                                              the wind.
by the generator in use. The simplest system with an induction
generator directly connected to the grid utilizes a capacitor                                                             VII.    MODEL IMPLEMENTATION
bank typically designed to compensate for generator no-load                                                    The WFG model generated has been successfully tested on
reactive power consumption. The additional reactive power                                                   several existing find farms networks. Figure 5 represents a
consumed when the generator is loaded must be taken from                                                    generic testing system. Figures 6 to 10 represent the dynamic
the connected the power system. Systems with power                                                          simulation results.
electronic converters offer much more powerful reactive
power control. Provided that the rating of the converter allows
it, they can fully compensate the generator for its reactive
                                                                                                                                    Mechanical
                                                         Rotor speed                                                               Power Output
                                                                                                              Wind                                                WT DFIG Rotor
                                                                                                            Mechanical                            ∑              Movement Equation
                                                                            Wind Velocity                                                 +
                                                                                                             Power
                                                                                                                                                   −
                                                                                                                                                                                   Rotor Speed
                                                   Maximum Power
                                                                              Pitch Control       Pitch Angle
                                                                                                                                              Active Power
                                                                                                                                                 Output

                  Active Power                                                                                        Frequency Trip
                     Output                                                                                                               Frequency Trip
                                       Desired Speed
                                      Control Scheme         Rotor Speed Reference
                                                                                                                                              Signal
                                                                                              Frequency/
                                                                                             Active Power                                                             Active Power Output
                                                                                               Control                                                                   to the Network
                                                              Rotor Speed
                                                                                                                 Ordered Active Power     WT DFIG/Network
                           Specified Collector Bus Voltage                                                                                   Interface
                                                                                                                Ordered Reactive Power                                Reactive Power Output
                                 Collector Bus Voltage                                                                                                 Trip Signal       to the Network
                                                                        Reactive Power
                                                                       and Power Factor                Voltage Protective
                                 Desired Power Factor
                                                                                                            Control                             Voltage Trip
                                                                            Control                                                                Signal
                             Modulated Reactive Power           Generator Terminal Voltage

                                                                                Electronic Converter Angle Control



                                                                        Fig. 1 DFIG Model Controls Block Diagrams




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2ND INTERNATIONAL CONFERENCE ON MODERN POWER SYSTEMS MPS 2008, 12-14 NOVEMBER 2008, CLUJ-NAPOCA, ROMANIA




                               Active Power
                                                              Maximum power
                                                              control tracking

                                 Pmax

                                                 Optimal power
                                                 control tracking
                                                                                        Constant speed control
                                                                                              tracking




                                                                                                   Rotor speed (p.u.)
                                          ωmin                          1.0       ωref ωmax

                                 DFIG Power Absorption                           DFIG Power Generation


                                              Fig. 4. DFIG Optimum Reference Speed Tracking




            Fig. 5. System Study Layout                                               Fig. 6. G1, G2 Rotor Angle and Electrical Power




 Fig. 7. G1, G2 Excitation Voltage, in p.u.                                                       Fig. 8. WFG Wind Speed, in m/sec




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       2ND INTERNATIONAL CONFERENCE ON MODERN POWER SYSTEMS MPS 2008, 12-14 NOVEMBER 2008, CLUJ-NAPOCA, ROMANIA




        Fig. 9. WFG Terminal Voltage, in p.u.                                     Fig. 10. WFG Reactive Power, in MVAR


                      VIII.   CONCLUSION                                              ACKNOWLEDGMENT
   With the increased use of wind power, particularly in wind-     The paper represents the results of the EDSA Micro
farms, the voltage and frequency behaviors of the power          Corporation team work and the model is implemented in the
system networks are likely to be affected significantly.         Paladin DesignBase library. The in field tests and the results
Doubly-Fed Induction Generators (DFIG) are mainly used for       demonstrated the validity of the model developed by the team.
wind energy conversion in MW power plants. DFIG has a
rotor inverter and a front-end converter while the stator is                                 REFERENCES
linked directly to the power system.                               [1] *** Paladin DesignBase. EDSA Micro Corporation, USA, 2008
   EDSA Micro Corporation - USA has incorporated state of              (www.edsa.com ).
the art models for Doubly Fed Induction Generator in both          [2] N.W. Miller, J.J. Sanchez-Gasca et al, “Dynamic Modeling of GE 1.5
                                                                       and 3.6 MW Wind Turbine-Generators for Stability Simulations,” 0-
EDSA’s advanced power flow and transient stability                     7803-7990-X/03, 2003 IEEE.
simulation programs. The DFIG dynamic model is                     [3] CIGRE Technical Brochure, Modeling New Forms of Generation and
comprehensive with highly flexible control system.                     Storage, Task Force 38.01.10, April 2001.
   All individual wind turbine generators (WTGs) in a large        [4] C.S. Demoulias and P. Dokopoulos, “Electrical Transients of Wind
                                                                       turbines in a Small Power Grid,” 0885-8969/96, 1996 IEEE.
wind farm are represented by an equivalent single WTG              [5] A. Miller, E. Muljadi et al, “A Variable Speed Wind Turbine Power
machine, which behaves in an integrated system for realistic           Control,” 0885-8969/97, 1996 IEEE.
approximation. The model neglects the dynamics of stator and       [6] J.G. Slootweg and W.L. Kling, “Modeling of Large Wind Farms in
rotor windings fluxes. Therefore, the DFIG model behaves in            Power System Simulations,” 0-7803-7519-X/02/, 2002 IEEE.
an algebraic, controlled source by the electrical and              [7] J.G. Slootweg, H. Polinder et al, “Dynamic Modeling of a Wind
mechanical control systems of DFIGs. DFIG injects active               Turbine with Doubly Fed Induction Generator,” 0-7803-7071-7/01/,
and reactive power to the bulk power network system. The               2001 IEEE.
electromagnetic power output instantaneously follows the           [8] R. Datta and V.T. Ranganathan, “Variable Speed Wind Power
                                                                       Generation Using Doubly Fed Wound Induction Machine—A
power output order from the control systems.                           Comparison with Alternative Schemes,” 0885-8969/02, 2002 IEEE.




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