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PSCAD modeling of doubly fed induction generators for wind turbines

application

Conference Paper · January 2014

DOI: 10.1049/cp.2014.0088

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 PSCAD Modeling of Doubly Fed Induction Generators for
Wind Turbines Application
Mostafa Kheshti*, Xiaoning Kang†, Zaibin Jiao†
*School of Electrical Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi, China

mostafa_kheshti@yahoo.com, kangxn@mail.xjtu.edu.cn, jiaozaibin@mail.xjtu.edu.cn

Keywords: DFIG, vector control method, wind turbine, direct-driven synchronous generator, apart from the cost
PSCAD software. saving of using a smaller converter.

Abstract To control the DFIG, the rotor current is controlled by

a power converter. One common way of controlling the
Dunhuang wind farm in Gansu province, China with rotor current is through the vector control. Several vector
237 GW wind energy potential has a great significance control schemes for the DFIG have been presented.
for northwest grid in China. In this paper, in line with Controlling the rotor current with stator-flux orientation is
Hexi 750kV Power Transmission Line Protections project, a common method [9], or by means of air-gap-flux
modeling of different wind turbines are studied. In this orientation. If the resistance of the stator can be assumed
research, modeling and analysis of doubly fed induction small, stator-flux orientation gives in principle orientation
generator for wind turbine applications is investigated. also with the stator voltage (grid-flux orientation). Wang
Based on the vector control method, the simulation is et al. [10] stated that the flux is influenced both by load
executed in PSCAD/EMTDC software and the results changes and stator power supply variations. For a
show the performance of the system. The data are used disturbance, the flux response is a damped oscillation. In
based on the real information from the wind farm site. previous research we proposed a new method of
Also, the smart grid roadmap is considered for this wind controlling the wind farm based on a fuzzy control system
farm to adjust itself with new situations hence smartening [11].
the grid. Furthermore, fault analysis and stability of the
system can be feasible. Here in this paper, the study on Key Technologies of
Hexi 750kV Power Transmission lines connected to the
1 Introduction Dunhuang Wind Farm in Gansu Province, China has been
executed. DFIG power conversion system as a vital part
Wind turbines can either operate at fixed velocity or of the wind farm is modeled in PSCAD software and
variable speed. In a fixed speed wind turbine, the vector controlled strategy for enhancing the power quality
generator is directly connected to the electrical power of the grid while injecting the required active power of
grid. In a variable speed wind turbine, the generator is the system is applied. The simulation results illustrate
controlled by power electronic equipments [1]. Some effectiveness of performance of the system. Also, reactive
advantages of variable speed wind turbine such as power control and dynamic power factor correction are
Possibilities to decrease stresses of the mechanical the other two important characteristics of the system.
structure, acoustic noise reduction and the possibility to
control active and reactive power make this type popular 2 System Description
[2]. Recently, the manufactures are developing new larger
wind turbines and they are all based on variable-speed Dunhuang-Hexi 750 kV Transmission line eases the
operation with pitch control using a direct driven integration and delivery of large scale wind power and
synchronous generator which are without gearbox or a interconnects the northwest grid to Xinjiang grid which is
doubly-fed induction generator (DFIG). Fixed-speed the largest province in China [4,5].
induction generators with stall control are considered as
unfeasible for these large wind turbines [3]. Nowadays, The wind farm includes asynchronous turbine with unit
doubly-fed induction generators are commonly used by rated capacity of 1.5 MW, the total 33 units, therefore
the wind turbine industry for larger wind turbines [4,5]. with total capacity of 49.5 MW. The rated voltage for
The significant advantage of doubly fed induction wound rotor induction generator is adjusted 690V, rated
generator, which made it desirable for manufactures and capacity will be 1.5 MVA, stator resistance sets 0.007 pu
users, is that the power electronics only has to handle and rotor resistance can be 0.007 pu, excitation
around 20-30 percent of the fraction of the total power impedance is tuned 2.5 per-unit, stator leakage resistance
[6,7,8]. equals 0.065 pu and rotor leakage resistance is 0.11 pu.
For wind turbine, its radius is 38.68 m, the air density ρ is
It means that the losses in the power electronic 1.225 g/m3, optimal velocity for doubly-fed machine is
equipment can be reduced compared to power electronic ɘ =1.167 and ୮ =0.38 . For simplicity, to avoid
equipments that have to deal with the total power as for a
 influencing the simulation analysis, the wind speed is ௗ௜೜ೝ
‫ݒ‬௤௥ ൌ ܴ௥ ݅௤௥ ൅ ߪ‫ܮ‬௥ ൅ ߱௦௟௜௣ ሺ‫ܮ‬௠ ݅௠௦ ൅ ߪ‫ܮ‬௥ ݅ௗ௥ ሻ (6)
considered ୵ ൌ11m/s and pitch angle is β=Ͳι . ௗ௧

௉
Figure 1 displays the schematic diagram of DFIG for ܶ௘ ൌ െ͵ Ǥ ‫ܮ‬௠ Ǥ ݅௠௦ Ǥ ݅௤௥ (7)
ଶ
wind turbine in PSCAD software.
௅మ೚
‫ܮ‬௠ ൌ (8)
௅ೞ

௅మ೚
σ=ͳ െ (9)
௅ೝǤ௅ೞ

ߣఈ௦ ൌ ‫׬‬ሺ‫ݒ‬ఈ௦ െ ܴ௦ Ǥ ݅ఈ௦ ሻ݀‫ݐ‬ (10)

ߣఉ௦ ൌ ‫׬‬൫‫ݒ‬ఉ௦ െ ܴ௦ Ǥ ݅ఉ௦ ൯݀‫ݐ‬ (11)

Equations (1-9) indicate the relationship between the

machine torque and the d-q axis voltages, currents and
Figure 1: PSCAD schematic diagram of doubly fed induction fluxes per phase values. With the help of equations (10)
generator for wind turbine application and (11), the stator flux angle is calculated.
ఒഁೞ
Although the operation of DFIG wind turbines is ߠ௦ ൌ –ƒିଵ (12)
ఒഀೞ
tolerably under grid balanced conditions, its performance
is not magnificent when the voltage at the point of The stator voltage after subtracting the resistive drop of
common coupling is influenced by voltage sags or the rotor (‫ݎ‬௔ Ǥ ݅௔ ) is derivative of the stator flux linkage per
network unbalances [6]. Under such situations, the phase.
electromagnetic transient of DFIG leads to high over
currents in the converter, which may cause its ௗఒೌ
‫ݒ‬௔ െ ‫ݎ‬௔ Ǥ ݅௔ ൌ (13)
disconnection to avoid damaging in semiconductors. ௗ௧

where, these equations are modeled in PSCAD software

3 Control strategies and diagrams
to determine the location of rotating flux vector ߶௦ which
The three phase rotor currents of the machine can be are illustrated in Figure 2.
settled into the direct and quadrature components id and
iq . The component id makes a flux vector in the air gap
which is aligned with the rotating flux vector linking the
stator and iq creates a flux vector at right angles of the
vector id. Vector cross product of the components id and iq
makes the torque in the machine. The reactive power
entering the machine is adjusted by id and machine torque
and power is influenced only by component iq. Hence, if Figure 2: a)
id and iq can be controlled precisely, then active and
reactive powers of the SSC are suitably controlled.

With the aid of a synchronous rotating d-q axis frame

which the axis d directed along the stator flux converter,
the induction machine is controlled. Therefore, a
decoupled control between the rotor excitation current
and electrical torque is obtained. The RSC tunes the
actuation, and the control needs the measurement of the
stator and the rotor currents, stator voltage and position of Figure 2: b)
the rotor. The stator magnetizing current can be fixed,
because the stator is linked and connected to the grid and
the effect of the stator resistance is small. Therefore we
have:

ߣ௦ ൌ ߣௗ௦ ൌ ‫ܮ‬௢ Ǥ ݅௠௦ ൌ ‫ܮ‬௦ Ǥ ݅ௗ௦ ൅ ‫ܮ‬௢ Ǥ ݅ௗ௥ (1)

Figure 2: c)
௅మ೚
ߣௗ௥ ൌ Ǥ ݅௠௦ ൅ ߪǤ ‫ܮ‬௥ Ǥ ݅ௗ௥ (2) Figure 2: Schematic control diagram to determine the location
௅ೞ

௅೚ of rotating flux vector.

݅௤௦ ൌ െ Ǥ ݅௤௥ (3)
௅ೞ
Figure 2-a, provides the present location of rotating
stator flux. Figure 2-b, shows integration of rotor speed,
߱௦௟௜௣ ൌ ߱௘ െ ߱௥ (4)
to achieve rotor position. Determining the relative
ௗ௜೏ೝ difference between stator flux and rotor position to
‫ݒ‬ௗ௥ ൌ ܴ௥ ݅ௗ௥ ൅ ߪ‫ܮ‬௥ െ ߱௦௟௜௣ ߪ‫ܮ‬௥ ݅௤௥ (5) resolve the rotor currents is depicted in Figure 2-c. And
ௗ௧
 Rotor side excitation vector control strategy is shown in
Figure 3.

Figure 5: PSCAD Structure of the PWM controller.

Figure 3: Rotor side excitation vector control method of
doubly-fed induction generator

For achieving stability of DFIG under fixed wind speed, 5 Simulation Results
whether it is with the goal to adjust active power, or with
the rotor rotating velocity and electromagnetic torque as The performance of the simulated wind turbine is
the goal, they are equal and the active power, the investigated in this part. Based on the information from
electromagnetic torque and rotor speed are comparable. the wind farm site the modeling and analysis has been
But for rotor speed as the aim of getting better dynamic studied. Figure 6 shows the generator characteristics in
performance and fast dynamic response, it can have a grid side. Referring to the figure,ܸ௚ , ݅௚ , ܲ௚ ,ܳ௚ , represent
better trend for the maximum wind energy. However its voltage, current, active power and reactive power of the
output may change dramatically and it has a high impact generator respectively, ߱௣௨ depicts the rotor speed in per-
on power grid that is not favorable for stability, however unit and ܷௗ௖ marks the dc link voltage. Active and
with the goal to adjust the active power and reactive powers of the rotor are illustrated in Figure 7.
electromagnetic torque, the dynamic response is slow, but The mechanical torque ܶ௠ is shown in Figure 8.
its output power changes slowly and has small effect on Parameters of the stator are displayed in Figures 9 and 10.
power network. In Figure 11, phase angle of the stator is shown. Figure 12
indicates the current control of the rotor which ݅௥ௗ , ݅௥௤ ,
4 Grid side control strategy ݅௥ௗ̴௥௘௙ , ݅௥௤̴௥௘௙ , ‫ݒ‬ௗି௥௘௙ , and ‫ݒ‬௤̴௥௘௙ are presented in this
figure. Figure 13 indicates the three phase rotor currents.
DC bus voltage and d-q axis current have relationship.
It is clear that ݅ௗ௖ includesactive current ‹ୢ and reactive
current ݅௤ , while considering the steady state voltage
fluctuations resulted by reactive power current in one
cycle is zero, i.e. considering only the voltage fluctuation
resulted by the active power, therefore:

‫ݑ݌ܥ‬ௗ௖ ൌ ݅ௗ െ ݅௅ (14)

This equation indicates the relationship between the

active current‹ୢ and the DC bus voltage—ୢୡ , control of the
active current adjusts the dc bus voltage, tuning the dc
bus voltage controls the active power output. This
strategy is displayed in Figure 4.

Figure 6: Grid side characteristics of the generator

Figure 4: Diagram of the GSC vector control

Figure 5 shows a PSCAD modeled of sinusoidal PWM

controller, which each of the phase voltage is compared
with a high frequency triangle waveform to provide the
firing pulse patterns.
Figure 7: Active and reactive powers of the rotor in DFIG

Figure 12: Current control of the rotor

Figure 8: Mechanical torque in wind turbine

Figure 13: Three phase rotor currents

6 Conclusion
As wind energy technologies are developing, various
projects are taking into consideration in China. Here in
this paper modeling and analysis of the doubly fed
Figure 9: Stator voltage and current induction generator has been investigated as a part of
Study on Key Technologies of Hexi 750kV Power
Transmission Line Protections project in China. The
analysis in this paper is used in real wind farm in Gansu
province, China to ease the issues and provide the system
to make it smarter and more reliable. The modeling
executed in PSCAD software and the simulation results
show the performance of the system. Investigation on the
smart grid and renewable energy integration will be
considered as the next steps of the research.

Figure 10: Active and reactive power of the stator Acknowledgement

This work is supported by National Natural Science
Foundation of China through grants No. 51037005,
51177127 and National Basic Research Program of China
( 973 Program ) (2012CB215105).

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