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OVERVIEW LVRT CAPABILITY OF DFIG TECHNIQUES

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									INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING
 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME
                           & TECHNOLOGY (IJEET)

ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)                                                     IJEET
Volume 4, Issue 3, May - June (2013), pp. 75-81
© IAEME: www.iaeme.com/ijeet.asp
Journal Impact Factor (2013): 5.5028 (Calculated by GISI)                ©IAEME
www.jifactor.com




           OVERVIEW LVRT CAPABILITY OF DFIG TECHNIQUES

                           Mustafa Jawad Kadhim, Prof.D.S.Chavan
                 Bharati Vidyapeeth Deemed University. College of Engineering
                               Pune-Satara Road, Pune-411043



  ABSTRACT

          Low Voltage Ride Through is an important feature for wind turbine systems to meet
  the conditions and requirements of the grid code. In case of wind turbine technologies using
  doubly fed induction generators the reaction to grid voltage disturbances is very sensitive
  which considers as a major drawbacks of using the Doubly Fed Induction Generators (DFIG).
  Protectiontechniques which include hardware or software must be implemented to protect the
  converter from tripping and provide uninterruptible operation to the DFIG during severe grid
  voltage faults. Methods for Ride Through operation of such system are presented.

  Keywords: doubly fed induction generator (DFIG); low voltage ride through (LVRT); grid
  defaults.

  I-INTRODUCTION

          Wind energy generation has been noted as the most rapidly growing renewable energy
  technology. The increasing penetration level of wind energy can have a significant impact on
  the grid, especially under abnormal grid voltage conditions. Thus, the power grid connection
  codes in most countriesrequire that wind turbines (WTs) should participate in grid voltage
  support in steady state and remain connected to the grid to maintain the reliability during and
  after a short-term fault [1].The latter requirement means that WTs have low voltage ride
  through (LVRT) capability and supply reactive currents to the grid as stated in the grid codes.
  Among the wind turbine concepts, the doubly fed induction generator (DFIG) is a popular
  wind turbine system due to its high energy efficiency, reduced mechanical stress on the wind
  turbine, separately controllable active and reactive power, and relatively low power rating of
  the connected converter [2], but due to the direct connection of the stator to the grid, the


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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME

DFIG suffers from a greatvulnerability to grid faults [3] and requires additional protection
    he
forthe rotor side power electronic converter.
        The structure is as follows. In Section II, the crowbar protection technique
isdescribed. FACTs devices are described in section III. Stator current feedback technique is
                     conclusion
investigated in IV. Aconclusion closes the investigation of the effectiveness of the proposed
techniques.


II-CROWBAR




                           Fig 1: DFIG using crowbar protection

        A protection system called active crowbar is one of the methods that used to enhance
              eration                    which
the DFIG operation during the LVRT whichdisconnects the rotor side converter in order to
                                       sq
protect it turningthe generator into a squirrel cage induction machine [4-5]. Thecrowbar may
comprise of a set of thyristors that will short-circuit the rotor windings when triggered and
thereby limit the rotor voltage and provide an additional path for the rotorcurrent. Different
values of the crowbar resistors result in adifferent behavior. Using this technology, the DFIG
can stayconnected to the grid and resume operation as soon as possible. But the main
disadvantage for this system is when the RSC disabled the machine draws a highshort circuit
                                               described
current when the crowbar is activated, as describedin [6], resulting in a large amount of
reactive power drawn fromthe power network, which is not acceptable when
consideringactual grid code requirements. Thus, other protection methodshave to be
                      through
investigated to ride-through grid faults safely andfulfill the grid codes.

III-FLEXIBLE AC TRANSMISSION SYSTEMS (FACTS)

         lexible
        Flexible AC Transmission Systems, called FACTS, got in the recent years a well-  well
known term for higher controllability in power systems by means of power electronic
                          devices
devices. Several FACTS-devices have been introduced for various applications worldwide. A
      er
number of new types of devices are in the stage of being introduced in practice.In most of the
applications the controllability is used to avoid cost intensive or landscape requiring
                                                                        substations
extensions of power systems, for instance like upgrades or additions of substations and power
                devices
lines. FACTS-devices provide a better adaptation to varying operational conditions and
improve the usage of existing installations. There are three configurations of the FACTS
                                                               s               7].
devices, shunt devices, series devices and combined shunt and series devices [7]




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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME

A-STATCOM
                        FACTS device
        The most used FACTS-device is the SVC or the version with Voltage Source
Converter called STATCOM. These shunt devices are operating as reactive power
compensators. The advantage of a STATCOM is that the reactive power provision is
independent from the actual voltage on the connection point.
The STATCOM configuration consists of a VSC, a dc energy storage device; a coupling
transformer connected in shunt with the ac system, and associated control circuits. Fig. 2
                                 D
shows the basic configuration of D-STATCOM.




                   Fig 2: STATCOM structure and voltage / current characteristic
TheVoltage Source Converter (VSC) converts the dc voltage across the storage deviceinto a
             phase                                          phase
set of three-phase ac output voltages. These voltages arein phase and coupled with the ac
system through the reactance of the coupling transformer. Suitable adjustment of the
phaseand magnitude of the STATCOM output voltages allowseffective control of active and
                                                            sy
reactive power exchangesbetween the STATCOM and the ac system[8].

B-Dynamic voltage restorer (DVR)
   Dynamic
        The DVR is a powerful controller that is commonly used for voltage sags mitigation
                                                                        D STATCOM,
at the point of connection. The DVR employs the same blocks as the D-STATCOM, but in
                                                                                       Fi
this application, the coupling transformer is connected in series with the ac system. Fig. 3
                                                                three phase
shows the basic configuration of DVR. The VSC generates a three-phase ac output voltage
which is controllable in phase and magnitude. These voltages are injected into the ac
                                                                         voltage
distribution system in order to maintain the load voltage at the desired voltage reference. If
the DVR device is used to compensatethe faulty grid voltage, any protection method in the
DFIGsystem can be left out. The advantages of such an external protectiondevice are thus the
reduced complexity in the DFIG system. Thedisadvantages are the cost and complexity of the
DVR [9].




                             Fig3: Basic configuration of DVR
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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME

C-Unified Power Flow controller (UPFC)
                             ller
        The UPFC is a combination of a static compensator and static series compensation. It
acts as a shunt compensating and a phase shifting device simultaneously.




                          Fig 4: Principle configuration of an UPFC

        The UPFC consists of a shunt and a series transformer, which are connected via two
                                             DC                        circuit
voltage source converters with a common DC-capacitor. The DC-circuit allows the active
                     ween
power exchange between shunt and series transformer to control the phase shift of the series
                                      4,
voltage. This setup, as shown in Fig 4, provides the full controllability for voltage and power
flow. The series converter needs to be protected with a Thyristor bridge. Due to the high
efforts for the Voltage Source Converters and the protection, an UPFC is getting quite
expensive, which limits the practical applications where the voltage and power flow control is
required simultaneously[10].

D-Magnetic Energy Recovery Switch (MERS)
                                  witch
       One of the series FACTS controllers. It consists of four power electronic switches and
                                                    single phase
a capacitor in aconfiguration identical to the single-phase full bridge converter. Its
arrangementin an electric circuit, however, is different, with only two of the
                                      connected
converter’sterminals utilized and connected in series. It has the characteristic of a
variablecapacitor and is related to FACTS controllers with series capacitors such as theGCSC
                 The
and the TCSC.The investigation of MERS indicated that it is able to increase the
                                  w                                     reestablishedpre-fault
LVRTcapabilityof wind farms with DFIG. This device successfully reestablishedpre
conditions in the whole system and the process of achievingthis was almost identical.
                                 50 Hz
However, found that a small 50-Hz distortion inthe generator’s torque was caused by the
                                   effectof
MERS. These are most likely the effectof harmonic distortion created by the device. It injects
           order
some fifth-order harmonicsinto the system, and it was found that these coincide with the
resonancefrequency of the simulation model. How the application of a different
                              tion
MERScapacitor or the operation in continuous mode would affect this is uncertain [11].




                             Fig 5: Typical MERS configuration

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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME

IV-STATOR CURRENT FEEDBACK TECHNIQUE

        The proposed technique aims to reduce the rotorcurrents by changing the RSC control
instead of installing additional hardware protection like a crowbar in the wind turbine system.
The solutionhas been presented in [12]. When a fault affects the generator the measured and
transformed statorcurrents are fed back as reference for the rotor current controller (stator
currents in stator flux orientation).The objective is to reduce stator current oscillations and
thus reduce the rotor currents aswell.If the DFIG system equations are combined as in [13], a
Laplace transformation is performed and somesimplifications are assumed, the following
equation for the stator currents can be obtained:


                                   1           ωs            L
                             isd =                      vsq − h ird .........()
                                                                            1
                                   Ls s2 + 2 Rs s +ω2      Ls
                                             L      s
                                                 s

                                                R
                                            s+ s
                                   1              Ls         L
                             isq =                      vsq − h irq.........( )
                                                                            2
                                   Ls s2 + 2 Rs s +ω2      Ls
                                             L      s
                                             s


                                                                             +             +
If the stator currents are fed back as rotor current reference values, i.e. ird = isd and irq = isq the
following equation for the stator currents can be obtained and the stator currents are reduced.

                                      1             ωs
                             isd =                                      .(
                                                             vsq..........3)
                                   Ls + Lh s2 + 2 Rs s +ω2
                                                  L      s
                                                  s
                                                    R
                                                 s+ s
                                      1                Ls
                             isq =                                      ..(
                                                             vsq.......... 4)
                                   Ls + Lh s2 + 2 Rs s +ω2
                                                  L      s
                                                  s



V-CONCLUSION

        Low Voltage Ride Through is an important feature for wind turbine systems to fulfill
grid code requirements. In case of wind turbine technologies using doubly fed induction
generators the reaction to grid voltage disturbances is sensitive. Hardware or software
protection must be implemented to protect the converter from tripping during severe grid
voltage faults. In this paper the proposed techniques have been investigated to show their
effectiveness to enhance the Doubly Fed Induction Generator (DFIG) capability of Low
Voltage Ride through (LVRT).


                                                    79
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME

REFERENCES
[1]Tsili, M.; Papathanassiou, S. A review of grid code technical requirements for wind farms.
IET Renew. Power Gen. 2009, 3, 308–332.
[2] Thomas, A. Wind Power in Power Systems; Wiley: New York, NY, USA, 2005.
[3]Mohseni, M.; Islam, S.; Masoum, M. Impacts of symmetrical and asymmetrical voltage
sags on DFIG-based wind turbines considering phase angle jump, voltage recovery, and sag
parameters. IEEE Trans. Power Electron. 2011, 26, 1587–1598.
[4] L. Shi, N. Chen and Q. Lu, "Dynamic Characteristic Analysis of Doublyfed Induction
Generator Low Voltage Ride-through", ScienceDirect, Energy Procedia 16 (2012) 1526 –
1534, doi:10.1016/j.egypro.2012.01.239.
[5] Christian Wessels, Fabian Gebhardt and Friedrich W. Fuchs, "Dynamic Voltage Restorer
to allow LVRT for a DFIG Wind Turbine", IEEE International Symposium on Industrial
Electronics (ISIE), 2010, doi: 10.1109/ISIE.2010.5637336.
[6] J. Morren and S. de Haan, “Short-circuit current of wind turbines with doubly fed
induction generator,” IEEE Trans. Energy Convers., vol. 22, no. 1, pp. 174–180, Mar. 2007.
[7] Y.H. Song, A.T. Johns, Flexible AC Transmission Systems (FACTS), IEE, London 1999.
[8] R. Mineski, R. Pawelek, I. Wasiak, “Shunt compensation for power quality improvement
using a STATCOM controller: modeling and simulation”, IEE Proc. on Generation,
Transmission and Distribution, Vol. 151, No. 2, March 2004.
[9] M. H. J. Bollen, Understanding Power Quality Problems Voltage Sags and Interruptions.
New York: Wiley, 2000.
[10] Alharbi, Y.M. ; Electr. &Comput. Eng. Dept., Curtin Univ., Perth, WA, Australia
; Yunus, A.M.S. ; Abu Siada, A. "Application of UPFC to improve the LVRT capability of
wind turbine generator" IEEE ,Conference Publications, date 26-29 Sept. 2012,in
Universities Power Engineering Conference (AUPEC), 2012 22nd Australasian.
[11] J. A. Wiik, F. D. Wijaya, R. Shimada, “An Innovative Series Connected Power Flow
Controller, Magnetic Energy Recovery Switch (MERS),” in Proc. Power Engineering Society
General Meeting, pp. 1-7, 2007.
[12] K. Lima, A. Luna, P. Rodriguez, E. Watanabe, R. Teodorescu, and F. Blaabjerg,
“Doubly-fed inductiongenerator control under voltage sags,” Energy 2030 Conference, 2008.
ENERGY 2008. IEEE, pp. 1–6, Nov. 2008.
[13] C. Wessels, F.W. Fuchs "LVRT of DFIG Wind Turbines - Crowbar vs. Stator Current
Feedback Solution –"Energy Conversion Congress and Exposition, (ECCE), 2010, IEEE,
conference date 12-16 sep 2010, in Atlanta, GA.
[14] Ameer H. Abd and D.S.Chavan, “Impact of Wind Farm of Double-Fed Induction
Generator (DFIG) on Voltage Quality”, International Journal of Electrical Engineering &
Technology (IJEET), Volume 3, Issue 1, 2012, pp. 235 - 246, ISSN Print : 0976-6545,
ISSN Online: 0976-6553.
[15] Nadiya G. Mohammed, “Application of Crowbar Protection on DFIG-Based Wind
Turbine Connected to Grid”, International Journal of Electrical Engineering & Technology
(IJEET), Volume 4, Issue 2, 2013, pp. 81 - 92, ISSN Print : 0976-6545, ISSN Online:
0976-6553.
[16] Nadiya G. Mohammed, Haider Muhamad Husen and Prof. D.S. Chavan, “Fault Ride-
Through Control for A Doubly Fed Induction Generator Wind Turbine Under Unbalanced
Voltage Sags”, International Journal of Electrical Engineering & Technology (IJEET),
Volume 3, Issue 1, 2012, pp. 261 - 281, ISSN Print : 0976-6545, ISSN Online: 0976-6553.


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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME

AUTHORS


             Mustafa Jawadkadhim Al-Tameemi, was born in Baghdad, Iraq on
             Jan 16,1986. He received B.Sc Degree in department of electrical engineering
             at University of Technology, Baghdad, Iraq in 2007. He is currently M.Tech
             electrical (Power Systems) candidate in BharatiVidyapeeth Deemed
             University, College of Engineering, Pune, India.




              Prof . D.S. Chavan: Ph D (Registered), ME (Electrical), BE (Electrical),
              DEE Associate Professor, Co-ordinator (R&D cell), Co-ordinator
              (PH.D.Programme management) BharatiVidyapeeth Deemed University
              College Of Engineering Pune 411043. He is pursuing Ph D. He received ME
              (Electrical)(Power systems) Achieved rank certificate in Pune University for




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