Overview lvrt capability of dfig techniques

International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME
75
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
INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING
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ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
Volume 4, Issue 3, May - June (2013), pp. 75-81
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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
DFIG suffers from a greatvulnerability to grid faults
forthe rotor side power electronic converter.
The structure is as follows.
isdescribed. FACTs devices are described in section III. Stator current feedback technique is
investigated in IV. Aconclusion closes the
techniques.
II-CROWBAR
Fig 1:
A protection system called active crowbar
the DFIG operation during the LVRT which
protect it turningthe generator into a sq
comprise of a set of thyristors that will
thereby limit the rotor voltage and
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.
disadvantage for this system is when the RSC disabled the machine draws a highshort circuit
current when the crowbar is activated, as described
reactive power drawn fromthe power network, which is not acceptable when
consideringactual grid code requirements.
investigated to ride-through grid faults safely andfulfill the grid codes.
III-FLEXIBLE AC TRANSMISSION SYSTEMS (FACTS)
Flexible AC Transmission Systems, called FACTS, got in the recent years a well
known term for higher controllability in power systems by means of power electronic
devices. Several FACTS-devices have been introduced for various applications worldwide. A
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
extensions of power systems, for instance like upgrades or additions of subst
lines. FACTS-devices provide a better adaptation to varying operational conditions and
improve the usage of existing installations.
devices, shunt devices, series devices and combined shunt and s
6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME
76
DFIG suffers from a greatvulnerability to grid faults [3] and requires additional protection
he rotor side power electronic converter.
The structure is as follows. In Section II, the crowbar protection technique
FACTs devices are described in section III. Stator current feedback technique is
conclusion closes the investigation of the effectiveness of the proposed
Fig 1: DFIG using crowbar protection
A protection system called active crowbar is one of the methods that used to enhance
eration during the LVRT whichdisconnects the rotor side converter in order to
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
can stayconnected to the grid and resume operation as soon as possible. But the main
this system is when the RSC disabled the machine draws a highshort circuit
current when the crowbar is activated, as describedin [6], resulting in a large amount of
requirements. Thus, other protection methodshave to be
through grid faults safely andfulfill the grid codes.
AC TRANSMISSION SYSTEMS (FACTS)
lexible AC Transmission Systems, called FACTS, got in the recent years a well
devices have been introduced for various applications worldwide. A
er of new types of devices are in the stage of being introduced in practice.In most of the
extensions of power systems, for instance like upgrades or additions of substations and power
devices provide a better adaptation to varying operational conditions and
improve the usage of existing installations. There are three configurations of the FACTS
devices, shunt devices, series devices and combined shunt and series devices [7]
June (2013), © IAEME
and requires additional protection
In Section II, the crowbar protection technique
FACTs devices are described in section III. Stator current feedback technique is
of the effectiveness of the proposed
of the methods that used to enhance
converter in order to
Thecrowbar may
the rotor windings when triggered and
provide an additional path for the rotorcurrent. Different
But the main
this system is when the RSC disabled the machine draws a highshort circuit
resulting in a large amount of
Thus, other protection methodshave to be
lexible AC Transmission Systems, called FACTS, got in the recent years a well-
devices have been introduced for various applications worldwide. A
er of new types of devices are in the stage of being introduced in practice.In most of the
ations and power
devices provide a better adaptation to varying operational conditions and
There are three configurations of the FACTS
7].

A-STATCOM
The most used FACTS
Converter called STATCOM. These shunt devices are operating as reactive power
compensators. The advantage of a STATCOM is that the reactive power
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
shows the basic configuration of D
Fig 2: STATCOM structure and voltage / current characteristic
TheVoltage Source Converter (VSC) converts the dc voltage across the storage deviceinto a
set of three-phase ac output voltages. These voltages arein phas
system through the reactance of the coupling transformer. Suitable adjustment of the
phaseand magnitude of the STATCOM output voltages allowseffective control of active and
reactive power exchangesbetween the STATCOM and the ac sy
B-Dynamic voltage restorer (DVR)
The DVR is a powerful controller that is commonly used for voltage sags mitigation
at the point of connection. The DVR employs the same blocks as the D
this application, the coupling transformer is connected in series
shows the basic configuration of DVR. The VSC generates a three
which is controllable in phase and magnitude. These voltages are injected into the ac
distribution system in order to maintain the load voltage at the desired volt
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
DVR [9].
Fig3:
77
The most used FACTS-device is the SVC or the version with Voltage Source
compensators. The advantage of a STATCOM is that the reactive power
independent from the actual voltage on the connection point.
the basic configuration of D-STATCOM.
phase ac output voltages. These voltages arein phase and coupled with the ac
reactive power exchangesbetween the STATCOM and the ac system[8].
Dynamic voltage restorer (DVR)
at the point of connection. The DVR employs the same blocks as the D-STATCOM, but in
this application, the coupling transformer is connected in series with the ac system. Fi
shows the basic configuration of DVR. The VSC generates a three-phase ac output voltage
distribution system in order to maintain the load voltage at the desired voltage
device is used to compensatethe faulty grid voltage, any protection method in the
reduced complexity in the DFIG system. Thedisadvantages are the cost and complexity of the
Fig3: Basic configuration of DVR
device is the SVC or the version with Voltage Source
compensators. The advantage of a STATCOM is that the reactive power provision is
e and coupled with the ac
STATCOM, but in
with the ac system. Fig. 3
phase ac output voltage
age reference. If
device is used to compensatethe faulty grid voltage, any protection method in the
are the cost and complexity of the

C-Unified Power Flow controller (UPFC)
The UPFC is a combination of a static compensator and static series compensation. It
acts as a shunt compensating and a phase shifting
Fig 4: Principle configuration of an UPFC
The UPFC consists of a shunt and a series transformer, which are connected via two
voltage source converters with a common DC
power exchange between shunt and series transformer to control the phase shift of the series
voltage. This setup, as shown in Fig 4
flow. The series converter needs to be protected with a Thyristor bridge. Due to the
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)
One of the series FACTS
a capacitor in aconfiguration identical to the single
arrangementin an electric circuit, however, is different, with only two of the
converter’sterminals utilized and connecte
variablecapacitor and is related to FACTS controllers with series capacitors such as theGCSC
and the TCSC.The investigation of
LVRTcapabilityof wind farms w
conditions in the whole system and the process of achievingthis was almost
However, found that a small 50
MERS. These are most likely the e
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
MERScapacitor or the operation in continuous mode would affect this is
Fig 5:
78
ller (UPFC)
acts as a shunt compensating and a phase shifting device simultaneously.
Principle configuration of an UPFC
voltage source converters with a common DC-capacitor. The DC-circuit allows the active
ween shunt and series transformer to control the phase shift of the series
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
witch (MERS)
series FACTS controllers. It consists of four power electronic switches and
a capacitor in aconfiguration identical to the single-phase full bridge converter. Its
and connected in series. It has the characteristic of a
The investigation of MERS indicated that it is able to increase the
LVRTcapabilityof wind farms with DFIG. This device successfully reestablishedpre
conditions in the whole system and the process of achievingthis was almost
However, found that a small 50-Hz distortion inthe generator’s torque was caused by the
MERS. These are most likely the effectof harmonic distortion created by the device. It injects
order harmonicsinto the system, and it was found that these coincide with the
tion in continuous mode would affect this is uncertain
Fig 5: Typical MERS configuration
circuit allows the active
ween shunt and series transformer to control the phase shift of the series
, provides the full controllability for voltage and power
flow. The series converter needs to be protected with a Thyristor bridge. Due to the high
consists of four power electronic switches and
phase full bridge converter. Its
d in series. It has the characteristic of a
MERS indicated that it is able to increase the
successfully reestablishedpre-fault
conditions in the whole system and the process of achievingthis was almost identical.
Hz distortion inthe generator’s torque was caused by the
ffectof harmonic distortion created by the device. It injects
order harmonicsinto the system, and it was found that these coincide with the
uncertain [11].

79
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:
)2.........(
2
1
)1.........(
2
1
22
22
rq
s
h
sq
s
s
s
s
s
s
sq
rd
s
h
sq
s
s
s
s
s
sd
i
L
L
v
s
L
R
s
L
R
s
L
i
i
L
L
v
s
L
R
sL
i
−
+




+
+
=
−
+




+
=
ω
ω
ω
If the stator currents are fed back as rotor current reference values, i.e. sdrd ii =+
and sqrq ii =+
the
following equation for the stator currents can be obtained and the stator currents are reduced.
)4..(..........
2
1
)3.(..........
2
1
22
22
sq
s
s
s
s
s
hs
sq
sq
s
s
s
s
hs
sd
v
s
L
R
s
L
R
s
LL
i
v
s
L
R
sLL
i
ω
ω
ω
+




+
+
+
=
+




++
=
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).

80
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.

81
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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