Overview lvrt capability of dfig techniques
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This document summarizes techniques for providing low voltage ride through (LVRT) capability for doubly fed induction generator (DFIG) wind turbine systems. It discusses three main techniques: 1) Active crowbar protection disconnects the rotor side converter during faults to protect it, turning the generator into a squirrel cage induction machine. This allows the DFIG to stay connected but draws a large reactive power. 2) Flexible AC transmission systems (FACTS) devices like STATCOM and dynamic voltage restorer (DVR) can provide external protection by injecting compensating voltages. Using a DVR avoids the need for internal DFIG protection methods. 3) Stator current feedback is investigated as another technique for
![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
& TECHNOLOGY (IJEET)
ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
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)
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IJEET
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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
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976
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
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
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
reactive power drawn fromthe power network, which is not acceptable when
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
known term for higher controllability in power systems by means of power electronic
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
applications the controllability is used to avoid cost intensive or landscape requiring
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]
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
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
values of the crowbar resistors result in adifferent behavior. Using this technology, the DFIG
But the main
this system is when the RSC disabled the machine draws a highshort circuit
resulting in a large amount of
reactive power drawn fromthe power network, which is not acceptable when
Thus, other protection methodshave to be
lexible 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 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
applications the controllability is used to avoid cost intensive or landscape requiring
ations and power
devices provide a better adaptation to varying operational conditions and
There are three configurations of the FACTS
7].](https://image.slidesharecdn.com/overviewlvrtcapabilityofdfigtechniques-130621024749-phpapp02/85/Overview-lvrt-capability-of-dfig-techniques-2-320.jpg)
![International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May
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:
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976
6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME
77
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
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
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 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
reactive power exchangesbetween the STATCOM and the ac system[8].
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-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
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 voltage
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
Fig3: Basic configuration of DVR
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
June (2013), © IAEME
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
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
Fig 2: STATCOM structure and voltage / current characteristic
TheVoltage Source Converter (VSC) converts the dc voltage across the storage deviceinto a
e 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
The DVR is a powerful controller that is commonly used for voltage sags mitigation
STATCOM, but in
with the ac system. Fig. 3
phase ac output voltage
which is controllable in phase and magnitude. These voltages are injected into the ac
age reference. If
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
are the cost and complexity of the](https://image.slidesharecdn.com/overviewlvrtcapabilityofdfigtechniques-130621024749-phpapp02/85/Overview-lvrt-capability-of-dfig-techniques-3-320.jpg)
![International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May
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:
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976
6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME
78
ller (UPFC)
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.
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-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
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
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
arrangementin an electric circuit, however, is different, with only two of the
and connected in series. It has the characteristic of a
variablecapacitor and is related to FACTS controllers with series capacitors such as theGCSC
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
resonancefrequency of the simulation model. How the application of a different
tion in continuous mode would affect this is uncertain
Fig 5: Typical MERS configuration
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
June (2013), © IAEME
The UPFC is a combination of a static compensator and static series compensation. It
The UPFC consists of a shunt and a series transformer, which are connected via two
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
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
consists of four power electronic switches and
phase full bridge converter. Its
arrangementin an electric circuit, however, is different, with only two of the
d in series. It has the characteristic of a
variablecapacitor and is related to FACTS controllers with series capacitors such as theGCSC
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
resonancefrequency of the simulation model. How the application of a different
uncertain [11].](https://image.slidesharecdn.com/overviewlvrtcapabilityofdfigtechniques-130621024749-phpapp02/85/Overview-lvrt-capability-of-dfig-techniques-4-320.jpg)
![International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME
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).](https://image.slidesharecdn.com/overviewlvrtcapabilityofdfigtechniques-130621024749-phpapp02/85/Overview-lvrt-capability-of-dfig-techniques-5-320.jpg)
![International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME
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.](https://image.slidesharecdn.com/overviewlvrtcapabilityofdfigtechniques-130621024749-phpapp02/85/Overview-lvrt-capability-of-dfig-techniques-6-320.jpg)
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Overview lvrt capability of dfig techniques
- 1. 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 & TECHNOLOGY (IJEET) ISSN 0976 – 6545(Print) ISSN 0976 – 6553(Online) 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) www.jifactor.com IJEET © I A E M E
- 2. 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 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 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 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 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 reactive power drawn fromthe power network, which is not acceptable when 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 known term for higher controllability in power systems by means of power electronic 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 applications the controllability is used to avoid cost intensive or landscape requiring 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] International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 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 values of the crowbar resistors result in adifferent behavior. Using this technology, the DFIG But the main this system is when the RSC disabled the machine draws a highshort circuit resulting in a large amount of reactive power drawn fromthe power network, which is not acceptable when Thus, other protection methodshave to be lexible 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 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 applications the controllability is used to avoid cost intensive or landscape requiring ations and power devices provide a better adaptation to varying operational conditions and There are three configurations of the FACTS 7].
- 3. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May 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: International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME 77 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 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 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 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 reactive power exchangesbetween the STATCOM and the ac system[8]. 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-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 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 voltage 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 Fig3: Basic configuration of DVR International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – June (2013), © IAEME 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 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 Fig 2: STATCOM structure and voltage / current characteristic TheVoltage Source Converter (VSC) converts the dc voltage across the storage deviceinto a e 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 The DVR is a powerful controller that is commonly used for voltage sags mitigation STATCOM, but in with the ac system. Fig. 3 phase ac output voltage which is controllable in phase and magnitude. These voltages are injected into the ac age reference. If 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 are the cost and complexity of the
- 4. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May 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: International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME 78 ller (UPFC) 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. 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-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 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 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 arrangementin an electric circuit, however, is different, with only two of the and connected in series. It has the characteristic of a variablecapacitor and is related to FACTS controllers with series capacitors such as theGCSC 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 resonancefrequency of the simulation model. How the application of a different tion in continuous mode would affect this is uncertain Fig 5: Typical MERS configuration International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – June (2013), © IAEME The UPFC is a combination of a static compensator and static series compensation. It The UPFC consists of a shunt and a series transformer, which are connected via two 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 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 consists of four power electronic switches and phase full bridge converter. Its arrangementin an electric circuit, however, is different, with only two of the d in series. It has the characteristic of a variablecapacitor and is related to FACTS controllers with series capacitors such as theGCSC 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 resonancefrequency of the simulation model. How the application of a different uncertain [11].
- 5. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME 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).
- 6. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME 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.
- 7. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME 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