Fault Ride Through of D F IG Wind Turbines during
symmetrical voltage dip with Crowbar or Stator
Current Feedback Solution
Christian Wessels, Student member, IEEE and Friedrich W. Fuchs, Senior member, IEEE
Institute of Power Electronics and Electrical Drives
Christian-Albrechts University of Kiel
Kaiserstrasse 2, 24143 Kiel, Germany
Email: chw@tf. uni-kiel.de, fwf@tf.uni-kiel.de
Abstract—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 d uring severe grid voltage fau lts.
In this paper two methods for low voltage ride through of
symmetrical grid voltage dips are investigated. As a basis,
an analysis of the rotor voltages during grid fault is given.
First, the conventional hardware method using a crowbar is
introduced. Then the stator current reference feedback solution
is presented. Both methods are investigated and compared by
simulation results using 2 MW wind turbine system parameters.
Measurement results on a 22 kW laboratory DFIG test bench
show the effectiveness of the proposed control technique.
I. INTRODUCTION
The increased amount of power from decentralised, re-
newable energy systems, as especially win d energy systems,
requires strong grid code requirements to maintain a stable
and safe operation of the energy network. T he grid codes
cover rules considering the fault ride through behaviour as
well as the steady state active power and reactive power
production. The actual grid cod es stipulate th at wind farms
should contribute to power system control like frequency
and voltage control to behave much as conventional power
stations. A detailed review of grid code technical requirements
regarding the connection of wind farms to the electrical power
system is given in [1]. For operation during grid voltage faults
it becomes clear that grid codes prescribe that wind turbines
must stay connected to the grid and should support the grid
by generating reactive power to support and restore quickly
the grid voltage after the fault.
Among th e wind turb ine concepts turbines using the doubly
fed in duction generator (DFIG) as described in [2] and [3]
and shown in Fig. 1 are dominant due to their variable speed
operation, the separately controllable active and reactive power
and their partially rated power converter. But, the reaction of
DFIGs to grid voltage disturbances is sensitive, as described in
[4] and [5] for symmetrical a nd unsymmetrical voltage dips,
and requir es additional protection for the rotor side power
electronic converter.
Fig. 1: Schema tic diagram of DFIG wind turbine system
Conventionally a resistive network called crowbar is con-
nected, in case of rotor overcurrents, to the rotor circuit and
the rotor side converter is disabled as described in [6],[7],[8]
and [9]. But the machine draws a high short circuit current
when the c rowbar is activated as described in [10] resu lting
in a large amount of reac tive power drawn from the power
network, which is not acce ptable when considering grid code
requirements. The lack of reactive power su pport capabilty
when using crowbar circuits has led to a renewed in te rest for
LVRT to ride throu gh grid faults safely and fulfill the grid
codes at the same time. There are several approaches limitin g
the rotor currents during transient grid voltage dip by cha nging
the rotor side converters control without using external pro-
tection devices. The rotor side converter can be protected by
feedforward of the faulty stator voltage [11], by considering
the stator flux linkage [ 12] or other methods dealing with an
improved control structure during unsymm e trical grid voltage
conditions [13], [14] and [15]. In [16] a metho d, based on the
conventional vector control, is proposed that aims to reduce
the rotor currents by using the measured stator currents as
reference for the current controllers. In this paper the stator
current reference feedback solution [16] is investigated and
compared to a conventional fault ride through of the DFIG
using a crowbar cicuit. First results have been presented in
[17] but detailed analysis is include d here.
The pape r is structured as follows. In section II the DFIG wind
turbine concept is introduce d. In chapter III an analysis of the
rotorvoltage dynamics during nominal and during symmetrical
grid volttage dip is given. Afterwards the rotor c onverter rating
is taken into acco unt. In chapter IV two solutions to protect
978-1-4244-5287-3/10/$26.00 ©2010 IEEE 2771
the r otor converter are presented. First the hardware solution
using a crowbar and then a software solution using the stator
current feedb ack are presented. Simulation results for a 2 MW
wind turbine in section V and measurement results on a 22
kW laboratory test be nch in section VI show the effectiveness
of the proposed technique in comparison to the LVRT of the
DFIG using a crowbar. A conclusion closes the paper.
II. DOUBLY FED INDUCTION GENERATOR
The investigated wind turbine system shown in Fig. 1
consists of the basic components like the tu rbine, a g e arbox (in
most systems), a DFIG generator and a back-to- back voltage
source converter with a DC link. A DC chop per to limit the DC
voltage across the D C capacitor and a crowbar are included .
The back-to-back converter consists of a rotor side converter
(RSC) and a line side converter (LSC) co nnected to the grid by
a line filter to reduce the harmonics caused by the converter.
The wind turbine system is connected to the high voltage grid
by two transformers. Due to the short per iod of time of voltage
disturbances the dyn amics of the mechanical part of the turbine
will be neglected and the mechanical torque brought in by the
wind is assum e d to be constant.
The RSC provides decoupled control of stator active and
reactive power. A cascade vector con trol structure with inner
current control loops is applied. The overall control structure
is shown in Fig. 2.
III. DFIG ROTOR VOLTAGE DYNAMICS
A precise knowledge about amplitude and frequ ency of the
rotor voltage is necessary to design and control the rotor side
converter. Therefore equations for the rotor voltage in normal
operation and under symmetric al stator voltage dip are derived
in the following and in [5]. Afterwards the rotor converter
rating is taken into account.
A. normal condition
From the per-phase e quivalent cir cuit of the DFIG in a static
stator orien te d r eference frame the following stator and rotor
voltage and flux equations can be derived.
~v
s
= R
s
~
i
s
+
d
~
ψ
s
dt
(1)
~v
r
= R
r
~
i
r
+
d
~
ψ
r
dt
− jΩ
~
ψ
r
(2)
~
ψ
s
= L
s
~
i
s
+ L
h
~
i
r
(3)
~
ψ
r
= L
r
~
i
r
+ L
h
~
i
s
(4)
where
~
ψ, ~v and
~
i represen t the flux, voltage and current vectors
respectively. Subscripts s and r denote the stator an d roto r
quantities respectively. L
s
= L
sσ
+ L
h
and L
r
= L
r σ
+ L
h
represent the stator and rotor inductance, L
h
is the mutual
inductanc e, R
s
and R
r
are the stator a nd rotor resistances and
Ω is the rotor electrical speed (nu mber of pole pa irs m ultiplied
by ω
mech
).
By introducing the leakage factor σ = 1 −
L
2
h
L
s
L
r
the rotor flux
can be described in dependence of the rotor current and the
stator flux
~
ψ
r
=
L
h
L
s
~
ψ
s
+ σL
r
~
i
r
(5)
By substituting (5) in (2) an equation for the rotor voltage can
be obtained,
~v
r
=
L
h
L
s
d
dt
− jΩ
~
ψ
s
+
R
r
+ σL
r
d
dt
− jΩ
~
i
r
(6)
that consists of two parts. The first pa rt is caused by the stator
flux
~
ψ
s
that is given in normal operation by the constantly
rotating vector:
~
ψ
s
=
V
s
jω
s
e
jω
s
t
(7)
The second part of (6) is caused by the rotor curre nt
~
i
r
. The
rotor resistance R
r
and the leakage factor σ are often small,
so the ro tor voltage does not differ considerably from the part
caused by the stator flux . Thus, the am plitude of the rotor
voltage in n ormal condition V
r 0
can be calculated as
V
r 0
≈ V
s
L
h
L
s
ω
r
ω
s
= V
s
L
h
L
s
s (8)
where s = 1 − (Ω/ω
s
) = ω
r
/ω
s
describes the slip and ω
r
the
rotor fr e quency.
B. Symmetrical Voltage Dip, Constant Phase Angle
Under a symmetric a l voltage dip the stator voltage is
reduced f rom normal am plitude V
1
to the faulty amplitude
V
2
as d e scribed in (9).
~v
s
=
V
1
e
jω
s
t
for t < t
0
V
2
e
jω
s
t
for t ≥ t
0
(9)
~
ψ
s
=
(
~
ψ
s1
=
V
1
jω
s
e
jω
s
t
for t < t
0
~
ψ
s2
=
V
2
jω
s
e
jω
s
t
for t ≥ t
0
(10)
Since the stator flux is a con tinuous value it cannot follow the
step function of the voltage. The evolution of the stator flux
can be derived by solving the differential equation (11)
d
~
ψ
s
dt
= ~v
s
−
R
s
L
s
~
ψ
s
(11)
that can be derived from (1) a nd (3). Due to the low influence
of the rotor current on the rotor voltage the open rotor
condition is a ssumed (
~
i
r
= 0). The solution consists of two
parts. The fir st part is the steady state stator flux after the
voltage dip, which is described by
~
ψ
s2
and the second p art is
the transition of the flux from
~
ψ
s1
to
~
ψ
s2
that is described by
(12)
~
ψ
s
=
~
ψ
s,diff
e
−tR
s
/L
s
=
~
ψ
s,diff
e
−t/τ
s
(12)
where
~
ψ
s,diff
is the difference of the stator flux before and af-
ter the voltage dip, described by (V
1
−V
2
)/jω
s
. Summarizing,
the stator flux is given by the sum of the two parts:
~
ψ
s
(t) =
V
2
jω
s
e
jω
s
t
+
V
1
− V
2
jω
s
e
−t/τ
s
(13)
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Fig. 2: Schematic diagra m of DFIG w ind tur bine control structure
When the dynamic stator flux from (13) is considered in
the rotor voltage equation of (6) (neglecting
~
i
r
and 1/τ
s
)
the dynamic behavior of the ro tor voltage under symmetrical
voltage dip is described by (15)
~v
r
=
L
h
L
s
d
dt
− jΩ
V
2
jω
s
e
jω
s
t
+
V
1
− V
2
jω
s
e
−t/τ
s
(14)
=
L
h
L
s
sV
2
e
jω
s
t
− (1 − s)(V
1
− V
2
)e
−t/τ
s
(15)
In a re ference frame rotating at rotor frequency the following
rotor voltage is obtained:
~v
r
=
L
h
L
s
sV
2
e
jω
r
t
− (1 − s)(V
1
− V
2
)e
−jΩt
e
−t/τ
s
(16)
The results of this analysis show that th e rotor voltage during
symmetrical voltage dip consists of two components. Th e first
part is p roportio nal to the slip and the remaining stator voltage,
thus for a deep voltage dip and a slip usually at -0.2 it is
small. The frequency of the first part is the slip frequen cy (at
a slip of -0.2 ω
r
= 10 Hz). The second part of (16) has a
high amplitude a t t=0 proportional to (1-s) and rotates at the
mechanical frequency Ω (at a slip of -0.2: Ω = 6 0 Hz). The
term is decaying exponentially with the stator time constant
of τ
s
. The maximum rotor voltage during symmetrical voltage
dip will occur at the beginning of the fault (t=0) and for a full
dip (V
2
= 0)
V
r m ax
=
L
h
L
s
(1 − s)V
1
(17)
C. Rotor Side Converter Rating
The nom inal power of the rotor side converter of a DFIG
is rated for a part of the stator power because the rotor power
is approximately pr oportional to the slip
P
r,n
≈ sP
s,n
(18)
that is chosen usua lly for wind turbine systems to s = ±0.3.
The req uired amplitud e of the rotor voltage is probably deter-
mined (with L
h
/L
s
≈ 1 in (8)) by
V
r
= sV
s
/N
sr
(19)
where N
sr
is the stator to ro tor turns ratio. The turns ratio
is usually set at 1/2 or 1/3 in practical win d tur bine driven
DFIGs to make full use of the DC link voltage a nd reduce the
converters c urrent rating. The required DC link voltage can be
determined by
V
conv
= m
V
DC
2
= V
r
(20)
where m is the modulation index of the pulse width modulation
(PWM) technique. The maximum value of the modulation
index is 1.0 for the carrier based sinusoidal PWM and 1.15
for the space vector modulation, both without overmodulation
technique s [18].
The findings of the sectio n enha nce the understandin g of rotor
overcurrents during symme trical grid voltage dip. Only if the
rotor side converter can provide a sufficient voltage level
controllability of rotor cu rrents can be obtained. I f the rotor
voltage exceeds the converter voltage high currents will flow
throug h the diodes into the dc link capacitor, damaging the
IGBT or the DC capacitor.
IV. DFIG PROTECTION
A. Crowbar
To protect the ro tor sid e converter from tripping due to
overcurrents in the rotor circuit or overvoltage in the DC link
during grid voltage dips a crowbar is installed in conventional
DFIG wind turbines, which is a resistive network that is
connected to the rotor windings of the DFIG. The crowbar
limits the voltages and provides a safe route for the currents
by b ypassing the rotor by a set of resistors. When the crowbar
is activated the rotor side converters pulses a re disabled and
the machine behaves like a squirrel cage induction machine
directly coupled to the grid. The magnetization of the machine
that was provided by the RSC in nominal condition is lost an d
the machine absorbs a large a mount of reactive power from
the stator and thus from the network [10], which can further
reduce the voltage level and is not allowed in actual grid codes.
Triggerin g of the crowbar circu it also means high stress to the
mechanical co mponen ts of the system as the sh aft and the
gear. Detailed analyses on the DFIG behavior during voltage
2773
dip and crowbar protection can be found in [6] and [10]. Thus,
from network and from ma chine mechanica l point of view a
crowbar triggering should be avoided.
Anyway, to compare the presented technique here with a con-
ventional DFIG wind turbine system protected by a crowbar
circuit, simulation results including crowbar protection are
examined. The refore the crowbar resistance is designed h ere.
Crowbar resistances are also designed in [9] and [10], but here
the resistance design is based on the an alytical findings on the
rotor voltage from the previous section.
There are two constraints that give an upper and a lower limit
to the crowbar resistance. As a first constraint the crowbar
resistance should be high e nough to limit the short circuit
rotor current I
r,max
. If the crowbar is activated, the crowbar
resistance R
cb
is added to the rotor circuit, re sulting in the
maximum rotor cur rent of (if R
r
is neglected)
I
r,max
=
V
r m ax
p
X
2
σr
+ R
2
cb
(21)
If the maximum rotor voltage during grid voltage dip from
(17) is considered the minimum crowbar resistance can be
derived as:
R
cb,min
=
s
L
h
L
s
(1 − s)V
1
I
r,max
2
− X
σr
2
(22)
As the second constraint, the crowbar resistance should be
low e nough to avoid too high voltage in the rotor circuit.
If the voltage across the crowbar terminals rises above the
maximum converter voltage high currents will flow through
the antiparallel diodes of the converter. A crowbar resistance of
R
crow
= 150R
r
is used in the simulations. Sim ulation results
for different crowbar resistances durin g a 10% voltage dip is
shown in Fig. 3. There are approaches limiting the operation
Fig. 3: Simulated rotor current during 10% voltage dip with
crowbar activated at t=0.5 s
time of the crowbar to return to normal DFIG operation with
active and reac tive power control as soon as possible. A
hysteresis control triggered by the rotor current is presented
in [8] and also applied in the simulations here. A reset of the
integral values of the RSCs cu rrent and power control before
restart is nece ssary to avoid overcurrents.
In the laboratory setup a passive crowbar circuit is used
that is triggered by a rotor overcurre nt. The crowbar can be
disabled manually by the user when saf e circumstances are
reestablished.
B. Stator Current Feedback Solution
The proposed technique aims to reduce the rotor currents
by changing the RSC control instead of installing additional
hardware protection like a cr owbar in the wind turbine system.
The solution has been presented in [16]. When a fault affects
the generator the m e asured and transformed stator currents are
fed back as reference for the rotor current controller (stator
currents in stator flux orientation). The objective is to reduce
stator cur rent oscillations and thus reduce the rotor curre nts as
well.
If the DFIG system equations (1)-(4) are combined, a Lapac e
transform ation is performe d and some simplifications are
assumed, the following equation for the stator currents can
be obtained:
i
sd
=
1
L
s
ω
s
s
2
+ 2 (R
s
/L
s
)s + ω
2
s
v
sq
−
L
h
L
s
i
r d
(23)
i
sq
=
1
L
s
s + R
s
/L
s
s
2
+ 2 (R
s
/L
s
)s + ω
2
s
v
sq
−
L
h
L
s
i
r q
(24)
If the stator currents are fed back as rotor current reference
values, i.e. i
∗
r d
= i
sd
and i
∗
r q
= i
sq
the following equation f or
the stator curren ts can be obta ined a nd the stator currents are
reduced .
i
sd
=
1
L
s
+ L
h
ω
s
s
2
+ 2(R
s
/L
s
)s + ω
2
s
v
sq
(25)
i
sq
=
1
L
s
+ L
h
s + R
s
/L
s
s
2
+ 2(R
s
/L
s
)s + ω
2
s
v
sq
(26)
The most important limitatio n lies in the fact that the rotor
converter voltage (20) must at least be as high as the maxi-
mum rotor voltage during voltage dip (17 ) to contain current
controllability. If current c ontrollability is assured, the stator
current feedback solution can re duce stator currents thus rotor
currents effectively. Otherwise, if the ro tor voltages exceed
the converter voltages, in case of deep voltage dips, hardware
protection solutions as the crowbar must be applied.
V. SIMULATION RESULTS
To show the effectiveness of the proposed technique sim-
ulations have been performed using MATLAB/Simulink and
PLECS for a 2 MW D FIG wind turbine system as shown in
Fig. 1. The simulation parameter are given in table I. The
control structure as shown in Fig. 2 is implemented. The
system performance of the DFIG is shown in Fig. 4 protected
by the conventional crowbar and in Fig. 5 protected by the
stator current f eedback solu tion during a three phase 50 %
voltage dip of 100 ms duration at the medium voltage level
(20 kV) (see Fig. 4,5 a)).
The DFIG reacts to the three p hase voltage dip with h igh stator
currents I
s
and thus high rotor currents are induced in the rotor
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0.4 0.5 0.6 0.7 0.8 0.9
−2
0
2
x 10
4
a) V
line
[V]
0.4 0.5 0.6 0.7 0.8 0.9
−500
0
500
b) V
s
[V]
0.4 0.5 0.6 0.7 0.8 0.9
−4000
−2000
0
2000
4000
c) I
s
[A]
0.4 0.5 0.6 0.7 0.8 0.9
−1000
0
1000
d) I
RSC
[A]
0.4 0.5 0.6 0.7 0.8 0.9
−1000
0
1000
e) I
crowbar
[A]
0.4 0.5 0.6 0.7 0.8 0.9
−4
−2
0
2
4
x 10
6
f) P,Q
s
[W,VA]
0.4 0.5 0.6 0.7 0.8 0.9
180
190
200
g) ω
mech
[rad/s]
Fig. 4: DFIG performance with Crowbar protection during 50
% three ph a se voltage dip
a) Line voltage b) Stator voltage c) Stator current
d) Roto r side converter current e) Crowbar current
f) Active and reactive stator power g) mechanical speed
circuit. When the rotor currents exceed the maximum level of
the hysteresis crowbar ( I
r,max
= 1400A) c ontrol the crowbar
is triggered to protect the RSC from overcurrents I
RSC
(Fig.
4 d),e)). T he crowbar has to be triggered several times during
the voltage dip. When the RSC is in operation the machine
magnetization is provided by the roto r but every time the
crowbar is triggered the RSC is disabled and the machine is
0.4 0.5 0.6 0.7 0.8 0.9
−2
0
2
x 10
4
a) V
line
[V]
0.4 0.5 0.6 0.7 0.8 0.9
−500
0
500
b) V
s
[V]
0.4 0.5 0.6 0.7 0.8 0.9
−4000
−2000
0
2000
4000
c) I
s
[A]
0.4 0.5 0.6 0.7 0.8 0.9
−1000
0
1000
d) I
RSC
[A]
0.4 0.5 0.6 0.7 0.8 0.9
−1000
0
1000
e) I
crowbar
[A]
0.4 0.5 0.6 0.7 0.8 0.9
−4
−2
0
2
4
x 10
6
f) P,Q
s
[W,VA]
0.4 0.5 0.6 0.7 0.8 0.9
180
190
200
g) ω
mech
[rad/s]
Fig. 5: DFIG performance with stator current reference pr o-
tection during 50 % three phase voltage d ip
a) Line voltage b) Stator voltage c) Stator current
d) Roto r side converter current e) Crowbar current
f) Active and reactive stator power g) mechanical speed
excited by the stator. Thus, continuous reactive power control
cannot b e provided during the voltage dip (see Fig. 4 f)) which
is not acc e ptable w hen considering the grid codes. The a ctive
power is o scillatin g as well so that a constant speed can not
be ensured.
In Fig. 5 the wind turbine system is protected by the proposed
stator current feedback solution. Th e ro tor currents are reduced
2775
