Analysis of Three Phase Grid Failure and Doubly Fed Induction Generator
Ride-through using Crowbars
Ralf Lohde, Simon Jensen, André Knop, Friedrich W. Fuchs
Institute of Power Electronics and Electrical Drives
Christian - Albrechts - University of Kiel
Kaiserstrasse 2
24143 Kiel, Germany
Tel.: +49 / (0) – 431.880.6105
Fax: +49 / (0) – 431.880.6103
E-Mail: rl@tf.uni-kiel.de
URL: http://www.tf.uni-kiel.de/etech/LEA
Acknowledgements
This work has been supported by the European Union and the state of Schleswig-Holstein. It is
part of the work of CEwind Competence Center Wind Energy of Universities in SchleswigHolstein, Germany.
Keywords
«Wind energy», «Doubly fed induction motor», «Fault handling strategy», «Wind generator
system»
Abstract
Doubly Fed Induction Generators (DFIG) are nowadays widely used in variable speed wind power
plants. The behaviour of these machines during grid failure is an important issue, since in case of
under voltage of the mains it is not allowed to simply disconnect the turbine but it is mandatory that it
keeps on delivering power to the mains supply system. To protect the rotor side converter, a crowbar
at the rotor has to be switched on. The performance of an active crowbar during voltage dips is
investigated for several parameter sets of machine, resistor and control.
1
Introduction
Doubly Fed Induction Generators (DFIG) have become a widely used generator type in wind energy
conversion. Especially the smaller converter power compared to a system with stator side converter is
very interesting.
Nowadays stringent grid codes require the turbine for a certain voltage range to stay connected to the
grid even through grid failures [1]. This implies some requirements for the safe operation of the rotor
side inverter of the DFIG, since the rotor current will become very large during these grid failures [2].
Usually a crowbar is used to protect the rotor side inverter from overcurrents and overvoltages [2, 3].
The crowbar 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 rotor current.
Different values of the crowbar resistors result in a different behaviour. Using this technology, the
DFIG can stay connected to the grid and resume operation as soon as possible.
The behaviour of the system during grid short circuit is mainly affected by resistor value. This is
modelled, simulated and analysed, in the case of super synchronous speed for five different machines
in the range of 1.5 to 5 MW and for different crowbar resistance.
The paper is organized as follows. An introduction was given in section 1, and a detailed system
description is given in section 2. Section 3 gives simulation results, which are summed up in a
conclusion in section 4.
2
System Model
This paper deals with a model which consists of different sub models. These are the wind rotor, the
mechanical drive train, the induction generator, the rotor side converter and the grid model. The
structure of a typical wind power plant is shown in Fig. 1. There are five different parameter sets for
the model of the wind power plant. We select wind power plants in a power range from 1.5 to 5 MW.
Drive Train
Crowbar
ΘR
d
z
ΘG
c
DFIG
Speedcontrol
vwind
Currentcontrol
MPP tracking
power-factor
control
Reference
value
Fig. 1: Structure of the wind power plant (WPP)
2.1
Turbine Model
The maximum useable power PRotor taken from the rotor out of the wind is given by [4, 5]:
PRotor 
1 3
vwind  R 2c p ,max
2
(1)
where ρ is the air density, R the radius of rotor blades and vwind the wind speed. Factor cp is the power
coefficient.
2.2
Drive Train
Despite of the complexity of the drive train system in a real wind power plant, the dynamic model is
reduced to a two mass system [6]. For investigation of dynamic behaviour the drive train needs to be
considered. In order to characterize the drive train it is necessary to know the gear transmission ratio z,
the stiffness c and the damping coefficient d of the shaft. The mechanical system is described by
equations (2) and (3) where T is the torque, Θ the inertia, ω the angular speed and γ the angle. Indices
R and G indicate rotor and generator side respectively.
2.3
TR  R   R  R  G  d   R   G  c
(2)
TG  G  G  R  G  d   R   G  c
(3)
Converter and its Control
The DFIG is controlled by the rotor side converter. This way the active and reactive stator power is
controlled indirectly by means of the inner rotor current control loop. The control is performed fieldoriented, where the rotor current loop is stator flux orientated. The grid side converter controls the DClink voltage to a constant value and may also control the active and reactive power taken from or
transferred to the grid. For this analysis the converter is not modelled with its switching function but as
continuous sinusoidal voltage source [2].
2.4
Crowbar
In case of a rotor overcurrent due to a grid fault the rotor side converter has to be disconnected from
the rotor. The rotor of the DFIG is switched from the rotor side converter to three phase resistor RCrow.
The external resistor RCrow is applied to reduce the rotor current magnitude after switching the
crowbar. Different values of RCrow are examined in this analysis. So the generator with activated
crowbar operates as an induction generator with rotor nearly in short circuit and with a large reactive
current [2, 7, 8].
2.5
Doubly Fed Induction Generator Model
The system of DFIG is described using a state space model. The stator voltage uS and rotor voltage uR
as well as the stator current iS and rotor current iR are space vectors with d- and q- components, here
oriented at stator flux. The state space equations of the electrical system are shown in equation (4).
The connection between the electrical system and the mechanical system is described by equation (6).
The angular speed ωm is the mechanical speed of the rotor and ωS is the angular speed of the grid
voltage [9, 10].

1
  
S

iSd    R k h   S
 
d  iSq  


  
kR
dt iRd
   
 
S
iRq
  k
m ,el R



 1
S 

1
 S
 k
 m ,el R

kR
 S
m ,el k S 



kS
 R
m ,el k S



1
 R
S kh  R

 1
L
 S
i


kS
 Sd
   0
 R   iSq  


  
 R   S k h  iRd
   kl
 iRq


 
  
1


 0
 R 

L2h
LS LR
Lh
L
, kR  h
LS
LR
kh 
L2h
Lh
, kl 
LS LR
LS LR
0
1
 kl
LS
0
0
LR
 kl
0
1

0 
 u 
Sd
 kl   
  u Sq 
 u  
0   Rd 
 u Rq 
1 
LR 
3
  iSd iRq
 
pLh iSq iRd
2
1
 m  Tel  TG 
(4)
Tel 
LS
L
, R  R
RS
RR
kS 
2.6
S  R kh

(5)
(6)

 R   S  p m
 m ,el  p m
Grid Model
The grid is modelled by an impedance and a three-phase voltage source (7), (Fig. 2).
failure
ugrid
uLgrid
uRgrid
Lgrid
Rgrid
iWPP
uWPP
15% ugrid
di 

uWPP  u grid   Rgrid  iWPP  Lgrid WPP 
dt 

(7)
Fig. 2: State space structure of the grid model
The impedance consists of a series resistor which is representing the ohmic copper losses and a line
inductance. The voltage source ugrid is assumed to be ideal.
2.7
Restrictions and Marginal Conditions
As will be shown in the next section a grid failure is causing high rotor and stator currents.
Equation (6) for Tel indicates the relation between electrical torque and current of the DFIG. The high
transient fluctuations of torque cause mechanical problems in the bearings and also in the gearbox and
the rotor.
The currents at undervoltage are mainly limited by the machine leakage inductance. It is well known
[7] that the leakage inductance as well as the magnetizing inductance are depending on the stator
current and magnetizing current respectively. These effects influence the electrical behaviour of DFIG.
These several effects are not considered within the state space model (4).
To consider only the behaviour of DFIG during the time in which the grid failure occurs the turbine
model (section 2.1) is replaced by a constant torque source. The parameter sets of the five machines
used in the simulation model are presented in Table I (Appendix).
3
Simulation Results and Analysis
This section deals with introductory considerations at first. The second part will present simulation
results of the electrical behaviour in case of stator voltage drop and draw conclusions for control
strategies for fault-ride-through. In the following the crowbar resistance will be given in p.u. scaled to
the rotor resistance.
3.1
Electrical behaviour of DFIG in case of rotor side converter failure
During a grid failure the DFIG reacts to two different occurrences. On the one hand the rotor side is
shortcut to protect the rotor side converter. On the other hand there is a voltage drop on the stator side,
so that with a limited current not enough electrical power is transferred to the grid. For introduction to
the behaviour, the switching of the rotor side crowbar is simulated without mains undervoltage. Thus
the crowbar performance can be analysed separately. The results of these considerations are serving to
describe a part of behaviour of DFIG in case of a grid failure as well as in case of a rotor side
converter failure.
These simulation results show the relation between the crowbar resistor value, the peak of the rotor
currents and voltages and the electrical torque gradient of the DFIG during converter failure. Several
simulations with the five machines have shown similar behaviour, so exemplary simulation results are
shown for machine type 4 (Table I).
The simulation scenario is defined as follows: (fault case one)
 nominal operating point of the wind power plant (Pn = 2MW, cos(φ)=1)
 rotor side of DFIG is switched to the crowbar (ton = 1s)
 stator side of DFIG is kept connected to grid
 maximum rotor speed before short circuit (n=2000 min-1)
 nominal grid voltage
Fig. 3: Fault performance versus time for fault case one (machine type 4)
a) Rotor currents; b) rotor voltages
In case of a rotor side converter failure and switching on of crowbar, the DFIG is changed to an
induction machine with an additional rotor resistance. Now the induction machine is trying to reach
the stationary operating of a machine with rotor nearly short cut near synchronous operation point
depending on the mechanical torque. This behaviour causes high rotor and stator currents, which are
necessary to reach the stationary operating point. The crowbar resistor is limiting these currents as
well as the electrical torque. The higher the crowbar resistor selected, the lower the rotor currents and
the electrical torque after switching are (Fig. 3.b, Fig. 4).
Fig. 4: Fault performance versus time for fault case one (machine type 4)
a) electrical torque; b) extension of Fig. 4.a (machine type 4)
Fig. 4 shows that for a low crowbar resistor of ten and twenty p.u. a dangerous state can occur. In this
case the torque will not be reduced enough. The torque gradient and the maximum rotor voltage are
rising with an increasing crowbar resistance (Fig 4.a, Fig. 4.b).
Fig. 5: Extraction of fault performance data depending on crowbar resistance at fault case one
(machine type 4)
a) torque gradient and maximum torque; b) maximum rotor current and maximum rotor voltage
It is known that high transient torque fluctuations cause mechanical problems in the bearings as well
as in the gearbox. It is necessary to consider this effect in design of the crowbar resistor. Fig. 5.a
shows the relation between crowbar resistance and torque gradient. The less the crowbar resistance the
less the torque gradient is.
Fig. 5.b shows the maximum rotor current and the maximum rotor voltage in relation to the crowbar
resistor value. By help of this diagram the range of the crowbar resistor value can be determined. A
small resistor causes high rotor currents, a big one causes high rotor voltages.
In case of the rotor side converter failure considered in this section, the optimal crowbar resistor value
is located between 50 and 150 p.u. as shown in Fig. 5.b.
3.2
Electrical behaviour of DFIG in case of grid failure
This section deals with the presentation and analysis of simulation results which allow to draw
conclusion to the value of the crowbar resistance. Several simulations with the five machines have
been executed and have shown similar behaviour, so exemplary simulation results are shown for
machine type 1 (for data see Table I). In case of a grid failure the grid voltage drops down the stator
voltage respectively, immediate dropping is assumed. The DFIG reacts with rising stator and rotor
currents [2, 3]. To protect the rotor side inverter the crowbar will be switched to rotor circuit of DFIG.
Comparisons between the simulation results of the switching of crowbar in section 3.1 and this section
indicates similar electrical behaviour of the DFIG depending on the crowbar resistance. The
simulation results show the relation between the crowbar resistor value and the electrical values of
DFIG, rotor currents and voltages and the electrical torque, during grid failure.
The simulation scenario is defined as follows: (fault case two)
 nominal operating point of the wind power plant (Pn = 1.5 MW, cos(φ)=1)
 stator voltage dip to 15% of nominal grid voltage (tf=1s)
 stator side of DFIG is kept connected to grid
 maximum rotor speed before short circuit (n=2000 min-1)
 rotor side of DFIG is switched to the crowbar when the rotor current is exceeding nominal
rotor current
Fig. 6: Fault performance versus time for fault case two (machine type 1)
a) rotor currents; b) rotor voltages
Fig. 6.a shows the absolute values of the rotor currents and Fig. 6.b shows the absolute values of the
rotor voltages for simulations with five different crowbar resistance values. Simulation results with
crowbar resistor Rcrow = 50 Rr show a rotor current peak of more than three times of nominal rotor
current, the decreasing rate is high however. With increasing crowbar resistance the overcurrents
become lesser or vanish.
Fig. 6.b shows the behaviour of the rotor voltage amplitude in relation to five different crowbar
resistances. The maximum overvoltage at intervening of the crowbar is directly proportional to the
resistance value (see Fig. 8).
Fig. 7: Fault performance versus time and extracted data for fault case two (machine type 1)
a) electrical torque versus time; b) maximum electrical torque depending on rotor resistance
Fig. 7 shows a dangerous state for a low crowbar resistor value of fifty p.u. In this case the torque will
not be reduced sufficiently. With the crowbar resistance of more than 200 p.u. the torque stays close to
its nominal value. More detailed analysis has shown a similar torque gradient for all crowbar
resistance values.
Fig. 8. shows the simulated maximum values extracted from the DFIG´s electrical values versus time
during a grid fault with switched crowbar for different crowbar resistance values. It can be seen that
the maximum value of the rotor voltage urstep increases with rising crowbar resistance, while the
maximum value of the rotor current irmax and its derivative dir/dt decrease. Especially the di/dt-value
reaches high values. From this figure the appropriate crowbar resistor value for fault case two can be
selected. With respect to the maximum torque (see Fig. 7.b) a crowbar resistance of one hundred to
two hundred and fifty p.u. has to be chosen.
Fig. 8: Maximum electrical values of DFIG at fault case two (machine type 1)
Simulations have been performed for the fault ride trough with this selected resistance for all sets of
machine parameters, shown in Fig. 9.
Fig. 9: Fault performance for fault case two (machine type 1, RCrow = 200 p.u. plant 1 to 5)
These simulations show different behaviour depending on the crow bar resistance. Firstly, the at 55 ms
and short after, directly after switching on of the crowbar, different kinds of oscillations can be seen.
Secondly, the recovery after low voltage ride through is different from type to type. Differences are
due to the different inertia of the simulated wind power plants. Nevertheless, these simulation results
shown in figure 9 depict the ability of several DFIG to ride through a three-phase grid fault.
4
Conclusion
This paper deals with the analysis of the performance of a wind power station with doubly fed
induction generator and its interaction with the mains in case of grid failure. The voltage drop related
problems during a grid fault can be split up into two parts in general. The first part is the drop of stator
(grid) voltage and the second part is the switch-in process of the crowbar resistor which converts the
DFIG configuration into an ordinary squirrel cage induction generator with increased rotor resistance.
First of all the switching in of the crowbar resistor at full grid and thus full stator voltage has been
presented and analysed. A dependency of the rotor currents and rotor voltages as well as the torque on
the rotor resistance has been shown. This can be used as an introductory analysis of the DFIG
behaviour.
Secondly, the drop down in stator voltages with additional switching in of the crow bar resistor has
been analysed. With simulations the increasing rotor current, rotor voltage and torque are presented.
and there dependency on the crowbar resistance. It has been shown that a low crowbar resistance leads
to higher electrical torque, overcurrents and low rotor voltages. On the other hand, high values for the
crowbar resistor will result in a lower electrical torque and rotor currents but also to higher rotor
voltages. An appropriate crowbar resistance is proposed based on the variety of simulations carried
out.
Appendix
Table I: Parameters of wind power plants
Type
Pmax
[MW]
Rs
[m]
Ls
[mH]
R’r
[m]
L’r
[mH]
Lh
[mH]
RFe
[]
p
Θem
[kgm²]
Us
[V]
Ur
[V]
D
[m]
M
[to]
Θ
[Mkgm²]
1
2
3
4
5
1,5
3,0
5,0
2,0
5,0
10,3
2,97
1,04
1,16
2,1
0,2801
0,1209
0,0828
0,0700
0,153
8,28
3,82
2,51
1,3
2,1
0,1177
0,0573
0,0604
0,0755
0,149
26,96
12,12
8,085
3
4,26
40
53
68
24
51
2
2
2
2
3
116
116
116
90
209
690√ 2
690√ 2
690√ 2
690√ 2
960√ 2
690√ 2
690√ 2
690√ 2
1800√ 2
2500√ 2
70
80
120
77
120
5,7
6,3
17,7
6,2
17,7
1,2
1,4
9,5
1,5
9,5
References
[1]
[2]
e.on.: Ergänzende Netzanschlussregeln für Windenergieanlagen, 2001. (in German)
Akhmatov V.: Variable-speed wind turbines with doubly-fed induction generators. part iv:
uninterrupted operation features at grid faults with converter control coordination, WindEngineering, 27(6).
[3] Erlich I., Wrede H., Feltes C.: Dynamic Behavior of DFIG-Based Wind Turbines during Grid
Faults, IEEE 2007 1-4244-0844
[4] Betz A.: Windenergie und ihre Ausnutzung durch Windmühlen, Ökobuch, Staufen bei Freiburg,
1926
[5] Gasch R.: Windkraftanlagen; Grundlagen und Entwurf, Teubner, Stuttgart, Germany 1995
(in German)
[6] Zhang G.: Speed Control of Two-Inertia System by PI/PID Control, IEEE Transactions on
industrial electronics, Vol.47, No.3, June 2000
[7] Koch F.W.: Simulation und Analyse der dynamischen Wechselwirkung von Windenergieanlagen
mit dem Elektroenergiesystem, PhD thesis, Universität Duisburg-Essen, Sep 2005. (in German)
[7] Bolik S.: Analytical Generator Model for fault simulation of Wind Turbines, EPE/PEMC 2004
[9] Arsudis D.: Doppeltgespeister Drehstromgenerator mit Spannungszwischenkreis-Umrichter im
Rotorkreis für Windkraftanlagen, PhD thesis, TU Braunschweig, 1989. (in German)
[10] Gevers D. N.: Beitrag zur Regelung einer doppeltgespeisten Asynchronmaschine ohne
Lagegeber für Windkraftanlagen. PhD thesis, ISLE Ilmenau, 2004. (in German)