Energy and Power Engineering, 2013, 5, 102-108
doi:10.4236/epe.2013.54B020 Published Online July 2013 (http://www.scirp.org/journal/epe)
Control Method of the DFIG Connected to a DC Link
through a Diode Bridge
G. D. Marques1, M. F. Iacchetti2
1INESC-ID, IST, University of Lisbon, Lisbon, Portugal
2Department of Electrical Engineering, Politecnico di Milano, Milano, Italy
Received February, 2013
ABSTRACT
This paper presents a control method for the Doubly-fed Induction Generator connected to a dc link through a diode
bridge on the stator. In th is system, the rotor is fed, at the slip frequency, by a PWM electronic converter and the stator
is directly connected to the dc link using a si mple diode bridge. The cost of power electronics is reduced in this system
when compared with the classic DFIG machine because the system uses less one PWM inverter and additionally it uses
a diode bridge. The application in mind is for microgrids. Microgrids need several elements that should work together.
The usual way to connect these elements is to use power electronic devices in a common dc link. This paper presents a
new form for the DFIG for this application and presents a control system for the inner control loop. Simu lation and ex-
perimental results show that the system can work acceptably using a stator frequency near the rated frequency of the
machine.
Keywords: Doubly-fed Induction Generator; Dc Link; Control
1. Introduction
A microgrid can be defined as a part of a distribution
network embedding multiple distributed generation as
renewable energy sources like photovoltaic panels, small
wind turbines etc., and storage systems. Most commonly
used energy storage devices in a microgrid are batteries,
supercapacitors, flywheels, and fuel cells. This type of
energy storage is termed as distributed storage and the
energy storage devices are termed as distributed storage
devices. These systems, with local loads, can be discon-
nected from the upstream network under emergency
conditions [1, 2].
The integration of renewable energy sources into a
microgrid poses a challenge because their output is in-
termittent and variable and, in principle, requires energy
storage to enable time-shift between energy production
and consumption. Several devices should be connected
together and this brings problems.
The power connection between microgrid components,
i.e. distributed generation sources, storages and loads,
can be done through a direct current (dc) link or an al-
ternating current (ac) link. In general, today these differ-
ent devices are Power Electronics controlled and each
one of these converters needs a dc link [3-9]. If the same
dc link is used considerab le savings can be obtained.
The Doubly-fed Induction Generator (DFIG) is a
well-known system. It has been applied to wind power
extraction plants working in generator mode converting
mechanical energy into electrical energy at adjustable
speed. Its main advantage is that, for a small range of
speed adjustment, it needs power electronics devices
rated to a fraction of the rating power of the wind turbine.
In this system, the stator is directly connected to the
network, and the rotor is connected to the network by an
ac/dc/ac frequency converter [10,11].
The usual application of the DFIG is connected to the
ac mains. There are many applications and this system
can now be consider ed as mature technology.
There are also other applications where the DFIG is
used in standalon e operation [11,12]. In this case there is
no ac network that imposes the stator frequency.
This paper presents a new structure for the DFIG and
one possible control system. The purpose is to use it
connected to a dc link in applications like microgrids.
This system brings considerable benefits because the cost
of power electronics is reduced as only a PWM ac/dc
converter is necessary. The purpose of the control system
is to regulate the DFIG torque and the stator frequency
near the rated frequency of the machine.
Section II presents the structure of the system. It uses
only an ac/dc PWM converter connecting the rotor to the
dc link and a simple diode bridge connecting the stator to
the dc link. The modeling and steady state laws are pre-
sented in section III. The structure of the control system

G. D. MARQUES, M. F. IACCHETTI 103
proposed in this paper is presented in section IV. Simula-
tions, section V, and experimental results, section VI,
show that the system can be applied in microgrids oper-
ating as generator in small wind turbines.
2. The Structure of the System
If the generator is to be connected to a dc link, only an
ac/dc converter is needed. The stator is connected to the
dc link using a diode bridge. The structure of the system
is presented in Figure 1. This figu re shows also the con-
ventions used in this paper. The active power on the sta-
tor terminals is considered positive when it flows from
the machine to the rectifier. However on the rotor a re-
verse convention is used. The torque is considered nega-
tive in the operation as generator. Because a diode bridge
is used in the stator terminals, only generator mode op-
eration is possible.
3. Modeling and Steady State
To represent the induction machine in steady state, the
equivalent circuit is used. It is shown in Figure 2.
According to the conventions adop ted in Figure 2:
21m
I
II (1)
3.1. Steady-state Simplified Model
The correspondent phasor diagram for the first harmonic
is shown in Figure 3. In this case the phase-shift due to
overlapping of the diodes during commutation, imposed
by the diode bridge between the stator voltage and first
harmonic of the current is neglected. This is only the first
approach to better understand the system. Experimental
results will show that this simplification is acceptable.
The most used control system of the DFIG is the stator
flux orientation. In this methodology, the control is per-
formed in the rotor circuits in the reference frame ori-
ented with the stator flux space vector. In this paper, an
attempt to control the system in a similar way will be
made. Using this methodology, the diagram shown in
Figure 3 is obtained, where impressed rotor current are
assumed. In the case of Figure 3 the following relations
involving the stator flux linkage s, the stator circular
frequency s and the voltages and currents can be writ-
ten:
1
1 11
;
mrd rq
ssm
s
sssmssr
II II
LI
ELI
UErI

 


 

d
LI
(2)
The active power in the stator is:
1111
33
rq
PUI UI (3)
Figure 1. Structure of the DFIM-DC.
Figure 2.  Equivalent circuit of the induction machine.
Figure 3. Phasor diagram without current shift.
So, it is possible to conclude:
 To control the voltage use s or Ird
 To control stator cu rr ent use I rq
 To control stator power use Ird  Irq
This will be true only if the reference frame is appro-
priately oriented with the stator flux. The diode bridge
gives two constraints, that is:
0; 0
rd rq
II (4)
All the well known considerations concerning the rat-
ing of the rotor side inverter hold also in this case, in
particular the rotor power is related to the slip by:
2
PsP
1

(5)
If we depict with n12 the stator/rotor turn ratio of the
DFIG and with U1n the rated line-to-line stator voltage,
the rms value of the rated line-to line rotor voltage U2n
equals
max max
21
12 12
.
2
dc
nn
ss
U
UU
nn
 (6)
In Equation (6), the relation Udc  2U1n has been con-

G. D. MARQUES, M. F. IACCHETTI
104
sidered as roughly valid. The same relation holds for the
maximum rms line-to-line ac voltage which can be ob-
tained at the rotor side by the space vector modulation
(i.e. U2n=Udc /2). Replacing this relation in Equation (6)
yields
12 max 0.33,ns (7)
where a typical range of 0.33 for the slip s has been
assumed. We conclude that a stator/rotor turn ratio
roughly equal to 0.33 allows the rotor converter and the
stator diode rectifier to share the same dc-bus.
3.2. Consideration of Overlapping of the Diodes
It is well known that overlapping diode commutations
happen in a diode bridge when a significant amount of
power is transferred to the dc bus [14, 15]. This phe-
nomenon is mainly due to the series ac inductance and in
this case it has important consequences because it re-
duces the effects of the harmonics on the DFIG. The ac
inductance is here represented by the leakage inductance
(20% in large machines). It produces also a little phase
shift between the first harmonics of stator voltage and
curren t: however such angle is normally small, even with
high commutation (overlapping) angles [13, 15]. Thus, in
this paper, this phase shift will be neglected. This ap-
proximation will be verified in the experimental results.
On the contrary, the commutation angle will be signifi-
cant and it will allow to drops the harmonic content in
the stator currents and then also the torque ripple.
4. The Structure of the Control System
4.1. Objectives of the Control System
The dc current and voltage are the two output variables
of this generator system. It is not possible to control both
because they are not independent variables: the dc link
imposes a constraint (relation) between them. In this pa-
per it is assumed that the purpose of this system is to be
connected to a dc link with constant dc voltage. In this
case the dc link voltage is constant, and it is supposed
that it will be controlled by other means, not from the
DFIG.
The first objective of the system is to control the speed
allowing its adjustment to the conditions of the wind tur-
bine. This is done with a speed controller, which will
operate in the reference torque. So the inner control sys-
tem is a torque control. If there were no torque and speed
controllers it is necessary the speed to be controlled by
the turbine connected to the generator. This is normally
not the case.
Because there is now a diode bridge, the frequency of
the stator is no more imposed by ac mains, whereas the
stator voltage is imposed by the Diode Bridge and dc link.
This results in a constant relation between the stator fre-
quency and the stator flux. In this control system the ob-
jective is to control the stator frequency near the rated
value of the machine. With this assumption the rated
power is guaranteed, and the stator flux will have appro-
priate values.
The control system should have the set point rotor
currents Ird* and Irq* (in a field oriented reference) as in-
puts, that will helps to drive the reference frame at rated
frequency and control the torque. The control method is
not the traditional stator FOC. In fact, the system meas-
ures the error between the reference frame where the
control is performed and the stator flux and corrects this
difference dynamically.
4.2. Reference Frame Stator Angle
Determination
Field orientation is based on reference frame transforma-
tions. Being the control system presented in the rotor,
and being s the reference position for the reference
frame where the control is performed, the transformation
angle is the slip position angle defined as:
.
s
rsm



 (8)
The angle

m is the rotor position angle. Because

m
can be measured, or estimated using sensorless methods,
the problem of reference frame determination reduces to
determine

s. The FOC principle states that this should
be the stator flux position angle
s


. Such an angle can
be calculated by a stator flux estimator based on the inte-
gration of the stator electromotive forces.
However in this case it is necessary to drive the stator
flux at 50Hz approximately, because it is not guaranteed
that it will rotate at the required frequency. The first
proposed approach is shown in Figure 4.
The reference angle *
s

is obtained integrating a con-
stant frequency of s; then *
s

is corrected with p ob-
tained with a low pass filter whose input is the difference
 between the actual stator flux position
s


and the
desired stator flux position s. If this angle  is main-
tained small, one can conclude that the system works
approximately in field orientation. The parameters of this
system are the gain ks and the time constant .
Figure 4. System to obtain the slip angle sr.

G. D. MARQUES, M. F. IACCHETTI 105
The complete control scheme is shown in Figure 5:
notice that the set point Ird* of the d-axis rotor current is a
degree of freedom in this scheme, whereas Irq* controls
the torque.
The implementation of the method proposed in Figure
4, is based in using the sine of the angle : in fact, sin
measures the deviation from FOC methodology. The
position errors and speed errors are defined in the block
diagram of Figure 4 as
* (rad) ; (p.u.)
psss ss 
    (9)
The position error p is of no importance, whereas its
derivative s measures the frequency error with respect to
the set point.
4.3. Additional Adjustment of the D-axis Rotor
Current Reference
The second approach uses an additional adjustment of
the d-axis current component with a PI controller whose
input is sin. The block diagram is presented in Figure 6.
5. Simulation Results
In this section some simulation results are presented
for illustrating the behavior of the system controlled with
the method proposed in this paper. The parameters of the
DFIG are reported in the Appendix.
Using the trial and error methodology the values of
ks=1 and =0.5 sec were obtained. The system has shown
to be very robust to the parameters. A large deviation
conducts to a small change in its behavior.
5.1. Using Only the Reference Angle
Determination System
Figures 7 to 9 show the results of the first control ap-
proach.
Figure 5. Control structure using a low pass filter in the
synchronizing loop.
Figure 6. Control structure using a low pass filter in the
synchronizing loop and an additional PI controller to adjust
the d-axis reference rotor current.
Figure 7. Stator and rotor currents.
Figure 8. Torque, stator flux and sin.
The d-axis rotor current component is fixed at the
rated value of the magnetizing current. At the instant t =

G. D. MARQUES, M. F. IACCHETTI
106
50ms a step on the reference q-axis rotor current is im-
posed. The results of Figure 7 show the current wave-
forms, Figure 8 shows the to rq u e Mem, the stator flux and
sin and Figure 9 shows the position and speed errors.
One can s ee that the frequ ency is high er than 50 Hz. Th e
influence of the diodes commutation is clearly shown
leading to a torque waveform with acceptable oscillations.
However there is an error of frequency as shown in Fig-
ure 7-top and Figure 9.
5.2. Using Also the D Rotor Adjustment System.
Introducing the adjustment on the direct rotor current
reference the results improve considerably. A PI with
high bandwidth, similar to the bandwidth of the current
controllers, was use d.
Figure 10 shows that the torque ripple is comparable
with that one of the first approach. Moreover sin0 i.e.
the field orientation is achieved. Also the average fre-
quency error in Figure 11 decreases to zero (i.e. the fre-
quency now is 50 Hz) and the stator flux tends to the
rated flux.
Figure 9. Position and speed errors.
Figure 10. Torque, stator flux and sin.
6. Experimental Results
This section presents some experimental results obtained
in a prototype using a 3.2 kW wound induction machine.
The machine parameters are given in the Appendix. The
control algorithm is implemented in Microchip ds-
PIC30F4011. To obtain experimental results in real time,
four PWM output channels with simple RC filters were
used. The actual rotor position is also measured using an
encoder with a 4096 step resolution. A more detailed
description of this prototype is found in [16]. Because the
rated stator voltage is 3 times higher than the rated volt-
age of the rotor, a step down transformer (380 V to 220
V) was used between the stator and the diode bridge. The
transformer introduces an additional equivalent stator
leakage about 10% of the total leakage of the induction
machine.
The voltage of the dc link was imposed by the dc net-
work of the laboratory that was adjusted to 200 V. This
dc network is obtained with a dc generator rated to 40
kW. Since the rated power of the rotor is smaller than the
stator rated power, a small flux is used for this experi-
mental validation. A value near 0.7 p. u. was chose n.
6.1. Results of the First Approach
Figure 12 shows the waveforms of the d-axis rotor cur-
rent, of the stator voltages and of sin when a step on the
d-axis rotor current reference is imposed. Because this
step is relatively small, the final voltage obtained is
smaller than the voltage necessary to start current on the
diode bridge. The machine is working at no-load. One
can see that the voltage waveforms are almost sinusoidal
as expected.
It is possible to conclude that the voltage response has
good behavior being fast and without undesired tran-
sients.
Figure 11. Position and speed errors.

G. D. MARQUES, M. F. IACCHETTI 107
Figure 13 shows a different transient for a step on the
q-axis rotor current. The system is working with constant
d-axis rotor current reference (0.28 p.u.), and, at t = 50
ms there is a step on the q-axis rotor reference current
from 4% to 80%. It is possible to conclude that the fre-
quency is almost 50 Hz, but sin is not null, showing that
the system is not working in field orientation. To have a
better view of the stator voltage and current waveforms
at steady state, Figure 14 is presented showing a zoom in
time of Figure 13.
From Figure 14 it can be concluded that the wave-
forms have a relatively small harmonic content. It is also
possible to verify that the phase shift between the voltage
and the cu rrent is small (a bout 4 deg), as supposed in the
section III-B.
6.2. Results of the Second Approach
Figure 15 shows a similar transient of Figure 13. In this
case the d-axis rotor reference current is no longer a free
Figure 12. Response to a step on the d-axis rotor current at
no-load.
Figure 13. Response to a step on q-axis rotor reference cur-
rent. The d-axis rotor reference current component is con-
stant (Idr* =0.28 p.u.)
Figure 14. Steady state waveforms.
Figure 15. Response to a step on iq using also the adjust-
ment of the d rotor current reference.
quantity, because it is adjusted using a PI controller. It is
possible to verify that the frequency is 50 Hz as desired,
and that the d-axis rotor current reference is adjusted in
order to obtain a sin near zero. This variable is near zero
before and after the transient.
7. Conclusions
The paper presents a control method for the DFIG con-
nected to a dc link through a diode rectifier on the stator
windings. Simulation and experimental results show that
it is possible to drive the stator flux at the rated frequency
of the machine using a simple controller that simultane-
ously adjusts the phase of the reference frame and the
rotor d-axis current reference. The waveforms of the sta-
tor current are not sinusoidal, because the presence of the
diode bridge, but have ac cept a ble harmonic content.
8. Acknowledgements
This work was supported by national funds through FCT
– Fundação para a Ciência e a Tecnologia, under project

G. D. MARQUES, M. F. IACCHETTI
108
PEst-OE/EEI/LA0021/2011.
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Appendix
Parameters of the 3.2 kW machine
Induction Machine: stator 380 V, 8.1 A, ro tor 110V, 19 A, 3.2 kW, four poles, 1400 rpm, Ls = 1.62 p.u., M = 1.17 p. u.,
rs = 0.06 p.u. rr = 0.05 p.u.