On the Emulation of an Isolated Wind Energy Conversion System: Experimental
Results
Roberto Galindo-del-Valle
Maria Cotorogea
* Universidad Tecnológica de Altamira, Blvd. de los
Ríos km 3+100, Puerto Industrial de Altamira.
*Instituto Tecnológico de Ciudad Madero, Ave. 1º. de
Mayo S/N, Cd. Madero, Tamaulipas, México.
E-mail: rogalva@hotmail.com
Infineon Technologies AG
Am Campeon 1-12
85579 Neubiberg
Germany.
E-mail: Maria.Cotorogea@infineon.com
Balduino Rabelo, Wilfried Hofmann
Technische Universität Dresden, Fakultät Elektrotechnik, Elektrotechnisches Institut, Professur Elektrische Maschinen und
Antriebe, Helmholtzstr. 9, Görgesbau, Raum 206/207, 01069 Dresden, Deutschland.
Abstract—This paper presents the emulation of an isolated wind
energy conversion system, which is composed by a doubly-fed
induction generator, a back-to-back converter connected to its
rotor, a LC filter to minimize the harmonic pollution in the
generated voltage and an isolated three-phase load. In first
instance, the test bench is described and its operational
capabilities are introduced. Afterwards, the control system
design is presented. Next, some associated experimental results
are shown as well. A special mention must be made to an
experimental study which considers the possibility of using the
self-excitation of the doubly fed induction generator to achieve
the black-start of the isolated wind energy conversion system.
Keywords- Induction generators; variable speed drives; wind
power generation; voltage control
I.
INTRODUCTION
Nowadays, there exist increasingly research efforts
focused on Wind Energy Conversion Systems (WECSs),
since wind power is one of the more feasible renewable
energies [1]. Currently, it is possible to obtain lower energy
costs than the corresponding to the best fossil fuel generation
technology in sites with good wind resources [2].
With regard to the operating speed range, WECSs can be
classified as constant-speed and variable-speed. Even more,
since variable-speed WECSs must use an electronic power
converter, they can be classified in full-power-handling
WECS (also called direct-in-line) and partial-power-handling
WECS, considering both the converter placement and ratings.
In a full-power-handling WECS, the power converter is in
series with the induction or synchronous generator, in order to
transform the variable amplitude/frequency produced voltages
into constant amplitude/frequency ones, and it must be able to
process the whole generated power. In a partial-powerhandling WECS, the converter to control the electrical
machine is in a secondary generator circuit, and it only
processes a portion of the total generated power (e.g. slip
power), which constitutes an advantage in terms of reduced
cost of the converter and increased efficiency of the system
[3].
This paper is focused on a partial power handling standalone WECS based on a doubly-fed induction generator
(DFIG).
Variable speed WECSs supplying an isolated load have
already been considered by other researchers. In [4], a stator
voltage direct control is proposed using PI regulators. It
offers a good dynamic performance, but is load dependent,
which causes some practical difficulties. In [5], a system,
where the rotor is fed from a battery through a PWM current
source inverter, is presented. Additionally, regulation of the
rms generated voltages is proposed, which results in
considerable voltage errors and load dependency. In [6]-[7],
several PI-based indirect stator voltage vector control
approaches are presented for a WECS in which a back-toback (B2B) converter is used to manage the power
interchange between the stator and rotor circuits. The
proposed control strategies produce good dynamic
performance with only slight load dependency.
In fact, the work presented here is based on the isolated
system treated in [6]-[7]. However, some modifications have
been made, which include the addition of a LC filter between
the B2B converter and the DFIG stator, the use of a PhaseLocked-Loop to obtain the stator voltage amplitude and
phase, and the utilization of a PI controller to assure the right
orientation of the DFIG (stator-flux-oriented) control system.
Moreover, in this study there are several items which are not
treated in the mentioned papers, like the start-up and turn-off
procedures and the analysis of the black-start capabilities by
using the self-excitation of the DFIG. To present the work,
this paper is organized as follows. In Section 2, the
considered system is described, and the associated test bench
is presented. In Section 3, the controller design is treated. In
Section 4, experimental results related with several tests are
shown and commented. Finally, in Section 5, the conclusions
are given.
II.
SYSTEM AND TEST-BENCH DESCRIPTION
The considered WECS is depicted in Fig. 1. In this
system, energy is collected by a wind turbine (WT) and
transferred to a DFIG by a drive train (DT). The DT also
increases the rotational speed by using a gear box (GB). The
DFIG supplies an isolated load (both main and auxiliary). The
system operation is controlled by the B2B converter,
composed by two VSI-PWMs: the front end (FEC) and the
machine side (MSC) converters.
The FEC manages the power flow between rotor and
stator circuits by a cascaded control system, in which the
inner loop controls the converter currents and the outer one
regulates the dc-link voltage [6]-[7]. In a similar fashion, the
MSC is used for regulating the DFIG stator voltage by means
of a cascaded control system, whose inner loop controls the
rotor currents, whilst the outer one indirectly regulates the
stator voltage amplitude. The LC filter is used to damp the
switching-generated high frequency harmonics.
The test bench was initially built to emulate a grid
connected system and presented in great detail in [8]. In order
to emulate the previously described isolated system a
modification was proposed. Fig. 2 shows the schematic
diagram of the test bench after this modification. Now it has
three operational modes: two of them are grid-connected and
the remaining one is isolated. It can be seen in Fig. 2 that if
the points I and II are connected to B and D, respectively,
then the system is grid connected with both the stator and the
rotor circuits tied to the secondary of the input transformer.
On the other hand, if the points I and II are connected to A
and D, respectively, then the system is grid connected again,
but now it has the stator directly connected to the grid, whilst
the rotor circuit (through the B2B) is connected to the
secondary of the transformer. Finally, the isolated system
operating mode is achieved by connecting the points I and II
to B and C, respectively.
In the experimental rig, the DFIG has a nominal power of
4 kW, and the B2B is a SEMISTACK-IGBT by SEMIKRON.
The pre-charger is an uncontrolled three-phase rectifier which
is manually operated. The capacitor bank in the LC filter is
able to furnish the entire magnetizing power to the DFIG, via
the stator. The WT is emulated by a 7.5 kW dc machine fed
by a 3-phase thyristor controlled bridge rectifier with a
separated field winding fed by a 1-phase controlled rectifier.
This drive can operate in four quadrants and also in field
weakening. At this stage, only a main three phase Yconnected resistive load is considered.
III.
CONTROL SYSTEM DESIGN
The controllers design was carried out by using the same
models and assumptions considered in [7] and [9]. The
system parameters are given in TABLE I. The root locus
method was used for tuning all the controllers. TABLE II
presents the gains obtained for each system.
For the sake of brevity, detailed mathematical treatment is
omitted, which can be found in [9]. Only the most important
considerations are listed.
1. For the DFIG control system, it has been used the well
known dq model of the induction machine.
2. The former control system considers a synchronous
reference frame aligned with the stator flux (s), whose
components are obtained by using (1).
TABLE II.
PARAMETERS USED IN THE CONTROLLERS DESIGN
Value
1.54
0.9
139e-3 H
0.4
1.4
6600 F
Parameter
rs
r’r
M
r1
r2
C0 (dc-link)
Parameter
Value
148e-3 H
141e-3 H
2.4
20e-3 H
8e-3 H
69F
Ls
L’r
nsr
L1
L2
Cf (LC Filter)
where: nsr is the stator to rotor turns ratio.
Auxiliary
Load
Main Load
LC Filter
Stator
High Speed
Side
Low Speed
Side
DFIG
GB
Rotor
WT
Back-to-back Converter
Legend
WT = Wind Turbine.
GB= Gear Box.
DFIG = Doubly Fed Induction Generator.
MSC= Machine Side Converter.
FEC= Front End Converter.
MSC
FEC
Controller
1
Controller
2
Figure 1. Wind Energy Conversion System (WECS) to supply an isolated
load.
Electrical Grid
Transformer
A
D
I
Stator
II
B
C
Isolated
Load
DFIG
Rotor
DC
Motor
PreCharger
LC Filter
L1
r1
r2
MSC
L2
FEC
Cf
Back-to-back
converter
Figure 2. Schematic diagram of the test bench.
s v s rs i s dt
(1)
where: v• s, i• s, and r• s are the stator voltage, current and
resistance, respectively, whilst = or .
3. The stator voltage is regulated indirectly by controlling
the magnetizing current. It is assumed that the stator
resistance voltage drop is negligible.
4. The q component of the rotor current is used to force
the reference frame orientation. Initially, the approach
used by Peña et al. in [6]-[7] was used. It consists in
calculating the proper set-point for i'rq by using (2),
which ensures that sq=0.
TABLE II.
Controlled variable
GAINS OF THE PI CONTROLLERS OBTAINED BY USING THE ROOT LOCUS METHOD
Requirements
DFIG rotor currents (ird, irq)
1.Damping =0.7071
Manipulated variable
Kp
Ki
MSC voltages (vrd, vrq)
38.5
Experimentally
adjusted to: 50
3465
Experimentally
adjusted to: 4500
2. Settling time ts=30 ms
1.Damping =0.7071
Stator voltage (through
magnetizing current: ims)
d-component of the
rotor current (ird)
0.2
8
2. Settling time ts=0.7 s
q-component of the stator
flux-linkages (sq)
q-component of the
rotor current (irq)
40
40
2.Settling time ts=4 s
FEC voltages (vnd, vnq)
2.5
800
d-component of the
FEC current (ind)
0.1
0.5
FEC currents (ind, inq)
1.Damping =0.7071
1.Damping =0.7071
2. Settling time ts=18 ms
1.Damping =0.7071
dc-link voltage (V0)
2. Settling time ts=1 s
L
irq S i sq
M
(2)
where: Ls and M are the stator and magnetizing
inductances, respectively.
However, the parameters uncertainty did not allow
achieving a good performance. In order to cope with
this, model (3) can be used to tune a controller, which is
able to force the stator-flux orientation:
d sq
dt
M rs
irq vsq e sd
LS
(3)
where: e is the synchronous angular speed.
5. By using this approach, the reference frame position
can be forced to be equal to the angle:
e e*dt
where:
e* 2 f e*
(4)
is the desired angular frequency.
This angle is used in all the reference frame
transformations concerning the DFIG control system.
6. The FEC control system is considered in a statorvoltage-oriented reference frame, which implies that a
decoupled control of the active and the reactive power
can be achieved.
7. Originally, for the orientation of the FEC control
system, [6]-[7] propose the use of angle (4) to estimate
the stator voltage vector position, by adding /2 to e.
However, in the laboratory test bench, better results
were obtained when a PLL was used to determine the
stator voltage vector position, since the stator resistance
voltage drop is not negligible in the considered DFIG.
Fig. 3 presents the block diagram corresponding to the
used PLL [10]. In this PLL, the PI controller drives the
stator voltage q-component to zero in order to achieve
the right orientation. So, the PI output constitutes the
angular frequency, which is integrated to obtain the
proper phase angle. Then, this angle is used in the Park
Transformation to determine the actual stator voltage
dq-components. Next, the current value of the qcomponent is used to calculate the new error. A feedforward term (=314.15 rad/s for 50 Hz systems) is
used to improve the PLL tracking capability.
8. The controller of the dc-link voltage produces a balance
between the power which is flowing inwards
(outwards) the FEC and the one that is flowing
outwards (inwards) the MSC in sub-synchronous
(super-synchronous) speed.
IV.
EXPERIMENTAL RESULTS WITH PI CONTROLLERS
The designed controllers were discretized by using the
trapezoidal method, and implemented in a dSpace DS1103
Board with a sampling frequency of 1.5 kHz. The IGBTs’
gate signals were obtained by considering SV-PWM and a
switching frequency of 4050 Hz. The DS1103 contains a
master IBM PowerPC processor running at 1 GHz and a slave
Texas Instruments TMS320F240 DSP running at 20 MHz.
Furthermore, the board has 36 ADC channels (20 with 16-bit
resolution and 16 with 10-bit resolution), and 50-bit for
digital IO purposes. Additionally, the DS1103 can be
programmed by using Real Time Interface (RTI) blocks from
the Simulink environment, which makes easier the
implementation tasks.
v*sq=0
+
PI
controller
-
+
1
s
+
314.15
vsd
Park
Tranformation
vsq
vsa
vsb
vsc
Figure 3. Three Phase Locked Loop.
In the following subsections some experimental results
are presented.
A. Start of the Stator Voltage Controller
The stator voltage controller was tested by using a dc-link
voltage of 330 V, which is maintained by the pre-charger,
whilst the dc-motor was driven the DFIG at 1400 rpm. A
three-phase resistive load of 40 was used. The current
controllers were started with their references set to 0A.
Fig. 4 shows the start of the voltage controller with a ramp
from 0 to 120 Vrms in 5 s. This ramp is initiated at t=1s, whilst
the set-points of the rotor current controllers are switched to
be automatically calculated by the controllers of the stator
voltage and of the q-component of the stator flux linkages
(which forces the right orientation). It can be seen that the d
component of the stator voltage is not zero, because the stator
resistance is not negligible, which produces an average stator
voltage error of approximately 6% in the steady state. In
addition, it can be noticed that both rotor current components
increase with the same pattern as vsq. For the stator voltage to
increase it is necessary to furnish some additional
magnetizing power, which is provided by i'rd. Meanwhile, as
the stator voltage increases, the active power consumed by
the resistive load is also increasing. This produces an
increment in isq which, in accordance with (2), makes i'rq
bigger.
B. Connection of the Complete System
In this test, the stator voltage controller is initially started
as described in the previous section. Then, the FEC current
controllers were enabled with their references set to 0A. Next,
the FEC currents are controlled to produce 120Vrms in the
filter capacitors, in phase with the stator voltage. This is
achieved by setting the ind reference to a small negative value
(to face the copper losses) and the inq reference to –e*· Cf ·
Vs*, which enables the FEC to “consume” the reactive power
associated with the filter capacitors.
Once the steady state has been reached in both
subsystems, they are connected to each other, whilst the precharger is disabled and the dc-link voltage controller is
activated. Figs. 5 and 6 present the results of this experiment.
The connection was carried out around t=1s. It can be seen in
Fig. 5 that after the connection the dc-link voltage drops
because it was disconnected from the pre-charger. In
consequence, a decrement in the generated stator voltage and
some variations in the system currents are produced, as
shown in Fig. 6. In this figure can be observed that the
reactive power associated currents (isd, i'rd, and inq) remain
almost the same and that their variations are produced only by
the transient in the generated voltage.
On the other hand, the active power related currents (isq,
i'rq, and ind) must increase their magnitudes in order for them
to be able to increase the dc-link voltage. In the end, the
steady state value of isq is slightly bigger than the previous
one, because now the stator is supplying not only the load, but
also the FEC. The same can be said of i'rq, whilst ind assumes
a positive value because now it must supply the necessary
active power to the dc-link.
Figure 4. Start of the stator voltage controller.
Figure 5. Transient behavior in stator and dc-link voltages during
connection.
Figure 6. Transient behavior in stator, rotor and FEC currents during
connection.
The phase voltage was 113 Vrms with 2.1% THD before
the connection and 112Vrms with 1.2% THD after it,
representing a regulation error of approximately 6%. This
decrement on the harmonic pollution is caused by the LC
filter and can be observed in Fig. 5. The load current was 2.93
Arms with 6% THD, before the connection, and 2.94 Arms with
9% THD after it.
C. Turn off Procedure
In order to disconnect the system easily, a turn off
procedure was proposed. It consists in an algorithm to
decrease the set-points of the generated stator voltage and of
the dc-link voltage. The slopes of the decreasing ramps were
chosen for the stator voltage to be 0 in 20 s, whilst the dc-link
voltage decreases to 0 in 30 s. Figs. 7 and 8 show the
experimental data obtained. It can be noticed that both stator
and dc-link voltages decrease monotonically until a point is
reached in which the required power to maintain them in their
set-points is not available any more. Then the currents have to
increase to recover the control. However, the dc-link is
discharged by their control efforts.
D. Black-Start Capability
The black-start consists in the system ability to start in a
completely independent way, which is very important in an
isolated system. Therefore, self-excitation (SE) of the DFIG
was considered. SE is a well-known phenomenon that is used
by squirrel-cage induction generators in some WECSs. It
requires the use of a three-phase capacitor bank connected to
the stator terminals, and it consists in a voltage build-up
originated either by the residual magnetic flux in the machine
or by the initial charge in the capacitor bank [11]-[12].
In the considered tests, SE was initiated both by the
sudden connection of the capacitor bank to the stator, whilst
the DFIG was running at a constant speed, or by increasing
the speed whilst the capacitor bank is already connected to
the stator.
In addition, two different ways to initially charge the dclink were verified: (a) using a Y-connected capacitor bank to
start the SE, and connecting the pre-charger (tied to the stator)
once the steady state has been reached, whereas the rotor
windings are short-circuited and the FEC is disconnected; or
(b) using a -connected capacitor bank, whilst the dc-link is
pre-charged via the free-wheeling diodes in the FEC.
Fig. 9 shows some of the experimental results. Stator
voltage orientation is considered. For this test the DFIG speed
was increased from 1300rpm to 1500rpm, with the conditions
described above in option (a). After the start transient, the
stator voltage amplitude is equal to the rated one (310 Vpeak),
and stator currents take the necessary value for the DFIG to
be able to receive the magnetizing power and to furnish the
copper losses. Then, in t=2.5s, the pre-charger is activated,
which constitutes a disturbance for the stator voltage, and
produces a spike in the d-component of the stator current in
order to supply the dc-link. Once the dc-link is charged, the
stator voltages and currents reach the steady state again.
Figure 7. Stator voltage and rotor currents behavior during the turn off.
Figure 8. DC-link voltage and FEC currents behavior during the turn off.
Figure 9. Stator voltage and current during the self-excitation phenomenon
and dc-link pre-charge. Stator voltage orientation is considered.
V.
CONCLUSIONS
In this paper it has been considered the experimental
emulation of an isolated WECS. The related test bench has
been described, and its control system design has been
commented. In addition, experimental results concerning the
start-up and turn-off procedures have been presented. In all
the tests the control system performance has been acceptable,
but it may be additionally improved either by modifying the
PIs tuning or by using another kind of controllers.
The use of the SE to produce the isolated system blackstart has also been explored. It could be used either a Yconnected capacitor bank, plus a pre-charger tied to the DFIG
stator, or a -connected capacitor bank, plus the freewheeling diodes in the FEC, in order to initially feed the dclink. Once the dc-link voltage has been fixed by the SE, the
FEC and MSC could start the switching. However, SE could
be lost. Additional experiments are required to explore this
situation.
ACKNOWLEDGMENTS
The first author whishes to thank the mexican Consejo
Nacional de Ciencia y Tecnología (CONACyT) and the
german Deutscher Akademischer Austausch Dienst (DAAD)
their invaluable economic support.
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