476
IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 57, NO. 2, FEBRUARY 2010
Extending the Life of Gear Box in Wind Generators
by Smoothing Transient Torque With STATCOM
Marta Molinas, Member, IEEE, Jon Are Suul, and Tore Undeland, Fellow, IEEE
Abstract—Gearboxes for wind turbines must ensure high reliability over a period of 20 years, withstanding cumulative and
transient loads. One main challenge to this is represented by
electromagnetic torque transients caused by grid faults and disturbances, which will result in significant stresses and fatigue
of the gearbox. Possibilities for limiting the torque transients in
fixed-speed wind generators have not been previously reported.
This paper presents a technique by which the transient torques
during recovery after a grid fault can be smoothed in a wind farm
with induction generators directly connected to the grid. A modelbased control technique using the quasi-stationary equivalent circuit of the system is suggested for controlling the torque with a
static synchronous compensator (STATCOM). The basis of the
approach consists of controlling the induction generator terminal
voltage by the injection/absorption of reactive current using the
STATCOM. By controlling the terminal voltage as a function of the
generator speed during the recovery process, the electromagnetic
torque of the generator is indirectly controlled, in order to reduce
the drive train mechanical stresses caused by the characteristics of
the induction machine when decelerating through the maximum
torque region. The control concept is shown by time-domain
simulations, where the smoothing effect of the proposed technique
on a wind turbine is seen during the recovery after a three-phaseto-ground-fault condition. The influence of the shaft stiffness in a
multimass drive train model is discussed, and the performance of
the control concept in the case of parallel connection of several
turbines is investigated to discuss the applicability in a wind farm.
Index Terms—Static synchronous compensator (STATCOM),
voltage source converter, wind energy.
I. I NTRODUCTION
A
LLEVIATION OF mechanical stresses in wind generation systems has traditionally been associated with
variable-speed wind turbine topologies by performing direct
control of the electromagnetic torque through power electronics. By partial or complete decoupling of the mechanical transients of the wind turbine from the electrical transients of the
power system, smoothing of the transient torques on the turbine
drive train is thus possible by controlling the electromagnetic
torque directly [1]–[5]. However, in fixed-speed wind generation systems based on induction generators directly connected
to the grid, there has been no obvious way to control the torque
for alleviating mechanical stresses in the drive train. This is
confirmed by the lack of literature on such a possible approach.
Still, many installations today are using this type of generation
Manuscript received December 1, 2008; revised October 15, 2009. First
published November 6, 2009; current version published January 13, 2010.
The authors are with the Department of Electric Power Engineering,
Norwegian University of Science and Technology, 7491 Trondheim, Norway.
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TIE.2009.2035464
system, and more are being planned for the future. Therefore,
having the possibility to control torque transients related to grid
faults could be beneficial for reducing the stresses in the drive
train of fixed-speed generators.
Fixed-speed generators are vulnerable to contingencies such
as faults and disturbances that might appear in the nearby grid
because they are locked to the grid voltage and frequency.
In these cases, the stresses on the generator and drive train
will be determined by the input mechanical torque and the
electrical connection to the grid. However, if, as proposed
in [6], an indirect control of the torque is implemented by
injecting/absorbing reactive current with a static synchronous
compensator (STATCOM), transient torques during recovery
after a grid fault can be smoothed. This way, it could be possible
to extend the life of the drive train by reducing cumulative and
transient loads related to grid faults [7].
Several studies of low-voltage ride through (LVRT) have
confirmed how reactive compensation can increase the torque
capability and, by that, the stability limit of induction generators [8]–[10]. As shown in [11] and [12], this will also increase
the maximum torque that occurs during the recovery process
when standard control strategies are implemented. This paper
investigates the implementation of indirect torque control (ITC)
by a STATCOM to wind turbines in a wind farm. The basic
assumption is to utilize a STATCOM originally installed for
providing LVRT capability [8], [12] for controlling the torque
of the machine only during the recovery process.
The STATCOM-based torque control is implemented as
a model-based control approach and added to the standard
STATCOM control structure as a new mode of operation. This
mode of operation was first presented in [6] and is labeled as
ITC since it is based on using the STATCOM to control the
voltage at the generator terminals and, by that, indirectly controlling the torque during the recovery process after a grid fault.
In [6], the concept of ITC was presented and applied as
a technique for torque transient alleviation in a single wind
turbine unit. In this paper, a more elaborated explanation of
the proposed concept is presented, and the properties of the
ITC are further investigated. Based on the results in [6],
the control concept is analyzed both for critical faults close to
the stability limit of the system and for shorter faults where the
operation of the STATCOM is not needed to ensure stability of
the induction generator. The torque control concept is further
extended to a wind farm composed of paralleled turbines, with
a STATCOM at the terminal of each generator. All STATCOMs
are using the proposed technique in its control structure as an
additional mode of operation that is activated only during the
recovery process after a grid fault. The parallel operation of
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MOLINAS et al.: EXTENDING LIFE OF GEAR BOX IN WIND GENERATORS
477
Fig. 1. Schematic configuration of the wind farm composed of two wind
generation systems in parallel, each with a STATCOM.
the STATCOM has raised new operational issues related to the
reactive power exchange between STATCOMs and the grid,
which are discussed in this paper.
Fig. 2. Block diagram of the control system, including ITC and normal
STATCOM.
II. S TATCOM -BASED T ORQUE C ONTROL C ONCEPT
Fig. 1 shows the schematic configuration of the wind farm
in which the ITC technique is implemented. The wind farm
investigated in this paper is modeled in PSCAD®™ by two
wind generation units in parallel, each with a STATCOM at the
terminals. One wind generation unit consists of an induction
generator driven by a wind turbine through a gear box, converting the low speed of the turbine shaft into a high speed
that matches the rotational speed of the induction generator.
The induction generator is connected to the grid through a
transformer, and a STATCOM is connected at the generator terminals to control the voltage level at the generator by injection
of reactive current [6], [13]–[15]. As reported in [8] and [12],
the STATCOM was originally installed to improve the transient
stability and the critical clearing time of the wind generator and,
by that, to increase the LVRT capability. High levels of reactive
compensation to improve the fault ride-through capability of
the system will, however, increase the maximum torque of the
generator during the recovery process [8], [11].
In order to temporarily avoid the high level of reactive
compensation during recovery, the control structure of the
STATCOM is expanded, as shown in the block diagram of
Fig. 2. The introduction of the ITC block, in addition to the normal STATCOM control, allows for torque transient alleviation
during the recovery process after a grid fault. This is possible
to achieve by reducing the voltage reference of the STATCOM
control system and, by that, the reactive compensation when
stability is ensured after fault clearing but before the grid
voltage and the speed of the generator have returned to the
prefault values. In this way, the STATCOM can improve the
torque capability of the induction generator when this is needed
to keep the system stable, and once stability is ensured, it
can reduce the maximum torque during recovery. The strain
on the drive train can thereby be reduced. This is particularly
relevant in the context of LVRT where wind turbines cannot
just disconnect from the grid to protect the installation from risk
of mechanical damage that might be caused by the cumulative
stress of repeated peak torque transients [7].
Fig. 3. Quasi-stationary equivalent circuit for the system under study, consisting of the traditional induction machine equivalent, the STATCOM modeled as
a current source, and a grid equivalent.
At the clearing instant of a fault and afterward, transients
of the electromagnetic torque will result in significant stresses
for the wind turbine mechanical system and can have harmful
effects on the lifetime of drive-train-sensitive components such
as the gearbox [16]–[18]. Gearbox fatigue is caused by stressing
of the gearbox teeth in response to torque overloads. For an
input torque in excess of the gearbox rating, the fatigue damage
increases in the extent to which the rating is exceeded and
also as the length of the time that the overload persists [19].
In addition to that, lifetime of the gearbox is reported to be
influenced by the load–duration distribution. The accumulated
duration of torque levels significantly influences the fatigue
load on the gearbox and, therefore, its lifetime [7]. Taking this
into account, not only the high transient torques will represent
stresses on the gearbox but also the cumulative torque stresses
under normal operation by adding up to the high transient
torques during recovery after a fault.
The short circuit initial torque transients of the induction
generator are not the target of the proposed ITC concept.
These transients will contribute to the cumulative stresses of
the system but cannot be influenced by the STATCOM when
the grid voltage is close to zero. This paper focuses on the
recovery process after fault clearing and the mitigation of
related transient torques observed in induction machines. A
three-phase grid failure is used as an example to put in evidence
the torque transients that appear after a grid fault is cleared.
However, the recovery process of the induction generator is
almost independent of the type of fault, which will have no significant influence on the performance of the proposed approach.
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478
IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 57, NO. 2, FEBRUARY 2010
III. M ODEL -BASED T ORQUE C ONTROL :
A NALYTICAL D ERIVATION
The equivalent circuit of the system in Fig. 3 is used to
derive the equations that will serve as the analytical basis for the
design of the model-based control. The analytical results will
then be used to generate a speed-dependent reference voltage
for the STATCOM control system under the ITC mode of
operation. For investigation of the concept, the analytical equations are implemented in PSCAD® and used to calculate the
speed-dependent voltage reference that is temporarily allowed
to override the normal voltage reference value when the ITC is
active, as shown in Fig. 2.
The process of deriving and applying the proposed concept
is explained hereafter starting from (1) which gives the relation
between the quasi-stationary electromagnetic torque and the
rotor current of the generator. From this equation, a reference
value for the quasi-stationary rotor current can be obtained as
a function of the slip of the machine, by specifying the value
of the electromagnetic torque. Introducing the rotor current i2
found from (1) into (2) will give the stator current i1 as a
function of the slip of the machine. By using the results from
(1) and (2), the corresponding value of the terminal voltage of
the generator can be found from (3). By using this equation as
a speed-dependent voltage reference value for the STATCOM,
indirect control of the generator torque can be obtained during
the recovery process
τem,i =
i1,i =
r2,i
|i2,i |2
si
r2,i
si + j(x2,i + xm,i )
jxm,i
(1)
i2,i
v 1,i = v STATCOMi,ref
= i1,i (r1,i + req,r,i + j(x1,i + xeq,r,i )) .
(2)
(3)
The core concept of the ITC is to limit the electromagnetic
torque during the recovery process after a grid fault. The
reference value for the torque must be set at a value that
limits the torque transients but, at the same time, ensures stable
deceleration of the generator until the system is recovered from
the influence of the fault. The validity of implementing the
simplified quasi-static model of the induction generator for calculation of critical speed and LVRT capability was discussed in
[12]. When using the same model to develop the ITC concept,
the validity of the quasi-stationary approach can be considered
more reasonable since the electromagnetic torque is controlled
to a value close to the mechanical torque.
As understood from this description, the concept of ITC is
derived from a rather simple set of equations. It is also seen
that this control concept, and the quasi-stationary model on
which it is based, gains its validity by limiting the torque of
the induction generator during the recovery process after a fault.
The control structure sketched in Fig. 2 also shows how the concept can be implemented only by temporarily modifying the reference value for the normal voltage control objective of the
STATCOM. The control is therefore presented as an additional
control feature of a STATCOM intended for voltage control and
improvement of LVRT capability of wind turbines. Although
very simple in the basic concept, there are no previously re-
ported interpretations of the induction machine quasi-stationary
equations for torque control purposes as presented in this paper.
As can be seen from the equations and also from Fig. 2, the
presented ITC concept will be dependent on the speed or the
slip of the induction machine. One drawback of the method
is therefore that practical implementation will require speed
sensors. The other main issue related to this concept is that,
since the decelerating torque of the generator is limited, the
recovery process of the system will be longer than for the
case of normal control of the STATCOM with a fixed voltage
reference value. The longer recovery time will also result in a
longer time with high reactive power flow in the system.
The ITC control concept as presented in this paper is independent of the inner current control structure of the STATCOM.
Therefore, this concept can be applied in combination with
any kind of established current control strategy and modulation techniques [20], [21]. In this paper, voltage-oriented
vector current control and carrier-based pulsewidth modulation
(PWM) with third harmonic injection are applied [22], [23].
For simulating the system in PSCAD®/EMTDC™, an average
model of the PWM is implemented to increase the simulation
speed, since switching transients are not of main importance to
the proposed concept [24].
IV. T ORQUE S MOOTHING C ONCEPT ON
S INGLE G ENERATION U NIT
To illustrate the main response of an induction generator to
a grid fault when the ITC is implemented, simulations of one
wind generation unit with a lumped mass model are carried
out with PSCAD®/EMTDC™. The investigated case is fault
clearing after a three-phase-to-ground fault condition at the
generator terminals (point A) in Fig. 1. The blue solid line and
the red dashed line in Fig. 4(a) show, respectively, the STATCOM current with the normal STATCOM control and the ITC,
for a critical fault that brings the system close to the stability
limit. It can be noticed that, some time after the clearing of
the fault, during the recovery process, the STATCOM current
with ITC goes from injection to absorption of reactive current
to reduce the torque. With the normal STATCOM control, there
will be only capacitive operation of the STATCOM, and the
speed of recovery of the system is the fastest possible. The recovery process with the ITC is longer than that with the normal
STATCOM control, because the decelerating torque is limited.
The corresponding curves in Fig. 4(b) show the difference
between the terminal voltages with the normal STATCOM
control and with ITC. The terminal voltage remains below rated
value for longer time due to the slower deceleration introduced
by the ITC control. The value of the remaining voltage depends
on the system parameters and the reference torque selected
when implementing the ITC. In the simulations presented in
this paper, the torque reference is chosen to be 1.15 pu, which
is very close to the mechanical torque of 1 pu.
In Fig. 5(a), the torque trajectories for both normal
STATCOM operation and for operation with ITC show how the
ITC is limiting the peak torques during the recovery process.
Fig. 5(b) shows the reactive current trajectory of the STATCOM
as a function of the generator speed. It can be seen that, with
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MOLINAS et al.: EXTENDING LIFE OF GEAR BOX IN WIND GENERATORS
479
Fig. 4. Time responses of STATCOM reactive current with normal STATCOM control and ITC, and time responses of terminal voltage with normal STATCOM
control and ITC for two different fault durations.
Fig. 5.
Torque and current trajectories during recovery process with normal STATCOM control and with ITC control for two different fault durations.
the normal STATCOM control, the current is kept around the
maximum value during the fault and the recovery process until
the speed of the system is back to the initial value. With the ITC,
the current during the recovery process is instead a function
of the speed as given by (4), shown at the bottom of the page
[6]. The same equation is obtained in [8] as an estimate of
the required compensation current as a function of mechanical
torque and generator speed.
As expected by the torque reference setting, the maximum
torque amplitude with ITC control is 1.15 pu at the beginning
of the recovery process. The torque is slightly reduced as the
system is getting closer to the rated speed and rated voltage
because of the dynamics of the machine when decelerating as
described in [12]. As a result of the almost constant torque of
the ITC, there is a characteristic linear change of speed of the
generator as can be seen in Fig. 6. With the normal STATCOM
|iSTATCOM |2 +
Fig. 6. Time response of generator speed for two different faults with and
without ITC.
control and for parameters used in this paper, the maximum
torque amplitude that appears during the recovery process is
about 43% above the rated torque. The propagation of these
2 · i2 (req,i2 · req,STATCOM + xeq,i2 · xeq,STATCOM )
(req,i2 · i2 )2 + (xeq,i2 · i2 )2 − |v g |
|iSTATCOM | +
=0
2
2
2
req,STATCOM + xeq,STATCOM
req,STATCOM
+ x2eq,STATCOM
(4)
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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 57, NO. 2, FEBRUARY 2010
torque transients in the drive train is determined by its torsional
characteristics, which is investigated in Section IV with a twomass model.
Figs. 4–6 also show results corresponding to a shorter fault
of 150 ms, which is well within the stability limit of the system.
For this fault, the system will be stable without compensation
by the STATCOM. The black dotted lines in the figures show
the response with normal control of the STATCOM, while the
green dashed curves show the response with the ITC concept.
As shown in Fig. 5(a), the 150-ms fault is cleared at a speed
where the torque capability of the induction generator is close
to the maximum value. Because the remagnetization of the
machine occurs at a speed where the torque capability is high
and, therefore, also at a higher voltage, the flux transients
are contributing more to the torque, and the maximum torque
during the recovery process is even higher than that in the case
of a fault close to the stability limit. In this case, the limiting
effect of the ITC on the torque is increased, since the torque
is brought into the same trajectory as the one described for the
case of the critical fault. As can be seen from both Figs. 4(a) and
5(b), this is achieved by bringing the STATCOM current from
maximum capacitive compensation during the fault to inductive
operation as soon as the fault is cleared. The influence on the
grid voltage is shown in Fig. 4(b), while the generator speed is
shown in Fig. 6.
These results indicate the general capability of the ITC in
limiting the torque during recovery processes after any fault that
is within the stability limit of the system. This is obtained without interfering with the primary objective of the STATCOM for
improving the LVRT capability. In order to further investigate
the effect of the ITC on the electromagnetic torque of the
generator when the torsional torque is taken into consideration,
a two-mass model of the wind generation system is introduced
in the next section [25].
V. R ESPONSE OF R EAL W IND G ENERATION S YSTEM
For application in a real wind farm, there are several factors that need to be taken into account when considering the
implementation of a control technique as the ITC. One of the
main issues will be the influence of the real drive train torsional
behavior of a wind turbine with a large gear ratio. Another issue
will be the control performance in a wind farm, where there
will be several units operating in parallel. This might lead to
interactions between the different units when the ITC control
is in operation, due to the control nature of the ITC that makes
the operation to depend on the effective Thévenin impedance as
seen from each of the units.
A. Influence of Two-Mass Model on ITC
When the wind turbine and the wind generator are modeled as a single-mass lumped model with a combined inertia
constant, stability analysis may give significant error when
compared to a multimass model [26], [27]. The effect of inertia constants, shaft stiffness, self-damping of the individual
masses, and mutual damping of the adjacent masses must be
taken into consideration when investigating the performance
of the ITC in real wind turbine generation systems. A two-
Fig. 7.
Two-mass model of the wind energy generation system.
TABLE I
M ACHINE AND G RID PARAMETERS U SED IN S IMULATIONS
mass drive train model, as shown in Fig. 7, is simulated in
PSCAD®/EMTDC™ with the parameters given in Table I
under a fault at the point of common coupling (PCC) in Fig. 1.
The simulation results shown in Figs. 8 and 9 illustrate
the influence of shaft stiffness and mutual damping on the
performance of the ITC control. In Fig. 8, the time responses
of turbine and generator speeds show how the initial response
after fault clearing is influenced by the shaft stiffness. With
the ITC, it is seen that the generator speed is allowed to
increase again after the initial torsional transient, but then,
the generator goes into the characteristic linear decrease of
speed. The speeds of the generator and the turbine are afterward
reduced simultaneously in a smooth way, and the oscillations
of the turbine speed around the generator speed, as shown in
the case with normal STATCOM control, are avoided. This
is confirmed by the results presented in Fig. 9 which shows
the time responses of the torque on the turbine shaft and the
generator. The effect of the ITC can be seen by the smoothed
torque compared to the normal STATCOM control as a result of
the torque limitation imposed by the ITC controller. This gives
an indication of the reduced torque stresses for the gearbox
compared to the stresses with normal STATCOM control. It
should be noted that the reduced stresses in the turbine shaft
are at the cost of a slower recovery process and lower terminal
voltage after fault clearing. The STATCOM current during the
fault and recovery process is shown in Fig. 10. During the fault,
both cases result in maximum amount of reactive current, but
after the fault clearing instant, the system with ITC responds
to the voltage reference calculated according to the proposed
algorithm, and the STATCOM goes from capacitive to inductive
operation depending on that voltage reference value.
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MOLINAS et al.: EXTENDING LIFE OF GEAR BOX IN WIND GENERATORS
481
results with reduced damping coefficient indicated that the wind
generation system can become oscillatory unstable and the ITC
does not perform as expected if the system has a shaft with
low damping. To improve the performance for low-damping
factors, the two-mass model of the drive train system should be
taken into account when designing the ITC control. This might
require design of a control structure operating with closedloop control and not only in a feedforward manner as the
presented model-based ITC concept. Then, it could be necessary to include feedbacks that would require either additional
measurements or design of suitable observers. More advanced
control methods could also be relevant for adding damping
and suppressing torsional oscillations while still obtaining the
objective of controlling the torque [28]–[33].
B. Operation of ITC in Parallel-Connected
Wind Generation Units
Fig. 8. Time responses of turbine and generator speeds with normal
STATCOM and ITC.
Fig. 9. Time responses of turbine shaft and generator torque with normal
STATCOM and ITC.
Fig. 10. Time responses of STATCOM currents with and without ITC.
The influence of the mutual damping has also been investigated in [6] by reducing the value of the damping coefficient.
As the damping coefficient was reduced, the responses became
more oscillatory as a result of the typical speed oscillations
originated by the torsional shaft of the two-mass model. The
For application in a wind farm, several identical turbines
operating in parallel should be considered. The main impedance
separating the different turbines will, in this case, be given by
the transformer impedances, and then, all turbines will see a
common substation transformer as part of the impedance of the
main grid.
Since most of the internal grid in a wind park is usually
by cable connection, local short circuit faults at the individual
turbines are more unlikely to happen than faults in the main
grid that will influence the entire wind farm by causing severe
voltage drops. Therefore, a situation with two turbines in parallel, as shown in Fig. 1, is simulated, and for simplicity, a grid
fault is applied at the PCC. When the turbines are operating at
the same conditions, they will, in general, behave equally, and
the ITC algorithm in the different turbines will have the same
effect. Therefore, almost exactly the same curves as shown in
Figs. 8–10 result from the simulation of each turbine.
In cases where each turbine in the farm has different operating condition, the ITC controllers of the different turbines
might interact. An example of this is shown in Figs. 11–14,
where the system is simulated for the same fault as before,
but where one turbine is operating with a constant mechanical
torque of 1.0 pu, while the other turbine is operating with
a torque of 0.9 pu. As can be seen from the simulations of
the system with normal STATCOM control, the two turbines
behave quite similarly with the main difference being a lower
maximum speed and lower maximum torque on the generator
with the lowest mechanical torque. In the case with ITC, it
can be seen that the generator and shaft torques are limited
to nearly the same maximum values regardless of the different operating conditions, which is clearly observed by the
shaft torque trajectory in Fig. 13. In Fig. 14, an interesting
behavior of the compensating current is observed. Generator 2
with lower operating torque has a faster recovery process
due to a larger braking torque compared to Generator 1,
since both ITC controllers are designed with the same torque
threshold. This brings as a consequence that Generator 2 ITC
will shift to normal STATCOM control faster than Generator 1
ITC. As a result of this, there is a transfer of reactive current
between the two STATCOMS. This is partly seen in the upper
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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 57, NO. 2, FEBRUARY 2010
Fig. 13. Torque trajectories of each turbine in the farm with and without the
ITC for two different operating torque conditions.
Fig. 11. Time responses of turbine and generator speeds with normal
STATCOM and ITC with two turbines in parallel at two different operating
torque conditions.
Fig. 12. Time responses of turbine and generator torques with normal
STATCOM and ITC with two turbines in parallel operating at two different
torque conditions.
plot of Fig. 14 when the STATCOM at Generator 2 runs into
the capacitive limit while the STATCOM at Generator 1 runs
into the inductive current limit. Since the ITC depends on
the equivalent grid impedance and the voltage drop caused by
the flow of reactive power, this problem cannot be completely
avoided and will be most significant when the impedance
between two units is small. The influence of this problem can
however be reduced if each ITC could be designed with torque
thresholds that are sensitive to their actual operating conditions.
If the torque reference of the different units is selected such
that the decelerating torque is the same, this will reduce the
difference in recovery time and therefore limit the period when
the controllers run into saturation.
The current that will be injected/absorbed by the STATCOM
under the ITC operation mode is given by (4), and as shown
from this equation, it will depend upon the parameters of the
Fig. 14. STATCOM current and reactive power profile at the PCC bus with
and without the ITC when two turbines operate in parallel at two different
torque conditions.
equivalent Thévenin impedance of the rest of the farm and
the grid combined, and the corresponding Thévenin-equivalent
grid voltage v g . This equation determines, at the same time,
the required rating of the STATCOM to operate under the ITC
modality.
The reactive current interchange at the PCC bus is shown
in the lower plot in Fig. 14. This interchange will depend
on the voltage reference set by the ITC and according to the
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MOLINAS et al.: EXTENDING LIFE OF GEAR BOX IN WIND GENERATORS
Fig. 15. Generator terminal voltages, PCC voltage, and grid power with and
without ITC for two turbines operating in parallel at two different torque
conditions.
impedance values of the Thévenin equivalent circuit of the
wind farm and grid combined. The resulting influence on
the voltage at the generator terminals can be observed from
the upper curves of Fig. 15. In the same figure, the overall
effect of the ITC on the PCC voltage is shown, indicating a
remaining voltage well within today’s grid code requirements.
The last plot in Fig. 15 shows the power flow into the grid for
both the normal STATCOM and the ITC. It is interesting to
note that, with the ITC, the power flow to the grid could be
kept under control regardless of the direct connection of the
generator to the grid. This is a feature that can be considered a
semidecoupling effect that is similar to the one obtained with a
converter-controlled grid interface.
The results observed in the last figures can also be relevant
when interconnecting wind farms with different sizes. The
possible interaction between units with ITC control and nearby
wind farms or other units with voltage regulation will also
require similar considerations.
VI. C ONCLUSION
A technique has been developed to alleviate torque transients
in fixed-speed wind generation systems with asynchronous
generators during the recovery process after a grid failure.
The main objective of the proposed controller is to reduce
stresses in the gearbox and other mechanical components of
the drive train. The new proposed technique labeled as ITC
483
has been first implemented by extending the standard control
capabilities of a STATCOM compensating a single generation
unit. Simulation results indicate how the STATCOM can allow
for the implementation of such a control strategy to reduce the
mechanical stresses on the drive train of a wind turbine during
recovery after a fault, but at the expense of a reduced voltage
level and a longer recovery time. Results from a critical fault
and a generic shorter fault indicate the general applicability of
the technique to adapt the STATCOM control to the needs of
the system during recovery after any kind of contingencies.
The ITC technique is additionally implemented and verified
by simulating a two-mass model and two turbines operating
in parallel for investigating the performance of the controller
in a wind farm. The limitations of the presented concept in
the case of shafts with low damping have been commented
and indicated the need for further work on more sophisticated
considerations in such conditions. Simulation results from operation of the ITC on two turbines in parallel indicate that some
interaction between the two STATCOM control systems and
the grid can occur if the different turbines are operating with
different mechanical torques. This might result in unintended
interchange of reactive current among units in the case of nonidentical operating conditions, and saturation of the STATCOM
current references might make the ITC incapable of limiting
the torque to the specified value. This problem will, however,
not compromise the stability of the generator and only lead to a
maximum torque that can be slightly higher than specified.
As one of the drawbacks of the ITC could be the slow
recovery process with low remaining voltage, a tradeoff between minimum required voltage level in the grid allowed flow
of reactive power depending on grid code requirements, and
smoothed transient torque should be attempted when implementing the ITC control in practice. Influence of low damping
in the mechanical drive train, controller parameter sensitivity,
and dependence on speed measurement are other factors which
will affect the performance in practical implementation of the
technique. In a further step, an experimental validation of the
technique in a reduced-scale laboratory model should be carried
out in order to investigate and propose a practical solution for
these.
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Marta Molinas (M’94) received the Diploma in
electro-mechanical engineering from the National
University of Asuncion, Asuncion, Paraguay, in
1992, the M.Sc. degree from Ryukyu University,
Okinawa, Japan, in 1997, and the Dr. Eng. degree
from Tokyo Institute of Technology, Tokyo, Japan,
in 2000.
In 1998, she was with the University of Padua,
Padua, Italy, as a Guest Researcher. From 2004 to
2007, she was a Postdoctoral Researcher with the
Norwegian University of Science and Technology,
Trondheim, Norway, where she is currently a Professor with the Department
of Electric Power Engineering. From 2008 to 2009, she was a JSPS Research
Fellow with the Energy Technology Research Institute, National Institute of
Advanced Industrial Science and Technology, Tsukuba, Japan. Her research
interests include wind/wave energy conversion systems, and power electronics
and electrical machines in distributed energy systems.
Prof. Molinas is actively engaged as a Reviewer for IEEE T RANSACTIONS
ON I NDUSTRIAL E LECTRONICS and IEEE T RANSACTIONS ON P OWER
E LECTRONICS. She is an AdCom member of the IEEE Power Electronics
Society.
Jon Are Suul received the M.Sc. degree from the
Norwegian University of Science and Technology,
Trondheim, Norway, in 2006, where he is currently
working toward the Ph.D. degree in the Department
of Electric Power Engineering.
In 2008, he was a guest Ph.D. student with the
Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan. From 2006 to 2007, he was
with Sintef Energy Research, working on simulation
of power electronic systems. His research interests
include control of power electronics converters in power systems and for
renewable energy applications.
Tore Undeland (M’86–SM’92–F’00) received the
M.Sc. and Ph.D. degrees from the Norwegian University of Science and Technology, in 1970 and 1977,
respectively.
He has been a Full Professor with the Department
of Electric Power Engineering, Norwegian University of Science and Technology, Trondheim, Norway,
since 1984, an Adjunct Professor with Chalmers
University of Technology, Göteborg, Sweden, since
2000, and a Scientific Advisor to Sintef Energy
Research. He is the coauthor of the well-known book
Power Electronics: Converters, Applications, and Design. His research interests
are in the areas of power electronics and wind energy systems.
Prof. Undeland is the President of the European Power Electronics society
and a member of the Norwegian Academy of Technological Sciences.
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