Torque Transient Alleviation in Fixed Speed Wind Generators by Indirect Torque Control with STATCOM Marta Molinas, Jon Are Suul and Tore Undeland Norwegian University of Science and Technology Department of Electrical Power Engineering, Trondheim, Norway,
[email protected] Abstract— Gearboxes for wind turbines must ensure high reliability over a period of 20 years withstanding cumulative and transient loads. Grid disturbances affecting wind turbines with induction generators directly connected to the grid generates electromagnetic torque transients that will result in significant stresses and fatigue of the gearbox. This paper presents a technique by which the transient torques during recovery after a grid fault can be smoothed for fixed speed wind turbines with induction generators directly connected to the grid. The technique labeled Indirect Torque Control (ITC) is suggested 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 basis of the approach consists of controlling the induction generator terminal voltage by the injection/absorption of reactive current using a STATCOM. By controlling the terminal voltage with the STATCOM, the electromagnetic torque of the generator is indirectly smoothed. The control concept is shown by simulations on a wind generation system model built in PSCAD, where the smoothing effect of the proposed technique is seen during the recovery after a three phase to ground fault condition. The influence of shaft stiffness and mutual damping on the proposed control is further investigated on a two mass model of the wind generation system. Keywords—Wind energy, Static Synchronous Compensator STATCOM; Voltage Source Converter
I. INTRODUCTION During the last decades, extensive research has been focused on operation of variable speed wind turbines. In addition to the possibility for increased energy capture, one of the main motivations for investigating variable speed wind turbine topologies has been to partly or completely decouple the mechanical transients of the wind turbine from the electrical transients of the power system. One clear benefit from variable speed operation is the possibility to control the electromagnetic torque directly and by that being able to smooth mechanical stresses on the wind turbine drive train. Since variable speed wind generators has been the subject of research for several years, extensive literature can be found on control and operation of different configurations and power electronics interfaces [1], [2]. However, induction generators directly connected to the grid are still used in
many modern wind turbine installations today. By being locked to the grid voltage and frequency, operating at approximately fixed speeds, they are vulnerable to contingencies such as faults and disturbances that might appear in the nearby grid. In the case of induction generators directly connected to the grid, the electromechanical torque can not be directly controlled by the stator currents, and the stress on the drive train will be determined by the input mechanical torque and the electrical connection to the grid. However, if a fast acting controllable source of reactive current is implemented at the stator terminals of the directly connected wind generators, indirect control of the electromagnetic torque can be possible by controlling the induction generator terminal voltage with the injection/absorption of reactive power. During recovery after a grid fault the drive train of the turbine can experience high torque caused by the torque-speed characteristics of induction machines, and such condition is the topic of investigation in this paper. The basis for the approach is a standard STATCOM control intended for improving the Low Voltage Ride Through (LVRT) capability of the wind turbine as investigated in [5],[10] and in that context an additional control feature for reducing the transient torque during the voltage recovery is suggested. The additional control feature suggested in this paper is labeled Indirect Torque Control (ITC), since the torque of the induction machine is influenced indirectly by utilizing a STATCOM to modulate the flow of reactive current and by that the voltage on the terminals of the induction machine. Several studies have confirmed how reactive compensation can increase the torque capability and by that the stability limit of induction generators [11],[12], but as shown in [5], [10] this will also increase the maximum torque that occurs during the recovery process. The basic idea of the ITC is therefore to reduce the compensation level of the STATCOM during the recovery process after a fault, after stability has been ensured but before the voltage is completely recovered and the speed has returned to the pre-fault value. Depending on the case, also the inductive region of STATCOM operation can be exploited to reduce the voltage at the machine terminals and limit the torque of the induction generator during the recovery process, on the cost of reactive power flow from the grid.
2349 c 2008 IEEE 978-1-4244-1742-1/08/$25.00
Wind turbine
'&OLQN
Lf 67$7&20
Gear
v a ,b Transformer
PCC
G
Electric Electric Grid Grid
PWM
Clark
vα , β
Cage Induction Generator
STATCOM
II.
INDIRECT TORQUE CONTROL BY USE OF STATCOM Figure 1 shows the schematic configuration of the wind generation system used to investigate the indirect torque control (ITC) [10]. The system consists of an induction generator directly connected to the grid and driven by a wind turbine through a gear box to convert the low speed of the turbine shaft into a high speed that matches the rated 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 [4]. As reported in [5], the STATCOM can be used to improve the transient stability and the critical clearing time of the wind generator and by that increase the LVRT capability. High levels of reactive compensation to improve the fault ridethrough capability of the system will however increase the maximum torque of the generator during the recovery process [3]. In this context, the control system of the STATCOM could be expanded, as indicated in the block diagram of Fig. 2, to include the ITC control in addition to the normal STATCOM control, to allow for torque transient alleviation during the recovery process after a grid fault. This can be achieved by reducing the voltage reference of the STATCOM control system after reclosing and by that the reactive compensation when stability is ensured but before the grid voltage and the speed of the generator has returned to the pre-fault values[10]. In this way the STATCOM can improve the torque capability of the induction generator when this is needed to keep the system stable, but also reduce the maximum torque during recovery once stability has been ensured and by that limit the strain on the drive train. This assumes relevance especially in the context of LVRT where wind turbines cannot just disconnect from the grid to protect the installation from mechanical damage that might be caused by the stresses of repeated peak torque transients.
Clark
iα , β
9ROWDJH 2ULHQWHG Park Park &XUUHQW &RQWURO i id vd q
9HFWRU Park-inv.
Three line to ground fault
Fig. 1. Schematic configuration of the system under study: directly connected induction generator with STATCOM
ia , b
-
* Vdc
ng
v*d
Vdc
Tref
,7&
ITC
PI
+
Vref
Vref = 1 pu
PI Vd
v*q
+
ωL
id *
iq*
Normal STATCOM
Fig. 2. Block diagram of the control system including ITC and normal STATCOM
At the reclosing instant of a fault sequence and afterwards, transients of the electromagnetic torque will result in significant stresses for the wind turbine mechanical system and can have harmful effects on the fatigue life of drive train sensitive components such as the gearbox [6][7][8]. 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 the overload persists [9]. In addition to that, lifetime of the gearbox is reported to be influenced by the load-duration distribution. The influence of the mean wind speed on the lifetime of gearbox is identified and found that the accumulated duration of torque levels significantly influences the fatigue load on the gearbox and therefore its lifetime [13]. 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 are not the target of the proposed ITC control. This paper focuses on the recovery process after breaker re-closing operation for being one of the cases that represents high transient torques for induction machines. A three-phase grid failure is used as example to put in evidence the torque transients that appear at the reclosing. The concept of the Indirect Torque control can be explained on basis of the equivalent circuit of Figure 3 [10]. This assumes the simplifications corresponding to quasi-static operation as discussed in [5], and can be considered reasonable when limiting the torque and giving
Fig. 3. Quasi stationary equivalent circuit for the system under study, consisting of the traditional induction machine equivalent, the STATCOM modelled as a current source and a grid equivalent
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2008 13th International Power Electronics and Motion Control Conference (EPE-PEMC 2008)
3 by fulfillment of the condition of a specific torque based on quasi-static considerations as presented in [10] will determine the required reactive current injection by the STATCOM when implementing the indirect torque control. From this criterion the required reactive current to ensure stability or to limit the torque below a specified level can be derived in the form of curves of required rating of reactive compensation as a function of speed at clearing of the induction generator. The value of this current is influenced by the selected setting of the reference of electromagnetic torque whose necessary condition is to be slightly higher than mechanical torque to ensure stability. The length of the recovery process will be affected by the difference between the mechanical torque and the selected value for Tem.
TABLE I. MACHINE AND GRID PARAMETERS USED IN SIMULATIONS Asynchronous machine Power : 2.26 MVA Generator inertia constant : J=2H = 6 s Self Damping : 0.008 pu
r1 = 0.008553 pu x1 = 0.07365 pu xm = 4.376 pu r2 = 0.01045 pu x2 = 0.1787 pu IJm = 1 pu
Two mass model HWT = 2.5 s HG = 0.5 s Kshaft = 50 Dmutual = 10 Grid
STATCOM Stationary current reference limitation of 1.0 pu reactive current. Vector control with inner current loop. Fault type and duration
xg = 0.1126 pu rg = 0.01126 pu
350 ms three phase to ground
τ em =
as a result slower deceleration of the generator. On this basis, the derivation of the control concept can be understood starting from the equation of the electromagnetic torque as given in (1). This equation can provide the required rotor current i2, from a reference value for the torque, that must be set to a slightly higher value than the mechanical torque (1.15 pu is used as reference in this case), after stability has been ensured and the recovery process has started. From the equivalent circuit of Figure 2 the current i1 can be obtained as given in equation (2). Current i1 in equation (3) will give the voltage v1 at the terminals of the STATCOM that will be used as reference voltage for the control of the STATCOM. In this way it is possible to indirectly control the torque of the induction generator by controlling the voltage at its terminals. As seen by this equation, the control structure of the ITC will require the information about the generator speed for calculation of the reference value. The system control structure is sketched in Fig. 2, where the signals needed for the ITC are indicated and the ITC part is enclosed in the dashed block on the left. The rest of the control structure corresponds to the normal STATCOM control. Equation (4) which is derived from the circuit of Figure
i STATCOM
2
+
(
2 ⋅ i2 req ,i 2 ⋅ req , STATCOM + xeq ,i 2 ⋅ xeq , STATCOM req2 , STATCOM
1
+
2 xeq , STATCOM
2
(1)
r2 + j ( x2 + xm ) i1 = s i2 jxm
(
(2)
(
v1 = i1 r1 + req , r + j x1 + xeq ,r
))
(3)
III. ILLUSTRATION OF CONCEPT Simulations performed with PSCAD/EMTDC on the model in Figure 1 provide an indication and illustration of the effect of the ITC technique on the recovery part of the transient torques for one set of generator and grid parameters as shown in Table I. The contingency investigated is breaker re-closing after a three phase to ground fault at the point of connection of the STATCOM. Figure 4a shows the STATCOM current with the normal STATCOM control and with the ITC control. It can be noticed that some time after the reclosing, 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,
)i
( req ,i 2 ⋅ i2 ) + ( xeq,i 2 ⋅ i2 ) + 2
STATCOM
Normal STATCOM control STATCOM used for ITC
req2 , STATCOM
+
2
− vg
2 xeq , STATCOM
(4)
=0
Normal STATCOM control STATCOM used for ITC
1.2
0.5
1
0.8
0
Voltage [pu]
Current [pu]
r2 i2 s
-0.5
0.6
0.4 -1 0.2
-1.5 -1
0
1
2
3 4 Time [s]
a)
5
6
7
8
0 -1
0
1
2
3 4 Time [s]
5
6
7
8
b)
Fig. 4. Time responses of terminal voltage with normal STATCOM control and ITC control, and time responses of STATCOM reactive current with normal STATCOM control and ITC control
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1.5
1
1.4
Current trajectory with normal STATCOM control Current trajectory with ITC
0.8
1.3 0.6
1.2 1.1
0.4 Inductive region 0.2
0.9
Current [pu]
Torque [pu]
1
0.8 0.7 0.6
0 -0.2 Capacitive region -0.4
0.5 0.4
-0.6
0.3
-0.8
0.2
0
-1
Torque trajectory with normal STATCOM control Torque trajectory with ITC
0.1 1
1.01
1.02
1.03
1.04 Speed [pu]
1.05
1.06
1.07
1.08
1
1.01
1.02
1.03
1.04 Speed [pu]
1.05
1.06
1.07
1.08
a) b) Fig. 5. Torque and current trajectories during recovery process with normal STATCOM control and with ITC 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 with the normal STATCOM control, because the decelerating torque is limited. Fig. 4b shows the difference between the terminal voltages with the normal STATCOM control and the ITC control. The terminal voltage remains below rated value for longer time due to the slower deceleration introduced by the ITC control. The value of the voltage depth 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. In Fig. 5a the torque trajectories for both cases; normal STATCOM and ITC control shows the difference in peak torques during the recovery process. With ITC control as expected by the torque reference setting, the maximum torque amplitude is of 15% at the beginning of the recovery process being reduced as it gets close to the rated speed and rated voltage. This reduction in torque is caused by the dynamics of the machine when decelerating as described in [5]. As a result of the almost constant torque of the ITC, there is a characteristic linear change of speed of the generator. With the normal STATCOM control and for parameters used in this study, the maximum torque amplitude that appears during the recovery process is about 43% above the rated torque. The propagation of these torque transients in the drive train is determined by its torsional characteristics, which is investigated in section IV with a two mass model. Figure 5b also shows the reactive current trajectory of the STATCOM as function of the generator speed. The part of the trajectory that corresponds to the recovery process is the same as the results from solving (4) for iq as a function of speed. Both types of control give maximum reactive current compensation during the fault and immediately after the fault clearing. With the normal control objective of the STATCOM to keep the voltage at the rated value, the compensation current is kept at the maximum value until the speed of the generator is reduced to the pre-fault value and the STATCOM is able to bring the terminal voltage back to the reference value. With the ITC, the compensation is reduced, and the STATCOM current even goes into the inductive region to limit the terminal voltage of the generator, and by that to limit the torque as clearly shown by the torque trajectory.
The torque transients after a grid fault as analysed in this paper, are a source of stresses in the gearbox that will be on top of the stresses during normal operation originated by the variability of wind speeds. The combined effect of these disturbances on the fatigue life of the gearbox needs to be investigated as well as detailed investigation of the performance of the new proposed approach on a multi-mass model for concluding on the validity of the proposed technique. 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 [14]. IV. INFLUENCE OF TWO MASS MODEL WIND GENERATION SYSTEM ON ITC When the wind turbine and the wind generator are modeled as one mass lumped model with a combined inertia constant, stability analysis based on such model may give significant error when compared to a multi-mass model [15],[16]. The effects of inertia constants, shaft stiffness, self-damping of the individual masses, and mutual damping of the adjacent masses, is taken into consideration when investigating the performance of the ITC in real wind turbine generation systems. A two-mass drive train model as indicated in Fig. 6 is built and simulated in PSCAD with the parameters given in Table I. The effects of shaft stiffness and damping factor on the ITC performance is investigated by considering a 3 phase to ground fault at the PCC in Fig. 1. Simulation results shown in Fig. 7 using the parameters indicated in Table I show the influence of stiffness and mutual damping on the performance of the ITC control. Time responses of turbine and generator speed show the
Kshaft
H
D
mutual
WT
ωWT
ωG
Fig. 6. Two mass model of the wind energy generation system
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2008 13th International Power Electronics and Motion Control Conference (EPE-PEMC 2008)
1.1
Normal STATCOM ITC
Genertator speed [pu]
1.08
1 0.9
1.06 0.8
1.04
1 0.98
0
1
2
3
4
5
6
7
8
1.1
10
0.6 0.5 0.4 0.3
Normal STATCOM ITC
1.08 Turbine speed [pu]
9
Voltage [pu]
0.7
1.02
0.2
1.06
0.1
1.04
0
1
2
3
4
5 Time [s]
1.02 1 0.98
0
1
2
3
4
5 Time [s]
6
7
8
9
10
0
1
2
3
4
5
6
7
9
10
ITC
10
0
-0.5
-1
Normal STATCOM ITC
1.5 Shaft torque [pu]
8
9
Normal STATCOM
STATCOM current [pu]
Genertator torque [pu]
0.5
0
8
0.5
1
-0.5
7
1
Normal STATCOM ITC
1.5
6
Fig. 10. Time responses of voltage at the PCC with normal STATCOM and ITC
Fig. 7. Time responses of turbine and generator speed with normal STATCOM and ITC
-1.5
1
0
1
2
3
4
5 Time [s]
6
7
8
9
10
0.5
Fig. 11. Time responses of STATCOM currents with and without ITC 0 -0.5
0
1
2
3
4
5 Time [s]
6
7
8
9
10
Fig. 8. Time responses of turbine shaft and generator torque with normal STATCOM and ITC
characteristic linear speed of the ITC after reclosing. Figure 8 shows the time responses of the turbine shaft and generator torques. 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 1.6 Normal STATCOM ITC 1.4
1.2 Shaft torque [pu]
Normal STATCOM ITC 0
1
0.8
0.6
0.4
0.2 0.98
1
1.02
1.04 1.06 Turbine speed [pu]
1.08
1.1
1.12
Fig. 9. Turbine shaft torque trajectory with normal STATCOM and ITC
imposed by the ITC controller. Figure 9 shows the turbine shaft torque trajectory with reduced torsional torque as a result of the ITC control. This gives an indication on the reduced torque stresses for the gearbox compared to the stresses with normal STATCOM control. The voltage at the PCC is shown in Fig. 10 with the characteristic reduced voltage with ITC control and slower recovery process compared to the normal STATCOM control. It should be noted that the reduced stresses in the turbine shaft is at the price of a slower recovery process and lower terminal voltage after reclosing. The STATCOM current during the fault and recovery process is shown in Fig. 11. During the fault, both controllers provide the maximum amount of reactive current but at the reclosing instant the current from the ITC control responds to the voltage calculated according to the algorithm proposed in this paper, and the STATCOM goes from capacitive to inductive operation depending on the voltage reference value. The critical mutual damping that makes the system to be on the stability limit with the ITC control is investigated and the simulation results corresponding to that case are shown in Fig. 12. Time responses of generator and turbine speeds are shown, where the ITC influence is seen by the oscillations compared to the normal STATCOM control. Reducing the value of the mutual damping below Dmutual = 5 will make the system to become unstable. Figure 13 shows the time responses of generator and turbine shaft torques in the stability limit for
2008 13th International Power Electronics and Motion Control Conference (EPE-PEMC 2008)
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Genertator speed [pu]
1.15
1
Normal STATCOM ITC
0.9
1.1 0.8
1.05
1 0
1
2
3
4
5
6
7
8
1.1
10
Normal STATCOM ITC
1.08 Turbine speed [pu]
9
Voltage [pu]
0.7 0.6 0.5 0.4
1.06
0.3
1.04 0.2
1.02 0.1
1 0.98
0
0
1
2
3
4
5 Time [s]
6
7
8
9
10
Normal STATCOM ITC 0
1
2
3
4
5 Time [s]
6
7
8
9
10
Fig. 15. Time responses of PCC voltage with and without ITC
Fig. 12. Time responses of generator and turbine speeds with and without ITC Normal STATCOM ITC
1
Normal STATCOM ITC
0.5 1 0.5 0 -0.5
0
1
2
3
4
5
6
7
9
10
-0.5
-1
1 0.5
-1.5
0 -0.5
0
1
2
3
4
5 Time [s]
6
7
8
9
10
Fig. 13. Time responses of turbine shaft and generator torque with normal STATCOM and ITC control
normal STATCOM control and ITC showing the tendency to oscillatory instability with the ITC. Figure 14 shows the shaft torque trajectory with a clearly reduced shaft torque with ITC but with considerable more oscillations compared to the normal STATCOM control. Figures 15 1.8
1.6
1.4
1.2 Shaft torque [pu]
0
Normal STATCOM ITC
1.5 Shaft torque [pu]
8
STATCOM current [pu]
Genertator torque [pu]
1.5
1
0.8
0.6
0
1
2
3
4
5 Time [s]
6
7
8
9
10
Fig. 16. Time responses of STATCOM currents with and without ITC
and 16 show the time responses of the PCC voltage and STATCOM current with and without the ITC control. The responses become more oscillatory as a result of the typical speed oscillations originated by the soft shaft of the two mass model and the recovery process lasts until the current oscillations are well damped. The results with reduced damping factor indicate that the wind generation system can become oscillatory unstable and the ITC does not perform as expected due to the soft shaft and little damping [17]-[19]. For having a realistic indication of the validity of the ITC control, parameters of mutual damping of wind turbines in the rating range of interest need to be tested by simulations. To improve performance for low damping factors, the equations of the two mass model should be included in the derivations of the ITC control logic and damping control could be included in the control structure of the ITC to suppress the vibration caused by mechanical resonance [18].
0.4
V. CONCLUSIONS
0.2 Normal STATCOM ITC
0 0.98
1
1.02 1.04 Turbine speed [pu]
1.06
1.08
1.1
Fig. 14. Turbine shaft torque trajectory with normal STATCOM and ITC control
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Torque transients developed in asynchronous generators during the recovery process after a grid failure has been investigated for a fixed speed wind generation system. These torque transients represent stresses for the gearbox and other mechanical components. To alleviate
2008 13th International Power Electronics and Motion Control Conference (EPE-PEMC 2008)
these torque transients an indirect torque control technique has been proposed and implemented by extending the control capabilities of a STATCOM. Simulation results indicate how the STATCOM can allow for the implementation of such 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. The terminal voltage of the generator is reduced by limiting the reactive compensation from the STATCOM. Simulation results on a one mass model provides a qualitative indication of the effect that torque transients would have on the gearbox, at the same time of providing a good proof of the effectiveness of the ITC in alleviating such torque transients. Simulation results on a two mass model system give an indication of the limitations of the ITC in a real system when the controller does not take into account the complete model of the system. The effect of mutual damping on the performance of the ITC is indicated by the oscillatory behavior that is typical of a two mass model system. Low damping factor will affect the proposed control in a negative way, requiring more sophisticated considerations for attaining a good performance in such conditions. Further work is mandatory for designing a controller that can take into account the two mass model into the controller structure. Controller parameter sensitivity and dependency on speed measurement are the other two factors which should be taken into consideration for further improving the performance of the ITC. A trade-off 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.
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
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[2]
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[7]
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F.D. Kanellos, S.A. Papathanassiou, N.D. Hatziargyriou, “Dynamic Analysis of a Variable Speed Wind Turbine equipped with a Voltage Source AC/DC/AC Converter Interface and a Reactive Current Control loop,” in 10th Mediterranean Electrotechnical Conference, vol. 3, pp. 986-989, 2000 S.A. Papathanassiou, M.P. Papadopoulos, “Dynamic Behavior of Variable Speed Wind Turbines under Stochastic Wind,” in IEEE Transactions on Energy Conversion, vol. 14, No. 4, pp. 16171623, Dec. 1999 M. Molinas, J. A. Suul, T. Undeland, “Wind farms with increased transient stability margin provided by a STATCOM”, in Proc. CES/IEEE 5th International Power Electronics and Motion Control Conference, IPEMC2006, 13-16 Aug. 2006, vol. 1, pp. 63-69 N.G.Hingorani, L.Gyugyi, Understanding FACTS: Concepts and Technology of Flexible AC Transmission Systems, Piscataway, NJ: IEEE Press, 2000, Chap. 5 M. Molinas, J.A. Suul, T. Undeland, “A Simple Method for analytical Evaluation of LVRT in Wind Energy for Induction Generators with STATCOM or SVC,” in Proceedings of the 12th European Power Electronics Conference EPE 2007, Aalborg, Denmark, September 2007 M. Papadopoulus, P. Malatestas, J. Tegopulos, “Stresses of self excited induction generators during abnormal supply conditions,” in Proc. of International Conference on Electrical Machines ICEM 1992, vol 3, pp. 1072-1076, 1992 J. Faiz, M. Ghaneei, A. Keyhani, “Performance analysis of fast reclosing transients in induction motors,” in IEEE Trans. on Energy Conversion, vol. 14, no. 1, pp. 101-107, March 1999. S. Papathanassiou, M. Papadopoulos, “Mechanical Stresses in Fixed-Speed Wind Turbines due to Network Disturbances,” in
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