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Engineering, Technology & Applied Science Research Vol. 9, No. 5, 2019, 4783-4788 4783  
  

www.etasr.com Chung: Voltage Enhancement on DFIG Based Wind Farm Terminal During Grid Faults 

 

Voltage Enhancement on DFIG Based Wind Farm 

Terminal During Grid Faults 
 

Phan Dinh Chung  

The University of Danang - University of Science and Technology 
Danang, Vietnam 

pdchung@dut.udn.vn 
 

 

Abstract—This paper proposes a method of retaining high 
enough voltage on the terminal of DFIG wind turbines during a 

fault interval on the connected grid. This method is developed by 

the ideal of dynamic voltage restorer equipment and was applied 

to a wind turbines group. The DC-link and the grid side 

converter of the first DFIG wind turbine in this group are 

utilized to support the other wind turbines. The converter is 
connected to a transformer, which is in series with the wind 

turbine group and the connected line, during an external fault. At 

the DC-link of the back-to-back converter of each DFIG wind 

turbine, a DC chopper is equipped to dissipate excessive active 

power during the fault interval. Moreover, the controllers of all 

wind turbines are modified to support all wind turbines in the 

group. The method is verified by MATLAB/Simulink. Simulation 
results implied that the proposed method can retain a high 

enough voltage on the terminal of DFIG while preventing 
overcurrent on the rotor side. 

Keywords-DFIG; DVR; fault-ride-through capability; voltage 

enhancement; wind turbine 

I. INTRODUCTION  

Wind industry develops rapidly in many countries and 
many wind farms have been deployed successfully. According 
to [1], China owns the world’s highest installed capacity with 
about 211GW (35.74%), followed by USA and Germany with 
about 96GW (16.34%) and 60GW (10.07%) respectively. In 
general, there are three kinds of generators used widely in wind 
turbines. These are: squirrel cage induction generator (SCIG), 
doubly-fed induction generator (DFIG) and permanent 
magnetic synchronous generator (PMSG) [2]. DFIG and 
PMSG wind turbines are more popular as they offer better 
control abilities [3]. However, in case of a voltage sag in the 
grid, the DFIG wind turbine’s response differs from the 
PMSG’s. In PMSG, the generator is isolated from the 
connected grid using a frequency converter and hence, the 
voltage sag does not have serious negative impact on generator 
[4, 5]. In contrast, the stator side of the DFIG wind turbine is 
directly connected to the external grid and hence, a voltage sag 
on the external grid has a serious negative impact on the 
generator, causing overcurrent in the rotor winding [6]. 
Therefore, supporting DFIG wind turbine during a voltage sag 
becomes a critical issue. Many methods have been proposed 
that support DFIG wind turbines during a voltage sag [7-14]. 
Conventionally, a crowbar was suggested to avoid the 

overcurrent in the rotor side [7, 8]. However, when the crowbar 
in the rotor side of DFIG is activated, DFIG becomes an SCIG 
wind turbine consuming a significant amount of reactive power 
from the external grid, reducing the generator’s terminal 
voltage. Another suggestion was the installation of a chopper at 
the DC-link of the back-to-back converter [9], which still 
allows overvoltage on the DC-link and damages the converters 
of the DFIG wind turbine. A combination of crowbar and 
chopper was studied in [10], but the problem of reactive power 
requirement for DFIG has not been solved yet. Series dynamic 
braking resistors (SDBR) were suggested to overcome the 
drawback of using crowbar and chopper, because a high 
enough voltage can remain at the generator terminal [11], but it 
requires two SDBR to be installed at both stator and rotor side. 
Fault current limiter (FCL) [12] was used to limit fault current 
and improve the rotor side converter (RSC) controllability, but 
it increased cost significantly. FACTS devices were also 
introduced to enhance the DFIG’s fault-ride through (FRT) 
ability [13, 14]. The use of a static VAR compensation (SVC) 
or STATCOM to support the DFIG wind turbine’s FRT 
capability could not obtain high efficiency because of low 
voltage at its terminal during an external short-circuit. Dynamic 
voltage restorer (DVR) can eliminate transience in the 
windings of DFIG and restore voltage quickly, and hence the 
back-to-back-converter’s controllability during an external 
fault becomes as good as in normal operation. However, DVR 
is quite expensive. 

In order to overcome the cost of DVR, many improvements 
have been proposed. In [15], DVR was employed but authors 
utilized the DC-link of DFIG wind turbine to replace the DC-
source of DVR, saving investment cost. In [16], authors 
utilized both the DC-link and the grid side converter (GSC) of 
the DFIG wind turbine to operate as a DVR, replacing both the 
DC-source and the converter in DVR. While these 
improvements can save investment cost, the use of these 
configurations in a wind farm brings some disadvantages. 
Firstly, it is quite expensive as DVR is used at each wind 
turbine. Secondly a lot of space is required in each wind turbine 
for the installation of the converter, the series transformer and 
auxiliary equipment, requiring a bigger nacelle and stronger 
foundation. This paper considers the use of a DVR to support 
the FRT capability of a wind turbine group, utilizing the DC 
link and the grid side converter of the first DFIG wind turbine 
in the group to support the others. This converter is connected 

Corresponding author: Phan Dinh Chung  



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to a series transformer between the wind turbine group and the 
connected line. Moreover, a chopper is installed at the DC-link 
of the back-to-back converter of each DFIG wind turbine, to 
consume active power during fault periods. The converter’s 
controller at each DFIG wind turbine is modified to minimize 
active power supply to the series transformer. This idea is 
verified by simulation in MATLAB/Simulink and the 
simulation results are compared to the conventional scheme 
that uses a crowbar and a chopper at each wind turbine. 

II. PROPOSAL OF FRT METHOD FOR WIND TURBINE GROUP 

We suppose that a wind farm consists of several groups of 
wind turbines. Each group consists of 4 DFIG-wind turbines 
and each wind turbine is connected to collector line through a 
step-up transformer. To support the FRT capability of all wind 
turbines in a group during external fault interval, a DVR is used 
as shown in Figure 1. The DC-link and GSC of the first DFIG 
wind turbine replaces the DC-source and the converter of DVR 
respectively. During normal operation, GSC of the first wind 
turbine is connected to the collector line through the low-side 
of the step-up transformer by switching the SW1 switch to pole 
A and closing SW2. When an external fault is detected, the 
SW1 switch is automatically switched to pole B so that the grid 
side converter is connected to the series transformer and the 
SW2 switch is opened. In this case, the DC-link, the GSC of 
the first wind turbine and the series transformer play the role of 
DVR, injecting voltage, so that a high enough voltage can 
remain on the collector line. 

 

 
Fig. 1.  Configuration of a wind turbine group  

III. DFIG WIND TURBINE  

A DFIG wind turbine consists of a variable speed turbine, a 
generator and a gearbox [17, 18]. The main objective of turbine 
is to convert wind power to mechanical power on its shaft. The 
relationship between mechanical power, ��,	and rotor speed, ��, is described as [19]: 

�� � �	
��
����,��
�� ���, (1) 

where 
, �, ��, �, and � are air density, blade’s length, power 
coefficient, tip-speed ratio, and pitch angle respectively. The 
relationship between �, ��, and wind speed �� is: 

� � ����  (2) 
When �  is constant, ����) has a unique maximum point 

with respect to �. This means that �� has a unique maximum 
point. The locus of maximum ��!"  versus ��  is described 
as[20]: 

��!" �#��� (3) 
where # � �	
��

����$�%,��
�$�%�

 and �&�' is the optimal value of �. 
The mechanical power on the shaft is converted to electrical 

power output in the stator and rotor winding of DFIG. In () 
frame, where (	axis is aligned with the stator flux vector, 
DFIG can be described as [20]: 

*�+, � ��-�+, ./+-�01+' .23
45
46 7
�80:.�82/3-�+, (4) 

*�+, � 7;�+;�,:;								-�+, �<
=�+=�,> (5) 

/ � ?� @ AB
C
AD ; 								3� 7

0 @1
1 0 : (6) 

�8 �@4546 �8=�,;    �� �
45
46 2�8=�, (7) 

where, ;,=,�  and 2  are corresponding to voltage, current, 
active power and slip of DFIG, �  and ?  correspond to 
resistance and inductance, G,2,H,( and ) stand for rotor side, 
stator side, multal, (	axis, and ) axis respectively, while �8 is 
the magnitude voltage of stator winding. Motion equation in 
DFIG wind turbine can be described as: 

I
	
+��J
+' � �� @�1@2��8 (8) 

where K is the inertia constant of the DFIG wind turbine. The 
DFIG’s grid side converter is connected to the connected grid 
through a filter, described in the () frame as [21]: 

*L+, ��M-L+, .?M +-N01+' .*O+, .�8?M3-L+, (9) 
*L+, � 7

;L+;L,:; -L+, �<
=L+
=L,>; *O+, � 7

;O+;O,: (10) 
�L � ;L+=L+ .;L,=L, (11) 

where ;,=,� and 2 are corresponding to voltage, current, active 
power and slip of DFIG and P,Q and R stand for filter, grid side 
and converter, respectively. The DC link in the back-to-back 
converter is described by: 



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�
	
+�0SJ
+' � �� @�L (12) 

where, �	is capacitance and �+O is DC voltage on the DC link. 
IV. CONTROLER MODIFICATION OF CONVERTERS 

The ability of active power generation depends on the stator 
voltage and hence if normal voltage can be retained on the 
terminal of the generator during an external fault interval, the 
generator can generate active power as in normal operation. 
However, if a voltage sag happens at the point of common 
coupling (PCC), active power cannot be supplied to the 
connected grid. Hence, the controllers of wind turbines should 
be modified in order to ensure the stable operation of wind 
turbine during and after an external fault. 

A. The First Wind Turbine in the Group 

During normal operation, the wind turbine’s rotor side 
converter adjusts rotor speed to obtain the maximum power 
point tracking of the wind turbine and retain constant voltage at 
the stator winding terminal, while the grid side converter is 
controlled to keep DC voltage at the rated value and ) 
component grid side current at zero. During an external fault 
interval, GSC is connected to the series transformer so as to 
inject voltage into the collector line. Hence, both the RSC 
controller and the GSC controller should be modified. The 
RSC controller aims to control DC and the stator’s side 
voltage, while GSC’s controller injects voltage in order to 
retain the collector line’s voltage at about the pre-fault value. In 
order for RSC to adjust rotor speed or DC voltage [22], =�, can 
be adjusted via ;�,, using (7) and (4). Maintaining a constant 
voltage at the stator winding terminal is carried out by =�+ via ;�+	[23]. The control diagram is shown in Figure 2(a), where 
control blocks including the rotor speed, DC voltage, rotor 
current and PI controls are used as in [22, 23]. In order for GSC 
to maintain a high enough voltage on the collector line, GSC 
should inject voltage into the collector line to compensate the 
error between pre-fault and actual value. The current in the 
collector line can occur with a negative component, and hence, 
GSC should reduce it. The control diagram for GSC is shown 
in Figure 2(b).  

(a) 

 

(b) 

 

Fig. 2.  Controller applying to the first wind turbine: (a) rotor side 

converter, (b) grid side converter 

Reference values are determined by: 

�L,�T�UM � V��!'U+	 @||XY+,,�||	 (13) 
;Y+,,� �Z;Y+,� ;Y,,�[\ (14) 
�L+,,�T�UM � �N,�]�^_||`a01,�|| ;Y+,,� (15) 

bO,cT�UM �Z=O+,�T�UM =O,,�T�UM[\ � 0 (16) 
where dL,�T�UM is the magnitude of the positive component of 
the GSC side voltage, d�!'U+  is the rated voltage in the 
collector line referring to the GSC side voltage, ||XY+,,�|| is the 
magnitude of the positive component voltage in the () frame 
of the connected line (or PCC), referring to the GSC side 

voltage, dL+,,�T�UM is the reference voltage in the () frame of 
the positive component voltage, eO,cT�UM  is the reference 
negative component current in the () frame of the connected 
line, while eO,c  represents the actual negative component 
current in the ()  frame of the connected line. The more 
detailed control diagram applying to GSC of the first DFIG 
wind turbine is shown in Figure 3, where: 

dL+,,� ��?MfeL+,,� .XL+,,� (17) 
∆dL+,,c � @�?MfeL+,,c .XL+,,c (18) 

hY,�,  and hY,c  are phase angles for positive and negative 
component voltage, respectively. 

 
Fig. 3.  Controller applied to the first wind turbine’s grid side converter 

B. Other Wind turbines in the Group 

At normal operation, wind turbines are controlled to 
generate maximum power output and maintain a constant 
voltage at the stator winding terminal. However, when an 
external fault occurs, since the voltage at the collector line is 
retained at a high value by the first wind turbine, other wind 
turbines can generate high active power. This active power 
cannot be transferred to the connected grid, due to the low 
voltage on the connected line, so the wind turbine’s controller 
objective of is to minimize the power output during the external 
fault. Therefore, RSC must control active power instead of the 
rotor speed as in normal operation, and GSC is controlled to 



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receive all active power output in the stator side instead of DC 
voltage control. It is noted that RSC controller’s reference 
power is set at minimum value. 

(a) 

 

(b) 

 

Fig. 4.  Controller for the second to last wind turbine: (a) ) component of 
rotor side converter and (b) ( component of grid side converter 

In order to control the rotor speed or the active power in the 
() frame, the ) component’s rotor current must be adjusted 
via	;�,, as shown in Figure 4(a). It is possible to adjust the DC 
voltage on the DC link or the active power in the grid side 

converter, the (  component grid side current via ;L, , as in 
Figure 4(b). Here, PI control is used for rotor speed control, 
active power, rotor current, and grid side current control, as 
mentioned in [24, 25]. Transferring from normal to abnormal 
operation mode is carried out when an external fault is 
detected. 

C. Chopper 

A chopper is installed in parallel with the DC-link at all 
wind turbines to dissipate all active power output generated by 
DFIG. A chopper and its controller are described in [26].  

V. VERIFICATION 

To verify the proposed model, MATLAB/Simulink was 
utilized to simulate a group of wind turbines with two methods, 
the proposed and the conventional. The conventional method 
which uses a crowbar at the rotor side and a chopper at the DC 
link of each generator is described in [26]. The parameters of a 
1MW DFIG-wind turbine used in simulation are given in [27]. 
It is supposed that an external short-circuit occurs at 2s and is 
cleared at 2.3s on the feeder, as in Figure 1. Simulation results 
are shown in Figures 5 to 9. Figure 5 shows the voltage at the 
terminal of the wind turbines. As it can be seen from Figure 
5(a), by using the proposed method the voltage on the stator 
winding terminal of all generators remains at around normal 
value during the external fault interval, while using the 
conventional method it is reduced to around 30V. Concerning 
the grid side’s voltage of the first wind turbine, as shown in 
Figure 5(b), with the use of the proposed method quite 
significant fluctuations exist at the beginning and at the end of 
the external fault. On the other wind turbines, the grid side 

voltage is retained at the stator winding’s terminal. Therefore 
using the proposed method, the voltage at the terminal of wind 
turbines is better than using the conventional method. Since the 
stator winding terminal voltage is reduced insignificantly at the 
beginning of the fault, the rotor current of all wind turbines at 
this time is smaller than the one obtained by using the 
conventional method, as shown in Figure 6. Therefore, using 
the proposed method, the overcurrent in the rotor side is 
mitigated significantly when the fault occurs. 

 

(a) 

 

(b) 

 

Fig. 5.  Voltage at terminal wind turbine system: (a) stator side of 

generator and (b) grid side converter of the first wind turbine 

(a) 

 

(b) 

 

Fig. 6.  Rotor current: (a) the first wind turbine and (b) other wind turbines 

Normally, during an external fault interval, as wind power 
is almost constant and the active power cannot be distributed to 
the grid, the rotor speed increases. This can be seen in Figure 7 
for both methods. However, using the proposed method, the 
voltage at both the stator and the grid side is retained at a high 
value and the active power generated by DFIG is dissipated on 
the chopper at the DC-link. Therefore, in the first wind turbine, 
the maximum value of the rotor speed during the fault interval 
is still smaller than using the conventional method, as shown in 
Figure 7(a). This benefit can be seen more clearly in the other 
wind turbines (Figure 7(b)). The rotor speed in these wind 
turbines when using the proposed method reaches only 
1820rpm while using the conventional method reaches 
1980rpm. 



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(a) 

 

(b) 

 

Fig. 7.  Rotor speed: (a) the first wind turbine and (b) other wind turbines 

As it can be seen from Figure 8 the active power supply to 
the grid is almost zero during the fault, in both methods, due to 
the low voltage on the connection line. However, in the 
conventional method the wind turbines still receive a small 
reactive power from the grid, while in the proposed method it is 
almost zero. When the external fault is cleared, after a short 
interval, wind turbines can supply stable power to the grid. 

 

(a) 

 

(b) 

 

Fig. 8.  Power output in total: (a) active power and (b) reactive power 

VI. CONCLUSION 

In this research, the DC-link and the grid side converter of 
the first DFIG wind turbine in a wind turbine group is utilized 
to support the others in the case of an external fault. They are 
connected with a series transformer to inject voltage into the 
collector line. Each back-to-back converter of each DFIG wind 
turbine is equipped with a DC-chopper to dissipate the 
excessive active power during the fault period. Moreover, the 
controllers of wind turbines are modified to retain a minimal 
active power generation. Simulation results in 
MATLAB/Simulink indicated that by using the proposed 
method the wind turbine group has better performance than 
using the conventional method. 

 

ACKNOWLEDGMENT 

This work is supported by The University of Danang - 
University of Science and Technology, code number of Project: 
T2019-02-47. 

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