Eman A. Awad1,
Ebrahim A. Badran 1,*,
and Fathi M. H. Youssef
J. Electrical Systems 10-4 (2014): 431-444
1
Regular paper
Mitigation of Temporary
Overvoltages in Weak Grids
Connected to DFIG-based Wind
Farms
JES
Journal of
Electrical
Systems
The level of temporary overvoltage (TOV) has a strong effect on wind turbine power system
reliability. TOV magnitude depends up on the state at which the circuit breaker contacts close. This
paper investigates the mitigation of TOV in weak grids connected to DFIG-based wind farms. Preinsertion resistor (PIR), Controlled switching of circuit breaker, Shunt reactor, and Surge arresters
are applied as protective devices to mitigate TOVs under various operation conditions such as
symmetrical or unsymmetrical switching of circuit breaker. Alternating Transient Program (ATP) is
used in this study to simulate of the investigated system and the elements of mitigation devices.
Keywords: Temporary Overvoltage, Wind Turbine, DFIG, ATP
Article history: Received 13 April 2014, Received in revised form 4 September 2014, Accepted 10 October 2014
1. Introduction
Weak grids are usually found in more remote places where the feeders are long and
operated at a medium voltage level [1]. Isolated wind farms away from the main grids are
considered weak grids because of long feeders [2].
Temporary overvoltages (TOVs) are one of the most frequently encountred problems during
switching procedures in wind turbine power system [3].
TOV has a frequency near to a harmonic of the power frequency, therefore, TOV also
called power frequency overvoltage [4]. TOVs can last for over a hundred cycles and have
slowly decaying amplitude. They can originate from line/ cable or transformer energisation,
faults, load rejection and ferrorresonance [5].
The magnitude and shape of TOVs vary with the system parameters and network
configuration. Even with the same system parameters and network configuration, TOVs are
highly dependent on the characteristics of the circuit breaker operation and the point-on-wave
where the switching operation takes place [3].
Circuit breaker plays the main role in changing the condition of operations. TOV magnitude
depends up on the state at which the circuit breaker contacts close [4]. When a transmission
line/cable is energized by closing the circuit breaker symmetrically, all the circuit breaker poles
attempts to close at the same instant of the voltage wave, TOVs are generated not only on the
transmission system but also in the supply network [6]. On the other hand, TOVs magnitude is
increased by the virtue of the fact that the three poles of the circuit breaker do not close
symmetrically [4]. Unsymmetrically switching as in the case of stack pole on the circuit breaker
can result in ferroresonance. Ferroresonance appear due to the interaction between the feeder
cable capacitance and the magnetic cores of generation unit transformers, particularly when the
Wind Turbine Generators (WTGs) are not connected. Ferroresonance produces voltage several
times normal magnitude on the open phases [5].
TOVs are a critical factor in wind plant insulation coordinates. Proper insulation
* Corresponding author: Ebrahim A. Badran, Electrical Engineering Department, Faculty of
Engineering, Mansoura University, Egypt, email: ebadran@mans.edu.eg; eabadran@ieee.org
1
Electrical Engineering Department, Faculty of Engineering, Mansoura University, Egypt.
Copyright © JES 2014 on-line : journal/esrgroups.org/jes
Eman A. Awad et al. Mitigation of Temporary Overvoltages in Weak Grids...
coordination is important to achieve expected life from wind plant equipment [7]. Pre-insertion
resistor (PIR), controlled switching of circuit breaker, shunt reactor, and surge arresters are
used as protective devices to mitigate TOVs under various operation conditions such as
symmetrical or unsymmetrical switching of circuit breaker [8, 9].
The use of doubly-fed induction generator (DFIG) is receiving an increased attention for
wind generation purposes [10, 11]. DFIG is basically a standard, wound rotor induction
machine with its stator windings directly connected to the grid and its rotor windings connected
to the grid by electronic converters through slip rings [12].
DFIG is one of the most commonly used technologies nowadays, as these offer advantages
such as the decoupled control of active and reactive powers and maximum power tracking.
These capabilities are possible due to the power electronic converters used in this type of
generator [13, 14].
In this paper, TOVs in weak grids connected to DFIG-based wind farms are investigated.
Also, TOV mitigation methods are introduced, applied and compared. ATP is used for
simulating the test power system, the connected DFIG-based wind farm, and the mitigation
devices [15].
2. The Investigated Offshore Wind Farm
2.1 Description of the Wind Farm
The investigated wind farm consists of 72 wind turbines (WTs) with a rated power of 2.3
MW. The turbines are arranged in a parallelogram, formed by eight rows with nine WTs each,
as shown in Fig. 1 [16, 17].
The WTs are connected in rows of 36 kV submarine cables. Each row is then connected to
the platform by one root cable. The park transformer (132/33/33 kV) is in the central position.
Each feeder is connected by a vacuum circuit breaker, followed by the root cable. There are
eight rows, from A to H, where A, B, C and D are connected to one MV winding, and E, F, G,
and H to the other medium voltage (MV) winding [16].
The submarine cable is connected on the bottom of each WT where the armor and the shield
of the cables are grounded. Then the transformer (2.5 MVA, 33/0.69 kV) is connected via
switch disconnector-fuse on the MV. The WTs are interconnected with 36 kV cables of 505 m
of length. Furthermore, the high voltage (HV) grid (132 kV) sea cable (10.5 km) and land cable
(18.3 km) are included in the model [17].
Fig. 1: The Investigated Offshore Wind Farm [17]
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J. Electrical Systems 10-4 (2014) 431-444
2.2 Modeling of the Wind Farm
ATP is used for modeling the components of the investigated offshore wind farm as
illustrated in this section. Fig. 2 shows the ATP model of the investigated offshore wind farm.
In this model, it is shown that:
1. The grid is modeled as a voltage source.
2. The HV, 132 kV, three-phase single-core sea and land cables are modeled as symmetrical,
distributed parameter and lumped resistance models.
3. The wind farm transformers are modeled using HYBRID transformer model.
4. The rows B, C, and D are modeled as one three-phase single-core cables because they are
connected in parallel. Also, the rows E, F, G, and H are modeled in the same way.
5. Only row A is modeled in details.
Fig. 2 shows the nine identical wind turbine transformers (WTTs) in row A. To reduce the
simulation time, the complexity of the wind farm model is reduced by equivalent models. The
wind turbine generators (WTGs) in the wind farm are aggregated into a three separate
equivalent model and the rest of the turbines in row A are modeled as very light loads connect
on the low voltage (LV) side of the WTTs. The light load is modeled as a high resistance of
1200 Ω. Therefore, the nine WTTs of row A are considered as transformers under no-load.
WTGs are considered as operating on an equivalent internal electrical network provided that
the incoming wind velocity is identical or similar on all the WTs [17].
Wind Turbine #A9
XFMR
V
M3
Y
I
Wind Turbine #A8
XFMR
V
XFMR
V
XFMR
V
XFMR
V
XFMR
V
XFMR
V
XFMR
V
Y
I
Wind Turbine #A7
Y
I
Wind Turbine #A6
Y
I
Wind Turbine #A5
Y
I
Wind Turbine #A4
Y
I
Wind Turbine #A3
Y
GROUP
groupdef
I
Wind Turbine #A2
Y
GROUP
groupdef
Root E,F,G,H
LCC
I
XFMR
M2
GROUP
groupdef
Wind Farm
Transformer Land
Cable
XFMR
Submarine
Cable
Wind Turbine #A1
Y
V
M2
LCC V
M1
V
V
Y
LCC
Switching
Point
V
Root B,C,D
Sea
Cable
Wind Turbine
Transformer
Fig. 2: The ATP model of the investigated offshore wind farm
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Eman A. Awad et al. Mitigation of Temporary Overvoltages in Weak Grids...
2.3- Modeling of Cables and Transformers
The MV single-core submarine cables are the most distinctive electric component in the offshore wind farms. The 33 kV three-phase single-core submarine cables connect between the
HV sides of the WTTs. The geometric configuration of the submarine cables which is
calculated based on the power transferred through them [17], and the cable parameters are
given in Table 1.
Table 1: Dimensions of the 33 kV Single-Core submarine Cable
Cross section of conductor [mm]2
Diameter of conductor [mm]
Insulation thickness [mm]
Diameter over insulation [mm]
Cross section of screen [mm]2
Outer diameter of cable [mm]
240.0
18.1
8.0
35.7
35.0
45.0
In submarine cables, the armor is usually quite thick. Therefore, it is assumed in this study
that, an armor of 5 mm steel wires thickness and a 5 mm outer insulation thickness are
incorporated into the three-phase single-core submarine cable design. So, the overall outer
diameter of the medium voltage single-core submarine cable is 65 mm. The MV (33 kV) singlecore submarine cables are modeled using the frequency dependent LCC model [15]. This
model represents the frequency dependence of single-core submarine cable parameters so this
model is much recommended for the electromagnetic transient studies [15].
The sea and land cables are modeled using the three phase Clark model [15] to avoid the
numerical errors due to the long length of the HV cables. The model parameters are calculated
from the cable dimensions and its materials using distributed parameters of three phase
transposed model at 50 Hz. Both dimensions and materials of the HV sea and land cables are
taken from [18]. Table 2 gives the positive and the zero sequence resistances, the inductive
reactance and the capacitive reactance of the 132 kV land and sea cables, respectively.
Both wind farm transformers and off-shore wind farm transformer are modeled using the
HYBIRD transformer model. The wind turbine transformers of row A are modeled using three
phase, two windings, HYBIRD model. The wind farm transformer is modeled as three winding
transformer using the HYBIRD model, which is suitable for low and medium frequency
transient studies [19, 20]. Table 3 gives the data required for transformers modeling.
Table 2: The 132 kV Single Core Cables Data
The 18.3 km/ 132 kV land cable (equivalent PI section model)
+ve sequence resistance
+ve sequence inductive reactance
+ve sequence capacitive reactance
zero sequence resistance
zero sequence inductive reactance
zero sequence capacitive reactance
0.087 [m ohm/m]
0.123 [m ohm/m]
9.092 [M ohm*m]
0.1865 [m ohm/m]
0.0551 [m ohm/m]
9.0915 [M ohm*m]
The 10.5 km / 132 kV sea cable (equivalent PI section model)
+ve sequence resistance
+ve sequence inductive reactance
+ve sequence capacitive reactance
zero sequence resistance
zero sequence inductive reactance
zero sequence capacitive reactance
0.0864 [m ohm/m]
0.123 [m ohm/m]
9.0915 [M ohm*m]
0.1827 [m ohm/m]
0.0619 [m ohm/m]
9.0915 [M ohm*m]
2.4 Modeling of DFIG-based Wind Turbine
A complete wind turbine model is used in this study [21]. The model includes the wind
speed model, the aerodynamic model of the wind turbine, the mechanical model of the
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J. Electrical Systems 10-4 (2014) 431-444
transmission system and models of the electrical components. The induction generator, PWM
(Pulse Width Modulated) voltage source converters, transformers, and the control and
supervisory system are modeled in details [21].
Fig. 3 illustrates the DFIG-based wind turbine model in ATP [21]. This model is used for
accurate simulation of the DFIG-based wind farm. Also, Table 4 summarizes the data used to
parameterize the DFIG.
Table 3: Wind Farm and WT Transformers Data
Wind Turbine Transformers Model
Connection method
Voltage
Rated power
Leakage reactance
Copper losses
No-load losses
Y/∆
0.69/33.0 [kV]
2.5 [MVA]
0.082573 [pu]
0.0084 [pu]
0.0022 [pu]
Wind Farm Transformer Model
Connection method
Voltage
Rated power
Leakage reactance
Copper losses
No-load losses
∆/Y/∆
33.0/132.0/33.0 [kV]
180.0 [MVA]
0.1 [pu]
0.004 [pu]
0.001 [pu]
Table 4: Parameters of the DFIG-based wind turbine
xxx8
xxx9
xxx10
xxx3
xxx45xxx46
xxx11
Turn-off
snubb er
xxx3
xxx46
xxx43
xxx3
U
xxx18xxx3
SAT
xxx7
xxx1 xxx43
xxx2
I
V
xxx20
xxx1
xxx2
+
xxx19
V
xxx1
xxx1
I
xxx1
Switching
xxx2
device (e.g.
IGBT) with
xxx6
conduction
characteristic
xxx3
Value
0.0025 Ω
0.097 mH
3 mH
0.0083 Ω
0.115 mH
4
2.400 Kg .m2
6.500 KVA
I
Parameter
Stator resistance
Stator leakage inductance
Magnetization inductance
Rotor resistance
Rotor leakage inductance
Electrical turn ratio
Moment of inertia (inertia constant = 2 s.)
Rated apparent power
xxx47
xxx4
xxx3
xxx3
xxx5
xxx3
xxx5
xxx17
xxx16
xxx15
xxx4
xxx5
xxx14
xxx4
xxx13
xxx4
Crowbar
xxx5
V
xxx24
V
xxx21 xxx21
xxx26
xxx33
xxx29
xxx24
I
xxx25
IM
xxx38
Power
xxx35 measurement
I
xxx26
ωxxx29xxx28
V
xxx34
xxx37
V
xxx38
xxx27
I
xxx29
xxx28
I
xxx22
xxx5
V
xxx32
xxx2
xxx36
xxx36
xxx35
xxx20
xxx44
Freewheeling
diode (with
characteristic)
xxx2
xxx32xxx30
xxx29
xxx12
xxx4
xxx29
xxx30
xxx28
xxx31
TACS/MODELS controllable
external torque (e.g. wind)
Electrical network
equivalent for the
mechanical system
Fig. 3: The DFIG-based wind turbine model in ATP [21]
435
Eman A. Awad et al. Mitigation of Temporary Overvoltages in Weak Grids...
3. Temporary Overvoltage Mitigation Methods
Traditionally, the TOVs are limited through the adoption of PIR in the line circuit breakers.
Another method to reduce TOV is the synchronous switching of circuit breakers. The
installation of shunt reactors is also used to mitigate TOV. Furthermore, the surge arresters are
used to limit the TOVs.
Pre-Insertion Resistor (PIR): PIR is an effective mitigation method, although it presents a
decreasing acceptance due to the high cost of implementation and maintenance [8]. The
closing resistors are inserted in series with the cable, normally being short circuited after 10 ms,
thereby damping TOV. In the present case, a 500 Ω resistor, with an insertion time equals the
switching time, is installed.
Synchronized Circuit Breakers: By means of switch synchronized controllers, both energizing
and de-energizing operations can be controlled with regard to the point-on-wave position to
mitigate harmful transients [6].
The suitable instant for controlled switching is the time in which the voltage across the
circuit breaker contacts for each phase is zero and the predicted time span between the closing
instant of the first and the last pole is as small as possible [22]. In this study, the closing order
is given when the voltage of each phase crosses consecutives zero.
Shunt Reactor: shunt reactor is used to damp TOVs and keep the system voltage within the
permissible limits [23]. In the present case, the shunt reactor is modeled, as in many studies, by
a simple lumped inductor (0.07 mH) with a series resistance (5 Ω). A parallel resistance (200
Ω) is added for realistic high frequency damping [24]. A shunt reactor is added nearby the
switching point.
Surge Arrester: Surge arrester provides a temporal path to earth which the superfluous charge
is removed from the system [25]. With the information of volt-ampere characteristic of surge
arrester, equivalent model of surge arrester is presented in [26, 27]. This model is used in this
investigation.
4. Mitigation of Energization TOVs Generated due to Symmetrical Switching
Operations
The TOVs generated due to symmetrical switching opertions in weak grids connected to
DFIG-based wind farms are investigated. The TOV due to the energization of row A is studied
and mitigated; this is carried out by the symmetrical closing the three phases of the circuit
breaker at the platform of row A. The symmetrical switching operation is applied at the voltage
peak by closing the circuit breaker phases when the voltage of each phase crosses consecutives
90o.
The three-phase voltage waveforms caused by the switching are investigated on both the
HV (33.0 kV) side and the LV (0.69 kV) side of the WTTs of row A.
Figs. 4 show the maximum TOVs at both the HV side and the LV side of the first WTT,
respectively, with and without using of mitigation methods.
Due to using PIR, the TOV at the HV side of the WTT of row A is reduced from 1.5266 pu
(29.081 kV) to 1.108 pu (21.11 kV) and from 1.576 pu (1.087 kV) to 1.1965 pu (0.826 kV) at
the LV side. The TOV due to using controlled switching of the circuit breaker at the HV side
of the WTT of row A which is reduced from 1.5266 pu (29.081 kV) to 0.9929 pu (18.915 kV)
and from 1.576 pu (1.087 kV) to 1.124 pu (0.7758 kV) at the LV side.
Using of shunt reactor reduces the TOV at the HV side of the WTTs of row A from 1.5266
pu (29.081 kV) to 1.305 pu (24.859 kV) and at the LV side is reduced from 1.576 pu (1.087
kV) to 1.3047 pu (0.90024 kV). Also, due to using the surge arrester, the TOV at the HV side
of the WTTs of row A is reduced from 1.5266 pu (29.081 kV) to 1.422 pu (27.087 kV) and at
the LV side from 1.576 pu (1.087 kV) to 1.1266 pu (0.77735 kV).
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J. Electrical Systems 10-4 (2014) 431-444
It can be concluded that the PIR and the controlled switching of circuit breaker introduce a
better mitigation performance. They reduce the TOVs to acceptable values (around 1.0 pu).
PIR and the controlled switching of circuit breaker have better mitigation effect than the shunt
reactor and the surge arrestor.
Fig. 4: TOVs at HV and LV sides of the first WTTs of row A with and without mitigation methods
5. Investigation of Ferroresonance TOVs Generated due to Unsymmetrical Switching
Operations
The ferroresonance TOVs due to unsymmetrical switching operations during deenergization procedures are investigated. The unsymmetrical switching operations are
performed at two different wind turbines, the first and the last wind turbines in row A, in the
offshore wind farm. The unsymmetrical switching is carried out by the de-energization of both
phases B, and C of HV or LV sides of WTTs.
The effect of ferroresonance on DFIG rotor angular speed and the turbine output voltage is
investigated. Figs. 5 and 6 show the ferroresonance TOVs effect on DFIG rotor angular speed,
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Eman A. Awad et al. Mitigation of Temporary Overvoltages in Weak Grids...
and the turbine output voltage of the first and last WTs, respectively.
When the switching is applied at HV side of the furthest (last) WTT, the rotor angular speed
lasts a long time transient, turning into a pulsating speed centered on the steady state speed.
This behavior differs from that of the first wind turbine; the rotor angular speed exposes a short
time transient. When the switching is applied at LV side of the first and the ninth WTTs, the
rotor angular speed increase rapidly.
When the switching is applied at LV side of the first or the last WTTs, the switching causes
the turbine output voltage in the open phases to increase equal six times the nominal voltage of
the turbine. This overvoltage is due to the interaction between the feeder cable capacitance and
the magnetic cores of WTG unit transformers, particularly when the transformer is still
connected to the wind farm while the WTGs are not connected. However, when the switching
is applied at HV side of the first or the last WTTs, there is a slight increase in the output
voltage.
It is seen from the exceeding results that the unsymmetrical de-energization at the LV side
of WTT is, by all means, much worse than that by the unsymmetrical de-energization at the HV
side of WTT.
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J. Electrical Systems 10-4 (2014) 431-444
Fig. 5: Ferroresonance effect on DFIG rotor angular speed and the turbine output voltage of the first
WT.
Fig. 6: Ferroresonance effect on DFIG rotor angular speed and the turbine output voltage of the last
WT
6. Mitigation of Ferroresonance TOVs Generated due to Unsymmetrical Switching
Operations
PIR, controlled switching of circuit breaker, shunt reactors, and surge arresters are used in
this study to mitigate the effect of ferroresonance TOVs, especially when the switching is
applied at LV side of WTTs, as it is the worst case.
Figs. 7 and 8 show the maximum ferroresonance overvoltage at LV side of the first and the
last wind turbine transformers, respectively, with and without using mitigation methods.
Due to using PIR, the ferroresonance TOV at LV side of the first WTT is reduced from
6.0674 pu (4186.5 V) to 1.0907 pu (752.58 V). The ferroresonance TOV due to using
controlled switching of the circuit breaker is reduced from 6.0674 pu (4186.5 V) to 0.974 pu
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Eman A. Awad et al. Mitigation of Temporary Overvoltages in Weak Grids...
(672.08 V). Using of shunt reactor reduces the ferroresonance TOV from 6.0674 pu (4186.5 V)
to 1.3482 pu (930.28 V). Due to using surge arrester, the ferroresonance TOV is reduced from
6.0674 pu (4186.5 V) to 2.3538 pu (1624.1 V).
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J. Electrical Systems 10-4 (2014) 431-444
Fig. 7: The maximum ferroresonance TOV at the first WTT with and without mitigation methods
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Eman A. Awad et al. Mitigation of Temporary Overvoltages in Weak Grids...
Fig. 8: The maximum ferroresonance TOV at the last WTT with and without mitigation methods
Due to using PIR, the ferroresonance TOV at LV side of the last WTT is reduced from
6.0465 pu (4172.1 V) to 1.0178 pu (702.3 V). The ferroresonance TOV due to using
controlled switching of the circuit breaker is reduced from 6.0465 pu (4172.1 V) to 0.996 pu
(687.3 V). Using of shunt reactor reduces the ferroresonance TOV from 6.0465 pu (4172.1 V)
to 1.3074 pu (902.1 V). Due to using surge arrester, the ferroresonance TOV is reduced from
6.0465 pu (4172.1 V) to 2.497 pu (1722.9 V).
PIR and controlled switching of circuit breaker show a better mitigation performance than
the other investigated methods. The reduction of TOVs due to shunt reactor is remarkable, but
still lesser than the reduction values achieved by PIR and controlled switching of circuit
breaker. Using surge arrester to reduce the ferroresonance TOV magnitude has limited
influence comparing with the other mitigation methods.
7. Conclusion
In this paper the mitigation of TOVs in weak grids connected to DFIG-based wind farms is
investigated. The DFIG-based wind turbine model in ATP is used for the simulation of the
wind farm. A wind farm which consists of 72 wind turbines is used for this analysis.
In order to investigate the mitigation of TOVs due to the different symmetrical and
unsymmetrical switching scenarios four different mitigation methods have been used and
compared by each other. These are the pre-insertion resistor (PIR), the controlled switching of
circuit breaker, the shunt reactor and the surge arrestor method. ATP is used in this study to
simulate the investigated system and the mitigation devices.
The results show that the TOVs due to the unsymmetrical switching are higher than those
due to the symmetrical switching. And those TOVs due to the unsymmetrical switching at the
LV side of the wind turbine transformer is higher than those at the HV side due to the
transformer interaction in the circuit (ferroresonance effect).
The comparison between the different mitigation methods show that the controlled
switching of the circuit breaker as well as the PIR method are the best mitigation means to the
TOVs.
It is recommended by the study of TOVs in wind farms and their mitigation methods to take
care of the TOVs caused due to ferroresonance unsymmetrical switching cases and to employ
the controlled switching of circuit breakers as far as possible.
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