j. electrical systems 10-4 (2014): 431-444 jes ebrahim a...

* Corresponding author: Ebrahim A. Badran, Electrical Engineering Department, Faculty of

Engineering, Mansoura University, Egypt, email: [email protected]; [email protected] 1 Electrical Engineering Department, Faculty of Engineering, Mansoura University, Egypt.

Copyright © JES 2014 on-line : journal/esrgroups.org/jes

Eman A. Awad1,

Ebrahim A. Badran 1,*

, and Fathi M. H. Youssef 1

J. Electrical Systems 10-4 (2014): 431-444

Regular paper

Mitigation of Temporary Overvoltages in Weak Grids

Connected to DFIG-based Wind Farms

JES

Journal of Journal of Journal of Journal of Electrical Electrical Electrical Electrical SystemsSystemsSystemsSystems

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. Pre-insertion 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

Eman A. Awad et al. Mitigation of Temporary Overvoltages in Weak Grids...

432

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]

J. Electrical Systems 10-4 (2014) 431-444

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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].

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Fig. 2: The ATP model of the investigated offshore wind farm


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2.3- Modeling of Cables and Transformers

The MV single-core submarine cables are the most distinctive electric component in the off-

shore 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]2 240.0

Diameter of conductor [mm] 18.1

Insulation thickness [mm] 8.0

Diameter over insulation [mm] 35.7

Cross section of screen [mm]2 35.0

Outer diameter of cable [mm] 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) single-

core 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 0.087 [m ohm/m] +ve sequence inductive reactance 0.123 [m ohm/m] +ve sequence capacitive reactance 9.092 [M ohm*m]

zero sequence resistance 0.1865 [m ohm/m] zero sequence inductive reactance 0.0551 [m ohm/m] zero sequence capacitive reactance 9.0915 [M ohm*m]

The 10.5 km / 132 kV sea cable (equivalent PI section model)

+ve sequence resistance 0.0864 [m ohm/m] +ve sequence inductive reactance 0.123 [m ohm/m] +ve sequence capacitive reactance 9.0915 [M ohm*m]

zero sequence resistance 0.1827 [m ohm/m] zero sequence inductive reactance 0.0619 [m ohm/m] zero sequence capacitive reactance 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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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 Y / ∆ Voltage 0.69/33.0 [kV]

Rated power 2.5 [MVA] Leakage reactance 0.082573 [pu]

Copper losses 0.0084 [pu] No-load losses 0.0022 [pu]

Wind Farm Transformer Model

Connection method ∆ / Y / ∆ Voltage 33.0/132.0/33.0 [kV]

Rated power 180.0 [MVA] Leakage reactance 0.1 [pu]

Copper losses 0.004 [pu] No-load losses 0.001 [pu]

Table 4: Parameters of the DFIG-based wind turbine

Parameter Value

Stator resistance 0.0025 Ω

Stator leakage inductance 0.097 mH

Magnetization inductance 3 mH

Rotor resistance 0.0083 Ω

Rotor leakage inductance 0.115 mH

Electrical turn ratio 4

Moment of inertia (inertia constant = 2 s.) 2.400 Kg .m2

Rated apparent power 6.500 KVA

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Fig. 3: The DFIG-based wind turbine model in ATP [21]


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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).


437

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 de-

energization 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,


438

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.


439

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


440

(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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Fig. 7: The maximum ferroresonance TOV at the first WTT with and without mitigation methods


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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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