protection of interconnected wind turbines against...

1

R.B. Rodrigues a, V.M.F. Mendes a, J.P.S. Catalão b,c,*

a Departmental Area of Electrical Engineering and Automation, Instituto Superior de Engenharia de Lisboa, R. Conselheiro Emídio

Navarro, 1950-062 Lisbon, Portugal b Department of Electromechanical Engineering, University of Beira Interior, R. Fonte do Lameiro, 6201-001 Covilha, Portugal

c Center for Innovation in Electrical and Energy Engineering, Instituto Superior Técnico, Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisbon, Portugal

Received 25 July 2011; received in revised form 3 March 2012

Abstract This paper is concerned with direct or indirect lightning strokes on wind turbines, studying overvoltages and electromagnetic transients. As wind power generation undergoes rapid growth, lightning damages involving wind turbines have come to be regarded with more attention. With the aim of providing further insights into the lightning protection of wind turbines, describing the transient behavior in an accurate way, the restructured version (RV) of the electromagnetic transients program (EMTP) is used in this paper. A new case study is presented with two interconnected wind turbines, considering a direct lightning stroke to the blade or considering that lightning strikes the soil near a tower. Comprehensive computer simulations with EMTP-RV are presented and conclusions are duly drawn. © 2012 Elsevier Ltd. All rights reserved. Keywords: Electromagnetic transients; Lightning protection; Wind energy

1. Introduction

Wind energy is one of the fastest growing renewable energy sources for power production [1],

particularly in European countries such as Ireland [2], Denmark [3] and Portugal [4,5].

Lightning strokes on power supply systems can produce dangerous overvoltages and damages on

equipments. Wind turbines are especially vulnerable to lightning, which can cause significant damage to

wind turbine components [6]. Available statistics reveal that between 4% and 8% of European wind

turbines are damaged by lightning every year [7].

* Corresponding author at: Department of Electromechanical Engineering, University of Beira Interior, R. Fonte do Lameiro, 6201-001 Covilha, Portugal. Tel.: +351 275 329914; fax: +351 275 329972.

E-mail address: [email protected] (J.P.S. Catalão).

Protection of interconnected wind turbines against lightning effects: overvoltages and electromagnetic transients study

2

Effective lightning protection of wind turbines is increasingly important nowadays [8,9], since areas of

favorable locations for wind turbines often coincide with areas of significant thunderstorm activity

[10,11]. Moreover, the escalating number of wind turbines in many countries Portugal makes their

reliability and safety of crucial importance [12,13].

Lightning protection of wind turbines presents problems that are not normally seen with other

structures. These problems are a result of the following [14]:

(i) Wind turbines are tall structures of more than 150 m in height;

(ii) Wind turbines are frequently placed at locations very exposed to lightning;

(iii) The most exposed wind turbine components such as blades and nacelle cover are often made of

composite materials. Although nowadays most of the wind turbine systems do have inbuilt

lightning protection systems, the lightning current is still able to produce severe damages;

(iv) The blades and nacelle are rotating;

(v) The lightning current has to be conducted through the wind turbine structure to the ground,

whereby significant parts of the lightning current will pass through or near to practically all wind

turbine components;

(vi) Wind turbines in wind farms are electrically interconnected and often placed at locations with

poor grounding conditions.

Modern wind turbines are characterized not only by greater heights, but also by the presence of ever-

increasing control and processing electronics. Consequently, the design of the lightning protection of

modern wind turbines remains a challenging problem [7].

The future development of wind power generation and the construction of more wind farms will

necessitate intensified discussion of lightning protection and the insulation design of such facilities [15].

Nevertheless, there are still very few studies in Portugal regarding lightning protection of wind turbines

using sophisticated numerical codes. Also, surge propagation during lightning strikes at wind farms

located in Portugal is still far from being clearly understood, given that the Portuguese Lightning

Location System (LLS) is in operation only since 2002, thus much work remains to be done in this area.

Direct and indirect lightning strokes can produce damages and/or malfunctions of the relevant

electrical and mechanical components [16]. Statistics of wind-turbine damages due to lightning have been

analyzed in the literature, along with the relevant risks [17]. An effective lightning protection system

should protect not only against the direct effects of lightning, but also against its indirect effects.

3

Scale models of electrical systems have been a popular tool to predict transients after different types of

perturbations [18]. For instance, a 3/100-scale model of an actual wind turbine that has blades with a

length of 25 m and a turbine that is 50 m high was considered in [19,20] for experimental and analytical

studies of lightning overvoltages.

However, in recent years, scale models have been progressively replaced by sophisticated numerical

codes capable of describing the transient behavior in an accurate way, such as the EMTP-RV, which

designates the restructured version (RV) of the electromagnetic transients program (EMTP) [21].

In this paper, a new case study is presented with two interconnected wind turbines, considering a direct

lightning stroke to the blade or considering that lightning strikes the soil near a tower. Comprehensive

computer simulations with EMTP-RV are presented and conclusions are duly drawn.

This paper is structured as follows. Section 2 presents the description of the wind turbines. Section 3

explains the EMTP-RV modeling. Section 4 illustrates the results obtained. Finally, concluding remarks

are given in Section 5.

2. Wind turbines description

Wind turbines with 2 MW of rated power are considered. The hub height varies between 70 to 138 m.

The rotor diameter is about 82 m. The rotor hub and annular generator are directly connected to each

other as a fixed unit without gears. The rotor unit is mounted on a fixed axle. The drive system has only

two slow-moving roller bearings due to the low speed of the direct drive. The annular generator is a low-

speed synchronous generator with no direct grid coupling. The output voltage and frequency change with

the speed, implying the need for electronic frequency conversion in order to make a connection to the

electric grid.

The LV/HV transformer is placed inside the tower at the bottom. It has 2500 kVA of rated power and

has a special design to fit the reduced dimensions and working conditions of the tower. The wind turbines

were modeled in 3D with AutoCAD, as shown in Fig. 1. Ensuring proper power feed from a wind turbine

into the grid requires grid connection monitoring, shown in Fig. 2. The electrical scheme of a LV/HV

substation inside the tower is shown in Fig. 3.

"See Fig. 1 at the end of the manuscript".



4

The wind turbine model is characterized by:

(i) A 690 V synchronous generator, sufficiently stable at 50 Hz, is considered;

(ii) A 690 V / 20 kV boost transformer is placed inside the tower;

(iii) The transformer model considers only electromagnetic transfer, and static transfer is ignored;

(iv) The interconnection to the power grid is through a 20/60 kV transformer;

(v) The grounding resistance considered for the earth electrode in the absence of lightning currents

is 1 Ω.

In addition, a standard lightning current waveform is assumed with wave front duration of 10 μs, half

wave-tail duration of 350 μs, and a peak value of 10 kA. The peak value considered is because, in

Portugal, 80% of lightning strikes have a peak current higher than 8-10 kA [13].

3. EMTP modeling

The EMTP has been extensively used to study transients in large scale power systems. In this paper,

the most recent version (EMTP-RV) is applied. The complete software is also named

EMTP/EMTPWorks, where EMTP designates the computational engine [21].

The following explains briefly the most important models used in this paper.

3.1 Lightning current source

The ICIGRE device was chosen to simulate the current lightning source. This device is used for

accurate calculations of the lightning performance of equipment.

The current front of the first stroke is given by:

nBtAtI (1)

where A and B are given by:

m

nS

tIn

nA max9.0

11 (2)

max9.0

11 ItSnt

B nmnn

(3)

The current tail equation is given by:

21

21t

ttt

tt nn

eIeII

(4)

Equation (4) is used when EMTP enters the tail zone at startn ttt .

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3.2 Wind turbine structure

To model the blade and the tower of a wind turbine, the Constant Parameter (CP) line is used, which is

a frequency independent transmission line model.

For the purpose of this paper, the CP line model can be successfully used. The frequency dependence

of the parameters was also not considered in [22], because the authors concluded that it has scarce

influence on the transient responses of the tower system. Besides, the same remark is provided in [23],

where the frequency dependence of the parameters is again not considered, since some studies have

shown that the skin effect has little influence on the lightning transient response.

The CP line is a distributed parameter model. The basic equations of the single phase distributed

parameter line are:

dt

txdILtxIRdx

txdV ,',', (5)

dt

txdVCtxVGdx

txdI ,',', (6)

The CP line parameters are calculated at a given frequency, which is better to take it above 1MHz

[19,20], and that is why it is labeled as frequency independent. The CP line parameters were calculated

taking into account technical information from the manufacturer, such as, material characteristics and

dimensions of components.

3.3 Ground electrode

Grounding systems are very important for wind turbines [24,25]. Precise modeling of the dynamic

performance of grounding electrodes under lightning currents must include both the time-dependent

nonlinear soil ionization and the frequency-dependent phenomena [26]. These phenomena might have

mutually opposing effects since the soil ionization effectively improves the grounding performance, while

frequency-dependent inductive behavior impairs it. In the case of lightning, the current that is injected in

the grounding electrodes is a fast-varying current pulse with high peak values. The dynamic response of

the grounding electrodes subjected to such current pulses is mainly influenced by:

(i) The soil ionization in the immediate proximity of the grounding electrode, which is related to the

current pulse intensity;

(ii) The lightning pulse propagation along the grounding electrode, which is related to the current

pulse front time.

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The ground electrode model used in this paper is very often used with lightning simulation purposes

for HV transmission lines and towers. It considers a nonlinear resistance using controlled resistance and

admittance. The presence of the current source provides an option for creating a piecewise linear

resistance function. Any segment k of such a function can be represented by the Norton circuit

equivalent:

kkkk IvYi (7)

The kY is actually the differential at the operating point k:

k

kk v

iY

(8)

When using the same ground electrode for safety and service purposes, the Portuguese regulation

requires a maximum value for earth resistance of 1 . This value is assumed in the absence of lightning

current flowing through it.

3.4 Surge arrester

The basic arrester model equation is given by (9), where ai is the arrester current and av is the

arrester voltage [27]:

aa vki (9)

For SiC (Silicon Carbide) arresters the value of is between 2 to 6. For MO (Metal Oxide) arresters

the value is 6010 . The k parameter is a constant used in fitting the arrester characteristic.

4. Case study

In this case study, two interconnected wind turbines are considered. The electrical scheme is shown in

Fig. 4.


The EMTP-RV circuit in Fig. 5 represents two interconnected wind turbines. In this case, lightning

strikes the ground near one wind turbine. For simplicity, the HV cable is represented by an electrical

model with concentrated parameters.


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No SPD is connected either in the LV or HV side. The purpose is to study the real influence of a CG

strike near the first tower on the second one.

With a peak value equal to 10 kA for the lightning current the second wind tower does not suffer

dangerous overvoltages. Results can be seen in Fig. 6. Even in presence of a peak value equal to 200 kA

for the lightning current, the maximum value considered in IEC standards for project, the second wind

turbine does not suffer dangerous overvoltages. Results can be seen in Fig. 7. The overvoltages at the first

wind turbine are considerable, as shown in Fig. 8, thus SPD should be installed in order to avoid damages

on the equipment.




The electrical scheme of Fig. 8 represents again two interconnected wind turbines, but in this case

lightning strikes directly the blade of one wind turbine. The peak value of the lightning current is assumed

to be 10 kA. In these conditions the simulations show that the second wind turbine does not suffer

dangerous overvoltages. Results can be seen in Fig. 9.



Even in presence of a peak value equal to 200 kA for the lightning current, the second wind turbine

does not suffer dangerous overvoltages. Results can be seen in Fig. 10. Nevertheless, SPD must be

installed to provide effective protection against dangerous overvoltages at the stroked wind turbine.


5. Conclusions

This paper presents a new case study with two interconnected wind turbines, considering a direct

lightning stroke to the blade or considering that lightning strikes the soil near a tower. The most recent

international standards have been used in this work. Comprehensive computer simulations are obtained

by using the most recent EMTP version: EMTP-RV. Reference values of international standards have

been adapted to Portuguese reality. Nevertheless, results are also true for other countries. The results have

show that the second wind turbine does not suffer dangerous overvoltages, even when considering a peak

value equal to 200 kA for the lightning current. To reduce the overvoltages to an acceptable value at the

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stroked wind turbine, SPD should also be installed, either in common or differential mode. The computer

simulations provided have proven to be very helpful on finding which are the most adequate protection

measures, and where they must be located.

Acknowledgements The authors would like to thank Prof. A. Machado e Moura for his valuable comments.

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main-shaft bearings for the development of specific lightning protection systems. IEEE Trans Electromagn

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Figure captions

Fig. 1. 3D model of the wind turbine.

Fig. 2. Grid connection monitoring on the wind turbine (Enercon).

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Fig. 3. LV/HV substation inside the tower.

Fig. 4. Electrical scheme with two wind turbines.

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Underground

Surface

Cable VV 1G35 (35 m)

Tow

er

Blade

Lightningstrike

Synchronous machine

Auxiliarytransformer

Power transformer

Non linear load

Ground electrode

Capacitivecoupling

Underground

Surface


Tow

er

Blade

Synchronous machine Power transformer

Non linear load

Ground electrode

Caoacitivecoupling

LXH

IOV

1x3

x95

(350

m)


+ A?i

m3

1 2

0.975/0.566

DY_1

+4uH

L3

+

10kA/10usIcigre2

VM

+ ?vm2

+

Rn1

0

SM

SM1

0.975kV2MVA

VM

+ m5?v

VM+?v

m4

+

C4

0.

1nF

+

75

+

IY

IYRn2

?i >i1if(u) 1

Fm1

+

C1

0.1n

F

+

C3

0.

1nF

+

Rn3

0

+

Rn4

0

+

41 V

M+

m6?v

VM

+m

7?v

VM

+m

9?v

VM+

m1?

v

1 2

0.975/28

YD_2

1 2

0.975/0.566

DY_2

+4uH

L1

VM+ ?vm10

+

Rn5

0

SM

SM2

0.975kV2MVA

VM

+ m11?v

VM

+?v

m12

+

C2 0.

1nF

+

75

+

IY

IYRn6

?i >i1if(u) 1

Fm2

+

C5

0.1n

F

+

C6

0.

1nF

+

Rn7

0

+

Rn8

0

+

41 V

M+

m13?v

VM

+m

14?v

VM+ m

15?v

VM+

m16

?v

1 2

0.975/28

YD_1

+13

4m

R2

+38

uH

L2

+22m R3

+22m R1

a

b

c

bc

a

a

b

c

bc

a

c

c

b

b

a

a

c

cb

b

a

a

Fig. 5. EMTP-RV circuit model for indirect lightning strike.

13

Fig. 6. Overvoltages at the: a) HV side of first wind turbine; b) control system of first wind turbine; c) HV side of second wind turbine.

14

Fig. 7. Overvoltages with I =200 kA at: a) HV side of first wind turbine; b) control system of first wind turbine; c) HV side of second wind turbine.

15

Underground

Surface


Tow

er

Blade

Lightningstrike

Synchronous machine


Power transformer

Non linear load

Ground electrode

Capacitivecoupling

Underground

Surface


Tow

er

Blade

Synchronous machine Power transformer

Non linear load

Ground electrode

Caoacitivecoupling

LXH

IOV

1x3

x95

(350

m)


+ A?i

m3

1 2

0.975/0.566

DY_1

+4uH

L3

+

200kA/10usIcigre2

VM

+ ?vm2

+

Rn1

0

SM

SM1

0.975kV2MVA

VM

+ m5?v

VM+?v

m4

+

C4

0.

1nF

+

75

+

IY

IYRn2

?i >i1if(u) 1

Fm1

+

C1

0.1n

F

+

C3

0.

1nF

+

Rn3

0

+

Rn4

0

+

41 V

M+

m6?v

VM

+m

7?v

VM

+m

9?v

VM+

m1?

v

1 2

0.975/28

YD_2

1 2

0.975/0.566

DY_2

+4uH

L1

VM+ ?vm10

+

Rn5

0

SM

SM2

0.975kV2MVA

VM

+ m11?v

VM

+?v

m12

+

C2 0.

1nF

+

75

+

IY

IYRn6

?i >i1if(u) 1

Fm2

+

C5

0.1n

F

+

C6

0.

1nF

+

Rn7

0

+

Rn8

0

+

41 V

M+

m13?v

VM

+m

14?v

VM+ m

15?v

VM+

m16

?v

1 2

0.975/28

YD_1

+13

4m

R2

+38

uH

L2

+22m R3

+22m R1

a

b

c

bc

a

a

b

c

bc

a

c

c

b

b

a

a

c

cb

b

a

a

Fig. 8. EMTP-RV circuit model for direct lightning strike.

16

Fig. 9. Overvoltages at the: a) HV side of first wind turbine; b) control system of first wind turbine; c) HV side of second wind turbine.

17

Fig. 10. Overvoltages with I =200 kA at: a) HV side of first wind turbine; b) control system of first wind turbine; c) HV side of second wind turbine.

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