a study on overvoltages in wind tower due to direct lightning stroke

2012 International Conference on Lightning Protection (ICLP), Vienna, Austria

A study on Overvoltages in Wind Tower due to Direct Lightning Stroke

Shozo Sekioka Department of Electrical & Electronic Engineering

Shonan Institute of Technology Fujisawa, Japan

[email protected]

Hitomi Otoguro, Toshihisa Funabashi Power Utility Sector (PUS) Business Unit

Meidensha Corporation Tokyo, Japan

[email protected]

Abstract—Lightning is one of the most serious problems for a wind turbine generator system. Direct lightning strokes to wind turbine generator systems sometimes cause serious damages. Power apparatuses and instruments in a wind tower as well as blades must be protected from the lightning from economical point of view. This paper discusses lightning overvoltages in apparatuses in a wind tower for direct lightning stroke to the tower. Simulations are carried out using the EMTP for such parameters as peak value of lightning current and grounding condition of metallic sheath of power cables in the tower.

Keywords-component; wind turbine; lightning surge; EMTP

I. INTRODUCTION The renewable energies such as the photovoltaic generation,

the fuel cells, and the wind turbine are expected to solve the global warming problem. Many wind turbines having a couple of MW capacity have been built all over the world. A lightning stroke to a wind tower sometimes causes damages in apparatuses and instruments in the wind turbine system. Lightning surges come into the wind turbine generator system as illustrated in Fig. 1. (a) overhead lines such as distribution and tele-communication lines (b) direct lightning stroke to a wind tower or a blade (c) ground potential rise caused by lightning hit to the ground or the tower (d) lightning back flow from the tower to the line

Figure 1. Lightning events in a wind turbine generator system and a

distribution line.

Direct lightning stroke to a distribution line or a nearby lightning which generates lightning-induced voltage sometimes causes lightning damages or outages in the distribution line. The top of a blade of MW-class wind turbine reaches 100 m, and the lightning frequently strikes a blade. The authors have studied insulation coordination design between a distribution line and wind turbines [1-3]. When a blade is greatly damaged, it costs much to repair it, and stops generating for long period. Wind turbine generator is not relatively damaged by lightning because it is set inside a nacelle. The grounding system is an important factor to determine lightning overvoltage and currents. Power and control cables with metallic sheath are frequently used. Grounding of the sheath greatly affects over-voltages in the cables.

This paper describes simulation results of lightning over-voltages in a wind tower caused by direct lightning hit to a tower considering a power cable condition.

II. SIMULATION CIRCUIT

A. Wind Turbine Generator System for Simulation Fig. 2 illustrates a simulation circuit to estimate lightning

overvoltages and currents in a wind turbine generator system. Lightning strikes a tower or a blade. Lightning current is injected into the top of the tower.

Zp Zp Zp Zp

Ra Rb Ra Rb RT

G

Figure 2. Simulation circuit.

B. Maintaining the Integrity of the Specifications 1) Distribution line

The Dommel model [4] and the J.Marti model [5] are available in the EMTP. The Dommel model treats only single frequency constants, and shows poorer accuracy than a frequency-dependent line model such as the J.Marti model. This paper uses the J.Marti model for the distribution line and the service line.

The length of an overhead distribution line is 1 km. The line is terminated by matching circuit at an end of the line to represent semi infinite length of line. The length between poles and a transformer is 50 m. Fig. 3 illustrates a configuration of the distribution line and the service line.

(a) Distribution line (b) Service line

Figure 3. Dimension and configuration of distribution and service lines.

2) Vertical conductors A wind tower is a tapered line. Based on experimental

study of a reinforced concrete pole, a tapered line for lightning surge analysis can be represented by a cylindrical pipe with radius, which is estimated by the average value of upper and bottom areas of the line [6]. The tower is used as a grounding conductor. A wind turbine is usually set at the top of a wind tower to directly connect to a rotor, and is connected to a transformer through a 3-phase power cable. As a result, 4 parallel vertical conductors exist in the wind tower as illustrated in Fig. 4. This paper considers a CVT cable of 22 mm2 as the power cable and a copper pipe with radius rTW of 1.6 m as the wind tower. Tower height is 60 m.

The surge impedance is a function of time due to TM field. For simplicity, a vertical conductor is modeled by a distributed-parameter line with constant surge impedance.

The self- and mutual surge impedances Z0s and Z0m of vertical conductors in the air are given by [7]

re

hZ s ⋅= ln600 (1)

de

hZ m ⋅= ln600 (2)

(a)Cross section of tower and cable

(b)Dimension of cable

Figure 4. Configuration of a power cable and a tower.

where h is the conductor height, r is the conductor radius, d is the distance between centers of the conductors, and e is the base of natural logarithm.

Line constants of cables can be obtained by using CABLE CONSTANTS [8], which is a subprogram in the EMTP. The equivalent conductor height for vertical conductors in order to use CABLE CONSTANTS is replaced by 0.5h/e [9], and the distance between the conductors in the subprograms is the same as that of the actual configuration. The Dommel model is used for the vertical conductors in this paper because of short conductor length.

3) Grounding System Reinforced concrete poles are used in the Japanese

distribution lines to sustain wires and power apparatuses. The concrete pole should be treated as a grounding lead conductor and a grounding electrode in lightning performance [9]. The grounding system of the distribution line is represented by a distributed-parameter line, and a grounding resistance. The surge velocity in the reinforced concrete pole is equal to the velocity of light in free space. The grounding resistance shows current dependence for high currents, and frequency dependence for steep-front currents. This paper does not consider these dependencies. The ground wire is grounded at each pole. The grounding resistance Ra for the surge arresters and the ground wire is 30 Ω. The surge impedance Zp of the

rT

DSCi

rTW h

rCC

rCi

rCS rCO

12.6m

0.5m 0.5m 1.5m

0.7m

Ground wire (ACSR 25mm

2)

Phase wire (Al-OC200mm

2)

0.6 m

CVT 200 m2

rcc=8.5mm, rCi=13.0mm, rCS=15.8mm, rCO=16.5mm

DSCi=2r/sqrt(3)=19.1mm

reinforced concrete pole is 200 Ω. The steady-state grounding resistance of Japanese wind turbine generator systems is often 2 Ω. The grounding resistance shows inductive variation for low steady-state value. Therefore, this paper uses 10 Ω.

4) Surge arrester and Surge Protective Device (SPD) Surge arresters of the distribution line are installed on odd

number reinforced concrete poles. The arrester and the SPD are represented by a series circuit of a voltage-controlled switch and a nonlinear resistance, which is given by current-voltage characteristics. Operation voltage of the distribution line arresters is 29 kV. Voltage-current curves of the arresters are shown in Fig. 5. Surge protective devices for low-voltage circuit are not simulated in this paper.

0

50

100

150

1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04

Current [A]

Vol

tage

[kV

]

0

1

2

3

Vol

tage

[kV

]

High-voltage Ar

Low-voltage Ar

Figure 5. Voltage - current curve of arrester and SPD.

5) Transformer Transformer is frequently represented by capacitances for

lightning surge analysis when the analysis is targeted only for the high voltage line. However, this simple model cannot deal with secondary transition voltages. Therefore, this paper adopts more accurate transformer model, which considers the secondary voltage and high frequency characteristics [10, 11]. Fig. 6 shows measured capacitances of Y-∆ transformers [11]. Capacitances of a transformer are given by a simple function of rated capacity [12]. Thus, the capacitances are estimated by approximate lines included in Fig. 6. Now, the rated capacity is 1 MVA from Table 1. Capacitances between high-voltage

windings and the ground and between low-voltage windings and ground are 500 pF, and Capacitance between low- and high-voltage windings 1000 pF. Table 1 is specifications of a transformer [13].

6) Generator A generator for lightning surges can be modeled by

reactance and capacitance [14]. This paper represents the generator by capacitance of 10 nF, which exists between a phase winding and the wind tower.

7) Lightning current The lightning current waveform is assumed to be a ramp

shape of 2/70 µs, which is the simplest model. The lightning is represented by a current source and a lightning channel impedance of 1 kΩ. Lightning-induced effect is not taken into account.

III. SIMULATION RESULTS The Electro-Magnetic Transients Program (EMTP) is used

to simulate lightning surges because many simulation models are available.

A. Simulation Cases Grounding of metallic sheath of a cable is an important

condition. Table II is simulation cases of grounding condition of the sheath of power cable in a wind tower. Crest value of lightning current is 10, 25, 50,100 kA.

B. Lightning Overvoltages for Low Lightning Current As a fundamental case, simulation for low lightning current

of 1 kA is done. Surge arresters and SPDs do not work due to the low current. Fig. 7 shows calculated results of voltage waveforms of (a) potential at tower top, tower base transformer (Tr) low- and high-voltage sides, (b) voltage to tower at tower top, Tr low- and high-voltage sides, and (c) cable core-sheath voltage at tower top and Tr low-voltage side. Fig. 8 shows peak value of the voltages.

From Figs. 7 and 8, the potentials are not affected by the grounding condition of the sheath. Transfer voltage in the Tr is relatively low. As a result, voltage to tower at Tr high voltage side becomes high. High frequency component is found in case of both-ends being connected to the tower.

TABLE I. SPECIFICATIONS OF TRANSFORMER

Connection method Y-∆ Rating power 1.0 MVA Rating voltage 600/6,600 V

Frequency 60 Hz % impedance 15.7 %

TABLE II. SIMULATION PARAMETER

Case 1 Case 2 Case 3 Case 4Tower top × × Tower bottom × ×

0.1

1

10

1 10 100

Rated capacity [MVA]

Cap

acita

nce

[nF/

phas

e]

High-voltage winding to ground ∆ Low-voltage winding to ground High- and low-voltage windings

Figure 6. Measured results of capacitances of transformers and approximate characteristics.

-5

0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40 45 50Time [µs]

Vol

tage

[kV

]

Tower top

Tower base

Tr low-voltage

Tr high-voltage side

top:open - bottom:opentop:short - bottom:opentop:open - bottom:shorttop:short - bottom:short

(a)Potential

-12

-10

-8

-6

-4

-2

0

2

0 5 10 15 20 25 30 35 40 45 50

Time [µs]

Vol

tage

[kV

]


Tower top


Tr low-voltage side

(b) Voltage to tower

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0 5 10 15 20 25 30 35 40 45 50

Time [µs]

Vol

tage

[kV

]

Tr low-voltage sideTower top

top:short - bottom:short

top:short - bottom:open

top:open - bottom:open

top:open - bottom:short

(c) Voltage between core and sheath

Figure 7. Calculated waveforms of voltages in tower for low lightning current.

C. Lightning Overvoltages for High Lightning Current Lightning current with several tens kilo-amperes some-

times strikes wind tower generator system. Surge arresters and SPDs suppress lightning overvoltages, and discharge currents are flown into the tower. Fig. 9 shows calculated waveforms of lightning overvoltages and currents for lightning current with 50 kA. The potentials are not dependent on the grounding condition of the cable sheath. Fig. 10 shows peak values of the voltages and currents.

0

5

10

15

20

25

30

1 2 3 4Case

Vol

tage

[kV

]

TW top

TW base

Tr low-V

Tr high-V

(a) Potential

0

2

4

6

8

10

12

1 2 3 4Case

Vol

tage

[kV

]

TW top

Tr low-V

Tr high-V


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1 2 3 4Case

Voltage

[kV]

TW top

Tr low-V


Figure 8. Peak values of calculated results of voltages in tower fcor low lightning current.

From Figs. 9 and 10, the voltage at the Tr high-voltage side is very high. Accordingly, surge arrester must be installed. The sheath should not be connected to the tower only at the tower base. The SPDs suppress the lightning overvoltages in the tower.

D. Lightning Overvoltages for High Lightning Current Fig. 11 is crest values of the voltages and currents in the

tower as a function of lightning current.

-400

-200

0

200

400

600

800

1000

1200

1400

0 5 10 15 20 25 30 35 40 45 50

Time [µs]

Vol

tage

[kV

]

Tower top

Tr low-voltage sideTower base


(a) Potential

-140

-120

-100

-80

-60

-40

-20

0

20

0 5 10 15 20 25 30 35 40 45 50

Time [µs]

Vol

tage

[kV

]



Tr low-voltage side, tower top


-5

-4

-3

-2

-1

0

1

2

3

4

5

6

0 5 10 15 20 25 30 35 40 45 50

Time [µs]

Vol

tage

[kV

]

Tr low-voltage sideTower top

top:short - bottom:short top:short - bottom:open




-12

-10

-8

-6

-4

-2

0

2

0 5 10 15 20 25 30 35 40 45 50

Time [µs]

Cur

rent

[kA

]

Tr high-voltage sideTr low-voltage sideTower top

(d)Current

Figure 9. Calculated waveforms of voltages in tower for high lightning current (50 kA).

0

200

400

600

800

1000

1200

1400

1 2 3 4Case

Vol

tage

[kV

]

TW top

TW base

Tr low-V

Tr high-V

(a) Potential

1

10

100

1000

1 2 3 4Case

Vol

tage

[kV

]

TW top

Tr low-V

Tr high-V


0

1

2

3

4

5

6

1 2 3 4Case

Voltage

[kV]

TW top

Tr low-V


0.001

0.01

0.1

1

10

100

1 2 3 4

Case

Curren

t [kA]

TW top

Tr low-V

Tr high-V

(d) Current

Figure 10. Peak values of calculated results of voltages in tower for high lightning current (50 kA).

0

1

2

3

0 20 40 60 80 100Lightning current [kA]

Cre

st v

alue

[kV

]


Tower top

Tower baseTr low-voltage side


(a) Potential

0

50

100

150


Cre

st v

alue

[kV

]

0

0.5

1

1.5

2

2.5



Tower topTr low-voltage side


0

1

2

3

4

5

6

7

8


Cres

t val

ue [k

V]



0

5

10

15

20

25


Cre

st v

alue

[kA

]

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7top:open - bottom:opentop:short - bottom:opentop:open - bottom:shorttop:short - bottom:short

Tr high-V sideTower top

Tr low-V side

(d)Current

Figure 11. Peak values of calculated results of voltages in tower as a function of lightning current.

The voltage and current in the Tr high-voltage side become high. Therefore, surge arrester at the high-voltage side should be carefully chosen. The grounding condition of the metallic sheath of power cable does not significantly affect lightning overvoltages.

IV. CONCLUSION This paper has discussed lightning overvoltages and

currents in a wind turbine generator system for a direct lightning hit to a wind tower. The simulation results using the EMTP show that the grounding condition of sheath of a power cable does not significantly affect lightning overvoltages and currents. Surge arrester at high-voltage side of a transformer needs high capacity due to ground potential rise of the wind tower.

REFERENCES

[1] S. Sekioka, and T. Funabashi, “A study on insulation coordination of a wind turbine generator system and a distribution line”, CIGRE Colloquium, S2-5, Kushiro, Japan, June 2009.

[2] S. Sekioka, and T. Funabashi, “A study on insulation coordination of a wind turbine generator system and a distribution line (II)”, in Proc. of SIPDA, pp. 279-284, Curitiba, Brazil, Nov. 2009.

[3] S. Sekioka, J.Takami, and S.Okabe, “Insulation coordination of a wind turbine and a power distribution line”, in Proc. of SIPDA, Fortaleza, Brazil, Oct. 2011.

[4] H. W. Dommel, "Digital computer solution of electro-magnetic transients in single- and multiphase networks", IEEE Trans. Power Apparatus and Systems, Vol. 88, No. 4, pp. 388-397, 1969.

[5] J. R. Marti, “Accurate modelling of frequency-dependent transmission lines in electromagnetic transient simulations”, ibid. Power Apparatus and Systems, Vol. 101, No. 101, pp. 147-157, 1982.

[6] K. Yamamoto, Z. Kawasaki, K. Matsuura, S. Sekioka, and S. Yokoyama, “Study on surge impedance of reinforced concrete pole and grounding lead wire on distribution lines by experiment on reduced scale model”, in Proc. of 10th Int. Symp. on High Voltage Engineering, Vol. 5, pp. 209-212, Montreal, Canada, 1997.

[7] A. Ametani, Y. Kasai, J. Sawada, A. Mochizuki, and T. Yamada, "Frequency-dependent impedance of vertical conductors and a multiconductor tower model," IEE Proc.- GTD, Vol. 141, No. 4, pp. 339-345, 1994.

[8] A. Ametani, "A general formulation of impedance and admittance of cables," IEEE Trans. Power Apparatus and Systems, Vol. 99, No. 3, pp. 902-910, 1980.

[9] S. Sekioka, “Lightning surge analysis model of reinforced concrete pole and grounding lead conductor in distribution line”, IEEJ Trans. Electrical and Electronic Engineering, Vol. 3, pp. 432-440, 2008.

[10] T. Ueda, T. Sugtimoto, T. Funabashi, N. Takeuchi, T. Sato, and K. Miyagi, “A study on transformer model for transfer voltages considering frequency characteristics”, IEEJ Trans., Vol. 117-B, No. 9, pp. 1294-1300, 1997 (in Japanese).

[11] T. Funabashi, “A study on modeling technique for power system transients analysis”, Thesis for Doctor Degree, 1999.

[12] A. Greenwood, Electrical transients in power systems, Second edition, John Wiley & Sons, Inc., 1991.

[13] Y. Yasuda, T. Hara, and T. Funabashi, ”Analysis on lightning surge propagation in wind farm”, IEEJ Trans., Vol. 125-B, No. 7, pp. 709-716, 2005 (in Japanese).

[14] T. Funabashi, N. Takeuchi, T. Sugtimoto, T. Ueda, L. Dube, and A. Ametani, “Generator modeling for transformer transfer voltage study”, IEEE Trans. Energy Conversion, Vol. 14, No. 4, pp. 1193-1198, 1999.

a study on overvoltages in wind tower due to direct lightning stroke

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