A STUDY ON INSULATION COORDINATION OF A WIND TURBINE ...

X International Symposium on
Lightning Protection
9 th -13 th November, 2009 – Curitiba, Brazil
A STUDY ON INSULATION COORDINATION OF A WIND TURBINE
GENERATOR SYSTEM AND A DISTRIBUTION LINE (II)
Shozo Sekioka 1 , Toshihisa Funabashi 2
1 Shonan Institute of Technology, Japan – sekioka@elec.shonan-it.ac.jp
2 Meidensha Corporation, Japan – t.funabashi@honsha.meidensha.co.jp
Abstract - Lightning damage is one of the most serious
problems for wind turbine generator systems. Direct
lightning stroke to a blade is mainly targeted in lightning
protection design and tests. Lightning surges sometimes
come from a distribution/transmission line to the wind
turbine system due to lightning hit to the line. In this
situation, an insulation coordination design is another aspect
for the lightning protection of the wind turbine system. This
paper discusses a coordination design of wind turbines in a
windfarm and a distribution line. Simulations are carried
out using the EMTP for such parameters as peak value of
the lightning current and lightning striking point.
1 INTRODUCTION
The renewable energy such a wind turbine generator, a
photovoltaic cell, and a fuel cell are actively constructed
all over the world to solve the global warming. The wind
turbine generates much higher electric power energy than
the other renewable energies. A windfarm is expected to
be a clean power station instead of a steam power one
from which a large amount of the CO 2 is exhausted. Most
of Japanese wind turbine generator systems have been
built along the coast of the Sea of Japan, because strong
wind blows from Siberia to the coast. However, the
winter lightning which has huge energy and coulomb,
frequently occurs in the area, and causes many serious
damages in the transmission and distribution lines, and
the wind turbine generator systems [1]. Summer lightning
also damages control circuits of the wind turbine
generator systems.
Lightning surges come into the wind turbine generator
system under the following situations as illustrated in Fig.
1.
(1) Overhead lines such as distribution and telecommunication
lines
(2) Direct lightning stroke to a wind tower or a blade
(3) Ground potential rise caused by lightning hit to the
ground or the tower
(4) Lightning back flow from the tower to the line
Fig. 1 - Lightning events in a wind turbine generator system and
a distribution line.
In case (1), a 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 authors investigated an
influence of the lightning back-flow current and the
ground potential rise on the lightning overvoltages in the
surge arresters of the distribution line, which is connected
to the wind turbine generator system, when the lightning
strikes the wind turbine tower [2]. The wind tower is high,
and the lightning frequently strikes a blade. On the other
hand, the distribution line is long. Accordingly, the
lightning surges coming from the distribution line to the
wind turbine generator system also should be considered
for the lightning protection design of the wind turbine
system. The authors study the insulation coordination in
case of a wind turbine generator alone [3]. There many
windfarms. This paper describes the insulation
coordination of the distribution line and the wind turbine
generator system in a windfarm, and discusses lightning
overvoltages in the wind turbine for the lightning stroke
to the distribution line. The EMTP [4, 5] is used to carry
out the lightning surge analysis.
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G
G
Zp
Zp
Zp
Zp
Ra
Ra
Ra
Ra
R
R
Fig. 2 - Simulation circuit.
2 SIMULATION CIRCUIT
2.1 Simulation Circuit for Direct Lightning Stroke to
a Distribution Line
Fig. 2 illustrates a simulation circuit to estimate lightning
overvoltages in a wind turbine generator system in a
windfarm for a direct lightning stroke to a shielding wire
of a distribution line. Table 1 shows specifications of a
transformer, which is used in [6]. The wind turbines and
the cables in the windfarm have the same specifications.
Table 1: specifications of transformer [6].
Connection method
Y-∆
Rating power
1.0 MVA
Rating voltage
600/6,600 V
Frequency
60 Hz
% impedance 15.7 %
Mutual leakage inductance
18.2 mH
2.2 Simulation Models
(a)Distribution line
The Dommel model [7] and the J.Marti model [8] are
available in the EMTP. The Dommel model deals with
line constants for only single frequency, and shows
poorer accuracy than the J.Marti model, which can take
the frequency dependence into account. Therefore, this
paper adopts the J.Marti model for the distribution line
and the service line.
The length of the overhead line is 1 km. The distribution
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 40m. Fig. 3 illustrates
a configuration of the distribution and service lines.
1 m
0.7m 0.7m
Ground wire (standard steel wire
22 mm 2 )
Medium voltage line
ACSR OC 32 mm 2
11m
CVT 22 mm 2
0.6 m
(a) Distribution line (b) Service line
Fig.3 - Arrangement of the distribution and service lines.
(b) Vertical conductors
A wind turbine is usually mounted at the top of a wind
tower to directly connect to a rotor. A reinforced concrete
pole is equivalent to a metallic pipe for lightning surges
based on experimental study [9]. The wind turbine is
connected to a transformer or an inverter 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 22mm 2 as the power
cable in the tower and a copper pipe with radius r TW of
1.6 m as the wind tower.
h'
r TW
Fig - 4 Configuration of a wind tower and a power cable
to obtain conductor constants.
r c
r s
The self- and mutual surge impedances Z 0s and Z 0m of
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vertical conductors are given by [10]
h
Z0 s = 60ln
(1)
e⋅
r
h
Z0 m = 60ln
(2)
e⋅
d
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 the vertical conductors can be obtained
by using CABLE CONSTANTS [11], which is a
subprogram in the EMTP. The conductor height h’ for the
vertical conductors used in the subprograms is replaced
by 0.5h/e [12], 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.
(c) Surge arrester
Surge arresters of the distribution line are installed on odd
number reinforced concrete poles. The arresters in the
high-voltage side of the transformer are not considered in
this paper. The distribution line arrester at the end of the
lone is located close to the transformer, and is expected to
protect the high-voltage side on the transformer. Lowvoltage
side arresters must be considered because direct
lightning stroke to the wind tower often causes lightning
overvoltages and damages in the wind turbine generator
system. The arrester is 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. 6. Surge protective devices for low-voltage circuit
are not simulated in this paper.
Voltage [kV]
25
20
15
10
5
High-voltage Ar
Low-voltage Ar
0
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]
Fig. 5 - Voltage - current curve of the surge arresters.
(d) 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 an accurate transformer model, which
considers the secondary voltage and high frequency
characteristics [13, 14]. Fig. 6 shows measured
capacitances of Y-∆ transformers [14]. Capacitances of a
transformer can be approximated by a simple function of
rated capacity [15]. Thus, the capacitances are estimated
by approximate lines included in Fig. 6.
Capacitance [nF/phase]
10
0.1
High-voltage winding to ground
∆ Low-voltage winding to ground
High- and low-voltage windings
1
1 10 100
Rated capacity [MVA]
Fig. 6 - Measured results of capacitances of transformers and
approximate characteristics.
Now, the rated capacity is 1 MVA form Table 1.
Capacitances between high-voltage windings and the
ground and between low-voltage windings and ground
are 500 pF, and that between low- and high-voltage
windings 1000 pF.
(e) Generator
A generator for lightning surges analysis can be modeled
by reactance and capacitance [16]. This paper represents
the generator by capacitance for simplicity. The
capacitance between a phase winding and the wind tower
is considered.
(f) Grounding system
Reinforced concrete poles are used in the Japanese
distribution lines to sustain wires and power apparatuses.
The reinforced concrete pole should be treated as a
grounding lead conductor and a grounding electrode in
lightning performance [12, 17]. The grounding system of
the distribution line is represented by a distributedparameter
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. This paper
does not consider the soil ionization [18] as well as
frequency dependence.
The ground wire is grounded at the each pole. The
grounding resistance R a for the surge arresters and the
ground wire is 30 Ω. The surge impedance Z p of the
reinforced concrete pole is 200 Ω [19].
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The grounding electrodes and the tower bases of wind
turbines are connected using a grounding conductor. Low
steady-state grounding resistance can be obtained using
the grounding conductor. Propagation characteristics of
the grounding conductor are quite different from those of
an insulated conductor because of the existence of shunt
conductance [20]. Voltages on the grounding conductor
are attenuated and deformed along the conductor.
Lightning current has short wavefront duration, and peak
voltage often appears before reflection voltage from the
receiving end of the grounding conductor reaches.
Therefore, effective length, which is defined as the length
above which no further reduction of impedance of a
grounding conductor is observed, should be considered to
prevent the wind turbines from lightning-caused damages
[21].
Fig. 7 shows an equivalent circuit of a horizontal
grounding conductor, where Z 0 is the surge impedance of
the loss-less line, v is the surge velocity on the line, and
∆x is the elementary length of the equivalent circuit. The
circuit is composed of shunt conductances due to finite
soil resistivity and a loss-less line [20]. The grounding
conductor is represented by a ladder circuit of the
equivalent circuit. The steady-state grounding resistance
R 0 of the grounding conductor is given by
−1
0 = ( Gl)
R (3)
where G is the shunt conductance per length.
The surge impedance and the propagation velocity of the
grounding conductor are 100 Ω, and 100 m/µs
respectively.
Z0, v, ∆x Z0, v, ∆x
G∆x k G∆x k+1 G∆x
Fig. 7 - An equivalent circuit of a long grounding conductor.
(g) Lightning current
The lightning current waveform is assumed to be a ramp
shape of 2/70 µs, which is the simplest model for summer
lightning. The lightning is represented by a parallel circuit
of a current source and a lightning channel impedance of
1 kΩ. Lightning current is injected into the top of a
reinforced concrete pole.
2.3 Simulation Conditions
This paper investigates an influence of the following
parameters on lightning overvoltages on a wind turbine
generator system. Standard values are shown in Table 2.
The grounding resistance of the wind turbine generator
system is often chosen to be 2 Ω. That for a lightning rod
is 10 Ω. This paper adopts 10 as a standard value. The
grounding resistance is 1, 2, 5, and 10 Ω. The transformer
is assumed to be set on the same base of the wind tower,
and the grounding of the transformer is common with the
wind tower.
This paper investigates influence of grounding system of
a wind turbine generator system on lightning overvoltages.
Five wind turbines are considered.
3 SIMULATION RESULTS
3.1 Single Wind Turbine Generator System
Fig.8 shows calculated results of peak value of lightning
overvoltages on single wind turbine generator for a
parameter of the grounding resistance R TB of the wind
turbine generator system. Voltage is defined by potential
difference between the observation point and the
grounding system.
Voltage [kV]
5
4
3
2
1
0
Table 2: standard values in simulation parameters.
Grounding
Striking Lightning
Generator
Resistance of
point Current
capacitance
Wind Turbine
1 st pole 25 kA 10 Ω 10 nF
cable sending end
cable receiving end
Tr secondary potential
1 10 100
Grounding resistance [Ω]
Fig. 8 - Calculated results of peak voltage for a parameter of
grounding resistance R TB .
From Fig. 8, the potential in the wind turbine generator
system increases as the grounding resistance R TB becomes
higher, because the ground potential rises. This potential
corresponds to the voltage in the telecommunication
equipment. The other voltages are independent of the
grounding resistance. Therefore, the grounding resistance
affects only potential rise for the lightning overvoltages
coming from the distribution line.
Fig. 9 shows calculated waveforms in case of R TB =10 Ω
and 100 Ω.
50
40
30
20
10
0
Potential [kV]
282

40
30
20
5
4
cable sending-end voltage
cable receiving-end voltage
X Tr secondary potential
50
40
Potential [kV]
10
0
‐10
‐20
0 10 20 30 40 50
Time [µs]
Voltage [kV]
3
2
1
30
20
10
Potential [kV]
‐30
‐40
(a) Potential
Voltage [kV]
4
3
2
1
0
‐2
‐3
‐4
10 Ω
100Ω
0 10 20 30 40 50
‐1
Time [µs]
10 Ω
100Ω
(b) Voltage
Fig. 9 - Calculated waveforms on single wind turbine
generator.
From Fig. 9, the voltage waveforms are not affected by
the grounding resistance.
3.2 Wind Turbine Generator System in Windfarm
Figs. 10 and 11 show calculated results of peak value of
the lightning overvoltages for a parameter of the
grounding resistance R CG of the grounding conductor in
case of R TB =10 and 100 Ω. The values for R CG =0 Ω and
100 Ω are results of single turbine and no grounding
conductor, respectively.
Voltage [kV]
5
4
3
2
1
0
cable sending-end voltage
cable receiving-end voltage
X Tr secondary potential
0 20 40 60 80 100
Grounding resistance [Ω]
Fig. 10 - Calculated results of peak voltage for a parameter of
grounding resistance R CG (R TB =10 Ω).
10
8
6
4
2
0
Potential [kV]
0
0 20 40 60 80 100
Grounding resistance [Ω]
Fig. 11 - Calculated results of peak voltage for a parameter of
lightning current (100 Ω).
It is clear from Figs. 10 and 11, the grounding resistance
R CG does not affect the voltage on the wind turbine. The
potential is reduced by using the grounding conductor.
Fig. 12 shows calculated waveforms of the voltage on the
first, third and fifth wind turbines in cases of R TB =R CG =
100 Ω.
Voltage [kV]
4
3
2
1
0
0 10 20 30 40 50
‐1
Time [µs]
‐2
‐3
‐4
Fig. 12 - Calculated waveforms of the voltage on first, third
and fifth wind turbines.
Fig. 12 shows that significant difference is not observed
among the wind turbines.
Fig. 13 shows calculated waveforms in cases that the
grounding resistance R CG is 10 Ω and 100 Ω when R TB is
100 Ω.
4 CONCLUSION
This paper has discussed lightning overvoltages on wind
turbines in a windfarm for a direct lightning stroke to a
distribution line. The simulation results using the EMTP
show that the grounding resistance of the wind turbine
generator system does not affect the lightning overvoltages,
but the potential increases as the grounding
resistance becomes higher.
5 REFERENCES
0
283

Potential [kV]
8
6
4
2
0
‐2
Time [µs]
‐4
‐6
‐8
(a) Potential
Voltage [kV]
4
3
2
1
0
‐2
‐3
‐4
10 Ω
100Ω
‐1
Time [µs]
10 Ω
100Ω
(b) Voltage
Fig. 13 - Calculated waveforms on wind turbines in
windfarm.
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