A novel lightning protection technique of wind turbine components
M.A. Abd-Allah, A. Said, Mahmoud N. Ali
Electrical Engineering Department, Faculty of Engineering at Shoubra, Benha University, Cairo, Egypt
E-mail: abdo_eng1987@yahoo.com
Published in The Journal of Engineering; Received on 4th November 2015; Revised on 10th November 2015; Accepted on
11th November 2015
Abstract: The lightning energy can be very harmful to wind turbine (WT) farm components; therefore an effective lightning protection technique is required. In this study, a novel technique for WT components protection is presented. This technique used ferromagnetic rings placed
around the WT blade roots. Ferrite ring was moulded into particular shapes from the powder of compounds of ferric oxide, manganese, and
zinc, and then sintered. The dimensions of rings used are 990 mm (inner diameter), 1030 mm (outer diameter), and 100 mm (thickness). The
effectiveness of the novel technique in overvoltage mitigation during lightning strokes is presented and discussed. The results show that the
overvoltage is effectively damped with using this technique. The transient overvoltage at control devices is reduced to 16% of its original
value, while at distribution system; it is reduced to 5% of its original value.
1
Introduction
2
With a rapid growth in wind power generation, lightning hazard to
wind turbines (WTs) has become a great importance due to their
great height, distinctive shape, and exposed location. When a WT
is struck by lightning, high lightning current flows through the
WT and causes considerable damage to electrical equipment
inside the WT structure and WT nacelle [1].
To decrease downtime, repairs and blade damage, a welldesigned WT lightning protection is a necessity. Modern WT
blades are made of insulating materials such as glass fibre reinforced plastic as a common material or wood epoxy. The lightning
protection of WT blades can be classified as receptor, metallic cap,
mesh wire, and metallic conductor, as reported in IEC-61400-24
standards [1].
In general, the problem of lightning protection of WT blades is to
conduct the lightning current safely from the attachment point on
the blade to the hub and then to the ground. The electromagnetic
fields radiated from the lightning current flows can cause a
serious problem on WT tower, due to its coupling with control
equipment or data cables inside the tower, which may resulted in
WT internal control system damages [2, 3].
Another serious problem is the ‘back-flow surge’, which causes
damage, not only to the struck WT, but also to neighbouring turbines. High resistivity soil often makes Surge Arresters (SAs) at
tower grounding systems operate in reverse and allow backflow
of surge current to the grid. The phenomenon of surge invasion
from a WT that is struck by lightning to the distribution line in a
wind farm is quite similar to the case of ‘back-flow surge’ [4].
Due to significant influence on the wind farm behaviour under
lightning, the transient response must be either accurately measured, which is very expensive and time consuming, or reliably predicted by simulations, which allows for parametric studies and
optimisation. Up to now the predominant approach for simulating
transient due to lightning is based on wind farm components modelling by their equivalent circuits. In this paper, a novel lightning
protection technique for WT components and connected systems
is presented and discussed. This technique uses ferromagnetic
rings placed around the WT blade roots. Ferrite rings have simple
construction and low cost.
J Eng 2015
doi: 10.1049/joe.2015.0175
Onshore wind farm under study
The simulation is carried out on one feeder of the final stage of
onshore wind farm. This feeder is composed of two identical
wind power generators. The layout of this model is shown in
Fig. 1 [5]. Boost transformers (1 MVA, 660 V/6.6 kV) are installed
in the vicinity of the WT tower. All boost transformers are connected to the grid transformer (10 MVA, 6.6 kV/66 kV) by transmission line [6]. Surge arresters are inserted at the primary and
secondary sides of both boost and grid-interactive transformers.
The modelling of onshore wind farm components, such as WT
tower, synchronous generators, transformers, grounding system
and surge arrester is shown in Fig. 2.
A current function model called Heidler is now used widely to
model a lightning current [3–5]. Equation (1) represents the lightning current. A 400 Ω lightning path resistance was connected
shunt to the simulated natural lightning [6]
i(t) = I0
(t/t1 )2
e−t/t2
[(t/t1 )2 + 1]
(1)
where I0 is the peak of current and t1, t2 are time constants of
current rising and dropping, respectively.
The Vestas V47-690/200 kW blade and tower used in Zafarana
wind farm [3, 7–9] including control systems, is modelled by
π-lumped equivalent circuit as shown in Fig. 3 [3].
The capacitors C01–C010 are grounding capacities of tower, C12 is
the capacitive coupling path between tower and shielding layer of
cable, C23 is the capacitive coupling path between shielding layer
of cable and inner conductor, z1, z2, and z3 are the impedance of
tower, impedance of shielding layer of cable, and the impedance
of cable conductor, respectively [2].
The overhead lines are considered and represented by singlephase positive wave impedance (i.e. Surge impedance) with the
light velocity
L/C V
1
v = √ m/s
LC
Z0 =
(2)
(3)
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1
Fig. 1 Lightning hit WT#1 [5]
where C and L are the capacitance and inductance of line, respectively, Z0 is the surge impedance, and v is the propagation velocity
[3, 6, 10, 11].
A simplified model of surge arrester was derived from IEEE
model [5–7]. The model circuit is shown in Fig. 4. This model is
composed of two sections of non-linear resistances designated by
A0 and A1 which are separated by inductances L1 and L0. A parallel
resistance Rp (about 1 MΩ) is added to avoid the numerical instability of the combination of the current source and non-linear elements. To check the validity of the model, surge arresters
protection levels at different locations, i.e. low voltage (LV),
medium voltage (MV) and high voltage (HV), are compared with
ABB manufacture data [6].
Ground system model is based on the non-linear performance of
the grounding resistance with high currents, i.e. HV, high frequency
model [4, 7].
3
Fig. 3 WT tower model
Lightning problem at wind farm component
Besides the serious damage of the blades, lightning strokes severely
affect the control equipment and data cables. Due to electromagnetic fields radiated from lightning current, the LV circuit, generators,
boost transformer of struck and non-struck turbine and the grid are
threatened. The breakdowns of the equipment can also occur due to
the rise of ground potential from high soil resistivity. As a case
study when a lightning stroke of 51 kA 2/631 μs hits the blade of
WT WT#1, the transient voltage reaches 14,000 kV at the top of
the tower as shown in Fig. 5a while 2.5 MV at the control device
as shown in Fig. 5b. Fig. 6 shows that the voltage across polyethylene (PE) layer of control cable is very high and exceed the
Fig. 4 Pinceti and Giannettoni model
withstand voltage, which cause insulation breakdown. The
ground potential rise (GPR) reaches 224 kV as shown in Fig. 7a.
The voltage at the generator of stricken turbine reaches 224 kV as
Fig. 2 ATPDraw circuit of two WTs with frequency dependant model
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2
J Eng 2015
doi: 10.1049/joe.2015.0175
Fig. 5 Voltage at different location in WT
a Voltage at the top of tower
b Voltage at the control device
shown in Fig. 7b, while it reaches 10 kV at generator terminal of
non-stricken turbine as shown in Fig. 7c. The voltage at the grid
reaches 30 kV as shown in Fig. 8. These overvoltages endanger different equipment in the system and need to be mitigated.
4
Fig. 7 Voltage at different location in distribution system
a GPR of stricken turbine
b Voltage at generator terminal of stricken turbine
c Voltage at generator terminal of non-stricken turbine
Lightning protection technique
The novel technique uses a ferrite rings around WT blade roots, as
shown in Fig. 9, which is presented in this paper to attenuate the
effects of the lightning strokes on all wind farm components. In
this technique, when lightning strokes strike a tower blade, the
lightning current reaches the ferrite ring from a receptor through a
conductive wire installed on the blade. When a current flows
through the WT blade and attached to ferrite ring the energy of
the lightning stroke is stored temporarily in a magnetic field in
the ring. This resulted in high wave energy consumption, which
leads to high lightning overvoltage damping.
Different ferromagnetic materials are introduced in [12–15]. Iron
cores consist of alloys of iron (Fe), and small amounts of nickel
(Ni), cobalt (Co), and chrome (Cr). They have high relative permeability µr and saturation induction and they are good electrical conductors. Ferrites are hard and brittle polycrystalline ceramics made
of iron oxides mainly. Other oxides are introduced such as manganese, zinc, or nickel. The most common compositions are
Fig. 6 Voltage across the insulation layer
J Eng 2015
doi: 10.1049/joe.2015.0175
Fig. 8 Voltage at grid
Fig. 9 Lightning protection technique for WT
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3
Fig. 10 Magnetic ring protection systems
Fig. 11 Dimensions and equivalent circuit of ferrite ring
NiZnFe2O4 and MnZnFe2O4. They can have high relative permeability (40–10,000), a very large range of high resistivity (1–107)
Ω m.
Ferrite is a high-frequency non-linear magnetic material. The
ferrite rings are used around conductors to absorb the transient
energy from lightning. Coating the blade roots with magnetic material as shown in Fig. 10 are used as a proposed novel technique
to meet the requirements of lightning protection.
Ferrite rings are commonly used as anti-interference components
in electronic circuits, which basically behave as large inductances
to resist common-mode electromagnetic interference. In this case,
the ring behaves like a special transformer with no load, where the
eddy loss, hysteresis loss, and other anomalous losses related to ferromagnetic materials are considered [15]. The losses can be represented
by a damping resistor as shown in Fig. 11, which is coupled to the
primary circuit at frequencies in a certain damping band much
higher than the power frequency. If the transient frequency falls in
the frequency band of the ring material, the transient due to lightning
is suppressed to a certain extent.
According to the classical calculation of eddy loss in a thin ringshaped lamination, the equivalent damping resistance [15]
R=
6r
D1
ln
pH
D2
(4)
where ρ is the resistivity; H is the thickness; and D1 and D2 are the
outer and inner radius of the ring. The equivalent inductance is
L=
mH
D1
ln
2p
D2
(5)
where µ is the permeability
According to the equivalent circuit of Fig. 11, a sufficient equivalent inductance L is necessary for hindering the travelling wave. If
R = 0, i.e. a short circuit to a travelling wave, there will be no transient suppression effect in this case, regardless of the value of L. If
R is ∞, the ferrite ring provides only an inductance effect, which
lead to limit the steepness of the transients, without effect on transient amplitude. When the resistance R is matched with the inductive reactance XL, a considerable wave energy consumption and
transient damping can be obtained. The choice of the ferrite material should be optimised to achieve this matching.
Ferrite ring used was moulded into particular shapes from the
powder of compounds of ferric oxide, manganese, and zinc, and
then sintered, of a relative permeability of 1000. The dimensions
of rings used are 990 mm (inner diameter), 1030 mm (outer diameter), and 100 mm (thickness). The lightning stroke is 51 kA, 2/631
μs. The results show that, the voltage at tower top is reduced from
14,000 to about 910 kV, i.e. a reduction by about 94% as shown in
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J Eng 2015
doi: 10.1049/joe.2015.0175
Fig. 12 Voltage at different location of WT when using ferrite ring
a Tower top voltage
b Control device voltage
c Insulation layer distribution voltages
Fig. 13 Voltage at different location of WT distribution system when using
ferrite ring
a GPR of stricken turbine
b Voltage at generator terminal of stricken turbine
c Voltage at generator terminal of non-stricken turbine
Fig. 12a. At control device the voltage is reduced from 2.5 MV to
about 390 kV, i.e. by about 84% as shown in Figs. 12b and c. Also
GPR is reduced from 224 to about 5.8 kV, i.e. by about 97% as
shown in Fig. 13a. This reduction will reduce the back flow
current to the WT component and the grid. The voltage at generator
of stricken turbine is reduced from 224 kV to about 700 V, i.e.
reduced by about 97% as shown in Fig. 13b. At generator terminal
of non-stricken turbine the voltage is reduced from 10 kV to about
700 V, i.e. reduced by about 93% as shown in Fig. 13c. At the grid
the voltage is reduced from 30 to about 9 kV, i.e. reduced by about
92% as shown in Fig. 14.
5
Fig. 14 Voltage at grid when using ferrite ring
Conclusions
A novel technique uses ferromagnetic rings around blade roots is
presented to protect the WT components and connected distribution
systems from lightning strokes. The most important and innovative
point of the proposed system is simplicity and low cost. Ferrite rings
reduced effectively the amplitudes of lightning overvoltage, the
steepness and the peak of the transient overvoltage. Ferrite rings
reduce the transient overvoltage by about 84% at control devices.
The peak value of the overvoltage transient at distribution system
is reduced by about 95%. The ground potential rise is reduced by
about 97%, which effectively reduces the back flow current to the
WT component and the grid.
J Eng 2015
doi: 10.1049/joe.2015.0175
6
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J Eng 2015
doi: 10.1049/joe.2015.0175