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Energy Harvesting Devices for High Voltage
Transmission Line Monitoring
Feng Guo, Student Member, IEEE, Hassan Hayat, and Jin Wang, Member, IEEE
Abstract— Sensor nodes with wireless communication
capability are essential elements in future intelligent
transmission line monitoring system. This paper focuses on
designing two types of transmission line mounted energy
harvesting devices to realize the self-power of these nodes.
First, the well-known electric field based energy harvesting
method is further analyzed to identify circuit design
guidelines. Then, a feasibility study of a low cost linear
generator that utilizes the magnetic field produced by high
voltage dc lines is presented. Although the power density of
the linear generator is low, a case study shows that it could be
utilized to continuously charge a battery for pulsed
communications.
Index Terms— Energy Harvesting, High
Transmission Line, Online Monitoring Units.
N
Voltage
I. INTRODUCTION
owadays, the implementations of renewable energy
sources are growing at an extremely fast pace around
the world. For wind power, the United States’ goal is that
by 2030 20% of electricity supply will be from wind [1].
For solar power, it is estimated that just in 2010, the
deployment of Photovoltaic (PV) modules is expected to
reach 3000 MW [2]. With expected deep penetration of
renewable energy, one common believe is that new high
voltage transmission systems are needed. The North
American Electric Reliability Corporation (NERC) projects
that the total number of miles of high voltage transmission
lines in the United States will increase by 9.5 percent
(15,700 circuit-miles) over the next ten years [3].
Ideally, as strategic assets, transmission lines need to be
monitored and maintained closely to ensure safe and
reliable operation. The related monitoring and maintenance
aspects include line temperature, line sag, icing, vibration,
corrosion in steel core, broken strand, corona, audible
noise, etc [4-9]. However, in reality, there is essentially no
real time monitoring in current transmission systems. The
common practice is helicopter or ground transportation
based line inspection. But routine line inspections are
pricy, and more importantly, usually fail to identify
developing problems or pinpoint fault locations in a timely
manner. Thus, the safety of the transmission network has
Feng Guo, Hassan Hayat, and Jin Wang are with the Department of
Electrical and Computer Engineering, the Ohio State University,
Columbus, OH 43210 (e- mail: [email protected], [email protected]).
978-1-4577-1002-5/11/$26.00 ©2011 IEEE
become one of the main concerns for the ever expanding
power grid. The solution is to build a real time monitoring
system for transmission line maintenance [10-16].
Recently, with the demand for a “Smart Grid”, the new
transmission lines planned for the next two decades have
made the real time monitoring related research more urgent
and meaningful [17].
For transmission line monitoring and maintenance, there
are two main sub-research directions: line robots and
distributed line-mount wireless sensors [18]-[20]. In these
devices, batteries are usually used as the major power
source [12]. The problem with the battery is that it needs to
be recharged from time to time, which restricts the
performance and increases the maintenance cost. In some
online monitoring units, current transformers are used to
charge the battery [13][14]. But when the line current is
low, these devices may not work well. This is especially
true for the case of transmission lines that are dedicated to
solar and wind power, considering the intermittency of
renewable energy sources. Besides, for the case of dc
transmissions, these current transformers will not work.
In summary, power supply still remains as a challenge
and bottleneck for transmission line monitoring and
maintenance [21][22]. The promising solution is in the high
voltage and power electronics combined energy harvesting.
II. EXISTING WORK
Energy harvesting is defined as the conversion of
ambient energy into a usable electrical form [23]. It is also
referred as energy scavenging. Typical energy harvesting
devices include PV panel, wind turbine, thermoelectric
generation, piezoelectric generation, electromagnetic field
harvester (current transformer), etc.
PV panel and horizontal axis wind turbine are generally
not considered as good power supplies for line monitoring
nodes because they need to be installed on the transmission
tower. Compared with direct line mounted devices, they
will have higher insulation and sensor costs.
Thermoelectric generation is also not suitable for
transmission line mounted energy harvesting because of the
wide temperature swings of both the high voltage
transmission line and ambient environment. Gas discharge
tube with controlled break down voltage is a possible
solution. But it is hard to scavenge a high frequency
current, which will bring problem for the transformer
design.
2
Similar to current transformers, electric field based
energy harvesting methods are only suitable for ac
transmission lines. For dc transmissions, electric field
based energy harvesting will become much less efficient.
In this case, wind or vibration based energy harvesting
methods, such as vertical wind turbine and piezoelectric
transducer, are more promising [25][26].
A promising method for ac line is the current
transformer. In [24], the authors demonstrated with
experimental results that a flux concentrator-core current
transformer with very small surface area (54×64 mm2) can
provide 257 mW when there is 1000 A in the transmission
line. Besides current transformer, another promising
method for ac line is the electric field based energy
harvesting. Electric field based energy harvesting relies on
capacitance coupling. In [4], the most up-to-date test
results in electric field based energy harvesting are
introduced. It is demonstrated that 380 mW could be
achieved with a 55 cm long tube shape energy harvester on
a 150 kV ac line.
In [4], the main power conditioning circuit has a very
simple structure as shown in Fig. 1. The reason that a
transformer is involved is mainly because the voltage drop
on the energy harvesting tube is usually in the order of
hundreds to thousands of volts. Because the rms value of
the load current keeps constant in the circuit (which will be
analyzed later), one challenge faced by this design is the
input voltage change during load dynamics. When the load
power is changing, the voltage across the energy harvesting
tube will change dramatically, which may lead the circuit
to work with very low efficiency or fail to work because of
under or over voltage.
Fig. 2. An alternative electric field energy harvesting method.
To summarize, a comparison between different high
voltage transmission line based energy harvesting methods
is shown in Table I. Considering size, complexity, power
rating, energy source availability, electric field based
energy harvesting is the most viable method for ac
transmission. However, the research on this topic is still at
its infancy. Most of the technical papers on this topic are at
proof of concept stage. Much more research work is still
needed. So far, no work has been done on energy
harvesting for the monitoring of high voltage dc
transmission lines.
In this paper, first, a detailed study on the electric field
based energy harvesting is provided. Though basic idea is
the same as energy harvesting tube presented in [4], more
detailed analysis is performed to investigate the design
considerations of the power conditioning circuit. Then for
the dc transmission lines, the paper presents a feasibility
study of utilizing wind energy and linear generator to
harvest energy. The proposed linear generator does not
involve expensive magnetic material but rely on the
magnetic field generated by the dc transmission line itself.
Fig. 1. Power conditioning circuit in [4].
In [15], [16], and [24], another configuration of electric
field based energy harvesting is presented. As shown in
Fig. 2, the basic idea is to use a floating capacitive structure
to harvest energy at a height relatively close to the ground.
The problem of this method is large electrode structure and
safety concerns because of the shortened distance between
high voltage potential to ground. The space between disks
of a multi-section insulator may also be used to harvest
energy. The available capacitance in this case is larger than
the aforementioned two methods. However, the voltage
distribution and resulting insulation strength will become a
serious issue if additional device is put on an insulator
string.
III. ENERGY HARVESTING AROUND HIGH VOLTAGE AC
TRANSMISSION LINE
Electric field based energy harvesting device works as a
capacitive voltage divider and can always harvest energy
TABLE I
COMPARISON BETWEEN DIFFERENT HIGH VOLTAGE TRANSMISSION LINE RELATED ENERGY HARVESTING METHODS.
Method
Size
Complexity of the
System
Power Rating
Energy Source
Availability
DC Network
Compliance
Magnetic field
+++
+++
++
++
-
Electric field
+
++
++
+++
-
Solar Energy
++
+
+++
-
+++
Horizontal Axis
Wind Turbine
-
-
+
++
+++
3
regardless the amplitude of the load current. The basic idea
is to use a simple energy harvesting tube (a simple metal
tube) to extract energy from electric field, which is shown
in Fig. 3(a). The principle of the harvester is shown in Fig.
3(b). When an energy harvesting tube is added to the
transmission line, the system could be seen as two
equivalent capacitors connected in series, where as Cct
stands for the equivalent capacitance between the high
voltage transmission line and the tube and Ctg represents
the equivalent capacitance between the tube and the earth
ground. The power conditioning circuit and its load, such
as sensors and communication units, can be seen as an
equivalent resistive load that are connected in parallel with
Cct.
(a)
(b)
Fig. 3. (a) Basic idea and (b) the principle of the energy harvesting tube
on ac transmission line.
In the following section, analysis on the equivalent
capacitance and associated simulation are presented. The
analysis and the simulation model are based on the voltage
ratings and heights of ac transmission lines in the United
States. The parameters of the analysis and simulations are
shown in Fig. 4.
A. Capacitance Analysis
The analysis work is mainly to find out the ballpark
number of the capacitances of Cct and Ctg.
Fig. 4. Simulation setup for electric field based energy harvesting.
The tube to line capacitance, Cct, can be roughly
calculated with a coaxial cylinder model shown in Fig.
5(a).
(a)
(b)
Fig. 5. The capacitance calculation model: (a) Coaxial model for tube to
line capacitance calculation and (b) Cylinder to ground model for tube to
ground capacitance calculation.
Assume the transmission line has a radius of r=0.02 m,
and the tube has an inner radius of Rin=0.15 m, the equation
for Cct can be written as
2πε L
Cct =
= 15.2 pF ,
(1)
Rin
ln
r
where L is the length of the tube, which is equal to 0.55 m.
The tube to ground capacitance, Ctg, can be roughly
calculated via a cylinder to ground model as shown in Fig.
5 (b). Using the method of image, assume the height of the
tube to ground is H=20 m, and the outer radius of the tube
is Rout=0.16 m, Ctg can be found as
2πε L
Ctg =
= 5.54 pF .
(2)
2H
ln
Rout
The calculation results are expected to be smaller than
the real numbers. The reason is that when the tube length
is comparable to the tube diameter, the edge effect of the
tube will add significant capacitance to the structure.
However, a rough calculation results can be used as
references for sanity check of the following simulation
results.
B. Simulations and Results Analysis
Simulations were carried out with Ansoft. Using the
same parameters with the calculation, the simulation results
show that the concerned capacitances are Cct=36 pF and
Ctg=11.3 pF. These numbers are in scale with the analysis
results and have reasonable larger value. So we have high
confidence in the simulation results and have used this
simulation model for the following two case studies.
1) Case 1: Line Voltage is Fixed at 230 kV
In this case, the equivalent load impendence (the
impedance in parallel with Cct, refer to Fig. 3 (b)), is
adjusted continuously to study the voltage drop cross the
Cct, and the energy that can be harvested. The results are
summarized in Table II. In the table, Vct is the voltage
between the tube and high voltage line, and I is the total
current that goes from the line to the tube at the equivalent
load impedance.
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TABLE II
TUBE TO LINE VOLTAGE AT DIFFERENT LOAD CONDITIONS.
Power
Resistance
I (uA)
Vct (V)
(W)
(MΩ)
0.5
282.7
565.6
0.16
1
565.2
565.6
0.319
1.5
847.3
564.8
0.479
3
1691
563.2
0.953
5
2807
561.8
1.576
It is shown that at the same line voltage, because the
load impedance is much smaller than the impedances of Cct
and Cct, the rms value of the current in the load keeps
almost constant. Therefore, if the power consumption is
increasing, the voltage stress on the tube will increase
almost linearly. This means two things: from energy
harvesting point of view, if more energy is needed, the
power conditioning circuits need to sustain higher voltage;
from efficiency point of view, the lower the efficiency of
the power conditioning circuit (more power consumption),
the higher the voltage stress on the same circuit. Thus, a
balance between the energy demand and the voltage stress
on the circuit needs to be considered during the design
process. Also, to minimize voltage stress at a desired
output power, the efficiency of the power conditioning
circuit needs to be optimized.
On the other hand, if there is no active power
consumption at all, according to the following equation:
Ctg
V
(3)
Vct = ll
3 Cct + Ctg
The voltage cross the tube could be as high as 31.7 kV
when the transmission line voltage is 230 kV. Please note
that this number is much larger than the voltages shown in
Table II. Fig. 6 shows a simulation comparison between
zero active load and one watt load. It is clearly shown that
without active load, the electric field concentration in the
tube would be much higher than the situation with active
load. Thus, compared with the case of finite load
resistance, the potential difference between the tube and the
transmission line would be extremely higher in the no load
case. For the circuit design and control, this means that a
dummy load needs to be switched on before the sensors
and communication units go into sleep modes.
2) Case 2: Load Power is Regulated at Around 1 W
In this case, the active load is fixed at around 1 W while
as the line voltage changes from 115 kV to 765 kV. The
purpose of this case study is to find out the different design
requirements for the power conditioning circuit at different
voltage levels. The simulated results are summarized in
Table III. It is shown that when the line voltage increases,
the voltage drop on the tube decreases from around 3.8 kV
to 0.56 kV. This means that the electric field based
harvesting is more suitable for higher voltage rating than
the magnetic field based harvesting (current transformer),
noting that current in Extra High Voltage (EHV) system for
renewable system can be very small from time to time.
Since at a constant load, the voltage stress on the tube
does go down with the line voltage, it is possible to build
one power conditioning circuit for multiple voltage levels.
TABLE III
TUBE TO LINE VOLTAGE AT DIFFERENT VOLTAGE LEVELS
Vcg (KV)
Resistance
(MΩ)
Vct(V)
I(uA)
Power
(W)
63.5
132.8
199.2
288.7
441.7
14.5
3.2
1.4
0.7
0.3
3769.5
1803.23
1186.42
860.35
564.33
261.5
564.2
847.2
1229.3
1881.3
0.98
1.02
1.01
1.06
1.06
In summary, combining the analysis and simulation, it
can be concluded that electric field based energy harvesting
is quite achievable:
1) at the same load power, the harvester’s voltage goes
down at higher line voltage. Thus there is possibility that a
single circuit design could fit several different transmission
voltage levels;
2) since lower efficiency will introduce higher voltage
stress and load dumping will result in instantaneous over
voltage, the design of the power conditioning circuit needs
to be highly efficient; and
3) the control would be quite delicate to shift power
between real load and dummy load or vise verse during
load transitions.
(a) Without active load
Fig. 6. Electric field strength inside and outside the tube (a) without and (b) with the active loading.
(b) With active load
5
C. Issues related with practical application
With different voltage levels and different power
transmission capability, not only the height of the conductor
will change, the line structure and conductor parameters will
also change a lot. For example, at some 230 kV and above
transmission lines, a bundle conductor is usually used. And
depending on the current level in the conductor, the diameter
of the conductor is different between transmission lines with
same voltage level.
The bundle structure will not greatly influence the tube size
design, but special attention needs to be paid. Keeping all
other parameters the same, assume the tube will be put on one
of the two bundled conductors, and the space between the two
conductors is 45 cm. The simulation results show that with the
bundle conductor, Cct becomes 33.1 pF, which keeps almost
the same, while Ctg decreases to 7.52 pF. This means at the
same voltage level, the available current in the energy
harvesting circuit will decrease. However, through proper
control, the same amount of power can be scavenged with
larger load resistance. Similar conclusion can be made for the
4-bundle or 6-bundle conductor. Besides, the electric field
strength around the tube is decreased with the bundle
conductor, so the insulation strength can also be decreased.
Thus, as long as the geometric size of the tube is suitable,
there is no special design consideration for the bundle
conductor.
The difference on the conductor diameter will not bring
much influence on the function of the tube either. Assume
another transmission line is using the conductor with a radius
of 1.4 cm. With the same tube size and voltage level, the
simulation result shows that the new values for Cct and Ctg are
34 pF and 12 pF, respectively. Compared to the 2 cm radius
case, the changes are both very small. Therefore, the same
tube design can be used for transmission lines with different
power level.
Insulation is another important issue that needs to be
thoroughly investigated, because the energy harvesting tube is
closely mounted on the high voltage transmission line.
Problems like corona and partial discharge will greatly disturb
the wireless communication and may damage the equipment
as well. Therefore, there should be no sharp edge on the tube,
and a corona ring can be used at the two ends of the tube.
More detailed insulation design will be studied in the future
work.
Effects of weather conditions, contamination, and moisture
also need to be considered for application purpose. Though
the tube itself can work as a shield and protect the circuit and
other components in it, additional components still needs be
designed in the future study.
IV. ENERGY HARVESTING AROUND HIGH VOLTAGE DC
TRANSMISSION LINE
Because dc current cannot go through the equivalent
capacitors, while the equivalent resistance between tube and
ground is nearly infinite, the aforementioned ac energy
harvester structure cannot be used in dc transmission line.
Thus, in this section, as shown in Fig. 7(a), a line mounted
linear generator utilizing wind energy is proposed.
(a) The machine structure
(b) Operation principle
Fig. 7. The proposed Linear Generator.
The generator uses the HVDC transmission line as the
stator, and a rectangular winding as the armature. To reduce
the cost, permanent magnets are not considered in this design.
When there is wind, with the help of the springs, the armature
can move back and forth in the static magnetic field produced
by the HVDC line, thus generating electricity. Though the
available power that can be scavenged is relatively small and
cannot be used as the main power source for the wireless
sensor node, it can be worked as a battery charger and make
the sensor nodes self-powered.
A. Analysis
When wind blows in perpendicular with the transmission
line, the relationship between wind speed and wind force can
be described as
1
FW = ρ (vW − v) 2 SW ,
(4)
2
where ρ is the air density, vw is the velocity of wind, v is the
velocity of the armature winding, and Sw is the frontal surface
area of the armature.
Assume the transmission line is infinite long and carries dc
current I, by Ampère's Law, the magnitude of flux density B
can be written as
6
μ0 I
,
(5)
2π r
where r is the distance between the point of concern and the
conductor.
If the winding moves in the magnetic field following the
direction in Fig. 7 (b), according to the Faraday’s Law, there
will be voltage generated in the winding. The induced voltage
can be described as
E = NBLv ,
(6)
where L is the length of the winding and N is the number of
the turns.
Because of the induced voltage, there will be current
flowing in the winding. Therefore, a magnetic force will be
added to the winding, which can be described as
E
FM = NBiL = NB L ,
(7)
R
where i is the current in the winding, and R is the total
equivalent resistance of the winding and external circuit.
If the reference point of the armature winding movement is
set at the point where the springs are at their natural length
and the transmission line is at the center of the winding, by
combing (4), (5), (6) and (7) together, the final expression of
magnetic force on the winding is converted into the following
format
μ02 I 2
μ02 I 2
L2 v
L2 v
2
+
, (8)
FM = N 2
N
4π 2 (W / 2 − x) 2 R
4π 2 (W / 2 + x) 2 R
where W is the width of the winding and x is the displacement
along the positive direction as shown in Fig. 7 (b).
According to Newton’s Laws of motion, notice that
v=dx/dt, the equation that defines the motion of the winding is
d2x 1
dx
m
= ρ (v − )2 S − kx
w
w
2
2
dt
dt
, (9)
N 2 μ02 I 2
1
1
L2 dx
(
)
−
+
4π 2
(W / 2 − x) 2 (W / 2 + x) 2 R dt
where k is the equivalent spring rate, and m is the mass of the
winding.
To solve this equation, noticing that compared with the
wind force and spring force, the magnetic force is negligible.
Thus, (9) can be rewritten as
d2x 1
dx
m
= ρ (v − )2 S − kx
w dt
w
2
2
dt
. (10)
1
1
dx
dx
= ρ S vw2 − ρ S vw
+ ρ S ( ) 2 − kx
w dt 2 w dt
2 w
Ignore the second-degree term of the derivative, the
solution for (10) is
B=
x(t ) = − Ae −ξω0t cos( 1 − ξ 2 ω0 t ) + A ,
where ω0 = k / m , ξ =
ρ S w vw
2 km
induced voltage can be written as
and A =
ρS v
2
w w
2k
E = NBLv
=
N μ0 IL
W
,
⋅ 2
2π
W
−ξω0 t
2
2
2
cos( 1 − ξ ω0 t )]
− A [1 − e
4
(12)
⋅ Aω0 e −ξω0 t sin( 1 − ξ 2 ω0 t + ϕ )
where ϕ = tan −1
− 1− ξ 2
.
ξ
Equation (12) shows that the induced voltage will increase
with the turns of the winding, the current in the transmission
line and the length of the winding. However, the change of
the number of turns and winding length will affect the mass of
the winding. Thus, the increment of induced voltage is not
linear with the number of turns and the length of the winding.
A design example of the proposed linear generator is
summarized in the in Table IV.
TABLE IV
SYSTEM PARAMETERS
N
W
(m)
L
(m)
k
(N/m)
m
(kg)
I
(A)
Vw
(m/s)
Sw
(m2)
2000
0.1
0.3
150
0.7
2000
7
0.06
R
(Ω)
1113
In the design, AWG #28 Aluminum wire is used in the
winding to reduce the total weight of the linear generator. The
2000 A dc current in the transmission line is chosen based on
typical HVDC projects data. The wind speed value is chosen
based on the data from American Wind Energy Association
(AWEA) [27]. Seven meter per second (15.7 mph) is a quite
moderate number for places with high wind energy potential.
With the system parameters in Table IV, (11), (12) and the
harvested energy can be plotted as shown in Fig. 8.
B. Simulation Verification
To verify the analysis presented in part A, simulations were
carried out with Ansoft. The results are shown in Fig. 9.
(a) Movement of the Winding
(11)
. Thus, the
(b) Induced Voltage
7
(c) Harvested Energy
Fig. 8. Mathematical model calculation results.
(c) Harvested Energy
Fig. 9. Ansoft Simulation Results.
Fig. 9 (a) shows a damped movement of the winding,
which is predicated by (11). Fig. 9 (b) shows the induced
voltage in the winding, with a maximum value of 1.8 V.
Though the voltage is oscillating, the frequency is relatively
small. This makes the power conditioning circuit easy to be
built and controlled. Fig. 9 (c) shows the total energy
harvested in the winding. Please note that some of the energy
will be consumed by the winding itself, and the maximum
available energy to external circuit is only half of what is
shown in the Figure. Also, because of the threshold voltage of
the semiconductor devices, when the induced voltage
decreases to certain level, the energy will not be able to be
used by the power conditioning circuit.
Compared the results in Fig. 8 and Fig. 9, it can be seen
that the calculation results and simulation results are
consistent with each other.
The mathematical model, which has been verified in part
B, is used to calculate this situation. The calculation results
are shown in Fig. 10. It clearly shows that with variable wind
speed, continuous energy harvesting is achieved. The average
output power for the simulated case is around 1 mW.
(a) Wind Speed
C. Real Operation with Variable Wind Speed
In reality, the wind speed changes from time to time.
Because this linear generator uses springs to store energy, this
change of wind speed will make the linear generator keep
oscillating and help the device to harvest energy continuously.
(b) Induced Voltage
(a) Displacement of the winding
(c) Harvested Energy
Fig. 10. Energy harvesting with variable wind speed.
(b) Induced Voltage
D. Feasibility Analysis
To prove the feasibility of this linear generator as a battery
charger, it is assumed that the wind speed is changing
frequently, and according to the calculation in part C, the
linear generator can continuously scavenge energy under this
condition. The main load of the circuit is the wireless
communication unit. The typical power consumption for an
XBee embedded SMT RF module [28] during transmitting
8
and receiving modes is approximate 105 mW, while the power
consumption at sleep mode is only around 3 µW. Assuming
the sensor unit needs to send and receive data every fifteen
minutes, and the average operation time for data
communication is 3 seconds [24], the energy consumption
during every 15 minutes period is around 318 mJ. If the
average output power of the linear generator is 1 mW, the
total energy scavenged during a 15 minutes interval will be
900 mJ, which is much more than what is required by the
wireless sensor node. Thus, it is safe to say that the linear
generator can harvest enough energy to recharge the battery
and make the wireless sensor node self-sustained.
V. CONCLUSION AND FUTURE WORK
In this paper, the power source problem for transmission
line monitoring units is introduced. And two viable devices to
realize self-power of these units are discussed.
First, an energy harvesting tube for ac transmission line is
studied. Design constrains of power conditioning circuit are
derived based on simulation results. At constant transmission
line voltage, the input voltage to the power conditioning
circuit will increase with load power. Thus, efficiency of the
circuit will have influence on voltage stress of the circuit. At
constant output power, the voltage stress of the power
conditioning circuit will decrease while the transmission line
voltage increases. Therefore, one circuit design could be
suitable for multiple voltage levels.
Then, a liner generator combining wind and magnetic field
energy harvesting is proposed and analyzed in detail. The
linear generator utilizes the magnetic field from the dc
transmission line, thus no expensive magnetic material is
needed in the construction. Though the power density of the
generator is low, a case study shows that it could provide
enough energy to charge batteries continuously.
Some issues should be considered for the practical
application of these two energy harvesters. Because they are
both mounted on the high voltage transmission line, insulation
problems like corona and partial discharge should be closely
studied. In the meantime, the influence of the operation
environment, like weather condition, contamination, line sag
and oscillation should also be considered. Detailed analysis
and design will be presented in future papers.
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
VI. REFERENCES
[1]
[2]
[3]
[4]
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“20% wind energy by 2030: increasing wind energy’s contribution to
U.S. electricity supply.” July 2008, [online]. Available:
http://www1.eere.energy.gov/windandhydro/wind_2030.html
B. Kroposki, R. Margolis, and D.Ton, “Harnessing the sun-- An
overview of solar technologies,” IEEE Power Energy Mag., vol. 7, pp.
22 – 33, May-June 2009.
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