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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 Voltage Transmission Line, Online Monitoring Units.

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: guof@ece.osu.edu, wang@ece.osu.edu).

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.

Energy Harvesting Devices for High Voltage Transmission Line Monitoring

Feng Guo, Student Member, IEEE, Hassan Hayat, and Jin Wang, Member, IEEE

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978-1-4577-1002-5/11/$26.00 ©2011 IEEE

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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. 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.

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].

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.

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 - - + + + + + +

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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

2 15.2 pFln

ctin

LCRr

πε= = , (1)

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

2 5.54 pF2ln

tg

out

LCH

R

πε= = . (2)

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.

Resistance (MΩ) Vct (V) I (uA) Power

(W) 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:

3

tgllct

ct tg

CVV

C C=

+ (3)

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 14.5 3769.5 261.5 0.98 132.8 3.2 1803.23 564.2 1.02 199.2 1.4 1186.42 847.2 1.01 288.7 0.7 860.35 1229.3 1.06 441.7 0.3 564.33 1881.3 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 (b) With active load

Fig. 6. Electric field strength inside and outside the tube (a) without and (b) with the active loading.

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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

21 ( )2W W WF v v Sρ= − , (4)

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

0

2I

Br

μπ

= , (5)

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

NMEF NBiL B LR

= = , (7)

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

2 2 2 22 22 20 0

2 2 2 24 ( / 2 ) 4 ( / 2 )MI IL v L vF N N

R RW x W xμ μ

π π= +

− +, (8)

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

2 2 2 2

02 2 2

2 1 2( )2 2

1 1( )4 ( / 2 ) ( / 2 )

d x dxm v S kxw wdtdtN I L dx

R dtW x W x

ρ

μπ

= − −

− +− +

, (9)

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

2 2

2 1 2( )2 2

1 1= ( )2 2w w

d x dxm v S kxw wdtdtdx dxS v S v S kxw w wdt dt

ρ

ρ ρ ρ

= − −

− + −

. (10)

Ignore the second-degree term of the derivative, the solution for (10) is

0 20( ) cos( 1 )tx t Ae t Aξω ξ ω−= − − + , (11)

where 0 /k mω = , 2

w wS vkm

ρξ = and 2

2w wS v

Ak

ρ= . Thus, the

induced voltage can be written as

0

0

02

2 2 20

20 0

2 [1 cos( 1 )]4

sin( 1 )

t

t

E NBLvN IL W

W A e t

A e t

ξω

ξω

μπ ξ ω

ω ξ ω ϕ

−

−

=

= ⋅− − −

⋅ − +

, (12)

where 2

1 1= tan

ξϕξ

− − −.

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) R

(Ω) 2000 0.1 0.3 150 0.7 2000 7 0.06 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

(b) Induced Voltage

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(c) Harvested Energy Fig. 8. Mathematical model calculation 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.

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.

(a) Displacement of the winding

(b) Induced Voltage

(c) Harvested Energy Fig. 9. Ansoft Simulation Results.

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

(b) Induced Voltage

(c) Harvested Energy Fig. 10. Energy harvesting with variable wind speed.

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

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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.

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