A Novel Design of Radio Frequency Energy Relays on Power Transmission Lines

energies

Article A Novel Design of Radio Frequency Energy Relays on Power Transmission Lines

Jin Tong 1,*, Yigang He 1,*, Bing Li 1, Fangming Deng 1,2 and Tao Wang 1

1 School of Electrical Engineering and Automation, Hefei University of Technology, Hefei 230009, China; [email protected] (B.L.); [email protected] (F.D.); [email protected] (T.W.) 2 School of Electrical and Automation Engineering, East China Jiaotong University, Nanchang 330013, China * Correspondence: [email protected] (J.T.); [email protected] (Y.H.); Tel.: +86-155-5651-8921 (J.T.)

Academic Editor: Chang Wu Yu Received: 1 April 2016; Accepted: 13 June 2016; Published: 21 June 2016

Abstract: In this paper, we investigate the energy problem of monitoring sensors on high-voltage power transmission lines and propose a wireless charging scheme for a Radio Frequency IDentification (RFID) sensor tag to solve a commercial efficiency problem: the maintenance-caused power outage. Considering the environmental influences on power transmission lines, a self-powered wireless energy relay is designed to meet the energy requirement of the passive RFID sensor tag. The relay can obtain the electric field energy from the transmission lines and wirelessly power the RFID sensor tags around for longer operating distance. A prototype of the energy relay is built and tested on a 110 kv line. The measurement results show that the energy relay can provide stable energy even with the influences of wind, noise and power outage. To our knowledge, it is the first work to power the RFID sensor tags on power transmission lines.

Keywords: wireless energy transmission; radio frequency identiﬁcation (RFID) technology; high-voltage power transmission line

1. Introduction Electricity demand in China has been on the rise with the rapid development of modern industry, while the safety of high-voltage power transmission is becoming vital because of the large supply area of power substations [1]. However, there are many uncontrollable factors which need to be monitored in real time, threatening the safety of power transmission lines, such as harsh environments [2]. Thus, lots of devices, such as current sensors and temperature sensors [2,3], have been applied in practice. However, the electricity quantity and operation life of power sources in these devices will influence the commercial efficiency of the monitoring system. A battery is a common choice to power the monitoring devices, but in a power transmission system, the power outage due to the replacement of batteries will cause commercial losses [4]. Several techniques and devices are proposed to avoid a maintenance-caused power outage. A magnetoresistive sensor is designed for monitoring sag and electric current on the transmission tower, far away from high-voltage devices, but few factors can be measured in this way [3]. A smart “stick-on” sensor stuck on to power appliances is designed for autonomous temperature monitoring for low-cost applications. However, this sensor is only suitable for low-voltage devices. Also, its large weight restricts the arrangement on transmission lines [5]. On the other hand, some energy harvesting technologies have been proposed to replace built-in batteries. Photovoltaic panels are widely used in power systems nowadays, but their large size and weight limit their installation [6]. Energy harvesters obtain energy from a surrounding electric field or magnetic field to power the monitoring devices [7,8]. However, the measured positions may not be

Energies 2016, 9, 476; doi:10.3390/en9060476 www.mdpi.com/journal/energies Energies 2016, 9, 476 2 of 14 suitable to install these kinds of harvesters. Piezoelectric generators are also not valid to power the monitoring devices on-line due to their poor power stabilities [9]. Radio frequency identification (RFID) technology provides a feasible solution to save on maintenance costs. RFID technology is widely used in smart grids, such as a power asset management system, an intelligent equipment patrol management system, etc. [10–12], and some RFID sensor tags have already been used as monitoring devices in some areas [13–15]. Two benefits are obvious in the application of the RFID sensor tags on power transmission lines. First, RFID sensor tags can decrease the maintenance times because the low-cost tags can be arranged as cold reserves [16]. When an error occurs in an on-line tag, the reserved tags can be activated to replace the failed tag. Second, the RFID sensor tag is able to passively communicate with the interrogator on a zero-powered backscatter mechanism, resulting in simple architecture, low power dissipation and low fabrication cost [17,18]. The passive tags can achieve an operation distance over 6 m [19–21] and the active tags can even communicate over 20 m [22–24], so data communication devices such as RFID readers can start maintaining without a power outage. However, active tags also need batteries, and the communication distance of the passive tags cannot satisfy the needs of long-distance monitoring on power transmission lines. Thus, in this paper, the proposed research work investigates designing an energy relay to power the passive RFID sensor tags. Radio frequency (RF) waves are chosen to transmit energy, because other charging technologies based on electromagnetic induction and magnetic resonance are obstructed by the awful environmental influences on power transmission lines [25,26]. This paper was organized as follows. Section2 presents the operating principle and environment of the energy relay. Then, in Section3, a prototype of the relay has been designed for 110 kv lines. Finally, Section4 presents experimental results of the operation of the relay on a 110 kv power line. The conclusion is presented in Section5.

2. Theoretical Considerations

2.1. Power Source of RFID Tags In this paper, the relay is designed to power the passive RFID senor tags around wirelessly, as it will enlarge the tags’ operation range and substitute the batteries in tags. This means the communication devices uploading the monitoring data can achieve a safe operating distance from the power lines. A comparison of the different power sources of RFID tags is provided in Table1.

Table 1. Comparison of different power sources of radio frequency identiﬁcation (RFID) tags on power lines.

Power Source Advantage Disadvantage Small, light, high power Lithium battery Cannot work for years without charging [16] density, low cost Not always available (nights, occlusion with Can work even when Photovoltaic panel snow etc.), can be destroyed by hail, requires power line is turn off high capacity energy storage [7] Light enough to Requires electric current in the conductor, Thermobattery installation, can work needs radiator [27] under harsh environment Not always stable, requires high capacity Vibration-power Can work even when energy storage, moving parts can hardly be generating battery power line is turn off mounted on a power line [9] Radio frequency (RF) energy Light, have simple Needs a self-powered relay powered from relay (This work) circuit architecture Energies 2016, 9, 476 3 of 14 Energies 2016, 9, 476 3 of 13 Energies 2016, 9, 476 3 of 13 2.2. Principle of RFID-BasedRFID‐Based Monitoring Networks 2.2. Principle of RFID‐Based Monitoring Networks Figure1 1 sketches sketches the the basic basic idea idea of of the the relay-powered relay‐powered RFID RFID monitoring monitoring networks, networks, consisting consisting of of severalFigure RFID 1 sensorsketches tags the and basic datadata idea communicationcommunication of the relay‐powered devices. RFID Relays monitoring are used networks, to obtain consisting energy from of electricseveral ﬁeldsRFIDfields sensor and radiate tags and RF energydata communication to powerpower thethe RFIDRFIDdevices. sensorsensor Relays tags,tags, are whichwhich used areto obtain incorporatedincorporated energy from with variouselectric sensorsfields and to monitorradiate theRF environment.energy to power The the measurement RFID sensor data tags, is read which by RFID are incorporated readers operated with atvarious aa safe safe sensors distance distance to and monitorand uploaded uploaded the environment. to the to monitoringthe monitoring The measurement center center with thewith data communication the is read communication by RFID devices readers devices like operated SCADA like RemoteSCADAat a safe Terminal Remote distance Terminal Units and oruploaded mobileUnits or base tomobile the stations. monitoring base stations. center with the communication devices like SCADA Remote Terminal Units or mobile base stations.

Figure 1. Principle of the proposedproposed relay-poweredrelay‐powered radioradio frequencyfrequency identiﬁcationidentification (RFID)(RFID) monitoringmonitoring networksFigure 1. Principle on power of transmission the proposed lines.lines. relay ‐powered radio frequency identification (RFID) monitoring networks on power transmission lines. Figure 2 shows the structure of the passive RFID sensor tag for application on transmission lines. Figure2 shows the structure of the passive RFID sensor tag for application on transmission lines. In orderFigure to realize 2 shows online the structure monitoring, of the the passive RFID RFIDsensor sensor tag is tagequipped for application with two on antennas transmission for energy lines. In order to realize online monitoring, the RFID sensor tag is equipped with two antennas for energy harvestingIn order to andrealize ultra online high monitoring, frequency communicatingthe RFID sensor respectively. tag is equipped The with energy two harvester, antennas forcomposed energy harvesting and ultra high frequency communicating respectively. The energy harvester, composed of ofharvesting the energy and harvester ultra high antenna, frequency a matching communicating network, andrespectively. a RF‐DC rectifier,The energy can harvester,transfer the composed received the energy harvester antenna, a matching network, and a RF-DC rectiﬁer, can transfer the received RF RFof the energy energy to aharvester stable DC antenna, voltage a supply matching for network,the passive and RFID a RF sensor‐DC rectifier, tag [28,29]. can transfer the received energy to a stable DC voltage supply for the passive RFID sensor tag [28,29]. RF energy to a stable DC voltage supply for the passive RFID sensor tag [28,29].

Figure 2. Block diagram of double‐antenna radio frequency identification (RFID) sensor tag. Figure 2. Block diagram of double-antennadouble‐antenna radio frequency identificationidentification (RFID)(RFID) sensorsensor tag.tag. 2.3. Model of RF Wave Transmission 2.3. Model of RF Wave Transmission 2.3. ModelThe RF of RFenergy Wave transmission Transmission model can be equal to a free space propagation model because of The RF energy transmission model can be equal to a free space propagation model because of the lackThe of RF obstacles energy transmission around the proposed model can relay. be equal The energy to a free transmission space propagation is susceptible model to because ambient of the lack of obstacles around the proposed relay. The energy transmission is susceptible to ambient theenvironmental lack of obstacles factors around like wind, the proposedrain and electromagnetic. relay. The energy In this transmission paper, the is rain susceptible fade can be to ambientignored, environmental factors like wind, rain and electromagnetic. In this paper, the rain fade can be ignored, environmentalbecause the wavelength factors like of wind, RF waves rain and (about electromagnetic. 69 cm for a 433 In this MHz paper, wave) the is rain much fade longer can be than ignored, the because the wavelength of RF waves (about 69 cm for a 433 MHz wave) is much longer than the becausediameter the of the wavelength raindrop of(less RF than waves 0.6 (about cm). Also, 69 cm rain for mainly a 433 MHzinfluences wave) extra is much‐long longercommunication than the diameter of the raindrop (less than 0.6 cm). Also, rain mainly influences extra‐long communication diameterover thousands of the raindrop of meters, (less while than our 0.6 cm).energy Also, transmission rain mainly range influences is less extra-long than 10 m communication [30,31]. Hence over the over thousands of meters, while our energy transmission range is less than 10 m [30,31]. Hence the thousandsmain factors of meters,need to while be considered our energy are transmission the influences range of is the less electromagnetic than 10 m [30,31 ].environment Hence the main and main factors need to be considered are the influences of the electromagnetic environment and factorswind swing. need to be considered are the influences of the electromagnetic environment and wind swing. windThe swing. basic model of wave transmission on power lines is shown in Figure 3. The relay radiates a conicalThe beam basic and model the radiatingof wave transmission area St can be on expressed power lines as: is shown in Figure 3. The relay radiates a conical beam and the radiating area St can be expressed as:

Energies 2016, 9, 476 4 of 13

d 22 S  (1) t 4 where d is the distance between the relay and the tags,  is the vertical beamwidth of transmitter antenna. The RF energy is mainly transmitted in the Fresnel Zone and the power line will not cause any loss because of the astatic reflector of its rough surface. The energy loss mainly results from the line‐of‐sightEnergies transmission2016, 9, 476 loss, so the received power Pr in the radiated area can be expressed4 of 14 as [32]:

2 PGGttr The basic model of wave transmissionPr on power lines is shown in Figure3. The relay radiates a (2) (4 )22dL conical beam and the radiating area St can be expressed as:

2 2 where Pt is the transmitted power of the relay, Gπdt isθ the antenna gain of the transmitter, Gr is the St “ (1) antenna gain of the receiver, d is the distance between4 the relay to receiver,  is the wavelength of the radiated wave,where Ld isis the the distance loss betweenof hardware the relay which and the tags,is independentθ is the vertical beamwidthwith the oftransmission transmitter process. antenna. The RF energy is mainly transmitted in the Fresnel Zone and the power line will not cause If ignoring the effect of hardware (L = 1) and taking dB as the unit, Pr can be described by the any loss because of the astatic reﬂector of its rough surface. The energy loss mainly results from the following equation:line-of-sight transmission loss, so the received power Pr in the radiated area can be expressed as [32]:

2 PPrt 10[logPtGtG Grλ t log G r ] (3) Pr “ (2) p4πq2d2L Assuming that energy is distributed averagely in the radiating area, the received energy from where Pt is the transmitted power of the relay, Gt is the antenna gain of the transmitter, Gr is the antenna the tags’ antenna Pra can be described as: gain of the receiver, d is the distance between the relay to receiver, λ is the wavelength of the radiated wave, L is the loss of hardware which is independent with the transmission process. If ignoring the AArr4 effect of hardware (L = 1) and taking dBPPra as the unit, rPr can22 be described P r by the following equation: (4) Sdt  Pr “ Pt ` 10rlogGt ` logGrs (3) where Ar is the active area of the receiver antenna.

FigureFigure 3. Basic 3. Basic model model of of wave wave transmission transmission on power on power line. line.

Assuming that energy is distributed averagely in the radiating area, the received energy from the When the radiated energy in 433 MHz is 30 dBm, Gt = 12 dBi, Gr = 1 dBi,  = 10°, the MATLAB tags’ antenna P can be described as: simulation results of thera relationship between the received power and transmission distance are shown in Figure 4. The received power reducedAr quickly4Ar with the increasing of transmitting distance Pra “ Pr “ 2 Pr (4) St πd2θ within the range from 3 to 8 m. The simulated Pra is 26.29 μW at the distance of 7 m, which is still larger than wherethe typicalAr is the minimum active area of operating the receiver antenna.power (20 μW) of an RFID sensor tag [23]. ˝ When the radiated energy in 433 MHz is 30 dBm, Gt = 12 dBi, Gr = 1 dBi, θ = 10 , the MATLAB simulation results of the relationship between the received power and transmission distance are shown in Figure4. The received power reduced quickly with the increasing of transmitting distance within the range from 3 to 8 m. The simulated Pra is 26.29 µW at the distance of 7 m, which is still larger than the typical minimum operating power (20 µW) of an RFID sensor tag [23].

Figure 4. Relationship between received power and transmission distance.

Energies 2016, 9, 476 4 of 13

d 22 S  (1) t 4 where d is the distance between the relay and the tags,  is the vertical beamwidth of transmitter antenna. The RF energy is mainly transmitted in the Fresnel Zone and the power line will not cause any loss because of the astatic reflector of its rough surface. The energy loss mainly results from the line‐of‐sight transmission loss, so the received power Pr in the radiated area can be expressed as [32]:

2 PGGttr Pr  (2) (4 )22dL where Pt is the transmitted power of the relay, Gt is the antenna gain of the transmitter, Gr is the antenna gain of the receiver, d is the distance between the relay to receiver,  is the wavelength of the radiated wave, L is the loss of hardware which is independent with the transmission process. If ignoring the effect of hardware (L = 1) and taking dB as the unit, Pr can be described by the following equation:

PPrt 10[log G t log G r ] (3)

Assuming that energy is distributed averagely in the radiating area, the received energy from the tags’ antenna Pra can be described as:

AArr4 PPra r22 P r (4) Sdt  where Ar is the active area of the receiver antenna.

Figure 3. Basic model of wave transmission on power line.

When the radiated energy in 433 MHz is 30 dBm, Gt = 12 dBi, Gr = 1 dBi,  = 10°, the MATLAB simulation results of the relationship between the received power and transmission distance are shown in Figure 4. The received power reduced quickly with the increasing of transmitting distance within the range from 3 to 8 m. The simulated Pra is 26.29 μW at the distance of 7 m, which is still larger thanEnergies the typical2016, 9, 476 minimum operating power (20 μW) of an RFID sensor tag [23].5 of 14

Energies 2016, 9, 476 5 of 13

2.4. InfluenceFigure of Electromagnetism 4.Figure Relationship 4. Relationship between between received received power power and and transmission transmission distance. distance.

2.4.The Influence electromagnetic of Electromagnetism compatibility (EMC) has been analyzed and verified in [7]. By using a metal case, the electromagnetic influence has been greatly reduced, and the case does not decrease but The electromagnetic compatibility (EMC) has been analyzed and verified in [7]. By using a metal rather increases the radiated power due to the enlarged active area of the antenna. Furthermore, the case, the electromagnetic influence has been greatly reduced, and the case does not decrease but rather metalincreases case can the offer radiated the inside power electrical due to the components enlarged active excellent area of protection the antenna. from Furthermore, EMC disturbances the metal [7]. case can offer the inside electrical components excellent protection from EMC disturbances [7]. 2.5. Influence of Wind 2.5. Influence of Wind Figure 5a shows the wind influence of the same line energy transmission between the relay and the tags. FigureCurvature5a shows change the wind in the influence wire will of thechange same the line relative energy transmission position of betweenthe relay the and relay the and tags. In the verticalthe tags. direction, Curvature the change relative in the height wire will between change the the relativerelay and position the tags of the changes relay and little the because tags. In the a wire vertical direction, the relative height between the relay and the tags changes little because a wire with with a low elastic coefficient is settled on the transmission tower. Along the wire direction, w1 is the a low elastic coefficient is settled on the transmission tower. Along the wire direction, θw1 is the static static radial direction, w2 is the radial direction in wind, and the angular variation a results from radial direction, θw2 is the radial direction in wind, and the angular variation ∆θa results from the the changechange ofof straight-line straight‐line distance distance from from the relay the to relay tags and to thetags sag and of thethe wire. sag Actually,of the wire. the straight-line Actually, the straightdistance‐line distance of energy of transmission energy transmission is less than is 10 less m, than and much 10 m, shorter and much than shorter the span than length the between span length betweentwo towerstwo towers (over 300(over m), 300 hence m), the hence angular the variation angular caused variation by the caused curvature by the change curvature can be ignored.change can be ignored.Based on Based the above on the analysis, above a analysis, high-gain, a narrow-beam, high‐gain, narrow directional‐beam, antenna directional can be used antenna in the can energy be used in thetransmission energy transmission over the same over wire the to same enhance wire the to density enhance of radiatedthe density energy. of radiated energy.

FigureFigure 5. Wind 5. Wind influence inﬂuence on on energy energy transmission: transmission: ( a) Transmission on on the the same same line; line; (b) ( Crossb) Cross section section of transmissionof transmission on ondifferent different lines. lines.

Figure 5b shows the cross‐section view of energy transmission between two lines. Assuming that two wires are in the same amplitude, the dotted lines represent the static position of the antennas and the full lines represent the maximum deviating position of the antenna in wind. The angular variation of the two radial directions  can be expressed as:

21  ()(())       (5)

where  is the angle of wind deflection, 1 is the angle between the radial direction and vertical direction without wind, and 2 is the angle between the radial direction and vertical direction with the maximum wind. Therefore, the beamwidth of the radiating antenna should be larger than double of the maximum swing angle max. The swing angle  can be described as:

g4 arctan (6) g1

where g1 is the wire gravity load per unit area as shown in Equation (7), and g4 is the wind load per unit area, as shown in Equation (8): Wg g 0 103 (7) 1 S

Energies 2016, 9, 476 6 of 14

Figure5b shows the cross-section view of energy transmission between two lines. Assuming that two wires are in the same amplitude, the dotted lines represent the static position of the antennas and the full lines represent the maximum deviating position of the antenna in wind. The angular variation of the two radial directions ∆θ can be expressed as:

∆θ “ θ2 ´ θ1 “ pπ ´ αq ´ pπ ´ pα ` βqq “ β (5) where β is the angle of wind deﬂection, θ1 is the angle between the radial direction and vertical direction without wind, and θ2 is the angle between the radial direction and vertical direction with the maximum wind. Therefore, the beamwidth of the radiating antenna should be larger than double of the maximum swing angle βmax. The swing angle β can be described as: g β “ arctan 4 (6) g1 where g1 is the wire gravity load per unit area as shown in Equation (7), and g4 is the wind load per unit area, as shown in Equation (8): W ¨ g g “ 0 ˆ 10´3 (7) 1 S where W0 is the weight of the wire, S is the sectional area of the wire, and g is gravitational acceleration.

αKv2 d g “ w g ˆ 10´3 MPa{m (8) 4 16S where α is uneven factor of wind velocity, K is the factor of wire size, vw is wind velocity, d is the external diameter of the wire. Based on the Equations (6)–(8), β can be described as:

g αKv2 d β “ arctan 4 “ arctan w (9) g1 16W0

Based on “code for designing of 110–750 kv overhead transmission line” (GB50545-2010) [31], the maximum design wind velocity (vw,max) of the wire which installed over 10 m high should not lower than 26.85 m/s. When the wind velocity is 26.85 m/s (vw = vw,max), α = 0.61, using LGJ-240/40 as the 2 wire, d = 21.66 mm, S = 277.75 mm , K = 1.05, W0 = 964.3 kg/km, the maximum swing angle βmax can be calculated as 32.95 based on Equation (9). ˝ Based on the analysis above, the beamwidth of radiating antenna should be over 65.90 (2 βmax). However, adopting wide-angle antennas will decrease the density of radiated energy, so we add bearings as the supporting structures of the relay, ensuring the antenna to radiate in a stable direction due to the gravity of the relay.

3. Prototype Design Figure6a shows the structure of energy harvester, which is mainly constructed with an aluminum-made tube. The surface of the tube is polished in order to avoid partial discharge and corona. The tube is installed on the wire with the supporting insulators and bearings to obtain energy from the time-varying electric ﬁeld. r1 is the radius of the wire, r2 is the radius of the tube, l is the length of the tube, d is the distance from the tube to the ground. C1 is the capacitance of the tube-formed capacitor, and C2 is the earth capacitance of the tube. Figure6b shows the architecture of the relay. The relay consists of the energy harvesting tube, a voltage conversion circuit and an energy transmitter. The voltage conversion circuit is used to provide a stable continuous energy to charge the ultra-capacitor Cu, Cu is used to store energy and power the transmitter continuously, and the transmitter is used to radiate energy with RF waves through the transmitter antenna. Energies 2016, 9, 476 6 of 13 where W0 is the weight of the wire, S is the sectional area of the wire, and g is gravitational acceleration. Kv2 d g w gMPam103 / (8) 4 16S where  is uneven factor of wind velocity, K is the factor of wire size, vw is wind velocity, d is the external diameter of the wire. Based on the Equations (6)–(8),  can be described as: g Kvd2 arctan4  arctan w (9) g1016W

Based on “code for designing of 110–750 kv overhead transmission line” (GB50545‐2010) [31], the maximum design wind velocity (vw,max) of the wire which installed over 10 m high should not lower than 26.85 m/s. When the wind velocity is 26.85 m/s (vw = vw,max),  = 0.61, using LGJ‐240/40 as the wire, d = 21.66 mm, S = 277.75 mm2, K = 1.05, W0 = 964.3 kg/km, the maximum swing angle max can be calculated as 32.95 based on Equation (9). Based on the analysis above, the beamwidth of radiating antenna should be over 65.90° (2 max). However, adopting wide‐angle antennas will decrease the density of radiated energy, so we add bearings as the supporting structures of the relay, ensuring the antenna to radiate in a stable direction due to the gravity of the relay.

3. Prototype Design Figure 6a shows the structure of energy harvester, which is mainly constructed with an aluminum‐made tube. The surface of the tube is polished in order to avoid partial discharge and corona. The tube is installed on the wire with the supporting insulators and bearings to obtain energy from the time‐varying electric field. r1 is the radius of the wire, r2 is the radius of the tube, l is the length of the tube, d is the distance from the tube to the ground. C1 is the capacitance of the tube‐ Energies 2016, 9, 476 7 of 14 formed capacitor, and C2 is the earth capacitance of the tube.

Figure 6. PrototypeFigure of 6. thePrototype proposed of the relay: proposed (a) relay:Structure (a) Structure of energy of energy harvester; harvester; (b ()b Architecture) Architecture of of energy relay. energy relay.

Figure 3.1.6b Energyshows Harvester the architecture on 110 kv Lines of the relay. The relay consists of the energy harvesting tube, a voltage conversion circuit and an energy transmitter. The voltage conversion circuit is used to The energy tube with a radius of 10 cm and a length of 60 cm can be equal to a ﬂoating capacitor. u u provide a stableThe capacitance continuous of the energy tube C1 is to given charge by the ultra‐capacitor C , C is used to store energy and power the transmitter continuously, and the transmitter is used to radiate energy with RF waves 2πε0εl through the transmitter antenna. C1 “ (10) ln pr2{r1q

l r ε 3.1. Energy Harvesterwhere is the on length 110 ofkv the Lines tube, 2 is radius of the tube, 0 is dielectric constant between tube and wire, and ε0 is the relative dielectric constant of air. Assuming that the wire is installed at a height of 10 m The energyabove tube the ground with ( da= radius 10 m), the of earth 10 cm capacitance and a length of the tube of 60C2 iscm express can be as: equal to a floating capacitor.

The capacitance of the tube C1 is given by 2πε0l “ C2 ´1 (11) cosh pd{r2q 2l C  0 Because d is much larger than r2, the voltage1 V harvested from the tube can be described as: (10) lnrr21 / C2 C2 ? V “ V0 “ Vac 2sinpωtq (12) C1 ` C2 C1 ` C2

where V0 is the real-time voltage of the wire, Vac is the voltage effective value of the wire. On a 110 kv line, assuming that r1 = 0.01083 m, r2 = 0.1 m, ε = 1, l = 0.6 m, d = 10 m the voltage V of the tube is 33.32 kv. When the relay is installed at a height of 10 m above the ground, C2 can be calculated to be 4.69 pF according to Equation (11). However, it is hard to test a harvester over 10 m high in laboratory, so we designed a capacitor composed of a copper box and a small copper plate to simulate the earth capacitance. The COMSOL Multiphysics simulation of the self-made capacitor is shown in Figure7. The size of the copper box is 1 m ˆ 0.5 m ˆ 1 m, the thickness of the box shell is 0.02 m and the size of the plate is 85 mm ˆ 85 mm ˆ 20 mm. When the plate is placed at the center of the box, the simulated capacitance is 4.90693 pF. Energies 2016, 9, 476 7 of 13 where l is the length of the tube, r2 is radius of the tube, 0 is dielectric constant between tube and wire, and 0 is the relative dielectric constant of air. Assuming that the wire is installed at a height of 10 m above the ground (d = 10 m), the earth capacitance of the tube C2 is express as：

20l C2  1 (11) cosh (dr /2 )

Because d is much larger than r2, the voltage V harvested from the tube can be described as：

CC22 VV0 Vtac 2sin(  ) (12) CC12 CC 12 where V0 is the real‐time voltage of the wire, Vac is the voltage effective value of the wire. On a 110 kv line, assuming that r1 = 0.01083 m, r2 = 0.1 m,  = 1, l = 0.6 m, d = 10 m the voltage V of the tube is 33.32 kv. When the relay is installed at a height of 10 m above the ground, C2 can be calculated to be 4.69 pF according to Equation (11). However, it is hard to test a harvester over 10 m high in laboratory, so we designed a capacitor composed of a copper box and a small copper plate to simulate the earth capacitance. The COMSOL Multiphysics simulation of the self‐made capacitor is shown in Figure 7. The size of the copper box is 1 m × 0.5 m × 1 m, the thickness of the box shell is 0.02 m and the size of the plate is 85 mm × 85 mm × 20 mm. When the plate is placed at the center of the box, the simulated Energies 2016, 9, 476 8 of 14 capacitance is 4.90693 pF.

FigureFigure 7. COMSOL 7. COMSOL Multiphysics Multiphysics simulation of of self-made self‐made capacitor. capacitor.

3.2. Voltage3.2. Conversion Voltage Conversion Circuit Circuit As shownAs shown in Figure in Figure 88,, the the voltage voltage conversion conversion circuit contains circuit a high-voltage contains potentiala high transformer‐voltage potential (T), a rectifier, a voltage regulator and an ultra-capacitor Cu. The primary side of the transformer transformerconnects (T), the a tuberectifier, with the a wire,voltage and theregulator secondary and side an is connectedultra‐capacitor to the rectifier. Cu. The A large-resistance primary side of the transformerresistor connectsR2 is used the to tube decrease with the the high wire, voltage and onthe the secondary primary side side of is the connected potential transformer. to the rectifier. A large‐resistanceThe capacitor resistorC2 and R the2 is primary used to coil decrease of transformer the high compose voltage a low on pass the filter primary to filter high side frequency of the potential transformer.and high The voltage capacitor pulse C current.2 and Thethe resistorprimaryR1 coiland diodeof transformerD1 are used tocompose eliminate a the low mal-operation. pass filter to filter The diodes D2–D5 compose the rectifier, a buck circuit consists of the voltage regulator with a giant high frequency and high voltage pulse current. The resistor R1 and diode D1 are used to eliminate the transistor G, an inductor L and a capacitor C. A clamping diode (D7) is used as an overvoltage mal‐operation. The diodes D2–D5 compose the rectifier, a buck circuit consists of the voltage regulator protection of 5 v. The ultra-capacitor Cu not only stores energy to cope with the power outage, but 7 with a giantalso regulates transistor the output G, an voltage. inductorD9 is L a voltage-regulatorand a capacitor diode C. operatingA clamping under diode reverse ( breakdownD ) is used as an overvoltagestate protection to ensure the of ultra-capacitor 5 v. The ultra charged‐capacitor well. The Cu not switch onlyS1 willstores be closedenergy to to power cope the with energy the power outage, buttransmitterEnergies also 2016 regulates, when9, 476 the the power output outage voltage. happens. D9 is a voltage‐regulator diode operating under8 of 13 reverse breakdown state to ensure the ultra‐capacitor charged well. The switch S1 will be closed to power the energy transmitter when the power outage happens.

FigureFigure 8.8. VoltageVoltage conversion conversion Circuit. Circuit.

When designed for a 110 kv line, the peak voltage on C1 can be calculated as 47.13 kv based on When designed for a 110 kv line, the peak voltage on C1 can be calculated as 47.13 kv based on the Equation (12), and its effective value of voltage is 33.32 kv. Thus, we choose a 4 M resistor (R2) the Equation (12), and its effective value of voltage is 33.32 kv. Thus, we choose a 4 MΩ resistor (R2) and a high‐voltage potential transformer (T) with the 1000:5 transformation ratio. When charging and a high-voltage potential transformer (T) with the 1000:5 transformation ratio. When charging load Z is 1 , L = 0.9 mH, C = 72 uF, the Multisim simulation results are shown in Figure 9, and the load Z is 1 Ω, L = 0.9 mH, C = 72 uF, the Multisim simulation results are shown in Figure9, and the charging voltage Vc of ultra‐capacitor is 4.7 v. charging voltage Vc of ultra-capacitor is 4.7 v. 5

4.5

3.5

2.5 Uo/V 2

1.5

0.5

0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 t/s Figure 9. Multisim simulation of charging voltage.

3.3. Energy Transmitter Differed from the traditional radio communication [33,34], the design of our wireless power transmitter is not necessary to consider the influence of noise. Figure 10 shows the structure of energy transmitter. The ultra‐capacitor Cu works as the energy supply for the entire circuit. A Microprogrammed Control Unit (MCU) is used to stabilize the output frequency and deal with the influences from outside, like the power outage. When a power outage happens, the MCU can stabilize the output of Cu to ensure the energy supply as well as the frequency of radiated waves for three minutes. A voltage‐controlled crystal oscillater (VCXO)—VG‐4513CA (EPSON, Nagano‐ken, Japan) is used to provide a stable RF signal of 433 MHz. A low dropout regulator (LDO)—LM1117T‐ADJ (National Semiconductor, Santa Clara, CA, USA), controlled by MCU, is responsible for adjusting the control voltage of VCXO. Some feedback from the output end of the LDO and the VCXO are connected to MCU. The waves are enhanced by a power amplifier (PA) (BLT53A), and then filtered and radiated by a band‐pass filter and an antenna. A small, light microstrip antenna is chosen as the transmitter antenna, because it can be easily installed in the tube to insulate from the strong electromagnetic environment.

Energies 2016, 9, 476 8 of 13

Figure 8. Voltage conversion Circuit.

When designed for a 110 kv line, the peak voltage on C1 can be calculated as 47.13 kv based on the Equation (12), and its effective value of voltage is 33.32 kv. Thus, we choose a 4 M resistor (R2) and a high‐voltage potential transformer (T) with the 1000:5 transformation ratio. When charging Energiesload Z2016 is 1, 9, 476, L = 0.9 mH, C = 72 uF, the Multisim simulation results are shown in Figure 9, and9 of the 14 charging voltage Vc of ultra‐capacitor is 4.7 v.

4.5

3.5

2.5 Uo/V 2

1.5

0.5

0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 t/s

Figure 9. Multisim simulation of charging voltage.voltage.

3.3. Energy Transmitter Differed fromfrom thethe traditionaltraditional radioradio communicationcommunication [[33,34],33,34], the design of our wirelesswireless powerpower transmittertransmitter is is not not necessary necessary to toconsider consider the influence the influence of noise. of noise. Figure 10 Figure shows 10 the shows structure the structureof energy transmitter. The ultra‐capacitor Cu works as the energy supply for the entire circuit. A of energy transmitter. The ultra-capacitor Cu works as the energy supply for the entire circuit. AMicroprogrammed Microprogrammed Control Control Unit Unit (MCU) (MCU) is is used used to to stabilize stabilize the the output output frequency frequency and deal with the influencesinfluences fromfrom outside,outside, likelike thethe powerpower outage.outage. When a power outage happens, thethe MCU can stabilize the output of Cu to ensure the energy supply as well as the frequency of radiated waves for three the output of Cu to ensure the energy supply as well as the frequency of radiated waves for three — minutes. A voltage-controlledvoltage‐controlled crystal oscillater (VCXO)—VG-4513CA(VCXO) VG‐4513CA (EPSON, Nagano-ken,Nagano‐ken, Japan) — isis usedused toto provideprovide aa stablestable RFRF signalsignal ofof 433433 MHz.MHz. AA lowlow dropoutdropout regulatorregulator (LDO)—LM1117T-ADJ(LDO) LM1117T‐ADJ (National(National Semiconductor, Santa Santa Clara, Clara, CA, CA, USA), USA), controlled controlled by by MCU, MCU, is responsible is responsible for foradjusting adjusting the thecontrol control voltage voltage of VCXO. of VCXO. Some Some feedback feedback from from the the output output end end of of the the LDO LDO and and the the VCXO areare connectedconnected to MCU. The waves are enhanced by a power amplifieramplifier (PA) (BLT53A), andand thenthen filteredfiltered and radiated by a a band band-pass‐pass filter filter and and an an antenna. antenna. A Asmall, small, light light microstrip microstrip antenna antenna is chosen is chosen as the as thetransmitter transmitter antenna, antenna, because because it can it can be beeasily easily installed installed in in the the tube tube to to insulate insulate from from the strongstrong electromagnetic environment. Energieselectromagnetic 2016, 9, 476 environment. 9 of 13

Figure 10. StructureStructure of the energy transmitter.

4. Experiment Experiment and and Results Results Figure 11a11a shows shows the the testing testing environment environment for for the the proposed proposed relay. relay. The Therelay relay is installed is installed on a steel on a‐ coredsteel-cored aluminium aluminium strand strand wire (LGJ wire‐240/40) (LGJ-240/40) and the design and the of design our RFID of temperature our RFID temperature sensor tag is sensor from thetag reference is from the [31]. reference The 110 [ 31kv]. voltage The 110 is kvprovided voltage by is a provided 110 kv transformer. by a 110 kv The transformer. energy is received The energy by ais 20 received cm × 20 bycm aantenna 20 cm ˆand20 analyzed cm antenna by a andreal‐ analyzedtime signal by analyzer—RSA5115B a real-time signal analyzer—RSA5115B (Tektronix, Johnston, (Tektronix,SC, USA). The Johnston, antenna SC, is located USA). 4 The m away antenna from is the located relay 4 when m away testing from the the influences relay when of noise, testing power the outageinﬂuences and of wind. noise, Several power ignition outage and coils wind. connected Several with ignition high coils‐voltage connected sources with are high-voltageused to generate sources the sparkare used discharge, to generate which the is spark the main discharge, cause of which the noise is the on main transmission cause of thelines. noise The on overall transmission weight of lines. the Theproposed overall relay weight is 2.79 of the kg, proposed which is less relay than is 2.79 the kg, maximum which is load less permitted than the maximum on the power load transmission permitted on line [30]. The photo of the energy transmitter and the far‐infrared temperature instrument (Victor (Xian, China) VC305) is shown in Figure 11b,c respectively.

Figure 11. Testing environment: (a) Experimental scene; (b) Photo of the energy transmitter; (c) Far‐ infrared temperature instrument.

Figure 12a shows the measurement results of the channel power at different measuring distances. The measured channel power at the distance of 6m is 26.92 μW (−1.57 dBm), which is higher than the typical minimum operating power of an RFID tag [23]. Figure 12b shows results of the channel power when testing the power outage influence. We simulated the power outage at the 3rd min and then restored the power at the 7th min. The results prove that the relay can continue operation for 3 min after the power outage, and can quickly recover within 5 s after the power is restored. When testing the influence of noise, the receive antenna is placed 4m away from the relay. Figure 12c shows the measured channel power is −7.95 dBm. As shown in Figure 12d, we measured the channel power again when spark discharges were generated, the measured channel power is −8.31 dBm. Comparing these two results, we can conclude that the channel power is reduced by an acceptable 7.74% during spark discharge.

Energies 2016, 9, 476 9 of 13

Figure 10. Structure of the energy transmitter.

4. Experiment and Results Figure 11a shows the testing environment for the proposed relay. The relay is installed on a steel‐ cored aluminium strand wire (LGJ‐240/40) and the design of our RFID temperature sensor tag is from the reference [31]. The 110 kv voltage is provided by a 110 kv transformer. The energy is received by a 20 cm × 20 cm antenna and analyzed by a real‐time signal analyzer—RSA5115B (Tektronix, Johnston, SC, USA). The antenna is located 4 m away from the relay when testing the influences of noise, power outage and wind. Several ignition coils connected with high‐voltage sources are used to generate the spark discharge,Energies 2016 which, 9, 476 is the main cause of the noise on transmission lines. The overall 10weight of 14 of the proposed relay is 2.79 kg, which is less than the maximum load permitted on the power transmission line [30].the The power photo transmission of the energy line [30]. transmitter The photo of theand energy the far transmitter‐infrared and temperature the far-infrared instrument temperature (Victor (Xian, China)instrument VC305) (Victor is shown (Xi’an, China) in Figure VC305) 11b,c is shown respectively. in Figure 11 b,c respectively.

Figure 11.Figure Testing 11. environment:Testing environment: (a) Experimental (a) Experimental scene; scene; (b) Photo (b) Photo of the of the energy energy transmitter; transmitter; (c) Far‐ infrared (temperaturec) Far-infrared temperatureinstrument. instrument.

Figure Figure12a shows 12a shows the the measurement measurement results results of the of channel the channel power at different power measuring at different distances. measuring µ distances.The The measured measured channel channel power at power the distance at the of 6m distance is 26.92 ofW( 6m´1.57 is dBm),26.92 which μW is(− higher1.57 dBm), than the which is typical minimum operating power of an RFID tag [23]. Figure 12b shows results of the channel power higher thanwhen the testing typical the powerminimum outage operating influence. Wepower simulated of an the RFID power tag outage [23]. atFigure the 3rd 12b min shows and then results of the channelrestored power the powerwhen at testing the 7th the min. power The results outage prove influence. that the relay We can simulated continue operation the power for outage 3 min at the 3rd min afterand thethen power restored outage, the and power can quickly at the recover 7th withinmin. The 5 s after results the power prove is restored.that the relay can continue operation forWhen 3 min testing after the the influence power ofoutage, noise, theand receive can quickly antenna isrecover placed 4mwithin away 5 froms after the the relay. power is restored.Figure 12c shows the measured channel power is ´7.95 dBm. As shown in Figure 12d, we measured the channel power again when spark discharges were generated, the measured channel power is When´8.31 testing dBm. the Comparing influence these of two noise, results, the wereceive can conclude antenna that is theplaced channel 4m power away is from reduced the byrelay. an Figure 12c showsacceptable the measured 7.74% during channel spark discharge.power is −7.95 dBm. As shown in Figure 12d, we measured the channel powerWhen again testing when the influence spark discharges of the wind, fourwere blowers generated, are used the to provide measured various channel wind velocities. power is −8.31 dBm. ComparingFigure 13 shows these the two harvested results, energy we from can the conclude receiving that antenna. the Thechannel energy power transmission is reduced has an by an acceptableexcellent 7.74% stability during under spark wind discharge. and the maximum measured error of the channel power is less than 15.31%. Also, it proves that the energy loss mainly results from the position change of the relay: when the relay rotates slightly, the received energy tends to be stable. Figure 14 compares the measurement results from the sensor tag and the far-infrared temperature instrument respectively. The RFID reader was placed 6 m away from the relay to read the measurement data. Figure 14a shows the results of temperature rise without wind and Figure 14b shows the results when the velocity of wind is 2.9 m/s and 6.2 m/s respectively. The results measured by the tag and the VC305 show an excellent consistency, and the maximum measured error is less than 4.4%. Powered by the proposed relay, the RFID sensor tag can operate at a maximum distance of 18 m away from the RFID reader, which is much larger than the required safe distance of 3 m in a power system [31]. Energies 2016, 9, 476 11 of 14 Energies 2016, 9, 476 10 of 13

Figure 12.Figure Results 12. Results of channel of channel power: power: ( a()a Measured) Measured from differentfrom different distance; ( bdistance;) Measured with(b) inﬂuenceMeasured with influenceof of power power outage; outage; (c) Measured (c) Measured without without noise; (d) noise; Measured (d) with Measured noise. with noise.

When testing the influence of the wind, four blowers are used to provide various wind velocities. Figure 13 shows the harvested energy from the receiving antenna. The energy transmission has an excellent stability under wind and the maximum measured error of the channel power is less than 15.31%. Also, it proves that the energy loss mainly results from the position change of the relay: when the relay rotates slightly, the received energy tends to be stable. Figure 14 compares the measurement results from the sensor tag and the far‐infrared temperature instrument respectively. The RFID reader was placed 6 m away from the relay to read the measurement data. Figure 14a shows the results of temperature rise without wind and Figure 14b shows the results when the velocity of wind is 2.9 m/s and 6.2 m/s respectively. The results measured by the tag and the VC305 show an excellent consistency, and the maximum measured error is less than 4.4%. Powered by the proposed relay, the RFID sensor tag can operate at a maximum distance

Energies 2016, 9, 476 11 of 13 of 18 m away from the RFID reader, which is much larger than the required safe distance of 3 m in a power system [31]. Energies 2016, 9, 476 11 of 13

Energiesof 18 m2016 away, 9, 476 from the RFID reader, which is much larger than the required safe distance of 3 m12 ofin 14 a power system [31].

Figure Figure13. Statistical 13. Statistical chart chart of of received received energy energy with with the wind. the wind.

Figure 14. Temperature measurement: (a) Measured without wind; (b) Measured in wind (2.9 m/s, 6.2 m/s).

5. Conclusions Figure 14. TemperatureFigureIn this 14.paper,Temperature a measurement: novel measurement:energy relay (a )is (aMeasured )proposed Measured to withoutwithout provide wind; awind; power (b) Measured ( sourceb) Measured for in wind RFID (2.9 insensor m/s,wind tags (2.9 m/s, on power6.2 m/s). transmission lines. The design of the relay is based on an analysis of the environmental 6.2 m/s). factors affecting power lines, including rain, high electromagnetism and wind. A prototype of the 5.relay Conclusions for 110 kv lines was built and tested in laboratory. The test results show that 26.92 μW of energy can be received at a distance of 6 m, which is enough to support an RFID sensor tag. There is little 5. Conclusions In this paper, a novel energy relay is proposed to provide a power source for RFID sensor tags influence on the transmission of energy from wind, noise or power outage. The results also show that on power transmission lines. The design of the relay is based on an analysis of the environmental In this thepaper, proposed a novel relay notenergy only prolongs relay is the proposed operation life to of provide the sensors, a powerbut also savessource on maintenancefor RFID sensor tags factors affecting power lines, including rain, high electromagnetism and wind. A prototype of the costs. In future, we plan to optimize the circuits and antennas to improve the radiated power density on power transmissionrelay for 110 kv lineslines. was The built design and tested of in the laboratory. relay Theis based test results on showan analysis that 26.92 µofW the of energy environmental and apply our design for higher voltage lines as well as bundle conductors. factors affectingcan be power received lines, at a distance including of 6 m, whichrain, ishigh enough electromagnetism to support an RFID sensorand wind. tag. There A isprototype little of the inﬂuence on the transmission of energy from wind, noise or power outage. The results also show that relay for 110Acknowledgments: kv lines was built This work and was tested supported in laboratory.by the National NaturalThe test Science results Funds showof China that under 26.92 Grant μNo.W of energy the proposed relay not only prolongs the operation life of the sensors, but also saves on maintenance can be received51577046, at a thedistance National of Defense6 m, which Advanced is enough Research toProject support under an GrantRFID No.sensor C1120110004, tag. There is little costs.9140A27020211DZ5102, In future, we plan the to National optimize Natural the circuits Science and Foundation antennas of toChina improve under the Grant radiated No. 61501162, power densitythe Key influence onandGrant the apply transmissionProject our of design Chinese for of Ministry higher energy voltage of fromEducation lines wind, as under well noise asGrant bundle orNo. power conductors.313018, outage.Anhui Provincial The results Science alsoand show that the proposedTechnology relay not Foundation only prolongs of China under the Grant operation No. 1301022036, life Scienceof the and sensors, technology but support also project saves of Jiangxion maintenance Acknowledgments:Province under Grant ThisNo. 20161BBE50076. work was supported by the National Natural Science Funds of China under costs. In future,Grant we No. plan 51577046, to optimize the National the Defense circuits Advanced and antennas Research Project to improve under Grant the No. radiated C1120110004, power density 9140A27020211DZ5102,Author Contributions: Jin the Tong National conceived, Natural designed Science and Foundation performed of the China experiments; under Grant Yigang No. He 61501162, provided the and apply ourKey design Grant Project for ofhigher Chinese voltage Ministry of lines Education as well under as Grant bundle No. 313018, conductors. Anhui Provincial Science and instructions for the design and financial support for this research; Bing Li helped to perform the measurements; Technology Foundation of China under Grant No. 1301022036, Science and technology support project of Jiangxi ProvinceTao Wang under helped Grant to analyze No. 20161BBE50076. the data; Jin Tong wrote the paper; and Fangming Deng provided help with revision Acknowledgments:of this manuscript. This work was supported by the National Natural Science Funds of China under Grant No. 51577046, the National Defense Advanced Research Project under Grant No. C1120110004,

9140A27020211DZ5102, the National Natural Science Foundation of China under Grant No. 61501162, the Key Grant Project of Chinese Ministry of Education under Grant No. 313018, Anhui Provincial Science and Technology Foundation of China under Grant No. 1301022036, Science and technology support project of Jiangxi Province under Grant No. 20161BBE50076.

Author Contributions: Jin Tong conceived, designed and performed the experiments; Yigang He provided the instructions for the design and financial support for this research; Bing Li helped to perform the measurements; Tao Wang helped to analyze the data; Jin Tong wrote the paper; and Fangming Deng provided help with revision of this manuscript.

Energies 2016, 9, 476 13 of 14

Author Contributions: Jin Tong conceived, designed and performed the experiments; Yigang He provided the instructions for the design and financial support for this research; Bing Li helped to perform the measurements; Tao Wang helped to analyze the data; Jin Tong wrote the paper; and Fangming Deng provided help with revision of this manuscript. Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations The following abbreviations are used in this manuscript:

RFID Radio Frequency Identiﬁcation Devices RF Radio frequency EMC Electromagnetic compatibility

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