Research on Power Output Characteristics of Magnetic Core in Energy Harvesting Devices

Transcription

1 Sensors & Transducers Vol. 74 Issue 7 July 04 pp Sensors & Transducers 04 by IFSA Publishing S. L. Research on Power Output Characteristics of Magnetic Core in Energy Harvesting Devices Rong-Ping GUO Yong-Xing CAO Qian PENG Tian-Chen YUE Tya-Dong LIU Ge-Hao SHENG Xiu-Chen JIANG Electric Power Research Institute Sichuan Electric Power Copany Chengdu 6007 China Departent of Electrical Engineering Shanghai Jiaotong University Shanghai 0040 China Tel.: E-ail: shenwen@6.co Received: 5 February 04 /Accepted: 30 June 04 /Published: 3 July 04 Abstract: Magnetic core is the doinant factor in the perforance of current transforer energy harvesting devices. The power output odel of the agnetic core is established and verified through experients. According to the actual application requireents the concept of power density is proposed. The relationships of power density to air gap aterial and diension of the agnetic core are analyzed and verified through experients. Copyright 04 IFSA Publishing S. L. Keywords: Inductive energy harvesting Magnetic core for energy harvesting Power density Output characteristics Energy supply.. Introduction Energy supply is one of the two key technologies for on-line onitoring in transission lines. Inductive energy harvesting technology which provides energy support for the onitoring device of the high voltage side is the ain approach for energy harvesting of transission lines on the high voltage side. The perforance of energy harvesting has direct effect on the stability and reliability of on-line onitoring in transission lines. Scholars of the world have carried out soe preliinary studies on inductive energy harvesting technology in recent years. Most researches focus on circuit design and verification of energy harvesting coils. However the studies on power output characteristics of energy harvesting and energy harvesting agnetic cores are rare. Literature [] revealed that axiu power was obtained when the load resistance was equal to the internal resistance of the electricity and energy harvesting coils and verified the conclusion with a Φ agnetic core. Literature [] carried out qualitative analysis of the relationships of the output power of inductive energy harvesting coil to factors such as agnetic conductivity and diension but no exact quantitative relationship was given. To prevent the energy harvesting coils fro entering the saturation condition when working literature [3-5] put forward the air gap approach to change the agnetization curve of agnetic cores. In this way the saturation proble of agnetic cores was solved. In literature [6-9] dual-coil configuration was adopted. When there was overcurrent on the priary side the copensation coil would be triggered which iproved the dynaic adaptation range of energy harvesting power supply. In literature [0] the secondary current was aintained within the preset range by CT ratio switchover. Literature [-] set up the power output odel of inductive energy 53

2 Sensors & Transducers Vol. 74 Issue 7 July 04 pp harvesting agnetic core and studied the effect on coil output power through the factors such as priary current and secondary coil turns. In consideration of line safety and signal quality there are strict liits on the weight of on-line onitoring devices. For exaple the weight of the onitoring device on the noral transission line should not exceed.5 kg; the weight of onitoring device on aeolian vibration line should not exceed kg. Therefore it is necessary to study the power output characteristics of energy harvesting coils and optiize the design of energy harvesting coils so as to iprove its power density. On the basis of the fundaentals principles of current transforer (CT) the power output characteristics of energy harvesting coils under power frequency condition are investigated in the study. The power output odel of energy harvesting coils is established. The effects of factors such as air gap and the aterial and diension of agnetic core on the output power of energy harvesting coils are deonstrated.. Power Output Model of Energy Harvesting Magnetic Core The basic working principle of CT energy harvesting device is shown as Fig.. y = a * x + b () NI + NI = NI () where N is turn I is the secondary output current and I is the excitation current. I can be decoposed into agnetization current coponent I μ and core loss I Fe. Phasor diagra is shown as the left side in Fig.. To siplify the analysis the internal resistance of secondary side and core loss are ignored. Siplified phasor diagra is shown as the right side in Fig.. E NI U IR E I μ I I I Fe Φ E NI E I I Φ Fig.. Phasor diagra and siplified phasor diagra. Φ I At this oent the agnetization current is the excitation current which is R i R O I E U RL I = μ I (3) The relationship to agnetization current to priary current and secondary current is expressed as I = I μ ( NI) (4) Fig.. Principle diagra of energy harvesting device. According to the basic theory of electric achinery [3] the line on the priary side passes though the agnetic core and the current is I. The induced electrootive force on the secondary side is E j fn = π Φ () where f is the 50 Hz power frequency and N is the turns of secondary coil; Φ is the axiu value of ain agnetic flux. The voltage on resistive load R L is U and the internal resistance of secondary coil is R. Magnetootive force balance equation is expressed as E π fn = Φ (5) The axiu value of ain agnetic flux is Φ = BS where S is the effective sectional area of agnetic core. Suppose that the agnetic core is working in linear region then the relationship between B and H is expressed as B = μh According to Apere's circuital law H l = NI μ (6) (7) where N is the turns of the priary side with the value of here. 54

3 Sensors & Transducers Vol. 74 Issue 7 July 04 pp The secondary output power is given by P = E I = π fμi S μ I I μ l (8) The agnetization current I μ is taken as an independent variable. The derivation is taken of forula (8) and assue P '( I ) = 0 μ. This will result in the axiu output power when I = μ I is solved that is of this odel used for calculating the power output of the agnetic core. Output Power /W theoretical value(0a) easured value(0a) P = π f μsi / l ax (9) The conclusions fro forula (8) and (9) are verified and an experiental circuit is established as Fig. 3. Magnetization Current /A Fig. 4. Measured value and theoretical value (I=0 A). Current Generator Fig. 3. Diagra of experiental circuit of output power. A V Output Power /W theoretical value(30a) easured value(30a) theoretical value(0a) easured value(0a) The strong current generator provides the priary current. The variable resistive load is connected to the secondary side. The experiental agnetic core is an annular icrocrystal alloy agnetic core after bisection using linear cutting. The inner diaeter is 55 the external diaeter 85 the width 0 00 turns average length of agnetic path 3. and effective sectional area is Strong current generator outputs the currents of 0 A 0 A and 30 A variable load and easure the load current and voltage. Fro forula (4) the excitation current can be obtained. The value is equal to the agnetization current. The relationship between the easured output power and agnetization current is shown as Fig. 4 and 5. It can be seen that the easured curve is in the sae diension as the curve calculated with Matlab. The axiu power value is a little saller than the theoretical value and the error is within 0 %. The variation trend of the easured curve slightly lags behind the one calculated theoretically. The reason for the lagging is that the excitation current is approxiate to the agnetization current in forula (4). However due to the existence of core loss coponent the agnetization current is saller than the excitation current. Because the agnetic core can always output its axiu power by controlling the load current [4] the overall shift of the curve towards the right due to the core loss will not affect the effectiveness Magnetization Current /A Fig. 5. Measured value and theoretical value (I=0 A I=30 A). 3. Power Density of Energy Harvesting Magnetic Core To copare the power output capability of the agnetic core under a certain priary current the power density λ of energy harvesting agnetic core is defined as the ratio of axiu power output P ax to the agnetic core diension V. P / ax π f μ SI l λ = = (0) V V 3.. Power Density and Air Gap Copared with the low reluctance of the soft agnetic aterial the air gap has extreely high reluctance. After introducing the air gap the effective agnetic pereability of the entire agnetic core decreases significantly. Fig. 6 shows the agnetic 55

4 Sensors & Transducers Vol. 74 Issue 7 July 04 pp core with air gap. The external diaeter of the agnetic core is denoted as D o the internal diaeter D i the average length of agnetic path l c air gap length as l g the relative agnetic pereability before cutting μ r absolute agnetic pereability μ and relative agnetic pereability after cutting μ rg. D o D i l c l g Fig. 6. Magnetic core with air gap. According to the ost coon siplified odel [5] of agnetic core with air gap the effective relative agnetic pereability of the agnetic core (including air gap) with air gap is expressed as μr μrg = lg + μr lc + lg w () The axiu output power of agnetic core with air gap under certain priary current is expressed as Fro forula () it can be seen that the axiu output power of agnetic core under a certain priary current is very sensitive to the length of air gap. The annular silicon steel agnetic core has an internal diaeter of 50 external diaeter of 70 width of 35 average length of agnetic path of 83 effective sectional area of and turns of 00. The experiental circuit is shown in Fig. 3 with variable load resistance. The output curve of agnetic core before cutting under 0 A priary current is easured. It can be seen there is no distortion in the voltage wavefor of the agnetic core at the axiu power output. The axiu output power is 869 W and the corresponding relative agnetic pereability of 3363 is obtained. Then the agnetic core is subject to bisection with linear cutting technology. Then packing papers of different thickness are added to the joint as shown in Fig. 7. The power output curve of agnetic core with packing paper under 0 A priary current is easured and copared with the curve before cutting. The result is shown in Fig. 8. The theoretical agnetic pereability and axiu output power of the agnetic core with packing papers of different thickness are calculated fro forula () and (). Then the theoretical values are copared with the easured axiu output power. The error is calculated and the result is as shown in Table. packing paper μ P ax = πfμrgμ0si / lc = πf SI / lc lg + μr lc + lg () Fig. 7. Magnetic core with packing paper. Output Power /W No cutting Output Power /W Magnetization Current /A Magnetization Current /A Fig. 8. Output power curves of agnetic core with packing paper. 56

5 Sensors & Transducers Vol. 74 Issue 7 July 04 pp It can be seen that the error is larger when the thickness of packing paper is used as the length of air gap to calculate the axiu output power. The cutting reversing will cause concave-convex stripes to the section during on-line cutting of agnetic core. Usually the surface roughness R Z of the section is μ - 50 μ. Therefore the air gap introduced by the surface roughness of section after linear cutting is taken into account. The packing paper is reoved. The axiu output power of silicon steel agnetic core easured after cutting under 0 A priary current is 58 W. It can be known fro forula () that the equivalent air gap length l eq caused by section roughness is about 0.0. The equivalent air gap length (thickness of packing paper plus the section roughness) l eq is taken as the odified equivalent air gap length l g. l g is used to calculate the theoretical axiu output power under 0 A priary current. The value is copared with the easured value and the result is shown in Table. It can be seen that when the equivalent air gap caused by linear cutting is taken into account the axiu theoretical power is alost the sae as the easured one in the experient with packing paper. The error is within 7 %. For the convenience of installation and disassebly the agnetic core of the energy harvesting device ust be subject to bisection before used on the lines. Since no cutting technology and polishing process can guarantee the absolute soothness of the section it is inevitable that there will be air gap introduced and the effective agnetic pereability and power density of agnetic core will be bound to decrease after cutting. The surface roughness R Z by laser cutting can be controlled within.6 μ μ but it costs too uch and the cutting thickness should not exceed 0 which is not suitable for cutting the agnetic core for energy harvesting. Under overall consideration linear cutting is preferred to cut the agnetic core. To decrease the equivalent air gap length caused by linear cutting the processes such as echanical polishing should be used to decrease the section roughness. Table. Maxiu theoretical value and easured value of axiu output power. l Theoretical relative Maxiu theoretical Measured axiu g agnetic pereability power /W power /W Error /% Table. Theoretical value and easured value of axiu output power with odified air gap length. l Theoretical agnetic Maxiu theoretical Measured axiu g pereability power /W power /W Error /% Power Density and Material of Magnetic Core At present the energy harvesting devices developed at hoe and abroad are usually ade of silicon steel icrocrystal alloy and peralloy as alternative aterials for agnetic core [6-7]. Under the condition of no cutting silicon steel has highly saturated agnetic induction intensity the icrocrystal alloy has high agnetic pereability and the agnetic pereability of peralloy is interediate between the two. Peralloy has the lowest saturated agnetic induction intensity. Fro the analysis of air gap it can be known that the agnetic core cutting will decrease the power output perforance. To copare the power output characteristics of the three different agnetic cores before cutting 0 saples are used for each type of the annular cores (no cutting) which are ade by the three aterials with internal diaeter of 55 external diaeter of 85 width of 0 and turns of 00. Then the circuit is shown as Fig. 3. The axiu output power of each agnetic core under 0 A priary current is observed. Whether the voltage wavefor has distorted when the axiu power is reached is observed. If the distortion occurs it eans that the working point of the agnetic core has entered into the saturation region. The above agnetic core saples are cut by linear cutting. The value of load resistance is changed to easure the power output curve of agnetic core under 0 A priary current. The axiu output power is obtained. Fig. 9 shows the typical output curve after cutting. Obviously the power output of silicon steel agnetic core after cutting is significantly higher than that of the other two aterials. The average values and variances of axiu output power P ax before cutting and those of axiu output power P ax of the saples ade by the three aterials after cutting are calculated along with P ax / Pax. The result is shown in Table 3. 57

6 Sensors & Transducers Vol. 74 Issue 7 July 04 pp Output Power /W Magnetization Current /A Silicon steel Microcrystal alloy Peralloy Fig. 9. Typical power output curves of agnetic cores in 3 aterials. Fro the standard deviations it can be seen that the degree of concentration of the saples is relatively ideal. The output characteristics of the 0 agnetic cores of each aterial are consistent. Before cutting the axiu output power of icrocrystal alloy agnetic core under 0 A priary current is basically the sae as that of silicon steel. However the axiu power point of icrocrystal alloy occurs in the saturation region while the latter is in the linear area. Suppose that the equivalent air gap lengths caused by linear cutting of silicon steel and icrocrystal alloy are equal. Then the agnetic pereability of icrocrystal alloy in linear region is significantly higher than that of silicon steel after cutting. Further the axiu output power of icrocrystal alloy agnetic core should be significantly higher than that of silicon steel after cutting. However the easured result is just the other way around. The average value of P ax of icrocrystal alloy agnetic core is only 57.5 % of that of P ax of silicon steel after cutting. The P ax / Pax of icrocrystal alloy agnetic core is only 7.%. All of these lead to the conclusion that the equivalent air gap length of icrocrystal alloy agnetic core caused by linear cutting is certainly larger than that of silicon steel. Using forula () it can be estiated that the equivalent air gap length caused by cutting silicon steel is about 0.0 while that of icrocrystal alloy is at least By observing the saple section after cutting it can be seen that the sections of silicon steel and peralloy saples are relatively sooth and clean while the section of icrocrystal alloy has an obvious rough surface. Mechanical polishing is perfored for the icrocrystal alloy agnetic core saple. Due to its large brittleness a large aount of fragents is produced during polishing. By easuring the axiu output power of these agnetic core saples under 0 A priary current after polishing the average value is 06.3 W and the standard deviation is 8. W. It can be concluded that the echanical polishing leads to no obvious iproveent in the output characteristics of icrocrystal alloy agnetic core. Because the inductive energy harvesting agnetic core is assebled to the line the agnetic core cannot be installed before cutting. The power density of peralloy agnetic core is the lowest after cutting so it is not an alternative aterial for high power density agnetic core. Although the icrocrystal alloy agnetic core without cutting has high agnetic pereability due to its large brittleness and poor ductility [8] its perforance will decline seriously after cutting. Besides there will be phases transforing fro aorphous state to crystalline state [9] at the kerf of icrocrystal alloy when cutting. This will cause its agnetic perforance to decline. Under the current cutting technology and polishing process silicon steel is preferred as the agnetic core aterial in inductive energy harvesting device to get the axiu power density. Degree has liit on agnetic core width. Besides when the average agnetic path length is larger than one hour the agnetic core will be easily saturated. Therefore under perissible conditions the internal and external diaeter should be decreased and the agnetic core width be increased to obtain the axiu power density when designing the energy harvesting agnetic cores. Material Table 3. Maxiu output power of agnetic cores ade of 3 aterials before and after cutting. Average value /W Before cutting P ax After cutting P ax Standard deviation /W Saturated or not Average value /W Standard deviation /W Saturated or not Silicon steel No No 9.8 Microcrystal alloy Yes No 7. Peralloy Yes No 7.4 P P ax ax /% 58

7 Sensors & Transducers Vol. 74 Issue 7 July 04 pp Conclusions This article establishes the power output odel of energy harvesting agnetic core. Through theoretical analysis and experient the relationships of power density to air gap the diension and aterial of the agnetic core are verified. The conclusions are as follows:. The power output characteristics of energy harvesting coil can be described precisely using its load odel.. Air gap has great effects on the output power of energy harvesting coil. Decreasing the air gap can iprove its unit power density. 3. The perforance of energy harvesting coils ade by silicon steel is iproved to a larger extent than that of icrocrystal and peralloy in sequence. Acknowledgeents This research was funded by a grant (No ) fro the National Natural Science Foundation of China and a grant (No. S03GR00) fro the International S&T Cooperation Projects of China. References []. N. M. Roscoe M. D. Judd. J. Fitch Developent of agnetic induction energy harvesting for condition onitoring in Proceedings of the 44 th International IEEE Universities Power Engineering Conference (UPEC) 009 pp. -5. []. J. Ahola T. Ahonen V. Sarkiaki et al. Design considerations for current transforer based energy harvesting for electronics attached to electric otor in Proceedings of IEEE International Syposiu on Power Electronics Electrical Drives Autoation and Motion (SPEEDAM'08) 008 pp [3]. L. Xian-Zhi D. Lin C. Wei-Gen et al. A novel schee of draw-out power supply utilized intransission line state onitoring Autoation of Electric Power Systes Vol. 3 Issue 008 pp [4]. W. Jintao S. Gehao Z. Yi Design and Developent of Power Supply for Transission Line Online Monitor Device Electric Engineering Issue 009 pp [5]. W. Zan Z. Fei W. Wei et al. Developent of Power Supply in High Voltage Side of Transission Lines Advances of Power Syste and Hydroelectric Engineering Issue 6 00 pp [6]. T. Xu-hui W. Bao-juan Z. Jing-chao etc. Design of Power Supply for Optical Current Transforer in High Voltage Side High-voltage Electrical Appliances Vol. 45 Issue pp [7]. R. Xiaodong C. Shuyong J. Tao Design of a High Side Energy Extracting Device for Active Electronic Current Transforer Power Syste Technology Vol. 3 Issue pp [8]. G. Yingxia Research of the High Voltage Circuit and Power Supply for Electronic Current Transforer Yanshan University 006. [9]. P. Tao X. Lei Y. Chun et al. Developent of HV Side Power Supply for Electronic Current Transforer Transforer Vol. 47 Issue 5 00 pp [0]. Q. Dong A Kind of Power Supply of Optic-electric Current Transforer for Accoodating Wide Bus Dynaic Current in Proceedings of the CSEE Vol. 6 Issue pp []. L. Yadong S. Gehao W. Kui etc. Design of Current Transforer Energy Harvesting Power Supply Based on Phase Angle Control Method Autoation of Electric Power Systes Vol. 35 Issue 9 0 pp []. L. Yadong Studies on Distributed Fault Location Theory and the Key Technologies for Transission Line Shanghai Jiaotong University 0. [3]. Z. Shunrong Electro echanics nd edition. Beijing: Science Press 007. [4]. L. Ya-dong S. Ge-hao W. You-jia etc. Current Transforer Draw-out Power Supply Design Based on Power-controlled Method Autoation of Electric Power Systes Vol. 34 Issue 3 00 pp [5]. W. M. Flanagan Handbook of Transforer Design and Applications nd edition New York McGraw- Hill 993. [6]. L. Jie L. Gang Z. Ming Ipact Analysis for the Airgap Width of Draw-out Power Supply Coil High Voltage Apparatus Vol. 48 Issue 0 pp [7]. G. Lin-Yun Y. Xiang-Gen Y. Xin-Rong et al. Research of Iproving Power Efficiency for Intelligent Device Self-power Supply Utilized in Power Distribution Network in Proceedings of the CSEE Vol. 9 Issue 009 pp. 7-. [8]. Z. Guoxiang A New Fe-based Aorphous Alloy with Excellent Soft Magnetic Properties Metallic Functional Materials Vol. 0 Issue 003 pp [9]. Liu Ji-Lei Zhu Zheng-Hou Yin Lei at al. Effect of wire saw technique on the soft agnetic properties of Fe78Si9B3 aorphous alloy under line frequency Journal of Nanchang University (Natural Science) Vol. 36 Issue 4 0 pp Copyright International Frequency Sensor Association (IFSA) Publishing S. L. All rights reserved. ( 59