A Novel Power Supply of Online Monitoring Systems for ... - IEEE Xplore

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 57, NO. 8, AUGUST 2010

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A Novel Power Supply of Online Monitoring Systems for Power Transmission Lines Lin Du, Caisheng Wang, Senior Member, IEEE, Xianzhi Li, Lijun Yang, Yan Mi, and Caixin Sun

Abstract—This paper presents a novel power supply of online condition monitoring systems for power transmission lines. The proposed power supply obtains energy from the magnetic field induced by transmission-line currents using a specially designed Rogowski coil. The design details of the power supply circuit including the unit for overvoltage and impulse line current protection are given in this paper. The impact to the power supply’s reliability caused by abnormal high transmission-line currents is also addressed. Experimental tests, including the short-circuit and impulse-current tests, have been conducted on the power supply to verify its performance under different operating conditions. Test results show that the power supply is capable of providing stable outputs with no saturation and low heat generation for the entire range of line conducting currents and can sustain severe conditions such as abnormal impulse currents. Index Terms—Impulse current, online monitoring system, power supply, Rogowski coil, transmission line.

I. I NTRODUCTION

A

STABLE power supply is one of the critical components for a transmission-line online monitoring system, which can continuously measure the line condition parameters such as the leakage current through insulators, thickness of ice on transmission lines, temperature of overhead conductors, etc. Since the whole online monitoring system needs to be mounted close to high-voltage transmission lines, it is not practical to employ conventional power sources. Due to the fact that the system is expected to continuously work outdoors under severe weather conditions, excellent reliability of the power supply is required to work for a long time (i.e., over ten years) without maintenance. Researchers have presented quite a few online monitoring methods addressing the measurement of condition parameters of transmission lines [1]–[5]; however, the power supply for monitoring systems has remained as a challenging Manuscript received April 18, 2008; revised January 30, 2009 and May 28, 2009; accepted October 5, 2009. Date of publication December 1, 2009; date of current version July 14, 2010. This work was supported in part by the National Basic Research Program of China through the 973 Program under Grant 2009CB724507, by the National Science Foundation Grant ECS-0823865, and by the Visiting Scholar Project administrated through the State Key Laboratory of Power Transmission Equipment and System Security and New Technology at Chongqing University. L. Du, L. Yang, Y. Mi, and C. Sun are with the College of Electrical Engineering, Chongqing University, Chongqing 400044, China (e-mail: [email protected]; [email protected]; [email protected]; suncaixin@ cqu.edu.cn). C. Wang is with the Department of Electrical and Computer Engineering and the Division of Engineering Technology, Wayne State University, Detroit, MI 48202 USA (e-mail: [email protected]). X. Li is with the Southwest Electric Power Design Institute, Chengdu 610041, China (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIE.2009.2037104

problem. Therefore, it is important to develop a reliable and maintenance-free power supply with desirable power output for practical transmission-line online monitoring systems. Powering the monitoring circuits by solar cells is a popular way at present [6], [7]. A solar-cell power supply is influenced by the weather, so that an energy storage device (normally a battery) is needed for the system to supply continuous and stable power. However, regular maintenance work is still required for the solar power supply, for instance, changing the battery due to its limited durability and cleaning the solar panel when it is covered by dust, snow, or ice. Powering active optical current transducers through laser lights has also been proposed [8]–[10], but this method is not fit for outdoor operation and has a high cost. In addition, the acquisition of power with contactless transfer has been introduced to aerospace and biomedical applications [11], [12]. However, this could generate an undesired interference with the online monitoring system. Another promising alternative is to obtain power energy from overhead current-carrying conductors. In this scheme, a specially designed Rogowski coil is mounted on a conductor to supply electric power by transferring the energy from the primary side (the conductor) to the secondary side. Although similar approaches have been presented in [13]–[15], systematic and detailed research has not been fully performed yet. Further investigation is necessary to promote the Rogowski coil power supplies (RCPSs) into practice. To generate stable power from an RCPS, three important issues need to be resolved. First, the RCPS should be able to provide stable outputs in a wide conductor current range. Second, the circuit should be protected from abnormal severe conditions such as impulse currents. The third is to keep the heat generated by the power supply low. In this paper, the realization of an RCPS, which will meet the practical needs of online monitoring systems, is described in a systematic way. Actual tests, including short-circuit and impulse-current tests, were carried out to verify the performance of the RCPS under different operating conditions. The test results are given and analyzed in this paper as well. II. D ESIGN P RINCIPLE OF THE ROGOWSKI C OIL In an RCPS, the Rogowski coil establishes the electromagnetic coupling between an overhead transmission conductor and the power supply system. The conductor, also the primary side of the coil, delivers the electric energy to the secondary side. The coil is a crucial element in the RCPS system, which determines the amount of power that the power supply can deliver to the online monitoring system.

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Fig. 1. Equivalent no-load model of a Rogowski coil.

A. Theoretical Analysis The fundamental no-load operating principle of a Rogowski coil (shown in Fig. 1) is similar to a current transformer (CT). In the coil, the primary side is controlled by ac current and is a one-turn winding. The major difference between a Rogowski coil and a CT is that the former does not require thick insulation between the primary and secondary sides since the RCPS and the whole online monitoring system stay at the same potential as the transmission line to be monitored. According to the electromagnetic theory, when the primary input is sinusoidal, the secondary root-mean-square (rms) voltage of the coil is U2 ≈ E2 = 4.44f N2 Φm

(1)

where E2 is the secondary rms-induced electromotive force, f is the power frequency of 50 or 60 Hz, N2 is the number of the winding turns on the secondary side, and Φm is the amplitude of the magnetic flux, which is represented as follows: Φm = Bm Sλ

(2)

where Bm is the flux density amplitude, S is the effective sectional area of the magnetic core, and λ is the lamination coefficient. Based on Ampère’s circuit law, the magnetic-field intensity can be obtained as √ (3) Hm l = 2N1 IE where Hm is the amplitude of the magnetic-field intensity; l is the average length of magnetic path; IE is the rms value of the excitation current, which is equal to the primary current I1 in a no-load condition; and N1 is the number of the winding turns on the primary side, which is one in this case. Then, Bm can be obtained from Hm as Bm = μ0 μr Hm = μHm

Fig. 2. Waveform of secondary voltage when the core is deeply saturated. The probe is set to ten times attenuation, and the primary current is only 70 A (10 V/div; 10 ms/div).

(4)

where μ0 is the vacuum permeability, μr is the relative permeability, and μ is the permeability of the material. The B−H curves of magnetic materials can be roughly divided into two zones: linear (unsaturated) and saturated zones. In the linear zone, the flux density B increases approximately proportional to the magnetic-field intensity H with a ratio μ. In the saturated region, however, the increase of Bm is flattened out while Hm increases. Therefore, it can be derived from (1)–(3) that E2 will not change much as the transmission conductor current changes when the Rogowski coil is saturated.

Fig. 3. Ideal piecewise linear B−H curves between the ordinary and proposed magnetic cores.

On one hand, the saturated coil helps limit the output voltage to some extent. On the other hand, it will also cause undesired problems such as distorted voltage waveforms and overheated core due to increased core loss, etc. [16]. Fig. 2 shows our test on a saturated magnetic core that causes a distorted voltage waveform. The peak spike voltage calls for a higher voltage rating and, possibly, a higher power rating for the following devices. This will accordingly result in a larger size circuit board, which is unfavorable to online system implementation because of the space and weight limitations based on the transmission-line operating requirements. B. Calculation of Rogowski Coil Parameters Since transmission-line monitoring systems generally consume very low power (the continuous power consumption is normally not more than 1 W), the load current is small. Therefore, the Rogowski coil can be treated as working at noload condition. Under this circumstance, the magnetic core of Rogowski coil can easily enter its saturation zone even if the primary current of the coil is only several amperes, which is much smaller than that of the transmission line. Therefore, it is required to have a specially designed magnetic core for an RCPS. In order to always keep the Rogowski coil in its linear region, it is necessary to have B ≤ Bs , where Bs is the critical saturation flux density, for the entire conductor current range (i.e., the entire range of I1 ). An air gap is added to the magnetic core to increase the magnetic reluctance of the core path to achieve a desired B−H characteristic curve, shown in Fig. 3.

DU et al.: NOVEL POWER SUPPLY OF ONLINE MONITORING SYSTEMS FOR POWER TRANSMISSION LINES

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Under the no-load condition, assume that the secondary side generates 6 V (6 V is based on the central power management requirement discussed in Section III) when the minimum primary current I1 = 100 A. The secondary side output voltage can be obtained as U2 = 6 V ≈ 4.44f N2 Bm Sλ.

(5)

Based on (3) and (4), (5) can be rewritten as N2 μeq ≈ √

l · U2 2 · 4.44 · f Sλμ0 I1

(6)

where μeq is the equivalent relative permeability of the overall flux path. For a given Rogowski coil, the right term of (6) results in a constant. In addition, the total magnetic reluctance of the flux path has two parts: the magnetic core reluctance (RmF e ) and the air-gap reluctance (Rmδ ). They can be calculated as RmF e = Rmδ =

l l−δ ≈ μ0 μF e S μ0 μF e S

(7)

δ μ0 S

(8)

where δ is the air-gap length, and l δ. The same effective sectional area S for the air gap and the magnetic core in (7) and (8) is assumed. The magnetomotive force is F = N1 I1 = I1 =

Bl . μeq μ0

(9)

The flux can be obtained from the magnetic path Hopkinson’s law as Φ = BS =

F . RmF e + Rmδ

(10)

Substituting (7)–(9) into (10), we can get μeq =

μF e = μF e · δl + 1

δ l

1 . + μF1 e

Fig. 4. Simulation results of the output voltage of the Rogowski coil. (a) Secondary output voltage waveforms. (b) Secondary output voltage rms values versus the primary current.

(11)

μeq can be approximately calculated as l/δ when μF e is much larger than l/δ, which is normally the case since μF e can be on the order of 104 or larger in the linear zone [17]. With regard to the annular Si–steel magnetic core in this paper, l = 251.2 mm, δ = 1 mm, S = 400 mm2 , and λ = 0.95. Thus, μeq ≈ 251.2. The required number of turns (N2 ) of the secondary winding can be obtained based on (6). The threshold conductor current beyond which the Rogowski coil is saturated can be determined as well. For instance, in addition to the parameters given in the previous paragraph, if the critical saturation flux density (Bs ) is 2.0 T, the outer diameter of the magnetic core D is 100 mm, the inner diameter d is 60 mm, and the core height h is 20 mm, then the threshold rms value of the conductor current can be calculated as √ 2πBs h ≈ 1102 A. I1,th = Bs S(RmF e + Rmδ ) = μ0 μeq ln D d (12)

C. Simulation Analysis A simulation study was carried out to verify the theoretical analysis. First of all, the Rogowski coil is modeled on the basis of the discussion in the previous section, with the following as the input parameters: the B–H curve of the magnetic material, physical dimension of the core, the lamination coefficient, the number of winding turns, diameter and resistivity of the windings, etc. Then, the current is injected to the primary side with power frequency at 100, 500, and 1000 A, respectively. The simulation results of the secondary output voltage waveforms are shown in Fig. 4(a). Fig. 4(b) shows the curve of the rms value of the secondary output voltage versus the primary current in the range from 100 to 1200 A. The output voltage waveforms, shown in Fig. 4(a) and (b), are not distorted when the current is below 1000 A, which indicates that the Rogowski coil does not enter the saturation region. However, the waveform is slightly distorted when the current is beyond 1000 A. Therefore, for the coil under study, the knee-point current (threshold current) is about 1000 A. Fig. 4(b) also shows that the secondary output voltage is about 7 V when I1 = 100 A, which basically matches the predefined design value of 6 V in (5). Note that the threshold current calculated by (12) is 1102 A while the simulation result shows a threshold current of approximately 1000 A. This discrepancy can be caused by the increase of the effective air-gap crosssectional area, which can be 5% (or more) larger than that of the magnetic core [17]. Nevertheless, they both show that

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Fig. 5. Principal block of the RCPS.

the proposed Rogowski coil is not saturated when the primary current is below 1000 A. The aforementioned simulation studies refer to the RCPS under no load. When the system is loaded, Bm and U2 are somewhat smaller than the no-load values. Nevertheless, as discussed before, the circuit power consumption is low; thus, the difference between the no-load and loaded conditions is negligible. III. P OWER S UPPLY C IRCUIT I MPLEMENTATION Fig. 6.

Block diagram of the central power management unit.

Fig. 7.

Diagram of the overvoltage protective unit.

A. Main Circuit Configuration Fig. 5 shows the circuit block diagram of the Rogowski power supply (RCPS). The Rogowski coil output voltage (the secondary voltage) is rectified and filtered to obtain a dc voltage output. Then, the dc voltage is interfaced by a step-down dc converter to obtain a stable +5-V output voltage. Finally, two channels of ±5 V are obtained by a dc/dc converter module. The protective units are designed to prevent possible overvoltages on the device caused by large currents or even lightning impulse currents. B. Central Power Management The rectified dc voltage varies between 7.5 and 75 V in the normal range of the primary side current. To obtain a +5-V output with high quality, a central power management device is needed. In our case, the requirements to the power management device are as follows: 1) a wide input voltage range; 2) capability of managing enough power; and 3) high efficiency. The authors choose the chip MAX5035B, which is an easy-to-use and efficient step-down dc/dc converter with a wide 6–75 V input voltage range. It can provide a +5-V output voltage and a maximum output current of 1 A. The ultrawide input voltage range of the unit allows the Rogowski coil to properly generate voltage within the primary current range. The wiring diagram of the central power management unit is shown in Fig. 6. After the unipolar +5-V power is obtained, a constantvoltage isolated dc/dc converter is used to deliver a bipolar ±5-V output. C. Overvoltage Protective Unit Abnormal large primary currents can cause overvoltages at the secondary side of the Rogowski coil. To protect the circuit

from possible overvoltages, a monitoring and protective module based on a solid-state relay (SSR) is employed, shown in Fig. 7. The coil of SSR detects the sampled Vin and trips on the preset threshold overvoltage protection value. The contacts of the SSR compose a monopole double throw switch. The switch has the two normally closed (NC) terminals connecting the coil and the power supply circuits, and the two normally open (NO) terminals that connect the coil output to a dissipative resistor. When the SSR coil trips, the SSR NC terminals are opened while the NO terminals are closed. By this means, the circuit is protected from the overvoltage condition. D. Impulse-Current Protective Unit Due to the short time duration of lightning impulse currents when they hit the overhead lines, the safety of the power supply equipment will be challenged by the intrusive electromagnetic wave if there is no additional protection measure taken. An 8 μs/20 μs 30-kA lightning impulse current generated by an impulse-current generator was used to investigate the effects of impulse currents upon the Rogowski coil. In the first test, the


Fig. 8.

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Actually measured B–H curve of the Rogowski coil.

coil secondary winding was distorted and destroyed. Some of the turns were broken at the edges of the magnetic core. The failure is caused by the great electromagnetic force induced by the impulse current. The coil structure was then redesigned and reinforced to improve the mechanical strength to withstand the large electromagnetic force. First, a buffer layer was filled up between the magnetic core and the winding. Second, the radius of the winding core was enlarged. Third, a protective shell was mounted for the RCPS to protect the winding from being stretched by the strong ampere force. Moreover, a set of transient voltage suppressors (TVSs) were installed in parallel with the Rogowski coil to give additional overvoltage protection to the system from impulse currents before the protective relays (the SSR shown in Fig. 7) are able to kick in to protect [18] the circuit. Since the response time of the TVSs is at the level of 10−12 s, they can effectively clamp the pulse voltage almost immediately when an impulse hits. The TVSs need to be bipolar and have large capacity of peak pulse power dissipation to endure transient impacts. Meanwhile, their nominal reverse cutoff voltage should also correspond to the maximal input voltage of the step-down converter of Fig. 5. After all the aforementioned measures were taken, the Rogowski coil successfully passed the impulse-current test. IV. T EST R ESULTS AND D ISCUSSION A. B–H Curve Measurement of Rogowski Coil A no-load test was carried out on the Rogowski coil with N2 = 267 in accordance with the theoretical calculation results obtained in Section II. A clamp-on amperemeter measured the primary current I1 , and an oscilloscope monitored the secondary voltage U2 . According to (1)–(3), the B and H values in the discrete series were calculated. Fig. 8 shows the B–H curve of the Rogowski coil. It is noted from the figure that, for the same flux density B, the magnetic intensity H is much larger in the proposed coil than that in an ordinary magnetic core without air gap. This feature enables the coil to accommodate a wide range of line currents. The test result also indicates that the Rogowski coil is guaranteed not to be deeply saturated within 1000 A of the primary current, which corresponds to 3.98 kA/m of H in Fig. 8. In addition, for the selected Si–steel core, the core loss is merely 1.5 W/kg when its flux density is 2.0 T at power frequency. Therefore, the temperature rise due to core loss is very low. After 2 h of continuous testing, the temperature rise of the coil does not exceed 3 ◦ C.

Fig. 9. Measured output power curve.

Fig. 10. I/O characteristic curve when the primary current rises from 0 to 680 A. In the figure, Ch1 and Ch2 symbolize the input and output voltages of the step-down converter, respectively. (Ch1: 20 V/div; Ch2: 2 V/div; 200 ms/div).

B. I/O Performance Tests An RCPS based on the principle shown in Fig. 1 was developed in this paper. The Rogowski coil output was connected to a variable resistor. The maximal output power was then measured at different primary currents. Fig. 9 shows the curve of the maximum power versus the primary current. The equivalent load resistance is also given in the figure. The RCPS can deliver 250-mW output power when the primary current is 60 A, which is enough for general data acquisition circuits. When the primary current is 200 A, the RCPS can then supply power as high as 2.5 W for a load with an equivalent resistance of 10 Ω. The transient response characteristic of the power supply was also tested. In the test, the primary current was raised from 0 to 680 A in a short time (about 100 ms). The waveforms of the input voltage to the step-down converter (of Fig. 5) and the output voltage are shown in Fig. 10. It is shown in the figure that the input voltage rises rapidly and smoothly, and the output voltage jumps to a stable 5 V almost instantly when the input rises beyond the threshold. This type of performance is extremely useful for providing a stable power supply for

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V. C ONCLUSION

Fig. 11. I/O waveform of step-down converter at the action of SSR, where Ch1 and Ch2 symbolize the input and output voltage of the step-down converter, respectively. (Ch1: 20 V/div; Ch2: 2 V/div; 400 ms/div).

A novel power supply using a particularly designed Rogowski coil for online transmission-line monitoring systems has been developed and described in this paper. With the help of the Rogowski coil, the proposed power supply obtains energy directly from the magnetic field induced by transmission-line currents and aims for long-time continuous operation without maintenance. Based on the theoretical calculations and studies, the Rogowski coil with a unique structure containing an air gap in the flux path has been developed. Actual tests have also been carried out to verify and improve the design of the power supply. Test results have indicated that the RCPS is capable of supplying stable output power in a wide range of transmissionline currents. The results have also shown that the RCPS can successfully sustain the electrical and mechanical stresses due to large and impulse currents through the specially designed coil structure and protection units for the system developed and described in this paper. R EFERENCES

Fig. 12. Output waveform of the Rogowski coil under an impulse current of 8 μs/20 μs 30 kA.

the transmission-line online monitoring systems, when the line current varies as the power system load changes. C. Electrical Tests on Protective Units Fig. 11 shows the input and output voltage waveforms of the step-down converter when the overvoltage protection unit discussed in Section III-C is tripped. The test result indicates that the SSR trips when Vin > 74 V and thereby cuts off the circuit behind from the Rogowski coil. However, the power to the rest of the circuit is not cut off immediately because of the large time constant of the filtering circuit. Therefore, Vin falls down slowly to approximately 62 V when the contacts of SSR switch back to the original state. The entire process will be repeated if the overvoltage still exists. The output of the step-down module gives a stable output at 5 V when its output fluctuates between 74 and 62 V, shown in Fig. 11. Note that this protection acts only when the primary current is above 1000 A, which seldom happens on a single conductor except for a transmission-line short-circuit current. Fig. 12 shows the output waveform of the Rogowski coil when it is under an impulse current of 8 μs/20 μs 30 kA [19]. Clamped by the TVSs, the output impulse voltage is limited below 80 V (the maximal input voltage of the step-down converter is 80 V). It can be seen that the impulse-current protective unit successfully protects the RCPS to from impulse currents.

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Lin Du received the M.S. and Ph.D. degrees in electrical engineering from Chongqing University, Chongqing, China, in 1996 and 2003, respectively. He is currently an Associate Professor with Chongqing University. His major research interests include high-voltage testing technique, online detection of insulation condition of electrical apparatus, insulation fault diagnosis for high-voltage equipments, and online overvoltage monitoring and recognition.

Caisheng Wang (M’02–SM’08) received the B.S. and M.S. degrees in electrical engineering from Chongqing University, Chongqing, China, in 1994 and 1997, respectively, and the Ph.D. degree in electrical engineering from Montana State University, Bozeman, in 2006. From August 1997 to May 2002, he was an Electrical Engineer with Zhejiang Electric Power Test and Research Institute, Hangzhou, China. Since August 2006, he has been a Faulty Member with the Department of Electrical and Computer Engineering and the Division of Engineering Technology, Wayne State University, Detroit, MI. His current research interests include modeling and control of power systems and electrical machines, alternative/hybrid energy power generation systems, and fault diagnosis and online monitoring of electric apparatus.

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Xianzhi Li received the B.S. and M.S. degrees in electrical engineering from Chongqing University, Chongqing, China, in 2005 and 2008, respectively. He is currently with the Southwest Electric Power Design Institute, Chengdu, China, working on the design of high-voltage transmission lines. His research is focused on the development of online monitoring system for the state parameters of transmission lines.

Lijun Yang received the M.S. degree in electrical engineering from Chongqing University, Chongqing, China, in 2004, where she is currently working toward the Ph.D. degree in the College of Electrical Engineering. Her research activities are in the fields of insulation aging for power transformer and online monitoring for electric devices.

Yan Mi was born in Hunan, China, on September 20, 1978. He received the B.S., M.S., and Ph.D. degrees in electrical engineering from Chongqing University, Chongqing, China, in 2000, 2003, and 2009, respectively. He is currently a Lecturer with the College of Electrical Engineering, Chongqing University. His areas of research include insulation online monitoring for high-voltage equipment and biomedical application of electrical engineering.

Caixin Sun received the B.S. degree in electrical engineering from Chongqing University, Chongqing, China, in 1969. He is currently a Professor with Chongqing University, leading the research group on high-voltage engineering. He is the author and the coauthor of over 200 publications. His research interest is in high-voltage engineering, particularly in online detection of insulation condition and insulation fault diagnosis for high-voltage equipment and discharge mechanism of outdoor insulation in complicated environment. Prof. Sun is a member of the Chinese Academy of Engineering and the Director of Electrical Power Engineering Committee of the National Science Foundation of China.