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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 30, NO. 1, FEBRUARY 2015

New Electronic Current Transformer With a Self-Contained Power Supply Chin-Tien Liang, Kun-Long Chen, Yuan-Pin Tsai, and Nanming Chen, Member, IEEE

Abstract—Compared with traditional current transformers (CTs), electronic current transformers (ECTs) are characterized by small volume, light weight, good isolation, good linearity, and easy digitization. Hence, ECTs are one of the primary devices for signal digitization for supervising intelligent substations. We design a coreless ECT using Hall sensors as well as its mechanism and circuit configuration in order to replace traditional CTs and other CTs. The proposed ECT achieves Class 0.5 accuracy for measuring CTs and accuracy Class 5P21 for protective CTs according to IEC Standard 60044-8. An additional power supply is required to drive the ECT. Considering that there are no usable power supplies for special measurement environments, we use the surrounding magnetic fields induced by the cable currents as the power supply. A self-contained power supply is proposed and integrated with a backup battery power supply and a battery protection circuit to ensure that power is supplied for different operating conditions. In addition, we design a protective device to protect the back-end circuits of the self-contained power supply in case of a fault occurring in the power system.

TABLE I COMPARISON OF THE MAIN CURRENT MEASUREMENT TECHNOLOGIES

Index Terms—Current measurement, current transformer (CT), electronic current transformer, Hall effect, iron core, self-contained power supply.

I. INTRODUCTION

I

N THE development of a smart grid, one of the important technologies for contemporary smart transmission grids is a wide-area measurement system (WAMS) using a phasor measurement unit (PMU) as its basis [1]–[3]. The measurement system depends on proper and precise sensing units, potential transformers (PTs), and current transformers (CTs) for accurate sensing of electrical signals [3], [4]. However, a traditional CT is primarily based on magnetic induction coils; thus, the current measurement range is narrow because of sensing nonlinearity and core saturation. An electronic current transformer (ECT) is characterized by small volume, light weight, good isolation, good linearity, and easy digitization; hence, it is one of the main devices for signal digitization for supervising intelligent substations, for dynamic grid observation for improving the reliability of protective relays, and for improving the control of the entire Manuscript received December 15, 2013; revised May 07, 2014; accepted June 14, 2014. Date of publication July 28, 2014; date of current version January 21, 2015. This work was supported by the National Science Council, Republic of China under Contract NSC 100-2221-E-001-010. Paper no. TPWRD-014002013. The authors are with the Department of Electrical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan (e-mail: [email protected]; [email protected]; [email protected]; [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/TPWRD.2014.2334894

power system. Currently, several existing ECT technologies resolve the core saturation problem, but they are not common in practical use owing to high cost and device complexity [5], [8]. Recently, some studies [9], [10] have proposed a new Hall sensor that can be used for current measurement. Other studies [11], [12] have revealed that external magnetic-field interference can be effectively eliminated by only using multiple, symmetrically distributed Hall sensors without an iron core architecture, and the signals obtained can then be processed. Based on this sensing concept, we completed the development of a new ECT that includes a sensing mechanism and its associated hardware circuit design. Further, this ECT has overall size and price advantages, which potentially allow it to replace traditional CTs and other ECTs. In addition, we conducted current accuracy class tests for the ECTs to verify their accuracy class in accordance with IEC Standard 60044-8 ECT Specifications [13]. Table I summarizes a comparison of the main current measurement technologies including traditional and electronic CTs. An additional power supply is required in order to drive Hall sensors. For some special measurement applications whose environments lack power sources, for example, current sensing of transmission lines for WAMS and cable condition monitoring in underground culverts, special power sources need to be designed for these types of special environments. Among all designs, a power-line energizing system based on a traditional CT is a popular method. However, this method cannot drive ECTs to meet metering and protective purposes for some conditions when not enough energy is converted, owing to a smaller cable

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LIANG et al.: NEW ELECTRONIC CURRENT TRANSFORMER WITH A SELF-CONTAINED POWER SUPPLY

current, and the iron core is saturated owing to a larger fault current [14]–[16]. We designed a self-contained power supply for some special measurement locations without usable power supplies for our proposed ECT and coupled it with a battery pack as a backup power supply when the cable current is insufficient and cannot induce enough energy. Moreover, the battery pack can be recharged from an inductive power supply and further coordinated with the self-contained power supply as a complete power-supply system. In addition, we also designed a protective device to protect the back-end circuits of the self-contained power supply in case of a fault occurring in the power system.

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Fig. 1. Architecture of the new ECT.

II. PRINCIPLES AND ARCHITECTURE OF THE PROPOSED ECT A magnetic field is present in the area surrounding an energized cable, and its intensity is proportional to the electric current flowing in this cable. Hence, the corresponding current is calculated from the intensity of the magnetic field measured using a Hall sensor. However, the interference from many magnetic-field sources exists in actual space, such as the magnetic fields produced from each phase current in three-phase or multiphase systems. Therefore, the traditional Hall CT still uses an iron core in order to enhance the magnetic field produced by the measured current and to eliminate the interference from ambient magnetic fields. For the design of the proposed ECT, a coreless Hall CT, when four symmetrically arranged Hall sensors are placed on the surface of a cable, and each cable spacing is greater than five times the cable radius, the interference from the ambient magnetic fields can be effectively eliminated by averaging the values of the four Hall voltages [11]. Thus, the actual current is obtained by simply applying the Biot-Savart Law for a long, straight cable; the formula is as follows [17]: (1) where is the radius of the cable, is the magnetic flux density at the distance from the cable center, is the current to be measured, and is the permeability of free space. The proposed ECT consists of four Hall sensors, a weighted adder, an offset voltage adjustment circuit, a filter, and a voltage follower, as shown in Fig. 1. The Hall voltages are summed by the weighted adder instead of performing the aforementioned averaging. In doing so, the interference from ambient magnetic fields can be equivalently and effectively eliminated, and a larger signal may be obtained to give a higher accuracy class when measuring smaller current. Finally, the ECT is connected to a filtering circuit with a cutoff frequency of 70 kHz to filter out the high-frequency noise. The signals are output to a data-acquisition card for readout. Since the channels of the data-acquisition card contain interference caused by common grounding, a voltage follower is connected to the back end of the output signal for buffer isolation, such that the signals will not be affected by common grounding. Moreover, an offset voltage adjustment circuit is used to eliminate the offset voltage of the Hall sensors. Normally, when the power supplied to Hall sensors is at a voltage of 5 V, each Hall sensor has a 2.5-V offset output voltage [18], and four of them will have a total dc voltage level of 10 V. Hence, the 12-V power-supply

Fig. 2. Configuration of the ECT mechanism.

Fig. 3. Photographs of the open-type mechanism and circuit configuration board with Hall sensors.

voltage division of the offset voltage adjustment circuit is used to eliminate this dc voltage level. III. MECHANISM AND CURRENT MEASUREMENT RANGE FOR THE PROPOSED ECT We first consider the mechanism design in addition to the circuit configuration for the new novel ECT so that it may be applied to actual current measurements in a power system. A diagram of the ECT mechanism is shown in Fig. 2, and the appearance is similar to a clamp-on meter; nevertheless, it is called an open-type mechanism in this paper. Fig. 3 shows the actual diagram of the proposed open-type mechanism and its internal circuit configuration board consisting of Hall sensors. The four Hall sensors are equidistantly and vertically distributed on two individual symmetrical printed-circuit boards (PCBs); also, plastic pillars are locked onto the PCBs in order to conveniently lock and fix the PCBs onto the mechanism. Since Hall sensors are separated into two PCBs, three conducting wires are used to connect the two PCBs, as shown in Fig. 3. In this design, two symmetrical U-shaped shells are assembled into a mechanism with upper and lower tough and flexible tabs capable of placing and centering the cable inside. Furthermore, the Hall sensors

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and their associated circuits were placed onto the mechanism to form an integrated measurement device. In the proposed design, the sum of the four Hall voltages is adopted; hence (1) should be modified as (2) As the distance between the center of the actual mechanism and each Hall sensor is 2.5 cm as shown in Fig. 2, from (2) (3) The selected Hall sensor is a commercial Hall sensor A1301, with a sensitivity of 2.5 mV/G; thus, the relationship between the measured currents and the output voltages of proposed ECTs could be derived as follows:

Fig. 4. Schematic diagram of all proposed circuits.

(4) Since the input range of the monitoring system is 10 V, the maximum measurable current is determined to be approximately 12 kA. Moreover, this value also fits the largest threephase fault current on the 11.95- and 23.9-kV sides in a primary power distribution substation. IV. POWER AND PROTECTION DESIGN FOR THE PROPOSED ECT As in the case of other ECTs, the proposed ECT requires an external power supply for proper operation. Therefore, we designed a self-contained power supply based on inductive power technology to supply energy induced from the magnetic fields generated by the current flowing in the power cable. The initial power supply design had a design goal of 600 A for the rated current subject to the specifications of the laboratory equipment. Since the self-contained power supply cannot provide enough energy when the current is small, we incorporated a battery pack in the design. When the current is smaller than a certain value, the battery pack is used to supply power. However, the inductive power supply will supply power to the ECT and charge the battery pack when the current is larger than a certain value. Under a fault condition, the inductive power circuit will induce a high voltage, and this high voltage can damage the proposed ECT. Hence, a protection mechanism is designed to protect the back-end circuits of the self-contained power supply; moreover, the battery pack will supply the ECT in case a fault occurs. The entire schematic diagram is shown in Fig. 4, and each component is introduced as follows: 1) small iron core and rectifier circuit: to induce ac power and then convert the ac power to dc power; 2) regulator circuit (1): to supply power to regulator circuit (2) and the battery pack; voltage range: input: 18-90 V, output 18 V; 3) regulator circuit (2): to supply power to the back-end associated circuit and Hall sensors; voltage range: input: 14.5–27 V, output: 12 V and 5 V; 4) back-end associated circuit: includes the offset voltage adjustment circuit, weighted adder circuit, and voltage follower; 5) overcurrent protection mechanism: when the current is greater than 850 A, the protection mechanism will be

Fig. 5. Actual diagram of the self-contained power supply.

activated; meanwhile, the battery pack will supply power to the regulator circuit (2). A. Self-Contained Power Supply As shown in Fig. 4, the self-contained power supply is designed to use a small iron core to convert the magnetic field induced by the current into energy. The power supply then supplies power to the ECT after the energy is processed by the rectifier and regulator circuits. Two sets of positive and negative power supplies are required; thus, two sets of windings are designed. The actual diagram of the self-contained power supply and its specifications are shown in Fig. 5. Since the back-end associated circuit is easily damaged under high-voltage conditions, equivalent burden resistances are adopted to replace the back-end-associated circuit during tests of the power-supply design. First, we need to know the power consumption of the entire circuit. An ac power supply is used to supply power to the entire circuit for estimating the required power, where the required positive power is 0.99 W and the required negative power is 0.25 W. After the required power is determined, the currents in the two windings are calculated with the magnetomotive force (mmf) formula in (2), and the corresponding burdens are estimated. (2) is the number of turns in the primary winding, that In (2), is, one power cable. and are the numbers of turns in the secondary and third windings, respectively; the two windings are used to supply positive and negative powers to the proposed ECT. Assuming the minimum current 20 A in the primary winding, the voltage outputs for positive and negative power


TABLE II POWER SUPPLIED BY THE SELF-CONTAINED POWER SUPPLY VERSUS THE CURRENT

supplies when 160 turns reaches the required operating voltage of 14.5 V obtained from several repeated measurements. Thus, . Moreover, from measurements of the entire circuit; thus, 0.1 A and 0.025 A. Thus, burden resistance , and burden resistance . Because of the limitations of the existing components, we adopt resistance values 100 and 402 for 99 and 400 , respectively, to carry out the actual measurements. Table II summarizes the measurement results. When the current reaches 25 A, the power generated by the self-contained power supply can meet the needs of the entire circuit; however, there is a slight error in the previous assumption in which the current is equal to 20 A. In the actual integration tests using the self-contained power supply, the results show that the ECT operates properly as long as the cable current reaches 20 A, which is even lower than 25 A. The reason is that part of the power is consumed by the resistances in the form of heat, and there might be some slight measuring error that exists in the measurement instrument. Therefore, the power supplied by the self-contained power supply would be greater than the values in Table II. B. Battery-Integrated Power Supply The battery pack and battery protection circuit are integrated into the self-contained power-supply circuit to supply power to the ECT when the current is small and cannot supply sufficient energy. A schematic diagram of the battery-integrated power supply is shown in Fig. 4. Since the input voltage of the back-end regulator circuit must be larger than 14.5 V, more than one battery is required in series to achieve the desired voltage when using a lead-acid battery (2 V), nickel-cadmium battery (1.2 V), or nickel-metal hydride battery (1.2 V), and the volume will be larger. Therefore, a lithium-ion battery with a higher-rated voltage (3.7 V) is adopted, and only four batteries in series are required to achieve the desired voltage. The endurance of the battery pack is obtained through calculation and then verified through testing. A dc power supply is used to supply power to the second voltage regulator in Fig. 4, and the front-end circuit of the second voltage regulator is disconnected. The test results indicate that the load currents for the positive and negative power supplies are 38 and 10 mA, respectively. Based on the discharge curve of the lithium-ion battery ICR18650K [19], the discharge capacity is roughly estimated to be about 2400 mA.h if the load current for the positive power supply is 38 mA at the rated voltage of 3.7 V. Hence, the discharge time is equal to (2400 mA. h)/(38 mA) h and the

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TABLE III CHARGING CURRENTS FOR THE BATTERY CORRESPONDING TO THE CURRENT

Fig. 6. Photograph of the protection mechanism and its left-side schematic diagram for the proposed ECT.

endurance is about 2.6 days. Moreover, the test results show that the endurance is about 60 h (equal to 2.5 days), so the error between the estimate and the testing is small. This error may be caused by slight differences in the characteristics among the four single batteries, and the fully charged state of the four single batteries may also have a slight difference. The charge capacity of the battery is the integral of the current, that is, for constant current conditions, the charge capacity is the product of the current and time. When the battery is charged at an input voltage of 14.5 V, the correspondence between the current and the charge current in the battery is summarized in Table III. We find that the charge current reaches 1.13 A when the current flowing in the cable is 600 A. Consequently, approximately 87% of the rated capacity of the battery is reached after charging for 2 h based on the constant-current and constant-voltage charging curve of a lithium-ion battery (ICR18650K) [19]. C. Protection Mechanism for the Proposed ECT In order to prevent the back-end circuit of the ECT from being affected by a large fault current when a fault occurs in a power system, a protection mechanism is required. This mechanism can be activated at the same time the fault current occurs to avoid damaging the ECT owing to the limitations of the voltage and current withstand capacities. We design a protection mechanism using the magnetic field induced by the current flowing in the power cable, as shown in Fig. 6. The protection mechanism includes two concentric structures, and the outer structure is formed by two silicon steel sheets. Moreover, two circular

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Fig. 7. Overall circuit diagram for the proposed ECT.

contacts are fixed on the inside of both silicon steel sheets, and the other two circular contacts are fixed on the outside of the inner structure. In addition, the two windings of the small iron cores are connected to the protection mechanism on both structures. When a fault occurs, the magnetic field is large enough to push the silicon steel sheets toward the power cable so that the contacts fixed on the outer structure will be shorted with the contacts fixed on the inner structure, as for the short-circuit condition of traditional CTs. Hence, the two windings are shorted, and the proposed ECT is protected from large fault currents. During this time, the battery continuously supplies power to the proposed ECT, and the performance of the ECT is not affected. A schematic diagram of the entire device is shown in Fig. 4. During testing, the inner and outer contacts will be shorted when the current flowing in the power cable exceeds 850 A; hence, no induced currents flow into the ECT. However, when the current drops below 550 A, the protective device will be reset to its original state. D. Circuit Architecture The entire circuit of the proposed ECT is shown in Fig. 7. Since the power sources of the back-end circuit include 12 V, two voltage regulators (7812) are used in the voltage regulating circuit, where the positive source is on the upper part of the circuit, and the negative source is on the lower part of the circuit, as shown in Fig. 7. Moreover, the entire proposed circuit is supplied by the self-contained power supply that consists of two power sources: A1A2 and B1B2. Therefore, the input voltages of the voltage regulating circuit will become higher as the current in the power cable increases, and the power of the increasing input voltages will be consumed by resistors and transistors in the form of heat. Traditionally, a positive power source is designed using an NPN transistor coupled with a voltage regulator (IC 78XX); the negative power source is designed using

a PNP transistor coupled with a voltage regulator (IC 79XX). With this design, the positive power source does work, but the negative power source will burn out because the voltage across the PNP transistor exceeds the maximum rating of the PNP transistor. When the current reaches 600 A, has already reached 70 V, which is higher than the maximum rating of 20 V for a commercial PNP transistor. For the aforementioned reasons, two sets of NPN transistors coupled with a voltage regulator (IC 7812) are used, and the positive end of the output of the upper voltage regulator (U3) is connected to the ground end of the output of the lower voltage regulator (U1). Therefore, a common ground will be established, and the positive and negative power sources are supplied. The voltage regulating circuit is divided into two stages, where the first regulating stage consists of transistors, resistors, and Zener diodes, and the second regulating stage mainly consists of IC 7812. The reason for two-state regulation is that the regulating ranges are different. In the first stage, the input voltage range for regulation is 18 to 90 V, and the output voltage for this stage is chosen to be 18 V. The reason for selecting 18 V is that this voltage is used to charge the battery, and the selected value must be higher than the output voltage of the four batteries in series. Furthermore, the voltage should not be too high; if not, more power will be consumed in the second regulating stage, resulting in a temperature rise in the voltage regulators. Since the voltage regulators and Hall sensors are packed together, the low consumption design in the voltage regulators can decrease the effect of temperature on the Hall sensors. The reason for selecting 90 V as the maximum voltage for this range is related to the protective device. The protective device will act when the current reaches 850 A, and the voltage before the protective device acts is approximately 90 V. In addition to the first regulating stage, the input voltage for the second regulating stage can also be supplied by the battery


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TABLE IV MEASURED DATA FOR 5%, 20%, 100%, AND 120% OF THE RATED CURRENT

pack. Although the rated voltage of the battery pack is 3.7 V 14.8 V, the actual voltage would be approximately equal to the maximum charge voltage of 4.25 V 17 V, under the condition that the battery pack is fully charged, and its discharging current is small. This floating voltage would result in smaller power consumption for the voltage regulators. The input voltage range of the voltage regulator IC 7812 is 14.5 to 27 V, and the corresponding output voltage is 12 V. V. TEST RESULTS AND DISCUSSION We carried out tests for the accuracy class and tested the effects of the inter-phase interference for the proposed ECT when the three-phase currents are balanced and unbalanced. Moreover, we tested the current waveforms measured by three different devices, including the traditional CT, proposed ECT, and linear CT when a three-phase balanced fault occurs. The R- and S-phases of a three-phase power source are used to set up the measurement system in Fig. 8. Each phase current generator consists of two transformers: the first transformer, an autotransformer, is used to supply the primary winding of the second transformer. The secondary winding of the second transformer adopts one turn of a cable. Therefore, when the transformer is energized, it will produce large current in the secondary winding. In the experiments, the traditional 2500/5-A and Class-0.2 CT is used as a reference instrument, and two proposed ECTs and one linear CT (Rogowski coil/3 kA /Class 0.3) are tested. All tests are described as follows. Case 1—Accuracy Class for a Rated Current of 600 A: In this case, only an R-phase power source is used. According to IEC Standard 60044–8, where the test items include 5%, 20%, 100%, and 120% of the rated current, corresponding current and phase errors are measured and summarized in Table IV. Current measurement results show that the proposed ECT meets accuracy Class 0.5 for measuring CTs. Case 2—Problem of Interference between Two Phases when Three-Phase Currents are Balanced: In this experiment, R- and S-phase power sources are used. As shown in Fig. 8, one proposed ECT is close to the S-phase cable, but another is far from the S-phase cable. Hence, the effect of the magnetic-field interference from the other phases on the accuracy of the proposed ECTs is tested. Table V lists the measurement results; compared with the results in Table IV, the current errors for

Fig. 8. Interphase interference measurement system.

the proposed ECT 1 between Tables IV and V are similar when the R- and S-phase currents are balanced and 5% of the rated current. However, the current error in Table V is larger than that in Table IV when the R- and S-phase currents proportionally increase. The reason for the increase in current error is that the magnetic-field interference from the other phase current becomes large when the R- and S-phase currents increase in a balanced manner. However, the measurement results for the proposed ECT still meet accuracy Class 0.5 for measuring CTs. Case 3—Problem of Interference between Two Phases when Three-Phase Currents are Unbalanced: We intend to test the magnetic-field interference from the S-phase current in the proposed ECT placed in the R-phase cable when the R- and S-phase currents are unbalanced. The measurement results for these conditions are listed in Table VI. When the measured R-phase current is 300 A, the current error for the proposed ECT 1 increases as the S-phase current increases. However, the proposed ECT, which consists of four Hall sensors symmetrically arranged on the R-phase cable, can eliminate the magnetic-field interference from the S-phase unbalanced current and still meet accuracy Class 0.5 for measuring CTs. Case 4—Problem of Fault Currents: Experiments are also conducted for fault currents, and Figs. 9–11 show the current waveforms measured by three different devices: the proposed ECT, traditional CT (2500/5 A and Class 0.2), and linear CT

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TABLE V MEASURED DATA FOR 5%, 20%, 100%, AND 120% OF THE RATED CURRENT WHEN AFFECTED BY INTERPHASE INTERFERENCE BETWEEN BALANCED PHASES

TABLE VI EFFECT OF INTERPHASE INTERFERENCE WHEN THE CURRENTS OF TWO PHASES ARE UNBALANCED

Fig. 9. R-phase fault current peak of 12 kA. Fig. 11. T-phase fault current peak of 12 kA.

TABLE VII COMPOSITE ERRORS AND ACCURACY CLASSES OF ECTS THREE-PHASE FAULT OCCURS

WHEN A

Fig. 10. S-phase fault current peak of 12.6 kA.

(Rogowski coil/1 MA/Class 1). Since the traditional CT experiences saturation problems in this test, a linear CT is used as a reference instrument; moreover, the measurement results are listed in Table VII. It can be seen in these figures that the second peak in the R-phase waveform and the first peaks in S- and T-phase

waveforms measured by the linear CTs are slightly bulging. A possible reason may be that the dc offset components of the fault currents cause the integrators in the linear CTs to generate errors. The measurement results show that the new-type ECT meets accuracy Class 5P21 for protective CTs. Moreover, the proposed ECT does not encounter saturation problems due to


TABLE VIII POWER CONSUMPTION OF THE PROPOSED ECT

its coreless structure designed as a linear CT. Hence, the current waveforms measured by the proposed ECT and linear CT nearly overlap. The proposed ECT does not have an integrator error problem caused by dc offsets, as in a linear CT. Case 5—Power Consumption of the Proposed ECT: In this experiment, the power consumption of the proposed ECT is measured for various operating environments. It can be seen from Figs. 4 and 7 that the input voltages of the regulator circuits could change as the load current changes in the power cable; therefore, the power consumption of the self-contained power supply will change as the load current changes. However, the power consumption of the back-end associated circuit can be maintained at a constant value. The actual measurement results are listed in Table VIII. VI. CONCLUSION In order to alleviate the core saturation problem in traditional CTs, the large cost of optical and linear CTs, and lack of practical applications for alternative CTs, a coreless ECT is designed in this research. Furthermore, we tested the accuracy class of the proposed ECTs in accordance with IEC Standard 60044-8. The ECT (rated current: 600 A) can meet accuracy Class 0.5 for measuring CTs and 5P21 for protective CTs. Due to the lack of costly and bulky iron cores, the proposed ECT not only can reduce equipment costs but can also save a lot of space in a substation or switchyard. Therefore, a tremendous amount of money can be saved by replacing traditional CTs with the proposed ECTs. Furthermore, a self-contained power supply was designed and implemented into the ECT to deal with the lack of power sources at certain measurement locations. In practical experiments, the self-contained power supply can supply continuous steady power to the ECT whenever light-load current flows in the power cable or a fault current occurs. As a result, this design solves the environmental restrictions of driving powers, which are needed for most ECTs.

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[4] T. Bi, H. Liu, X. Zhou, and Q. Yang, “Impact of transient response of instrument transformers on phasor measurements,” presented at the IEEE Power Eng. Soc. Gen. Meeting, Minneapolis, MN, USA, Jul. 2010. [5] T. W. Cease and P. Johnston, “A magneto-optic current transducer,” IEEE Trans. Power Del., vol. 5, no. 2, pp. 548–555, Apr. 1990. [6] W. F. Ray, “Rogowski transducers for high bandwidth high current measurement,” IEEE Colloq. Low Frequency Power Measurement and Analysis, Nov. 2, 1994. [7] H. Kirkham, “Current measurement methods for the smart grid,” presented at the IEEE Power Eng. Soc. Gen. Meeting, Calgary, AB, Canada, Jul. 2009. [8] J. Schmid and K. Kunde, “Application of non conventional voltage and currents sensors in high voltage transmission and distribution systems,” presented at the IEEE Int. Conf. Smart Meas. Future Grids, Bologna, Italy, Nov. 2011. [9] Y. P. Tsai, K. L. Chen, and N. Chen, “Design of a hall effect current microsensor for power networks,” IEEE Trans. Smart Grid, vol. 2, no. 3, pp. 421–427, Sep. 2011. [10] K. L. Chen, Y. P. Tsai, and N. Chen, “Application of power current microsensors to current measurements in gas insulated switchgears,” J. Chinese Inst. Eng., vol. 35, no. 8, 2012. [11] K. L. Chen and N. Chen, “A new method for power current measurement using a coreless hall effect current transformer,” IEEE Trans. Instrum. Meas., vol. 60, no. 1, pp. 158–169, Jan. 2011. [12] K. L. Chen, Y. P. Tsai, N. Chen Suratsavadee, K. Korkua, and W.-J. Lee, “Using coreless hall effect sensor for accurate current measurement in zigbee based wireless sensor network,” presented at the Ind. Appl. Soc. Annu. Meeting, Orlando, FL, USA, Oct. 2011. [13] Instrument Transformers-Part 8: Electronic Current Transformers, IEC Standard 60044-8, 2002. [14] K. W. Klontz, D. M. Divan, D. W. Novotny, and R. D. Lorenz, “Contactless power delivery system for mining applications,” IEEE Trans. Ind. Appl., vol. 31, no. 1, pp. 27–35, Jan./Feb. 1995. [15] J. Chen, R. Yan, J. Li, and Q. Yang, “Power-line energizing system for active power electronic current transformer,” presented at the Int. Conf. Elect. Mach. Syst., Wuhan, China, Oct. 2008. [16] Z. Wu, K. Thomas, R. Sun, V. A. Centeno, and A. G. Phadke, “Analysis and design of a current transformer fed power supply from high AC voltage cable,” presented at the IEEE Int. Symp. Ind. Electron., Hangzhou, China, May 2012. [17] D. K. Cheng, Field and Wave Electromagnetics. Reading, MA, USA: Addison-Wesley, 1989, pp. 321–345. [18] A1301 and A1302 Datasheets, Allegro MicroSystems, Inc., 2005. [19] ICR18650K datasheets, 2011, E-ONE Molienergy Corp.

Chin-Tien Liang was born in Changhua City, Taiwan, in 1981. He received the B.S.E.E. and M.S.E.E. degrees from National Taiwan University of Science and Technology (NTUST), Taipei, in 2007 and 2012, respectively. His research interest is in current sensors and their applications in power systems.

REFERENCES [1] A. Mao, J. Yu, and Z. Guo, “PMU placement and data processing in WAMS that complements SCADA,” presented at the IEEE Power Eng. Soc. Gen. Meeting, San Francisco, CA, USA, Jun. 2005. [2] P. Castello, P. Ferrari, A. Flammini, C. Muscas, and S. Rinaldi, “An IEC 61850-compliant distributed PMU for electrical substations,” presented at the 2012 IEEE Int. Workshop Appl. Measurements Power Syst., Aachen, Germany, Sep. 2012. [3] Z. Wu, K. Thomas, R. Sun, V. A. Centeno, and A. G. Phadke, “Three-phase instrument transformer calibration with synchronized phasor measurements,” presented at the IEEE Power Energy Soc. Innovative Smart Grid Technol., Washington, DC, USA, Jan. 2012.

Kun-Long Chen was born in New Taipei City, Taiwan, in 1982. He received the B.S.E.E. degree from Feng-Chia University, Taichung, Taiwan, in 2004, and the M.S.E.E. and Ph.D. degrees in electrical engineering from National Taiwan University of Science and Technology (NTUST), Taipei, Taiwan, in 2006 and 2011, respectively. Since 2011, he has been a Researcher with the Industrial Technology Research Institute, Taiwan. His research interests are current sensors, electricity metrology for smart grid, and the nonintrusive load monitoring system.

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Yuan-Pin Tsai was born in Chiayi, Taiwan, in 1965. He received the B.S.E.E., M.S.E.E., and Ph.D. degrees in electrical engineering from National Taiwan University of Science and Technology (NTUST), Taipei, in 1992, 2004, and 2013, respectively. Since 1992, he has been with Taiwan Electrical and Mechanical Engineering Services, Inc., Taipei, where he is a Project Manager. His research interest is in power system protection and electrical safety.

Nanming Chen (S'76–M'80) was born in Taiwan in 1951. He received the B.S.E.E. degree from National Taiwan University in 1973, the M.S.E.E. from Virginia Polytechnic Institute and State University, Blacksburg, VA, USA, in 1977, and the Ph.D. degree in electrical engineering from Purdue University, West Lafayette, IN, USA, in 1980. He was with Pacific Gas and Electric Company, San Francisco, CA, USA, and San Francisco State University. Since 2008, he has been a Chair Professor of National Taiwan University of Science and Technology, Taipei. He is also a Chair Professor of National United University, Miaoli, Taiwan. His research interest is in power engineering and electrical railway systems. Dr. Chen was a Director on the Board of Directors of Taiwan Power Company, Taipei, Taiwan, from 2004 to 2007, and the Stability and Reliability Committee of Taiwan Power Company from 2001 to 2003. He was also appointed as a member of the 1999 blackout investigation committee and 2001 3rd nuclear powerplant blackout investigation committee.