Article Design and Experimental Study of a Current Transformer with a Stacked PCB Based on B-Dot

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1 Article Design nd Experimentl Study of Current Trnsformer with Stcked PCB Bsed on B-Dot Jingng Wng 1, Dincheng Si 1, *, Tin Tin 2 nd Rn Ren 2 1 Stte Key Lbortory of Power Trnsmission Equipment nd System Security nd New Technology, Chongqing University, Chongqing 444, Chin; jingng@cqu.edu.cn 2 Chongying Electric Power Design Institute, Chongying 41121, Chin; pperfecty@163.com (T.T.); rnrnxinxing@126.com (R.R.) * Correspondence: sidincheng@cqu.edu.cn or sidincheng@163.com; Tel.: Acdemic Editor: Vittorio M. N. Pssro Received: 15 Jnury 217; Accepted: 29 Mrch 217; Published: 1 April 217 Abstrct: An electronic current trnsformer with B-dot sensor is proposed in this study. The B-dot sensor cn relize the current mesurement of the trnsmission line in non-contct wy in ccordnce with the principle of mgnetic field coupling. The multiple electrodes series-opposing structure is pplied together with differentil input structures nd ctive integrting circuits, which cn llow the sensor to operte in differentil mode. Mxwell softwre is dopted to model nd simulte the sensor. Optimiztion of the sensor structurl prmeters is conducted through finite-element simultion. A test pltform is built to conduct the stedy-stte chrcteristic, on-off opertion, nd linerity tests for the designed current trnsformer under the power-frequency current. As shown by the test results, in contrst with trditionl electromgnetic CT, the designed current trnsformer cn chieve high ccurcy nd good phse-frequency; its linerity is lso very good t different distnces from the wire. The proposed current trnsformer provides new method for electricity lrceny prevention nd on-line monitoring of the power grid in n electric system, thereby stisfying the development demnds of the smrt power grid. Keywords: non-contct; mgnetic field coupling; current trnsformer; current monitoring 1. Introduction In power system, the ccurte mesurement of power-frequency current is n importnt pproch in ensuring the relible opertion of power distribution network. The ccurcy, convenience, nd speed of current trnsformers ply significnt role in electric energy mesurement, system monitoring nd dignosis, nd power system fult nlysis in smrt power grids. Given tht trditionl electromgnetic current trnsformers (CTs) contin n iron core, their ferromgnetic effect tends to produce liner sturtion nd ferromgnetic resonnce problems s well s insultion nd volume nd bndwidth response problems [1]. The method of current mesurement by using Rogowski coil hs been proposed [2,3], resulting in high mesurement ccurcy nd good trnsient response. However, this kind of method bsed on the Rogowski coil trgets high-frequency lightning current mesurements, pulse current mesurements, nd prtil dischrge mesurements on power cbles, nd the current loop to be mesured must pss through the coil [4 13]. Current mesurement methods bsed on the B-dot principle hve been studied [14 16], resulting in high mesurement ccurcy nd bndwidths reching MHz. A B-dot coil is ctully specil type of Rogowski coil tht clcultes the wire current vi retroduction by sensing the chnge of mgnetic field generted by the current-crrying wire. Compred with norml Rogowski coil, B-dot coil does not need to pss through the coil [17]. Most studies bsed on the B-dot principle Sensors 217, 17, 82; doi:1.339/s

2 Sensors 217, 17, 82 2 of 17 focus on high-frequency current mesurement (MHz, ka level) by sensor under the self-integrl stte, such s in lightening current nd plsm dischrge pulse current [18 21]. In comprison, only few studies hve exmined the performnce of B-dot sensors in mesuring currents in differentil mode. In the pper, B-dot sensor is proposed bsed on mgnetic field coupling, nd non-contct electronic current trnsformer with stcked printed circuit bord (PCB) is proposed. By nlyzing the working principle nd the working mode of the B-dot sensor nd its influencing fctors, method for differentil input nd multiple electrode series-opposing is introduced, with the im of designing trnsformer tht cn be operted in differentil mode for mesuring the power frequency current of power system. The proposed trnsformer hs the following dvntges: good phse-frequency chrcteristic, good linerity, simple structure, nd esy instlltion. It offers n lterntive for electricity lrceny prevention nd on-line monitoring of power grid in n electric system nd meets the development requirements of smrt power grids. 2. Principle of Current Mesurement 2.1. Principl for B-Dot The B-dot probe is Rogowski coil with specil structure, nd mesures the current indirectly by mesuring the chnging mgnetic field built by the chnging current [17]. The principle for the B-dot current mesurement is shown in Figure 1. r Figure 1. B-dot coil mgnetic field induction. Here, r is the distnce from the plne of the probe to the conductor, r is the rdius of the probe coil, S is the re enclosed by the probe coil, B is the mgnetic induction intensity generted by the current t given moment, nd R is pure resistnce. According to the lw of electromgnetic induction, when r << r, the induced probe voltge in the mgnetic field is given by To mke M ds 2 r, then et () d () di () dt 2 r dt ds (1) di() t et () M (2) dt In Eqution (1), coil mgnetic flux is ψ, S is the ctive re of the probe coil, nd r is the distnce from probe center to the electrified wire. From Eqution (2), the output voltge of the probe is proportionl to the differentil wire current. Thus, by clculting the integrl of the induced electromotive force of the probe, the wire current cn be clculted.

3 Sensors 217, 17, 82 3 of Sensor Design Bsed on the principle of B-dot coil current mesurement, the PCB ir-core coil sensor is designed. Its structure is shown in Figure 2. The top lyer coil is reversely connected in series with the corresponding bottom coil through vis, nd totl of eight ir-core coils re plced successively in series to form loop. Obviously, the electromotive force of the coils is superimposed in sequence. By pplying the Rogowski coil principle, the returning turn coils cn eliminte the interference mgnetic field from the externl environment [22]. In the present pper, the ide of the returning turn design is dopted. Hence, the d, d coils re dopted s compensting coils. When the sensor is subject to externl disturbnce in the mgnetic field, the d coil cn counterct the interference flux in the, b, c coils to certin extent. i(t) b b d d? c c 顶层 底层 Figure 2. Series-opposing structure of the printed circuit bord (PCB) ir-core coil. As shown in Figure 3, n ir-core coil is equivlent to N squre coils. Here, r is the distnce from the point on the coil to the wire, is the width of the outermost lyer of the ir-core coil, c is the distnce between the turns of the coil, d is the distnce from the outermost coil to the center of the wire, nd N is the number of turns of single ir-core coil. In Figure 2, the distnces of ech ir-core coil to the wire re given by d, db, dc, dd respectively, where db is equl to dc. I i(t) d c c I i(t) d+kc -2kc r dr b-2kc () single-core coil (b) clcultion of rectngulr coil flux Figure 3. Principle of mgnetic induction in single-core coil. According to the lw of electromgnetic induction, the flux of single turn coil in Figure 3b is given by

4 Sensors 217, 17, 82 4 of 17 () t B() t ds dbkc dkc S () it ( 2 kc) dr 2 r (3) The totl flux of single ir-core coil is expressed s it () dbkc n1 n1 () t ( 2 kc)ln( ) k k 2 d kc d() t et () dt From the mthemticl reltionship, we know tht when d =, the flux Φ will rech the mximum vlue, so the induced voltge is the mximum vlue. Induced electromotive force of PCB ir-core coil is given by et () e() t e() t e() t e() t (5) b c d n1 it () d kc db kc dd kc et ( ) ( 2 kc) ln( ) 2ln( ) ln( ) k d kc db kc dd kc (6) (4) 2.3. Sensor under Self-Integrting Mode As shown in Figure 4, when the vlue of smple resistnce R is smll nd 1/ωC >> R, then ic, nd ir i2, nd when the current chnge rte is lrger, nmely, when di2/dt is quite lrge, we hve di1 di2 M L (7) dt dt Then the smple resistnce voltge is expressed s M u i2r ir 1 (8) L Now, the output voltge of the smple resistnce is in direct proportion to the mesured current; here, coil self-inductnce L serves s n integrl. Hence, the externl integrting circuit is not required, becuse the circuit itself chieves integrl function. Such sttus is clled self-integrting sttus [11,13,14]. Under this sttus, the sensor is suitble for the mesurement of high-frequency current rther thn low-frequency current. i1 M e i2 r L ic ir C R u 2.4. Sensor under Differentil Mode Figure 4. Equivlent circuit of the sensor under self-integrting mode. When R is lrge (R >> r) nd the vrition of di2/dt is smll (reltively low-frequency), the PCB ir-core coil equivlent circuit is similr to pure resistnce circuit. At this time, the sensor output voltge is given by

5 Sensors 217, 17, 82 5 of 17 di () t (9) 1 u1 () t M dt At this moment, n integrting circuit must be connected to the sensor so tht the output voltge nd mesured current cn hve direct rtio with ech other. This kind of working condition is clled differentil sttus or externl integrl sttus. In Figure 5, the trnsfer function in the mesurement link is expressed s G 1 s U1 s Ms I1 s 2 L r LCs rc s 1 R R (1) The ctive integrl link is given by G 2 s u s R2 1 u s R R Cs (11) The trnsfer function of the whole system is given by 1 2 G s G s G s MR2 s R1 (12) 2 L r RCs 2 1LCs rc s 1 R R The trnsfer function is required to drw Bode figure, s shown in Figure 6. i 1 M i2 L r e i C c i R u R 1 R 1 OPA R 2 C u Figure 5. Equivlent circuit of the sensor with ctive integrl. O LG db 2 db/ dec 1 l RC 2 1 2lg RC 2 M MR MR 2lg 2lg RC 2 R r R r h db / dec 1 LC Figure 6. Amplitude-frequency chrcteristic of sensor with n ctive integrtor (lg is bse 1 logrithm). Bsed on R >> r, the system s upper nd lower cut-off frequency re simplified respectively s

6 Sensors 217, 17, 82 6 of 17 1 fl 2 RC 2 1 fh 2 LC (13) The frequency bnd is given by 1 1 f fh fl (14) 2 2 LC RC 2 In Eqution (13), the lower cut-off frequency of the system is determined by the feedbck resistnce R2 nd the integrl cpcitnce C, wheres the upper cut-off frequency is determined by the coil prmeters. The system sensitivity is given by MR2 1 M S G j (15) R R C RC In Eqution (14), the sensitivity of the system cn be improved by reducing R1, nd the lower cut-off frequency cn be reduced by incresing the R2. Therefore, the integrl link cn be utilized to properly djust the cut-off frequency. Using the Bode function of MATLAB softwre to nlyze the trnsfer function of the system, we find tht the upper cut-off frequency of the sensor cn rech MHz, nd the lower cut-off frequency is close to Hz. Therefore, when the sensor is operting in differentil mode, it cn meet the requirements of low frequency current mesurement, which is suitble for mesuring nd monitoring power frequency current Anti-Interference Sensor Anlysis Interference mgnetic field cn be decomposed into two components: prllel to the PCB bord nd perpendiculr to the PCB bord Mgnetic Field Component Prllel to the PCB Bord When the mgnetic field is prllel to the PCB bord, s shown in Figure 7, the direction of the mgnetic field is prllel to the ir-core coils, nd the mgnetic flux pssing through these coils is zero; hence, no induced electromotive force is generted. b b c d c d Interference mgnetic field component verticl to PCB bord () Interference mgnetic field component prllel to PCB bord (b) Interference mgnetic field component verticl to PCB bord Figure 7. Influence of the externl interference mgnetic field.

7 Sensors 217, 17, 82 7 of Mgnetic Field Component Verticl to the PCB Bord When the externl mgnetic field is uniform mgnetic field, the mgnetic fluxes through the four ir-core coils hve the sme size, nd the induced electromotive forces re equl in size nd opposite in direction. Due to the reverse series of the coils, the induced electromotive forces cncel ech other out, s shown in Figure 7b. Therefore, the ir-core coils cn resist the interference of the uniform mgnetic field. The externl interference current is tken s n exmple for this nlysis. When the externl mgnetic field is not uniform, the current in the plne of the PCB bord cn produce mgnetic field perpendiculr to the ir-core coils. As shown in Figure 8, the interference current i(t) is locted on the side of the PCB bord. The distnces of interference currents to ech ir-core coil re d, db, dc nd dd. The length of the outer coil is, the width is b, the turn pitch is c. Figure 8. The clcultions of interference mgnetic. According to Eqution (3), it () d kc (16) n1 () t ( 2 kc)ln( ) k 2 r d kc it () d kc (17) n1 b b() t ( 2 kc)ln( ) k 2 r db kc it () d kc (18) n1 c c () t ( 2 kc)ln( ) k 2 r dc kc it () d kc (19) n1 d d () t ( 2 kc)ln( ) k 2 r dd kc The totl flux generted by the interference current in the ir-core coil is () t 2[ () t () t () t ()] t (2) b c d In Figure 9, the mgnetic field intensity decys rpidly long the rdil direction of the wire, nd t distnce of.5 m from the wire, it decys to bout 3%. The influence of the mgnetic field generted by other wires cn be ignored when the other wires re wy from the wire to be mesured by more thn.5 m. The trnsmission lines re usully overhed lines in the opertion of the power system, nd the wire spcing is much lrger thn.5 m. Therefore, the mesurement ccurcy cn be chieved s long s the distnce is mintined. Once the PCB ir-core coil sensor is pplied in more complex scenes, the djcent phse interference must be considered.

8 Sensors 217, 17, 82 8 of Design of the Current Trnsformer Figure 9. Mgnetic field curve round the wire in 1 A Sensor Optimiztion Bsed on Finite Element Simultion The Ansoft Mxwell softwre is used to build the model of the conducting wire. The wire rdius is set to 2.5 mm, nd the current is set to 1 A. The vrition curve of the mgnetic field intensity round the wire is shown in Figure 9. As cn be seen, the mgnetic field intensity decys rpidly long the rdil direction of the wire. At the distnce of.5 m from the wire, it decys to bout 3% of the mximum vlue. Therefore, when the current is mesured by the sensor, the mesuring points should be in the vicinity of the conductor region. According to Eqution (7), mjor fctors influencing the sensor mutul induction coefficient re coil length, distnce between turns, nd the number of turns. Coil length nd number of turns cn influence the PCB size, wheres the distnce between turns cn ffect the frequency response of the sensor. Finite element in Ansoft Mxwell softwre is pplied to optimize the bove prmeters nd gurntee proper mutul induction coefficient. The simultion settings re s follows: connecting trnsmission wire of 5 m with current of 5 Hz nd 1 A, wire rdius of 2.5 mm, region size tht is 3% of the clcultion model, nd border region clcultion of to simulte the sitution wherein the mgnetic field t the infinite point is, s shown in Figure 1. Bsed on the simultion, the model is further optimized nd improved to obtin higher voltge output. The distribution of the mgnetic field is shown in Figure 11. The clcultion of the mgnetic flux density of the sensor coil is performed by using the Mxwell softwre. Then, the output voltge of the sensor is clculted bsed on the flux dt obtined. After considering ll the prmeters nd the output voltge, set of suitble prmeters is selected to determine the structure of PCB ir-core coil. The coil prmeters re shown in Tble 1. Figure 1. Finite element clcultion model.

9 Sensors 217, 17, 82 9 of 17 b d c Figure 11. PCB ir-core coil mgnetic simultion clcultion. Tble 1. Design prmeters for single-lyer PCB ir-core coil structure. Coil Number of Turns (n) Length (cm) Electrode Width (mil) Distnce Between Electrodes (mil) Bsed on the prmeters, the totl mgnetic flux of the coil in, b, c, d, which re obtined from the simultion re , , nd Wb, respectively. Thus, the totl mgnetic influx of the CT bsed on PCB plnr-type ir-core coil is given by The mutul induction efficient is expressed s b c d Wb M. (21) I The output voltge of the whole trnsformer is given by di( t) d1sin(1 t) et () M M Amplitude: E 4.3 mv dt dt Finlly, the PCB ir-core coil sensor is fbricted by clculting the coil prmeters [23]. The impednce of the sensor coil itself is mesured by using Agilent 4294A (Agilent Technologies Inc., Snt Clr, CA, USA), s shown in Figure 12. Mesurement is done within the scope of 4 Hz to 4 MHz. The induction nd cpcitnce prmeters hve certin fluctutions. The result is shown in Tble 2. Figure 12. Prmeter mesurement of the PCB ir-core coil sensor.

10 Sensors 217, 17, 82 1 of 17 Tble 2. Inner impednce prmeter of PCB ir-core coil. PCB Air-core coil Resistnce Induction Fluctuting Devition Cpcitnce Fluctuting Devition Ω μh 5 μh 37.2 pf 8 pf 3.2. Device Design for the Electronic Trnsformer In considertion of the low output voltge of the single-block PCB sensor, multi-block PCBs re connected in series to improve output voltge [24]. The trnsformer structure is designed s shown in Figure 13, nd consists of the sensor, signl cquisition unit, nd signl processing unit. The output signl of the sensor is collected by circuit with differentil mplifying structure [25,26]. The differentil mplifier circuit is minly composed of differentil mplifier AD62 (Anlog Devices Inc., Norwood, MA, USA) nd gin resistor. The AD62 performs differentil opertion on the output signl to remove the common-mode signl with the sme polrity in the interfering signl. Simultneously, the differentil mode signl with opposite polrity is mplified, nd this differentil mode signl is useful signl we wnt to obtin. This not only increses the output of the trnsformer, but lso restrins the common mode interference, such s high-frequency noise signl. Figure 13. A model of the current trnsformer structure. For the integrl link, n ctive integrtor with n inertil element cts s circuit for restoring the originl signl. When mesuring the current of the high-voltge trnsmission line, s n electronic trnsformer, introducing the AD module nd WIFI technology is convenient, fter which the dt is sent to the remote terminl using WIFI technology. 4. Test Result Anlysis 4.1. Test Pltform A current test pltform is built to simulte the current mesurement environment of the power distribution network. The structure of the pltform is shown in Figure 14. Both the trditionl electromgnetic current trnsformer (CT, HL-3,.2 level) [27], nd the designed trnsformer re used t the sme time to conduct current mesurement for the power wire in the pltform. Here, the WIFI technology is no longer needed becuse of the low power output. Frequency Power Power line CT Power Lod PCB current trnsformer Signl cquisition circuit Oscilloscope Figure 14. Test pltform structure.

11 Sensors 217, 17, of 17 The test pltform is shown s Figures 15 nd 16; it uses the progrm-controlled convertor (type: JJ98DD53D, Shndong Jingjiu Science nd Technology Co., Ltd., Jinn, Chin) s the power supply nd the high-power resistnce is considered the lod. The oscilloscope (Tektronix, DPO214B, Beverton, OR, USA) is pplied s wveform disply nd mesuring equipment. The CT is used s the mesurement stndrd to relize the clibrtion of the designed trnsformer. The mplitude, phse position, nd wveform re compred to verify the performnce of the designed trnsformer. Vrible frequency power supply Oscilloscope High power lod Sensor CT DC power supply Figure 15. Test pltform. Figure 16. Sensor test physicl picture Stedy-Stte Performnce Test On the bove test pltform, the sensor is fixed bove the wire, nd the verticl reltive distnce D is kept invrint, s shown in Figure 14. The current output is djusted grdully. In ddition, the secondry CT is connected to resistive lod for I-V conversion. The output wveform of the designed trnsformer nd the CT is displyed vi oscilloscope, s shown in Figure 17. The oscilloscope chnnel 1 shows the output wveform of the designed trnsformer, wheres chnnel 2 shows the CT output wveform. ()

12 Sensors 217, 17, of 17 (b) (c) Figure 17. The stedy stte wveform of the designed trnsformer nd the current trnsformer (CT). () f = 5 Hz, I = 1 A, D = 5 mm; (b) f = 5 Hz, I = 23 A, D = 5 mm; (c) f = 15 Hz, I = 9 A, D = 5 mm. In Figure 17, the output wveform of the trnsformer is bsiclly the sme s tht of the CT, which indictes tht the trnsformer hs good stedy-stte response chrcteristics. If φ <, the phse of the designed trnsformer s output voltge is delyed. As stipulted in IEC Stndrd [28], rtio error ε% nd phse position difference φ re respectively defined s KU n s Ip % 1%, (22) I p. (23) s p Here, Us is the output voltge of the designed trnsformer, Kn is the rted trnsformtion rtio, Ip is the current in the wire, φp is the phse position of the designed trnsformer, nd φs is the phse position of the stndrd CT. First, we djust the power to current chnges between A nd 4 A; then, when the output current chnges, the mplitude nd frequency of both the CT nd the trnsformer output voltge re recorded, nd the wveform is sved. Finlly, the experimentl dt re collted, nd the rtio difference nd phse difference of the trnsformer re clculted. The results re shown in Tble 3. In this Tble, In is the rted current, Im is the wire current, nd Up is the output voltge of the trnsformer. Tble 3. Accurcy mesurement for the current trnsformer (D = 1 mm). Mesuring Point Im/A Up/mv ε %(±) φ/( ) 2% In % In % In % In % In % In % In % In % In

13 Sensors 217, 17, of 17 The results bove show the following: 1) In Tble 3, within the rted current rnge from 2% to 12%, the trnsformer rtio error ε% <.9%, nd the phse difference φ < 3. Thus, the designed trnsformer hs high mesurement ccurcy. 2) In Figure 17 nd Tble 3, smll error cn be found in the phse position, which my be due to the prsitic cpcitnce between the coils nd the resulting coil inductnce Linerity Test On the test pltform, the linerity performnce is tested. The test is done vi three steps: 1) djust the distnce from the sensor (PCB bord) to the wire, tking 5 mm s unit (chnge the height of lifting column in Figure 16); 2) grdully djust the output current from A to 4 A; nd 3) record the dt. The experimentl dt when the distnce D is 5 mm re shown in Tble 4. Tble 4. Output voltge of the trnsformer under different currents (D = 5 mm, f = 5 Hz). Uc/mV Up/mV I/A Uc/mV Up/mV I/A In fitting the experimentl dt of trnsformer nd CT, the horizontl xis represents the output voltge of the trnsformer, wheres the verticl xis represents the CT. The fitting curve is shown in Figure 18. The rtio of the liner fitting curve is the correction fctor of the trnsformer error. D=5mm 4. D=1mm U p /V U p /V U c /V U p /V () 5 mm 3. D=15mm U p /V...4 U c /V (b) 1 mm 2. D=2mm U c /V (c) 15 mm...4 U c /V (d) 2 mm

14 Sensors 217, 17, of D=25mm 1. D=3mm 1. U p /V U p /V U c /V (e) 25 mm U c /V (f) 3 mm Figure 18. Curve fitting of current t different distnces Moreover, linerity is tested when D hs the vlues of 1, 15, 2, 25, nd 3 mm. The dt re shown in Tble 5. Tble 5. Trnsformer fitting dt under vrious distnces. Dstnce/D Fitting Correction Coefficient Congruence Men Error Mximum Phse Difference φ ( ) 5 mm mm mm mm mm mm The test results show the following: (1) As shown in Figure 18, ech point is extremely close to the fitting line, which indictes good linerity of the designed trnsformer. (2) In Tble 5, the designed trnsformer mintins smll phse error t different distnces. However, s cn be seen in Tble 5, the first-order fitting correction fctor with the CT is grdully reduced. This finding mens tht when the distnce from the designed trnsformer to the wire becomes closer, the trnsformer linerity lso improves. However, s the distnce increses, the phse error tends to enlrge. Chnces re tht when the distnce is closer, the mgnetic induction line through the sensor is denser, nd then the impct of interference on the environment surrounding the sensor becomes reltively wek. When the distnce is fr, the sensor becomes more susceptible On-off Opertion Test On the test pltform, the trnsient performnce of the designed trnsformer is tested under the on-off stte. The sensor is fixed bove the wire, nd the reltive position remins constnt. Under different current levels, the power is switched off suddenly so s to cut off the current, then the oscilloscope is used to record the wveform response of voltge ttenution for both the designed trnsformer nd the CT.

15 Sensors 217, 17, of 17 Figure 19. Switching trnsient wveform (I = 3 A, D = 5 mm, f = 5 Hz). In Figure 19, the results show tht the output wveform of the trnsformer closely follows the chnge of current, nd its output voltge is reduced within certin period to 1% of the pek before the fult in very short period of time. Furthermore, the wveform no longer fluctutes. Therefore, the designed trnsformer cn effectively reflect nd trck the chnges in the current in the wire Frequency Test This experiment is done to verify the performnce of the trnsformer under different frequencies. According to the nlysis of the test dt of the linerity of the trnsformer, the smller distnce D hs smller phse error, so the selected distnce for testing is set to is 5 mm. The corresponding dt liner fitting is shown in Figure 2. U p /V 2 45Hz 5Hz 55Hz 6Hz 15 65Hz 7Hz 1Hz U c /V Figure 2. Voltge fitted curve under different frequencies In Figure 2, the output voltge of the designed trnsformer hs good linerity under different frequencies. This finding mens tht the trnsformer hs good frequency response under low frequency. Limited by the output power nd frequency of the power supply, there is no longer need to conduct more performnce tests of the designed trnsformer t higher current nd higher frequency. 5. Conclusion nd Future Prospects In this pper, by studying the reltionship between the current in the wire nd its surrounding mgnetic field, non-contct electronic current trnsformer bsed on the mgnetic field coupling principle is proposed. According to the experimentl results, compred with the trditionl CT, when the designed trnsformer is used to mesure the power frequency current, the results

16 Sensors 217, 17, of 17 demonstrte tht it hs very good stedy-stte nd trnsient chrcteristics s well s high ccurcy. At the sme time, the results show tht it hs excellent linerity when the mesuring distnce is chnging. In ddition, the current trnsformer hs good ccurcy nd smll phse error. In further studies, more performnce tests of the designed trnsformer shll be crried out t higher currents nd higher frequencies. To improve the nti-interference cpcity of the sensor, reserch on phse compenstion to the sensor will lso be done, long with n investigtion into the structurl optimiztion of the sensor. Acknowledgments: This work ws supported by nd the Ntionl Nturl Science Foundtions of Chin (Grnt No ). Author Contributions: Jingng Wng is the hed of the reserch group tht conducted this study, nd he proposed the improved structure of the sensor. Rn Ren nd Tin Tin conducted experimentl dt collection. Dincheng Si minly crried on the simultion of sensors, dt nlysis, the design of the hrdwre circuit, nd the min writing of the mnuscript. And ll uthors contributed to the writing nd revision of the mnuscript. Conflicts of Interest: The uthors declre no conflict of interest. References nd Notes 1. Wng, Z.; Zhng, X.; Wng, F.; Ln, X.; Zhou, Y. Effects of ging on the structurl, mechnicl, nd therml properties of the silicone rubber current trnsformer insultion bushing for 5 kv substtion. SpringerPlus 216, 5, Ymmoto, K.; Ued, N.; Ametni, A.; Ntsuno, D. A Study of Lightning Current Distribution t Wind Turbine Foot: Influence on Current Mesurements Using Rogowski Coil. Electr. Eng. Jpn. 212, 18, Metwlly, I.A. Tpe-wound Rogowski coil for mesuring lrge-mgnitude pulsed currents. Instrum. Exp. Tech. 216, 59, Ardil-Rey, J.A.; Albrrcín, R.; Álvrez, F.; Brrueto, A. A vlidtion of the spectrl power clustering technique (SPCT) by using Rogowski Coil in prtil dischrge mesurements. Sensors 215, 15, Metwlly, I.A. Self-Integrting Rogowski Coil for High-Impulse Current Mesurement. IEEE Trns. Instrum. Mes. 21, 59, Yutthgowith, P.; Pttndech, N.; Kunkorn, A.; Leeljindkrirerk, M. Design nd construction of Rogowski s coil with compensted RC integrtors for mesuring impulse current. In Proceedings ot the Interntionl Power Engineering Conference (IPEC 27), Singpore, 3 6 December 27; pp Liu, Y.; Lin, F.; Zhng, Q.; Zhong, H. Design nd construction of Rogowski coil for mesuring wide pulse current. IEEE Sens. J. 211, 11, Moonmirt, P.; Kunkorn, A.; Yutthgowith, P. A wide bndwidth Rogowski coil with n ctive integrtor for mesurement of impulse currents. In Proceedings of the Asi-Pcific Interntionl Conference on Lightning (APL 213), Seoul, Kore, June 213; pp Liu, Y.; Xie, X.; Hu, Y.; Qin, Y.; Sheng, G.; Jing, X. A novel trnsient fult current sensor bsed on the PCB Rogowski Coil for overhed trnsmission lines. Sensors 216, 16, Shfiq, M.; Kutt, L.; Lehtonen, M.; Nieminen, T.; Hshmi, M. Prmeters Identifiction nd Modeling of High-Frequency Current Trnsducer for Prtil Dischrge Mesurements. IEEE Sens. J. 213, 13, Moreno, M.V.R.; Robles, G.; Albrrcín, R.; Rey, J.A.; Trif, J.M.M. Study on the self-integrtion of Rogowski coil used in the mesurement of prtil dischrges pulses. Electr. Eng. 216, 1 1, doi:1.17/s Mrrcci, M.; Tellini, B. Anlysis of precision Rogowski coil vi nlyticl method nd effective cross section prmeter. In Proceedings of the 216 IEEE Interntionl Instrumenttion nd Mesurement Technology Conference (I2MTC), Tipei, Tiwn, My 216; pp Metwlly I A. Multi-lyer self-integrting Rogowski coils for high pulsed current mesurement. Instrum. Exp. Tech. 215, 58, Yo, C.; Xio, Q.; Mi, Y.; Yun, T.; Li, C.; Sim, W. Contctless Mesurement of Lightning Current Using Self-integrting B-dot Probe. IEEE Trns. Dielectr. Electr. Insul. 211, 18, Zhng, H.; Xi, L; Shen, Y.; Li, Q.; Wng, Y.; Zhng, L.; Liu, K. Anlysis nd Process of B-Dot Wveformsin High-Current Injector. IEEE Trns. Plsm Sci. 216, 44,

17 Sensors 217, 17, of Huiskmp, T.; Beckers, F.J.C.M.; vn Heesch, E.J.M.; Pemen, A.J.M. B-Dot nd D-Dot Sensors for (Sub)Nnosecond High-Voltge nd High-Current Pulse Mesurements. IEEE Sens. J. 216, 16, Ahmd, A.A.; Robert, A.S. Clibrtion of Electromgnetic Dot Sensor Prt 2: B-dot Mode. IEEE Sens. J. 214, 14, Hrdin, R. Mgnetic Field Genertion nd B-dot Sensor Chrcteriztion in the High Frequency Bnd. Mster s Thesis, Air Force Institute of Technology, Wright-Ptterson AFB, OH, USA, Mrch Wgoner, T.C.; Stygr, W.A.; Ives, H.C.; Gillilnd, T.L.; Spielmn, R.B.; Johnson, M.F.; Reynolds, P.G.; Moore, J.K.; Mourning, R.L.; Fehl, D.L.; et l. Differentil-output B-dot nd D-dot monitors for current nd voltge mesurements on 2-MA, 3-MV pulsed-power ccelertor. Phys. Rev. Spec. Top. Accel. Bems 28, 11, Lne, B.; Cmpbell, C.; Swd, I.; Ventzek, P.L.G. Mesurement of sptil nd temporl evolution of electromgnetic fields in 1 MHz plsm source using B-dot nd double dipole probes. J. Vc. Sci. Technol. A 216, 34, He, J.; Guo, R.; Mesurement of rc velocity in rotting-rc pulsed-power switch bsed on B-dot probes. IEEE Trns. Plsm Sci. 21, 38, To, T.; Zho, Z.; Pn, Q.; Tng, J.; Zhng, Y. Design of PCB Rogowski coil nd nlysis of nti-interference. Dingong Jishu Xuebo/Trns. Chin. Electrotech. Soc. 211, 26, Wng, R.; Prbhkrn, S.; Burdick, W.; Rymond, N. Rogowski current sensor design nd nlysis bsed on printed circuit bords (PCB) In Proceedings of the 214 IEEE Energy Conversion Congress nd Exposition (ECCE), Pittsburgh, PA, USA,14 18 September 214, pp Bschirotto, A.; Dllgo, E.; Mlcovti, P.; Mrchesi, M.; Venchi, G; Rossini, A. Multilyer PCB Plnr Fluxgte Mgnetic Sensor. In Proceedings of the 26 Ph.D. Reserch in Microelectronics nd Electronics (PRIME), Otrnto, Itly, June 26; pp Kubo, T.; Furukw, T.; Itoh, H.; Hiso, W.; Wkuy, H. Numericl electric field nlysis of power sttus sensor observing power distribution system tking into ccount mesurement circuit nd pprtus. In Proceedings of the SICE Annul Conference, Tokyo, Jpn, September 211; pp Koziy, K.; Bei, G.; Aslkson, J. A low-cost power qulity meter with series rc-fult detection cpbility for smrt grid. IEEE Trns. Power Deliv. 213, 28, GB , Current Trnsformers; Ntionl Bureu of Qulity Inspection, Beijing, Chin, IEC , Prt 8: Electronic Current Trnsformers; Interntionl Electrotechnicl Commission: Genev, Switzerlnd, by the uthors. Licensee MDPI, Bsel, Switzerlnd. This rticle is n open ccess rticle distributed under the terms nd conditions of the Cretive Commons Attribution (CC BY) license (