Wideband UHF Antenna for Partial Discharge Detection

applied sciences

Article Wideband UHF Antenna for Partial Discharge Detection

Zhen Cui 1, Seungyong Park 1, Hosung Choo 2 and Kyung-Young Jung 1,* 1 Department of Electronics Computer Engineering, Hanyang University, Seoul 04763, Korea; [email protected] (Z.C.); [email protected] (S.P.) 2 School of Electronic and Electrical Engineering, Hongik University, Seoul 04066, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-2-2220-2320

Received: 29 January 2020; Accepted: 27 February 2020; Published: 2 March 2020

Abstract: This paper presents a ultra-high frequency (UHF) antenna for partial discharge (PD) detection and the antenna sensor can be used near a conducting ground wire. The proposed UHF antenna has advantages of easy setup and higher-frequency detection over the high-frequency current transformer (HFCT) sensor. First, a variety of loop-shaped antennas are designed to compare each near field coupling capability. Then, a new UHF antenna is designed based on the loop-shaped antenna, which has the best near field coupling capability. Finally, the proposed UHF antenna is fabricated and measured. It provides a wide impedance bandwidth of 760 MHz (740–1500 MHz). Its simple setup configuration and wide bandwidth frequency response in the UHF band can provide a more efficient means for PD detection.

Keywords: partial discharge; ultra-high frequency (UHF) antenna

1. Introduction With the significant increase in electric power consumption, power equipment is currently being developed towards large capacity, ultra-high voltage, and unmanned control. If such large capacity power equipment fails to work, in general, it takes considerable time to repair the entire system and resume electric power supply. Among many factors that cause the failure in the power equipment, partial discharge (PD), which is a localized discharge in an electrical dielectric insulation system under high-voltage (HV) field, is the major cause of power equipment failure. Power plant accidents are closely related to the dielectric breakdown in HV power equipment, and thus there has been increasing demand for real-time PD diagnostics for HV power equipment [1,2]. There are several techniques for detecting the PD, including optical, acoustic, electrical, and electromagnetic (EM) wave detection methods [3]. Among them, the electromagnetic wave detection method is widely used due to its high detection sensitivity and robust anti-interference ability [4]. Therefore, high frequency (HF) and ultra-high frequency (UHF) sensors have popularly been studied in PD detection systems. PD events in HV power equipment typically produce very short current pulses with a rise time of less than 1 nanosecond, which can emit EM waves in the GHz frequency band [5]. When this PD occurs, the UHF sensor detects the EM waves emitted by the PD [6], so the UHF antenna is a core part of the UHF PD detection system, and the performance of the UHF antenna can directly affect the detection performance of the system [7–12]. There are two approaches to detect the PD in the EM wave detection method. One is to install a PD sensor on the surface of the metallic chassis of HV power equipment for directly detecting the PD inside HV power equipment. The other is to place the PD sensor near a conducting ground wire for detecting the PD current flowing through the wire. Figure1 shows a PD detection approach using a high-frequency current transformer (HFCT), which is one of the common techniques for detecting the PD currents

Appl. Sci. 2020, 10, 1698; doi:10.3390/app10051698 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, x FOR PEER REVIEW 2 of 10 onAppl.Appl. the Sci. Sci.conducting 20202020, 10, 10, x, FOR 1698 ground PEER REVIEW wire. When the HFCT is applied to the PD detection system of HV power2 of2 of10 10 equipment, the HFCT should completely surround the conducting ground wire with various shapes andon the sizes, conducting which increases ground thewire. complexity When the and HFCT inco isnvenience applied to of the the PD experiment detection [13].system of HV power equipment,on the conducting the HFCT ground should wire. completely When thesurround HFCT isthe applied conducting to the ground PD detection wire with system various of HV shapes power andequipment, sizes, which the increases HFCT should the complexity completely and surround inconvenience the conducting of the experiment ground wire [13]. with various shapes and sizes, which increases the complexity and inconvenience of the experiment [13].

Figure 1. Partial discharge (PD) detection mechanism using the high-frequency current transformer

Figure 1. Partial discharge (PD) detection mechanism(HFCT). using the high-frequency current transformer Figure 1. Partial discharge (PD) detection mechanism using the high-frequency current transformer (HFCT). In this work, a novel UHF antenna is designed(HFCT to). detect the PD in HV power equipment, which is placedIn thisnearby work, the a conducting novel UHF antennaground iswire, designed rather to than detect completely the PD in surrounding HV power equipment, the conducting which is In this work, a novel UHF antenna is designed to detect the PD in HV power equipment, which wireplaced as shown nearby the in the conducting conceptual ground diagram wire, rather of Figure than completely2. Herein, surroundingthe PD detection the conducting can be easily wire as is placed nearby the conducting ground wire, rather than completely surrounding the conducting implementedshown in the since conceptual it does diagramnot need of to Figure be clamped2. Herein, to the the conducting PD detection ground can be wire, easily different implemented from sincethe wire as shown in the conceptual diagram of Figure 2. Herein, the PD detection can be easily HFCT.it does In notaddition, need to the be proposed clamped toUHF the antenna conducting can grounddetect the wire, PD di signalsﬀerent in from the theconducting HFCT. In ground addition, implemented since it does not need to be clamped to the conducting ground wire, different from the wirethe in proposed the GHz UHFfrequency antenna band can that detect exceeds the the PD detection signals in range the conductingof the HFCT. ground wire in the GHz HFCT. In addition, the proposed UHF antenna can detect the PD signals in the conducting ground frequency band that exceeds the detection range of the HFCT. wire in the GHz frequency band that exceeds the detection range of the HFCT.

(a) (b)

FigureFigure 2. Conceptual 2. Conceptual (diagrama)diagram for for the the PD PD detection detection in inthe the conducting conducting ground ground( wire:b) wire: (a) (HFCTa) HFCT and and (b) ( b) proposedproposed ultra ultra-high-high frequency frequency (UHF (UHF)) antenna. antenna. Figure 2. Conceptual diagram for the PD detection in the conducting ground wire: (a) HFCT and (b) First,proposedFirst, various various ultra -high shapes shapes frequency of of loop loop ( antennasUHF antennas) antenna. are are examined examined to compare to compare their near their field near coupling field capabilities.coupling capabilities.Through the Through preliminary the preliminary parametric parametric study, a novel study, UHF a novel antenna UHF is proposedantenna is based proposed on the based basic on loop theantenna basicFirst, loop structure, various antenna shapes which structure ofhas loop, thewhich best antennas has near the field best are couplingexaminednear field capability. coupling to compare capability. The their proposed nearThe proposed field UHF couplingantenna UHF is capabilities.then fabricated, Through and the preliminary PD detection parametric performance study, is measured a novel UHF using antenna the experimental is proposed setup based that on can the basic loop antenna structure, which has the best near field coupling capability. The proposed UHF Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 10 antennaAppl. Sci. 2020 is then, 10, 1698 fabricated, and the PD detection performance is measured using the experimental3 of 10 setup that can imitate the conducting ground wire of the HV power equipment. Experimental results demonstrate that the proposed UHF antenna can effectively detect the PD currents. imitate the conducting ground wire of the HV power equipment. Experimental results demonstrate 2.that Basic the Loop proposed Antenna UHF antenna can effectively detect the PD currents. 2. BasicAs alluded Loop Antenna previously, we will design a loop-shaped antenna suitable for PD detection in the conducting ground wire. Toward this purpose, we considered various shaped loop antennas, As alluded previously, we will design a loop-shaped antenna suitable for PD detection in the including circle, square, rectangle, rhombic, and elliptical loop antennas [14–19]. Since we conducting ground wire. Toward this purpose, we considered various shaped loop antennas, including investigated the UHF antenna for PD detection from 500 –1500 MHz, various simple loop antennas circle, square, rectangle, rhombic, and elliptical loop antennas [14–19]. Since we investigated the were designed at 1 GHz, the center frequency of the operating band. In other words, the total UHF antenna for PD detection from 500 –1500 MHz, various simple loop antennas were designed circumference was approximately 300 mm, one wavelength at the frequency. In order to choose a at 1 GHz, the center frequency of the operating band. In other words, the total circumference was proper loop-shaped antenna, we investigated the detection capabilities of various antennas approximately 300 mm, one wavelength at the frequency. In order to choose a proper loop-shaped mentioned above. The current source was injected into the conducting ground wire, and the coupling antenna, we investigated the detection capabilities of various antennas mentioned above. The current performance of the loop antennas was calculated by the ratio of the received voltage (at the antenna source was injected into the conducting ground wire, and the coupling performance of the loop port) to the injected current (into the conducting ground wire). The distance between the injected antennas was calculated by the ratio of the received voltage (at the antenna port) to the injected current current source and the plane of the antenna is d, as shown in the simulation models in Figure 3. The (into the conducting ground wire). The distance between the injected current source and the plane voltages received at the loop antennas were calculated by integrating the electric field in the antenna of the antenna is d, as shown in the simulation models in Figure3. The voltages received at the loop port using the HFSS field calculator function [20] as follows: antennas were calculated by integrating the electric field in the antenna port using the HFSS field calculator function [20] as follows: VdZEl V = E dl where E is the electric field in the antenna port and dl ·indicates the differential displacement along a linewhere in theE is antenna the electric port. field As inshown the antenna in Figure port 4, rectangle and dl indicates #1 yields the the di ffbesterential performance. displacement along a line in the antenna port. As shown in Figure4, rectangle #1 yields the best performance.

(a) (b)

Figure 3. SimulationSimulation models models of of loop loop antennas: antennas: ( (aa)) t topop view view and and ( (bb)) s sideide view. view. Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 10

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Figure 4. Coupling performance of each antenna.

The spectral response of coupling performance for rectangle #1 was also investigated in Figure 5. As d increased, the coupling performance decreased, especially for shorter wavelengths. Note that FigureFigure 4. Coupling4. Coupling performance performance of eachof each antenna. antenna. the reduction of the received voltage was negligible as the distance increase since the distance difference was very small compared to the wavelength. The simulation results show that the TheThe spectral spectral response response of couplingof coupling performance performance for for rectangle rectangle #1 was#1 was also also investigated investigated in Figure in Figure5. rectangle #1 antenna could have a good ability to sense the near field, but the bandwidth of this basic As5.d Asincreased, d increase thed coupling, the coupling performance performance decreased, decrease especiallyd, especially for shorter for shorter wavelengths. wavelengths. Note thatNote the that loop antenna was narrow. In the next section, we focused on increasing the operating bandwidth of reductionthe reduction of the received of the received voltage was voltage negligible was negligible as the distance as the increase distance since increase the distance since the diﬀ erence distance the antenna. wasdifference very small was compared very small to the compared wavelength. to The the simulation wavelength. results The show simulation that the resultsrectangle show #1 antenna that the couldrectangle have a#1 good antenna ability could to sensehave a the good near ability ﬁeld, to but sense the the bandwidth near field, of but this the basic bandwidth loop antenna of this was basic narrow.loop antenna In the next was section, narrow. we In focusedthe next on section, increasing we focus the operatinged on increasing bandwidth the operating of the antenna. bandwidth of the antenna.

FigureFigure 5. 5.Coupling Coupling performance performance by by rectangle rectangle #1 #1 versus versus frequency. frequency.

Figure 5. Coupling performance by rectangle #1 versus frequency. Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 10 Appl. Sci. 2020, 10, 1698 5 of 10 Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 10 3. Wideband ultra-high frequency (UHF) Antenna 3. Wideband ultra-high frequency (UHF) Antenna 3. WidebandThe bandwidth Ultra-High of Frequency the basic loop (UHF) antenna Antenna is extremely narrow because of its resonant structure, asThe mentioned bandwidth in theof the previous basic loop section. antenna In this is work,extremely in order narrow to enhance because the of bandwidthits resonant of structure, the antenna, The bandwidth of the basic loop antenna is extremely narrow because of its resonant structure, as mentionedthe rectangle in the #1 previousloop antenna section. was In modified. this work, in order to enhance the bandwidth of the antenna, as mentioned in the previous section. In this work, in order to enhance the bandwidth of the antenna, the rectangleTo increase #1 loop theantenna bandwidth, was modified. first, we designed the antenna (modified antenna 1 in Figure 6b) on the rectangle #1 loop antenna was modified. theTo FR4increase (a relative the bandwidth, permittivity first, of we4.4, design a thicknessed the of antenna 1.6 mm, (modified and a loss antenna tangent 1 of in 0.02) Figure and 6b change) on d To increase the bandwidth, first, we designed the antenna (modified antenna 1 in Figure6b) on the FR4the position(a relative of permittivitythe feed line. of It is4.4, worth a thickness noting thatof 1.6 the mm, employment and a loss of tangent FR4 could of 0.02) lead and to size change reductiond the FR4 (a relative permittivity of 4.4, a thickness of 1.6 mm, and a loss tangent of 0.02) and changed the positionand the variationof the feed of line. the Itfeeding is worth point noting could that lead the to employment a change in ofthe FR4 return could loss. lead In tospecific, size reduction the antenna the position of the feed line. It is worth noting that the employment of FR4 could lead to size reduction and couldthe variation have two of the resonance feeding pointpoints could at 0.7 lead GHz to aand change 1.29 inGHz, the returnas shown loss. in In Figurespecific, 7a the. However, antenna the and the variation of the feeding point could lead to a change in the return loss. In specific, the antenna couldantenna have two matching resonance perfo pointsrmance at was 0.7 stillGHz not and good, 1.29 especiallyGHz, as shown at the centerin Figure frequency. 7a. However, Therefore, the the could have two resonance points at 0.7 GHz and 1.29 GHz, as shown in Figure7a. However, th antenna antennaantenna matching was further performance modified was by still adding not good, two stubs especially (modified at the antenna center frequency. 2 in Figure Therefore, 6c). This antennathe matching performance was still not good, especially at the center frequency. Therefore, the antenna antennacould was increase further the modified effective by electrical adding length two stubs and (modifiedimprove the antenna antenna 2 inimpedance Figure 6c matching). This antenna from 0.76 was further modified by adding two stubs (modified antenna 2 in Figure6c). This antenna could couldto increase 1.2 GHz. the However, effective S 11electrical was still length greater and than improve −10 dB thein the antenna range impedanceof 0.93–1.10 matching GHz. In order from to0.76 tackle increase the effective electrical length and improve the antenna impedance matching from 0.76 to to 1.2this GHz. issue, However, the strip S11 width was still of thegreater antenna than −was10 dB increased in the range to further of 0.93 improve–1.10 GHz. the In antenna order to impedance tackle 1.2 GHz. However, S11 was still greater than 10 dB in the range of 0.93–1.10 GHz. In order to tackle this matching.issue, the stripThe final width design of the of antenna the proposed was− increased antenna was to further performed improve by using the antennaHFSS optimization. impedance The this issue, the strip width of the antenna was increased to further improve the antenna impedance matching.S11 parameters The final anddesign the of smith the proposed charts of antennathe step was-by- stepperformed design by antennas using HFSS are shown optimization. in Figure The 7. As matching. The final design of the proposed antenna was performed by using HFSS optimization. 11 S11 parametersshown in the and figures, the smith S parameterscharts of the got step lower-by- stepand designloci were antennas located are further shown into in theFigure center 7. As of the The S parameters and the smith charts of the step-by-step design antennas are shown in Figure7. 11 11 shownsmith in thechart figures, as the Santenna11 parameters design got evolve lowerd. Noteand loci that were the final located antenna further could into have the Scenter < −1 of0 dB the from As shown in the figures, S11 parameters got lower and loci were located further into the center of the smith0.71 chart to 1.5 as theGHz. antenna design evolved. Note that the final antenna could have S11 < −10 dB from smith chart as the antenna design evolved. Note that the final antenna could have S11 < 10 dB from 0.71 to 1.5 GHz. − 0.71 to 1.5 GHz.

(a) (b) (c) (d) Figure(a) 6. The geometry of loop(b antennas:) (a) basic loop antenna;(c) (b) modified antenna( 1;d )( c) modified FigureFigureantenna 6. 6. TheThe 2;geometry geometryand (d) finalof of looploop antenna. antennas: antennas: (a ()a basic) basic loop loop antenna; antenna; (b (b) modified) modified antenna antenna 1; 1; (c ()c )modified modified antennaantenna 2; 2; and and (d (d) )final final antenna. antenna.

(a) (b)

FigureFigure 7.(aAntenna) 7. Antenna matching matching performance: performance: (a)S (11a) parameters.S11 parameters. (b()b ( Smithb) ) Smith chart. chart.

Figure 7. Antenna matching performance: (a) S11 parameters. (b) Smith chart. Appl. Sci. 2020, 10, 1698 6 of 10 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 10

The geometry of the ﬁnalfinal UHF antenna is shown in Figure 88 and and thethe designdesign parametersparameters ofof thethe proposed antenna are listed in Table1 1.. FigureFigure9 9shows shows that that the the real real part part of of the the impedance impedance was was near near 50 ohm and the the imaginary imaginary part part of of the the impedance impedance was was near near 0 0ohm ohm in in the the range range of of710 710–1500–1500 MHz. MHz. It Itindicates indicates that that the the antenna antenna ha hadd good good impedance impedance matching matching in inthe the frequency frequency range range of of interests. interests.

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The geometry of the final UHF antenna is shown in Figure 8 and the design parameters of the proposed antenna are listed in Table 1. Figure 9 shows that the real part of the impedance was near 50 ohm and the imaginary part of the impedance was near 0 ohm in the range of 710–1500 MHz. It indicates that the antenna had good impedance matching in the frequency range of interests.

Figure 8. The geometry of the proposed UHF antenna. Table 1. Design parameters of the proposed UHF antenna.

Parameters Value (mm) Parameters Value (mm) Lf 138 W1 45 Wf 89.5 W2 5.5 L1 103.5 W3 6 L2 40 W4 8.5 L3 5.5 W5 10.5 L4 6 W6 12.5 L5 26.5 W7 8.5

L6 0.5 W8 1 L7Figure 8. The geometry 5 of the proposed W9 UHF antenna. 3.5 L8 20.5

Figure 9. Input impedance of the proposed UHF antenna.

Table 1. Design parameters of the proposed UHF antenna.

Parameters Value (mm) Parameters Value (mm) Lf 138 W1 45 Wf 89.5 W2 5.5 L1 103.5 W3 6 L2 40 W4 8.5 L3 5.5 W5 10.5 L4 6 W6 12.5 L5 26.5 W7 8.5 FigureFigureL6 9. 9.Input Input impedance impedance0.5 of of the the proposedW8 proposed UHF UHF antenna.1 antenna . L7 5 W9 3.5 TableL8 1. Design parameters20.5 of the proposed UHF antenna.

Parameters Value (mm) Parameters Value (mm) Lf 138 W1 45 Wf 89.5 W2 5.5 L1 103.5 W3 6 L2 40 W4 8.5 L3 5.5 W5 10.5 L4 6 W6 12.5 L5 26.5 W7 8.5 L6 0.5 W8 1 L7 5 W9 3.5 L8 20.5 Appl.Appl. Sci. 2020 Sci., 201020, 1698, 10, x FOR PEER REVIEW 7 of 107 of 10

4. Fabrication and Measured Results 4. Fabrication and Measured Results The proposed UHF antenna was printed on a FR4 substrate with the dimensions of 89.5 mm × The proposed UHF antenna was printed on a FR4 substrate with the dimensions of 138 mm × 1.6 mm, and the photograph of the fabricated antenna is shown in Figure 10. The S11 of the 89.5 mm 138 mm 1.6 mm, and the photograph of the fabricated antenna is shown in Figure 10. antenna× was measured× in an anechoic chamber (W = 3 m, L = 6 m, and H = 3 m) using a vector network The S11 of the antenna was measured in an anechoic chamber (W = 3 m, L = 6 m, and H = 3 m) analyzer (Agilent Technologies E5071C). Figure 11 depicts the simulated and measured S11 using a vector network analyzer (Agilent Technologies E5071C). Figure 11 depicts the simulated and parameters of the proposed antenna. It is shown that S11 was less than −10 dB from 740 to 1500 MHz. measured S parameters of the proposed antenna. It is shown that S was less than 10 dB from 740 The measurement11 result agreed well with the simulation result.11 Slight discrepancies− between the to 1500 MHz. The measurement result agreed well with the simulation result. Slight discrepancies simulation and measurement could be caused by fabrication and measurement errors. between the simulation and measurement could be caused by fabrication and measurement errors.

(a) (b)

FigureFigure 10. The 10. prototypeThe prototype of the of proposedthe proposed UHF UHF antenna: antenna: (a) top (a) viewtop view and (andb) bottom (b) bottom view. view.

Figure 11. Simulation and measurement results (S11) of the proposed UHF antenna. Figure 11. Simulation and measurement results (S11) of the proposed UHF antenna. In order to estimate the PD detection ability, we set up a test system in our laboratory, as shown inIn Figure order to12. estimate We use thed a PDRG316 detection coaxial ability, cable weto imitate set up athe test conducting system in ourground laboratory, wire of as the shown HV power in Figureequipment. 12. We used Toward a RG316 this purpose, coaxial cable the protective to imitate dielectric the conducting layer and ground the metal wire of net the in HV the powermiddle of equipment.the coaxial Toward line were this purpose, peeled off the so protective that the EM dielectric waves layer could and be theemitted metal to net the in ou thetside. middle At the of the end of coaxialthe linecoaxial were cable, peeled we connect oﬀ so thated thea 50 EMohm waves termination could beto emittedimitate the to thewire outside. that is grounded At the end to ofthe the earth. coaxialThe cable, other we end connected of the coaxial a 50 ohm cable termination was connected to imitate to port the 1 wire of the that vector is grounded network to analyzer. the earth. The The otherfabricated end of antenna the coaxial was cable placed was below connected the to center port 1 of of the the coaxial vectornetwork cable, same analyzer. as Figure The fabricated 3. The SMA connector of the antenna was connected to port 2 of the vector network analyzer. The PD detection

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Appl.antenna Sci. 2020,was 10, x placedFOR PEER below REVIEW the center of the coaxial cable, same as Figure3. The SMA connector8 of of 10 the antenna was connected to port 2 of the vector network analyzer. The PD detection ability could be ability could be inferred by the S21 parameter. Note the test system employed in this work was not inferred by the S21 parameter. Note the test system employed in this work was not exactly same as exactlythe previoussame as the simulation previous in simulation Section2 becausein Section there 2 because were discontinuities there were discontinuities of the peeled of coaxialthe peeled cable. coaxialNonetheless, cable. Nonetheless, the test setup the in our test work setup could in our obtain work the could trends obtain of the coupling the trends performance of the coupling since the performance since the S21 parameter was proportional to the coupling coefficient of Section 2. S21 parameter was proportional to the coupling coeﬃcient of Section2.

FigureFigure 12. 12.PD PDexperimental experimental set- set-upup for the for theantenna antenna test. test.

We investigated the S21 parameters for the different distances (10 mm, 20 mm, and 30 mm) We investigated the S21 parameters for the different distances (10 mm, 20 mm, and 30 mm) between the peeled coaxial cable and the antenna. Figure 13 shows the actual measurement picture for between the peeled coaxial cable and the antenna. Figure 13 shows the actual measurement picture the distance of 10 mm. As shown in Figure 13, the 10 mm-thick Styrofoam was placed under the ends for the distance of 10 mm. As shown in Figure 13, the 10 mm-thick Styrofoam was placed under the of the coaxial cable and fixed with tape, which made the coaxial cable to be firmly placed 10 mm above ends of the coaxial cable and fixed with tape, which made the coaxial cable to be firmly placed 10 mm the antenna. Note that more Styrofoam was stacked for the tests with longer distances. As shown above the antenna. Note that more Styrofoam was stacked for the tests with longer distances. As in Figure 14, the S21 parameters lay between 10 and 40 dB in both simulation and measurement shown in Figure 14, the S21 parameters lay between− −1− 0 and −40 dB in both simulation and results. Discrepancies between the measurement results and simulation results might be caused by measurement results. Discrepancies between the measurement results and simulation results might fabrication and measurement errors including antenna misalignment, the non-perfectly straight peeled be caused by fabrication and measurement errors including antenna misalignment, the non-perfectly coaxial cable, etc. It should be noted that it would be useful to provide a exact methodology to extract straight peeled coaxial cable, etc. It should be noted that it would be useful to provide a exact the coupling coefficient of Section2 using the available measurement equipment. In future work, methodology to extract the coupling coefficient of Section 2 using the available measurement this extraction methodology should be addressed and studied. equipment. In future work, this extraction methodology should be addressed and studied.

Figure 13. Actual measurement picture. Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 10

ability could be inferred by the S21 parameter. Note the test system employed in this work was not exactly same as the previous simulation in Section 2 because there were discontinuities of the peeled coaxial cable. Nonetheless, the test setup in our work could obtain the trends of the coupling performance since the S21 parameter was proportional to the coupling coefficient of Section 2.

Figure 12. PD experimental set-up for the antenna test.

We investigated the S21 parameters for the different distances (10 mm, 20 mm, and 30 mm) between the peeled coaxial cable and the antenna. Figure 13 shows the actual measurement picture for the distance of 10 mm. As shown in Figure 13, the 10 mm-thick Styrofoam was placed under the ends of the coaxial cable and fixed with tape, which made the coaxial cable to be firmly placed 10 mm above the antenna. Note that more Styrofoam was stacked for the tests with longer distances. As shown in Figure 14, the S21 parameters lay between −10 and −40 dB in both simulation and measurement results. Discrepancies between the measurement results and simulation results might be caused by fabrication and measurement errors including antenna misalignment, the non-perfectly straight peeled coaxial cable, etc. It should be noted that it would be useful to provide a exact Appl.methodology Sci. 2020, 10 , to 1698 extract the coupling coefficient of Section 2 using the available measurement9 of 10 equipment. In future work, this extraction methodology should be addressed and studied.

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Figure 13.13. Actual measurement picture.

Figure 14. Simulation and measurement results (S21). Figure 14. Simulation and measurement results (S21). 5. Conclusions 5. Conclusions In this work, a wideband UHF antenna for PD detection was proposed. This antenna could provideIn a thiswide work, bandwidth a wideband of 760 MHz UHF(740–1500 antenna forMHz). PD The detection proposed was antenna proposed. was This an o ff antennaset structure could toprovide the conducting a wide ground bandwidth wires of so 760 that MHz it did (740not have–1500 to MHz). be clamped The proposed to the conducting antenna groundwas an wires offset tostructure detect PD. to Such the conducting a simple structure ground makes wires it so easy that to it carry did andnot usehave for to the be PDclamped detection to the system conducting of HV powerground equipment wires to detect nearby PD. conducting Such a simple ground structure wires. makes The proposed it easy to UHF carry antenna and use can for bethe used PD detection for PD detectionsystem of on HV conducting power equipment ground wires nearby in any conducting power equipment ground wires. systems The such proposed as electric UHF vehicles. antenna can be used for PD detection on conducting ground wires in any power equipment systems such as Authorelectric Contributions: vehicles. The presented work was carried out in collaboration of all authors. Z.C. performed the simulations. S.P., H.C. and K.-Y.J. participated to the conception, fabrication and experiment. Z.C. wrote the paper, which was edited by all co-authors. All authors have read and agreed to the published version of the manuscript. Author Contributions: The presented work was carried out in collaboration of all authors. Z.C. performed the Funding:simulations.This researchS.P., H.C. was and funded K.-Y.J. by participated Basic Science to Research the conception, Program fabrication through the and National experiment. Research Z.C. Foundation wrote the of Korea (NRF) funded by the Ministry of Education (No. 2015R1A6A1A03031833 and No. 2017R1D1A1B03034537). paper, which was edited by all co-authors. All authors have read and agreed to the published version of the Conflictsmanuscript. of Interest: The authors declare no conflict of interest.

Funding: This research was funded by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2015R1A6A1A03031833 and No. 2017R1D1A1B03034537).

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Wang, M.; Vandermaar, A.J.; Srivastava, K.D. Review of condition assessment of power transformers in service. IEEE Electr. Insul. Mag. 2002, 18, 12–25. 2. Ha, S.-G.; Cho, J.; Lee, J.; Min, B.-W.; Choi, J.; Jung, K.-Y. Numerical study of estimating the arrival time of UHF signals for partial discharge localization in a power transformer. J. Electromagn. Eng. Sci. 2018, 18, 94–100. 3. Soomro, I.A.; Ramdon, M.N. Study on different techniques of partial discharge (PD) detection in power transformers winding: Simulation between paper and EPOXY resin using UHF method. Int. J. Concept. Electr. Electron. Eng. 2014, 2, 57–61. 4. Judd, M.D.; Yang, L.; Hunter, I.B.B. Partial discharge monitoring of power transformers using UHF sensors part 1: Sensors and signal interpretation. IEEE Electr. Insul. Mag. 2005, 21, 5–14. 5. Chai, H.; Phung, B.T.; Zhang, D. Development of UHF sensors for partial discharge detection in power transformer. In Proceedings of the 2018 Condition Monitoring and Diagnosis (CMD), Perth, Australia, 1–5 September 2018. 6. Yang, F.; Peng, C.; Yang, Q.; Luo, H.; Ullah, I.; Yang, Y. An UWB printed antenna for partial discharge UHF detection in high voltage switchgears. Prog. Electromagn. Res. C 2016, 69, 105–114. 7. Sinaga, H.H.; Phung, B.T.; Blackburn, T.R. Partial discharge localization in transformers using UHF detection method. IEEE Trans. Dielectr. Electr. Insul. 2012, 19, 1891–1900. Appl. Sci. 2020, 10, 1698 10 of 10

References

1. Wang, M.; Vandermaar, A.J.; Srivastava, K.D. Review of condition assessment of power transformers in service. IEEE Electr. Insul. Mag. 2002, 18, 12–25. [CrossRef] 2. Ha, S.-G.; Cho, J.; Lee, J.; Min, B.-W.; Choi, J.; Jung, K.-Y. Numerical study of estimating the arrival time of UHF signals for partial discharge localization in a power transformer. J. Electromagn. Eng. Sci. 2018, 18, 94–100. [CrossRef] 3. Soomro, I.A.; Ramdon, M.N. Study on different techniques of partial discharge (PD) detection in power transformers winding: Simulation between paper and EPOXY resin using UHF method. Int. J. Concept. Electr. Electron. Eng. 2014, 2, 57–61. 4. Judd, M.D.; Yang, L.; Hunter, I.B.B. Partial discharge monitoring of power transformers using UHF sensors part 1: Sensors and signal interpretation. IEEE Electr. Insul. Mag. 2005, 21, 5–14. [CrossRef] 5. Chai, H.; Phung, B.T.; Zhang, D. Development of UHF sensors for partial discharge detection in power transformer. In Proceedings of the 2018 Condition Monitoring and Diagnosis (CMD), Perth, Australia, 1–5 September 2018. 6. Yang, F.; Peng, C.; Yang, Q.; Luo, H.; Ullah, I.; Yang, Y. An UWB printed antenna for partial discharge UHF detection in high voltage switchgears. Prog. Electromagn. Res. C 2016, 69, 105–114. [CrossRef] 7. Sinaga, H.H.; Phung, B.T.; Blackburn, T.R. Partial discharge localization in transformers using UHF detection method. IEEE Trans. Dielectr. Electr. Insul. 2012, 19, 1891–1900. [CrossRef] 8. Jahangir, H.; Akbari, A.; Werle, P.; Akbari, M.; Szczechowski, J. UHF characteristics of different types of PD sources in power transformers. In Proceedings of the Iranian Conference on Electrical Engineering (ICEE), Tehran, Iran, 2–4 May 2017; pp. 1242–1247. 9. Rozi, F.; Khayam, U. Development of loop antennas for partial discharge detection. Int. J. Electr. Eng. Inform. 2015, 7, 29–41. [CrossRef] 10. Li, J.; Jiang, T.; Wang, C.; Cheng, C. Optimization of UHF Hilbert Antenna for Partial discharge detection of transformers. IEEE Trans. Antennas Propag. 2012, 60, 2536–2540. 11. Muslim, J.; Khayam, S.U.; Hikita, M. Enhanced bowtie UHF antenna for detecting partial discharge in gas insulated substation. In Proceedings of the 48th International Universities’ Power Engineering Conference (UPEC), Dublin, Ireland, 2–5 September 2013. 12. Khosronejad, M.; Gentili, G.G. Design of an Archimedean spiral UHF antenna for pulse monitoring application. In Proceedings of the 2015 Loughborough Antennas & Propagation Conference (LAPC), Loughborough, UK, 2–3 November 2015. 13. Alvarez, F.; Garnacho, F.; Ortego, J.; Sanchez-Uran, M.A. Application of HFCT and UHF sensors in on-line partial discharge measurements for insulation diagnosis of high voltage equipment. Sensors 2015, 15, 7360–7387. [CrossRef][PubMed] 14. Balanis, C.A. Antenna Theory: Analysis and Design, 3rd ed.; John Wiley & Sons, Inc: New Jersey, NJ, USA, 2005. 15. Hur, J.; Byun, G.; Choo, H. Design of small CRPA arrays with circular microstrip loops for electromagnetically coupled feed. J. Electromagn. Eng. Sci. 2018, 18, 129–135. [CrossRef] 16. O, Y.; Jin, Y.; Choi, J. A compact four-port coplanar antenna based on an excitation switching reconfigurable mechanism for cognitive radio applications. Appl. Sci. 2019, 9, 3157. [CrossRef] 17. Samanta, G.; Mitra, D. Dual-band circular polarized flexible implantable antenna using reactive impedance substrate. IEEE Trans. Antennas Propag. 2019, 67, 4218–4223. [CrossRef] 18. Lee, H.; Zahid, Z.; Kim, H. Loop-Type ground radiation antenna for a C-Shaped ground plane. J. Electromagn. Eng. Sci. 2019, 19, 1–5. [CrossRef] 19. Sung, Y. Dual-band circularly polarized stack-ring antenna. J. Electromagn. Eng. Sci. 2019, 19, 37–41. [CrossRef] 20. Available online: http://www.ansys.com/products/electronics/ansys-hfss (accessed on 30 July 2019).

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