Pulse Current of Multi-Needle Negative Corona Discharge and Its Electromagnetic Radiation Characteristics

energies

Article Pulse Current of Multi-Needle Negative Corona Discharge and Its Electromagnetic Radiation Characteristics

Chuang Wang 1, Xi Chen 1,*, Jiting Ouyang 1, Tie Li 2 and Jialu Fu 1

1 State Key Laboratory of Mechatronics Engineering and Control, Beijing Institute of Technology, Beijing 100081, China; [email protected] (C.W.); [email protected] (J.O.); [email protected] (J.F.) 2 Xi’an Institute of Electromechanical Information Technology, Xi’an 710065, China; [email protected] * Correspondence: [email protected]; Tel.: +86-010-6891-8017

Received: 20 October 2018; Accepted: 6 November 2018; Published: 12 November 2018

Abstract: Negative corona discharge occurs widely in high voltage transmission lines and other “high voltage” uses, which can cause strong electromagnetic interference (EMI). In this research, the pulse current of multi-needle negative corona discharge and its electromagnetic (EM) radiation characteristics were studied and compared with that of single-needle negative corona discharge. A dipole radiation model was established to analyze the EM radiation characteristics of the negative corona discharge. The results show that the Trichel pulse discharge process of one discharge needle in multi-needle discharge structure will inhibit the discharge of the other discharge needles. It is only when the voltage reaches a certain threshold will the current and EM radiation ﬁelds of multi-needle discharge structure with a signiﬁcant increasing of amolitude. The frequency of EM radiation of negative corona discharge is not affected by the number of needles, but is only related to ambient air pressure. This research provides a basis for detecting corona discharge sources in different conditions.

Keywords: corona discharge; Trichel pulse; multi-needle; EM radiation

1. Introduction Corona discharge, which is a relatively low-energy discharge process, occurs only in a strong electric field near a electrode with small radius of curvature [1] is a common discharge phenomenon in plasma generator [2], ozone generator [3], electrostatic precipitator [4], electrostatic printing, electrostatic charge of aerosol particles [5], high voltage transmission lines [6], and charged aircraft [7]. Corona discharge usually performs simultaneously in a multi-needle (multi-point) format [8–10]. Negative corona discharge currents exhibit a regular pulse form, called Trichel pulse [11], which can cause strong electromagnetic (EM) interference. The EM radiation characteristics of multi-point and single-point corona discharge are significantly different. On the one hand, multi-needle negative corona discharge can cause the loss of power high voltage equipment, radio interference and television interference [12]. On the other hand, multi-needle negative corona discharge EM radiation signals can be used as target signals for high-voltage transmission line detection and high-altitude aircraft target recognition [13]. Therefore, it is of great significance to study the pulse current of multi-needle negative corona discharge and its EM radiation characteristics. Reid A.J. [14], Mutakamihigashi T. [15], and Nugraha F.A. [16] studied the EM radiation characteristics that were produced by partial discharge (PD). IEC 62478 standard [17] and Tungkmawanich A. [18] proposed methods for PD detection using EM radiation signals of PD. However, their research focuses more on strong discharge, such as spark discharge, but less on EM radiation characteristics of corona discharge. Yasuyuki T. et al. [19] experimentally studied the

Energies 2018, 11, 3120; doi:10.3390/en11113120 www.mdpi.com/journal/energies Energies 2018, 11, 3120 2 of 16 spectrum of the negative corona discharge radiation signal under the needle-plate discharge structure and compared it with the spark discharge EM radiation signal. Although they obtained the spectrum of the negative corona radiation signal, they did not analyze the mechanism of the radiation signal. He W. [20] studied the characteristics of alternating current corona discharge pulses and its radio interference level in a coaxial wire-cylinder gap. They focused on the radio interference current at a frequency of 0.5 MHz. Zhang Y. [21] studied the relationship between the spectrum of the single-needle negative corona radiation signal and the discharge parameters. He believed that the spectrum of the discharge EM radiation signal is only related to the rising time of the Trichel pulse current. But, the study did not analyze the discharge EM radiation process. The above studies did not pay attention to the EM radiation characteristics of multi-needle DC negative corona discharge. At present, researches on multi-needle (multi-point) discharge mainly focus on the volt-ampere characteristics of discharge and the interaction analysis of discharge process [9,22]. In addition, Albarracin R. [23] studied the EM radiation characteristics of multi-needle corona discharge under AC voltage. His research provides a good basis for the study of EM radiation characteristics of multi-needle corona discharge. However, further research is needed on the EM radiation characteristics of multi-needle negative corona discharge. The pulse current characteristics of the multi-needle negative corona discharge should be studied, and the corresponding relationship between EM radiation signal and pulse current should be analyzed. The effects of multi-needle discharge structure on pulse current and EM radiation characteristics of negative corona discharge must be further studied. In this paper, we focus on the current characteristics of multi-needle negative corona discharge and its EM radiation characteristics. A test system for negative corona discharge radiation characteristics was set up. The effects of discharge parameters on the single-needle negative corona discharge Trichel pulse current and its EM radiation ﬁeld were tested. The current superposition of multi-needle negative corona discharge and the interference of the EM radiation ﬁeld were studied. The EM radiation model of dipole negative corona discharge was established, and the relationship between the EM radiation characteristics of negative corona discharge and the parameters of the discharge system was analyzed. Effects of multi-needle discharge structure on the EM radiation characteristics of negative corona discharge were studied. This study can provide promising methods for the evaluation of insulation condition of electric apparatus as well as the detection of the discharge sources.

2. Experimental Setup The experimental system is shown in Figure1a, which consists of a high voltage generating module, a discharging module, a signal detection module, and a shielding module. The high voltage generating module includes the high voltage power supply and the high voltage capacitor. During the experiment, the high voltage capacitor is charged at ﬁrst with the high voltage power supply. The high voltage power supply is turned off after the voltage reaches a predetermined value. The high voltage capacitor is used to supply the discharge needle to eliminate the EM radiation interference that was generated by the high voltage power supply. The capacitance is 20 µF and the withstand voltage is 20 kV. The discharging module is designed according to IEC 60270 standard [24], which includes a corona discharge object and a vacuum box, as shown in Figure1b,c. Discharge experiments of discharge needles of different tip radius can be carried out through the change of the discharge needle on the discharge object, and multi-needle simultaneous discharge experiments can also be performed. The displacement platform on the corona discharge object can be used to precisely control the gap of the electrodes. The vacuum box can control the air pressure of the discharge environment in the range of 0.01 MPa to 0.1 MPa. The grounding plate is grounded through a 1 kΩ sampling resistor. Energies 2018, 11, 3120 3 of 16 Energies 2018, 11, x FOR PEER REVIEW 3 of 16

(a)

(b) (c) (d)

FigureFigure 1.1. Experimental set-up: (a) Schematic of the experimental set-up; ( b) Corona discharge object; ((cc)) VacuumVacuum box; box; and, and, (d ()d Photograph) Photograph of theof the experimental experimental set-up set-up and and background background electromagnetic electromagnetic (EM) noise(EM) ofnoise microwave of microwave anechoic anechoic chamber. chamber.

TheThe signalsignal detection detection module module includes includes an oscilloscope an oscilloscope (Tektronix (Tektronix MDO3104, MDO3104, Tektronix, Tektronix, Tektronix, Beaverton,Tektronix, Beaverton, OR, USA), OR, a spectrum USA), a spectrum analyzer analyzer (Agilent (Agilent E4447A, E4447A, Agilent, Agilent, Santa Clara, Santa CA,Clara, USA), CA, andUSA), a receivingand a receivingantenna (Disconeantenna Antenna (Discone OX-08-02, Antenna 20 OX-08-02, MHz to 1000 20 MHz.MHz Kelixun, to 1000 Chengdu, MHz. Kelixun, China). TheChengdu, antenna China). has a The S11 parameterantenna has lower a S11 than parameter 2 in the lower 20 MHz than to 2 1000in the MHz 20 MHz band. to The1000 oscilloscope MHz band. measuresThe oscilloscope the voltage measures waveform the voltage on the samplingwaveform resistor on the to sampling calculate resistor the discharge to calculate current. the The discharge corona dischargecurrent. The EM corona radiation discharge signal EM is received radiation by signal the wide-band is received diskcone by the wide-band antenna, and diskcone the distance antenna, d betweenand the thedistance antenna d between and the dischargethe antenna device and can the be discharge adjusted. device The signal, can be received adjusted. by theThe antenna, signal, isreceived collected by by the the antenna, oscilloscope is collected after being by analyzedthe oscilloscope by the spectrumafter being analyzer.In analyzed order by the to eliminatespectrum theanalyzer.In inﬂuence order of external to eliminate electromagnetic the influence interference of external (EMI) electromagnetic on the test results,interference all experiments (EMI) on the were test proceededresults, all inexperiments a microwave were anechoic proceede chamber.d in a microwave anechoic chamber.

Energies 2018, 11, x FOR PEER REVIEW 4 of 16

EnergiesEnergies2018 2018, 11, 11 3120, x FOR PEER REVIEW 4 of4 16of 16 3. Experimental Results and Analysis 3. Experimental Results and Analysis 3.3.1. Experimental Single-Needle Results Negative and Corona Analysis Discharge Trichel Pulse Current and Its Radiation Characteristics 3.1. SingleThe initial-Needle stage Negative of negative Corona corona Discharge discharge Trichel is Pulsethe Townsend Current and discharge Its Radiation stage, Characteristiwhere the dischargecs 3.1. Single-Needle Negative Corona Discharge Trichel Pulse Current and Its Radiation Characteristics current is usually within 1 μA and is relatively stable. As the applied voltage increases, the discharge The initial stage of negative corona discharge is the Townsend discharge stage, where the discharge entersThe initialthe Trichel stage of pulse negative stage. corona Figure discharge 2 shows is the the Townsend Trichel pulse discharge current stage, and where its radiated the discharge signal current is usually within 1 μA and is relatively stable. As the applied voltage increases, the discharge currentwaveform. is usually Trichel within pulses 1 µ Ahave and a is short relatively rising stable. time, Aswhich the appliedis typically voltage tens increases,of ns. The the falling discharge time is enters the Trichel pulse stage. Figure 2 shows the Trichel pulse current and its radiated signal enterstypically the Trichel a few pulsehundred stage. ns, Figure much2 longer shows than the Trichel the rising pulse time. current Trichel and itspulses radiated have signal a stable waveform. repetition waveform. Trichel pulses have a short rising time, which is typically tens of ns. The falling time is Trichelrate. In pulses the Trichel have apulse short stage, rising EM time, waves which will is be typically radiated tens to the of ns.surroundings. The falling The time waveform is typically of athe typically a few hundred ns, much longer than the rising time. Trichel pulses have a stable repetition fewdischarge hundred EM ns, radiation much longer signal than that the was rising received time. Trichel by the pulses antenna have corresponds a stable repetition to the Trichel rate. In pulse the rate. In the Trichel pulse stage, EM waves will be radiated to the surroundings. The waveform of the current waveform with the same repetition frequence. Therefore, it is believed that the discharge Tricheldischarge pulse stage,EM radiation EM waves signal will bethat radiated was received to the surroundings. by the antenna The corresponds waveform of to the the discharge Trichel EMpulse radiationradiation signal signal that is wasgenerated received by bythe the Trichel antenna pulse corresponds process. The to thefrequency Trichel spectrum pulse current of the waveform discharge current waveform with the same repetition frequence. Therefore, it is believed that the discharge EM radiation signal under atmospheric pressure is shown in Figure 3. The figure indicates that the withradiation the same signal repetition is generated frequence. by the Therefore, Trichel pulse it is process. believed The that frequency the discharge spectrum radiation of the signal discharge is generatedfrequency by of the the Trichel single pulse-needle process. discharge The radiation frequency signal spectrum is mainly of the within discharge 200 EMMHz. radiation There is signal a large EM radiation signal under atmospheric pressure is shown in Figure 3. The figure indicates that the energy distribution near 70 MHz (the fundamental frequency of the EM radiation signal), and also a underfrequency atmospheric of the sing pressurele-needle is shown discharge in Figureradiation3. Thesignal ﬁgure is mainly indicates within that 200 the MHz. frequency There ofis a the large certain energy distribution near 132 MHz. (second-harmonic generation of the EM radiation signal). single-needleenergy distribution discharge near radiation 70 MHz signal (the is fundamental mainly within frequency 200 MHz. of Therethe EM is aradiation large energy signal), distribution and also a nearcertain 70 MHz energy (the distribution fundamental near frequency132 MHz. (second of the- EMharmonic radiation generation signal), of andthe EM also radiation a certain signal). energy distribution near 132 MHz. (second-harmonic generation of the EM radiation signal).

Figure 2. Trichel pulse current and its EM radiation signal. Figure 2. Trichel pulse current and its EM radiation signal. Figure 2. Trichel pulse current and its EM radiation signal.

FigureFigure 3. 3.The The spectrum spectrum of of negative negative corona corona discharge discharge EM EM radiation radiation signal. signal.

Figure 3. The spectrum of negative corona discharge EM radiation signal.

EnergiesEnergies2018 2018, 11, ,11 3120, x FOR PEER REVIEW 55 of of 16 16 Energies 2018, 11, x FOR PEER REVIEW 5 of 16 The Trichel pulse that is generated by negative corona discharge is related to factors such as The Trichel pulse that is generated by negative corona discharge is related to factors such as appliedThe voltage,Trichel pulseair pressure, that is andgenerated tip radius by ofnegative the discharge corona needle discharge, etc. Figure is related 4 shows to factors the relatio suchnship as applied voltage, air pressure, and tip radius of the discharge needle, etc. Figure4 shows the relationship appliedbetween voltage, the repetition air pressure, frequency and tip of radiusthe Trichel of the pulse discharge and the needle applied, etc voltage.. Figure 4The shows repetition the relatio frequencynship between the repetition frequency of the Trichel pulse and the applied voltage. The repetition frequency betweenof Trichel the pulserepetition increases frequency with of the the raise Trichel of pulseapplied and voltage the applied, while voltage. the amplitude The repetition of a single frequency pulse of Trichel pulse increases with the raise of applied voltage, while the amplitude of a single pulse ofdecreases Trichel pulse slightly. increases This iswith consistent the raise with of applied the research voltage results, while of the Leob amplitude [25]. The of spectrum a single pulse of the decreases slightly. This is consistent with the research results of Leob [25]. The spectrum of the decreasesdischarge slightly. radiation This signal is consistentat different withpulse th repetitione research frequencies results of is Leob shown [25] in. TheFigure spectrum 5. The intensity of the discharge radiation signal at different pulse repetition frequencies is shown in Figure5. The intensity dischargeof the discharge radiation EM signal radiation at different signal pulse decreases repetition slightly, frequencies the frequency is shown of in theFigure EM 5. radiation The intensity signal of the discharge EM radiation signal decreases slightly, the frequency of the EM radiation signal does ofdoes the not discharge change, EM the radiationspectral lines signal become decreases denser, slightly, and the the average frequency power of of the the EM discharge radiation radiation signal not change, the spectral lines become denser, and the average power of the discharge radiation signal doessignal not increases change, theas the spectral increase lines of become the repetiti denser,on frequency and the average of Trichel power pulse of currentthe discharge that is radiationgenerated increases as the increase of the repetition frequency of Trichel pulse current that is generated by the signalby the increases negative as corona the increase discharge. of the It repetitialso indicateson frequency that the of changesTrichel pulsein gap current distance that of is electrode generated do negative corona discharge. It also indicates that the changes in gap distance of electrode do not affect bynot the affect negative the intensity corona discharge.and frequency It also of theindicates radiation that signal. the changes in gap distance of electrode do thenot intensityaffect the and intensity frequency and offrequency the radiation of the signal. radiation signal.

Figure 4. The relationship between Trichel pulse repetition frequency and applied voltage. FigureFigure 4.4. TheThe relationshiprelationship betweenbetween TrichelTrichel pulsepulse repetitionrepetition frequencyfrequency and and applied applied voltage. voltage.

FigureFigure 5. 5.Frequency Frequency spectrum spectrum of of discharge discharge radiation radiation signal signal at at different different pulse pulse repetition repetition frequencies. frequencies. Figure 5. Frequency spectrum of discharge radiation signal at different pulse repetition frequencies. As shown in Table1, when the air pressure is reduced from 0.1 MPa to 0.04 MPa, the rising time As shown in Table 1, when the air pressure is reduced from 0.1 MPa to 0.04 MPa, the rising time of the Trichel pulse increases from 52 ns to 265 ns, while the pulse amplitude remains unchanged. of theAs Trichelshown inpulse Table increases 1, when from the air 52 pressure ns to 265 is ns,reduced while from the pulse 0.1 MPa amplitude to 0.04 MPa, remains the risingunchanged. time ofStudies the Trichel show pulse that theincreases discharge from EM 52 nsradiation to 265 ns,signal while spectrum the pulse is onlyamplitude related remains to the rising unchanged. time of Studies show that the discharge EM radiation signal spectrum is only related to the rising time of

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Studies show that the discharge EM radiation signal spectrum is only related to the rising time of Trichel pulse [19]. The main frequency position of the EM radiation signal that is generated by the negative corona discharge shifts from 70 MHz to 45 MHz, while the power of the discharge EM radiation signal decreases from 1.926 nW to 0.218 nW.

Table 1. Negative corona discharge Trichel pulse current and its radiation characteristics under different tip radius and different air pressures.

Tip Radius Air Pressure Pules Amplitude Rising Time Power of the Radiation Frequency of Radiation (µm) (MPa) (µA) (ns) Signal (nW) Signal (MHz) 94 0.04 241 264 0.218 4584 94 0.06 245 118 0.697 52,102 94 0.08 243 78 1.0524 65,125 94 0.1 240 52 1.926 70,132 120 0.1 290 52 2.395 70,132 165 0.1 410 53 4.796 70,132 210 0.1 550 54 8.581 70,132

As the tip radius of the discharge needle increases from 62 µm to 210 µm, the amplitude of the Trichel pulse increases from 240 µA to 550 µA, while the rising and falling time of the pulse remains unchanged. This result is consistent with that of Lama [26] and Dordizadeh [27]. The frequency of the negative corona discharge EM radiation signal remains unchanged and the power of the radiation signal increases from 1.926 nW to 8.581 nW. Therefore, the frequency of the single-needle negative corona discharge EM radiation signal will be determined by the air pressure. The intensity of the EM radiation signal of the negative corona discharge is affected by the air pressure and the tip radius of cathode. The applied voltage and the gap distance of the electrode have no effect on the EM radiation signal frequency, but affect the intensity and the average power of the discharge radiation signal.

3.2. Multi-Needle Negative Corona Discharge Characteristics In practical applications, corona discharge often has a multi-needle (multi-point) structure. The waveform of a multi-needle discharge is significantly different from single-needle discharge. We firstly measured the double-needle discharge Trichel pulse current. To distinguish the Trichel pulse currents that were generated by different discharge needles, we chose two discharge needles with tip radius of 168 µm and 94 µm, respectively, for experiments (the larger the tip radius is, the larger the amplitude of the Trichel pulse current will be). The discharge condition is: applied voltage 7.4 kV, gap distance 10 mm, ambient pressure 0.1 MHz, and relative humidity 42%. The discharge current is shown in Figure6a. It can be clearly indicates that the amplitude of the Trichel pulse current that is generated by the discharge needle with a smaller radius is about 270 µA, and the pulse interval time is 1.32 µs., while the large radius are respectively 440 µA and 6.08 µs. Under this voltage, the Trichel pulse currents that were generated by the two discharge needles appear separately. Through further increasing the applied voltage, the discharge current waveform is shown in Figure6b. It indicates that as the applied voltage increases, the amplitude of the Trichel pulse current that is generated by the smaller radius tip decreases from about 270 µA to about 245 µA, and the pulse interval time decreases to 0.69 µs. At the same time, when the applied voltage rises to a certain value, the Trichel pulses that were generated by the discharge needle with larger tip radius and smaller tip radius superimpose with each other. The maximum amplitude of the superimposed pulse current can reach 535 µA and the minimum 490 µA, which is significantly larger than the unsuperimposed pulse. We also measured the discharge current of the multi-needle discharge structure. The test result is similar to the double-needle discharge. As the applied voltage increase, the smaller tip radius needle begins to discharge first. The other needles begin to discharge with further increases of applied voltage. The Trichel pulse current between the needles occurs separately in a certain voltage range. However, as the applied Energies 2018, 11, 3120 7 of 16

Energies 2018, 11, x FOR PEER REVIEW 7 of 16 voltage continues to increase, the Trichel pulse current generated by the different discharge needles willEnergies be superimposed, 2018, 11, x FOR PEER and REVIEW the amplitude of the superimposed pulse will increase signiﬁcantly. 7 of 16

(a) (b)

Figure 6. Negative corona discharge current waveform under double-needle discharge structure: (a) Applied voltage of(a )7.4 kV; (b) Applied voltage of 8.7 kV. (b) FigureFigure 6. 6.Negative Negative coronacorona dischargedischarge current waveform under under double double-needle-needle discharge discharge structure: structure: (a) (aApplied) AppliedWhen voltage voltage the appliedof of 7.4 7.4 kV; kV; voltage (b (b) )Applied Applied is lower, voltage voltage the of of 8.7 Trichel 8.7 kV kV.. pulse processes of the two needles will not happen at the same time. The EM radiation signal, which corresponds to the Trichel pulse current, is WhendetermWhen theined the applied by applied the voltage Trichel voltage ispulse lower, is current lower, the Trichel generated the Trichel pulse by processes pulse each needle. processes of the When two of needles the applied two will needlesnot voltage happen will rises atnot to an thehappen sameappropriate time.at the The same value, EM time. radiationthe TrichelThe EM signal, pulse radiation whichprocesses signal corresponds of, thewhich two tocorresponds needles the Trichel will tooccur pulse the simultaneously.Trichel current, pulse is determined current, The Trichel is bydeterm thepulse Trichelined current by pulse the waveform currentTrichel generatedpulse and current the byEM eachgenerated field needle. signal by When generateach needle. theed applied during Whenvoltage the the simultaneous applied rises to voltage an appropriatedischarge rises to ofan the value,appropriatetwo the needles Trichel value, are pulse the shown Trichel processes in pulseFigure of processes the7. The two pulse needlesof thes pointedtwo will needles occur by arrow will simultaneously. occur 1 and simultaneously. arrow The2 are Trichel the The Trichel pulseTrichel pulse currentpulsegenerated current waveform waveformby two and different the and EM the field discharge EM signal field needles generatedsignal generatrespectively, duringed during the and simultaneous the largersimultaneous pulse discharge pointed discharge of by the the of two thearrow needlestwo0 needles is arethe shownsup areerimposed shown in Figure in Figpulse7.ure The that7. pulsesThe is generatedpulse pointeds pointed by by the arrow by simultaneous arrow 1 and 1 and arrow arrow discharge 2 are 2 are the of the Trichelthe Trichel two pulse discharge pulse generatedgeneratedneedles. by by twoIt two can different differentbe seen discharge thatdischarge the needlesdischarge needles respectively, respectively,EM radiation and and signal the the larger largergenerated pulse pulse pointed by pointed the bynon theby-superimposed the arrow arrow 0 is0 the isTrichel superimposedthe sup pulseerimposed current pulse pulse that has isthatamplitude generated is generated of by about the by simultaneous 4.5the mV, simultaneous and discharge for the discharge superimpo of the two of dischargesedthe pulsetwo discharge needles.current, the Itneedles. canamplitude be seen It can that canbe the seen reach discharge that 7.5 the mV. EM discharge radiation Therefore, EM signal whenradiation generated the signal double by thegenerated-needle non-superimposed simultaneous by the non-superimposed Trichel discharge pulse pulse currentTrichelcurrents has pulse amplitude superimpose, current ofhas about amplitudethe generated 4.5 mV, of and aboutEM for radiation 4.5 the mV, superimposed fieldand willfor thealso pulse superimpo be superimposed, current,sed the pulse amplitude and current, the intensity can the reachamplitudewill 7.5 mV.increase can Therefore, reachsignificantly. 7.5 when mV. the Therefore, double-needle when simultaneous the double discharge-needle simultaneous pulse currents discharge superimpose, pulse thecurrents generated superimpose, EM radiation the field generated will also EM be radiation superimposed, field will and also the intensitybe superimposed, will increase and significantly. the intensity will increase significantly.

Figure 7. Trichel pulse current and radiation signal of the double-needle discharge structure. Figure 7. Trichel pulse current and radiation signal of the double-needle discharge structure.

The double-needle discharge radiation spectrum is shown in Figure8. The air pressure is The double-needle discharge radiation spectrum is shown in Figure 8. The air pressure is 0.1 MPa 0.1 MPa andFigure the 7.tip Trichel radius pulse of current both discharge and radiation needles signal isof 60theµ doublem. It-needle indicates discharge that the structure spectrum. of and the tip radius of both discharge needles is 60 μm. It indicates that the spectrum of the EM radiationThe double signal-needle of the discharge double-needle radiation negative spectrum corona is shown discharge in Figure is mainly 8. The distributed air pressure within is 0.1 200 MPa MHz , andthe the signal tip radius intensity of both is large discharge in the needles frequency is 60 band μm. around It indicates 70 MHz, that t andhe spectrum there is alsoof the a EMcertain radiation signal of the double-needle negative corona discharge is mainly distributed within 200 MHz, the signal intensity is large in the frequency band around 70 MHz, and there is also a certain

Energies 2018, 11, 3120 8 of 16

Energies 2018, 11, x FOR PEER REVIEW 8 of 16 the EM radiation signal of the double-needle negative corona discharge is mainly distributed within 200frequency MHz, the distribution signal intensity in the frequen is largecy in band the frequency around 140 band MHz. around Under 70 the MHz, same anddischarge there iscondition, also a certainthe double frequency-needle distribution negative corona in the frequencydischarge has band the around same frequency 140 MHz. as Under the single the same-needle discharge negative condition,corona discha the double-needlerge. However, negative when corona the Trichel discharge pulse has processes the same of frequency two discharge as the single-needle needles occur negativesimultan coronaeously, discharge. the spectral However, intensity when of thethe Trichelgenerated pulse EM processes radiation of twosignal discharge increases needles significantly occur simultaneously,when the pulse the process spectral is intensity superimposed, of the generated and the spectral EM radiation intensity signal near increases 70 MHz signiﬁcantly is increased when from the−60 pulse dBm process of the issingle superimposed,-needle discharge and the structure spectral intensityto –57 dBm near of 70the MHz double is increased-needle. from −60 dBm of the single-needle discharge structure to –57 dBm of the double-needle.

FigureFigure 8. 8.Frequency Frequency spectrum spectrum of of discharge discharge radiation radiation signal signal of of the the double-needle double-needle discharge discharge structure. structure.

TheThe frequency frequency ofof the EM EM radiation radiation signal signal generated generated by the by double the double-needle-needle negative negative corona discharge corona dischargeis only related is only to related the ambient to the ambientair pressure. air pressure. When the When air pressure the air pressureis determined, is determined, the frequency the frequency of the EM ofradiation the EM signal radiation generated signal by generated the double by-needle the double-needle discharge will discharge remain unchanged. will remain The unchanged. intensity is Therelated intensity to the is relatedair pressure, to the the air tip pressure, radius theof cathode tip radius, and of applied cathode, voltage. and applied voltage. ItIt is is found found in in the the experimental experimental test test that that the the multi-needle multi-needle simultaneous simultaneous discharge discharge current current characteristicscharacteristics and and EM radiation EM radiation characteristics characteristics are basically are consistent basically with consistent those of with the double-needle those of the dischargedouble-needle structure. discharge When the structure. applied Whenvoltage the is low, applied the pulse voltage processes is low, of the the pulse discharge processes needles of do the notdischarge occur at theneedles same do time, not and occur the at radiation the same signals time, are and generated the radiation by the signals discharge are pulse generated process by of the eachdischarge discharge pulse needle process alone. of As each the applied discharge voltage needle rises, alone. different As the discharge applied needle voltage pulse rises, processes different maydischarge occur simultaneously, needle pulse processes and the pulsemay occur currents simultaneously, will be linearly and superimposed, the pulse currents which will can be result linearly in ansuperimposed, increase in the which discharge can radiation result in ﬁeld an increaseintensity. in If the the applied discharge voltage radiation is increased field intensity. continuously, If the thenapplied the corona voltage discharge is increased will enter continuously, the glow stage,then the the dischargecorona discharge current willwill beenter in a the DC glow state, stage, and the the EMdischarge radiation current signal will will be disappear. in a DC state, The and frequency the EM ofradiation the EM signal radiation will signaldisappear. of the The multi-needle frequency of negativethe EM corona radiation discharge signal is of only therelated multi-needle to the air negative pressure, corona while dischargethe intensity is isonly affected related by numberto the air ofpressure discharge, while needles, the tip intensity radius is of affected cathode, by gap number distance of of discharge electrode, needles, and applied tip radius voltage. of cathode, Therefore, gap whendistance the airof electrode pressure, is and determined, applied voltage. the frequency Therefore, of thewhen EM the radiation air pressure signal is determined, will also be determined,the frequency andof the the detection EM radiation of the signal negative will corona also be discharge determined, can be and realized the detection by detecting of the the negative characteristic corona spectrumdischarge of can the negativebe realized corona by de dischargetecting the radiation characteristic signal. spectrum of the negative corona discharge radiation signal. 4. Discussion 4. Discussion 4.1. Mechanism Analysis of EM Radiation Generated by Negative Corona Discharge 4.1.The Mechanism space between Analysis the oftwo EM electrodesRadiation Generated of negative by corona Negative discharge Corona canDischarge be divided into a ionization zone, a plasma zone, and a drift zone [28]. As shown in Figure9, the ionization region is near the tip The space between the two electrodes of negative corona discharge can be divided into a of the discharge needle. At the beginning of the discharge, the seed electrons near the electrode collide ionization zone, a plasma zone, and a drift zone [28]. As shown in Figure 9, the ionization region is near the tip of the discharge needle. At the beginning of the discharge, the seed electrons near the electrode collide with the neutral air molecules under the action of a strong electric field. The neutral molecules are ionized into positive ions and electrons, the positive ions move toward the negative

Energies 2018, 11, 3120 9 of 16 with the neutral air molecules under the action of a strong electric field. The neutral molecules are ionizedEnergies into 2018 positive, 11, x FOR ions PEER and REVIEW electrons, the positive ions move toward the negative electrode9 of under16 the action of the electric field, and the electrons continue to ionize other neutral molecules under the actionelectrode of the electric under field.the action In a of very the short electric time, field, the and number the electrons of electrons continue increases to ionize sharply, other neutral forms an molecules under the action of the electric field. In a very short time, the number of electrons ionosphere near the tip of the discharge needle, and the external circuit current increases sharply. increases sharply, forms an ionosphere near the tip of the discharge needle, and the external circuit This process is called electronic avalanche. The thickness of the ionization region of the negative corona current increases sharply. This process is called electronic avalanche. The thickness of the ionization dischargeregion is of usually the negative about corona 1.5 mm discharge [1]. Since is theusual electricly about field 1.5 mm is relatively [1]. Since strongthe electric and field the charge-to-massis relatively ratio ofstrong the electron and the charge is relatively-to-mass large ratio inof thisthe region,electron theis relatively velocity large of electrons in this region, in this the region velocity can of reach 0.01 timeselectrons of light in this speed region [29 can,30 ].reach After 0.01 leaving times of the light ionization speed [29,30] region,. After the leaving electrons the willionization enter theregion, plasma region.the The electrons electric will field enter in the the plasma plasma region. region The cannot electric provide field in sufficient the plasma energy region for cannot the electronsprovide to completesufficient the impact energy ionization. for the electrons Therefore, to complete the electrons the impact combine ionization. with the Therefore, neutral theair molecules electrons to form negativecombine with ions the in theneutral plasma air molecules region, then to form enter negative into the ions drift in region, the plasma and region, move towardsthen enter the into anode underthe the drift action region of the, and electric move field. towards Since the the anode charge-to-mass under the action ratio of of positive the electric ions field. and negative Since the ions charge-to-mass ratio of positive ions and negative ions is much larger than that of electrons, the is much larger than that of electrons, the velocity of positive ions and negative ions is negligible when velocity of positive ions and negative ions is negligible when compared to electrons moving at high compared to electrons moving at high speed in the ionization region. Therefore, it can be considered speed in the ionization region. Therefore, it can be considered that for each pulse process in the that forentire each discharge pulse process region, in the the discharge entire discharge EM radiation region, field the is generated discharge due EM to radiation the exponential field is growth generated due toand the the exponential rapid movement growth of electrons and the in rapid the process movement of the electron of electrons avalanche in the in processthe ionization of the region electron avalancheand the in plasma the ionization region. region and the plasma region.

FigureFigure 9. Negative9. Negative corona corona dischargedischarge region division division..

WilsonWilson P.F. etP.F al.. et [31 al.] [31] equate equate the the EM EM radiation radiation field field generatedgenerated by by spark spark discharge discharge with with a dipole a dipole radiationradiation model. model. The The length length of of the the dipole dipole is is equal equal to to thatthat of the the discharge discharge gap. gap. According According to his to his findings, we equate the EM radiation field that is generated by negative corona discharge with the findings, we equate the EM radiation field that is generated by negative corona discharge with the dipole radiation model, as shown in Figure 10. However, the length of the negative corona dipole dipolemodel radiation is different model, from as shownthat of inthe Figure spark discharge10. However, dipole the model. length The of dipole the negative length dl corona = 1.5 mm dipole model(thickness is different of the from ionization thatof region), the spark the direction discharge is alon dipoleg the z model.-axis of the The cylindrical dipole lengthcoordinates,dl = 1.5and mm (thicknessthe center of the of ionizationthe dipole is region), the coordinate the direction origin. For is along this configuration, the z-axis of the the fields cylindrical are found coordinates, to be [31]: and the center of the dipole is the coordinate origin. For this configuration, the fields are found to be [31]:  2 2  0z3 i (u) 1 i (u)    0  3 z  i (u)  z  1  i (u)  E(r, t ) a dl2 2    az dl  2  1  2   2  1   * * * 2Rn R cR uo * 2  nh R2 Ri Rh 2 cR i u o ( ) = η0 ρz 3i(u) + 1 ∂i(u) + η0 3z − i(u ) + z  − 1  ∂i(u) E r , t a ρdl 2π R2 R2 cR ∂u a zdl 2π R2 1 R2 R2 1 cR ∂u (1) 1 i (u) 1 i (u) (1) *  *  1 ρ ni(u) 1 ∂i(u) o  H( r , Ht)(r, = t)a adl dl 2+   φ 2π2R RR R2 cR ∂u u 

Energies 2018, 11, x FOR PEER REVIEW 10 of 16 Energies 2018, 11, 3120 10 of 16    where E(r, t ) (V/m) is the electric field strength of the EM wave, H(r, t ) (T) is the magnetic field * * *  wherestrengthE( r ,oft) the(V/m) EM wave is the, R electric (m) is the ﬁeld distance strength from of the the discharge EM wave, pointH( rto, tthe) (T) observation is the magnetic point r ﬁeld(ρ,strength φ, z),  of the(Ω)EM is wave,the freeR (m) space is the wave distance impedance, from theand discharge i(u) (A point) is the to the discharge observation current * 0 pointtimer- (dependentρ, ϕ, z), η0 waveform(Ω) is the evaluated free space at wave time u impedance, (s), where u and = t −i( Ru)/c (A)indicates is the the discharge time lag current when the time-dependent waveform evaluated at time u (s), where u = t − R/c indicates the time lag when the radiation signal propagates to point* r . radiation signal propagates to point r .

 r (ρ，φ，z)

dl z=0

Figure 10. Negative corona discharge dipole radiation model. Figure 10. Negative corona discharge dipole radiation model. According to Equation (1), the space around the discharge needle can be divided into a near-field (d < λ/According2π) and a far-field to Equation region (1), (d > theλ/ space2π), where aroundλ is the the dischargewavelength needle of the canradiation be divided signal [ into31]. a In thenear near-field-field ( d  region,/ 2  ) the and EM a radiation far-field region signal is ( d mainly / 2  determined), where  by isi ( theu). wavelength The intensity of is the attenuatedradiation as signalR2. While [31]. inIn thethe farnear field-field are region,∂i(u)/ the∂t and EM Rradiation. signal is mainly determined by i(u) . TheSubstitute intensity the is attenuated experimentally as R2 measured. While in Trichelthe far field pulse are current i(u) /into  t and the dipoleR . radiation model to calculate the EM radiation field generated by the Trichel pulse. Figure 11 shows the Trichel pulse Substitute the experimentally measured Trichel pulse current into the dipole radiation model to current of the discharge needle with a tip radius of 210 µm under 0.1 MPa. The current amplitude is calculate the EM radiation field generated by the Trichel pulse. Figure 11 shows the Trichel pulse 550EnergiesµA and 2018 the, 11, risingx FOR PEER time REVIEW is 53 ns. 11 of 16 current of the discharge needle with a tip radius of 210 μm under 0.1 MPa. The current amplitude is 550 μA and the rising time is 53 ns. Substitute the Trichel pulse current into Equation (1) to calculate the discharge radiation field intensity waveform at different detection distances. As shown in Figure 12, the calculated EM radiation field intensity forms a pulse form, and the amplitude of the radiation signal is approximately inversely proportional to the detection distance. The EM wave propagates in the space at the speed of light, and the EM radiation field delay time is different at different positions.

FigureFigure 11. 11.Single Single Trichel Trichel pulse pulse current current waveform. waveform.

Figure 12. Discharge EM radiation field waveform.

4.2. Comparison of Calculation Results of Dipole Radiation Model with Experimental Test Results Document [21] has studied the effect of Trichel pulse current parameters on the discharge radiation spectrum. This paper mainly analyzes the relationship between the amplitude of discharge EM radiation field and Trichel pulse parameters. When the single Trichel pulse current waveform is the same, as the pulse repetition frequency increases, the average power of the radiation signal will increase, but the repetition frequency of the pulse will not affect the EM radiation signal that is generated by each pulse current, which is consistent with the experimental results (Figure 5). Figure 13 shows the relationship between the amplitude of EM radiation signal and the rising time of Trichel pulse. The tip radius of the needle used in the experiment is 94 μm. Trichel pulse current produced by negative corona discharge and its EM radiation characteristics were measured at ambient pressure of 0.1 MPa, 0.08 MPa, 0.06 MPa, and 0.04 MPa, respectively. We measured 20 sets of data at each ambient pressure, each of which was 10 μs long and contained about 20 Trichel pulses. The rising time of Trichel pulse measured at ambient pressure of 0.1 MPa, 0.08 MPa, 0.06 MPa,

Energies 2018, 11, x FOR PEER REVIEW 11 of 16

Energies 2018, 11, 3120 11 of 16

Substitute the Trichel pulse current into Equation (1) to calculate the discharge radiation field intensity waveform at different detection distances. As shown in Figure 12, the calculated EM radiation field intensity forms a pulse form, and the amplitude of the radiation signal is approximately inversely proportional to the detection distance. The EM wave propagates in the space at the speed of light, and the EM radiation field delayFigure time 11. isSingle different Trichel at pulse different current positions. waveform.

FigureFigure 12. 12.Discharge Discharge EM EM radiation radiation field field waveform. waveform. 4.2. Comparison of Calculation Results of Dipole Radiation Model with Experimental Test Results 4.2. Comparison of Calculation Results of Dipole Radiation Model with Experimental Test Results Document [21] has studied the effect of Trichel pulse current parameters on the discharge radiation Document [21] has studied the effect of Trichel pulse current parameters on the discharge spectrum. This paper mainly analyzes the relationship between the amplitude of discharge EM radiation spectrum. This paper mainly analyzes the relationship between the amplitude of discharge radiation field and Trichel pulse parameters. When the single Trichel pulse current waveform is EM radiation field and Trichel pulse parameters. When the single Trichel pulse current waveform is the same, as the pulse repetition frequency increases, the average power of the radiation signal will the same, as the pulse repetition frequency increases, the average power of the radiation signal will increase, but the repetition frequency of the pulse will not affect the EM radiation signal that is increase, but the repetition frequency of the pulse will not affect the EM radiation signal that is generated by each pulse current, which is consistent with the experimental results (Figure5). generated by each pulse current, which is consistent with the experimental results (Figure 5). Figure 13 shows the relationship between the amplitude of EM radiation signal and the rising time Figure 13 shows the relationship between the amplitude of EM radiation signal and the rising of Trichel pulse. The tip radius of the needle used in the experiment is 94 µm. Trichel pulse current time of Trichel pulse. The tip radius of the needle used in the experiment is 94 μm. Trichel pulse produced by negative corona discharge and its EM radiation characteristics were measured at ambient current produced by negative corona discharge and its EM radiation characteristics were measured pressure of 0.1 MPa, 0.08 MPa, 0.06 MPa, and 0.04 MPa, respectively. We measured 20 sets of data at at ambient pressure of 0.1 MPa, 0.08 MPa, 0.06 MPa, and 0.04 MPa, respectively. We measured 20 each ambient pressure, each of which was 10 µs long and contained about 20 Trichel pulses. The rising sets of data at each ambient pressure, each of which was 10 μs long and contained about 20 Trichel time of Trichel pulse measured at ambient pressure of 0.1 MPa, 0.08 MPa, 0.06 MPa, and 0.04 MPa pulses. The rising time of Trichel pulse measured at ambient pressure of 0.1 MPa, 0.08 MPa, 0.06 MPa, was 52 ns, 78 ns, 118 ns and 264 ns, respectively. The power of the measured EM radiation signals is 8.824 mV, 6.504 mV, 4.936 mV, and 2.97 mV, respectively. After normalizing the amplitude of EM radiation signals, the yellow curve in Figure 13 is obtained. The Trichel pulse currents that were measured at different ambient pressures are taken into Equation (1) to calculate the electric field at a distance of 3 m from the discharge point. According to the parameters of the antenna, the calculated electric field is filtered from 20 MHz to 1000 MHz. Normalizing the filtered electric field can obtain the normalized theoretical EM radiation signal amplitude, as shown in Figure 13. Energies 2018, 11, x FOR PEER REVIEW 12 of 16 Energies 2018, 11, x FOR PEER REVIEW 12 of 16 and 0.04 MPa was 52 ns, 78 ns, 118 ns and 264 ns, respectively. The power of the measured EM and 0.04 MPa was 52 ns, 78 ns, 118 ns and 264 ns, respectively. The power of the measured EM radiation signals is 8.824 mV, 6.504 mV, 4.936 mV, and 2.97 mV, respectively. After normalizing the radiation signals is 8.824 mV, 6.504 mV, 4.936 mV, and 2.97 mV, respectively. After normalizing the amplitude of EM radiation signals, the yellow curve in Figure 13 is obtained. The Trichel pulse amplitude of EM radiation signals, the yellow curve in Figure 13 is obtained. The Trichel pulse currents that were measured at different ambient pressures are taken into Equation (1) to calculate currents that were measured at different ambient pressures are taken into Equation (1) to calculate the electric field at a distance of 3 m from the discharge point. According to the parameters of the the electric field at a distance of 3 m from the discharge point. According to the parameters of the antenna, the calculated electric field is filtered from 20 MHz to 1000 MHz. Normalizing the filtered antenna, the calculated electric field is filtered from 20 MHz to 1000 MHz. Normalizing the filtered Energieselectric2018 field, 11 can, 3120 obtain the normalized theoretical EM radiation signal amplitude, as shown in Figure12 of 13. 16 electric field can obtain the normalized theoretical EM radiation signal amplitude, as shown in Figure 13.

Figure 13. Theoretical calculated and experimentally tested “relationship between Trichel pulse FigureFigure 13. 13.Theoretical Theoretical calculated calculated and and experimentally experimentally tested tested “relationship “relationship between between Trichel Trichel pulse rising pulse rising edge and radiation signal amplitude”. edgerising and edge radiation and radiation signal amplitude”.signal amplitude”. Figure 14 shows the relationship between the amplitude of the EM radiation signal and the FigureFigure 14 14 shows shows the the relationship relationship between between the the amplitude amplitude of the the EM EM radiation radiation signal signal and and the the amplitude of the Trichel pulse current. We measured the Trichel pulse current and its radiation amplitudeamplitude of the the Trichel Trichel pulse pulse current. current. We measured measured the Trichel Trichel pulse pulse current current and and its its radiation radiation characteristics of needles with a tip radius of 62 μm, 94 μm, 120 μm, 165 μm, and 210 μm. We measured characteristicscharacteristics of needles with with a a tip tip radius radius of of 62 62 μm,µm, 94 94 μm,µm, 120 μm,µm, 165 μmµm,, and 210 µμm.m. We measured 20 sets of data at each tip radius, each of which was 10 μs long and contained about 20 Trichel pulses. 2020 setssets of data at each tip radius, each of which was 10 μsµs long and contained contained about about 20 20 Trich Trichelel pulses. The measured amplitudes of the Trichel pulses are 180 μA, 250 μA, 290 μA, 410 μA, and 550 μA, TheThe measuredmeasured amplitudes of the Trichel pulses are 180 μA,µA, 250 μA,µA, 290 μA,µA, 410 μAµA,, and 550 μA,µA, respectively. The amplitude of the EM radiation signal is 7.83 mV, 8.824 mV, 10.965 mV, 14.216 mV, respectively.respectively. The amplitude of the the EM EM radiation radiation signal signal is is 7.83 7.83 mV, mV, 8.824 8.824 mV, mV, 10.965 10.965 mV, mV, 14.216 mV mV,, and 19.016 mV. After normalizing the amplitude of EM radiation signals, the yellow curve in Figure 14 andand 19.01619.016 mV.mV. AfterAfter normalizingnormalizing thethe amplitudeamplitude ofof EMEM radiationradiation signals,signals, thethe yellowyellow curvecurve inin FigureFigure 1414 is obtained. Bring the Trichel pulse current generated by needles with different tip radius into isis obtained. obtained. Bring Bring the the Trichel Trichel pulse current generated by needles needles with with different different tip tip radius radius into into Equation (1) to calculate the electric field at a distance of 3 m from the discharge point. The calculated EquationEquation (1)(1) toto calculatecalculate the electric fieldfield at a distance of 3 m fromfrom thethe dischargedischarge point.point. The calculated electric field is filtered and normalized to obtain the blue curve in Figure 14. electricelectric fieldfield isis filteredfiltered andand normalizednormalized toto obtainobtain thethe blueblue curvecurve inin FigureFigure 14 14..

FigureFigure 14. 14. TheoreticalTheoretical calculated calculated and and experimentally experimentally tested tested “relationship“relationship between between Trichel Trichel pulse pulse Figure 14. Theoretical calculated and experimentally tested “relationship between Trichel pulse amplitudeamplitude andand radiationradiation signalsignal amplitude”. amplitude”. amplitude and radiation signal amplitude”. The band of the antenna used in the experimental test is 20~1000 MHz. When the antenna was placed at a distance of 3 m from the discharge device, the received radiation signal shows far-ﬁeld characteristics. The discharge EM radiation signal was dominated by ∂i(u)/∂t. If the ∂i(u)/∂t becomes smaller, then the amplitude of the EM radiation signal will be smaller. As the air pressure decreases, the rising time of the Trichel pulse becomes larger, and other parameters of the Trichel pulse remain unchanged. As shown in Figure 13, when the rising time of the pulse increases from 52 ns to 264 ns, the amplitude of the theoretically calculated EM radiation signal decreases by 69%, and the amplitude of the experimentally tested EM radiation signal decreases by about 66%. Energies 2018, 11, x FOR PEER REVIEW 13 of 16

The band of the antenna used in the experimental test is 20~1000 MHz. When the antenna was placed at a distance of 3 m from the discharge device, the received radiation signal shows far-field characteristics. The discharge EM radiation signal was dominated by i(u) /  t . If the i(u) /  t becomes smaller, then the amplitude of the EM radiation signal will be smaller. As the air pressure decreases, the rising time of the Trichel pulse becomes larger, and other parameters of the Trichel pulse remain unchanged. As shown in Figure 13, when the rising time of the pulse increases from 52 ns to 264 ns, the amplitude of the theoretically calculated EM radiation signal decreases by 69%, and the Energiesamplitude2018 , of11 ,the 3120 experimentally tested EM radiation signal decreases by about 66%. The theoretical13 of 16 and experimental results were basically consistent. When the tip radius of the cathode increases, the amplitude of the Trichel pulse increases, and other parameters of the Trichel pulse remain The theoretical and experimental results were basically consistent. When the tip radius of the cathode unchanged. When the amplitude of the Trichel pulse increases from 180 μA to 550 μA, the i(u) /  t increases, the amplitude of the Trichel pulse increases, and other parameters of the Trichel pulse remain increases, the intensity of the EM radiation signal increases, as shown in Figure 14. The theoretically unchanged. When the amplitude of the Trichel pulse increases from 180 µA to 550 µA, the ∂i(u)/∂t calculated amplitude of the radiation signal increases approximately linearly, and the measured increases, the intensity of the EM radiation signal increases, as shown in Figure 14. The theoretically value is consistent with the theoretical value. calculated amplitude of the radiation signal increases approximately linearly, and the measured value is consistent with the theoretical value. 4.3. Effects of Multi-Needle Discharge Structure on EM Radiation Characteristics of Negative Corona 4.3.Discharge Effects of Multi-Needle Discharge Structure on EM Radiation Characteristics of Negative Corona Discharge The ionization during negative corona discharge generates a large number of electrons and positiveThe ions ionization in the duringionization negative region. corona Since dischargethe charge generates-to-mass ratio a large of electrons number ofis electronsmuch smaller and positivethan that ions of in ions, the ionizationwhich move region. slowly Since in the the charge-to-mass electric field andratio form of electrons positive is much ion clouds smaller in than the thationization of ions, region. which The move electrons slowly leave in the the electric ionization field andregion form during positive the movement ion clouds to in the the anode ionization and region.combine The with electrons the neutral leave the particles ionization to form region negative during theions, movement form a negative to the anode ion cloud and combine outside with the theionization neutral region. particles The to space form charge negative of the ions, positive form a ion negative cloud and ion cloudthe negative outside ion the cloud ionization will generate region. Thea spatial space electric charge offield, the whichpositive has ion an cloud effect and on the the negative original ion electric cloud field will generate[32]. In the a spatial multi- electricneedle field,discharge which structure, has an effect the ion on cloud the original formed electric by the fielddischarge [32]. needleIn the multi-needle in the pulse phase discharge will structure, affect the theelectric ion cloudfield around formed the by nearby the discharge discharge needle needle, in the thereby pulse affecting phase will the affect Trichel the pulse electric process. field around As shown the nearbyin Figure discharge 15, when needle, a discharge thereby needle affecting is subjected the Trichel to a pulse pulse process. process, As the shown positive in Figureion cloud 15, whenformed a dischargeby it has little needle effects is subjected on the electric to a pulse field process, in the ionization the positive direction ion cloud of the formed other by discharge it has little need effectsle, and on the electric negative field ion in cloud the ionization has a significant direction weakening of the other effect discharge on the needle, electric and field the negative in the ionization ion cloud hasdirection a significant of the other weakening discharge effect needle. on the Therefore, electric field in the in the case ionization of a small direction applied ofvoltage, the other the dischargeprevious needle.discharge Therefore, needle pulse in the process case of a has small an applied inhibitory voltage, effect the on previous the other discharge [22]. As needle the applied pulse processvoltage hasincreases, an inhibitory the effect effect of on the the negative other [22 ion]. As cloud the applied is reduced, voltage and increases, the pulse the processes effect of the of negative the two iondischarge cloud isneedles reduced, occur and simultaneously. the pulse processes of the two discharge needles occur simultaneously.

(a) (b)

Figure 15. 15.Double-needle Double-needle Trichel Trichel pulse pulse process: process: (a) Trichel (a) Trichel pulse pulse process process under small under applied small voltage;applied (voltage;b) Trichel (b pulse) Trichel process pulse under process large under applied large voltage. applied voltage.

In order to to prove prove that that the Trichel pulse pulse discharge discharge process process of one one discharge discharge needle in the multi-needlemulti-needle discharge structure will inhibit that of the other dischargedischarge needle, we measured the PRPD of the single-needle and double-needle discharge structure under the same applied voltage, as shown in Figure 16. The tip radius of the discharge needle selected for measurement is 158 µm and 165 µm, respectively. Firstly, a single needle discharge experiment was performed using the needle with a tip radius of 158 µm and a Trichel pulse sequence with a discharge voltage of 8.6 kV was obtained, as shown in Figure 16a. The Trichel pulse interval of the single-needle discharge structure is between 1400 ns and 1600 ns when the applied voltage is 8.6 kV. The experiment was then carried out using a two-needle discharge structure consisting of two discharge needles with tip radius of 158 µm and 165 µm. The Trichel pulse sequence of the double-needles discharge structure at an applied Energies 2018, 11, x FOR PEER REVIEW 14 of 16

PRPD of the single-needle and double-needle discharge structure under the same applied voltage, as shown in Figure 16. The tip radius of the discharge needle selected for measurement is 158 μm and 165 μm, respectively. Firstly, a single needle discharge experiment was performed using the needle with a tip radius of 158 μm and a Trichel pulse sequence with a discharge voltage of 8.6 kV was obtained, as shown in Figure 16a. The Trichel pulse interval of the single-needle discharge structure is between 1400 ns and 1600 ns when the applied voltage is 8.6 kV. The experiment was then carried Energies 2018, 11, 3120 14 of 16 out using a two-needle discharge structure consisting of two discharge needles with tip radius of 158 μm and 165 μm. The Trichel pulse sequence of the double-needles discharge structure at an applied voltagevoltage of of 8.6 8.6 kV kV is is shown shown in in Figure Figure 16 16b.b. The The Trichel Trichel pulse pulse indicated indicated by by the the blue blue arrow arrow is is generated generated by by thethe discharge discharge needle needle with with a a tip tip radius radius of of 165 165µ μmm with with interval interval is is of of approximately approximately 3000 3000 ns. ns. The The Trichel Trichel pulsespulses indicated indicated by by the the red red arrow arrow in the in ﬁgure the figure are generated are generated by the by discharge the discharge needle needlewith a tip with radius a tip ofradius 158 µ m. of The 158 time μm. intervalsThe time of intervals these Trichel of these pulses Trichel vary greatly.pulses vary When greatly. there is When another there Trichel is another pulse generatedTrichel pulse by another generated discharge by another needle discharge between theneedle two between Trichel pulses, the two the Trichel interval pulses, between the the interval two pulsesbetween can the reach two 1800 pulses ns; can otherwise, reach 1800 it is ns about; otherwise 1400 ns., it Such is about measurement 1400 ns. Such results measurement can explain results that whencan explain a discharge that needlewhen a is discharge subjected needle to a pulse is subjected process,the to pulsea pulse process process, of thethe otherpulse needle process will of bethe suppressed.other needle The will interval be suppressed. of the Trichel The pulseinterval will of increase.the Trichel The pulse EM radiationwill increase. signal The corresponds EM radiation to thesignal pulse corresponds current signal to the of negative pulse current corona signal discharge. of negative When corona the pulse discharge. current signalWhen is the superimposed, pulse current thesignal EM radiationis superimposed, signal will the also EM beradiation superimposed. signal will also be superimposed.

(a) (b)

FigureFigure 16. 16.Negative Negative corona corona discharge discharge Trichel Trichel pulse pulse current current waveform waveform when when loading loading voltage voltage is 8.6 is kV: 8.6 (akV:) Single (a) Single needle; needle; (b) Double (b) Double needles. needles.

5.5. Conclusions Conclusions InIn this this paper, paper, the the pulse pulse current current characteristics characteristics of of multi-needle multi-needle negative negative corona corona discharge discharge and and its its EMEM radiation radiation characteristics characteristics are are studied. studied. BasedBased on on the the study study of of EM EM radiation radiation characteristics characteristics of of single-needle single-needle negative negative corona corona discharge discharge byby Zhang Zhang Y. Y [.21 [21]], we, we studied studied the the pulse pulse current current and and its its EM EM radiation radiation characteristics characteristics of of multi-needle multi-needle negativenegative corona. corona. Different Different from from Abdel-Salam’s Abdel-Salam’s [ 8[8,22,22]] research research on on the the volt-ampere volt-ampere characteristics characteristics of of multi-needlemulti-needle negative negative corona corona discharge, discharge, we analyzed we analyzed the pulse the current pulse characteristicscurrent characteristics of the negative of the coronanegative discharge corona anddischarge studied and the studied corresponding the corresponding relationship relationship between EM between radiation EM signal radiation and pulsesignal current.and pulse The current. effect of The multi-needle effect of multi discharge-needle structure discharge on s thetructure EM radiation on the EM characteristics radiation characteristics of negative coronaof negative discharge corona is analyzed.discharge is analyzed. InIn the the multi-needle multi-needle discharge discharge structure, structure, the the Trichel Trichel pulse pulse discharge discharge process process of of one one discharge discharge needleneedle in in the the multi-needle multi-needle discharge discharge structure structure will inhibit will inhibit the discharge the discharge of the other of the discharge other discharge needle. Whenneedle. the W appliedhen the voltage applied is low,voltage the negativeis low, the corona negative discharge corona Trichel discharge pulse processesTrichel pulse of the processes respective of dischargethe respective needles discharge are mutually needles suppressed, are mutually and suppressed, the discharge and needle the discharge pulse processes needle occur pulse separately. processes Whenoccur the separately. applied voltageWhen the increases applied to vo a certainltage increases threshold, to thea certain pulse threshold, processes ofthe different pulse processes discharge of needlesdifferent will discharge occur simultaneously, needles will occur the simultaneously, multi-needle discharge the multi current-needle discharge will be linearly current superimposed, will be linearly andsuperimposed, the generated and EM the radiation generated ﬁeld EM will radiation also be linearlyfield will superimposed. also be linearly The superimposed. frequency of The the radiationfrequency signal after multi-needle simultaneous discharge superposition is the same as the frequency of the single-needle, but the amplitude is increased.

The negative corona discharge radiation process can be equivalent to the dipole antenna radiation model. The dipole length is the sum of the thickness of the ionization region and the plasma region. The radiation field in the near-field region is dominated by i(u) and the discharge radiation field in the far-field region is dominated by ∂i(u)/∂t. The amplitude of the EM radiation signal is related to the tip radius of the cathode and the air pressure. The larger the tip radius is, the larger the amplitude of the Energies 2018, 11, 3120 15 of 16 pulse current and the intensity of the EM radiation signal will be. As to the air pressure, which gets lower, the rising time of the pulse will be longer, while the change of current as time, and the intensity of the radiation signal will be smaller. The spectral characteristics of the EM radiation of the multi-needle negative corona discharge are only related to the air pressure. The intensity of the EM radiation signal is related to the tip radius of the cathode, the air pressure, and the number of discharge needles. The results of this work may provide promising method for evaluation of insulation condition of electric apparatus, as well as detection of the discharge sources.

Author Contributions: Conceptualization, C.W. and X.C.; Methodology, C.W., T.L. and J.O.; Software, C.W. and J.F.; Validation, C.W., J.O. and X.C.; formal analysis, C.W. and J.F.; Investigation, J.O.; Resources, C.W. and X.C.; Data curation, C.W. and X.C.; Writing—original draft preparation, C.W. and X.C.; Writing—review and editing, C.W. and T.L.; Visualization, C.W.; Supervision, X.C.; Project administration, X.C.; Funding acquisition, C.W., X.C. and J.O. Funding: This work was financially supported by grants from National Natural Science Foundation of China (#U1630130, #51777010, #51707008, #51407009). Conflicts of Interest: The authors declare no conflict of interest.

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