Supply Systems of Non-Thermal Plasma Reactors. Construction Review with Examples of Applications

applied sciences

Review Supply Systems of Non-Thermal Plasma Reactors. Construction Review with Examples of Applications

Henryka Danuta Stryczewska Department of Electrical Engineering and Electrotechnology, Lublin University of Technology, 20-618 Lublin, Poland; [email protected]; Tel.: +48-85384289

Received: 4 March 2020; Accepted: 27 April 2020; Published: 7 May 2020

Featured Application: Review presents power supply systems of non-thermal plasma reactors for industrial applications with examples where they have already been applied as energy-eﬃcient, low cost and reliable systems.

Abstract: A review of the supply systems of non-thermal plasma reactors (NTPR) with dielectric barrier discharge (DBD), atmospheric pressure plasma jets (APPJ) and gliding arc discharge (GAD) was performed. This choice is due to the following reasons: these types of electrical discharges produce non-thermal plasma at atmospheric pressure, the reactor design is well developed and relatively simple, the potential area of application is large, especially in environmental protection processes and biotechnologies currently under development, theses reactors can be powered from similar sources using non-linear transformer magnetic circuits and power electronics systems, and ﬁnally, these plasma reactors and their power supply systems, as well as their applications are the subject of research conducted by the author of the review and her team from the Department of Electrical Engineering and Electrotechnology of the Lublin University of Technology, Poland.

Keywords: non-thermal plasma reactors (NTPR); dielectric barrier discharge (DBD); gliding arc discharge (GAD); atmospheric pressure plasma jet (APPJ); special transformers; discharge ignition; RF power supply; AC/DC/AC inverters; pulsed supply; resonance power supply

1. Introduction Non-thermal plasma, commonly known as “cold”, has been used for almost two hundred years. Its main source in the nineteenth century was arcing, and the first application, already at the beginning of the nineteenth century, arc lamps, which illuminated the streets of the world metropolises, such as Paris, London and New York [1,2]. The industrial applications of plasma electrical discharge were initiated by the brothers Siemens-Werner and William, who built the first ozonizer (1857) and electric arc furnace (1878). At the beginning of the 20th century, first in Nice (1907) and then in Sankt Petersburg (1908), plasma reactors with barrier discharges were introduced to drinking water treatment technology. The technology of using electric discharges to produce ozone is still used today, and so far there is no alternative [3]. The basis for the division of plasma applications for technological purposes is its interaction with gas molecules, which results in a change in the chemical properties, momentum and energy of plasma particles. The transformation of physical and chemical properties of plasma particles is the basis of chemical technologies such as surface modification, etching, thin film deposition, powder production, ozone generation, plasma gas and wastewater treatment, thermal waste treatment and others. The momentum and energy transformations of plasma particles are used practically to obtain plasma beams for biomedical engineering (plasma sterilization), laser technologies, rocket propulsion, for radiation generation and in plasma light sources [1–4].

Appl. Sci. 2020, 10, 3242; doi:10.3390/app10093242 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 3242 2 of 25

In industrial practice, metallurgy and plasma light sources are among the oldest but still successfully used plasma technologies. The last decades of the twentieth century have brought new applications for already used electrical discharges, which have always been the main source of plasma for technological purposes. These include such large areas of application as: microelectronics, technologies for producing semiconductors, superconductors, organic, bio- and nano-materials. The twenty-ﬁrst century is the era of new materials and technologies for their production, which are currently based mostly on plasma and laser technologies. The latter are closely related to plasma, which is often a medium that strengthens the laser beam (pumping lasers), and are included directly in the plasma technology [1–3]. At present non-thermal (NT) plasma treatment is widely applied in environmental processes of air, water and soil decontamination and in biology and medicine. Environmental applications include:

Decomposition of nitrogen and sulfur oxides in flue gases and volatile organic compounds (VOCs) • emitted during various industrial processes—many hazardous organic compounds are readily attacked by exciting species, free radicals, electrons, ions, and/or UV photons generated in low temperature plasma [5–7]; Generation of hydrogen H and elemental sulfur S from hydrogen sulfide H S; • 2 Conversion of the greenhouse gases (CO ,C H ) to synthesis gas or liquid fuels and their • 2 2 4 valorization [8–10]; Water purification by ozone treatment. The ozone synthesis typically takes place in electrical • discharges in air or oxygen. They produce strongly oxidizing agents, such as OH*, H*, O*, O3, and hydrogen peroxide H2O2. Moreover, the strong electric fields of electric discharges and UV radiation are also lethal to several kinds of microorganisms in water [5,11]; Removal of hazardous organic pollutants from waste water—pulsed corona discharges, dielectric • barrier discharges (DBDs), and contact glow discharge electrolysis techniques are being studied also for the purpose of water cleaning; Soil treatment—the afterglow of discharges that generate a non-thermal plasma in air or in • oxygen is the source of sterilization agents, like ozone, NO, activated oxygen and nitrogen, and is a reasonable alternative to conventional chemical methods [12–15].

Virucidal, fungicidal, and bactericidal properties of non-thermal plasma makes it eﬀective in the processes of sterilization and disinfection as well as in treatment of many diseases [16–21], like:

Sterilization of human and animal tissues, blood and surface wounds; • Assisting skin cancer therapy; • Odontology—caries therapy; • Coating of implants, contact optical lens, dentures with biocompatible films; • Live tissue engineering—fabrication of bioactive agents and medicines, immobilization of • biological molecules, cell surface modification to control their behavior, improvement of blood adhesion; Sterilization of medical and surgical instruments, especially made of materials and fabrics not • resistant to high temperature; Medical diagnostics—fabrication biosensors based on polymers and thin amorphous films for • medical and bio-chemical analysis.

In the agriculture and food industry the plasma treatment is used for [22–30]:

Food pasteurization, disinfection, and preservation—O is used in common refrigerators as • 3 a deodorizing and antimicrobial agent; Food storage packages sterilization; • Storing agriculture products—ozone increasing storage life, sanitizing fruit and vegetable surfaces; • Plant growth enhancement and fruit formation processes; • Seed germination; • Appl. Sci. 2020, 10, 3242 3 of 25

Ozone-aided corn steeping process—to replace current SO application. • 2 Research in the ﬁeld of technological applications of plasma concerns two main issues:

New solutions of plasma reactors (PRs) and electric discharges used in them, in which plasma, • with the required plasma-chemical parameters and high time-space effectiveness, will be produced efficiently at atmospheric pressure [31–34]; Efficient and controllable plasma reactor’s power supply systems with a wide range of changes in • the properties (voltage shape and frequency, range of regulation), which are an inseparable part of the plasma-chemical installation that determines industrial implementation [35–45].

In the review basic types of systems used to supply plasma devices are presented. These are, among others, systems using special transformers and non-linearity of the magnetization characteristics of their cores, as well as systems based on semiconductor technology, operating at DC, AC and pulse voltage, with mains, increased and high frequency. Also, the selected applications of the discussed systems, which were studied and implemented by the author of the article and her research team, has been presented.

2. Non-Thermal Plasma Reactors Plasma reactors (PRs) are very unusual electrical devices. Their macroscopic characteristics are strongly non-linear and their work is generally carried out at high AC, DC or pulsed voltage, often with increased or high frequency. The discharge power, which is a measure of the eﬃciency of the plasma reactor, is regulated by the value of voltage or current, depending on the type of discharge, while the proper cooperation of the plasma reactor that in industrial applications is a high power device, with mains requiring additional devices such as reactive power compensation systems or ﬁlters, reducing distortion of the mains current. The reliable operation of the plasma reactor depends on the characteristics of the power supply system (PSS) and, on the other hand, the PSS responds to such an unusual receiver as the PR is. Therefore, the feed system must be designed and constructed together with the PR. In practice, various power sources are used to supply PRs, which can be divided into two main groups:

Transformer systems using the properties of magnetic circuits; • Systems with power electronics elements. • The above division is arbitrary, because both mentioned systems contain magnetic and power electronic elements. The latter, due to the enormous progress in the field of power electronics, are increasingly used to power plasma reactors. The use of the latest, fully controllable semiconductor components, such as GTO, MOSFET and IGBT transistors, allows the construction of power systems for plasma reactors with high power and high switching frequencies. Converter power supplies can provide the current–voltage characteristics and good current, voltage, power and frequency regulation required by the plasma reactor. They also enable automation of the plasma reactor operation by controlling the position of the electrodes, the composition and amount of the treated gas flow, and the plasma temperature. Recently, systems with impulse energy have been finding more and more applications in the processes of non-thermal plasma generation. Owing to the improvement of semiconductor and integrated circuit technologies, as well as the properties of magnetic materials, the use of pulse energy to power plasma reactors increases [46]. To make a selection and design a PSS for a given plasma process, one needs to specify the requirements and parameters of the plasma receiver, the most important of which are:

Supply voltage—constant, sine, impulse; • The presence of additional ignition system of the discharge or its absence; • NTP reactor power and attainable power of the PSS; • Appl. Sci. 2020, 10, 3242 4 of 25

The ability to adjust the current value and maintain its continuity in the entire area of the plasma • reactor operation; Ability of the power source to work in automatic control and regulation systems and adjustment • of parameters to various process gases and their mixtures; Correct cooperation with the power supply network; • High eﬃciency; • Simplicity and safety of use; • Low capital and operating costs. • These numerous requirements make the supply systems of non-thermal plasma reactors quite complex. In addition to the electricity supply system, they contain gas supply systems, gas velocity regulation and protection systems.

3. Transformer Power Supply Systems for Plasma Reactors Regardless of the type of electrical discharges used to generate the plasma, each PR power supply system for industrial applications is equipped with a transformer, which in its proper implementation is the simplest PSS for both single and multielectrode PRs. It transforms the voltage to the required level that allows ignition of discharges, and owing to a special design it can limit the current in the circuit with discharge. Using diﬀerent transformer connections, it is possible to supply both two-, three- and multi-electrode plasma reactors. Transformer power supply systems for gliding arc reactors can be equipped with separate, high-frequency, ignition systems, made in the form of electronic modules giving an ignition voltage with a frequency (20–40) kHz, which increases the possibility of occurrence of ignition and at the same time reduces the size of the ignition system [5,34,36,43,47]. Regulation of the power delivered to the discharge space can be carried out in transformer systems by changing the supply voltage (autotransformer, taps on the primary side) or by using power electronic controllers. Among the transformer PSS, an integrated system (IPSS) for feeding non-thermal PRs will be discussed, which, when properly designed, can be used as a feeder for GAD or DBD plasma reactors.

3.1. Integrated Power Supply Systems In the basic solution of the IPSS, three single-phase power transformers (Tr W), with magnetic circuits ensuring free return paths for higher harmonics of the magnetic ﬂux, are fed from a symmetrical three-phase network [43,47]. The primary windings of the transformers are connected in a star (Y), while the secondary windings, depending on the type of non-thermal plasma reactor and the number of electrodes, can be connected in an open triangle (d) for DBD reactor—Figure1a or 2—electrode GlidArc and in a star (y) when fed with a three- and multi-electrode GlidArc—Figure1b. Idea of the IPSS is based on the non-linearity of the power transformers’ cores. IPSS for DBD reactor is the well-known solution of the magnetic frequency tripler (mf3). Output voltage of the supplier has the tripled frequency of mains (150/180Hz), therefore the DBD plasma reactor running at three times increased frequency can be supplied at more than 30% lower voltage than at 50/60Hz having the same active power P [43]. Moreover, the voltage to the DBD plasma reactor can be easily adjusted (VR) by the impedance connected in series in the neutral conductor of the primary side of the PS, between the PEN and N terminals. In this way we get increase of frequency and voltage as well as its regulation in the one power supply of DBD reactor. In addition, reactive power compensation required in other transformer systems because of increased magnetic ﬂux density is not necessary for the capacitive receiver that is the DBD reactor. Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 26

having the same active power P [43]. Moreover, the voltage to the DBD plasma reactor can be easily adjusted (VR) by the impedance connected in series in the neutral conductor of the primary side of the PS, between the PEN and N terminals. In this way we get increase of frequency and voltage as well as its regulation in the one power supply of DBD reactor. In addition, reactive power

Appl.compensation Sci. 2020, 10, required 3242 in other transformer systems because of increased magnetic flux density5 of 25is not necessary for the capacitive receiver that is the DBD reactor.

(a) (b)

Figure 1. 1. IntegratedIntegrated power power supply supply system system (IPSS) (IPSS for) dielectricfor dielectric barrier barrier discharge discharge (DBD) ( plasmaDBD) plasma reactor (PR)reactor (a) ( andPR) for(a) gliding and for arc gliding discharge arc (GAD) discharge PR ((bGAD), where:) PR Tr(b), W—power where: Tr transformer, W—power VR—voltagetransformer, regulation,VR—voltage Tr regulation, I—ignition Tr transformer, I—ignition WE—working transformer, WE electrodes,—working IE—ignition electrodes, electrode, IE—ignition YNd-3-phase electrode, transformerYNd-3-phase with transformer star-delta with connection star-delta with connection neutral wire with (N) neutral on the wire primary (N) on side, the YNyn-3-phase primary side, transformerYNyn-3-phase with transformer star-star connection with star with-star neutral connection wires with on the neutral both transformer wires on the sides both [43 transformer,48–51]. sides [43,48–51]. IPSS of GAD Multielectrode Plasma Reactors 3.1.1.Plasma-chemical IPSS of GAD Multielectrode reactions generated Plasma by Reactors the GAD take place directly in the polluted gas, therefore this kindPlasma of-chemical non-thermal reactions plasma generated reactors have by the found GAD applications take place in directly the treatment in the polluted of flue gases gas, emittedtherefore by this industrial kind of non processes-thermal of plasma coal and reactors biomass have combustion found applications (SO2, NO inx removing),the treatment painting of flue andgases varnishing emitted by (VOCindustrial abatement), processes wastes of coal utilizationand biomass [47 combustion,52]. In all (SO these2, NO processesx removing), non-thermal painting plasmaand varnishing should be (VOC generated abatement), in relatively wastes large utilization volume of[47 chamber,,52]. In all with these dimensions processes in non the- rangethermal of centimetersplasma should or evenbe generated meters, unlikein relatively dimensions large volume of DBD reactorsof chamber, discharge with dimensions gaps of millimeters in the range range. of Multielectrodecentimeters or andeven multistage meters, unlike GAD dimensions plasma reactors of DBD are reactors used in thesedischarge applications gaps of tomillimeters increase the range. time ofMultielectrode the penetration and of multistage polluted gas GAD by theplasma non-thermal reactors plasma,are used what in these is achieved applications by the to connection increase the of thetime discharge of the penetration chambers in ofseries polluted with the gas gas by flow, the or non in parallel,-thermal to plasma, divide the what polluted is achieved gas stream by into the smallerconnection streams of the [5 discharge]. chambers in series with the gas flow, or in parallel, to divide the polluted gas streamPlasma into reactors smaller with streams GAD have [5]. different requirements for the power supply system than reactors with DBD.Plasma The reactors ignition with of the GAD discharge have should different be realized requirements at a voltage for of the several powe kilovoltsr supply (depending system than on thereactors distance with between DBD. The the ignition working of electrodes), the discharge while should the voltage be realized on the dischargeat a voltage after of ignitionseveral kilovolts amounts to(depending several hundred on the distance volts. This between disproportion the working of ignition electrodes), and stable while operation the voltage voltages on the as discharge well as theafter strong ignition non-linearity amounts to of several the conductance hundred ofvolts. the dischargeThis disproportion creates a difficult of ignition task and for the stable power operation supply system,voltages whichas well must as the have strong the non properties-linearity of of both the conductance a high-voltage of the ignition discharge system creates and a a difficult system task that deliversfor the power the power supply to system, the discharge which must chamber have and the followsproperties the of rapid bothtime a high changes-voltage of igni thetion non-linear system current-voltageand a system that characteristics delivers the power of the GAD.to the discharge chamber and follows the rapid time changes of the nonThe-linear name current “integrated”-voltage was characteristics introduced of by the the GAD authors. with the idea of this system [43,48–51], for theThe following name "integrated” reasons, fundamental was introduced for by the the GAD auth plasmaors with reactor the idea operation: of this system (1) Integration [43], [48-51 of], thefor the ignition following function reasons, in the fundamental power supply, for (2)the sustaining GAD plasma discharge reactor current operation: at each (1) I passagentegration through of the zero,ignition and function (3) ability in the to limit power short-circuit supply, (2) current sustaining of PSS. discharge current at each passage through zero, and (3)IPSS ability for theto limit three-electrode short-circuit GADcurrent PR of consistsPSS. of four transformers—three power and one ignition—FigureIPSS for the1 b.three Primary-electrode and GAD secondary PR consists windings of offour the transformers power transformers—three power are connected and one inignition a star,— withFigure the 1b secondary. Primary and winding secondary terminals windings connected of the topower the plasmatransformers reactor are electrodes connected WE1, in a WE2star, with and W3.the secondary The ignition winding transformer terminals TR connected I is connected to the toplasma a voltage reactor that electrodes is induced WE1, between WE2 theand PEN W3. andThe Nignition terminals transformer of the primary TR I sideis connected of the power to a transformersvoltage that becauseis induced of the between non-linearity the PEN of the magnetization characteristics of their magnetic circuits. The UI voltage at the output of the ignition transformer having a 150/180Hz frequency, is sufficient to ionize the inter-electrode space, enabling ignition of the discharge between power transformers, which are designed and built for a voltage several times lower than the ignition voltage—Figure2a. Three times higher frequency of the ignition Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 26 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 26 and N terminals of the primary side of the power transformers because of the non-linearity of the magnetizationand N terminals characteristi of the primarycs of theirside ofmagnetic the power circuits. transformers The UI voltage because at ofthe the output non- linearityof the ignition of the transformermagnetization having characteristi a 150/180Hzcs of theirfrequency, magnetic is sufficient circuits. toThe ionize UI voltage the inter at -theelectrode output space, of the enabling ignition ignitiontransformerAppl. Sci. 2020of the, 10having, 3242discharge a 150/180Hz between frequency, power transformers, is sufficient whichto ionize are the designed inter-electrode and built space, for a enabling voltage6 of 25 severalignition timesof the lowerdischarge than between the ignition power voltage transformers,—Figure which 2a. T hreeare designed times higher and bfrequencyuilt for a voltage of the ignitionseveral times voltage lower in relation than the to ignitionthe power voltage voltage—Figure frequency 2a. Timproveshree times the higher ignition frequency reliability of and the reducesignitionvoltage inpower voltage relation failure. in to relation the Courses power to voltageofthe working power frequency voltageelectrodes’ improves frequency voltages the improves ignitionare shown reliability the in ignitionFigure and 2b. reliability reduces power and reducesfailure. Coursespower failure. of working Courses electrodes’ of working voltages electrodes’ are shown voltages in Figure are shown2b. in Figure 2b.

(a) (b) (a) (b) Figure 2. (a) Characteristics of the IPSS (A-B ignition voltage characteristic, B-C operating voltage characteristic);Figure 2. (a)(a) Characteristics (b) phase voltages of the u IPSSf1(t), u (A-B(Af2(t)-B ignitionandignition uf3(t) voltagevoltage on the characteristic,characteristic,3-electrode GAD B–CB-C reactor operating durin voltageg the discharge in air at flow rate of 10 lit/minf1 , supplyf2 voltagef3 frequency f = 50Hz. characteristic); ( (bb)) phase phase voltages voltages u u(t),f1(t), u u(t)f2 (t)and and u (t) uf3 (t)on onthe the3-electrode 3-electrode GAD GAD reactor reactor durin duringg the dischargethe discharge in air in at air flow at ﬂow rate rate of 10 of lit/min 10 lit/min,, supply supply voltage voltage frequency frequency f = 50Hz. f = 50Hz. Figure 3 presents courses of electrode current, phase voltage, and instantaneous power recordedFigure for 3 3 presents a presentsthree- courseselectrode courses of GAD electrode of electrode plasma current, reactor current, phase fed phase voltage, from voltage andtransformers instantaneous, and instantaneouswith powercores recordedmade power of metallicrecordedfor a three-electrode glass for a ( athree) and GAD-electrode a conventional plasma GAD reactor plasma transformer fed from reactor transformers sheet fed (b from). According with transformers cores to made the with of oscillograph metalliccores made glass from ( ofa) Figuremetallicand a conventional 3, glass the material(a) and transformer aof conventional the power sheet( b transformersupply). According transformer sheet to the (b oscillograph). core According has a fromsignificant to the Figure oscillograph impact3, the material on from the instantaneousFigureof the power 3, the supply phase material voltage transformer of the of the power core GAD has supply PR. a signiﬁcantIn the transformer case impactof a core core on material the has instantaneous a significantmade of metallic phase impact voltageglasses, on the ofit willinstantaneousthe GADcarry PR.much In phase themore case voltage harmonics of a coreof the materialthat GAD come PR. made from In the of ignition metallic case of dischargesa glasses, core material it will systems. carry made muchReducing of metallic more the harmonicsglasses, content it ofwillthat higher comecarry frommuch harmonics ignition more inharmonics discharges a system that systems. with come amorphous Reducingfrom ignition cores the content discharges is possible of higher systems. by harmonics using Reducing an ignition in a systemthe content system with operatingofamorphous higher at harmonics coresa technical is possible in frequency a system by using of with 50 an Hz amorphous ignition. In the system case cores of operatingtransformer is possible at asheet by technical using cores, an frequency higher ignition harmonics of system 50 Hz. areoperatingIn the suppressed case at of a transformer technical by the power frequency sheet supply's cores, of higher50 magnetic Hz. harmonicsIn the circuit case are ofand suppressedtransformer do not occur by sheet the in powercores,phase supply’shighervoltages harmonics— magneticFigure 3arecircuitb. suppressed and do not by occurthe power in phase supply's voltages—Figure magnetic circuit3b. and do not occur in phase voltages—Figure 3b.

(a) (b) (a) (b) FigureFigure 3. 3. OscillographsOscillographs of ofelectrode electrode current current IGAD I,GAD phase, phase voltag voltagee UGAD and UGAD dischargeand discharge power p powerGAD(t), whenFigurepGAD (t), feeding 3. whenOscillographs the feeding GAD theof reactor electrode GAD from reactor current transformers from IGAD transformers, phase with voltag cores with e made UGAD cores of:and made( adischarge) Amorphous of: ( apower) Amorphous glass, pGAD ((t)b), transformerwhenglass, (feedingb) transformer sheet; the GADon both sheet; reactor oscillographs on both from oscillographs transformers frequency frequency withof ignition cores of voltage made ignition of: is voltage (equala) Amorphous to is 20 equal kHz toglass,, process 20 kHz, (b) 3 transformerprocess gas—Argon, sheet; on ﬂow both rate oscillographs 1 m /h, RMS frequency voltage U ofGAD ignition= 250 V,voltage RMS currentis equal I GADto 20= kHz2.2 A,, process supply voltage frequency f = 50 Hz. Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 26

Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 26 gas—Argon, flow rate 1m3/h, RMS voltage UGAD=250V, RMS current IGAD=2.2A, supply voltage frequency f=50 Hz. Appl.gas Sci. —2020Argon,, 10, 3242 flow rate 1m3/h, RMS voltage UGAD=250V, RMS current IGAD=2.2A, supply voltage7 of 25 frequency f=50 Hz. IPSS can also be used in three-wire network without an available neutral point, then the role of the neutralIPSSIPSS can can point also also isbe be played used used in inby three three-wire an -artificialwire network network star point without without (N2) an, an whichavailable available can neutral be neutral imple point, point,mented then then fromthe the role three role of thecapacitorsof the neutral neutral connected point point is isplayed in played a star by (Figure by an an artificial artiﬁcial 4). star star point point (N2) (N2),, which which can can be be imple implementedmented from from three capacitors connected in a star (Figure 44).).

Figure 4. 4. IPSSIPSS with with artiﬁcial artificial neutral neutral point point from from three three capacitors capacitors connected connected in star—N2 in star artiﬁcial—N2 artificial neutral point,neutral N1 point, and N1 n—neutral and n— pointsneutral according points according to primary to primary and secondary and secondary side of transformers side of transforme Tr1, Tr2,rs Figure 4. IPSS with artificial neutral point from three capacitors connected in star—N2 artificial Tr1,and Tr3Tr2, connected and Tr3 connected in star [43 in]. star [43]. neutral point, N1 and n—neutral points according to primary and secondary side of transformers Tr1, Tr2, and Tr3 connected in star [43]. Other integrated circuit solutions for feeding six and ninenine-electrode-electrode reactors are shown in Figure5a,b [5,43]. FigureOther 5 (a ) integratedand (b) [5], circuit[43]. solutions for feeding six and nine-electrode reactors are shown in Figure 5 (a) and (b) [5], [43].

(a) (b)

Figure 5. (a) IPSSIPSS forfor(a) six six electrode;electrode; ( b(b)) nine nine electrode electrode GAD GAD reactor reactor (TrZ—ignition (TrZ—ignition(b) transformer) transformer [5), 43[5],]. [43]. Figure 5. (a) IPSS for six electrode; (b) nine electrode GAD reactor (TrZ—ignition transformer) [5], Power supply system from Figure5b owing to the three-stage connection of working [43]. electrodes—twoPower supply of three system working from electrodesFigure 5b andowing one to ignition the three electrode-stage in connection each stage—allows of working to electrodesenlargePower the— spacetwo supply of covered three system working by from the discharge electrodesFigure 5b and andowing can one be to ignition used the to three electrode utilize-stage exhaust in connection each gases stage at of— highallows working flow to electrodesenlargerates, so the as— tospacetwo extend of covered three the residence working by the discharge electrodes time of contaminated and and can one be ignition used gas into electrode theutilize plasma exhaust in space, each gases stage which at— high meansallows flow its to enlargerates,better so decontamination. the as tospace extend covered the residence by the discharge time of conta and minatedcan be used gas into theutilize plasma exhaust space, gases wh ichat highmeans flow its rates,better sodecont as toam extendination. the residence time of contaminated gas in the plasma space, which means its 3.2. A Five-Column Transformer as a Power Supply for a GAD Plasma Reactor better decontamination. 3.2.AA Five modification-Column Transformer of the integrated as a Power system Supply is for the a useGAD of P alasma five-column Reactor transformer in the plasma 3.2.reactorAA F ivemodific supply-Column system.ation Transformer of Inthe typical integrated as a designs Power system S ofupply five-column is for the a useGAD of transformers, P alasma five -Rcolumneactor magnetic transformer flux closing in the throughplasma external columns is not in use. It was proposed to use the energy of the external columns’ fluxes in reactorA modificsupply ationsystem of. Inthe typical integrated designs system of five is- thecolumn use oftransformers, a five-column magnetic transformer flux closing in the through plasma a three-phase five-column transformer. The idea of this solution is presented in Figure6. The voltage reactor supply system. In typical designs of five-column transformers, magnetic flux closing through induced in the windings of the external columns can be used to power the ignition system of the plasma Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 26 external columns is not in use. It was proposed to use the energy of the external columns’ fluxes in a Appl.three Sci.-phase 2020, 10five, x -FORcolumn PEER transformerREVIEW . The idea of this solution is presented in Figure 6. The voltage8 of 26 induced in the windings of the external columns can be used to power the ignition system of the Appl.plasmaexternal Sci. reactor.2020 columns, 10, 3242 In is this not wa in y,use. the It energy was proposed of the magnetic to use the flux energy closing of tinhe these external columns columns’ will befluxes used8 ofin to 25a ionizethree-phase the inter five-electrode-column transformerspaces of the. The GAD idea reactor. of this solution is presented in Figure 6. The voltage induced in the windings of the external columns can be used to power the ignition system of the plasmareactor. reactor. In this way,In this the wa energyy, the ofenergy the magnetic of the magnetic flux closing flux closing in these in columns these columns will be will used be to used ionize to ionizethe inter-electrode the inter-electrode spaces spaces of the GADof the reactor. GAD reactor.

(a) (b)

Figure 6. A 5-column transformer with external wound columns and a magnetic circuit made of wound cores; (a) windings connection—external column windings connected in series; (b) (a) (b) arrangement of working and ignition coils on wound cores, 1/2—concentrically wound primary and Figuresecondary 6. A Awindings, 55-column-column 3 — transformer transformerwindings of with with external external external columns wound wound designed columns columns for and and supplying a a magnetic ignitio circuitn systems made ofof woundGAD plasma cores; cores; (reactora )(a windings) w [23,44]indings connection—external. connection—external column column windings windings connected connected in series; (b in) arrangement series; (b) arrangementof working and of working ignition and coils ignition on wound coils cores, on wound 1/2—concentrically cores, 1/2—concentrically wound primary wound and primary secondary and windings,secondaryTests conducted 3—windingswindings, on 3 a— of11windings external kVA laboratory columnsof external designed modelcolumnsfor confirmed designed supplying for the ignition supplying validity systems ignitioof the of GADn idea systems plasmaof using of a five-columnGADreactor plasma [ 23 transformer,44 ].reactor [23,44] to power. 3-electrode GAD reactors. In particular, they proved that it is possible to use the energy of magnetic fluxes closing through the outer columns of the transformer to igniteTests the GAD. conducted Figure on on 7a a apresents 11 11 kVA kVA the laboratory laboratory courses ofmodel model ignition confirmed confirmed voltage andthe the validity(b) validity between of of the electrodes the idea idea ofof usingvoltage using a offivea five-columnGAD-column reactor transformer transformer in argon as to a to powerprocessing power 3- 3-electrodeelectrode gas [44,47] GAD GAD. reactors. reactors. In In particular, particular, they they proved proved that that it it is possibleThe to method use the ofenergy the reactor of magnetic ignition fluxes fluxes system closing designing through doesthe outer not columns impose restrictionsof the transformer and it to is possibleignite the toGAD. supply Figure PRs 77aa presentspresents without thethe separate coursescourses ignition ofof ignitionignition circuits voltagevoltage as andand well (b)(b) as betweenbetween with two electrodeselectrodes-electrode voltagevoltage and singleof GAD -electrode reactor inignition argon assystems. a processing gas [[44,47]44,47].. The method of the reactor ignition system designing does not impose restrictions and it is possible to supply PRs without separate ignition circuits as well as with two-electrode and single-electrode ignition systems.

(a) (b)

Figure 7. (a)(a) I Ignitiongnition voltage Uign waveformwaveform (distance (distance betw betweeneen ignition ignition electrodes electrodes 5 mm) when supplied from external windingswindings connectedconnected inin seriesseries ofof thethe ﬁve-columnfive-columntransformer transformer ( b(b)) voltage voltage U UGAD (a) (b) GAD between electrodes of GAD reactor in argon,argon, at aa thethe primaryprimary voltagevoltage ofof 90V,90V, [[44]44].. Figure 7. (a) Ignition voltage Uign waveform (distance between ignition electrodes 5 mm) when

suppliedInThe all method analyzed from of external thecases, reactor windingsthe tested ignition connected five system-column in designing series transformer of the does five not-designcolumn impose performstransformer restrictions the (b )basic voltage and functions it is U possibleGAD of theto supply powerbetween PRssupply, electrodes without i.e. :of separateenables GAD reactor initial ignition in ionization argon circuits, at a theof as inter wellprimary- aselectrode withvoltage two-electrode spaces,of 90V, [44] ignition. and of single-electrode the discharge ignition systems. In all analyzed cases, the tested five ﬁve-column-column transformer design performs the basic functions of the power supply, i.e.,:i.e.: enables enables initial initial ionization ionization of of inter inter-electrode-electrode spaces, ignition of the discharge Appl. Sci. 2020, 10, 3242 9 of 25

Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 26 and its maintenance during the PR operation cycle, limiting the short-circuit current while ensuring and its maintenance during the PR operation cycle, limiting the short-circuit current while ensuring the correct current value for plasma processes. the correct current value for plasma processes. As an example of GAD plasma application, an installation of the exhaust gas purification from As an example of GAD plasma application, an installation of the exhaust gas purification from paint shop is presented [5,53]. GAD plasma reactor construction shown in Figure8a consisted of one paint shop is presented [5,53]. GAD plasma reactor construction shown in Figure 8a consisted of one central ignition electrode and three working electrodes placed around the center of the discharge central ignition electrode and three working electrodes placed around the center of the discharge chamber in 2π/3 radians distance from each other. Technical specification of GAD reactor is given in chamber in /3 radians distance from each other. Technical specification of GAD reactor is given in Table1. Table 1. Table 1. Technical specification of GAD reactor [53]. Table 1. Technical specification of GAD reactor [53]. Height of Reaction Chamber 500 mm Height of Reaction Chamber 500 mm Diameter of reaction chamber 114 mm DiameterIgnition of electrode reaction material chamber Tungsten114 mm 1 mm WorkingIgnition electrodeselectrode material acidT proofungsten steel 1 0H18Nmm NumberWorking of electrodes working electrodes material acid proof 3steel 0H18N NumberLength ofof workingworking electrodes electrodes 2103 mm Distance betweenLength electrodes of working in the ignitionelectrodes/quenching zones 5mm210/50mm mm 3 Distance between electrodesGas flow in the rate ignition/ quenching zones (0.5–20)5mm/50mm m /h Gas flow rate (0,5-20) m3/h PlasmaPlasma burn-up burn-up ofof gasesgases emittedemitted fromfrom factory was tested tested using using pilot pilot-plant-plant set set-up-up installed installed in in foundry’sfoundry's production production line. line. GasesGases pollutedpolluted during painting painting and and drying drying of of foundry foundry molds molds were were mixed mixed withwith hot hot air air and and emitted emitted to to the the atmosphere atmosphere via via three three chimneys. chimneys. Average Average flow flow rate rate of gasof g wasas was 2000 2000 m3/ h, temperaturem3/h, temperature at the outlet at the wasoutlet 150 was degree 150 degree centigrade. centigrade Main. pollutantsMain pollutants were heptane,were heptane, toluene, toluene, xylene, butanol,xylene, terpentine,butanol, terpentine, and ethyl and acetate. ethyl Inacetate. Table 2In, laboratory Table 2, laboratory scale experimental scale experimental results are results presented are forpresented propane for and propane butane and as pollutants. butane as pollutan The ratiots. of The pollutants ratio of pollutants varied in dependencevaried in dependence on the used on paint.the Molds’used paint.drying Molds' system drying with inlets system (I), withoutlets inlets (O), (I), and outlets GAD PR (O) placed, and GAD on one PR of placed the output on one chimneys of the is presentedoutput chimney in Figures is8 b.presented in Figure 8b.

(a) (b)

FigureFigure 8. 8.( a()a) GAD GAD plasma plasma reactor;reactor; ((bb)) dryingdrying systemsystem in the the paint paint shop, shop, (I1, (I1, I2, I2, I3) I3)—hot—hot air air inlets, inlets, (O1, (O1, O2,O2 O3)—polluted ,O3)—polluted air air outlets. outlets.

PaintedPainted molds molds were were consequentlyconsequently transported via via primary primary drying drying unit unit to to main main drying drying tunnel, tunnel, where hot air was dosed through the I1-I3 inlets. Polluted air from the main tunnel was evacuated where hot air was dosed through the I1-I3 inlets. Polluted air from the main tunnel was evacuated through the chimneys O1-O3. The pilot plant plasma treatment system was installed on the O1 outlet. through the chimneys O1-O3. The pilot plant plasma treatment system was installed on the O1 outlet. InvestigatedInvestigated gas gas consisted consisted of approximately of approximately 13% 13% ethyl et acetate,hyl acetate, 10% 10%benzene, benzene 65%, heptane, 65% heptane, 6% toluene, 6% 1%toluene, 1-butanol 1% 1 and-butanol less than and 0.5less % than of other 0.5 % chemicalof other chemical compounds compounds on average on average (Table3 ).(Table 3). AsAs it it can can be be seen seen from from TablesTables2 2and and3 ,3, achieved achieved treatment treatment eefficiencyfficiency and and percentage percentage pollutants pollutants content, both at laboratory conditions and in the pilot plant plasma treatment system were quite content, both at laboratory conditions and in the pilot plant plasma treatment system were quite satisfactory and it tended to decrease with relatively low ranging from 4 to 28 kWh for 1 kg of satisfactory and it tended to decrease with relatively low ranging from 4 to 28 kWh for 1 kg of mineralized hydrocarbons (Table 2). Pollutants, after 30 minutes treatment time, were partly Appl. Sci. 2020, 10, 3242 10 of 25 mineralized hydrocarbons (Table2). Pollutants, after 30 min treatment time, were partly removed from the exhaust gas stream enabling hot air recovery in the future. The energetic efficiency of the process was considered sufficient and in a good accordance with literature data [54]. Pollutants’ concentration was much below the ignition level, which assured the safety of applied solution (Table3). No signs of electrodes’ corrosion were noticed during the treatment procedure.

Table 2. Laboratory experimental results [53].

PROPANE BUTANE Initial concentration (volume) % 1.26 0.48 0.85 0.345 Abatement level % 54 22 47 19 Flow rate m3 /h 74 88 75 87 Active power kW 4.3 3.8 4.2 4.1 Gas temperature (inlet) ◦C 25 22 28 28 Gas temperature (outlet) ◦C 410 188 400 210 SEI kWh/m3 0.057 0.044 0.056 0.047 SER kWh/kg 4.3 21 5.4 28

Table 3. Gas chromatography results of volatile organic compound (VOC) treatment [53].

Measurement 1 Measurement 2 at Air Mixture Velocity 4 m/s at Air Mixture Velocity 6m/s Chemical Retention Amount Retention Amount Compound Time of Pollutants % Time of Pollutants % before after before after min min Treatment Treatment Treatment Treatment methyl ethyl ketone 13.78 0.38 7.38 13.749 0.28 4.70 ethyl acetate 14.85 13.39 9.20 14.828 13.37 8.96 1-butanol 18.47 1.09 2.14 18.462 1.06 1.20 benzene 21.12 10.25 9.90 21.118 10.15 10.47 heptane 21.33 66.36 62.0 21.338 68.10 65.8 toluene 21.97 6.32 5.70 21.976 6.59 6.24 butyl acetate 23.32 0.23 0.09 23.325 0.29 0.06 ethyl benzene 23.62 0.23 0.16 23.621 0.25 0.20 xylene 24.44 0.58 0.44 24.446 0.61 0.53

4. Power Electronics Supply Systems for NTPR The use of static semiconductor voltage and frequency converters to NTPR is becoming more common and promising because of the enormous and ongoing progress in the field of semiconductor technologies, especially high power. There are many, based on power electronics, solutions of NTPR supply systems. Plasma reactors with DBDs, surface and co-planar discharges, atmospheric pressure plasma jets (APPJ) are often energized from frequency resonant converters, RF and pulse power systems, flyback and forward converters. Plasma reactors with arc discharges can be supplied with both direct and alternating voltage and pulse voltage [43,55]. Block diagrams of two basic constructions of DC power system of arc plasma reactors are presented in Figure9. They consist of a bridge controlled on the secondary side of the matching transformer—Figure9a or an AC controller on the primary side of the matching transformer—Figure9b. Other solutions of DC arc power supply systems are modifications of the above mentioned structures. Among the alternate current (AC) power supply system’s the AC/DC/AC are used to energize three and multielectrode arc plasma reactor. Highly efficient AC/DC/AC converters are more and more commonly applied in numerous applications of the plasma-chemical process. They enable adjustment of the voltage waveform, as well as the current, voltage, power and frequency regulation, for the purpose Appl. Sci. 2020, 10, 3242 11 of 25 Appl. Sci. 2020,, 10,, xx FORFOR PEERPEER REVIEWREVIEW 11 of 26 regulation,of a given plasma for theprocess. purpose Block of a given diagram plasma of sixth process. electrode Block GAD diagram power of supply sixth electrode system is GAD presented power in supplyFigure 10system[43,55 is]. presented in Figure 10 [43,55].

(a)

(b)

Figure 9. DCDC power power systems systems of of arc arc discisc dischargeharge reactor reactor (ADR) (ADR) with with::: ((a) (Aa) controlled A controlled bridge bridge (R) on (R) the on secondarythe secondary side side of the of transformer the transformer (Tr); (Tr); (b) with (b) with an AC an ACcontroller controller (I) on (I) the on primarythe primary side side of the of transformerthe transformer...

Figure 10. BlockBlock diagram diagram of of AC/DC/AC AC/DC/AC power power supply supply systems systems for for sixth sixth electrode electrode GAD GAD r reactor,eactor, where L—lineL—linelinechoke; choke; choke; F—controlled F F—controlled rectifier, rectifier, C—filters C—filters/DC/DC bus; bus; T—converters T—converters with withthe ability the ability to adjust to adjustthe off setthe betweenoffset between the output the output signals, signals, Tr—boosting Tr—boosting transformer; transformer PC—capacitors;; PCPC—capacitors pre-charging pre-charging system; system;PS—protection PS—protection system insystem disconnected in disconnected states; M—microcontroller states; M—microcontroller [55]. [55]..

The block diagram of the pulse power supply of a two-electrodetwo-electrode micro-glidingmicro-gliding arc discharge µ reactor ( GAD), in in which to to realize realize the main main converter converter a a symmetrical symmetrical push push-pull-pull topology topology was was used, used, is presented in Figure 1111..

Figure 11. BlockBlock diagram diagram of of the the pulse pulse powe powerr supply as a functional block chain chain...

The block block marked marked in in Figure Figure 1 111 as as EMI EMI (1) (1) is isresponsible responsible for for the the filtration filtration of voltages of voltages enterin enteringg the rectifier,the rectifier, marked marked as AC/ as ACDC/DC block block (2) (2)... TheThe The PFCPFC PFC blockblock block (3) (3) soothes soothes the the ripple rippless coming coming from from the the rectifier, rectifier, improves the the power factor factor,,, and provides a resourceresource ofof quicklyquickly availableavailable directdirect current.current. The block marked as PWM (4) shapes the alternating voltage, usually rectangular with with the the desired frequency and dutyduty cycle,cycle, while while the the last last block block marked marked AC AC/AC/AC (5)is (5) a highis a high frequency frequency pulse transformer.pulse transformer. The design The designof the switching of the switching power power supply supply for the for NTP the reactor NTP reactor is based is onbased this on standard this standard block diagram,block diagram, and is andpresented is presented in more in detail more indetail the Sectionin the section4.2, while 4.2, the while selected the selected problems problems of designing of designing such power such powersupplies, supplie especiallys, especially high frequency high frequency transformers transformers are presented are presented in chapter in chapter 5. 5. Typical resonant converter topology to energize DBD plasma reactor reactor is presented in in Figure 12 and is described in more detail in the Sectionsection 4.3 together with the example of application. Appl. Sci. 2020, 10, 3242 12 of 25

Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 26

FigureFigure 12. ResonantResonant converter converter topology topology for for DBD DBD plasma plasma reactor reactor [ 5566]]..

The block diagram of thethe RFRF radioradio frequencyfrequency power power supply supply system system of of the the APPJ APPJ plasma plasma reactor reactor is isshown shown in in Figure Figure 13 1 and3 and example example of of its its application application is presentedis presented in in Section section 4.4 4.4. .

FigureFigure 13.13. Block diagramdiagram of of the the RF-frequency RF-frequency power power supply supply of atmospheric of atmospheric pressure pressure plasma plasma jets (APPJ). jets (APPJ). 4.1. AC/DC/AC PSS for a GAD Plasma Reactor 4.1. AC/DC/AC PSS for a GAD Plasma Reactor 4.1. AC/DC/ACIdea of the PSS AC /forDC a/ ACGAD inverter Plasma isReactor presented in Figure 14. System can operate both as a current andIdea voltage of the source. AC/DC/AC At the inverter beginning, is presented plasma in reactor Figure should 14. System be supplied can operate from both voltage as a current source. andCurrent-voltage voltage source. characteristic At the beginning, of the plasma plasma reactor reactor changes should considerably be supplied during from its voltage operation source. from Currentthe ignition-voltage to the characteristic extinction ofof thethe discharge.plasma reactor Discharge changes development considerably from during short its arc,operation in the from zone theof ignition, ignition toto the glowextinction discharge of the requires discharge. from Discharge the supply development to follow-up from discharge short arc, voltage in the increasezone of ignition,proportional to the to glow the discharge discharge length. requires At from the thesteady-state supply to operation, follow-up voltage discharge changes voltage depend increase on proportionalthe processing to gasthe parameters,discharge length. while At current the steady should-state be keptoperation, at constant voltage value, changes as to depend ensure plasmaon the processingto be in non-thermal gas parameters, and non-equilibrium while current shou state.ld be In kept general, at constant the current value, inverter as to ensure is energized plasma to from be incurrent non-thermal source, whichand non is- realizedequilibrium with state. the series In general, impedance the current coil attached inverter between is energized DC voltage from current source source,and inverter which input. is realized Output with current the series has the impedance rectangular coil waveform attached whilebetween voltage DC voltage waveform source depends and invonerter the load input. properties. Output Usually,current has capacitors the rectangular are connected waveform in parallel while to voltage the load waveform of the current depends inverter. on theFrom load the properties. discharge pointUsually, of viewcapacitors capacitance are connected connected in parallel in parallel to the to the load plasma of the reactor current electrodes inverter. Fromcan significantly the discharge change point its of characteristics: view capacitance higher connected capacitance in par canallel decrease to the plasma the inter-electrode reactor electrodes voltage canand impairsignificantly the thermal change conditions its characteristics of the GAD: higher [55]. capacitance can decrease the inter-electrode voltageIn theand case impair the inverterthe thermal operation conditions as a current of the GAD source, [5 the5]. output voltage of the supply in no-load is of rectangular form and output current waveform is forced as sine independently on the load character by means of pulse-width voltage modulation (PWM). Since, the GAD plasma reactor’s current has practically sine waveform at the whole range of operation parameters, while voltage waveform is distorted considerably—Figure 15a, therefore it is easier to control the power to the discharge through current value changes than voltage. Moreover, the distorted discharge current, as it is when GAD PR is energized from the sine voltage source—Figure 15a, could faultily inﬂuence the control circuits of the inverter that may possibly result in the damage of its electronic modules. Courses of discharge voltage and current in the reactor energized from the AC/DC/AC inverter operating as a current source and a voltage source are shown in Figure 15a,b respectively. Strong voltage distortion is observed, while discharge current is sine as it is programmable forced in the supplier—Figure 15a. Appl. Sci. 2020, 10, x FOR PEER REVIEW 13 of 26

Appl. Sci. 2020, 10, 3242 13 of 25 Appl. Sci. 2020, 10, x FOR PEER REVIEW 13 of 26

Figure 14. Scheme connection of the AC/DC/AC power supply system of the sixth electrode GAD rector for decomposition of volatile organic compounds (VOCs) emitted during industrial processes [55].

In the case the inverter operation as a current source, the output voltage of the supply in no-load is of rectangular form and output current waveform is forced as sine independently on the load character by means of pulse-width voltage modulation (PWM). Since, the GAD plasma reactor’s current has practically sine waveform at the whole range of operation parameters, while voltageFigure waveform 14. SchemeScheme is connectiondistortedconnection considerably of of the the AC AC/DC/AC/DC/ACFigure power power 15 supplya, supplytherefore system system it of is the easierof sixththe sixthto electrode control electrode GAD the rectorpowerGAD to the rectordischargefor decomposition for decomposition through of current volatile of volatile value organic changesorganic compounds compounds than (VOCs) voltage. (VOCs) emitted emitted during during industrial industrial processes processes [55]. [5 5].

(a) (b)

FigureFigure 15. 15.Discharge Discharge voltagevoltage (blue(blue signal)signal) andand currentcurrent (yellow signal) waveforms while while energized energized formform AC AC/DC/AC/DC/AC inverter: inverter: ((aa)) InIn thethe currentcurrent sourcesource regime—currentregimecurrent is is sine; sine; ( (bb)) in in the the voltage voltage source source regime—voltageregimevoltage is sine; process gas gas—ArgonArgon at at flow ﬂow rate rate 2m 23 m/h.3 /h.

4.2. Micro-GlidingMoreover, the Arc distorted Discharge discharge (µGAD) Powercurrent, Supply as it Systemis when with GAD Push-Pull PR is energized Inverter from the sine voltageTwo source electrode—Figure micro-gliding 15a, couldarc faultily discharges influence (µGADs) the control have circuits found ofrecently the inverter applications that may for possibly result in the damage of its electronic modules. Courses of discharge voltage and current in agricultural and biomedical solutions [24–29]. They can be used for treatment and bactericidal the reactor energized from the AC/DC/AC inverter operating as a current source and a voltage decontamination of surfaces(a) not resistant to high temperatures, like TEFLON,(b) paper or materials for source are shown in Figures 15a and b respectively. Strong voltage distortion is observed, while the packagingFigure 15. ofDischarge food products, voltage (blue for seed signal) germination and current and (yellow plants signal) grow waveforms support as while well forenergized biomedical discharge current is sine as it is programmable forced in the supplierFigure 15a. applications,form AC/DC/AC like blood inverter: coagulation, (a) In the wound current healing,source regime sterilizationcurrent ofis sine; tissues (b) andin the medical voltage equipment,source in dermatologyregimevoltage and is even sine; skinprocess cancer gasA therapyrgon at [flow18,19 rate,57 ,2m58].3/h. In the Figure 16 results of ﬁnal temperature measurements on the surface of the sample with E-coli dropletsMoreover, in the functionthe distorted of gas discharge composition current, are presented.as it is when Evidently, GAD PR the isE-coli energizedbacteria from colony the sine was voltageexterminated source because—Figure of 15 thea, plasma could faultilyprocess, influence but not the the heating control process circuits [59 of–61 the]. inverter that may possibly result in the damage of its electronic modules. Courses of discharge voltage and current in the reactor energized from the AC/DC/AC inverter operating as a current source and a voltage source are shown in Figures 15a and b respectively. Strong voltage distortion is observed, while discharge current is sine as it is programmable forced in the supplierFigure 15a. Appl. Sci. 2020, 10, x FOR PEER REVIEW 14 of 26

4.2.Appl. Micro Sci. -20G20liding, 10, x AFORrc PEERDischar REVIEWge ( GAD) Power Supply System with Push-Pull Inverter 14 of 26 Two electrode micro-gliding arc discharges (GADs) have found recently applications for 4.2. Micro-Gliding Arc Discharge (GAD) Power Supply System with Push-Pull Inverter agricultural and biomedical solutions [24−29]. They can be used for treatment and bactericidal decontaminationTwo electrode of surf microaces- glidingnot resistant arc discharges to high temperatures, (GADs) have like found TEFLON, recently paper applications or materials for for theagricultural packaging and of biomedical food products solutions, for seed [24−29 germination]. They can be and used plants for treatment grow support and bactericidalas well for biomedicaldecontamination applications, of surf likeaces blood not resistant coagulation to high, wound temperatures, healing like , sterilization TEFLON, paperof tissues or materials and medical for equipmentthe packaging, in dermatology of food products and even, for skin seed cancer germination therapy [ 1 and8, 19 ] plants, [57,5 8 grow]. support as well for biomedicalIn the Figure applications, 16 results like of blood final coagulation temperature, wound measurements healing , sterilizationon the surface of tissues of the and sample medical with E-coliequipment droplets, in in dermatology the function and of gas even composition skin cancer aretherapy presented. [18, 19 ]E, viden[57,58]tly,. the E-coli bacteria colony Appl. Sci.In2020 the, 10Figure, 3242 16 results of final temperature measurements on the surface of the sample with14 of 25 was exterminated because of the plasma process, but not the heating process [59,60,61]. E-coli droplets in the function of gas composition are presented. Evidently, the E-coli bacteria colony

was exterminated because of the plasma process, but not the heating process [59,60,61].

(a) (b) (a) (b) FigureFigure 16. 16. ((aa)) Final Final temperature temperature in in degrees degrees centigrade centigrade on on the the surface surface of of the the sample sample in in function function of of gas gas Figure 16. (a) Final temperature in degrees centigrade on the surface of the sample in function of gas compositioncomposition—N—N2,2,O O22,, an andd CO2 CO2 and their mixtures mixtures—,—, (b) (b) EE-coli-coli bacteriabacteria samples samples treated treated by by µGADs,GADs, distancedistancecomposition between between—N2, the the O2 top , an of ofd electrodes electrodesCO2 and their and mixturesthe sample —, 21 (b) mm E-coli [5 [59 9bacteria]].. samples treated by GADs, distance between the top of electrodes and the sample 21 mm [59]. PowerPower suppliers suppliers with with p push-pullush-pull topology topology are are well well-known,-known, but but their their uses uses for for supplying plasma plasma reactorsreactorsPower are relatively suppliers rarerare with [[62 62,63push,63].-].pull TheThe topology supplysupply shown shownare well on on-known, Figure Figure 17but 17 owes theirowes its uses its beneficial beneficial for supplying properties properties plasma to itsto itstopology.reactors topology. Itare has relatively It also has the also rare special the [62,63 special feature]. The of featuresupply not having shown of not the on having switching Figure the17 overvoltagesowes switching its beneficial overvoltages totally pro extinguished,perties totally to extinguished,asits it happens topology. inas typical Itit happens has push-pull also in the typical special inverters. push feature- Thepull overvoltageinverters. of not having The peaks overvoltage the that switching are dangerous peaks overvoltages that for are the dangerous transistor totally forareextinguished, the the transistor only ones as toare it behappens the cut only off inand ones typical the toremaining be push cut- pulloff ones andinverters. the move remaining The to the overvoltage secondary ones move peaks coil. to Thethat the correctly aresecondary dangerous shaped coil. for the transistor are the only ones to be cut off and the remaining ones move to the secondary coil. Theovervoltages correctly areshaped used overvoltages for improvement are used of ignition for improvement in GAD reactor of ignition [45,64 in]. GAD reactor [45,64]. The correctly shaped overvoltages are used for improvement of ignition in GAD reactor [45,64]. TheThe power power supply supply with with the the block block diagram diagram shown shown in Figure in Figure 11 has 11 a modularhas a modular structure, structure, whose The power supply with the block diagram shown in Figure 11 has a modular structure, whose mutualwhose mutuallocation locationis shown isin shownFigure 1 in7a. Figure Development 17a. Development of the discharge of theis presented discharge in is Figure presented 17b. As in mutual location is shown in Figure 17a. Development of the discharge is presented in Figure 17b. As canFigure be seen17b. from As can Figure be seens 17 fromand 1 Figures8, the AC/DC 17 and bus18, the(1) consists AC/DC busof two (1) components: consists of two a rectifier components: with can be seen from Figures 17 and 18, the AC/DC bus (1) consists of two components: a rectifier with protectivea rectifier withelements protective and elementsa filter, which and a filter,is equipped which iswith equipped a DC withside acapacit DC sideor capacitorpre-charging pre-charging system, protective elements and a filter, which is equipped with a DC side capacitor pre-charging system, twinsystem, current twin control current modules control modules on the primary on the primary side (2) side contain (2) contain IGBTs IGBTs with special with special drivers drivers in their in twin current control modules on the primary side (2) contain IGBTs with special drivers in their structure.theirstructure. structure. Their Their Theirinputs inputs inputs first first of firstof which which of which has has a a has0°0° phasephase a 0◦ phase and the and second the second oneone 180°. 180°. one 180◦.

(a(a) ) (b(b) )

FigureFigureFigure 17. 17. ((a a()a) Block) Block Block diagram diagram diagram of ofof the the the of of of NT NTPNTPP power power supply supply for for 2 2 2 electrode electrode electrode GAD GAD GAD reactor, reactor, reactor, based based based on on on pushpush—pullpush–pull–pull topology, topology,topology, 1 1—AC—1—AC/DCAC/DC/DC bus bus with with with,, ,2 2 2—twin——twintwin currentcurrent current control control modules,modules, modules, 3 3— 3—PWM—PWMPWM control control control system, system, system, 4—auxiliary DC voltage, 5—HV and HF transformer with divided primary winding, (b) development of the µGAD in air at ﬂow rate 450 dm3/h—above photo and below image recorded with a fast speed camera [64].

4—auxiliary DC voltage, 5—HV and HF transformer with divided primary winding, (b) development of the GAD in air at flow rate 450 dm3/h—above photo and below image recorded with a fast speed camera [64].

They constitute links of the power path, and at their output alternating voltages VAC’ and VAC” are obtained with the frequency and duty cycle values set by the PWM control system (3). PWM controllers are implemented as two-output astable generators. Depending on the set value, pulse width modulation, and dead times, they perform the function of frequency control and soft start function.Appl. Sci. 2020 This, 10 module, 3242 also has an error amplifier used in the feedback loop. With its help, the signal15 of 25 di can be used to influence the PWM value and control the primary current of the high frequency transformer.is an auxiliary Block DC 4 voltage is an auxiliary source. DC Mains voltage voltage source. is applied Mains tovoltage its input is applied and the to output its input receives and the two output receives two stabilized direct voltages VDC and VDC* galvanically separated from the network stabilized direct voltages VDC and VDC* galvanically separated from the network voltage as well as voltage as well as from each other. The high-frequency transformer (5) receives on the primary side from each other. The high-frequency transformer (5) receives on the primary side two counter-current two counter-current alternating voltages, which transformed to the secondary side as alternating HV alternating voltages, which transformed to the secondary side as alternating HV supply the NTP reactor. supply the NTP reactor. In the middle tap of the transformer, the primary winding has a current transducer LEM (Figure 18), In the middle tap of the transformer, the primary winding has a current transducer LEM which is used to scale the instantaneous value of the primary current to the differential di signal. (Figure 18), which is used to scale the instantaneous value of the primary current to the differential The signals marked as arrows in the Figure 17a do not symbolize electrical connections, but only di signal. The signals marked as arrows in the Figure 17a do not symbolize electrical connections, demonstrate the path of energy conversion and control relationships established with the blocks. As it but only demonstrate the path of energy conversion and control relationships established with the is mentioned, the important feature of the proposed push-pull pulse power supply is periodically blocks. As it is mentioned, the important feature of the proposed push-pull pulse power supply is obtained switching overvoltages (SO) that can be used for the GAD ignition improvement. Phenomenon periodically obtained switching overvoltages (SO) that can be used for the GAD ignition is based on the parasitic parameters. improvement. Phenomenon is based on the parasitic parameters. The fast-changing current in the coils representing leakage inductances and its derivative causes The fast-changing current in the coils representing leakage inductances and its derivative causesthe energy the pulsations energy pulsations and the formation and the of formation a flow that of can a be flow called that commutation can be called overvoltage. commutation In their overvoltage.shaping to the In formtheir ofshaping dampened to the resonance, form of dampened the RLC elementsresonance, in the emitterRLC elements collector in circuitthe emitter (R5, C2 collectorand R6, circuit C3) in (R5, Figure C2 18and contribute R6, C3) in a Figure lot. Basically, 18 contribute they shapea lot. Basically, the course they of voltageshape the in course fluctuation. of voltageThe switching in fluctuation. overvoltages The switching are transformed overvoltages to the secondary are transformed side and to increase the secondary the basic side secondary and increasevoltage tothe the basic level secondary ensuring voltage ignition toin the the level GAD ensuring reactor ignition [64,65]. in the GAD reactor [64, 65].

Figure 18. Simpliﬁed diagram of high-voltage pulse power supply of two electrode µGAD plasma Figure 18. Simplified diagram of high-voltage pulse power supply of two electrode GAD plasma reactor [64]. reactor [64]. The switching overvoltages ignition improvement (SOII) simpliﬁes the structure of a matching transformer, since the turn ratio can be of a lower value. Figure 19a depicts the output voltage waveform obtained in a PSPICE environment simulation of the system presented in Figure 18. The appearing overvoltages (Figure 19) can be observed on the full secondary winding, but the observation is not very precise because of the parasitic capacitances existing on the secondary side of the HV transformer. Therefore they were recorded for 1 turn of the secondary winding in the no-load state. In the voltage waveform a boost by more than 60% is observable—Figure 19b. This is a really high improvement due to ignition reasons. Chosen issues connected with the design of HV and HF matching transformer for a push-pull pulse power supply are presented in chapter 5 [45,64]. Appl. Sci. 2020, 10, x FOR PEER REVIEW 16 of 26

The switching overvoltages ignition improvement (SOII) simplifies the structure of a matching transformer, since the turn ratio can be of a lower value. Figure 19a depicts the output voltage Appl. Sci. 2020, 10, 3242 16 of 25 waveform obtained in a PSPICE environment simulation of the system presented in Figure 18.

(a) (b)

Figure 19. (a)(a) Output voltage waveform obtained in a PSPICE environment simulation, (b) same voltage waveform (signal 3) measured in 1 turn of the secondary sideside ofof thethe HVHV transformer.transformer.

4.3. DBDsThe appearing Power System overvoltages with Frequency (Figure Resonant 19) can Inverter be observed on the full secondary winding, but the observationElectrical is parametersnot very precise of the because DBD reactor of the depend parasitic on physical,capacitances chemical existing and on technological the secondary conditions side of theof the HV non-thermal transformer plasma. Therefore processes. they were These recorded conditions for are 1 type turn of of gas,the its secondary composition, winding electrode’s in the nomaterial-load state. and contaminations, In the voltage waveform temperature, a boost humidity, by more flow than rate, 60% and is all observable of them have—Figure an eff ect19b. on This reactor is a performancereally high im andprovem ignitionent due voltage to ignition value. reasons From technological. Chosen issues point connected of view, with it is importantthe design to of have HV andthe overallHF matching system transformer efficiency as for high a push as possible.-pull pulse The power use of supply electrical are resonance presented allows in chapter higher 5 [45 power,64]. supply efficiency and higher power density factor of the power electronic converter. Capacitive load, 4.3. DBDs Power System with Frequency Resonant Inverter as the DBD reactor represents, naturally creates a freely oscillating resonant circuit with the inverter outputElectrical inductive parameters elements. of Changes the DBD in the reactor capacity depend of the on reactor physical, substantially chemical a andffect technological the resonant conditionsfrequency ofof the the resonant non-thermal circuit plasma and, as proces a resultses. on These the operation conditions of the are inverter type of [ 36gas,,56 ].its As composition, an example electrode’sthe diagonal material bridge and resonant contaminations, converter fortemperature, an excitation humidity, of DBD flow reactor rate, used and inall theof them diesel have engine an effectexhaust on treatment reactor performance system has been and presented. ignition voltage Figure value. 20a shows From diagonal technological bridge point resonant of view, converter it is impconstructionortant to have and coursesthe overall of currentsystem andefficiency voltage as ofhigh DBD as possible. reactor energized The use of from electrical this systemresonance (b). allowsDBD reactor higher construction power supply for diesel efficiency engine and exhaust higher treatment power density is shown factor in Figure of the 21 a. power electronic converter.The overall Capacitive efficiency load, of asthe the power DBD supply reactor system, represents, including natur theally effi ciency creates of a resonant freely oscillating converter resonantand the high circuit voltage with transformer,the inverter wasoutput 68%—Figure inductive 21elements.b. Resonant Changes power in supplythe capacity system of topology the reactor for substantiallyDBD reactors affect allows the continuous resonant frequency resonant frequencyof the resonant tracking circuit and and, operation as a result in reactor on the short–circuitoperation of thestate. inverter An additional [36,56]. As benefit an example is the property the diagonal of transistor bridge synchronizationresonant converter with for the an resonantexcitation capacitor of DBD voltagereactorAppl. Sci. used instead 2020 ,in 10 , of thex FOR the diesel widelyPEER engineREVIEW used exhaust method treatment of distorted system reactor has current been waveformpresented.synchronization. Figure 20a shows17 of 26 diagonal bridge resonant converter construction and courses of current and voltage of DBD reactor energized from this system (b). DBD reactor construction for diesel engine exhaust treatment is shown in Figure 21a.

(a) (b)

FigureFigure 20. 20. ((aa)) DiagonalDiagonal bridge resonant resonant converter converter for for DBD DBD reactor reactor excitation excitation,, (b) ( bDBD) DBD current current and andvol voltagetage waveforms waveforms [56 []56. ].

The overall efficiency of the power supply system, including the efficiency of resonant converter and the high voltage transformer, was 68%—Figure 21b. Resonant power supply system topology for DBD reactors allows continuous resonant frequency tracking and operation in reactor short–circuit state. An additional benefit is the property of transistor synchronization with the resonant capacitor voltage instead of the widely used method of distorted reactor current waveform synchronization.

(a) (b)

Figure 21. DBD reactor construction for diesel engine exhaust treatment (b) results of efficiency measurements of power electronic diagonal bridge resonant converter [36,56].

4.4. RF Power Supply System of APPJ APPJ belongs to a group of discharge plasma reactors whose demands for power supply system are very special, mainly because of the strong sensitivity of the V-I characteristics of power supply to changes of the receiver electrical parameters and necessity to limit value of current, to ensure a non-thermal plasma generation in the working space of APPJ [60−63[ [66,67] [68-75]. The RF power supply should provide the appropriate voltage and power necessary for sustaining stable process parameters. Output characteristic of the sine wave generator is similar to the characteristics of the real current source with the characteristic impedance equal to 50 ohms and to maintain the relevant plasma process parameters the impedance of the receiver should be matched. Appl. Sci. 2020, 10, x FOR PEER REVIEW 17 of 26

(a) (b)

Figure 20. (a) Diagonal bridge resonant converter for DBD reactor excitation, (b) DBD current and voltage waveforms [56].

The overall efficiency of the power supply system, including the efficiency of resonant converter and the high voltage transformer, was 68%—Figure 21b. Resonant power supply system topology for DBD reactors allows continuous resonant frequency tracking and operation in reactor short–circuit state. An additional benefit is the property of transistor synchronization with the Appl.resonant Sci. 2020 capacitor, 10, 3242 voltage instead of the widely used method of distorted reactor current waveform17 of 25 synchronization.

(a) (b)

FigureFigure 21. 21.DBD DBD reactor reactor construction construction for for diesel diesel engine engine exhaust exhaust treatment treatment ( b(b)) results results of of e ﬃ efficiencyciency measurementsmeasurements of of power power electronic electronic diagonal diagonal bridge bridge resonant resonant converter converter [36 [,3656,5].6].

4.4.4.4. RF RF Power Power Supply Supply System System of of APPJ APPJ APPJAPPJ belongs belongs to to a groupa group of of discharge discharge plasma plasma reactors reactors whose whose demands demands for for power power supply supply system system areare very very special, special, mainly mainly because because of of the the strongstrong sensitivitysensitivity of the V V-I-I characteristics of of power power supply supply to tochanges changes of of the the receiver receiver electrical electrical parameters parameters and and necessity necessity to to limit limit value value of of current, current, to to ensure a a non-thermalnon-thermal plasma generation inin thethe workingworking spacespace ofof APPJAPPJ [[6060–−6363,[66 [6–6,756].7] [68-75]. TheThe RF RF power power supply supply should should provide provide the the appropriate appropriate voltage voltage and and power power necessary necessary for for sustainingsustaining stable stable process process parameters. parameters. Output Output characteristic characteristic of of the the sine sine wave wave generator generator is is similar similar to to thethe characteristics characteristics of of the the real real current current source source with wi theth the characteristic characteristic impedance impedance equal equal to 50 to ohms 50 ohms and and to maintain the relevant plasma process parameters the impedance of the receiver should be matched. Appl.to Sci. maintain 2020, 10 , x the FOR relevant PEER REVIEW plasma process parameters the impedance of the receiver should18 of 26 be matched.

(a) (b)

Figure 22. 22. ((aa)) APPJ APPJ power power supply supply system system with with separated separated resonant resonant sub-circuit. sub-circuit. (b) Picture (b) P oficture the reactor.of the reactor. The inner rod electrode was powered by a regulated RF supply via impedance matching network (FigureThe 13 inner). RF-frequency rod electrode power was supply powered of APPJ by a has regulated been represented RF supply with via a impedance resonant sub-circuit matching networkdepicted in(Figure Figure 1 223).a. RF The-frequency most stable power operation, supply which of APPJ resulted has in been the lowest represented ratio of with reﬂected a resonant power, subdepending-circuit depicted on the feed in gasFigure type, 22 wasa. The achieved most stable at frequency operation, range which 12–15 resulted MHz [ 60in, 61the,73 lowest,74]. ratio of reflectedRF atmospheric power, depending pressure onplasma the feedjet has gas been type, used was to enhance achieved wettability at frequency of cellulose-based range 12–15 paperMHz [6as0 a−6 perspective1], [73−74]. platform for antibiotic sensitivity tests [69–77]. Helium and argon were the carrier gasesRF for atmospheric oxygen and pressure nitrogen. plasma Goniometric jet has tests been were used performed to enhance for wettability pure water of and cellulose rape seed-based oil paperdroplets. as a RF perspe plasmactive jet allowedplatform to for decrease antibiotic surface sensitivity contact tests angle [6 without9−76]. Helium changes and in otherargon features were the of carriertested material.gases for Figure oxygen 23 andpresents nitrogen. results Goniometric of contact angle tests measurementswere performed in functionfor pure of water treatment and rape time seedfor rapeseed oil droplets. oil 1 sRF after plasma contact jet ofallowed a droplet to decrease with the surface paper surface contact [ 66angle,71,73 wi].thout changes in other features of tested material. Figure 23 presents results of contact angle measurements in function of treatment time for rapeseed oil 1 second after contact of a droplet with the paper surface [66] [71,73].

Figure 23. Contact angle versus treatment time for rapeseed oil (90 g/m2) measured 1 second after a droplet contact with the paper surface, at 3 cm distance between the paper surface and a nozzle of the APPJ [71,73].

5. Selected Issues of Designing Power Systems for NT Plasma Reactors The design of electromagnetic power supplies for plasma reactors with barrier or gliding arc discharge involves, in principle, determining the design parameters of the transformers forming the power supply, either in the form of a magnetic multiplier, a five-column core, an integrated system of four single-phase transformers as well as transformers in the power electronic supplies. The work of these special transformers is based on the use of higher harmonics of the magnetic flux that induce an operating voltage with an increased frequency (in the case of reactors with barrier discharges) or the ignition voltage of discharges, as is the case in the integrated system and in the system with 5 column transformer. Therefore, the choice of material for the cores and the level of magnetic flux density becomes particularly important to ensure a high harmonic value of the magnetic flux. Appl. Sci. 2020, 10, x FOR PEER REVIEW 18 of 26

(a) (b)

Figure 22. (a) APPJ power supply system with separated resonant sub-circuit. (b) Picture of the reactor.

The inner rod electrode was powered by a regulated RF supply via impedance matching network (Figure 13). RF-frequency power supply of APPJ has been represented with a resonant sub-circuit depicted in Figure 22a. The most stable operation, which resulted in the lowest ratio of reflected power, depending on the feed gas type, was achieved at frequency range 12–15 MHz [60−61], [73−74]. RF atmospheric pressure plasma jet has been used to enhance wettability of cellulose-based paper as a perspective platform for antibiotic sensitivity tests [69−76]. Helium and argon were the carrier gases for oxygen and nitrogen. Goniometric tests were performed for pure water and rape seed oil droplets. RF plasma jet allowed to decrease surface contact angle without changes in other Appl.features Sci. 2020 ,of10 ,tested 3242 material. Figure 23 presents results of contact angle measurements in function18 of of 25 treatment time for rapeseed oil 1 second after contact of a droplet with the paper surface [66] [71,73].

FigureFigure 23. 23.Contact Contact angle versus versus treatment treatment time time for rapeseed for rapeseed oil (90oil g/m (902) measured g/m2) measured 1 second after 1 s aftera a dropletdroplet contact contact withwith the paper surface, surface, at at 3 cm 3 cm distance distance between between the paper the paper surface surface and a andnozzle anozzle of the of the APPJ [ [7711,73,73].].

5. Selected5. Selected Issues Issues of of Designing Designing Power Power Systems Systems forfor NT P Plasmalasma R Reactorseactors TheThe design design of of electromagnetic electromagnetic power power suppliessupplies for plasma plasma reactors reactors with with barrier barrier or orgliding gliding arc arc dischargedischarge involves, involves in, in principle, principle, determining the the design design parameters parameters of the of transformers the transformers forming forming the the power supply, either either in in the the form form of of a amagnetic magnetic multiplier multiplier,, a five a five-column-column core, core, an anintegrated integrated system system of fourof four single-phase single-phase transformers transformers as as well well as as transformerstransformers in in the the power power electronic electronic supplies. supplies. The The work work of theseof these special special transformers transformers is is based based on on the the useuse of higher harmonics harmonics of of the the magnetic magnetic flux flux that that induce induce an operating voltage with an increased frequency (in the case of reactors with barrier discharges) or an operating voltage with an increased frequency (in the case of reactors with barrier discharges) the ignition voltage of discharges, as is the case in the integrated system and in the system with 5 or the ignition voltage of discharges, as is the case in the integrated system and in the system with column transformer. Therefore, the choice of material for the cores and the level of magnetic flux 5 columndensity transformer. becomes particularly Therefore, important the choice to ensure of material a high harmonic for the cores value and of the the magnetic level of magneticflux. flux density becomes particularly important to ensure a high harmonic value of the magnetic flux. The content of higher harmonics in the magnetic flux depends not only on the non-linearity of the magnetization characteristics, but also on the non-linear characteristics of the plasma reactor, and in the presence of a high frequency ignition circuit, also on the harmonics induced by this circuit, which can affect the operating voltage waveforms, and indirectly in the shape of a magnetic flux. As it results from the analyzes and tests of the gliding reactor power systems, the transformer core material can significantly change the voltage waveforms in the reactor and the ability of the power supply to carry higher harmonics generated in it (Figure3). The material for transformer cores largely determines frequency processing, power transfer, losses and efficiency of the power supply [44,45]. The core material should meet the requirements of work with significant saturation of magnetic flux density of the magnetic circuit, and the shape of the magnetization characteristics and location of the operating point affect the output power and operating parameters of the power supply. The optimal core material should be characterized by a large value of the third harmonic B3h of magnetic flux density, low magnetizing current and small loss. These criteria are well met by materials with a high rectangularity coefficient, referred to as the ratio of magnetic flux density remanence Br to its saturation Bs. For commonly used electrical power transformer cores, the squareness factor is within 0.85–0.9, which ensures the third harmonic of magnetic induction approximately equal to 21% of the fundamental one. Increasingly, amorphous materials (metallic glasses) are used for transformer magnetic cores, which are characterized by a magnetization curve with high rectangularity and low magnetic flux density saturation. This allows the power supply to work with low losses and low magnetizing current. Currently, amorphous materials are available and their price is not much higher than conventional magnetic materials. GADs burn between electrodes located at a fixed distance and in media with constant pressure. Under these conditions, the discharge voltage practically does not depend on the wide-range of current changes. Therefore, the limitation of the current value on which the discharged power depends must be outside of the discharge. In the case of AC powered plasma reactors, an important issue is the reduction of Appl. Sci. 2020, 10, 3242 19 of 25 non-current interruptions that deionize the discharge space, deteriorating the stability of the plasma reactor and the quality of the plasma generated in it. Also, an important issue in the design of transformer power supplies for GAD plasma reactors discharge is to ensure the required leakage reactance that is defined as related to magnetic flux closing in air gaps and outside the core, linear inductance multiplied by angular frequency. According to previous studies, this leakage reactance should be 30–40% of short-circuit reactance for integrated power supply transformers [43,75,76,78]. Similar leakage reactance values are required in power supplies based on a five-column transformer [44]. This value is higher than in classic power transformers and lower than in arc welding power supply systems. Such a value of leakage reactance is difficult to obtain in constructions of coaxial windings, where it does not exceed 15%, or in structures with separated windings, where it is over 60%. The leakage reactance of the windings supplying the working electrodes of GAD reactor is shaped by appropriate allocation of the primary and secondary windings on the transformer columns. The highest value of leakage reactance is obtained in the case of disk windings. When there is one primary winding disc and one secondary winding disc, the leakage reactance is so high that in conventional transformers it causes an unacceptably high voltage loss. By changing the number of discs, one can change the value of reactance within wide limits while in concentric windings, can influence to some extent the value of leakage reactance by introducing spatial asymmetry of the windings. Implementation of the matching HV and HF transformer to the pulse push-pull powers supply should assume the use of a core material that will be able to transfer frequencies in the range from (10–200) kHz, while frequencies in the range (10–25) kHz transfer most of the energy from the primary to the secondary in the form of a square wave. The ability to distribute high frequency pulses to the secondary side is only maintained because of the support of the reactor ignition by using commutation overvoltages. Many power ferrite powders are able to provide these parameters. The 3C90 core material was chosen for use in the push-pull power supply [45,59,64] whose own resonance frequency allows operation in a transformer up to 0.2 MHz. Impulse transformers made on theAppl. ferrite Sci. 2020 powder’s, 10, x FOR PEER core REVIEW allow achieving magnetic flux density B exceeding 0.3 T.20Moreover, of 26 the transformer for the NTP power supply is not a typical device, because of the high-voltage secondary coil,transforme which requiresr for the special NTP power electrical supply insulation is not a typical strength. device, Additionally, because of the high high voltage-voltage requires secondary the use of a corecoil, which forming requires a large special window electrical in relation insulation to thestrength. small Additionally, cross-sectional high area voltage of therequires column the use to house windings.of a core Primary forming and a large secondary window windings in relation shouldto the small be placed cross-sectional on separate area columns,of the column which to house minimizes windings. Primary and secondary windings should be placed on separate columns, which the risk of high voltages being transferred to the inverter side. Schematic structure of the secondary minimizes the risk of high voltages being transferred to the inverter side. Schematic structure of the windingsecondary of the windinghigh frequency of the hi HVgh transformer frequency HV is depicted transformer in Figureis depicted 24A, inwhile Figure HV 2 transformer4A, while HV picture is presentedtransformer in Figure picture 24 is B[presented45,64]. in Figure 24B [45,64].

(A) (B)

FigureFigure 24. 24.(A )(A Schematic) Schematic structure structure of of the secondary secondary winding winding of of the the high high frequency frequency transformer, transformer, wherewhere a) total a) total windings’ windings’ cross-section cross-section with with the the core, core, b) b) enlarged enlarged section section fragment fragment withwith oneone coil, c) layer layer diagram, where 1—core, 2—carcass 3—insulation coil, 4—winding wire, 5 and 6—interlayer insulation, (B) – picture of the HF transformer [45,64].

Other issues related to the design of transformers for plasma reactor power systems do not have special features.

6. Conclusion Non-thermal and non-equilibrium plasma can be generated by electrical discharges of almost any type. Cold plasma generation in vacuum or high vacuum conditions does not pose significant difficulties, even in high volume discharge chambers, but fabrication such a vacuum is expensive and requires special plasma reactor designs. Hence, there is a need to look for reactors in which cold non-equilibrium plasma can be generated under atmospheric pressure. Promising plasma sources for industrial scale applications are reactors with discharges in the presence of a dielectric barrier, atmospheric pressure plasma jets and gliding arc in multi-electrode systems. In most applications, the design of the discharge chamber of the plasma reactor and its electrodes is not difficult, however, the use of plasma reactors on an industrial scale is determined by its power supply system, which must meet specific requirements, resulting mainly from the non-linearity of the receiver, which is the plasma reactor, working at high voltage and frequency and the need for ignition. Equally important in industrial conditions is the electromagnetic compatibility of plasma reactor supplies, which in the case of power electronic systems with microprocessor control, may require shielding and the use of filters to remove radiated and conducted interference from a non-linear receiver to the power electronic and then to the mains [79,80]. The main element of all plasma reactor power supply presented in the review is the transformer, which in appropriate implementation can meet most of the requirements for plasma power supplies. Owing to the transformer’s cores nonlinearity, simple, reliable, low cost and efficient power systems, especially suitable for industrial applications, can be constructed. Furthermore, transformer-based systems are much more resistant to radiated and conducted Appl. Sci. 2020, 10, 3242 20 of 25

diagram, where 1—core, 2—carcass 3—insulation coil, 4—winding wire, 5 and 6—interlayer insulation, (B)—picture of the HF transformer [45,64].

Other issues related to the design of transformers for plasma reactor power systems do not have special features.

6. Conclusions Non-thermal and non-equilibrium plasma can be generated by electrical discharges of almost any type. Cold plasma generation in vacuum or high vacuum conditions does not pose significant difficulties, even in high volume discharge chambers, but fabrication such a vacuum is expensive and requires special plasma reactor designs. Hence, there is a need to look for reactors in which cold non-equilibrium plasma can be generated under atmospheric pressure. Promising plasma sources for industrial scale applications are reactors with discharges in the presence of a dielectric barrier, atmospheric pressure plasma jets and gliding arc in multi-electrode systems. In most applications, the design of the discharge chamber of the plasma reactor and its electrodes is not difficult, however, the use of plasma reactors on an industrial scale is determined by its power supply system, which must meet specific requirements, resulting mainly from the non-linearity of the receiver, which is the plasma reactor, working at high voltage and frequency and the need for ignition. Equally important in industrial conditions is the electromagnetic compatibility of plasma reactor supplies, which in the case of power electronic systems with microprocessor control, may require shielding and the use of filters to remove radiated and conducted interference from a non-linear receiver to the power electronic and then to the mains [79,80]. The main element of all plasma reactor power supply presented in the review is the transformer, which in appropriate implementation can meet most of the requirements for plasma power supplies. Owing to the transformer’s cores nonlinearity, simple, reliable, low cost and efficient power systems, especially suitable for industrial applications, can be constructed. Furthermore, transformer-based systems are much more resistant to radiated and conducted interference generated by electrical discharges. Some disadvantage of a transformer power supply is its relatively narrow range of the discharge power control. AC/DC/AC inverters can operate both in the sine voltage or sine current regime. In the voltage regime inverter is suitable to supply DBD reactor while the current regime is most suitable to supply GAD reactor. Taking into account the electromagnetic compatibility, power supply system of GAD reactor with nonlinear transformers seems to be the better solution than electronic power inverter. Electronic modules of AC/ DC/AC inverter are sensitive for GADs’ origin interferences, that could propagate both through conduction and radiation and at some conditions can even destroy the supplier [80]. Push-pull topology using IGBT transistors to supply miniature gliding arc reactor allows to obtain the commutation auxiliary ignition in conditions of work with air as the plasma gas. Power supply has a high efficiency of energy processing above 91%, a wide scope of adjustment properties in the range of 13 kHz to 26 kHz and confirmed desired features in cooperation with the µGAD reactor. It guarantees the ignition of the discharge and maintains cyclical operation in a wide range of non-linearity parameters of the arc discharge resistance and generated plasma is very homogeneous both in air and in helium as processing gases [45]. The use of electrical resonance allows for higher power supply efficiency and higher power density factor of the power electronic converter. Resonant power supply system topology for DBD reactors, proposed in [56] allows continuous resonant frequency tracking. Precise identification of the APPJ power system parameters allow constructing efficient systems for the plasma treatment of the heat-sensitive surfaces. Appl. Sci. 2020, 10, 3242 21 of 25

Research on the properties and applications of non-thermal and non-equilibrium plasma at atmospheric pressure requires interdisciplinary cooperation of scientists representing both basic and applied sciences, let us mention: plasma chemistry and physics, biochemistry, microbiology, medicine, chemical technology, environmental engineering, material engineering, agricultural engineering, bioengineering, metrology and electrical engineering. The latter is represented by the author of this review, who, together with a team from the Department of Electrical Engineering and Electrotechnology of the Lublin University of Technology, for over 30 years conducts research on the generation of non-thermal plasma by means of electrical discharges, in particular on the power supply systems of these special energy receivers, in cooperation with the above-mentioned representatives of the basic and applied sciences. The research has resulted in many monographs, publications, doctorates and patents, most of which are cited in this review [9,11–16,20–30,34,36,37,41–45,47–53,55,57–61,65–68,71–82].

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

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