Plasma torch ignition by a half bridge resonant converter ... - IEEE Xplore

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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 27, NO. 4, AUGUST 1999

Plasma Torch Ignition by a Half Bridge Resonant Converter Joel Pacheco-Sotelo, Rosendo Peña-Eguiluz, Leonardo Peña Eguiluz, Armando Segovia de los R´ıos, and German Cota Sánchez

Abstract—In this paper, the design, implementation, and performance of a half bridge resonant converter (HBRC) used as an electronic ignition system for arc plasma torch generation is presented. The significance of the design lies in its simplicity, versatility, and low cost. The system operates as a high voltage supply attached to electrodes before gaseous breakdown and as an open circuit when an electric arc is established. The operation of the HBRC does not interfere with either the electric supply line or the electric measurement devices. Advantages and limitations of the converter are reviewed. The experimental impedance variation in the medium as a function of frequency and some experiences in striking arcs are also presented. Index Terms— Breakdown voltage, current source, electrical discharge, half bridge resonant converter, resonance frequency, spark gap, thermal plasma torch.

I. INTRODUCTION

B

REAKDOWN phenomenon is the last stage in a succession of events that initiates when the impedance between electrodes is reduced by mechanical or electrical techniques and finishes when an arc is produced and self-sustained with an external power supply attached to the electrodes. The high energy density contained in the electric arc can be used in diverse industrial processes like plasma metallurgy [1]–[4], arc lamps [5], high intensity electric arcs [6], [7], chemical compose production [4], [8], [9], and waste destruction [10], [11], etc. In our case, thermal plasma is used in the destruction of dangerous waste [12] with our main objective being to construct a high performance degradation system complying with international regulations. Plasma torch operates in a closed pyrolisis reactor, being impossible to start the arc in a mechanical way. Because of this restriction, it is necessary to have a starter based on external ionization of the electrode gap. Under the action of a uniform electric field at high pressures, above 100 torr, Paschen’s law no longer applies. The breakdown voltage, over limited distances, is given by the formula [13] kV

(1)

Under these circumstances, the breakdown voltage seems but after some to be only a function of the gap distance Manuscript received May 11, 1998; revised April 16, 1999. The authors are with National Institute for Nuclear Research, Mexico City CP 11801, México (e-mail: [email protected]). Publisher Item Identifier S 0093-3813(99)06999-4.

experiences, the breakdown voltage also depends on both the cathode material and the type of gas [14]. When the electric field is greater than the value predicted in (1), other phenomena, like streamers and Corona effects can be formed. The ignition system proposed must react in such a way that these secondary effects should be avoided. II. OUTLINE

OF THE

PHENOMENON

Arcs are difficult to start because they require: 1) a power supply with a high voltage to initiate the phenomenon and 2) large current at low voltage to maintain the plasma arc in steady state. A number of initiating mechanisms have been developed to overcome this problem. The most common way to generate the gaseous breakdown is obtained using high voltage impulses at very high frequency. A conventional method for striking an arc is using a spark gap. This device consists of two conductors and one over-stressed dielectric. The spark gap has a limitation on the time required to recover the electrical strength, which implies that the applied voltage must be removed to recover the open state. A conventional is charged to a circuit is shown in Fig. 1. The capacitor this voltage is transferred to the load relatively low voltage and a pulse transformer The high voltage on the by a breaks the stressed dielectric contained in secondary side of the spark gap and induces a high voltage pulse on the charge to obtain the discharge in the torch. This via transformer capability and hence a limitation on circuit has a limited the generation of narrow pulses (lower than 100 s). Normally, the spark gap frequency has a high level of interference that is injected into the main supply, due to the abrupt variation in the current. This may disturb the performance of control systems and other peripheral devices [15]. To overcome the limitations of the spark gap starter, a half bridge series resonant converter HBRC is proposed. As can be seen in Fig. 2, the converter is supplied by a constant voltage Because ionization is achieved in a short time, it is not necessary to use a permanent voltage source; in this way, is , obtained by a simple circuit composed of the thyristor , and the capacitor As is fired, the resistor is charged through until the voltage is reached (80 V). When this condition is accomplished, the operation of HBRC can be started simply by pressing the push button The HBRC has two MOSFET transistors IRF240, connected oscillates with a in a half-bridge topology. The capacitor A step-up (1 : 50) transformer is ferrite core inductor

0093–3813/99$10.00  1999 IEEE

PACHECO-SOTELO et al.: PLASMA TORCH IGNITION

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Fig. 1. Ionization circuit by spark gaps.

Fig. 2. Half bridge resonant converter circuit.

connected in series and its output is attached to the electrodes through the capacitor To drive the half bridge, a monolithic high voltage and high-speed driver IR2110 was used. The floating bootstrap power supply is provided by a low and a high-voltage diode Each time voltage capacitor node goes low, charges to approximately 15 V the When the upper MOSFET is through the limiting resistor is reverse biased and acts like switched on, the diode an isolated power supply to drive the upper MOSFET. This circuit provides the breakdown voltage needed to create the arc ignition. Once the ionization channel is generated, a low impedance path is created through the electrodes, and the HBRC delivers the current until the gap impedance is low enough to permit the entrance of the high power supply which delivers enough energy to sustain the plasma (Fig. 2). and Taking into account the current mesh labeled as considering only the primary side of transformer, Fig. 3(a) represents the voltage at midpoint of is obtained, where and is the voltage across the primary the HBRC Based on the Fresnel Diagram [16], side of transformer it is possible and depending on the operation frequency to obtain the two different electric behaviors, represented in Fig. 3(b) and (c). According to the electrical diagram, it can

be deduced that (2) Since the resonance frequency is

then, (2) can be written as (3) In the case of a short circuit in the electrodes, the voltage and (3) becomes (4) The amplitude of the short-circuit current is 90 delayed with respect to deduced (where

can be by (5)

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(a)

(b)

(c)

(d)

Fig. 3. (a) Equivalent electric inverter diagram, (b) Fresnel’s diagram for current characteristic.

Fig. 4. Van der Pol diagram transition from open circuit to discharge.

!OP > !RES ; (c) Fresnel’s diagram for !OP < !RES ; and (d) natural limited


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Fig. 5. Half bridge resonant converter probe system.

Fig. 6. Gap impedance variation. TABLE I OPERATIONAL MONTAGES CONDITIONS

Dividing (3) by

VARIABLES

and using (5), it can be shown that (6)

AND

TABLE II ELEMENTS USED

IN THE

HBRC

Defining and (6) represents an ellipse. Fig. 3(d) shows a portion of this ellipse and are greater than zero. when The knowledge of these characteristics allows us to define and the current for a given charge. the voltage value

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Fig. 8. Waveforms of the voltage vDS in the middle point of HBRC (CH1) and of inductor current iL (CH2).

III. EXPERIMENTAL PROCEDURE (a)

(b) Fig. 7. (a) Electric arc between vertical electrodes under flowing gas action. (b) Voltage and current traces showing current source behavior.

Because the gap resistance changes, according to ionization, the maximum current passing through the electrodes which can eventually becomes the short circuit current be evaluated by (5). This expression shows that a natural is obtained as a function of the limitation on the current This relation can be varied from 0.7–1.2 by rate a voltage controlled oscillator. When this ratio has a value of one, a minimal impedance path is obtained between electrodes (resonance condition).

A circuit model was simulated in SPICE1 to demonstrate the transition from the open circuit to discharge. In order to know the HBRC behavior, a Van der Pol diagram (Fig. 4) is utilized. This diagram shows the variation of the state variables present in the resonant circuit. The inductance current and the capacitor voltage give the evolution during the discharge phenomenon. The circular trajectory manifests the energy interchange between the inductor and the capacitor at each period of resonance. The transition from open circuit to the discharge is not abrupt, but very soft, with limited current increments at each cycle, until the discharge is established. Since one tour of the circle represents the time corresponding to the period of working frequency s), then the elapsed time to produce a stable discharge is about 100–125 s. A prototype of a HBRC using MOSFET and the control already described, was constructed to generate the breakdown voltage in a plasma torch, and permits the entrance of a high power electric supply to sustain the thermal plasma. Before the final implementation, two experimental settings were carried out. The first one consists of measuring the voltage between two vertical copper electrodes, where the outputs of HBRC were branched as shown in Fig. 5. Impedance changes were measured experimentally by varying from 0.7–1.2 times the the HBRC operation frequency (40 kHz). A great reduction of the resonant frequency approaches gap impedance was observed when The impedance variations were obtained from the ratio; these values are shown in Fig. 6, where it is possible to see that a minimal gap impedance was obtained close to the resonance frequency, just when the transition from voltage source to current source behavior occurred. A second experimental setup used the cylindrical electrodes of the plasma torch. The plasma torch chamber was filled with argon gas at atmospheric pressure to reduce the hold off dielectric 1 DesignLabTM , Evaluation Software, Version 7.1; MicroSim Corporation, Irvine, CA USA, 1996.


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Fig. 9. Thermal plasma jet.

capability. The ionization was easily obtained (see operation conditions marked in Table I). All the values of the elements used are given in Table II. If a permanent voltage source is used, a high inrush curwould be required. rent to charge the large capacitor filter This problem can be resolved with the circuit used in Fig. 2, which limits the inrush current and charges the capacitor with sufficient energy (voltage thus permitting the HBRC operation during the ionization time. IV. RESULTS Fig. 7(a) represents the electric arc between two vertical copper electrodes under the action of argon gas flowing transversally at 25 lpm of flow rate. Even in this situation, the electric arc is not extinguished; voltage and current traces of the arc are shown to demonstrate the behavior of the HBRC as a current source [Fig. 7(b)]. The oscillogram in Fig. 8 shows the voltage in the middle point of the HBRC in channel CH1 (attenuation 20 and the current in channel CH 2 (attenuation 2 The overshoots normally produced during MOSFET commutation, do not exist and the current oscillates near resonance frequency with a nearsinusoidal waveform. The harmonic content is reduced. The commutation between the ignition system and the principal power supply is automatically achieved by using the electrical diagram described in Fig. 2. Once the high power supply is started, the plasma torch can be maintained in stable condition, as can be observed in Fig. 9.

V. CONCLUSION The electronic system presented in this paper allows the establishing of the ionization path in order to create an electric arc between the torch electrodes, with important advantages over conventional systems. It avoids mechanical adjustments wearing away; it also reduces the electric interference with the electric line supply, and allows the automatic entrance of the plasma power supply once the electric arc is produced. The HBRC presents a natural limitation in case of current surge caused by the arc impedance variations even in the case of electrodes in short circuit. The commutation of the MOSFET transistors is obtained at minimal current and voltage, getting a low power dissipation. For this reason the electronic system can create the ionization path with only one capacitor charge. The behavior of the HBRC changes from voltage source, when the operation frequency is lower than the resonant to a current source at frequency [17]. This is a particular advantage of the HBRC and is very suitable for this application. This system reduces the electrode erosion because it avoids physical contact of electrodes, and there is no need for sophisticated mechanical design for outside reactor ignition. Likewise, in case of plasma torch extinction, the HBRC restores the plasma torch automatically. ACKNOWLEDGMENT The authors are grateful for the assistance of A. Cruz, F. Ramos, and G. Castañeda from the National Institute for

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Nuclear Research. They also acknowledge the observations given by the anonymous referees that led to an improved paper. REFERENCES [1] V. Dembovsky, Plasma metallurgy: The principles, C. Laird, Ed. New York: Elsevier Sci., 1985. [2] M. Boulos, P. Fauchais, and E. Pfender, Thermal Plasmas Fundamentals and Applications. New York: Plenum, 1994, vol. 1, pp. 33–43. [3] F. Kassabji, G. Laroche, M. Lancre, Les Plasmas Dans L’industrie. Paris, France: Elektra-DOPEE, 1991, pp. 109–200. [4] P. Fauchais and A. Vardelle, “Thermal plasmas,” IEEE Trans. Plasma Sci., vol. 25, pp. 1262–1267, 1997. [5] J. J. Damelincourt, D. Lein, M. Skowronek, “Les lamps a` décharge et leur application comme sources de lumière,” in L’arc e´ lectrique et ses Applications. Paris, France: Centre National de la Recherche Scientifique, 1984, tome 2. [6] Pfender, “Electric arcs and arc gas heaters,” in Gaseous Electronics, N. Hirsh and H. Oskam, Eds. New York: Academic, 1978, vol. 1. [7] S. Vacquié, A. Lefort, M. Pitanre, J. Chapelle, “Etude physique de l’arc e´ lectrique,” in L’arc e´ lectrique et ses Applications. Paris, France: Centre National de la Recherche Scientifique, 1984, tome 1. [8] P. Fauchais et J. F. Coudert, “Les transformations physiques et chimiques dans les plasmas thermiques produits par arc e´ lectrique,” in L’arc e´ lectrique et ses Applications. Paris, France: Centre National de la Recherche Scientifique, 1984, tome 2. [9] V. Arrondel, “Hydropyrolyse d’hydorocarbures lourds par torche a plasma,” in Les Plasmas Dans L’industrie. Paris, France: ElektraDOPEE, 1991, pp. 354–362. [10] R. Benocci, G. Bonizzoni, E. Sindoni, “Thermal plasmas for hazardous waste treatment,” in Proc. Int. School Plasma Phys. Singapore, Ed. World Scientific, Sept. 1995. [11] L. Laouilleau, “Destruction de déchets,” in Les Plasmas Dans L’industrie. Paris, France: Elektra-DOPEE, 1991, pp. 320–330. [12] J. Pacheco et al., “DC plasma Torch, Theoretical and experimentals results,” in XXIII Int. Conf. Phenomena Ionized Gases, Toulouse, France, July 1997, vol. II, pp. 120–121. [13] J. R. Roth, Industrial Plasma Engineering. Philadelphia, PA: Inst. Phys. Publ., 1995, vol. 1, pp. 281–282. [14] W. Sarjeant and R. Dollinger, High Power Electronics. Blue Ridge Summit, PA: Tab Books, 1989, pp. 171–197. [15] E. J. Dede, F. Arenas, J. Garcia, J. Gonzalez, and J. Williams, “Starter unit free of interferences for plasma installations,” J. High Temp. Chem. Processes, vol. 1, pp. 543–548, Sept. 1992. [16] M. Metz, B. Escaut et P. Marty, Electrotechnique et e´ lectronique de puissance. Toulouse, France: Inst. National Polytechnique de Toulouse ENSEEITH, 1993, vol. 2. [17] J. Pacheco, “Alimentation de magnétron a` haute stabilité,” Ph.D. dissertation, Institute National Polytechnique de Toulouse ENSEEITH, France, 1993.

Joel Pacheco-Sotelo received the B.Sc. degree in industrial electronics and the M.Sc. degree in power electronics from the Technological Institute of Chihuahua, México, in 1974 and 1983, respectively, and the DEA and Ph.D. degrees in electronics from the National Polytechnique Institute of Toulouse, France in 1993. From 1974 to 1998, he worked as a Scientist in several projects related to electronics at the National Institute of Nuclear Research (ININ). In 1987, he joined the Technological Institute of Toluca, where he is currently Professor of Power Electronics. Since 1995, he has been responsible for the development and applications of the Thermal Plasma Laboratory at ININ.

Rosendo Pena-Eguiluz ˜ received the B.Sc. degree in electronic engineering and the M.Sc. degree in power electronics from the Technological Institute of Toluca, México, in 1992 and 1996, respectively. He is a research member of the Thermal Plasma Applications Laboratory at the National Institute for Nuclear Research, where he has been involved with the development of power supply systems and associated electronic devices for plasma torches and design of microcontroller based systems.

Leonardo Pena ˜ Eguiluz received the B.Sc. degree in electronic engineering from the Technological Institute of Toluca, México, in 1997. He is a member of Quality Control Department of Alcatel-Indetel México, S. A. de C. V., where he has been involved with calibration and the testing of quality control of measuring equipment and production systems.

Armando Segovia de los R´ıos received the electrical and mechanical engineer degree from Technological Institute of Toluca, México, in 1980, the M.Sc. degree in automatic control in 1984 from the Technological Institute of La Laguna, and the Ph.D. degree in control systems from the Technological Institute of Compiègne, France, in 1995. Since 1995, he has been a Researcher in the Automation and Instrumentation Department at the National Institute for Nuclear Research and a Professor-Researcher at the Post Graduate Center of the Technological Institute of Toluca.

German Cota Sánchez received the B.Sc. degree in chemical engineering from the Technological Institute of Los Mochis, Mexico, and the M.Sc. degree from the National University Autonomous of México. He is a research member of the Thermal Plasma Applications Laboratory at the National Institute of Nuclear Research, where he has been involved in the development and research of Thermal Plasma Chemical Processes for the past four years. He has designed and built destruction of hazardous waste equipment by thermal plasma in which he investigated the destruction of different hazardous wastes, energy and matter balances, and efficiency destruction.