Over-voltage Trigger Device for Marx Generators

Transcription

1 Journal of the Korean Physical Society, Vol. 59, No. 6, December 2011, pp Over-voltage Trigger Device for Marx Generators M. Sack, R. Stängle and G. Müller Karlsruhe Institute of Technology (KIT), Institute for Pulsed Power and Microwave Technology, Hermann-von-Helmholtz-Platz 1, Eggenstein-Leopoldshafen, Germany (Received 15 December 2010, in final form 3 October 2011) For treatment of plant tissue with pulsed electric fields in large scale, synchronized operation of several Marx generators is required in order to minimize the low-field regions inside the electroporation reactor and to overcome the current limitations of a single pulse circuit. Hence, the Marx generators must be triggered. If long-term operation without service is to be achieved, these generators are triggered by over-volting the first spark gap without the use of an additional electrode for ignition, which would cause increased wear. A trigger device for over-volting the first spark gap of a Marx generator has been designed. It has been tailored to replace the charging coils between the 1 st and the 2 nd stage of an already existing Marx generator. In order to overcome the requirements for insulation to ground potential, the trigger device is powered by the charging current of the Marx generator. This paper describes some design issues. PACS numbers: Kq, Ek Keywords: Over-voltage trigger, Marx generator DOI: /jkps I. INTRODUCTION For many applications, a Marx generator has to be synchronized to an external event. This is accomplished by triggering. Usual trigger methods like three-electrode spark gaps or trigatrons require a third electrode for ignition. This trigger electrode is subjected to increased wear compared to the main electrodes because the arc is concentrated on the small surface of the ignition electrode. Switching electrodes for Marx generators in industrial applications are designed for long-term repetitive operation and, hence, are equipped with switching electrodes that have a homogeneous-field profile according to Borda or Rogowski [1,2], so the switching arcs are distributed equally across the whole electrode surface, and the electrode is subjected to a homogeneous wear across its surface. In order to avoid the use of a trigger electrode, a fast-rising over-voltage is applied to the spark gap of the 1 st stage of the Marx generator to initiate its self-breakdown [3]. This over-voltage can be coupled into the circuit by replacing one or both charging coils between the 1 st and the 2 nd stage by using pulse transformers in combination with a pulse generator. After ignition, the other spark gaps will switch on in the usual manner due to the over-voltage caused by the uncharged stray capacitances. martin.sack@kit.edu; Fax: Fig. 1. Pulse generator arrangements: (a) single pulse generator and (b) two pulse generators. II. EXPERIMENTS AND DISCUSSION Figure 1 shows two possible arrangements for combinations of pulse generators and transformers between stages 1 and 2. An over-voltage induced by a single pulse transformer according to Fig. 1(a) is added to the charging voltage of the stage capacitor, increasing at appropriate polarity the voltage across the spark gap. After ignition of the spark gap, the transformer has to insulate the stage voltage during discharge. Hence, either the combination of the transformer and the pulse generator needs to have a sufficiently high impedance or an external impedance must be switched in series. The branch

2 Over-voltage Trigger Device for Marx Generators M. Sack et al across the charging coil between stages 1 and 2 formed by C2 and L2 acts as a load for the trigger generator and forms, together with the impedance of the branch with the trigger transformer L1, a voltage divider reducing the voltage across the spark gap, so the induced voltage of the transformer needs to be sufficiently higher. To overcome this disadvantage, the second branch can also be equipped with a trigger transformer, and both transformers are then powered synchronously. This parallel configuration has the additional advantage that smaller devices can be used, each delivering half of the current only, and for their design it is not necessary to consider an additional voltage drop. A trigger pulse generator can be equipped with either closing switches and capacitors to form a voltage source or with opening switches and inductances to form a current source. In the case of a voltage source, a current limitation for transient stage insulation by either an external impedance or a sufficiently large stray inductance of the transformer is mandatory. The high inner impedance of the current source supersedes the use of an additional series impedance. For low jitter, a fast rise of the trigger pulse and an immediate generation of seed electrons for the discharge are required. The voltage rise is determined on one hand by the stray capacitance of the Marx generator at the output of the 1 st stage and the capacitance of the transformers high-voltage coils, and on the other hand by the current charging them. The generation of seed electrons can be fostered easily by mounting a corona ring [4]. This ring is connected to the cathode of the spark gap and is fixed around the center of the spark gap in such a way that ultraviolet light emitted by a glow discharge around the wire of the ring frees seed electrons at the electrode surfaces. For demonstration purposes, an over-voltage trigger using an ignition coil for a combustion engine has been set up. Figure 2 shows a photo of an 8-stage Marx generator with a charging voltage of 15 kv per stage. It is equipped with charging resistors, and the ignition coil is switched in series to the ground-side resistor according to the simplified schematic. When closing the transistor T1, energy is stored in the magnetic field of coil L1. The secondary voltage has a polarity opposite that at triggering. Hence, the voltage across the 1 st spark gap is diminished slightly according to transformer ratio, primary voltage, and the ratio of the voltage divider formed by R1 and R2. When the transistor is opened, the primary current is transferred to the secondary side, charging the stray capacitance. The voltage at the output of the ignition coil may rise to 30 kv, which is twice the charging voltage per stage. This is limited by the circuitry at primary side. According to the voltage divider ratio 1+R1/R2 the voltage is halved, resulting in an over-voltage of 15 kv across the spark gap. The selfbreakdown voltage has been adjusted to 17 kv, so at a charging voltage of 15 kv, the spark gap is over-volted by nearly a factor of two, which is in the same range Fig. 2. (Color online) Small over-voltage triggered Marx generator for demonstration purposes. as the over-voltage applied to the other spark gaps as part of the usual ignition sequence of the Marx generator. Seed electrons for fast ignition of the remaining spark gaps are generated by the light emitted from the already ignited spark gap. For seed electron generation at the 1 st spark gap, instead of a corona wire, a corona around the tip of a small wire at some distance from the spark gap has been used. When igniting, there is again a change of polarity across the trigger transformer and the transistor T1. A protective diode D1 limits the voltage across T1 in the reverse direction, thereby behaving as a voltage source. A charging resistor is required to limit the current. III. COMPONENT DESIGN In an existing experimental electroporation device for sugar beets, two 7-stage Marx generators with a stage capacitance of 140 nf and a charging voltage of 50 kv per stage have been operated in the free-running mode.

3 Journal of the Korean Physical Society, Vol. 59, No. 6, December 2011 Fig. 4. (Color onine) Magnetic field distribution when powering (a) the primary winding and (b) the secondary winding. Fig. 3. Disk-like and cone-like shapes of the high-voltage winding. In order to synchronize both generators, an over-voltage trigger device has been developed to replace the charging coils between the 1 st and the 2 nd stage. The cylindrical charging coils are fixed on an insulating rod through their center axis. In order to fit this arrangement, a core-less design of the trigger transformer has been chosen. To omit additional current-limiting elements, a pulse generator design with opening switches has been used. 1. Pulse Transformer Two designs for the trigger transformer with differences concerning the high-voltage winding have been considered (Fig. 3). For one design, the high-voltage winding consists of a disk-like structure, so the insulation of the adjacent wires is stressed by the voltage of one turn only, and the winding can be manufactured quite easily. For good magnetic coupling between the primary and the secondary winding, a low distance between both windings is of advantage, but it is limited by electrical insulation requirements. Moreover, the insulation has to be designed for polarity reversal after ignition of the spark gap. For the second design, the magnetic coupling between the primary and the secondary winding has been improved by giving the high-voltage winding a cone-like shape. The distance between the turns of the primary and the secondary winding scales with the voltage. In order to limit the length of the transformer and to increase the transformer ratio, the secondary winding has been folded once. Based on calculations, the number of turns for the primary winding has been chosen to be 72, and for the secondary winding, it is 279. Figure 4(a) shows the calculated flux density B when storing energy to the primary winding (FEM-software: MAXWELL 2D). The simulation has been made for a primary current of 70 A; the secondary current has been set to zero. Figure 4(b) shows the magnetic flux density B during trigger pulse generation. The primary current is zero, and the secondary current has been assumed for the calculation to be 10 A. For both cases, the stored magnetic energy can be calculated as the integral of the square of the magnetic flux density over the cross section area of the coil: W = 1 2 µ 0 B 2 da. (1) Based on the stored energy, the inductance can be calculated according to L = 2 W I 2 = µ 0 I 2 B 2 da. (2) The inductance of the primary winding has been calculated to be 398 µh, and that of the secondary winding to be 4.32 mh. The induced voltage in a winding scales with the change of the magnetic flux through each of its turns. Under the assumption of an induced voltage of 300 V in the primary turn with the biggest change of flux, a voltage of 19.3 kv across the primary winding and 54.8 kv across the secondary winding would be achieved, which is in the order of the charging voltage per stage. The mutual inductance has been calculated to be 1.13 mh by using L 21 = U 2 L 1 I 2 = 0. (3) U 1

4 Over-voltage Trigger Device for Marx Generators M. Sack et al Fig. 6. (Color online) Electric field distribution: (a) capacitive field distribution and (b) induced voltages. Fig. 5. Simplified schematic of the trigger pulse generator. The coupling factor k can be calculated to be k = 0.87 by using k = L 21. (4) L1 L 2 2. Pulse Generator and Power Supply In order to use fast-switching IGBTs with 1200 V blocking voltage as opening switches, the primary winding has been split into several parts, each powered by a separate small pulse generator. For an operating voltage of approximately 1000 V, each part consists of 3 turns of the primary winding, resulting in 24 small pulse generators. According to Fig. 5, each pulse generator consists of a capacitor (C1, C2, C3) and an IGBT (T1, T2, T3) connected in series to a diode. The diode blocks the voltage after polarity reversal. For simplicity, all small pulse generators are switched synchronously to each other. They are driven by a MOS field effect transistor (MOSFET), (T5) via a ferrite-core transformer (Tr2). After T5 is switched off, an induced negative voltage is applied to the gates of the IGBTs, fostering fast blocking. For equal currents in all parts of the primary winding of the pulse transformer (Tr3), having the same capacitances and inductances in all pulse circuits is crucial, but the flux through each turn varies with the position of the turn along the length of the primary winding. To balance the inductances between all circuits, turns near the center of the primary winding have been combined with turns from the ends, rather than just combining adjacent turns to one part of the primary winding. By this method it, the induced voltages of the parts of the primary winding differ by only 6.8% rather than by 37.2%, as in the case of adjacent windings. Due to the incomplete coupling between the primary and the secondary winding, the induced voltage is diminished according to the transformer s stray inductance. Hence, according to Eq. (3), a voltage of 74.2 kv needs to be applied to the high-voltage winding in order to obtain the same voltage across the primary winding as for a pulse generation of 54 kv. This is sufficiently high to protect the pulse generators against over-voltage. The pulse generator is powered by the charging current of the Marx generator. The power drawn from the generator must be sufficient to charge the capacitors of the pulse generator within the charging time of the Marx generator. The power is controlled by varying the voltage drop. C4 is charged by the current and discharged via Tr1 when reaching the appropriate voltage. Tr1, T4, and D5 work as a fly-back converter to reduce the voltage for charging. An auxiliary power supply is needed to power up the device. The high-voltage winding of the pulse transformer and C4 form a low-pass filter to protect the circuitry from the stage voltage. Additionally, C4 bypasses the trigger pulse. 3. Distribution of Electric Field When applying a fast-rising pulse to a transformer, an inhomogeneous voltage distribution governed by undesired capacitive coupling and travelling waves may occur. In order to adapt the electric field distribution in the transient case to the one in the quasi-static case, a cylindrical electrode has been put inside the inner transformer winding, forming together with the primary winding a cylinder capacitor. Its length is short when comparing the rise time of the pulse and its travelling time along the capacitor. The position of the wires of each turn of the secondary winding has been chosen in such a way that its potential due to the induced voltage is nearly the same as the potential due to capacitive coupling. Hence, all wires are pre-charged by the electric field to approximately the same potential as in the quasi-static case. Figure 6 shows the calculated electric field strength for the (a) transient capacitive and the (b) quasi-static inductive case when applying 76.6 kv to the secondary winding. The potential along the high-voltage winding

5 Journal of the Korean Physical Society, Vol. 59, No. 6, December 2011 Table 1. Current through charging branches. R L Û load Î load t h I RMS at f rep = 20 Hz [A] [Ω] [kv] [ka] [µs] I load I L1 I L2 I L3 I L4 I L Fig. 7. Simplified schematic for the PSPICE simulation. differs by not more than +/ 5 kv (+/ 10%) between the capacitive and the inductive case. In order to omit eddy currents, the inner cylindrical electrode is made of single wires. At the primary winding forming the outer cylindrical electrode, on average, every third winding is grounded via the capacitors of the pulse generators. 4. Thermal Design Considerations For the thermal design of the trigger transformers, the ohmic losses are of importance. The current through the charging coils consists of a superposition of the DC charging current, which depends on the stage, and the pulse current when the stage voltage is applied. The pulse current through the trigger transformers and charging coils has been calculated by means of a PSPICE simulation (Fig. 7). The trigger transformers have been modelled with their secondary inductance in series with the resistance of the wire (4.3 mh, 2.5 Ω). The inductance and the resistance of each charging coil are 1.8 mh and 1.4 Ω. Table 1 lists the RMS currents through selected charging paths. According to Fig. 7, all coils except the ground-side trigger transformer of the 1 st stage and the charging coil of the last stage on high-voltage side are arranged in a parallel connection. With all switches closed, the currents of both parallel paths are concentrated in L1 and L12, heating both circuit elements more than their counterparts. As the impedance of trigger transformer L2 is larger than the impedance of charging coil L3, a part of the current is shifted to L3, increasing its losses. All currents through charging paths not listed in Table 1 are approximately equal to the current through L4, so the total secondary losses for R L = 60 Ω of the high-voltage side trigger transformer are only 2.7 W, and, hence, much smaller than the losses of the groundside trigger transformer of 18.1 W. The high-voltage side Fig. 8. (Color online) Measured voltage at the 2 nd stage and the current. charging coil L12 between the last and the second last stage has, with 16.2 W, approximately twice the loss of the other charging coils. IV. DEVICE TEST An over-voltage trigger device has been implemented into a 7-stage Marx generator [1,4]. Figure 8 shows the current through the load and the voltage measured at the 2 nd stage. The test setup has been adjusted in such a way that the 1 st spark gap fires when an over-voltage of 40 kv is applied. As the pulse circuit is grounded near the load, an inductive voltage drop appears in the voltage measurement along the ground-side connection to the Marx generator during current rise. V. CONCLUSION Two different designs of an over-voltage trigger device have been described. One design is intended for demonstration purposes and uses an ignition coil as a trigger transformer. The other design has been tailored to an existing 7-stage Marx generator for repetitive operation. Future activities will deal with further improvements of the design, e.g., inductive coupling.

6 Over-voltage Trigger Device for Marx Generators M. Sack et al REFERENCES [1] M. Sack, C. Schultheiss and H. Bluhm, IEEE Trans. Ind. Appl. 41, 725 (2005). [2] A. Küchler, Hochspannungstechnik (VDI-Verlag, Düsseldorf, 1996). [3] M. Sack, C. Schultheiss and H. Bluhm, in Proceedings PPC 03 (Dallas, TX, June 15-18, 2003), p [4] M. Sack and H. Bluhm, IEEE Trans. Ind. Appl. 44, 1074 (2008).