Design and Development of a Compact Trip Pulse Generator

International Journal of Electronics and Electrical Engineering Vol. 4, No. 6, December 2016

Design and Development of a Compact Trip Pulse Generator Madhu Palati1, G. R. Nagabhushana1, and Archana Sharma2 1

Dept. of Electrical & Electronics Engineering, School of Engineering and Technology, Jain University, Bangalore562112, India 2 Energetics and Pulse Power Section, APPD, BARC, Mumbai, Maharashtra-400085, India Email: [email protected]

(due to two capacitors coming in series) appears across the second spark gap, and breakdown occurs in that gap. This repeats for subsequent stages of Marx and is known as “erecting of Marx”. Therefore the stage voltage of each capacitor gets added up and appears across the load. The ideal no-load output voltage across the load is equal to n*Vc, where n is number of stages and Vc is the stage charging voltage The design, development, limitations and work carried out on triggering of the first spark gap electrodes of the Marx generators for generation of fast pulses, by the earlier researchers has been briefly discussed in next paragraph: Osmokrovic et al. [1] discussed the testing of two three electrode spark gap models, first model with third electrode inside the main electrode and the second model with a separate third electrode. Several characteristics were determined experimentally and comparative analysis was made. Sack et al. [2] discussed about the drawback of three electrode gap i.e. the trigger electrode gets subjected to more wear compared to the main electrode because the arc gets concentrated on the small surface of the trigger electrode. The design of trigger device for over-volting the first gap was replaced by charging inductor with a pulse transformer in combination with a pulse generator. Sack et al. [3] presented the design of trigger generator for over voltage triggering of first gap of Marx generator used in repetitive applications. Pulse transformer equipped with IGBT switches was used to generate trigger pulses to cause over voltage across the first spark gap electrodes. Sack et al. [4] discussed the short life of conventional triggered spark gaps. A new trigger method has been developed and presented some preliminary experimental results by inclusion of triggering system for the existing Marx generator Choyal et al. [5] designed & developed the first gap triggering mechanism for a 300kV Marx generator by means of pulse transformer, which produced a 6kV pulse and was applied between the first spark gap electrodes. The UV light is passed through all the gaps that caused pre-ionization of all remaining gaps, resulted in simultaneous sparking of all gaps. A hollow ceramic tube of 1.2mm diameter was inserted through the bore of one of the first spark gap electrodes. A 0.5mm wire was

Abstract—Pulsed power engineering has found many applications of great importance in areas of defense, nuclear physics, civilian, industrial and medical etc. In all these applications Marx generators are the primary source of generation of pulsed voltage/current. For the erection of Marx generator, a proper triggering mechanism is implemented for the first spark gap electrode, which gives consistent breakdown for all the remaining spark gaps in the Marx column. This paper discusses the design and development of a simple, inexpensive and compact trip pulse generator. This triggering mechanism provides the control triggering to operate the Marx generator at definite time. The experimental results reveal that the output voltage of trip pulse generator is able to make air breakdown in the first electrode gap. 

Index Terms—Marx generator, spark gap electrodes, controlled triggering, trip pulse generator, breakdown voltage

I.

INTRODUCTION

Fast pulses (nano and sub-nanosecond rise times) have many applications both civilian and defense. Most common method of generating high voltage pulses is using a Marx generator. The Marx generator (proposed by Prof. Erwin Marx in 1923 at the Technical University of Braunschweig, Germany) works on the principle of charging several capacitors in parallel and discharging them in series so that voltages add up. The schematic of a four stage Marx generator is shown in Fig. 1.

Figure 1. Schematic diagram of a four stage Marx generator

The capacitors get charged through the charging resistors, RC. After reaching the desired voltage the first spark gap is self triggered. However, if controlled triggering is required, the first gap is usually triggered by an external means using a three electrode gap or Trigatron gap based triggering scheme. Twice the voltage Manuscript received June 24, 2015; revised March 22, 2016. ©2016 Int. J. Electron. Electr. Eng. doi: 10.18178/ijeee.4.6.505-509

505


inserted in the hollow ceramic tube to form the third terminal for which a negative voltage of 6kV was applied. Thomas Baby et al. [6] developed the triggering mechanism with pulse repetition frequency ranging from 0.1Hz to 1kHz. Pulses of 5µs duration with rise time less than 1µs were generated from the timer circuit. This voltage pulse is fed to the gate of the SCR that was wired to the primary of a pulse transformer, which produces a transient voltage of 4kV and is fed to the trigger pin of the spark electrode. Rowan Sinton et al. [7] developed a custom built first stage spark gap i.e. a three electrode gap. By varying the gap length, it was able to trigger reliably ranging from 10 to 90kV. The trigger signal was delivered thru a fiberoptic cable. From above, it is very clear that triggering mechanism is required for triggering the first electrode gap. In this paper an attempt is made to develop a custom made trip pulse generator for triggering the first electrode gap of a 10 stage Marx generator of rating 200kV, 20J. II.

METHODS OF CONTROLLED TRIGGERING

B. Trigatron Gap In this arrangement, one of the spark gap i.e. earthed electrode has a bore at center and the schematic and tripping circuit of trigatron gap are shown in Fig. 3 and Fig. 4 respectively. The trigger electrode is fitted into this hole through a bushing on application of trip pulse to the trigger electrode by means of tripping circuit, the field gets distorted between the HV electrode and the earthed electrode results in spark over in the main gap. This method requires lower trip pulse voltage compared to three electrode gap scheme [8], [9].

Figure 3. Schematic of tripping circuit with trigatron gap

The methods for controlled triggering of first stage of Marx generator are listed below:  Using a three electrode gap  Using a trigatron gap  Spark gaps with movable frame A. Three Electrode Gap The first stage spark gap of a Marx generator is fitted with a three electrode gap and is shown in Fig. 2. The central electrode is maintained at a potential in between that of a top and bottom electrode of three electrode gap. Breakdown is achieved at any instant by applying a trigger pulse of peak voltage not less than one fifth of the charging voltage to the central electrode. This three electrode gap requires more space and an elaborate construction [8], [9].

Figure 4. Tripping circuit using a trigatron gap

The capacitor C1 is charged to 5 to 10kV, when the switch S is closed a pulse is applied to CRO through the capacitor C2 and at the same time Capacitor C3 gets charged and a trigger pulse is applied to the trigatron gap. The delay time for triggering can be obtained by varying R3 and C3 and the residual charge can be discharged into high resistance R2 [10]. C. Trigatron Spark-Gaps Mounted on Movable Frame In this arrangement, one of the spark gaps electrode are mounted on a movable frame, once the capacitors gets fully charged, the spark gap distance is reduced by moving the movable frame. In order to have consistency of sparking, irradiation from an ultraviolet lamp is provided from the bottom to all the gaps. This method is difficult and does not assure consistent and controlled triggering [8], [9]. Most often Trigatron gap scheme is used, and this is expensive. In our work, the principle of three electrode gap is used to breakdown the air gap in the first spark gap electrode. Another method of triggering first stage spark gap electrode without third electrode is explained by Eugene et al. [11] and the schematic circuit is shown in Fig. 5.

Figure 2. Tripping circuit with three electrode gap

By closing the switch S, the thyratron conducts and Capacitor C produces a decaying pulse of positive polarity to initiate the Oscillogram time base and negative pulse through the capacitor C1, which gets applied across the top electrode and central electrode and the gap conducts.

©2016 Int. J. Electron. Electr. Eng.

506


the capacitor is suddenly disconnected, this will cause the magnetic field to collapse and induce high voltage transient in the secondary coil.

Figure 5. Schematic of trip circuit without trigger electrode

The ground side charging inductor of the first stage of Marx generator is replaced by a pulse transformer which is auxiliary connected to the pulse generator. The pulse transformer super imposes a voltage pulse to the charging voltage of the first stage capacitor C1 of Marx generator. Therefore this over voltage across the first spark gap causes the spark gap to fire. III.

Figure 7. Schematic of trip pulse generator

The output transient of ignition coil is applied to the set of five capacitors connected in series each of rating 0.01µF, 4kV. Twenty resistors of carbon type, each of rating 100kΩ,2W are connected in series and the whole series combination of these resistors are connected across the set of the capacitors mentioned above, to avoid the reverse flow of current. Also, to discharge the remaining stored energy into these resistors after post application. A 10 ohms wire wound non-inductive resistor, is connected in series with this combination to limit the current. The high voltage lead is connected to the copper rod of 3mm diameter. ±10mm adjustment for vertical and horizontal displacement of the copper rod is provided in the stand, to focus the spark at the first stage spark gap. Fig. 8 shows the experimental setup of the trip pulse generator.

EXPERIMENTAL MODEL

Control triggering circuit i.e. Trip pulse generator is designed for triggering the first spark gap electrode of the existing Marx at our laboratory. The schematic view of the Marx with trip pulse generator and schematic of trip pulse generator are shown in Fig. 6 and Fig. 7 respectively.

Figure 8. Experimental setup of Marx generator with trip pulse generator

Figure 6. Schematic view of Marx generator with trip pulse generator

IV.

Single phase supply is fed to the primary of isolation transformer (230V/110V) and the secondary side voltage is rectified to DC and the 1000µF capacitor gets charged. When the switch gets closed, the capacitor discharges into the ignition coil. Ignition coil consist of primary coil of less number of turns and are of thick wire and secondary coil of more number of turns and are of thin wire. When the input voltage from the capacitor is applied to the primary thru the switch, it creates strong magnetic field in the primary and when the supply from ©2016 Int. J. Electron. Electr. Eng.

RESULTS & DISCUSSIONS

The end of the copper rod of the trip pulse generator is placed very close to the one of the electrodes of the first spark gap of the Marx generator. The output of the Trip pulse generator is tested using P6015A, 1000X High voltage probe of Tektronix make. Control triggering is done by using the control button, by pressing the control button, the spark occurs between the tip of the copper rod and one of the electrodes of the first spark gap. The High voltage waveform is captured by the HV probe and is 507


displayed on the Digital storage oscilloscope and the same is shown in the Fig. 9. The distance between the copper rod and the spark gap electrode was 5mm and 1000X probe was used to measure the waveform, from the waveform the magnitude of the output pulse is 16kV (each unit is of 5V and multiplication factor of probe is 1000, for 3.2 units the voltage is 16kV).

gap of the Marx generator. The trip pulse generator gave an output voltage of 16kV for a 5mm gap between the electrodes of the trip pulse generator & the Marx first stage spark gap electrode. The experimental and theoretical values of the voltages are in close agreement. After the occurrence of breakdown of first spark gap, the remaining spark gaps of Marx generator were self triggered due to overvoltage across them. Based on the requirements, the output voltage of the trip pulse generator can be increased by adding the capacitors mentioned in the circuit. ACKNOWLEDGMENT The work has been carried out by the financial support of Department of Atomic Energy (DAE), Board of Research studies in Nuclear Sciences (BRNS). We are highly thankful for them. Author is grateful to the Director, EEE HOD & Management of School of Engineering & Technology, Jain University, Bangalore for their constant support and encouragement, in carrying out this research work. REFERENCES

Figure 9. Experimental output voltage of the trip pulse generator

P. Osmokrovic, N. Arsic, and N. Kartalovic, “Triggered three electrode spark gaps,” in Proc. IEEE 10th Pulsed Power Conference, July 3-6, 1995, pp. 822-827. [2] M. Sack, R. Stangle, and G. Miller, “Overvoltage trigger device for Marx generators,” Journal of the Korean Physical Society, vol. 59, no. 6, pp. 3602-3607, December 2011. [3] M. Sack and G. Miller, “Design and test of a modular trigger generator for over-voltage triggering of Marx generators,” in Proc. IEEE Power Modulator and High Voltage Conference, June 2012, pp. 320-323. [4] M. Sack, C. Schultheiss, and H. Bluhm, “Wear-Less trigger method for Marx generators in repetitive operation,” in Proc. IEEE 14th Pulsed Power Conference, June 15-18, 2003, pp. 14151418. [5] Y. Choyal, et al., “Development of a 300kV Marx generator and its application to drive a relativistic electron beam,” Sadhana, vol. 30, no. 6, pp. 757-764, December 2005. [6] T. Baby, T. Ramachandran, P. Radhakrishnan, V. P. N. Nampoori, and C. P. G. Vallabhan, “A low inductance long life triggered spark gap switch for Blumlein driven lasers,” Measurement Science and Technology, vol. 2, pp. 873-875, 1991. [7] R. Sinton, V. H. Ryan, W. Enright, and P. Bodger, “A Marx generator for exploding wire experiments,” in Proc. Asia-Pacific Power and Energy Engineering Conference, 2011. [8] M. S. Naidu and V. Kamaraju, High Voltage Engineering, 4th ed., Tata McGraw Hill, 2009, p. 182. [9] E. Kuffel, W. S. Zaengl, and J. Kuffel, High Voltage Engineering Fundamentals, Second ed., Newness Publications, 2008, pp. 70-72. [10] C. L. Wadhwa, High Voltage Engineering, Third ed., New Age International Publishers, 2010, pp. 119-120. [11] E. Vorobiev and N. Lebovka, Electro Technologies for Extraction from Food Plants and Biomaterials, Springer Publications, 2008, pp. 246-249.

[1]

The air density factor, δ [8] is given by:

𝑑=

𝑝 760

[

293

273+𝑡

]

(1)

k=d

(2)

Vo (corrected) = Vo (STP)* k % 𝐸𝑟𝑟𝑜𝑟 =

𝑉𝑜 (𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑)−𝑉 𝑉𝑜 (𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑)

∗ 100

(3) (4)

where p is the pressure in torrs, t is the room temperature in degrees centigrade, k is the correction factor, VO (STP) is the breakdown voltage of air at standard temperature (20°C) and pressure (760 torrs), VO (corrected) is the corrected value of breakdown voltage of air at room temperature and pressure and V is the experimental value at room temperature and pressure. The measured room temperature t is 26°C and pressure p is 750 torrs. From (1), the air density factor is 0.968. For values of d greater than 0.95, correction factor is same as air density factor [8]. Therefore, from (2) the value of correction factor is same as air density factor. The breakdown voltage of air VO (STP) for a sphere gap spacing of 5mm is 16.8kV and VO (corrected) calculated from (3) is 16.26kV. The obtained value from experimental waveform and the calculated value are in agreement and the change is very nominal i.e.0.26kV and % error calculated from (4) is minimal i.e. 1.6%. This voltage was sufficiently enough to break the first gap of the Marx generator available at our laboratory and at the remaining gaps the voltage got added up and self breakdown took place simultaneously. V.

Madhu Palati received the B.Tech degree in Electrical & Electronics Engineering from Sri Venkateshwara University, Tirupati, India, in 2003 and M.E from M.S.University, Baroda, India in 2005. He has worked as a software Engineer in Keane India Ltd, Gurgaon for a period of one and half years and in IBM Private Limited, Bangalore for a period of three years. He is currently working as Assistant Professor and also working towards the PhD degree in the department of Electrical & Electronics

CONCLUSION

In this work an attempt has been made to develop a simple compact, inexpensive and portable trip pulse generator for controlled triggering of the first stage spark ©2016 Int. J. Electron. Electr. Eng.

508


line insulation, and laboratory simulation of NEMP and lightning. He has been responsible for setting up several high-voltage laboratories in India.

Engineering, School of Engineering & Technology, Jain University, Bangalore.

G. R. Nagabhushana received the B.Sc. degree from Mysore University, Mysore, India, in 1960, and the B.E. (Electrical), M.E. (Electrical High Voltage Engineering), and Ph.D (High Voltage Engineering) degrees from the Indian Institute of Science, Bangalore, India, in 1963, 1965, and 1973, respectively. He was with the Department of High Voltage Engineering, Indian Institute of Science, Bangalore, India, prior to his retirement in July 2004. He was Chairman of the Department from 1989 to 1996 and again from 1999 to July 2004. Presently, he is an Emeritus Fellow of the All India Council of Technical Education in the Department. Currently he is working as visiting professor at school of Engineering & technology, Jain University. His main areas of interest have been vacuum insulation, pollution performance of the transmission

©2016 Int. J. Electron. Electr. Eng.

509

Dr. Archana Sharma is an electrical engineer with Ph.D, in High Voltage Engineering from Indian Institute of Science, Bangalore. Presently she is head, Pulse Power Systems Section, APPD, BARC. She is the recipient of Homi Bhabha Science and Technology Award-2011. She is working in the design and development of intense pulsed power system for high power microwaves and flash X-rays radiography applications. She is currently involved in the developmental activities of compact, repetitive and mobile. She is also part of Intentional Electromagnetic Interference (IEMI) studies using HPM and UWB sources for various electronics circuitry and their shielding techniques.