SIMULATION, DESIGN AND TEST OF A MOV PULSE SHAPING DEVICE FOR HIGH POWER MICROWAVE GENERATORS

M. Giesselmannl, T. Heeren, E. Kristiansen, J. Dickens D. Castro, D. Garcia, M. Kristiansen.

Pulsed Power Laboratory, Departments of Electrical Engineering and Physics Texas Tech University, Lubbock, Texas, 79409-3 102, USA

$Email: MichaelG@coe.ttu.edu

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Abstract This paper describes a Metal Oxide Varistor (MOV)

pulse shaping device, which is to be used for generating a flat-top voltage pulse for High Power Microwave devices. The MOV pulse-shaping device is attached to a coaxial pulse compression system. The pulse compression system consists of an inductive energy storage section, an exploding wire fuse and an output spark gap. The paper contains a detailed discussion of the pulse compression system followed by the description of the MOV pulse- shaping device. The MOV elements that are used for this pulse shaping application are the SIOV-E32VR602 type made by Siemens.

I. EXPERIMENTAL SETUP Figure 1 shows a drawing of the experimental setup.

The system consists of a primary storage capacitor (Maxwell 16.5 pF, 45 kV), a triggered closing switch, the energy storage inductor, the exploding wire fuse, the MOV stack, a peaking gap, and a 13 R resistive load resistor. For all of the results reported here, the peaking

.790%9499.2@961O.OO@l999 IEEE. 14

gap was closed. This was done to be able to observe the heating phase of the fuse. The fuse section, consisting of the lower Fuse-T and upper fuse cylinder, as well as the enclosure of the peaking gap were tilled with a SF6- Air mixture at an approximate l/l ratio. The coaxial enclosure of the load resistor was filled with transformer oil. The fuse was constructed of a filament of thin copper wires. The fuse wires and the support structure were embedded in fine sand used for sandblasting (Potters Industries, 2WS80) to aid in the suppression of the arc after the explosion of the fuse.

To analyze the performance of the system, we measured the voltage on the output of the triggered closing switch located directly behind the primary storage capacitor. We also measured the currents at four positions in the system using Pearson current monitors. A Pearson monitor model 1423 was used to measure the current on the input of the primary of the storage inductor. The currents in the fuse and in the load were measured with Pearson monitors model 4418 and 4997 respectively. A forth Pearson monitor with a ratio of O.OOlV/A was used to measure the current in the MOVs.

Figure 1. Drawing of the complete Pulse Forming System.

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II. EXPERIMENTAL RESULTS From the examination of the experimental setup as well

as from the study of previous results [l] it is obvious, that the process of current-interruption by the fuse is critical for the performance of the system. Ideally, the fuse should open at the instant of the current peak in the inductor L Storer absorb no energy and instantaneously commutate the current from the inductor into the load. Under these ideal conditions the ratio of the load voltage and the initial charging voltage of the capacitor Csad would be:

(1)

However, with the parameters in our circuit, the resistance of the fuse has a noticeable influence on the current buildup during the heating phase (prior to the explosion of the fuse wires). In addition, the fuse will absorb a considerable amount of energy during the turn off phase. Therefore, we can not expect to get close to the voltage ratio represented by the formula shown above. In order to find the optimum parameters for the fuse, we studied the influence of the total cross section of the fuse as well as the influence of the total number of fuse wires. The rationale behind variation of these parameters is:

9 If the total cross section of the fuse is too low, it interrupts too early and not enough current has build up in the inductor.

l If the total cross section of the wires is too large, the fuse will not open or open after the peak current has occurred.

l If more fuse wires are used for a given total cross section, each individual wire carries less current which should be easier to interrupt.

For our investigations we used copper wires with both 36 gauge and 44 gauge. The following equation closely relates the US-gauge standard to the actual cross section of the wire [2]:

SO- Gauge

A rmil(Gauge) ‘=2 t 1 3 (2)

The resulting area is given in circular mils. 1 circular mil represents the area of a circle with a diameter of lmil = l/1000 *inch.

Figure 2. Voltage and current traces for best fuse performance.

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12

2

0

240 m

180 ,~/r------260

0 100 200 300 400 500 600

Cross-section [cmil]

Figure 3. Peak load current as a function of total cross-section for 36 gauge and 44 gauge wires,

700 800

16

l 14

.

,/‘--

12

2

0 I I 1 I I I I -I

35 36 37 38 39 40 41 42 43

Wire Gauge [AGWJ

Figure 4: Peak load current as a function of wire gauge # with constant total cross section

Figures 3&4 show the results of a comprehensive set of experiments to determine the optimum number of fuse wires and the optimum total cross section. Figure 3 shows the peak current obtained in the 13 Q load resistor as a function of the total cross section of the fuse wires for 36 gauge and 44 gauge wires. It can be seen that the peak is more pronounced for 44 gauge wires and occurs at a slightly lower total cross section. Figure 4 shows that the

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peak current in the load increases, if for a given total cross section, more wires with a smaller diameter are used.

III. MOV PULSE SHAPING Figure 5 shows a normalized V-I characteristic of one

of the MOV units we used. The SIOV-E32VR602 MOVs, which have been used in the experiment, have a cylindrical shape with a diameter of 34 mm and a height

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of 44 mm. The reference voltage for one MOV is given by the data sheet to be 25.6 kV at a current level of 1 mA. This corresponds to 0.5 or 50% voltage level in Figure 5.

SIEMENS

b&.w Swr saw ‘04 If 1 1,: lil lW ‘.+xX ‘“WN

Pi

Figure 5. Normalized V-I Characteristic of the MOVs.

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Figure 6 shows a set of current traces obtained with MOVs. The fuse for this shot was made up of 10 wires of 36 gauge copper wire. Without MOVs, the load voltage obtained with these settings is about 130 kV. In Figure 6, this voltage is limited to about IOOkV with the use of 7 MOVs connected in series. The peak current in the MOVs is 5kA.

IV. CONCLUSIONS We build and tested a pulse conditioning system for use

with a high power microwave system containing an MOV based pulse-shaping device. We performed a comprehensive performance study to determine the best fuse parameters varying both the total cross section as well as the diameter of individual fuse wires. The MOV based pulse-shaping device proved to be an effective means to limit the voltage at a pre-set level.

Figul

+--- Current at the Trigger Gap, 6.4kNdiv

Current in the Fuse, 6.4kNdiv

Current in the Load, 2kNdiv

-Current in the MOVs. 5k#div

.“I, “...aar... ;,. -A..-... -../ .--.- -_..*, ..*, .*r.. 1 .*.- .?a-.~_ .,,,. .rl_..- ,a-.-.h.;.,*r. . .* F..

re 6. Experimental Results with MOVs.

V. REFERENCES [I] M. Kristiansen, J. Dickens, T. Hurtig, M.

Giesselmann, E. Kristiansen, “Simulation, Design and Construction of a Pulsed Power Supply for High Power Microwaves Using Explosively Driven Magnetic Flux Compression”, 1998 MegaGauss Conference, Tallahassee, Fl, Oct. 18-23, 1998.

[2] Rudy Severns, P.O. Box 589, Cottage Grove, Oregon 97424, Professional Education Seminar on Magnetics Design, 1998 Applied Power Electronics Conference, Anaheim, Ca., Feb. 14-18, 1998.

VI. ACKNOWLEDGMENT This work was solely funded by the Explosive-Driven Power Generation MURI program funded by the Director of Defense Research & Engineering (DDR&E) and managed by the Air Force Office of Scientific Research (AFOSR). We also like to acknowledge the generous donation of the MOVs by SIEMENS, Berlin, Germany.

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