PULSE POWER CONDITIONING WITH A TRANSFORMER FOR AN INDUCTIVE ENERGY STORAGE SYSTEM

M. Giesselmanr?, J. Zhang?, 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

‘Permanent address: Applied Physics Department, National University of Defense Technology, Changsha, 410073, Hunan, China :Email: MichaelG@coe.ttu.edu

Abstract One of the key technologies in a high power microwave

system is the Pulsed Power Conditioning System (PPCS). For a system driven by an explosive flux compression generator, the PPCS may consist of an energy storage inductor, a fuse type opening switch and a sharpening spark gap. This paper presents the investigation of a PPCS with a pulse transformer. Before the construction of a prototype, the behavior of the PPCS has been simulated using the PSpice circuit simulation code. A transformer with a primary inductance L,=3.5 pH, secondary inductance L,=85 pH, and a coupling coefficient K=0.75, was designed and used in our experiments. The transformer was designed with two coaxial windings. Simulation results as well as experimental waveforms are shown.

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 pulse transformer, a fuse, a peaking gap and a 13 R resistive load,

o-7eos549am9mo.o939 IEEE. 14

For all of the results reported here, the peaking gap was closed, in order to observe the heating phase of the fuse. The fuse section, consisting of the lower fuse-T and the upper fuse cylinder, as well as the enclosure of the peaking gap were filled with an SF6 - Air mixture. 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, 2W580) 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 three 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 transformer. The currents in the fuse and in the load were measured with Pearson monitors model 44 18 and 4997, respectively.

Figure 1. Drawing of the Complete Pulse Forming System.

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II. CONSTRUCTION OF THE PULSE TRANSFORMER

Figure 2 shows a photograph of the construction of the pulse transformer. The pulse transformer has a primary winding consisting of eight turns made of copper sheet- metal wound in a spiral. The width of the primary copper sheet is 5.08 cm and its thickness is 1 mm. The primary is wound on an insulating, cylindrical support structure in which a spiral groove, that matches the dimensions of the primary winding, has been machined. The secondary winding is located on a separate insulating support structure. It is mounted inside of the primary winding. The secondary winding has 50 turns made of solid copper wire with 2.8 mm diameter. Towards the high voltage end, (located on the left side of Figure 2), the diameter of the support structure of the secondary winding is

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gradually decreasing, to increase the isolation distance from the primary winding and the dielectric strength. Delrina was used as the material for both support structures. The fully assembled transformer was filled with transformer oil before it was energized. The transformer oil was de-gassed by applying a vacuum on the filler port for two days. Figure 3 shows the fully assembled transformer integrated into the pulse forming system. In order to avoid eddy currents, that would degrade the performance of the transformer, the outer enclosure of the transformer was made out of PVC instead of stainless steel, which was used for the rest of the system.

Figui *e 2. Photo@

Figure 3. Photograph of Transformer integrated in the Pulse Forming System.

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III. COMPUTER MODELING OF THE SYSTEM

In order to predict the performance of the system and to understand the processes during the discharge, we have extended existing computer models for flux compression generators to cover the system reported here [ 11. The main modification of the simulation circuit was the inclusion of a model for the transformer. We arrived at the values of the transformer model through theoretical considerations involving the geometry of the construction and measurements on the assembled unit. Figure 4 shows the top level schematic circuit that was used to simulate the system. We used a model involving coupled linear inductors since the transformer has an air core. The values

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of the model parameters are shown in Figure 4. Figure 5 shows the results of the simulation of the circuit shown in Figure 4.

agreement.

IV. EXPERIMENTAL RESULTS Figure 6 shows oscilloscope traces of a test shot taken

on the system. The data was taken using the above described instrumentation and recorded on an HP Infinium digital storage oscilloscope with 2 Gs/sec sampling speed. A comparison of the experimental results with the results of the circuit simulation shows a good

Power Conditioning System with Capacitor Bank, Transformer and Resistive Load:

cap Lstray --J---T 350nH

I---

2 Trigger-Gap tClose=5us

Cbank - IC=2OkV

16.5uF

_ Fuse

AWtl;Juse-Cul

AW-44 VWC3=6 L-mm-463 InActance-1 OonH ljwO.0 13 C-Heat=1 2

Ll_VALUE=3,5uH L2_VALUE=EEuH COUPLING=0.75

r rgure 4. bcnemattc or Pulse hormmg system tor Srmulatton.

Current at the output of the Trigger Gap i I #

SEL>> j I -30KAI----------------------------------------------------------------------------------~ t-i I (Lstray)

30K”;-----------------.------------------------------------------------------------------;

Voltage at the Trigger’Gap

I -20K”I---------------~-------------------------------------------------------------------1

q U(Trlgger) ,0KAT----------------.------------------------------------------------------------------~

I I Current in the Fuse

I I

-5KII/----------------,----------------,----------------,----------------,----------------; 0s 10us 20us 30~s 4011s 50~s

q I(AWG~Fuse~Cul.Rxl.R_decoup) o I(R-Load) Tine

Figure 5. Results of the Circuit Simulation.

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Voltage at the Output of the Trigger Gap, 6kWDiv

)Zurrent at the Output of the Trigger Gap, 12.8kA’Div

V. CONCLUSIONS We built and tested a pulse conditioning system for use

with a high power microwave system involving a transformer. We performed computational analysis and experimental investigations and found the results in good agreement.

VI. RJZFERElNCES [l] 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”, presented at 1998 MegaGauss Conference, Tallahassee, Florida, October 18 - 23, 1998.

VII. 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)

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