Young-Su Roh

Analysis of Output Pulse of High Voltage and Nanosecond Blumlein Pulse Generator

A high voltage and nanosecond Blumlein pulse generator has been developed to produce an output pulse whose voltage level is greater than 250 kV and pulse duration 5 ns. The generator consists of various components such as a charging circuit, a pulse transformer, and a spark gap switch. As a heart of the generator, a Blumlein pulse forming line has been constructed in the cylindrical form using three cylindrical aluminum electrodes that are placed concentrically. Unlike the ideal Blumlein line, the output pulse of an actual Blumlein line may be affected by stray inductances and capacitances of switching and charging components, thereby degrading the performance of the generator. In this paper, PSPICE simulations have been performed to examine effects of stray inductances and capacitances on waveforms of output pulses. Simulation results show that the pulse waveform is significantly distorted mainly by the stray inductance of the spark gap switch.

Improvement of Power Spectrum in Ultrashort Pulse Reflectometry Signals Using Three Chirp Configuration

The flat power spectrum of the transmitter output signal for the desired frequency range is ideal to achieve the best performance of ultrashort pulse reflectometry. However, the power spectrum of a typical pulse generator decreases significantly as frequency increases. A configuration of three chirped waveforms was employed to improve the power spectrum of the transmitter signal at higher frequencies. To determine the amplification gain required for higher frequency components, three chirped waveforms were theoretically generated and their power spectra were measured using numerical band-pass filters. Based on the results of numerical computations, the three chirp configuration was successfully applied to the design of the transmitter for a broadband system..

Design and Test of an Electric Field Sensor for the Measurement of High-Voltage Nanosecond Pulses

D-dot sensors were designed and tested for the measurement of nanosecond high-voltage pulses. Computer simulation results showed that the I-type sensor has an acceptable response in a wide range of frequency among three types of sensors such as I, ∇, and T. The I-type sensor has coaxial cylinder shape, which consists of a brass inner conductor, a Teflon middle dielectric, and an aluminum outer conductor. Since I-type showed good linearity up to 1.2 GHz, we calibrated the sensor using relatively low frequencies. The attenuation ratio of the integrated signal of the D-dot sensor was calibrated against a standard high-voltage probe (Tektronix P6015, 75-MHz bandwidth). The measured attenuation ratio and standard deviation were 7.70×1012 and 0.0608×1012, respectively. The measured attenuation ratio was in good agreement with the calculated ratio within 7.5%. The operational characteristics of the sensor were tested by measuring nanosecond voltage pulses generated from a Blumlein pulse forming line. We measured high-voltage pulses having 300 kV, 5-ns pulsewidth, and 300-ps rise time using the designed D-dot sensor. The accuracy of the sensor enabled detection of several tens of picosecond differences in the rise time of the high-voltage pulses resulting from different gap distances in the peaking switch.

A Study on the Characteristics of Organic Insulating Materials Carbonized by a Leakage Current

Organic insulating materials which are utilized as insulating materials for the low voltage show unique carbonization characteristics when they are carbonized by a leakage current. Therefore the use of the carbonization characteristics makes it possible to examine the electrical fire which is caused by a leakage current flowing on the surface of the organic insulating material. In order to understand such carbonization characteristics, in this paper, experiments have been done to carbonize typical organic insulating materials such as phenol resin, PVC, and acrylic resin, and the carbonization patterns and the IR absorption spectrum of specimens have been analyzed. According to the analysis of the carbonization patterns, the phenol resin shows the so-called `spider-leg` carbonization pattern due to a thermosetting property. In contrast to the phenol resin, the thermoplastic property makes it difficult to observe a clear carbonization pattern to verify carbonizing causes on the surfaces of PVC and acrylic resins. In this case, the IR absorption spectrum can be analyzed to examine the specimen carbonized by a leakage current. The analysis result shows that absorption peaks appear at the wave numbers of and , which can be an important factor to verify the carbonizing causes.

A Peaking Switch to Generate a High Voltage Pulse of Sub-nanosecond Rise Time

A triaxial Blumlein pulse forming line has been designed to generate a pulse whose voltage is ~300 kV, pulse duration is ~5 ns, and rise time is ~500 ps. It turns out, however, that the rise time of the pulse becomes much longer than 500 ps due to parasitic inductances and capacitances existing inside the system. A peaking switch has been developed to shorten the rise time of the pulse from Blumlein pulse forming line. In the peaking switch, a wedge-shaped dielectric material (MC 901 nylon) is employed to surround the electrode on the antenna side. This shape inhibits an abrupt change of the output impedance, thereby minimizing the reflection of the output pulse. Experimental results show that the peaking switch is capable of improving the rise time of the pulse at a level of 500 ps.

Simulation on Surface Tracking Pattern using the Dielectric Breakdown Model

The tracking pattern formed on the dielectric surface due to a surface electrical discharge exhibits fractal structure. In order to quantitatively investigate the fractal characteristics of the surface tracking pattern, the dielectric breakdown model has been employed to numerically generate the surface tracking pattern. In dielectric breakdown model, the pattern growth is determined stochastically by a probability function depending on the local electric potential difference. For the computation of the electric potential for all points of the lattice, a two-dimensional discrete Laplace equation is solved by mean of the successive over-relaxation method combined to the Gauss-Seidel method. The box counting method has been used to calculate the fractal dimensions of the simulated patterns with various exponent η and breakdown voltage O b . As a result of the simulation, it is found that the fractal nature of the surface tracking pattern depends strongly on η and O b .

Study on the Chirped Waveform of the USPR Pulse using the Impulse Response of a Waveguide

In ultrashort-pulse reflectometry (USPR), a chirped waveform transformed from the USPR source impulse signal via waveguide makes it possible to employ millimeter-wave mixers for the frequency up-conversion process. Consequently, the frequency bandwidth of the USPR system is sufficiently wide to cover a large portion of the electron density profile of the plasma. Some physical aspects of the chirped waveform, such as maximum amplitude and length, are critical factors to determine the performance of the system. In this paper, the propagation of the USPR impulse signal through a rectangular waveguide is numerically studied to derive the chirped waveform using the impulse response of the waveguide. The results of numerical computation show that the chirped waveform significantly depends on the waveguide cutoff frequency as well as the waveguide length.

A Study on Discrimination between Short-Circuit and Overload based on the Characteristics of the Fusing Current of an Electrical Wire

In the case that an overcurrent flows through in electrical wire due to short-circuit or overload, the wire can be fused, thereby causing an electrical fire. In the present article the characteristics of the fusing current of an electrical wire have been studied to discriminate between short-circuit and overload. In the experiment the fusing time was measured as the currents determined by Preece`s equation were supplied to bare wires of various diameter. As the results of experiment, the measured fusing currents well satisfied the Onderdonk`s equation. By comparing the measured results and the short current the IEC recommends, it is shown that the variable to determine the short current for a bare copper wire, k is appoximately 300. The fusing current of an electrical wire which becomes a short circuit within 5sec can be expressed as a function of diameter based on the value of k. Consequently, the equation for the fusing current provides a criterion to discriminate between short-circuit and overload.

Surface Discharge Characteristics of Phenolic Resin Treated by Heat and Its Structure Analysis

For clearing the cause of firing due to the tacking on the surface of phenolic resin, this paper describes the analysis through a couple of methods ; FT-IR; DTA; photograph analysis, etc. Phenolic resin has been widely employed as a case of low voltage appliances. In the experiment it was confirmed that its surface was carbonized and graphitized by the external fire. In the FT-IR test a graphite specimen thermally treated at showed the 2 different IR absorption peaks at . In normal phenolic resin, the exothermic peak appeared at , while in graphite specimen, it appeared at in DTA test. From the results, the electrical fire causes could be cleared and it is expected to protect the human life and property from the electrical fire by using the important data.

A Research on the Actual Conditions of the Electric Shock Hazard of a Guard Lamp

In this paper, guard lamps at three areas have been investigated to understand the actual conditions of the electric shock hazard of the guard lamps. Upon the basis of the obtained data, the actual conditions of the guard lamps were precisely analyzed in terms of the electric shock hazard. As a result of the investigation, it was turned out that many guard lamps were not installed in organized ways due to the lack of legal regulations, resulting in the increase of electric shock hazard. Therefore, it is necessary, from a point of electric shock protection view, to establish the regulations of equipment installation and management, which can be specifically applied to guard lamps.