Doctoral Dissertations

Date of Award

12-1989

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Electrical Engineering

Major Professor

T.V. Blalock

Committee Members

Igor Alexeff, J.M. Rochelle, L.L. Riedinger

Abstract

When an insulating fluid is electrically stressed to near its breakdown level, prebreakdown phenomena occur. Currents flowing under these conditions are referred to as prebreakdown currents, and their measurement is important to the understanding of the physical processes leading up to breakdown. Preliminary results indicated that for dc-stressed hexane the currents were pulse-like in nature and that the accuracy with which they could be measured was limited by the bandwidth and noise level of the measurement system. This research comprises a study of fundamental measurement limitations and the development of an optimized measurement system.

A typical system used to measure prebreakdown currents consists of a high voltage source, a test cell and electrodes (to contain and electrically stress the fluid), a detection resistor (to convert the currents to a voltage) and a recorder (to capture a record of the currents). Sometimes a preamplifier is included to allow viewing of lower level currents. This research began with a study of the factors limiting the bandwidth and noise level of such a measurement system. A simple circuit model for the test cell was developed and used. The model consists of a current source shunted by a small capacitance. Two potential preamplifier configurations (voltage-sensitive and transimpedance) were considered for use. For the voltage-sensitive configuration, the bandwidth is limited by the test cell capacitance shunting the detection resistor, and the thermal noise of the resistor places a lower bound on the system noise level. Analysis revealed that for the same bandwidth, the transimpedance preamplifier could provide a lower noise level and therefore a lower minimum detectable current.

A model describing the transimpedance preamplifier in terms of an open-loop gain function, feedback impedances, source impedances and noise power spectral densities was developed. A simplified version of the model was analyzed algebraically to give equations describing the frequency response and noise performance of the preamplifier. A computer-aided-design tool was developed to perform similar analyses of the complete model numerically. The noise analysis revealed that the input device for the transimpedance preamplifier should have low input noise power spectral density and low input capacity and an FET would be preferred over a bipolar device. A number of different types of FETs were characterized, and it was found that the use of GaAs MESFETs would produce a transimpedance preamplifier with the lowest noise level in a dc to 100 MHz bandwidth.

A 90-MHz bandwidth transimpedance preamplifier using a GaAs MESFET input and a 5 kΩ feedback resistor was developed and incorporated into a system consisting of a cell, the preamplifier, two wideband buffer amplifiers, a 200-Msample/s transient digitizer and waveform reconstruction software. The system has an overall bandwidth of 65 MHz, including the digitizer, and a minimum detectable current of less than 250 nA rms for a signal-to-noise ratio of five. Waveform reconstruction software was used to quadruple the effective sampling rate from 5 ns per point to 1.25 as per point. This does not increase the bandwidth, but provides a better representation of the high-frequency components of the digitized waveform than simple linear interpolation does.

The measurement system was tested both with artificially generated current pulses and by measuring prebreakdown currents in hexane with either positive or negative high voltage. Pulses with extremely short durations (7 ns fwhm) and amplitudes ranging from tens of μA down to near the noise level of the measurement system were observed. The usual pulse shape appeared to be the system impulse response, which implies that a still greater bandwidth would be required to see the actual shape of the current pulses.

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