T.V. Blalock

Observation of partial discharge in hexane under high magnification

Partial discharges are observed in hexane by shadow photography under the application of DC voltages. A nonuniform field geometry is used and the growth of cavities associated with partial discharges at a point cathode was photographed at 200* magnification. The use of an image-preserving optical delay allows a record of the conditions which exist in the liquid prior to the initiation of the low-density streamer to be obtained. The cavity growth at a point cathode is anisotropic, which suggests that electrostatic forces are of primary importance in driving its expansion. The onset of instabilities in the cavity wall is suggested. The initial partial discharge current pulse precedes or is concurrent with the growth of the cavity. An upper bound of 60 ns between first current pulse and the growth of the cavity is obtained. >

Prebreakdown current pulses in n-hexane and other dielectric fluids

A point-plane electrode geometry was used to apply negative DC stresses to n-hexane, trimethylpentane, and transformer oil. The resulting prebreakdown currents and light were observed with a state-of-the-art, low-noise amplifier and a high-speed digitizer. Currents appear as bursts of fast (on the order of 100 ns) negative pulses with progressively increasing amplitudes, and a wide bandwidth (at least 10 MHz) is required to resolve individual pulses. Light pulses occur coincidentally with current pulses, but are narrower. The effect of adding particles to the sample or increasing the applied high voltage is primarily to increase the burst rate, with little or no effect on other pulse characteristics such as shape, amplitude, or rate of increase. These observations suggest a model describing the source of the current pulses as a series of discharges inside an expanding low-density region. >

Low-noise wide-band amplification system for acquiring prebreakdown current pulses in liquid dielectrics

A low-noise wideband amplification system consisting of a preamplifier with input protection circuit and an amplifier were developed for measuring the prebreakdown currents in liquid dielectrics. The system design considerations and experimental results are presented. The preamplifier was voltage-sensitive with a voltage gain of 154 V/V, and had a bandwidth of 56 MHz and an equivalent noise voltage spectral density of 0.93 nV/Hz/sup 1/2/. The amplifier had a voltage gain of about 12 and a bandwidth of 58 MHz and was capable of driving mod 2 mod V across two paralleled 50 Omega loads. The overall system had a gain of 92400 V/A when loaded with 50 Omega , and it had a minimum detectable current of 1.9 mu A for an overall bandwidth of about 29 MHz. >

Observations of partial discharges in hexane under high magnification

Partial discharges are observed in hexane by shadow photography under the application of DC voltages. A nonuniform field geometry is used and the growth of cavities associated with partial discharges at a point cathode was photographed at 200* magnification. The use of an image-preserving optical delay allows a record of the conditions which exist in the liquid prior to the initiation of the low-density streamer to be obtained. The cavity growth at a point cathode is anisotropic, which suggests that electrostatic forces are of primary importance in driving its expansion. The onset of instabilities in the cavity wall is suggested. The initial partial discharge current pulse precedes or is concurrent with the growth of the cavity. An upper bound of 60 ns between first current pulse and the growth of the cavity is obtained.<<ETX>>

Possible models for the earliest prebreakdown events in DC stressed hexane

Observed phenomena in liquid hexane at a point cathode include the formation of a low-density region (LDR), a chain of growing electrical pulses, a succession of light flashes, and a final disruption and termination of the process. The authors address three separate effects: the inception of the process; the growth of the low-density region, with electrical pulses and light emission; and the break-up of the low-density region. It is shown that it is possible to predict the onset, growth, pulse and light phenomena, termination of growth, and break-up of the low-density region in terms of electrical and fluid dynamical processes.<<ETX>>

Pressure effects on partial discharges in hexane under DC voltage

The pressure dependence of partial discharges (PD) has been experimentally investigated at a needle electrode in hexane from subatmospheric pressure (near hexane vapor pressure) to several atmospheres. Each PD produces a phase transition in the liquid near the needle, which is photographed in synchronism with a characteristic pattern of current pulses. An image-preserving optical delay allows photography to commence just before or at inception of the discharge. Individual current pulses comprising a characteristic pattern are resolved. The cathode event consists of a short pressure-insensitive inception phase, a pressure-sensitive growth at a decreasing rate, and finally a detachment and dissipation, sometimes with noticeable contraction before detachment; increased pressure reduces the growth rate and lifetime. The accompanying characteristic current pulse pattern always ceases during the growth of the PD. For the anode event, less extensive data similarly show slowing of growth with increased pressure and a (different) characteristic current pulse pattern.<<ETX>>

Nonuniform field prebreakdown current pulses in negatively stressed n-Hexane

To better understand how breakdown starts in an electrically-stressed liquid, the lowest measureable levels of prebreakdown currents were studied. Knowledge of the factors that influence and enhance prebreakdown activity and an understanding of the mechanisms leading up to breakdown would allow development of liquid dielectrics with higher strength and improved aging properties.

High-bandwidth measurement of low-level prebreakdown currents in liquid dielectrics

The development of a measurement system optimized for making low-noise, high-bandwidth measurements of prebreakdown currents in liquid dielectrics under DC stress is described. The system consists of a 90-MHz bandwidth transimpedance amplifier with a 5000-V/A gain, 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 300 nA for a signal-to-RMS noise ratio of five. Waveform reconstruction software is used to quadruple the effective sampling rate from 5 ns per point to 1.25 ns per point. Initial experimental results from the use of this measurement system are presented. Prebreakdown currents were measured in hexane using a point-plane electrode geometry with the needle connected to the preamplifier input and either positive or negative high voltage applied to the plane. Pulses with extremely short durations (<8 ns full width at half-maximum amplitude) and amplitudes ranging from tens of microamperes down to near the noise level of the measurement system were observed. The usual pulse shape appears to be the system impulse response, which implies that the current pulses are very fast indeed.<<ETX>>

New results in the search for electrical signals during water tree growth

Polyethylene samples, printed in one region with needles to greatly accelerate water treeing, were exposed to high voltage (60 Hz AC and DC) and a water treeing solution. A very-low-noise, wide-bandwidth detection system was developed to monitor signals occurring in the sample via two probes located under the sample, one at a reference region and the other at the treeing region. Two distinct signals have been found that originate only in the active needle printed region. It has not been determined conclusively however, that the detected signals are due to water treeing, just that they originate at the water treeing site.<<ETX>>

Electrical signals during water tree growth

A possible diagnostic is being tested that could provide in real non-invasively a temporal record of water tree initiation, growth, sition to an electrical tree without disturbing the usual arrangem insulation between two electrodes. The technique, detecting imag in existing electrodes, is very challenging and appears to require noise, high bandwidth detectors and careful interpretation of ele conditions. A previously described test system was used to attempt detectic image current signals while water trees grew in two polyethylene s ples. Two independent probes and detectors were used on the San but only one near p~ active water treeing region. Calibration tests signal originating in the sample and at one probe was detected excl that probe, but signals originating efsewhere were detected simultar both probes. Thus from signals observed exclusively in the probe at trees, the da ta suggest that fast pulses originate in the sample a t th during water treeing, and a preliminary observation of their rate was made. Each of two samples was placed with its water treeing rr a different detector probe, and the exclusive pulses were always foi channel at the tree site. INTRODUCTION A possible diagnostic is being developed that could monitor treeing activity in polyethylene (PE) sensitively, non-invasively, ai time in the usual test or application geometries. It is based on a successful diagnostic for prebreakdown studies in liquids [1,2]. In CEIDP meeting the design and operation of a test system for wat in PE was described. [3]