E.O. Forster

Partial discharge distributions in liquid dielectrics

Different dielectric fluids used in HV electrical components were subjected to AC stresses, and the partial discharge (PD) spectra produced under nonuniform and uniform field conditions were recorded with the help of an electrical detector and a multichannel analyzer (MCA) as a function of applied stress. The analysis of the collected data confirms that PD activity in a liquid is a random phenomenon as well as a breakdown process. The presence of PD may be related with the presence in the liquid of bubbles near the electrode interface and with a low density region (vapor phase), which is the first step in the breakdown development. This confirms that PD occurs only in a gas and not in a solid or liquid. In this paper, which is part of an ongoing research project, it is shown that MCA can be a helpful tool in PD measurements and analysis, allowing the demonstration of how the partial discharge inception voltage (PDIV) significantly changes according to the adopted inception criteria. >

The effect of molecular structure on the properties of dielectric fluids

The dielectric properties of six test fluids (MDBT, DTE, DINP, PXE, PFPE, and PPMS) were evaluated under AC and DC stresses. The data suggest that DC conduction is less sensitive to molecular structure effects than to impurities while AC conduction reflects more molecular characteristics. Within a family of fluids such as liquid aliphatic hydrocarbons, both the AC and DC breakdown level are adversely affected by impurities which increase their conductivities. Among different types of fluids, molecular structure has a significant impact on DC impulse breakdown. Conventional (50 or 60 Hz) AC breakdown appears to be much less sensitive to structural effects. >

Streamer formation in perfluoropolyether under impulse conditions

A family of perfluorinated fluids that recently has become available, called perfluoropolyethers, contain oxygen in addition to fluorine and carbon and are potentially interesting as electrical insulating fluids. A study of their behavior under electrical pulse conditions is reported. Under the prevailing nonuniform field conditions, the breakdown voltage was lower when the needle was a cathode than when it was an anode. The streamer structures and their inception voltages are studied in some detail. The effect of pressure on the streamer structure is evaluated. >

A study of partial discharge measurements in dielectric liquids

An investigation was undertaken to study the partial discharge behavior of some well-defined dielectric fluids such as phenyl xylene ethane, ditolyl ether, mono and dibenzyl toluene and perfluoro polyether. The partial discharge inception and extinction voltages of these fluids were determined under uniform and nonuniform field conditions. The results of this study are presented and their significance is discusses in detail. >

The effect of the definition used in measuring partial discharge inception voltages

At present, several definitions of partial discharge inception voltage (PDIV) have been advanced. To establish the value of each of these definitions, four practical electrical insulating fluids are selected, and their PDIV is determined according to three of these different specifications. The results suggest that the use of PDIV as a quality criterion generally is not warranted. The significance of these findings is discussed. >

Study of the time to breakdown in transformer oil under impulse conditions

The time to breakdown in dielectric fluids has been studied extensively, and it has been shown that the time elapsed between the start of the pulse and breakdown decreases with increasing voltage. Basically, two regimes can be distinguished, depending on whether breakdown occurs on the rise of the pulse or on its tail. The first regime occurs at much higher voltages than the latter one and it usually occurs at small electrode separations. In the first regime the time to breakdown decreases more or less regularly with increasing crest voltages while in the second one the time to breakdown shows interesting statistical fluctuations. The significance of these fluctuations is discussed. An attempt is made to show that these temporal fluctuations at more or less constant crest voltage are a reflection of the electron emission characteristics of the cathode. >

Current pulse measurement system using LED in dielectric liquids under impulse voltage conditions

The time response and the charge sensitivity of an LED (light emitting diode) current pulse measurement system (LCPMS) are evaluated. In addition, the current pulse and the streamer initiation in dielectric liquids are measured simultaneously by using this system and a high-speed shadowgraph system under impulse voltage conditions. It is concluded that the LCPMS can measure partial-discharge (PD) pulses under impulse voltage conditions. The charge sensitivity of the LCPM is markedly increased by applying an optimal DC drive current to the LED. PD inception and streamer initiation were measured in cyclohexane under impulse voltage conditions. A PD current pulse of 8.2 pC was measured during negative streamer initiation.

The inception and structure of prebreakdown streamers in perfluorinated polyethers

Electrical measurements of perfluorinated polyethers were performed to determine the suitability of this class of material as a liquid dielectric. Measures of the breakdown strength and streamer inception voltage are obtained under electrical impulse stress. The breakdown strength is nearly independent of polarity. Under high magnification photography the cathode-originated streamers are seen to have a subsonic mode of growth. The transition to fast growth occurs on time and spatial scales shorter than those found in liquid hydrocarbons. The gas-phase electrical properties and the high heat of vaporization of this material are considered as mechanisms for this behavior.<<ETX>>

Simultaneous measurements of streamer and partial discharge processes in perfluorinated polyether

In this paper, the effect of polarity on prebreakdown phenomena in perfluorinated polyether (PFPE) under nonuniform field was investigated. The inception and propagation of streamers were observed using a high speed and high magnification shadowgraph system. The current pulses were also measured simultaneously using a LED current pulse measurement system. The streamer inception voltage for both polarities were -9.99 kV and +12.86 kV, respectively. The deviation of time delay from the voltage application to the streamer inception was larger for a positive point than a negative. The initial shape of the negative streamer was a filament which emanated from a needle tip. On the other hand, for a positive point, a filamentary streamer started at about 9 /spl mu/m away from a needle tip, and propagated toward a needle tip. These experimental results suggested that the inception of a positive streamer needs a much higher field than a negative one due to the higher ionization potential of the fluorine atom in PFPE as well as breakdown voltage.<<ETX>>

Ultrahigh frequency study of PD pulses

Conventional partial discharge (PD) measurements are carried out with instruments of relatively limited bandwidth. There arises the question as to the temporal characteristics of the actual pulses and the significance of the results so obtained. To gain a detailed knowledge of the rise time, duration and amplitude of these pulses, a study has been undertaken using as detector a 500 MHz digitizing oscilloscope in combination with a point-plane electrode geometry. In this paper the pulse characteristics of PD in air were studied as a function of polarity under dc and ac conditions.<<ETX>>