Fabio Ciani

Analysis of partial discharge phenomena in paper-oil insulation systems as a basis for risk assessment evaluation

Partial discharge (pd)-based risk assessment in electrical insulation systems becomes effective only if the nature of the defect generating pd can be ascertained. In oil/paper insulation systems this is a fundamental item since e.g. harmfulness of pd generated by gas bubbles is much smaller than that of pd occurring between paper layers. For such reasons, partial discharge analysis of oil/paper insulated systems often lacks a clear correlation between cause (i.e., defect type) and effect (partial discharge recording). Results relevant to partial discharge measurements performed on lab-scale objects having different electrode configuration are presented in the paper with the purpose to spotlight characteristic features aimed at identifying the basic nature of partial discharge-generating defects in oil/paper insulated systems.

Diagnosis of EHV and HV Transformers Through an Innovative Partial-Discharge-Based Technique

Online partial discharge (PD) analysis of power transformers is sometimes complex due to the presence of large disturbance and noise as well as to uncertainty in PD source identification. In this paper, a novel approach to PD detection and analysis in online power transformers is presented. The proposed approach provides hardware and software tools to remove disturbance, noise, separate contributions from different sources and, eventually, identify PD sources. Besides, a description of the hardware tools used to assemble the diagnostic system, the tests (in the lab and on real transformers) carried out to build the reference database needed for identification system training will be discussed, providing rules for PD source identification. A number of working cases will be presented to show the effectiveness of the proposed method.

First electron availability and partial discharge generation in insulation cavities: effect of light irradiation

An experimental investigation of the behavior of partial discharges occurring in insulation cavities as a function of light irradiation conditions is presented in this paper. Partial discharge inception voltage (PDIV) measurements were performed for different irradiation conditions of the test objects (visible light, of variable intensity, ultra violet (UV) light or absence of irradiation), defect geometry and conductivity of insulating material. It is shown that changes of such factors involve considerable alteration of partial discharge inception features, in terms of both inception voltage and delay time. Depending on applied field, defect geometry, insulating material characteristics and irradiation condition, in fact, partial discharge inception voltage may be influenced prevailingly by background radiation, as generally stated in literature, or by field-assisted electron detrapping from material-cavity interface.

The effect of space charge on phenomenology of partial discharges in insulation cavities

The effect of space charge accumulated in insulation cavities on partial discharge activity is dealt with in the paper. Partial discharges were measured on non-polarized and DC-polarized XLPE specimens. In order to characterize space charge distribution, pulsed-electroacoustic (PEA) space charge measurements were also performed on polarized and PD-aged specimens. The results show that space charge can affect, although in some complex way, partial discharge phenomenology. Moreover, space charge measurements on stressed specimens can help in the inference of degradation mechanisms (e.g., trap formation at the cavity surface) associated to partial discharge activity.

Electric strength, space charge and surface discharge characterization of nanostructured epoxy-silicate insulating materials

A set of surface discharge ageing procedures, space charge observations and dielectric strength measurements has been implemented on a type of insulating nanofilled epoxy-resin (nanostructured by compatibilised synthetic layered silicates) and on its non-filled base resin. Promising improvements in the electrical behaviour of the nanofilled epoxy, particularly as concerns surface discharge endurance, were detected. Regarding space charge, it was found that its accumulation rate as a function of ageing time is lower for the nanostructured resin. Finally, the nanofilled material evidenced a highly homogeneous behaviour of the dielectric strength data distribution.

Artificial intelligence algorithms for the recognition of PD-generating defects in rotating machines

This paper presents an innovative technique for partial discharge source identification in rotating machines. Source identification is based on artificial intelligence tools that rely upon fuzzy inference algorithms and operate through a tree-like structure. The third level of the tree-like structure, specific for rotating machine defects, is described in this paper with emphasis on bar-to-bar discharges. Its effectiveness in the identification of defects generating PD, even in the presence of noise or overlapped PD phenomena, is discussed on the basis of lab tests performed on defective generator bars.

First electron availability and PD generation in insulation cavities

An investigation on the behavior of partial discharges occurring in insulation cavities as a function of environmental conditions is presented in this paper. In particular, focus is made on peculiar conditions where semi-transparent insulation is subjected to different visible light intensity, is irradiated by UV or is kept in the dark. Experiments are carried out on cells with an artificial defect, obtained producing a spherical void in epoxy resin. It is shown that not only PD inception voltage is sensitive to fight wavelength and intensity, but also PD average delay time changes considerably. It can be speculated that inception voltage can be related prevailingly to either background radiation or field-assisted electron detrapping from defect surfaces, depending on defect geometry and environmental conditions.

Comparison and electrical modeling of new polymeric cellular electrets

A new family of cellular polymers - that, upon appropriate charging, reveals interesting piezoelectric properties - has been developed recently. Its major characteristic is thermal stability up to high temperatures, which broadens the potential field of application. These materials become electrets upon charging under high levels of electric field. Investigation of charging mechanisms is fundamental to reach optimization of electret arid piezoelectric properties. This paper presents results and considerations relevant to contact charging of these new materials, pointing out the relation between partial discharge and space charge formation, as well as time and temperature stability of electret properties. The concepts of threshold for partial discharge inception and space charge accumulation, the modeling of avalanches in cavities, and the rate of charge generation are exploited trying to explain charging mechanism and. therefore, they provide useful indication for optimization of material manufacturing.

Improved identification level for defects generating partial discharges in solid insulation systems

Partial discharge analysis should provide a detailed description of insulation defects to ascertain failure risk accurately. The authors have devised previously a fuzzy tool that identifies fundamental properties of discharges generated by defects, i.e., internal, surface or corona discharges. This paper describes a new fuzzy inference engine that is aimed at deciding whether an internal cavity is close to an electrode and, in any particular case, which electrode (high or low voltage). A physical explanation of the rules used to build the fuzzy engine is provided, then examples of application to real apparatus containing artificial defects are shown.

Diagnosis of EHV and HV Transformers Through an Innovative Technique: Perspectives for Asset Management

The application to power transformers of a novel system for partial discharge detection and analysis is presented in this paper. The system exploits the diversity of partial discharge pulses to separate phenomena relevant to disturbance and/or multiple sources. By this way, measurement data can be treated in a way that allows accurate diagnostic results to be obtained. Besides signal processing tools, this system is also endowed with automatic identification routines that have been specialized for paper/oil insulated systems. Working cases are presented to show the effectiveness of the proposed system.