Partial Discharge Inception Electric Field Research Articles

A Comparative Analysis of Partial Discharge in 13 Combined Insulation Structures of 11 Materials Used in Cast-Resin Dry-Type Transformers

Partial discharge (PD) in cast-resin dry-type transformers (CRDTs) is an important issue that needs to be overcome. In this study, PD in insulation materials used in CRDT windings is investigated. Four types of samples are prepared using 11 different types of materials, including epoxy resin, three-ply glued polyester insulation, prepreg insulation, glass fiber mesh, polyester adhesive tape, polyester film, polyimide film, rectangular conductors with polyester insulation, round conductors with polyester insulation, round conductors with enamel insulation, and foil conductors without insulation, and subjected to PD measurements. The samples are placed into a mold for resin casting that is similar to those used to fabricate large cast-resin windings, and the material behavior is tested. PD measurements are carried out in the high-voltage laboratory of the BEST Transformer Company. The measured data are used as inputs to models of the samples that are simulated using finite element analysis software to determine the PD inception electric field. The PD patterns of the samples are compared with well-known PD patterns in the literature. Finally, a comparative analysis of the materials is performed, and strong and weak materials against PD are identified. The results of this study show that laminated insulation materials or fiberglass meshes, both with epoxy resin, which have PD inception at a mean value of 3.4 kV/mm of electric field intensity, are more susceptible to PD than the other insulation materials investigated. In contrast, insulated conductors or nonlaminated polyester or polyimide films, all with epoxy resin, which have PD inception above a mean value of 5.3 kV/mm of electric field intensity, are robust to PD.

Effects of filler composition, loading, and geometry on the dielectric loss, partial discharge, and dielectric strength of liquid metal polymer composites

With the goal of enabling the design of liquid metal polymer composites (LMPCs) with desired electrical and mechanical properties while preventing premature dielectric aging and failure, the dielectric breakdown characteristics of LMPCs containing galinstan (GaInSn), barium titanate (BTO), iron (Fe), and mixtures thereof are investigated. The dielectric properties including dielectric loss (tan δ), partial discharge inception electric field (PDIE), and breakdown electric field (Ebd) are investigated. According to the study, LMPCs with a higher overall filler concentration tend to show higher tan δ, lower PDIE, and lower Ebd. The study also demonstrates that these dielectric properties vary by the type of filler even with an equally maintained filler concentration. This is due to the distinct filler conductivity, permittivity, and geometry that cause differences in the electric field distribution within the composites. To provide further insights into the observed experimental results, finite element analysis (FEA) models were developed and analyzed.

Dielectric Characteristics of Liquid Nitrogen/Synthetic Paper Composite Insulation System for Extra High Voltage HTS Cable

For extra-high-voltage high-temperature superconducting (HTS) cables, not only high electrical insulation performance but also low dielectric loss is required. Therefore, we focus on a porous synthetic paper with lower dielectric loss, e.g., Tyvek polyethylene nonwoven fabric, than that of polypropylene laminated paper (PPLP), which has been used for a conventional HTS cable insulation system. In this paper, we compare partial discharge inception characteristics for different liquid nitrogen/synthetic paper composite insulation systems. The partial discharge inception electric field strength in the composite insulation system depends on the electric field strength in the porous area of synthetic paper.

Influence of Combined AC–DC Voltages on Breakdown Characteristics of Epoxy Resin in LN2

In a dc superconducting cable system, the ripple waves contained lots of ac components that can superimpose dc voltage and current and are widely applied in rectifiers and other power electronic devices, which threaten the reliability and safety of a high-temperature superconducting (HTS) insulation system. In this paper, the partial discharge (PD) characteristics of the epoxy resin are studied with different ac/dc proportions in liquid nitrogen and room temperature. The results show that the PD inception electric field and breakdown strength of the epoxy resin increase as the dc component increases in both liquid nitrogen and room temperature. According to the phase-resolved PD distribution obtained in the inception stage, PDs show different behaviors at a higher dc component. The mean and total PD amplitude decrease as the dc component increases in both the inception stage and prebreakdown stage; they depend on the electric field modified by space charges. Compared with the PD characteristics in room temperature, they are weaker in liquid nitrogen, indicating the PD detection of insulation materials should be enhanced to eliminate the latent destruction in the HTS system. The results establish the foundation for diagnosis and assessment of insulation materials in liquid nitrogen with combined ac-dc voltages.

Partial discharge characteristics in composite insulation systems with PPLP for HTS cable

The electrical insulation system of high-temperature superconducting (HTS) cable consists of liquid nitrogen (N2(l)) and polypropylene laminated paper (PPLP). Partial discharge (PD) may occur in butt gaps of the insulation layers and its characteristics imply the insulation performance of HTS cables. N2(l) cooling system is installed in the power system and N2(l) will flow through the cables during the system operation. Filling the HTS cable with N2(l) in order to perform pre-shipment inspection is timeconsuming and costly for cable manufacturers. Therefore, the authors are trying to find a cost effective method for pre-shipment inspections. One alternative is to use high pressure gaseous nitrogen (N2(g)) instead of N2(l). This article investigates PD characteristics such as PD inception electric field (PDIE) and PD extinction electric field (PDEE) in butt gaps of HTS cables in 0.1-MPa to 0.3-MPa N2(l) and 0.1-MPa to 1.0-MPa N2(g) environments. For assessing the surface/volume effects, PD characteristics are measured with changing the size of butt gaps. It turns out that PDIE and PDEE in N2(g) are linearly correlated with those in N2(l) at any gas pressure in our testing, and PDIE in 1.0-MPa N2(g) is almost 30% of that in 0.2-MPa N2(l). It suggests that PD characteristics in N2(l) can be extrapolated from those in N2(g).

Partial discharge inception electric field strength of water droplets on polymeric insulating surfaces

Water droplets are formed on the surface of polymeric insulators under wet conditions, which enhance the electric field especially in close vicinity of the contact line between air, water droplet and insulator surface. It is experimentally found that the maximum allowable E-field strength tangential to a polymeric insulating surface in presence of a single water droplet on the flat surface with wettability class 1 is between 6 and 7.5 kV/cm (peak). Some important parameters that have an influence on PD inception are experimentally investigated to find more reliable results. A simple prediction method of partial discharge inception E-field strength of a single water droplet on polymeric insulating surfaces based on the streamer criterion for inception is also proposed and discussed. The investigated PD inception E-fields in this contribution can be used for prediction of partial discharge inception voltage of polymeric insulators under wet conditions.

Partial Discharge Characteristics in Composite Insulation Systems with PPLP for HTS Cable

The electrical insulation system of high-temperature superconducting (HTS) cable consists of liquid nitrogen (N2(l)) and polypropylene laminated paper (PPLP). Partial discharge (PD) may occur in butt gaps of the insulation layers and its characteristics imply the insulation performance of HTS cables. N2(l) cooling system is installed in the power system and N2(l) will flow through the cables during the system operation. Filling the HTS cable with N2(l) in order to perform pre-shipment inspection is time-consuming and costly for cable manufacturers. Therefore, they are trying to find a cost effective method for pre-shipment inspections. One alternative is to use high pressure gaseous nitrogen (N2(g)) instead of N2(l). This article investigates PD characteristics such as PD inception electric field (PDIE) and PD extinction electric field (PDEE) in butt gaps of HTS cables in 0.1 to 0.3 MPa and 0.1 MPa to 1.0 MPa N2(g) environments. For assessing the surface/volume effects, PD characteristics are measured with changing the size of butt gaps. It turns out that PDIE and PDEE in N2(g) are linearly correlated with those in N2(l) at any gas pressure in our testing, and PDIE in 1.0 MPa N2(g) is almost 30% of that in 0.2 MPa It suggests that PD characteristics in N2(l) can be extrapolated from those in N2(g).

Investigation of Partial Discharge and Fracture Strength in Piezoelectric Ceramics

Although piezoelectric ceramics are widely used, the structural reliability of these materials in long‐term service is still a concern. In this study, a nondestructive method for assessing material strength reliability using partial discharge (PD) tests is proposed. The PD inception electric field and the flexure strength are both controlled by the defects in the ceramics which suggests that the PD test can be an alternative nondestructive method for evaluating the structural reliability of piezoelectric ceramics. PD and four‐point flexure tests were carried out on piezoelectric ceramics. The influence of the PD test on samples was investigated by comparing the microstructure and electrical properties before and after the PD test. Samples which were not PD tested were also fractured and compared. It was found that no strength degradation was caused by the PD test. Weibull analysis revealed a correlation between the distribution in values for PD inception electric field and flexure strength.

Application of contact analysis on evaluation of breakdown strength and PD inception field strength of solid-solid interfaces

Interfaces between solid insulating materials are generally weak regions in electrical insulation systems, particularly if the electrical stress is applied in parallel direction. This paper presents a theoretical mechanical model for estimating average size of air-filled voids at the interface as a function of surface roughness and contact pressure. It is argued that the interfacial breakdown strength (BDS) is governed by the discharge inception stress (E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</sub> ) of the void with the most likely estimated dimension. The estimated values of the breakdown strength and partial discharge inception electric field strength (PDIE) were compared with the results from measurements using XLPE specimens with interfaces energized in longitudinal direction. The measured ratio of increase of breakdown strength was found to be in good agreement with the estimated breakdown values of the most likely interface's void. Additionally, it was found out that the air pressure inside voids was not affected by applied contact pressure. The estimated PDIE values was found to be in agreement with the measured values, in the case of rough surfaces, but not for smooth surfaces. The results indicate that the mechanical contact approach using the motif description of surface roughness may improve the understanding of the factors influencing the PDIE of electrically stressed interfaces.

Dielectric Characteristics of HTS Cables Based on Partial Discharge Measurement

We have discussed dielectric characteristics of liquid nitrogen (LN/sub 2/)/polypropylene (PP) laminated paper composite insulation system for the practical electrical insulation design of high temperature superconducting (HTS) cables. Focusing on the partial discharge (PD) inception characteristics and mechanism, the volume effect and its saturation on PD inception electric field strength (PDIE) was evaluated at atmospheric and pressurized condition. PD inception characteristics under lightning impulse voltage application were also investigated using an electrical and optical PD measuring system, and compared with those under ac voltage application.

Fundamental Insulation Characteristics and Impulse Ratio of CO2 Gas

SF6 gas has been used as an excellent insulating and arc-quenching gas for electric power apparatus such as SF6 gas insulated switchgear (GIS). However, since SF6 gas was specified to be a greenhouse gas because of its high global warming potential (GWP), environmental-friendly gases or gas mixtures alternative to SF6 gas are strongly required to be developed. Especially, the development of insulation gases without SF6 gas is an important task for the future electric power system.We focused our research on CO2 gas and CO2-based gas mixtures such as CO2/O2, CO2/N2 with small GWP as SF6 substitutes, and investigated partial discharge (PD) and breakdown (BD) characteristics under lightning impulse and ac voltage applications. In this paper, PD inception electric field (PDIE), BD electric field (BDE) and the impulse ratio were obtained in CO2, N2, SF6 gases and CO2-based gas mixtures, and discharge mechanisms of CO2 gas were discussed. Experimental results revealed that impulse ratio in CO2 gas was higher than those in SF6 and N2 gases, and dielectric-coating effect in CO2 gas was higher than that in SF6 gas. These results were attributed to the lower generation probability of initial electrons in CO2 gas.

V-t characteristics of partial discharge inception in liquid nitrogen/ PPLP/sup /spl reg// composite insulation system for HTS cable

In this paper, we studied partial discharge (PD) inception characteristics and V-t characteristics of PD inception in liquid nitrogen (LN/sub 2/)/polypropylene laminated paper (PPLP/sup /spl reg//) composite insulation system for high temperature superconducting (HTS) cable. Experimental results revealed that the magnitude of initial PD was in the range of 2.0-30 pC irrespective of butt gap condition and the initial PD was generated at the first and third quadrant of voltage phase. PD inception electric field strength (PDIE) without butt gap was 5-10% higher than that with butt gap and thicker butt gap gave larger PDIE drop. Moreover, PDIE with butt gap depended on the butt gap thickness and the number of PPLP/sup /spl reg// layers. The reason is explained by the existing probability of weak points of electrical insulation at butt gap. Finally, lifetime indices n of V-t characteristics at PD inception were obtained as 80-100 irrespective of butt gap condition. These values showed enough flat characteristics of V-t phenomena of electrical insulation.

Partial discharge inception characteristics under butt gap condition in liquid nitrogen/ PPLP/sup /spl reg// composite insulation system for high temperature superconducting cable

The partial discharge (PD) inception characteristics are studied in liquid nitrogen (LN/sub 2/)/polypropylene laminated paper (PPLP/sup /spl reg//) composite insulation system for high temperature superconducting (HTS) cable. Experimental results revealed that the magnitude of the initial PD increased as the PD inception electric field strength was increased, because the injected energy increased. Initial PD was generated at the first and third quadrant of applied AC voltage phase. The probability of initial PD at the positive and negative voltage phase was almost the same. The reason is because liquid nitrogen is a nonpolar molecule and we used symmetric electrode configuration with uniform electric field distribution. Finally, it was pointed out that PD inception electric field strength (PDIE) depended on the volume of the butt gap because of the increasing probability of weak points of electrical insulation, and PDIE linearly decreased with increasing stressed volume of the butt gap in the log-log scale.

油浸絶縁構成における部分放電発生に及ぼす粒子形状と材質の影響

The effect of particles' shape on partial discharge inception electric field was investigated in oil immersed insulating system. Particles' shape effect can be evaluated by shape- parameter for copper particles. Various particles were evaluated in material, size and density on partial discharge inception electric field. The metallic particle causes the initiation voltage of partial discharge to be lower than the dielectric particles. However the property can be deteriorated by dense dielectric particles remarkably. The motion of dense dielectric particles was observed with high speed camera at the instance of partial discharge initiation. It is found that partial discharge depends on the motion of particles rather than the bridge formation of particles across the gap. Positively charged particle arrives at pressboard on negative electrode to cause partial discharge.

油浸絶縁構成における部分放電開始電界に及ぼす球状金属粒子の数と粒径の影響

油浸絶縁構成における部分放電開始電界に及ぼす球状金属粒子の数と粒径の影響