Ja-Yoon Koo

An experimental investigation on the degradation characteristic of the outdoor silicone rubber insulator due to sulfate and nitrate ions

Since 1960, polymer insulator has been employed as an outdoor insulator for high voltage ascribed to its excellent aging characteristics compared to those of ceramic insulator. However, in spite of various investigations related to the polymer insulator, its aging mechanism due to acid rain have not been well elucidated. In this paper, aging characteristics of outdoor silicone rubber insulator resulting from the acid rain have been investigated with regards to sulfuric and nitric solutions. For this purpose, silicone rubber was aged by being immersed in the sulfuric and nitric solutions with different level of pH (5.6, 4.0, 2.5, 1.5) during 1500 hours and various tests were performed such as tracking, contact angle, tan/spl delta/, breakdown voltage, SEM, EDS, FT-IR. Regarding the effect of the acidity of nitric solution, the surface of test samples were slightly damaged up to pH 2.5 and ruined at pH 1.5. However, in case of sulfuric solution, the degree of the surface degradation was observed to be consistently increased. And thus, for the former, it could be expected that there is certain critical value between pH 1.5 and pH 2.5, which likely could induce serious surface degradation For both cases, it was observed that the hydrophobicity and tracking resistance of the silicone rubber insulator are well retained regardless of the acidity of the solution.

An Application of the Novel Techniques Detecting Partial Discharge Employable to GIS Using Optical Sensor

A novel technique has been proposed and related experimental works have been performed in order to detect the partial discharges and the location of the possible defects introduced into the Gas Insulated Switchgear. For this purpose, a prototype HY Pockels sensor has been developed and then employed in order to investigate its field applicability for finding the location of the defects using a 170kV GIS mock-up. Our proposed sensor enables us to measure the electric field variation due to the PD occurrence. In addition, the different PD patterns are observed, which might be dependant on the location and the distance of the sensor with respect to the PD source. Throughout this work, its linear response has been proved to be admissible as a function of the applied voltage. And also the position of the PD source might be distinguished by comparing the PD patterns.

Discrimination of Insulation Defects in a Gas Insulated Switchgear (GIS) by use of a Neural Network Based on a Chaos Analysis of Partial Discharge (CAPD)

In this work, experimental investigation is carried out in order to design and fabricate the UHF sensor that is able to detect the partial discharges produced from 10 artificial defects introduced into the real scale 70kV GIS mock-up under high voltage within a well shielded room. As well, in order to verify the on-site applicability of our method, the newly proposed CAPD (chaos analysis of partial discharge) is combined with spectral analysis for identifying the nature of 10 artificial defects under investigation. The PD pattern recognition of each defect has been fulfilled by applying our ANN software. The result indicates that the recognition rate reaches up to 80% by the newly proposed method while the traditional PRPD analysis method allows us to obtain 41%. In consequence, it can be pointed out that the proposed method seems likely to be applicable to the real GIS at the site.

A study on internal temperature monitoring system for power transformer using pptical fiber Bragg grating sensors

A precise information on spatial distributions of temperature are important for diagnosing power transformers and evaluating its life because a power system failure will result in an enormous loss in tangible as well as social. The optical fiber Bragg grating (FBG) sensors, which have been studied intensively for last decade, can be very efficient tools for applications to above mentioned purpose because these are immune to EMI and can be highly multiplexed, which enables efficient quasi-distributed temperature sensing along tens of km range. We proposed a fiber-optic temperature monitoring system with an array of over 4 sensor gratings. The temperature-induced Bragg wavelength variations are accurately monitored by a scanned tunable wavelength filter. Differential measurement with temperature-stabilized reference gratings and a Gaussian curve-fitting algorithm has been used to enhance measurement accuracy, which obtained temperature resolution of ~0.6degC, and linearity error less than 0.4 %.

Comparison of diagnostic method for the pattern analysis due to artificial defects introduced at the interface in XLPE cable joint using laboratory model

In this work, in order to realize the possible defects at the cable joint interface, four different types of artificial defects are provided such as conducting, insulating substances, void and scratches. The analysis related to the PD patterns has been performed by means of phase resolved partial discharge analysis (PRPDA) and chaotic analysis of partial discharge (CAPD) as well. The latter has been proposed by our previous communication. As a result, it could be pointed out that each defect has shown particular characteristics in its pattern respectively and that the nature of defect causing partial discharge could be identified more distinctively when the CAPD is combined with conventional statistic method, PRPDA.

A possible application of the PD detection technique using electro-optic Pockels cell with nonlinear characteristic analysis on the PD signals

In this paper, new Partial Discharge (PD) detection technique using Pockels cell was proposed and considerable apparent chaotic characteristics were discussed. For this purpose, PD was generated from needle-plane electrode in air and detected by optical measuring system using Pockels cell, based on Mach-Zehnder interferometer, consisting of He-Ne laser, single mode optical fiber, 50/50 beam splitter and photo detector. In addition, the presence of chaos of the PD signals has been investigated by examining their means of qualitative and quantitative information. For the former, return map and 3-dimensional strange attractor have been drawn in order to investigate the presence of chaotic characteristics relevant to PD signals, detected through CT and Pockels sensor respectively, in the normalized time series. The presence of strange attractor, indicates the existence of fractal structures in its phase space. For the latter, several dimension values of strange attractor were verified sequentially. Throughout this paper, it is likely that the chaotic characteristics regarding the PD signals under air are verified.

Comparison Between PD Inception Voltage and BD Voltage of PPLP in

With the rapid development of world economics, demand for electricity in metropolitan areas has increased dramatically. The high-temperature superconductivity (HTS) cable is one of the most promising technologies for solving the bottleneck of the transmission system. However, power system planners have not yet considered the HTS cable as a mature technology due to its weak insulation characteristics with respect to those of conventional power cables. The electrical insulation design is one of the major issues for HTS cable. It is evident that the insulation in the dc cable should be investigated further at this time. For the insulation design of the HTS cable, it is necessary to investigate the breakdown (BD) characteristics and the partial discharge (PD) inception voltage of the liquid nitrogen (LN2)/polypropylene laminated paper (PPLP) composite insulation system. In this paper, BD voltage and PD inception voltage of PPLP immersed in LN2 was compared under ac and dc stresses. Various kinds of PPLP specimens considering the different layers and the butt gaps were prepared. The PD inception voltage was measured in a shield room according to IEC 60270. Consequently, it was investigated that characteristics of the PD inception voltage and the BD voltage was rather different when ac and dc voltages were respectively applied in LN2, and these results could be used for the insulation design of ac and dc HTS cables.

\hbox{LN}_{2}

Cryogenic dielectric insulation skills play a significant role in the development of superconducting electric equipment for transmission and distribution electric network. Nowadays, newly developed nano-composites have shown enhanced electrical, thermal, and mechanical properties of polymer insulation materials. And the application of nano-composites in high voltage power systems could be implemented in the near future. Among the various nano-composites, epoxy nano-composites have been paid much attention as a new insulating material for high voltage insulation. In this paper, we presented experimental results of epoxy nanocomposites in liquid nitrogen and determined the possible applications of nano-composites as insulating material for superconducting equipment. In order to determine their dielectric breakdown properties in liquid nitrogen, various kinds of epoxy based nano-composites have been made by mixing SiO2, Al2O3, TiO2 fillers, respectively. AC withstand voltage test and partial discharge (PD) inception voltage tests have been performed to verify the insulation breakdown characteristics of nano-composites in cryogenic environment. Consequently, it was deduced that the breakdown strength of epoxy nano-composites have been improved compared to epoxy with micro-fillers in cryogenic environment. And epoxy nano-composites immersed in liquid nitrogen showed a similar breakdown voltage characteristic when compared with that of insulating oil. In addition, the effects of nano-composites have been varied according to the concentration of nano-fillers and their content of fillers.

Considering HTS Cable Insulation

In this work, in order to realize the possible defects at the cable joint interface, four different types of artificial defects are provided such as conducting, insulating substances, void and scratches. The analysis related to the PD patterns has been performed by means of Phase Resolved Partial Discharge Analysis (PRPDA) and Chaotic Analysis of Partial Discharge (CAPD) as well. The results show that it could be pointed out that each defect has shown particular characteristics in its pattern respectively and that the nature of the defect causing partial discharge could be identified more distinctively when the CAPD is combined with a conventional statistical method, PRPDA.

High Voltage Dielectric Characteristics of Epoxy Nano-Composites in Liquid Nitrogen for Superconducting Equipment

Inverter driven induction motors with high speed switching and advanced PWM techniques are widely used for variable speed applications. Recently, the insulation failures of stator windings have attracted much concern due to high dv/dt of IGBT PWM inverter output. In this paper, the detailed insulation test results of 25 low-voltage induction motors are presented. Five different types of insulation techniques are applied to 25 motors. The insulation characteristics are analyzed with partial discharge, discharge inception voltage, AC current, and dissipation factor tests. Also, breakdown tests by high voltage pulses are performed.