In-Jin Seo

Comparative Evaluation Between DC and AC Breakdown Characteristic of Dielectric Insulating Materials in Liquid Nitrogen

Due to the existence of AC loss in superconducting materials when an alternating voltage is applied, high cryogenic costs are inevitable to operate superconducting devices in AC networks. Therefore applications of superconducting devices in DC electric power networks could be regarded as the optimum choice for superconducting devices, because superconductors show exactly zero resistance to a DC source. Recently, DC superconducting devices such as DC cables have received noticeable attention as DC power transmission lines. In order to develop DC superconducting devices, the DC insulation characteristics in cryogenic liquids should be clarified. However, up to now, limited research has been reported in this field. In this paper, to clarify the different breakdown characteristics of DC and AC applications, various kinds of cryogenic dielectric sheets including Kraft, Kapton (polyimide) and Nomex (polyamide) papers have been prepared. Furthermore, a penetrating breakdown test for three kinds of sheets and turn-to-turn breakdown tests have been performed in liquid nitrogen (LN2). Consequently, it was found that the prepared sheets have shown 1.7-2.7 times higher dielectric strength than those of AC. Moreover, the Nomex and Kraft sheets have shown a remarkable increase in their dielectric strength in liquid nitrogen compared to air. However, the dielectric strength of the Kapton sheet did not show a remarkable increase in liquid nitrogen. From the turn-to-turn breakdown test, it was proved that the dielectric strength has been linearly increased according to the wrapping number of the sheets and that the DC breakdown voltage was 1.1-2.5 times higher than AC.

Identification of Insulation Defects in Cryogenic Dielectric Materials for the HTS Power Applications

Recently, various high temperature superconducting (HTS) power applications have been developed and prepared for field tests and commercial applications. Comparing to conventional power applications, it could offer several advantages such as reduced size and weight, high efficiency, decreased losses, no oil, nonflammable and decrease of CO2 emissions. Besides overload operation is possible with no loss of lifetime. For HTS power applications in low temperature and high voltage environments, partial discharge (PD) measurements in cryogenic dielectric materials of HTS power applications are very important because PD was regarded as primary source for ageing and breakdown of cryogenic materials. One of the diagnostic methods for safety of the power components, the detection of PD taking place inside the apparatus has been widely investigated. The first method, phase resolved partial discharge (PRPD) Analysis was developed in the early 1970s taking the phase information of the applied AC voltage into account. We also proposed a pattern analysis method named chaotic analysis of PD(CAPD) for PDs occurred in liquid nitrogen, considering three normalized parameters obtained from the values between two consecutive PD pulses: amplitude difference (Pt), occurring time difference (Tt) and correlation between Tt and Pt. This pattern analysis method can identify the type of defects by means of PD pattern classification without employing the phase information of the applied voltage signal. For the experimental investigation, three artificial defects have been fabricated considering possible defects formed during the manufacturing process of HTS power applications: turn to turn insulation, floating particle and protrusion. And PD signals originated from these artificial defects are measured and analyzed by means of CAPD. Throughout this work, it seems that the correlation between the consecutive PD pulses, depending on the nature PDs, could be clarified by CAPD.

Identification of Insulation Defects Based on Chaotic Analysis of Partial Discharge in HVDC Superconducting Cable

High-voltage direct current (HVDC) technology is considered to have some important advantages over traditional high-voltage alternating current, such as higher overall efficiency and smaller power losses for long-distance transmission. In addition, applications of superconducting cables in dc electric power networks may realize real zero impedance, and the economic and technical advantages could be maximized. Therefore, many research institutes have tried to develop advanced superconducting cables for HVDC grids with higher reliability, by considering insulation diagnosis in order to avoid unexpected failures. As one of the plausible diagnostic methods for power cables applied to the ac grid, the detection of partial discharges (PDs) taking place inside the apparatus has been widely investigated. With regard to the related PD pattern analysis, a phase resolved PD analysis (PRPDA), which was first developed in the early 1970s, accounts for the phase information of the applied ac voltage. In 2001, we also proposed a method for pattern recognition, i.e., chaotic analysis of PD (CAPD), that considers three normalized parameters obtained from the values between two consecutive PD pulses: amplitude difference (Pt), occurring time difference (Tt), and applied voltage difference (Vt). However, none of the proposed methods of pattern analysis can be employed for PD under dc stress. Therefore, in this paper, we propose a modified CAPD for the related pattern recognition of possible defects inside a joint box and termination of an HVDC superconducting cable. PDs are produced from four artificial defects and are then detected by a self-designed and fabricated sensor, for which the analysis was performed based on our newly modified CAPD.

Experimental and analytical study of the DC breakdown characteristics of polypropylene laminated paper with a butt gap condition considering the insulation design of superconducting cable

It has been reported that the insulation design under DC stress is considered as one of the critical factors in determining the performance of high-voltage direct current (HVDC) superconducting cable. Therefore, it is fundamentally necessary to investigate the DC breakdown characteristics of the composite insulation system consisting of liquid nitrogen (LN2)/polypropylene-laminated-paper (PPLP). In particular, the insulation characteristics under DC polarity reversal condition should be verified to understand the polarity effect of the DC voltage considering the unexpected incidents taking place at line-commutated-converters (LCC) under service at a DC power grid. In this study, to examine the variation of DC electric field strength, the step voltage and polarity reversal breakdown tests are performed under DC stress. Also, we investigate the electric field distributions in a butt gap of the LN2/PPLP condition considering the DC polarity reversal by using simulation software.

Assessments of Low-Temperature Aging Test Method for the Dielectric Materials Immersed in Liquid Nitrogen

Among the various factors influencing the service life of the electric equipment, the performance of dielectric insulation materials has an important role to determine their whole service life. Degradation of insulating materials is fatal to insulation breakdown and finally lead to stop their operations. So the evaluation methods for the degree of degradation and aging for solid, oil and gas insulation have been developed and utilized very effectively to determine their performances. In order to determine the degradation of insulating materials immersed in extremely low temperature media such as liquid nitrogen, the abrupt temperature change from cryogenic to normal room temperature should be considered. But the assessments of low-temperature aging test method for the dielectric materials immersed in liquid nitrogen considering these conditions were not fully reported. Therefore, for the fundamental step to establish the suitable degradation test methods for cryogenic dielectric materials, we focused on the evaluation of aging test methods for dielectric materials exposed to low temperature environments considering thermal shock by cool-down and warm up test, thermal stress test and electric field stress test. In order to verify the effect of various degradation methods on cryogenic dielectric materials, Kraft, Kapton(polyimide) and Nomax type 410 sheets have been prepared and degradation experiments have been conducted. Then dielectric breakdown tests were performed. According to the test results, it was found that thermal shock degradation could be effective factor determining the degradation of cryogenic dielectric materials.

Surface Breakdown Characteristics of Composite Insulating Papers in LN 2 for a Stop Joint Box of DC HTS Power Cable

One of the key components for dc HTS cable for long-distance line is a stop joint box (SJB). Up to now, the SJB has not been commercialized yet due to some difficulties of dc electrical insulation matters in LN2. Basically, the structure of SJB would be similar to the SJB of oil-filled cable because they adopt liquid insulation and sectionalized cooling separations. The insulation structure of conventional SJB based on ac electric field was no more suitable for dc; hence, there is a need to design suitable insulation structure, which can be applied for dc. In this work, in order to find a solution to reduce dc electric field concentration on the polypropylene laminated paper (PPLP) above epoxy spacer, double layer structure composed of PPLP and Kraft were suggested. To verify the effectiveness of this structure for relieving dc electric field, simulation and experiments were performed. From analytical works, the Kraft layer between PPLP and epoxy has shown noticeable electric field mitigation. Furthermore, surface breakdown tests considering PPLP/Kraft combined layers were conducted to verify the insulation properties of double layer structure. Finally, it was deduced that Kraft layer was effective to enhance surface breakdown voltage. Surface breakdown characteristics of PPLP and Kraft were relevant to the surface roughness of insulating materials.

Experimental investigation on the DC breakdown of silicone Polymer composites employable to 500kV HVDC insulator

HVDC technology has been widely used for the bulk power transmission due to its advantages over the conventional HVAC such as higher efficiency with larger current carrying capability and lower electrical losses for long-distance transmission. In this regards, many research institutes have put their intensive efforts to commercialize relevant components in order to realize better HVDC transmission system. One of the critical components for HVDC transmission systems is HVDC polymer insulators which shows rather severe requirements compared to HVAC ones. It has been perceived uneasy to achieve relevant insulation properties with proper design of power apparatus under HVDC. Especially, there is few research works presented concerning the insulation properties of insulator which is a fundamental power component in HVDC system. In this work, dielectric insulation characteristics of various silicone composites have been experimentally investigated under DC stress, of which the results could supply essential base for developing HVDC polymer insulator. For this purpose, polymer composite samples have been prepared in different ways by mixing silicone and ATH respectively and then have been under DC stress to evaluate breakdown characteristics. For the comparison both in DC and AC environment, withstand voltage test and surface discharge test under AC were also performed.

Evaluation of the Reliability of Insulating Gases for the Bushing in Cryogenic Environment

Conventional high-voltage bushings, which adopt SF6 gas as an insulation media, cannot be directly applied in cryogenic environment due to the liquefaction of SF6 gas. In our previous study, it was shown that SF6 gas could not be used as an insulation gas in cryogenic environment, but instead, CF4 gas has shown excellent insulation properties with little liquefaction phenomena, and have a possibility to be applied for the insulation gas for high-voltage bushings in cryogenic environment. However, in order to apply CF4 gas for cryogenic bushing, it should be determined whether CF4 gas can maintain its insulation characteristics in long-term operation. In order to reveal the long-term insulation properties of various gases, which have a possibility to be applied in high-voltage bushings in cryogenic environment, the long-term breakdown characteristics of CF4 gas, SF6 gas, N2 gas, and dry-air were compared over time in a cryogenic environment. For an experiment, a high-vacuum cryostat and a cryo-cooler were prepared to prevent the evaporation of liquid nitrogen for long-term duration. An fiberglass reinforced plastics bushing was installed on the cryostat to apply high voltage up to 60 kV. A rod to plane electrode was set in a cryostat to determine the breakdown voltage of insulating gases. The lightning impulse and ac overvoltage tests have been performed and the test results were analyzed. From the test, the reduction of breakdown voltage of CF4 gas were observed as time passed by, and it was more severe in SF6 . However, stable breakdown characteristic have been shown with N2 gas. Therefore, increasing the pressure of N2 would be a viable option for increasing the breakdown voltage in a cryogenic environment.

Pattern recognition rate comparison for particial discharges at cryogenic temperature

The application of the superconducting power apparatus is now being considered as one of the promising tool for enlarging the limited transmission capacity of the traditional electric power apparatus due to the several technical advantages such as reduced size, weight, high efficiency and so on. Therefore, since more than two decades, many research institutes try to improve performance by carrying out experimental investigations related to the reliability of the apparatus under cryogenic temperature. One of them is Partial discharge (PD) detection which is considered as the indication of the insulation state of the apparatus, however, very few reports have been reported based on the results obtained under cryogenic temperature. In this work, 3 different types of artificial defects are put into Liquid Nitrogen in order to produce PD under AC applied voltage: protrusion, floating electrode, and turn to turn. PD signals are detected by use of our specially designed sensor and then its pattern recognition is made based on PRPDA (Phase Resolved PD Analysis). Regarding the related recognition rate, NN (Neural Networks) is employed for learning process. Moreover, other patterns from the unknown defects are also put into network for its comparison. On the other hand, difference in recognition rate depending on three methods of NN has been noticed enabling us to deduce their related recognition rate.