Seung-Myeong Baek

Electrical breakdown properties of liquid nitrogen for electrical insulation design of pancake coil type HTS transformer

In the electrical insulation design of a pancake coil type high temperature superconducting (HTS) transformer, knowledge of the dielectric behavior of both liquid nitrogen (LN/sub 2/) and gaseous nitrogen (GN/sub 2/) is very important. Also, the breakdown strength under a quench conditions is an important factor of the insulation engineering. Since spacers are used in the pancake coil type HTS transformer, the liquid nitrogen and the spacer must be considered together in the design of the insulation and cooling of this type transformer. This paper describes the results of an experimental study on the electrical breakdown phenomena and properties of liquid nitrogen with the electrode of the pancake coil made with Ag sheathed Bi-2223 HTS tape. When bubbles occur, the breakdown characteristics of LN/sub 2/ in a simulated cooling channel are examined to understand the optimal dimensions of the cooling channel. Open and closed cooling channels were made to compare the breakdown voltage to each other. The breakdown voltage in the open cooling channel appeared higher than the breakdown voltage in the closed cooling channel. The open cooling channel type is recommended for higher stability of equipment. This research will be useful in the electrical design of pancake coil type HTS transformers that are cooled by LN/sub 2/.

The Insulation Design of 154 kV HTS Transformer and on Load Tap Changers

The HTS transformer is one of the promising HTS power applications that are expected to be commercialized in the near future. Among the auxiliary functions of a transformer, the voltage regulation function is known to be the most important one. For the voltage regulation of HTS transformers, they are normally equipped with the on-load tap changers (OLTC). It is also required that, in order to develop a HTS transformer as a complete system, not only the basic technologies for all the components including OLTC and bushing, but also the technologies for the insulation of these components at a cryogenic temperature should be well established as applicable. Taking such facts into account, the breakdown characteristics of the insulation components have been evaluated for such application areas as turn-to-turn insulation, layer-to-layer insulation, c, drive shaft-to-drive shaft insulation, tap connecting terminal insulation and others. In overall, this paper discusses the electric insulation design and experimental results with regard to HTS transformer, OLTC and bushing.

A study on the composite dielectric properties for an HTS cable

In order to optimize the insulation design of a cold dielectric high temperature superconducting (HTS) cable, the composite insulation system has been investigated according to the arrangement of laminated polypropylene paper (LPP) and kraft paper in liquid nitrogen. LPP is a prominent insulating material with a high dielectric strength and low dielectric loss, which has been used previously as a HTS cable insulating materials. In addition kraft paper has been used for oil-field (OF) cable as insulating materials for a long time. In this paper, the dielectric properties on composite insulation system according to the arrangement of LPP and kraft paper was compared. The economic and dielectric performances of two insulating materials were considered and applied toward a HTS cable. Furthermore, from an economic satisfaction point the most suitable arrangement, and compared the dielectric properties of the mini-model cables were examined.

Insulation test of reciprocal and concentric winding arrangement for a HTS transformer

In Korea, the Hyosung Industrial, Korea Polytechnic University and Gyeongsang National University are developing a power distribution and transmission class HTS transformer that is one of the 21st century superconducting frontier projects. For the development, it is necessary to establish the dielectric technology at cryogenic temperature such as insulating design, cooling system, manufacture, compact, and so on. Also, verification of insulating stability is an important dielectric technology. Therefore, we prepared two models, one is concentric arrangement, the other is reciprocal arrangement, from Kapton insulated Cu tape for a small simulated HTS transformer and measured their insulation characteristics such as PD, ac (50 kV, 1 min) and impulse (154 kV, 1.2/spl times/50 /spl mu/s) withstand test. Before manufacture of each model, we have analyzed insulation composition and investigated electrical characteristics such as breakdown of LN/sub 2/, polymer and surface flashover on FRP in LN/sub 2/. We are going to compare with measured each value and apply the value to most suitable insulating design of the HTS transformer.

Surface flashover characteristics in liquid nitrogen for application of superconducting pancake coils

Abstract For the development of superconducting power apparatus, it is necessary to establish the dielectric technology in coolants like LN 2 . Among the dielectric technology, surface flashover characteristics are studied with several simplified spacers at the structural aspects. Double pancake coil can apply to transformer and fault current limiter, etc. To design dielectric system of high temperature superconducting transformer consisting of double pancake coils, this study discusses an effective insulator composition. Circular shape insulator divided into two parts should be inserted between coils and the insulator should cover electric stresses concentrated at the circumference of the coils which are in the same section of double pancake coils facing each other.

Comparison of Insulation Test of Mini-Models With Different Winding for a HTS Transformer

Recently, universities and companies are developing a power distribution and transmission class high temperature superconducting (HTS) transformer that is one of the 21st century superconducting frontier projects in Korea. For the development, it is necessary to establish the dielectric technology at cryogenic temperature such as insulating design, cooling system, manufacture, compact, and so on. Also, verification of insulating stability is one of important dielectric technology. Therefore, we prepared three models for a small simulated the HTS transformer, one is concentric arrangement, another is reciprocal arrangement, the other is Z continuous winding arrangement from Kapton insulated Cu tape and measured their insulation characteristics such as partial discharge (PD), AC (50 kV, 1 min) and impulse (154 kV, 1.2times50 mus) withstand test. Before manufacture each model, we have been analyzed insulation composition and investigated electrical characteristics such as breakdown of LN2 , polymer and surface flashover on fiberglass reinforced plastic (FRP) in LN2. We are going to compare with measured value and apply the value to most suitable insulating design of the HTS transformer

Characteristic analysis of solid materials for electrical insulation of HTS magnets

Following the successful development of practical high temperature superconducting (HTS) wires, there have been renewed activities in developing superconducting power equipment. HTS equipment has to be operated in a coolant such as liquid nitrogen (LN/sub 2/), or cooled by conduction-cooling method such as using Gifford-McMahon (G-M) cryocooler to maintain the temperature below critical level. In this paper, the dielectric strength of some insulating materials, such as polyimide film, epoxy, and Teflon tape in LN/sub 2/ were measured. The insulation materials for HTS current lead cooled by cryocooler needs to satisfy two opposing requirements: electrical insulation and heat conduction. To meet the two requirements, a thermal link that consists of oxide free copper (OFC) sheets, polyimide films, glass GFRP plates, and interfacing material was fabricated. Polyimide film was used as an electrical insulator for the current lead. Surface flashover voltage of glass fiber reinforced plastic (GFRP), the basic property of designing HTS solenoid coil, was also analyzed. Epoxy is a good insulating material but fragile at cryogenic temperature. The multi-layer insulating method for current lead has been suggested to compensate for this fragile property. It consists of Teflon tape layer and epoxy layer fixed by fiber material. Based on these measurements, conduction-cooled HTS current lead for 1.2 kV class DC reactor type high temperature superconducting fault current limiter (HTSFCL) and 6.6 kV class HTS magnets for same type HTSFCL cooled by sub-cooled LN/sub 2/ were successfully fabricated and tested.

Electrical insulation characteristics in the simulated electrode system of HTS double pancake coil

For the experiment four types of spacer were distinguished by an arrangement that could improve the dielectric strength by making the path of flashover longer. One of the spacers could be applied to an insulator between windings of a high temperature superconducting (HTS) transformer. The flashover characteristic of each type was investigated and the flashover phenomena were observed to understand breakdown mechanism in liquid nitrogen (LN/sub 2/). In the first, the flashover characteristic in LN/sub 2/ was compared with that in air using the simulated electrode made from five turns of HTS tape. The dielectric strength of spacers was improved with an arrangement in air but not in LN/sub 2/. Through the observation of flashover phenomena, a micro gap between the spacer and the coil electrode generating bubbles was regarded as the main cause. In the second, the effect of the micro gap on flashover was investigated with a plane-plane electrode system. The micro gap decreased the flashover voltage by 70 percent. Finally, the dielectric strength of the spacer in LN/sub 2/ also could be improved by filling of a small amount of epoxy in the gap between the coil and the spacer.

AC surface flashover strength and barrier effect of LN2 for HTS transformer with simulated electrode

Abstract In the response to an increasing demand for electrical energy, much effort aimed to develop and commercialise HTS power equipments is going on around the world. For the development, it is necessary to establish the dielectric technology in LN2. Hence many types of dielectric tests should be carried out to understand the dielectric phenomena at cryogenic temperature and to gather various dielectric data. Among the many types dielectric tests, the characteristic of surface flashover and the barrier effect were conducted with the simulated electrode after analysing the insulating configuration of the pancake-coil-type HTS transformer. The influence of a barrier on the dielectric strength was measured according to the size of the barrier, the position of the barrier and the effect of the back-electrode. It was shown that the effectiveness, namely the ratio of the breakdown voltage in presence of barrier to the voltage without barrier, is highest when the barrier is placed at the needle electrode side. The effect increased up to 1.8 times when collar length is 10 mm. The flashover characteristic with back-electrode was remarkably lower than the characteristic without one in the case the electrodes located at the same surface of dielectric plate. On the contrary, in the case the barrier was placed between the electrodes, the characteristic was even improved slightly.

Experimental research on the surface discharge characteristics for a HTS cable termination

As energy demands increase and environmental concerns heighten, an underground high-temperature superconducting (HTS) cable will provide the necessary alternative to meet power supply needs. Within conventional cable technology, the terminations are important components. HTS cable terminations are required when the insulated shield HTS cables connect with other conductors such as a bus or uninsulated overhead lines. HTS cable terminations must span a temperature range from 77 to 300/spl deg/K. The termination is insulated with insulating oil or air, cryogenic gaseous nitrogen, and liquid nitrogen. Difficult conditions for high-voltage insulation had to be overcome with HTS cable. Different environments can substantially raise the flashover possibility at a HTS cable termination. This paper will report on experimental investigations of the surface flashover of terminations with various surface lengths and different glass fiber-reinforced plastics thicknesses in air, transformer oil, liquid nitrogen (LN/sub 2/), and complex conditions. This paper examined the surface flashover characteristics under ac voltage application for the Oil-LN/sub 2/ type using a model electrode containing a particle. The particle initiated flashover voltage versus a termination without a particle. Particle position was also investigated.