Jong-Man Joung

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/.

Manufacturing and test of model double-pancake coils of HTS transformer for cryogenic insulation design

The feasibility of the development of HTS transformer has been grown and now its researches are going on actively around the world. Also the development research of 22.9 kV, 1 MVA HTS transformer is going on in Korea. To realize the development, various breakdown tests at liquid nitrogen temperature should be carried out and the breakdown mechanism should be understood. Moreover the insulation design technology that apply the basic dielectric experimental data to insulation design of it should be developed. Therefore, in this paper, breakdown tests such as ac breakdown, surface flashover and impulse breakdown in LN/sub 2/ were conducted using simulated electrodes and then the model double pancake coils were wounded on FRP former. Finally, electrical insulation tests of the model coils were performed with impulse 154 kV and ac 50 kV.

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.

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.

Electrical insulation design and withstand test of model coils for 6.6 kV class HTSFCL

Electrical design and withstand test of mini-model coils for high temperature superconducting fault current limiter (HTSFCL) have been studied. Electrical insulating factors of the HTS coil for HTSFCL are turn-to-turn and layer-to-layer. The electrical insulation of turn-to-turn depends on surface length, and the electrical insulation of layer-to-layer depends on surface length and breakdown strength of LN/sub 2/. Therefore, two basic characteristics of breakdown and flashover voltage were experimentally investigated to design electrical insulation for 6.6 kV Class HTSFCL. We used Weibull distribution to get electric field strength for insulation design. And mini-model coils for HTSFCL were designed by using Weibull distribution and were manufactured to investigate breakdown characteristics. The mini-model coils were passed in impulse withstand test as well as AC withstand test.

A Risk Evaluation Model of Power Distribution Line Using Bayesian Rule -Overhead Distribution System-

After introducing diagnosis equipment power failure prevention activities for distribution system have become more active. To do facility diagnosis and maintenance work more efficiently we need to evaluate reliability for the system and should determine the priority line with appropriate criteria. Thus, to calculate risk factor for the power distribution line that are composed of many component facilities its historical failure events for the last 5 years are collected and analysed. The failure statics show that more than 60% of various failures are related to environment factors randomly and about 20% of the failures are refer to the aging. As a strategic evaluation system reflecting these environmental influence is needed, a system on the basis of the probabilistic approach related statical variables in terms of failure rate and failure probability of electrical components is proposed. The figures for the evaluation are derived from the field failure DB. With adopting Bayesian rule we can calculate easily about conditional probability query. The proposed evaluation system is demonstrated with model system and the calculated indices representing the properties of the model line are discussed.