Pressurizing and sub-cooling effects on electrical breakdown of LN2 in modeled HTS coils

Pressurizing and sub-cooling of liquid nitrogen are considered to have advantageous aspects in electrical insulation and the critical current in high temperature superconducting (HTS) coils. This paper deals with experimental studies on bubble behavior and partial breakdown (PBD) characteristics in a simulated electrode system of HTS coils immersed in pressurized liquid nitrogen, following our previous reports with saturated liquid nitrogen at atmospheric pressure. The tested electrode system consists of a coaxial coil layer to a cylindrical electrode with an insulation barrier and spacers. A heater is mounted inside the coil electrode to generate boiling which appears on quenched superconducting coils. Liquid nitrogen is pressurized to 0.2 MPa under constant temperature of ~77 K. The experimental results show that the partial discharge (PD) voltage is affected markedly by the bubble behavior which depends on the applied pressure to liquid nitrogen. Pressurization suppresses bubble growth in the insulation space and increases the inception voltage of PD. The charge quantity associated with a single PD at a given applied voltage increases with the applied pressure, and deterioration of solid insulation such as spacers and barriers is accelerated compared to that at atmospheric pressure if the PBD occurred     

A study of thermal bubble behavior in the simulated electrodesystem of HT superconducting coils

Bubble behavior is studied theoretically and experimentally with an electrode system which consists of concentric coil layer-to-cylindrical electrodes with insulation barrier and spacers immersed in liquid nitrogen for the simulation of the insulation environment in high temperature superconducting coils at the quenching state. The results show that bubble behavior is affected severely by 60 Hz electric field and pressure; it is categorized into two types according to the width l_c of the cooling channel between coil layer and cylindrical electrode. When l_c is larger than the diameter of bubble released from the coil electrode, bubbles rise by buoyancy at a low applied voltage, but they are trapped in grooves between coil-turns at higher applied voltages. Trapped bubbles move along the groove if there is no obstruction, but otherwise move out of the groove. When l_c is smaller than the bubble diameter, the bubbles are trapped in the groove due to surface tension, and now along the groove even at no applied voltage. At higher applied voltages, the bubbles are trapped tightly and an electrohydrodynamic (EHD) instability is excited on the bubble surface if the spacer exists. Bubbles stream from the groove due to the effects of EHD instability and buoyancy     

DC breakdown voltage characteristics of saturated liquid helium in the presence of metallic particles

This paper deals with the breakdown voltage characteristics of saturated liquid helium in the presence of a needle-shaped or spherical metallic particle to obtain insulation design data for pool-cooled, low-temperature superconducting coils and to find the predominant factor affecting the breakdown voltage of liquid helium at a given state. The results show the following. (1) The generation of bubbles at the instant of collision of a particle with an electrode is caused mainly by the kinetic energy released from the moving particle to the liquid helium, while the electrostatic energy accompanying the microdischarge between the particle and the electrode has a lesser effect on it. (2) The lowest breakdown voltage at different particle conditions appears in the case of a free needle particle. The low breakdown voltage is caused by the bubble triggered by the particle collision, the high electric field at the tip of the needle particle, and the rich initial electrons supplied by the microdischarge.     

Effect of a thin insulation film on thermal bubble‐triggered breakdown phenomena in liquid nitrogen

The effect of a thin insulation film on breakdown phenomena in liquid nitrogen in the presence of thermally induced bubbles is investigated with a cylinder‐to‐plane electrode. Bubbles were produced by a heater mounted in the cylinder, which is partially covered with an insulation film. The results show that the film's effect on the breakdown voltage is insignificant until a pore is formed on the film by a previous breakdown, but it becomes substantial after pore formation. The reduction rate in breakdown voltage after the pore formation depends on the gap length and heater power. The minimum breakdown voltage drops to the magnitude of the breakdown voltage in the gaseous phase at normal boiling temperature. The breakdown mechanism is discussed on the basis of bubble observation and numerical calculation of suspended‐bubble motion. © 1999 Scripta Technica, Electgr Eng Jpn, 127(4): 18–28, 1999     

A novel technique for partial discharge and breakdown investigation based on current pulse waveform analysis

A novel technique for partial discharge (PD) measurement and analysis (PD-CPWA; PD current pulse waveform analysis) is developed and introduced in this paper. PD-CPWA is expected to be utilized to discuss PD mechanisms and physics in electrical insulating materials, focusing on the PD current pulse waveform and its time transition from PD inception to breakdown (BD). In this paper, the concept and principle of PD-CPWA are described, and the applications of PD-CPWA to (1) epoxy spacer samples under thermal and electric combined stresses in GIS, (2) creepage PD on epoxy spacers in SF/sub 6/ gas and (3) liquid nitrogen/polypropylene laminated paper composite insulation system for high temperature superconducting cables are introduced and discussed.     

Insulation design of pool-boiling He-cooled superconducting coils.Coil parameters affecting insulation spacing [SMES application]

An insulation design procedure for SC (superconducting) coils for electric power apparatus is proposed and applied to a small scale SMES (superconducting magnetic energy storage system) consisting of two modules of pool-boiling helium-cooled SC coils. Coil operation parameters affecting insulation spaces are discussed on the basis of an estimated time-dependent ratio of insulation withstand voltage to induced voltage for different quenching states according to the proposed procedure     

液氮温区超导复合绝缘材料沿面放电特性

随着超导输电技术的不断发展,对各种超导设备中使用复合绝缘材料的绝缘特性提出了更高要求。复合绝缘的放电击穿往往发生在固体绝缘材料和液氮2种工作介质的交界面。而在液氮温度下,绝缘材料又有着相对独特的放电特性。针对这种情况,设计了极不均匀沿面电场,研究了3种不同的固体绝缘材料在液氮环境中沿面放电的特性,包括其放电起始电压、放电随电压发展过程、击穿电压等特性,并进一步分析3种材料差异的原因,为实际应用提供了参考。     

Gaseous electrical discharge characteristics in air and nitrogen at cryogenic temperature

In designing superconducting electrical power apparatus, the knowledge of cryogenic gas and liquid insulation characteristics is essential. The authors have studied the discharge characteristics of relatively long-gap configurations in air and nitrogen at a cryogenic temperature. A sphere-to-sphere electrode with a gap length of 20 to 150 mm is used for measurements in uniform electric field. The breakdown voltage characteristics basically obey Paschen's law at cryogenic temperature for 50 Hz, AC, DC and lightning impulse voltage applications. A rod-to-plane electrode with a gap length of 20 to 330 mm is used for measurements in nonuniform electric field. In air at cryogenic temperature and nitrogen gas at both room and cryogenic temperature, streamer-like corona discharge appears near the tip of the rod electrode before the breakdown, and the breakdown voltage increases linearly with gap length. In air at room temperature thin film-like corona discharge, however, appears near the tip of the rod electrode before breakdown, and the breakdown voltage becomes higher than the other case. In order to examine the variation of corona discharge characteristics, some additional experiments are conducted. As a result, it becomes clear that thin film-like glow corona discharge appears when electronegative gas is contained and sufficient electrons are supplied from the cathode.     

Dynamic breakdown characteristics of liquid helium induced by aquench of superconducting wire and coil

For practical insulation design of superconducting power apparatus, it is necessary to take into account an inherent phenomenon known as quench, the transition from the superconducting to the normal state. We investigated quench-induced dynamic breakdown and prebreakdown characteristics of liquid helium (LHe). Experimental results revealed that the quench of the superconductor drastically reduced the breakdown voltage of LHe in the thermal bubble disturbance. Moreover, dynamic breakdown characteristics in a quasi-uniform field using a superconducting coil were investigated. The results revealed that the breakdown was induced in the quench-initiated region where the largest thermal energy was generated     

Insulation properties and degradation mechanism of insulating spacers in gas insulated switchgear (GIS) for repeated/long voltage application

Gas insulated switchgear (GIS) has been increasingly being used in recent years and high reliability and rational insulation design are required. This research provided V-N characteristics (the dielectric breakdown voltage versus number of repetitions characteristics) regarding the internal insulation and creeping insulation of the epoxy spacer, which are the main insulation elements of GIS, in order to help set LIWV (lightning impulse withstand voltage) and SIWV (switching impulse withstand voltage). The insulation strength decreased the most when the lighting impulse voltage was applied to internal insulation of the spacer, however, taking the absolute values into consideration, it turned out that the impact of frequent surges is slight. In the experiment in which alternating current voltage is applied for a long period of time, it was found that there is no decline in the insulation properties even after the voltage is applied for the equivalent of 30 years when the electric field intensity is 12 kVrms/mm or less although the combination with the multiple lightning impulse application may bring about damages to the spacer insulation. The degradation mechanism caused by generation of micro-pits was also understood through simultaneous microscopic observation of the surface and of the interface between the electrode and epoxy     

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.     

Evaluation and testing of turn insulation of large AC motors

This paper reviews the results of tests performed on the stator windings of a 13.8 kV, 15000 HP variable speed slipring induction motor in order to assess its insulation condition and quantify the impulse strength of the motor winding. A surge voltage distribution test showed severe nonlinearity of turn voltage where up to 120% of the applied voltage was achieved across the first coil with 0.1-μs risetime. The effect of a surge capacitor was also demonstrated when a substantial reduction of the first coil voltage could be obtained at low risetime of 0.1 μs. Measurements of breakdown voltage (BDV) of coils at different locations gave indirect evidence that turn insulation degradation was most likely surge related. Finally, the paper contains a proposal of an alternative testing criteria to quantify impulse strength of turn insulation of large AC machines     

Prebreakdown and breakdown phenomena under uniform field in liquid nitrogen and comparison with mineral oil

This paper presents a study of breakdown and prebreakdown phenomena (streamers) in liquid nitrogen and mineral oil under quasi uniform electric field, under ac and impulse voltage. Streamers preceding breakdown are studied up to 0.5 MPa by high-speed visualization and recording of emitted light. In these conditions, breakdown in LN/sub 2/ is mainly due to negative streamers, initiated at lower voltage than the corresponding positive voltage. Hydrostatic pressure has a limited effect on breakdown voltage, such as in mineral oil. It is shown that the ratio of impulse to ac breakdown voltage in LN/sub 2/ is surprisingly low (close to 1), whereas in the same conditions ac breakdown voltage in mineral oil is lower than impulse breakdown voltage. Practical consequences for the design of HV insulation in superconducting systems are discussed.     

The effect of surge testing on the voltage endurance life of stator coils

Surge tests with short rise times are used to ensure turn insulation integrity in form-wound vacuum-pressure impregnated (VPI) coils prior to resin treatment. Use of this test migrated from the evaluation of fully processed generator bars and random wound stators. The dielectric properties of the insulation in these systems are fully developed at the time of testing. This is not true of "green" (unimpregnated) VPI coils. Although surge testing is a valuable manufacturing quality test, an excessive transient electric field in uncured coils may cause irreversible changes to the insulation. These changes may affect the life of the insulation system after complete impregnation and cure. This paper describes laboratory tests and finite element simulations that examine the effect of applied surge test voltage and number of applied pulses on voltage endurance (VE) life. The sympathetic voltage response in a single coil as a function of its location within a winding is described for a stator undergoing green surge testing as part of a quality assurance program. Results of VE testing and sample dissection data are presented. Test voltage was identified a more significant factor on VE life than the number of pulses applied.     

Breakdown mechanism of liquid nitrogen viewed from area and volumeeffects

We investigated the area and volume effects on the breakdown strength in liquid nitrogen (LN₂) to discuss the breakdown mechanism in cryogenic liquids for superconducting power apparatus. We measured breakdown voltages in LN₂ with and without thermal bubbles over a very wide range of the electrode size. Experimental results revealed that the breakdown mechanism changed from an area dominant to volume effective region at larger electrode configurations in LN₂. Moreover, we discussed the contribution rate of area and volume effects to the breakdown strength in LN₂. It was suggested that a mutual contribution of area and volume effects appeared in breakdown characteristics in LN₂ under thermal bubble conditions, as a phenomenon peculiar to cryogenic liquids. Consequently, we pointed out that it is very important to consider both thermal bubbles and electrode surface condition for HV insulation of superconducting power apparatus     

Electrical breakdown characteristics of nitrogen and air at cryogenic temperature in a quasi‐uniform electric field

The breakdown voltages of the longer‐gap configurations in gaseous nitrogen and air that are necessary in designing superconducting electrical power apparatuses are measured at temperatures of 293 and 93 K. The quasi‐uniform electric field made by a sphere‐to‐sphere electrode with a diameter of 150 mm and a gap length of about 10 to 100 mm is used in the measurement of the breakdown voltages. When 50‐Hz ac and dc voltages are applied to the sphere‐to‐sphere gap, the breakdown voltages in nitrogen and air obey Paschen's law even at cryogenic temperatures (93 K). When a 1.4/50‐μs lightning impulse voltage is applied to the gap, the 50% breakdown voltage of nitrogen also obeys Paschen's law under UV irradiation of the cathode electrode. However, the breakdown voltage in air at 93 K is higher in the case of lightning impulse voltage applications, and the delay from impulse voltage application to breakdown occurrence is apparently longer at 93 K than at 293 K. © 2000 Scripta Technica, Electr Eng Jpn, 132(4): 28–33, 2000     

Analysis of steep-fronted voltage distribution and turn insulationfailure in inverter-fed form-wound AC motor

As a potential source of turn insulation failure, the steep-fronted voltage at the motor terminal generated by pulsewidth modulation drive switching or cable ringing is not uniformly distributed among the coils of the windings and among the turns of the coils. This paper presents a detailed electromagnetic analysis of the parameters of a transient model of coil and coil group which includes iron loss, skin, and proximity effects. The finite-difference method is applied to solve the model for the interturn and intercoil voltage distributions. The simulation is compared to a simplified lattice diagram analysis and to test results. A turn insulation surge-withstanding capability study is then presented with a flexible insulation test system that models the repetitive steep-fronted voltage stress and temperature stress     

DC pre-breakdown phenomena and breakdown characteristics in thepresence of conducting particles in liquid nitrogen

DC pre-breakdown phenomena and breakdown characteristics in the presence of free conducting particles in liquid nitrogen are studied experimentally. The results show that a microdischarge occurs when a charged particle is approaching an oppositely charged electrode. An intense microdischarge can trigger a complete breakdown of the gap. The breakdown voltage of a uniform field gap with a free metallic particle of mm size might be reduced well below that of a point-to-plane gap without a particle in liquid nitrogen. Heavy contamination by a metallic powder produces a large reduction in the breakdown voltage with a horizontal spacer surface. However carbon powder is less hazardous compared to metallic powder     

高温超导电缆及其低温绝缘研究现状

概述了世界上高温超导电缆的研究历史和现状,介绍了高温超导电缆本体的基本结构及绝缘要求,分析了高温超导电缆主绝缘的结构及存在的问题。针对高温超导电缆中使用的液氮和几种低温固体绝缘材料,分别介绍了其在低温环境下介电性能的相关研究进展。总结发现：液氮的击穿场强受到气泡和电极材料的影响;液氮下绝缘材料的直流击穿场强高于交流击穿场强;聚酰亚胺在液氮下的交直流击穿场强高于聚丙烯层压纸;低温会抑制环氧树脂中电树枝的生长。     

Switching surge overvoltages on the feeding system of a MAGLEV train

This paper investigates switching surge overvoltages due to a section switchgear between coils and a feeding cable, and surge overvoltages due to a ground fault in the feeding cable. The maximum overvoltage due to switchgear closing is about 25 kV on the coil and 15 kV on the cable sheath. The coil overvoltage becomes greater as a source voltage reaches its peak. The overvoltages due to switchgear breaking is dependent on the breaking current and they reach 115 kV on the coil and 15 kV on the cable sheath when the current is 50 A. The fault surge overvoltage is about 24 kV on the coil which is smaller than the switching overvoltage. On the other hand, the cable sheath overvoltage reaches 32 kV, which is the largest among various overvoltages investigated in this paper, and could result in sheath insulation breakdown. The coil overvoltage due to current breaking by a section switchgear may cause coil insulation breakdown when the breaking current is large. Both the coil and sheath overvoltages can be controlled to less than the insulation level by arresters. © 1998 Scripta Technica, Electr Eng Jpn, (4): 58–66, 1998