V-t characteristics of partial discharge inception in liquid nitrogen/ PPLP/sup /spl reg// composite insulation system for HTS cable

In this paper, we studied partial discharge (PD) inception characteristics and V-t characteristics of PD inception 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 initial PD was in the range of 2.0-30 pC irrespective of butt gap condition and the initial PD was generated at the first and third quadrant of voltage phase. PD inception electric field strength (PDIE) without butt gap was 5-10% higher than that with butt gap and thicker butt gap gave larger PDIE drop. Moreover, PDIE with butt gap depended on the butt gap thickness and the number of PPLP/sup /spl reg// layers. The reason is explained by the existing probability of weak points of electrical insulation at butt gap. Finally, lifetime indices n of V-t characteristics at PD inception were obtained as 80-100 irrespective of butt gap condition. These values showed enough flat characteristics of V-t phenomena of electrical insulation.     

Partial discharge inception characteristics of LN/sub 2//polypropylene laminated paper composite insulation system for high temperature superconducting cables

Partial discharge (PD) inception characteristics of liquid nitrogen (LN₂)/polypropylene (PP) laminated paper composite insulation system for high temperature superconducting (HTS) cables were investigated in terms of the volume effect and the V-t characteristics. The electrical and optical measurements of PD inception characteristics showed that initial PD could be generated between PP laminated paper layers, as well as in a butt gap. Using a parameter called statistically stressed liquid volume (SSLV) based on the discharge probability in both butt gaps and LN₂-filled thin layers between PP laminated papers, we could systematically analyze and evaluate the volume effect on PD inception stress (PDIE). Furthermore, experimental results revealed that n values of V-t characteristics at PD inception were as high as 80-110. On the other hand, the lower n values obtained at breakdown were interpreted by the intensified PD development in thermal bubbles generated after the PD inception     

Electrical insulation characteristics of cold dielectric high temperature superconducting cable

For the optimization of electrical insulation design for high temperature superconducting (HTS) cable, evaluation of electrical insulation characteristics especially for butt gap of LN/sub 2/ impregnated cold dielectric (CD) which consists of the wrapped tape insulation impregnated with LN/sub 2/ plays an important role. This paper presents partial discharge (PD) inception and breakdown characteristics in LN/sub 2/ impregnated butt gap model which modeled a weak point of the wrapped tape insulation impregnated with LN/sub 2/ and cable model with short length with polypropylene laminated paper (PPLP/sup /spl reg//), Nomex/sup /spl reg// paper and cellulose paper. PD current pulse was found to have a steep rise time of /spl sim/ ns and amplitude of /spl sim/ tens /spl mu/A at PD inception voltage region. Little dependency of breakdown stress on the insulating material is found. PD inception stress is almost independent of insulation thickness of 1 to 3 mm. The requirement insulation thickness for 66 kV class HTS cable is estimated to be /spl sim/ 5 mm under PD-free condition from viewpoint of long-term reliability.     

Impulse partial discharge characteristics and their mechanisms under non-uniform electric field in N/sub 2//SF/sub 6/ gas mixtures

We have investigated impulse partial discharge (PD) and breakdown (BD) characteristics of a needle-plane gap in N/sub 2//SF/sub 6/ gas mixtures under positive lightning impulse voltage application, and discussed their physical mechanisms. The 50% probability PD inception voltage (PDIV/sub 50/), leader discharge onset voltage (LOV) and BD voltage (BDV/sub 50/) were measured and analyzed as a function of gas pressure and SF/sub 6/ content. Experimental results revealed the stepwise propagation process of the impulse PD and enabled us to classify the impulse PD in N/sub 2//SF/sub 6/ gas mixtures into two types, the streamer discharge and the leader discharge. We also discussed the impulse PD propagation mechanisms in terms of PD parameters such as propagation length, time interval and current pulse magnitude, and suggested a sequential relationship in the PD propagation process under non-uniform electric field.     

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     

Partial discharge inception and propagation characteristics of magnet wire for inverter-fed motor under surge voltage application

In inverter-fed motor coils, surge voltages with the rise time of several tens or hundreds of nano-second may cause partial discharge (PD) and degradation of electrical insulation performance of the inverter-fed motor coils. This paper discusses PD inception characteristics as well as PD propagation characteristics after PD inception for magnet wire of inverter-fed motor under surge voltage application. Experimental results firstly revealed that PD inception voltage (PDIV) decreased with the increase in the length of enamel-coated wire, which was evaluated in terms of the stressed wire contact length under surge voltage application, i.e. size effect. We proposed a regression line for the size effect on PDIV for the electrical insulation design of inverter-fed motor coils. Secondly, PD propagation characteristics were also investigated under the higher voltage application, and their mechanisms were discussed in terms of generation probability of initial electrons, space charge behavior in the wedge-shaped air gap, charging on the enamel surface and so on     

人工盐雾环境下环氧树脂的绝缘特性

采用超声波盐雾发生器模拟沿海地区环境,在直流电场条件下测试了盐雾电导率对环氧树脂小绝缘间隙的绝缘特性的影响;借助单筒连续变倍视频显微镜观察实验前后试样表面状况,分析绝缘破坏过程。实验结果表明:盐雾液滴在环氧树脂表面的接触角随盐雾电导率的增大而减小;放电起始电压随盐雾电导率的增大而降低,随绝缘间隙的增大而升高;绝缘破坏电压随盐雾电导率的增大而降低,随绝缘间隙的增大而升高。     

纤维杂质对油纸复合绝缘局部放电特性影响研究

为研究纤维杂质对油纸绝缘局部放电特性的影响,搭建了直流油纸绝缘实验平台,观测了油中纤维杂质产生及成桥过程,就杂质成桥的动力学基础进行了分析,并对杂质影响下的油纸绝缘局部放电频数、平均放电量及最大放电量等特征参数进行了研究。结果表明,直流电压下纤维杂质在油中受电场力、重力、浮力及油黏性阻力作用。当杂质所在处场强大于2.12×105 V/m时,纤维杂质以缓慢速度向高压电极运动,且沿电压梯度最大方向成桥;纤维杂质畸变电场可使油纸绝缘局部放电频数上升、放电量下降。进一步实验表明,纤维小桥可导致油隙内泄漏电流增加,引起局部发热,并使油中水分局部气化为微小气泡。直流电压下低电导率气泡承担较强电场,先于变压器油发生放电,宏观表现为更加密集且强度较低的局部放电。     

振荡冲击电压下典型绝缘模型局部放电解析(英文)

The aim of this paper was to give an overview on partial discharges under oscillating impulse voltage.Three models(void in solid,needle-plate in air and oil) were presented,which describe the stochastic discharge process and represent internal discharges in solids and corona in air or silicon oil.Moreover,an air cored Rogowski coil and a sampling resistor for partial discharge(PD) measurement were developed and introduced in this paper.PD inception and extinction voltages(PDIV,PDEV) under single oscillating impulse voltage and AC voltage were investigated with different test samples.Experimental results firstly revealed that the PD inception voltage(PDIV) decreased with increasing applied voltage;secondly the PD inception voltage for three different insulating materials,showed an escalating trend with increasing frequency of the applied voltage.It was proven that the characteristics of PD under oscillating impulse voltage were identical to the features under AC voltage,which could be measured with the phase resolved partial discharge analysis(PRPDA) technique.Based on the reorganization and analysis of PDs under oscillating impulse voltage,the information about insulation defects was extracted from the measured data and used for estimating the risk of insulation failure of the equipment.     

Insulation characteristics of perfluorocarbon liquid/solid insulation for nonflammable large power transformer

A new type nonflammable large power transformer system is proposed in which the excellent cooling characteristics and insulation properties of nonflammable perfluorocarbon (PFC) liquid is utilized. In the system, transformer windings are put in an insulating cylinder filled with PFC liquid. The core includes cooling panel through the liquid. The space between the insulating cylinder and tank is filled with compressed SF⁶ gas. This paper describes the insulation characteristics of main insulation and inter-winding insulation models which simulate the transformer. It is found that (1) higher withstand voltage is expected by means of insulation barriers in the main insulation model; (2) partial discharge in inter-winding insulation model is beginning to behave in a constant electric field strength on the point of 0.1 mm liquid gap length.     

Impulse partial discharge and breakdown characteristics of rod-plane gaps in N/sub 2//SF/sub 6/ gas mixtures

Impulse partial discharge (PD) and breakdown (BD) characteristics of rod-plane gaps in N/sub 2//SF/sub 6/ gas mixtures were investigated for different gap geometry, gas pressure and SF/sub 6/ gas content. Experimental results revealed that the 50% probability breakdown voltage increased with gas pressure, which agreed with the theoretical values with consideration of discharge time lag for impulse voltage application. For the calculation of discharge inception voltage, the volume-time theory was successfully applied to the N/sub 2//SF/sub 6/ gas mixture. Furthermore, impulse PD light emission image was observed together with PD current pulse and light intensity waveforms. The long discharge with stepwise propagation and double-peak PD pulses could be observed, which corresponded to the streamer/leader transition leading to breakdown.     

老化绝缘纸表面的沿面放电试验分析

在故障变压器检修及事故调查中发现,变压器内绝缘纸界面发生沿面闪络后会留下烧灼痕迹,对此分析了油纸绝缘界面沿面闪络的起因及影响因素。测量了在添加和不添加绝缘纸板条件下绝缘油的击穿电压和起始局放电压,分析不同热老化程度的绝缘纸沿面闪络电压和起始局放电压的变化规律。研究结果表明:添加新绝缘纸不会改变油间隙的电气强度,但是老化后的绝缘纸会大幅降低油间隙的沿面闪络电压以及起始局放电压,在老化过程中产生的大量水分子可能是导致上述试验现象的重要原因。     

异型波电压下气隙缺陷的局部放电检测方法设计

设计了三种异型波,将其应用于含气隙缺陷的交联聚乙烯绝缘材料局部放电检测试验中,旨在探索一种便捷、无损、准确、可靠的针对气隙缺陷进行局部放电检测的方法。首先,制作了含气隙缺陷的绝缘试样模型,在工频交流电压下对气隙缺陷试样进行局部放电试验;其次,设计并制作了三种异型波,利用所设计的异型波对相同的试样进行局部放电试验;最后,对比分析了在工频交流电压及三种异型波电压下的局部放电试验结果。研究结果表明：不对称振荡波为检测含气隙缺陷绝缘试样的最佳异型波;异型波电压与交流电压下的放电等效性较好,最佳异型波电压下的局部放电起始放电电压更小、放电量更大、放电次数更多、相位分布更广。综上,异型波电压可用于提高局部放电检测的准确性,在去噪与无损检测等方面具有一定的优势。     

空气中电晕放电与绝缘液体中流注放电的测量和分析

Insulation is one of the most important parts in a high voltage equipment.There are gaseous,liquid and solid insulations which are commonly used.In a high voltage transformer for example the insulating materials are all used.During operation of a high voltage equipment high electric stress may occur.Under extreme condition failure of the insulation may take place.Excessive electric field in air may cause corona discharges while in liquid insulation discharges may take place in the form of streamer.This paper reports experimental results on the corona and streamer discharges in air and silicone oil.The discharges were artificially generated around a needle tip in a needle-plane electrode system with gap length of 4 mm under sinusoidal and triangular voltages.The needle was made of steel with tip radius of 3 μm and curvature angle of 30°.The needle was made by Ogura Jewelry.The discharge pulses were measured using personal-computer based partial discharge(PD)measurement system with sensitivity of better than 0.5 pC.The system is able to measure discharge in time sequential.Phase-resolved analysis of the discharges was done to interpret the physical processes behind the discharges.The experimental results showed that corona discharges took place at negative half cycles.The discharges were concentrated around 270° of phase angle of applied voltage.The discharge magnitude and discharge number of corona clearly dependent on the instantaneous of applied voltage.These were strongly supported by the application of triangular voltage.Streamer discharges occurred at both positive and negative half cycles.The discharges pulses concentrated around the peak of applied voltage at phase angle of 90° and 270°.Experimental results under sinusoidal and triangular voltages revealed that streamer discharge magnitude as well as probability of occurrence was strongly dependent on the instantaneous applied voltage.     

Verification tests and insulation design method of cold dielectric superconducting cable

A High‐Temperature Superconducting (HTS) cable has a bulk power transmission capacity as a candidate for the replacement of aged cables and/or for the increase of the power transmission capacity, and its diameter is preferred to be smaller than the inner diameter of the duct for the existing cables. To reduce the diameter of HTS cable, the cold dielectric (CD)‐type electrical insulation in which a cable core is immersed into liquid nitrogen (LN₂) should be adopted, and the thickness of its electrical insulation layer has to be optimized. Since a partial discharge (PD) in the electrical insulation layer of the CD‐type HTS cable is considered as a major cause for the aging of the insulation layer, PD‐free design must be adopted for the CD‐type HTS cable. This paper describes a design method for the electrical insulation layer of the CD‐type HTS cable adopting the PD‐free design under AC stress, based on the experimental results such as a PD inception stress (PDIE), an impulse breakdown stress, and PD extinction characteristics under AC stress superimposed with an impulse stress. Moreover, the proposed design method was applied to a 500‐m HTS cable and was verified by a field test. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 164(2): 25–36, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20512     

油纸绝缘气隙模型局部放电特性试验研究

为了研究直流输电系统中换流变压器油纸绝缘的局部放电特性,设计构造了典型的油纸局部放电模型———气隙放电模型。针对同一模型分别施以交流电压、直流电压以及冲击衰减振荡电压,采用IEC 60270-2000推荐的脉冲电流法分别检测与分析该模型局部放电的不同特征参数。对比研究了在交流、直流及振荡冲击电压下油纸绝缘气隙放电模型的局部放电脉冲特征参量包括局部放电起始电压、熄灭电压、放电波形、放电水平、放电重复率等,对实现换流变压器的局部放电在线监测与绝缘状态综合评估有一定的指导意义。     

特高压变压器油纸绝缘典型缺陷局部放电特征

变压器绝缘在制造及运行过程中难免产生缺陷,本文针对特高压变压器典型夹持结构进行了典型含缺陷油纸绝缘局部放电特性分析。根据特高压变压器夹持结构绝缘建立了实验室研究模型,用以模拟绝缘含空腔及金属异物等典型绝缘故障模型。采用脉冲电流法对缺陷试样进行了局部放电特性测试,获得了局部放电起始电压、放电量q-相位φ、放电次数n-相位φ、放电次数n-放电量q等特征参数,提取出峭度K_u、偏斜度S_k、相位中值μ、不对称度Q等统计算子特征参数,对比了局部放电不同阶段下各绝缘缺陷的放电特征图谱。结果表明,含有空腔及金属杂质的缺陷对油纸绝缘局部放电影响明显,其中金属杂质影响最严重。通过对比不同试样的平均放电量相位分布图谱H_qn(φ)与放电次数相位分布图谱H_n(q)可有效对局部放电原因进行分类。     

换流变压器油纸绝缘复合电场击穿特性及其试验方法

为了查明换流变压器油纸绝缘在交流叠加直流电场下的击穿特性,利用交流、直流及交流叠加直流等复合电场,对油隙、浸油纸板和油纸复合绝缘进行了击穿试验研究。结果表明：油隙的击穿场强几乎不随交流电压分量变化;浸油纸板击穿场强随交流电压分量增加迅速下降;受浸油纸板击穿特性影响,油纸复合绝缘击穿场强随交流电压分量减小明显提高。说明油隙的击穿场强与交流电压分量基本无关,但浸油纸板的击穿场强受交流电压分量影响严重。依据试验结果,进一步对换流变压器交流和直流电压试验项目的有效性进行了讨论。经分析发现换流变压器主绝缘的油隙与浸油纸板在运行电压下出现的最高电压仍为交流叠加直流电压。交流和直流电压试验的电压幅值能够满足要求,但电压波形与承受的运行电压相差较大。认为交流电压试验考核油隙绝缘介电强度是有效的,而直流电压试验考核浸油纸板绝缘介电强度并不充分。     

Insulation optimization by electrode contour modification based onbreakdown area/volume effects

This paper shows how to determine the electrode contour with the best insulation performance on the basis of an area effect and a volume effect in the breakdown field strength. Previous electric field optimization techniques have provided us only with an optimum electric field distribution, For the design of power apparatus insulation, we have to consider the dielectric breakdown characteristics rather than the electric field distribution. As a first step, we developed a new optimization technique which enables us to obtain an optimum electrode contour with the highest breakdown strength while taking into account the area and volume effects of breakdown strength of insulating media. From the results, we have confirmed that the proposed optimization method improved the breakdown voltage more than did the electric field optimization. This leads to an effective insulation design of electric power apparatus     

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.