【作者】 王亚林；

【导师】 尹毅；

【作者基本信息】 上海交通大学 ， 电气工程， 2018， 博士

【摘要】 聚合物材料由于具有诸多优良特性,因而在电力工程、能源工程和航空航天工程等领域有着广泛的应用。聚合物绝缘电缆作为超高压直流输电的主要组成部分,在运行过程中承受着一定强度的电场。直流电场下聚合物材料容易积累空间电荷,引发局部电场畸变,使热电子运动以及能量的储存与释放加强,进而加速绝缘老化,严重影响电缆寿命。聚合物在辐射环境下诸如航天器中也得到大量应用,然而高能粒子辐射、等离子体撞击和紫外线照射等因素极易导致聚合物材料表面和内部积累大量电荷,引起静电放电。造成空间电荷积累的主要原因是材料内部存在或深或浅的陷阱,空间电荷的行为与材料内部陷阱能态分布具有很强的关联性。对材料内的陷阱分布与空间电荷分布进行联合测量与研究有助于全面地了解材料中载流子的输运过程。然而鲜有研究者对同一试样中的空间电荷分布与陷阱能态密度进行联合测量,相关设备的缺失使得该项研究难以进行。为此,本文针对聚合物绝缘中的空间电荷行为与陷阱分布的关联性这一关键问题,将聚合物的空间电荷行为与微观陷阱能态密度相结合,研制了能够对同一试样的空间电荷与陷阱能态密度进行联合测试的仪器。并以交联聚乙烯(cross-linked polyethylene,XLPE)和纳米MgO/XLPE复合介质为研究对象,对两种材料的空间电荷与电导,空间电荷与热刺激电流进行联合测量。研究了空间电荷与电导的联系,以及空间电荷与陷阱能态密度的关系,比较了两种材料中的空间电荷行为,并对载流子的输运特性进行了分析。聚合物空间电荷与陷阱能态密度测试仪器的研制难点主要在于协调空间电荷测量与电流测量这两种测量方式之间的矛盾。为解决该矛盾,本文首先通过仿真和理论计算分析了空间电荷测量时产生的脉冲电场对电流测量的影响,发现脉冲电场引起的脉冲电流幅值较大,将极大地干扰对流过试样的电流的测量,甚至可能损毁电流测量仪器。因此,采用高压电极、空间电荷测量极、电流测量极和接地保护极构成的“四电极”结构,将空间电荷测量区域与电流测量区域分离开。另外,采用快速切换分时测量的策略,将空间电荷测量与电流测量分时进行。采用液氮与电加热器件相结合的控温方式,实现了对实验样品以0.5~10℃/min的速率线性升温。此外,又研制了基于Blumlein传输线的高重复频率纳秒脉冲电压源,使用高开关速度和重复频率的MOSFET器件,使得脉冲重复频率最高可达3 MHz,为空间电荷快速测量提供了激励源。通过对经典的单一陷阱能级的热刺激电流表达式进行分析后发现,不同类型的电荷来源引起的热刺激电流表达式相似。在此基础上,提出了基于非负线性最小二乘迭代算法的热刺激电流分析方法。该方法无需人为判断电流峰的位置和个数,自动对整条热刺激电流曲线进行分析,能够获得整个禁带范围内的陷阱能态密度谱,具有重复性好、能规避人为误差以及自动排除无效数据等优点,可以用于聚合物陷阱能态密度的分析。使用联合测试设备对XLPE和纳米MgO/XLPE复合介质在20到60℃下进行空间电荷与高场电导联合测量,发现纳米MgO/XLPE复合介质的空间电荷积累阈值电场大于XLPE的空间电荷积累阈值电场,表明一定粒径和浓度的纳米MgO的添加抑制了空间电荷的积累。在相同温度下,纳米MgO/XLPE复合介质的电流密度-电场关系曲线的转折电场高于XLPE的转折电场。通过对两种材料的电流密度-电压标度曲线进行拟合,发现纳米MgO颗粒的添加引入了新的陷阱能级,纳米MgO/XLPE复合介质内的陷阱在空间上分布较XLPE更为均匀。另外,又使用空间电荷和电流联合测量的结果,发现载流子关于局部电场的负微分迁移率是空间电荷包形成的主要原因。使用联合测试设备对XLPE和纳米MgO/XLPE复合介质进行了空间电荷与热刺激电流联合测量与分析。在极化阶段,XLPE试样的阳极附近积累了大量的正电荷并逐渐向试样内部迁移,而降温阶段空间电荷的迁移减缓。在热刺激阶段,当温度大约高于60℃时空间电荷开始剧烈减少。纳米MgO/XLPE复合介质在极化阶段和降温阶段均没有明显的空间电荷积累。使用基于非负线性最小二乘迭代算法的热刺激电流分析方法计算了两种材料的陷阱能态密度,发现XLPE在1.15 eV能级存在数量较多的深陷阱,而纳米MgO/XLPE复合介质在不同能级上的陷阱密度较为均匀。由空间电荷与陷阱能态密度的联合分析可知,XLPE积累的空间电荷主要分布在深陷阱内,而纳米MgO/XLPE复合介质由于添加了纳米MgO颗粒,引入了更多的陷阱能级,抑制了空间电荷的积累。更多还原

【Abstract】 Polymeric materials are widely applied in many fields such as electric power engineering,energy engineering and aerospace engineering due to their superb properties.The polymer cable,as a crucial part of the high voltage direct current power transmission system,endures high electric field strength during work time.Polymers are vulnerable to space charge accumulation in high direct current electric field,which will distort the local electric field,enhance the thermal movement of carriers,intensify the energy storage and release process,age the insulation and eventually impose critical threats on lifetime.Polymers are also used in radiant environment situations such as spacecrafts.However,the radiation of high energy particles,collision of plasma,and irradiation of ultraviolet lights make them easy to accumulate charges on the surface and in the bulk of the polymers,thus causing electrostatic discharge.Space charge accumulation phenomenon mainly results from the existence of shallow and deep traps and space charge behavior is strongly related to the distribution of traps in polymers.It is very beneficial to perform simultaneous measurement of space charge and trap energy density,and it will help to comprehensively understand the mechanism of carrier transport in detail.However,the lack of proper equipment hinders the development of simultaneous measurement of space charge and trap energy density.This paper focuses on the crucial issue of the relation between space charge behavior and trap energy density distribution,and elaborates a measurement system that can simultaneously measure space charge and trap energy density of the same specimen.The space charge,conduction current and trap energy density distribution of cross-linked polyethylene(XLPE)and nano-MgO/XLPE composite were measured by this system.The relation between space charge and conduction current,and the relation between space charge and trap energy density distribution were carefully investigated to analyze the carrier transport process.The most difficult part of the development of simultaneous measurement system lies in how to coordinate the experiment setup contradiction of space charge measurement and current measurement.For solving this problem,firstly,we performed the simulation and theoretical analysis of the interference of voltage pulse(which is indispensable in space charge measurement)in current measurement.It is found that the voltage pulse results in a current pulse that can affect the current measurement or even destroy the current measuring equipment.Therefore,a “four electrodes” setup,including high voltage electrode,space charge measurement electrode,current measurement electrode and grounded protection electrode,is adopted to isolate the space charge measurement area and current measurement area.In addition,a time-division measuring strategy is adopted to alternately perform space charge measurement and current measurement.The liquid nitrogen and a heating coil are set to control the temperature of the specimen during measurement.Besides,a nanosecond voltage pulse generator with high repetition rate is developed based on the Blumlein transmission line.The pulse generator is realized by a fast MOSFET that can make the generator operate in 3 MHz repetition rate.This pulse generator realizes the fast measurement of space charge.According to the classic thermally stimulated current analysis theory of single energy level,it is found that the thermally stimulated current equations caused by different charge sources are similar.A modified thermally stimulated current analysis method is proposed on the basis of an iterative algorithm,solving non-negative linear least square problem.This method does not require preknowledge of the number and location of the thermally stimulated current peak,and can automatically analyze the whole thermally stimulated current curve.It has good repeatability and can avoid human error.This analysis method can be used to calculate the trap energy density distribution in polymers.Through the simultaneous measurement of space charge and conduction current in XLPE and nano-MgO/XLPE composite at the temperature range of 20 to 60 °C,it is found that the electric field threshold of space charge injection of nano-MgO/XLPE is higher than that of XLPE,which means that the addition of nanoparticles can inhibit the space charge accumulation to some extent.As for the conduction current characteristics,it is found that the turning electric field in J-E curve of nano-MgO/XLPE is higher than that of XLPE.Moreover,according to the analysis of J-V scaling curves,it can be induced that deep traps exist in the bulk of both materials and the spatial trap distribution in nano-MgO/XLPE is more even than that in XLPE.Besides,according to the experiment results of the simultaneous measurement of space charge and current,it is found that the negative differential mobility is the main reason of space charge packet formation.The simultaneous measurements of space charge and thermally stimulated current of XLPE and nano-MgO/XLPE are performed in the temperature range of-60 to 100 °C.The space charge measurement results show that many positive charges are accumulated near the anode in polarization stage and gradually migrate to the bulk of XLPE.During cooling stage,the movement of space charge slows down.During thermal stimulation stage,the amount of space charge decreases dramatically.Whereas,not so much space charge accumulated in nano-MgO/XLPE during the polarization stage and cooling stage.The trap energy density distribution shows that there are many deep traps in 1.15 eV energy level in XLPE,whereas the traps in nano-MgO/XLPE are distributed more evenly in energy levels.The combined analysis results show that most of the space charges in XLPE are distributed in deep traps,whereas less space charge is accumulated in nano-MgO/XLPE due to the excess trap energy levels induced by the addition of nanoparticles.更多还原