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电气设备的接地与接零,是为了防止人身触电事故、保证电气设备正常运行所采取的重要措施。笔者针对这一问题,结合我国传统的低压供电方式和目前推荐的先进技术方案进行分析。1三相三线制供电系统的保护接地保护接地,就是将电气设备在正常运行时不带电而绝缘损坏时可能带电的金属部分(例如各种电气设备的金属外壳、配电装置的金属构架等)与接地体(接地网)可靠地连接起来,以保证工作人员等触及时的安全,在农村10kV以下电源中性点不接地的场合广泛采用。在实际生产中,如单台或为数不多的设备,也可采用接地体进行保护接地。当设… 相似文献
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三相交流电力系统中性点与大地之间的电气连接方式,称为电网中性点接地方式。中性点接地方式涉及电网的安全可靠性、经济性;同时直接影响系统设备绝缘水平的选择、过电压水平及继电保护方式、通讯干扰等。一般来说,电网中性点接地方式也就是变电所中变压器的各级电压中性点接地方式。因此,在变电所的规划设计时选择变压器中性点接地方式中应进行具体分析、全面考虑。 相似文献
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低压配电线路的接地故障是指相线对地或与地有联系的导电体之间的短路,它包括相线与大地、PE线、PEN线、配电和用电设备的金属外壳、敷线管槽、建筑物金属构件、上下水和采暖、通风等管道以及金属层面、水面等之间的短路。当发生接地短路时,与它有关联的电气设备和管道的外露 相似文献
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接地装置的接地电阻等于接地体(人工接地体或自然接地体)与大地之间的电阻和接地线的电阻之和。接地线的电阻一般很小,可以忽略不计,所以,通常所说的接地电阻是指接地体与大地之间的电阻。 相似文献
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小电流接地系统中,多数变电站未装设接地选线装置,在发接地信号时需要人工进行判断。由于多种故障都会引发接地信号,所以运行人员一定要熟悉接地信号的判断与处理,不要误以为都是线路接地,按照接地故障处理,造成不必要的停电。以下就发接地信号的几种故障判断与处理,谈一下笔者的认识。 相似文献
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随着科学技术的进步,现代建筑物向自动化、节能化、信息化和智能化方向发展。在这些建筑物内往往设置有许多电子设备,如电子计算机、广播与通信设备等。对于由这些电子设备组成的系统,采用合理的接地方法是非常重要的。1电子设备接地技术1.1电子设备一般应具有下列几种接地 相似文献
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在10kV配电线路运行中,由于线路长、分布广,会出现各种各样的故障,而线路接地是最常见的故障.根据变电所电压互感器反映的现象,准确、快速地判断接地信号动作的原因,对于我们保证电网安全运行,提高供电可靠性具有十分重要的意义.1 线路接地故障造成接地信号动作造成单相接地的故障很多,如:一相导线断线落地,树枝碰及导线,导线因风偏对杆塔放电,跌落保险、瓷瓶等被击穿,变压器等电机设备损坏等.发生金属性接地故障时相电压为0,非故障相电压升高为3~(1/2) 倍相电压,零序电压为100 V.非金属性(经过渡电阻)接地时,相电压低,但不为0,非故障相电压升高接近于线电压,出现零序电压,造成接地信号动作.2 铁磁谐振造成接地信号动作 相似文献
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《Journal of Agricultural Engineering Research》1998,69(3):249-254
A model to represent the discolouration of dried powder of carrots as a function of temperature was developed. Two types of carrots, Asian and European, containing large amount of lycopene and β-carotene, respectively, were tested. The dried powder was stored under an atmosphere with O2and at three different temperatures, 5,20 and 35°C. The surface colour of the powder was monitored during storage by a Hunter colour ratioa/b. In the proposed model, the rate of oxidation was assumed to be represented by the product of the fraction of concentration of oxidized pigment at arbitrary time and a specific rate of oxidation of pigment. The rate parameters in the proposed model were calculated by fitting the experimental data with smooth curves by trial and error at the end of the induction period but before discolouration took place. The frequency constant and activation energy, expressing the temperature dependence of the rate parameters, were expressed by the Arrhenius relation. The simulation results of a Hunter colour ratioa/bof dried powder at 25°C showed good agreement with the experimental data, suggesting that the proposed model was effective for the prediction of discolouration of dried powder of both types of carrot. The use of the induction period in the manner described may enable the shelf life of dried powder to be predicted. 相似文献
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感官检验棉花的品级,这是农村常见的一种检测手段.它的一个主要特点就是存在不确定性.这是由于检验员的品级检验水平存在一定的差异,其中有很多因素制约着检验的结果.文章对影响感官检验结果的因素进行了系统的分析. 相似文献
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The fractional outflow rate of water from the rumen, measured as the rate of disappearance of the chromium complex of ethylenediamine tetra-acetic acid, was related to a number of dietary factors. A multiple regression equation was derived which expressed fractional outflow rate as a function of the intakes of dry matter, ash, acid-detergent fibre and crude protein.The data analysed consisted of 243 observations for sheep on 59 diets, including forage, concentrate and mixed diets. The ranges covered by the data were: fractional outflow rate 0.7–3.6 day?1; dry matter intake 560–1400g/day; ash 4–30%, acid-detergent fibre 7–41%, crude protein 5–26 % of dry matter; there were no observations for sheep of less than 34 kg liveweight.The equation accounted for 56·3 % of the total variance. Between-sheep variance could not be calculated for the whole of the data but was undoubtedly substantial since, for a sixth of the data, it was known to account for 38 % of the total variance. 相似文献
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从灾害经济学的角度探讨了城镇供水优化调度的必要性和可能性,认为在缺水不可避免的情况下,对供水进行优化调度可以减小缺水带来的损失,产生较好的减灾效益,特别是可以大大减小严重干旱的灾害效应。并根据水资源的特性,提出了缺水损失的计算模式以及优化调度的原则和方法。 相似文献