UP-RATING OF OVERHEAD LINES

伴随着环境压力的对电力消耗的不断增长的需求强加了对电力系统修改和增长的持续需求，部分原因是由于放松管制导致的操作实践的变化，部分原因是由于分布式发电的增加使用，这正在改变对传输，特别是配电线路的需求。但多年来，由于公众对视觉和其他环境影响的关注，安装新线路的机会变得非常有限，很明显，系统容量的扩大将在基本上不需要新线路的情况下得到满足。输电线路和配电线路的电压额定值和绝缘配合是通过综合考虑施加在线路上的电压应力及其电强度。由于开关瞬变或雷电浪涌引起的过电压会产生应力。操作过电压的大小由系统元件的特性控制，在最高工作电压下更为关键。另一方面，雷电过电压的幅度要大得多，对配电系统来说更为繁重。IEC 60071对特定电压水平所用的间隙和间隙提出了建议，然后，各个操作员将根据当地条件采用高于这些建议的安全系数。例如，污染可能会使击穿电压降低50%。这些采用的间隙通常非常大，可以使用现代设备和实践进行优化。研究人员多年来一直在研究通过改进绝缘协调和使用聚合物绝缘体以及更有效的保护装置（如ZnO电涌放电器）来实现紧凑型线路和变电站的可能性。因此，本方案建议将紧凑型线路概念应用于现有线路的升级改造。它将涉及开关和雷电浪涌的统计研究，这些研究考虑到影响过电压幅度的各种参数，例如断路器的闭合时间和线路可能状态的分析，以便最大限度地减少重新闭合捕获电荷的风险。系统的应力和强度的统计变化将结合在电压-频率图中，以确定故障风险，该风险必须在经济约束范围内最小化。应力将被呈现为一定的过电压发生的概率，并且其沿着线路的分布将由现代ZnO电涌放电器的明智放置来控制。另一方面，电气强度可以表示为概率击穿曲线。它主要是通过考虑塔架和沿着的临界间隙，对故障曲线进行考虑而得出的。这些原则已经研究了多年，但目前的压力正在引起对传统限制和方法的重新评估。现代ZnO避雷器在控制过电压方面的优异性能和新型聚合物绝缘子的上级防污染性能也支持这一点。该方案还将包括实验室和现场实验方案，以测试和验证用于提高功率线路紧凑设计的新设备和配置。该计划的主要成果是建立经过充分研究的基本原则，从而为未来的输电和配电线路提供高效和安全的设计。该计划的基础由卡迪夫大学的HIVES提出，然后通过与国家电网、四家英国DNO、ESB和三家线路建设公司的行业团体进行讨论来主持，这些公司的观点都包含在拟议的计划中。

The ever increasing demand for electricity consumption accompanied by environmental pressures impose a continuing need for electrical systems modification and growth, partly because of changing operational practices resulting from de-regulation and, partly, due to the increased use of distributed generation, which is changing the demands on transmission and, especially, distribution lines. But for many years now, the opportunities for installation of new lines have become very limited because of public concern over visual and other environmental impacts, and it is clear that extensions to system capacity will have to be met substantially without new lines.The voltage rating and the insulation coordination of transmission and distribution lines is determined by a combined consideration of the voltage stress applied to the line and its electrical strength. The stress arises from overvoltages due to switching transients or lightning surges. The magnitude of the switching overvoltage is controlled by the characteristics of the system components, and is more critical at the highest operating voltages. Lightning overvoltages, on the other hand, are of much larger magnitudes and are more onerous to distribution systems.IEC 60071 makes recommendations for the gaps and clearances to be used for specific voltage levels, and individual operators will then adopt safety factors above and beyond these recommendations, depending upon local conditions. Pollution, for instance, may reduce the breakdown voltage by up to 50%. These adopted clearances are usually very generous and can be optimised using modern equipment and practice.The investigators have researched for many years the possibilities for compact lines and substations through improved co-ordination of insulation and the use of polymeric insulators and more effective protective devices such as ZnO surge arresters. This programme, therefore, proposes to apply the compact line concepts to the up-rating of existing lines. It will involve statistical studies of switching and lightning surges that account for various parameters which affect the overvoltage magnitudes, such as closing times for circuit breakers and analysis of the possible state of the line in order to minimize the risk of re-closing onto trapped charge. The statistical variations of stress and strength of the system will be combined in a voltage-frequency plot to determine the risk of failure, which has to be minimized within economic constraints. The stress will be presented as the probability of a certain overvoltage occurring, and its distribution along a line will be controlled by the judicious placement of modern ZnO surge arresters. Electrical strength, on the other hand, can be presented as a probabilistic breakdown curve. It will be primarily derived from consideration of the breakdown curves taking into account the critical clearances at the tower and along the line. These principles have been studied over the years, but present-day pressures are causing a re-evaluation of the conventional limits and methodology. This is also supported by the excellent performance of modern ZnO surge arresters in controlling overvoltages and the superior pollution performance of new polymeric insulators. The programme will also include laboratory and field experimental programmes to test and characterise the new devices and configurations to be used for the compact design of the uprated lines. The main output of the programme is to establish well researched fundamental principles that will allow an efficient and safe design for the future transmission and distribution lines.The basis of this programme has been proposed by HIVES, Cardiff University and then moderated by discussions with an industry group involving National Grid, four UK DNOs, ESB and three line construction companies, whose views are embedded in the proposed programme.