UP-RATING OF OVERHEAD LINES

随着环境压力，对电力消费的需求不断增加，电力系统不断需要改进和发展，部分原因是由于取消管制而改变了运营做法，部分原因是分布式发电的使用增加，这正在改变对输电，特别是对配电线路的需求。但多年来，由于公众对视觉和其他环境影响的担忧，安装新线路的机会变得非常有限，显然，在没有新线路的情况下，必须基本上满足系统容量的扩展。输配电线路的额定电压和绝缘协调取决于对线路施加的电压应力和其电气强度的综合考虑。这种应力是由开关瞬变或雷电浪涌引起的过电压引起的。开关过电压的大小由系统组件的特性控制，并且在最高工作电压时更为关键。另一方面，雷电过电压的幅度要大得多，对配电系统来说也更加繁重。IEC 60071就特定电压水平所使用的间隙和间隙提出了建议，然后个别操作员将根据当地情况采用高于或超过这些建议的安全系数。例如，污染可能会使击穿电压降低高达50%。这些采用的间隙通常非常宽大，可以使用现代设备和实践进行优化。调查人员多年来一直在研究通过改进绝缘配合、使用聚合物绝缘子和更有效的保护装置(如氧化锌避雷器)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上调评级。它将涉及对开关和雷击浪涌的统计研究，这些浪涌考虑了影响过电压大小的各种参数，例如断路器的关闭时间，以及分析线路的可能状态，以将重新关闭被困电荷的风险降至最低。系统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下降至最低。应力将表示为某一过电压发生的概率，其沿线路的分布将由现代氧化锌避雷器的合理布置来控制。另一方面，电气强度可以用概率击穿曲线表示。它将主要通过考虑击穿曲线，并考虑塔和沿线的临界间隙而得出。多年来，人们一直在研究这些原则，但目前的压力正在导致对传统限制和方法的重新评估。现代氧化锌避雷器在控制过电压方面的出色性能以及新型聚合物绝缘子的卓越污染性能也支持了这一点。该方案还将包括实验室和实地试验方案，以测试和确定将用于紧凑型线路设计的新装置和配置。该计划的主要成果是建立经过充分研究的基本原则，以便为未来的输电和配电线路进行高效和安全的设计。该计划的基础是由加的夫大学提出的，然后通过与一个行业组织的讨论进行协调，该行业组织涉及国家电网、四家英国DNO、ESB和三家线路建设公司，他们的观点被纳入拟议的计划中。

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.