High energy line surge arresters to improve reliability and protection against switching surges on a 500kV transmission line

摘要

Line Surge Arresters (LSA) are commonly used for increasing the reliability of a transmission line by improvement of protection against lightning induced flashovers and following interruptions. Therefore the energy handling capability of such an arrester must be designed to meet the requirements from atmospheric disturbances but mostly neglects the effects of switching overvoltages. The effect of overvoltages on long high voltage and extra high voltage transmission lines induced by switching events is well-known and usually considered in projecting a transmission line. However, higher utilization, grind expansion as well as increasing reliability requirements reveal new challenges for transmission system operators (TSO). This paper describes investigations and actions on a transmission line operated by a US-based utility initiated by considerations regarding the transmission lines' reliability program by the US North American Electric Reliability Corporation (NERC). The overall issue under review of this program was to check actual field conditions and confirm adequate clearances required by the National Electrical Safety Code (NESC) for system voltages over 100kV, dependent on the switching surge overvoltage levels. An in-depth analysis of the utility's 500kV transmission lines has been carried out using an Electromagnetic Transients Program (EMTP) calculating the switching surge factors. Thereof some options in a wide range were derived, including construction works at the towers to increase clearances to ground. Because of the long distances of the lines, shunt reactors and series capacitors are installed and their behaviour has been considered in the model as well. Based on that, the TSO decided to apply high energy LSAs for limiting the switching surge overvoltages to levels that would avoid the need for much more expensive capital improvements to increase clearance to ground. Normally LSAs are designed to handle lightning surges with limited charges due to the parallel installation of many arresters. This applies especially for shielded lines with their backflashover dominated behaviour. For limiting switching overvoltages a different approach is needed. Moreover the correct location of the arresters and the span to be protected in an efficient way is the goal of the prior study and the basis for installation. The utility and the arrester manufacturer jointly decided about the design of the surge arrester to be developed, a high strength polymer housed station class surge arrester with high energy handling capability of 13 kJ/kVMCOV. As agreed between the utility and the manufacturer, the energy rating test was repeated per the latest IEEE/ANSI standard C62.11-2012. More details about this new test and its relation to the respective IEC standards will be described below. In order to monitor the parameters of the real switching surge stresses a monitoring system has been added to the arresters which is described more detailed below. Finally, after some details about the installation in difficult terrains in the state of Arizona, the paper will summarize how this joint development of a TSO, a network consulting company and a product manufacturer contributes to the efforts to meet new US requirements for transmission line's reliability.