Overview on Transient Overvoltages and Insulation Design For a High Voltage Transmission System William Sones T &D Technical Services Entergy Services, Inc. Jackson, MS, USA wsones@entergy.com Sze Mei ‘Cat’ Wong, Ph.D, PE Transmission Design Basis Entergy Services, Inc. New Orleans, LA, USA swong@entergy.com Abstract— This paper presents a high level overview of overvoltages and insulation performance and offers a brief overview of insulator technology and development. Frequent lightning and switching, an ageing system, and contaminated insulators are some of the factors that impact transmission line performance and reliability. A lightning case study is included in this paper with results to illustrate the importance of insulation coordination to ensure the reliable operation of Entergy’s transmission system. I. INTRODUCTION Entergy operates a system composed of more than 15,500 miles of high-voltage transmission lines and 1,550 transmission substations. Transmission system reliability is closely monitored by Entergy and The SERC Reliability Corporation (SERC). When a transmission line has an unacceptable outage frequency, investigations are initiated so that a solution may be proposed to reduce the outage frequency of that line. Lightning overvoltages, switching overvoltages, contaminated or incorrect insulators which are sometimes coupled with damaged or missing tower grounds are some of the reasons for line outages within Entergy’s transmission system. Properly designed transmission lines must be able to withstand, with a high degree of reliability, power frequency voltage stresses and transient stresses. Shield wires must be strategically placed to intercept lightning strokes before they come in contact with the transmission conductor. Overvoltages and insulation performance must be balanced in the design phase to retain a high level of reliability. Entergy’s transmission substation design group and transmission line design group follow IEEE standards and internal standards in the design of transmission lines and substations based on a mass of experimental data, modeling and calculation techniques, operating and design experience and economics. II. INSULATION COORDINATION The IEC [1] definition of insulation coordination is “the selection of the electric strength of equipment in relation to the voltages which can appear on the system for which the equipment is intended, taking into account the service environment and the characteristics of available protective devices”. It can be read as: the coordination of electric strength with electric stress. Fig. 1 shows how the range of electric stress is related to electric strength to achieve a practical low level of failure. Figure 1. Strength and stress balance in insulation coordination [1] Overvoltages are either external or internal. External overvoltages typically are from lightning and internal overvoltages are typically caused by switching operations. The shapes of voltage transients are classified as oscillatory, fast- front surge and slow front surge. For testing purposes, both the IEC and IEEE represent voltage and current surges by representative impulses and, in the case of IEEE, a representative “ring wave”. Their definitions and shapes are shown in Fig. 2. Figure 2. Voltage Impulse, Current impulse and Ring Wave A. Lightning Overvoltages Lightning strokes cause 30% to 40% of all transmission and distribution outages and is a major component of overall reliability [3]. Lightning produces a surge on an overhead 978-1-4244-8286-3/10/$26.00 ©2010 IEEE 12