Live Line Clearance Evaluation Transient Study
Power system apparatus, such as transformers, switchgear, and cables may be exposed to various types of transients. High-frequency transients and very steep overvoltages may cause problems for electrical equipment because they can cause local overstressing of the insulation system. Utility transmission line circuit breaker opening-and-closing operations are an important potential source of these high rate-of-rise transients.
A high-frequency transient analysis study was completed for a utility that is evaluating the latest changes to the industry-accepted transient overvoltage (T Factor) recommendations for performing maintenance work on energized transmission lines. IEEE Std. 516 (Guide for Maintenance Methods on Energized Power Lines), IEEE Std. 1427 (Guide for Recommended Electrical Clearances and Insulation Levels in Air-Insulated Electrical Power Substations), the National Electric Safety Code (NESC), and OSHA Standards allow reduced minimum safety clearances for live line work when engineering studies of the switching transients have been performed.
The study included an analysis of overvoltages and arrester energy duties during various transmission line energizing operations, including the effects of MOV surge arresters and circuit breaker closing resistors. The study investigated energizing 230kV, 138kV, and 115kV transmission lines of varying lengths with the goal of determining the statistical switching surge overvoltages required for a review of electrical live line clearances. The transient analysis for the study was completed using the EMTP-RV simulation program.
Energizing and reclosing transients for transmission lines are often a combination of line energizing, transformer inrush, and load inrush. Line energizing transient overvoltages typically decay to negligible values in about ½ cycle and they generally do not pose significant problems for customer equipment. Transients generated by energizing overhead lines are characterized by high frequencies because of the small effective shunt capacitance of these circuits. These high frequency surges are quickly attenuated by the transmission line resistance and circuit loads. Longer circuits have higher capacitance values and can look similar to capacitor banks when they are energized.
High-frequency transient simulation models of the required substations and adjacent power system equipment were created. The transient simulation models consisted of equivalent sources and transfer impedances, π-section and traveling wave transmission line models, substation transformers, shunt capacitor banks, MOV surge arresters, and the appropriate circuit breaker and substation load characteristics.
The transient simulation cases involved statistical energization of a number of 230kV, 138kV, and 115kV transmission lines with the circuit breakers at the remote ends of the transmission lines open, resulting in radial transmission line energizing (refer to Figure 1). The statistical switching included 200 random circuit breaker closing operations over a one-cycle pole span. The transient models also included minimal load levels, so it can be assumed that the worst-case transients were simulated during the line energizing cases. Important study results included finding the minimum, mean, maximum and standard deviation transmission line energizing transient overvoltages, as well as the highest “98% Level” (CP98) values at the open ends of each line termination for each line energizing case.
The Power System Engineering group at EnerNex has extensive expertise performing power system studies investigating the high-frequency transients associated with transmission line circuit breaker operations. These studies often require use of sophisticated digital simulation tools (e.g., EMTP-RV, PSCAD). Computer simulations provide a convenient means to characterize transient phenomena, determine resulting problems, and evaluate possible mitigation alternatives.
Figure 1 – Radial Transmission Line Energizing Transient Voltage Waveform