Circuit-breaker requirements for alternative configurations of a 500 kV transmission system

Abstract

This paper describes the study made to assess the electrical stresses imposed on line circuit breakers of a new SOOkV, AC expansion of the FURNAS transmission system, to be possibly installed in the Brazilian central region. The study has considered three configurations of the transmission lines, two of which assuming the use of compact towers. In one of these two cases the lines would have high surge-impedance loading. Digital simulations have been made with the aid of the Alternative Transients Program (ATP) to calculate the switching transients appearing as a result of line energization and reclosing, clearing of terminal and short-line faults, no-load line opening in normal and faulted conditions, and out-of phase switching. Four alternatives have been considered for the limitation of overvoltages on closing, assuming either the sole use of surge arresters on each line end or the use, in addition to the arresters, of pre-insertion resistors, synchronizers or time relays to provide a staggered closing of the breaker poles. The stresses obtained were compared with those applied by the standard IEC tests or with the maximum admissible levels usually considered by the utility for its transmission branches at the planning stage.

Mitigation of switching overvoltages in transmission lines via controlled switching

“…In order to avoid the pre-arcing effects, it was considered that the dv CB /dt is greater than the maximum value of the power system voltage derivative [15].…”

“…Concerning the surge arresters, two situations were analyzed: 396 kV and 420 kV class arresters. The 396 kV arrester has a protection level of 783 kV at 2 kA and the 420 kV arrester, which is normally used at the 500 kV Brazilian Power System [15], has a protection level of 830 kV at 2 kA.…”

The Resonant Overvoltage in Non-Sinusoidal Mode of Main Electric Network

“…Investigation of resonant overvoltage in SHVTL. The main direction of previous studies of overvoltage [2][3][4][5][6][7][8][9] using mathematical modeling was focused on the development and application of mathematical models for quantitative results in order to limit overvoltage at the main frequency. They did not consider the physical processes of the development of resonance overvoltage on even harmonic components, although the occurrence of this type of overvoltage is known for a relatively long time.…”

“…It is shown that the necessary condition for the occurrence of overvoltage is the coincidence of two factors: the length of the SHVTL and the angle of switching of the poles of the circuit breaker. It should be noted that the length of the SHVTL is one of the determining factors for overvoltage not only in non-sinusoidal, but also in asymmetrical modes [6][7][8][9], although the source of distortion of the mode, and the appearance of the corresponding resonant circuits in these cases are different. But the length of the line is determined when designing the track of the line, and the factor of the possibility of occurrence of overvoltage is not taken into account.…”

“…Thus, for each type of overvoltage, measures for their warning on this SHVTL have to be developed and implemented separately. An event that will be effective for overvoltage in a non-sinusoidal mode will be ineffective in the asymmetric mode and vice versa [6][7][8]. In modern main electric networks, structural measures, which are connected with the change of the working circuit and preventing the appearance of an abnormal mode, should be used first of all.…”