engleski

Energy stresses of station surge arresters due to temporary overvoltages

Usually the voltage rating of station surge arresters is chosen higher than the amplitude of the temporary overvoltages (TOV) which can occur in the power system. This approach leads to the use of arresters with a high protection levels, especially if the system is not solidly grounded, which do not reduce the level of slow-front and fast-front overvoltages as efficiently as arresters with lower protection level would do. Compact upgraded lines have shorter clearances, compared to standard lines of the same voltage level, so they require the use of efficient techniques to reduce overvoltages in order to avoid phase to ground or phase to phase flashovers. In any complex electromagnetic system, a sudden change in state gives rise to transient oscillations which, in turn, can cause high overvoltages unless suitably damped. For EHV systems it has been common practice for many years now to equip circuit-breakers with closing resistors, as a means of controlling such system transient interactions during closing or re-closing operations. The closing resistors are inserted in series with the load circuit being switched for a short period of time, before closing the main contacts of the breaker – thereby damping the transient overvoltages. Optimum overvoltage control requires correct choice of the resistor value in relation to the source impedance level, the line length and the line parameters. Although a well-proven technology, breakers equipped with closing resistors inevitably involve relatively complex mechanical constructions. In recent years several large utilities have experienced problems with the long term mechanical reliability of closing resistor mechanisms (especially in older circuit-breakers) – with adverse impact on overall system reliability – and have begun to examine alternative approaches to switching surge overvoltage control. Foremost among these is the installation of suitably chosen zinc oxide surge arresters at both ends of the transmission line being switched. Given the availability now of ZnO arresters having low protective levels and high discharge energy capabilities, this proves to be a particularly economical and very simple way of controlling transmission line switching surges – without any dependence upon complex mechanisms or sensitive timing systems. Slow-front overvoltages due to energization or reclosing of the line may be reduced by using various techniques like point-on-the-wave switching or breakers equipped with pre-insertion resistors, but slow-front overvoltages due to faults on adjacent lines, which are most of the time not of interest for standard overhead lines, might get critical in case of compact or upgraded lines. Obviously they cannot be reduced by the techniques listed above and the use of station arresters with a low protection level at the terminals of the line might be the only option. This low protection level requires the use of arresters whose rated voltage might be below the level of TOV and this issue has to be studied carefully due to energy stress absorbed by surge arresters.

temporary overvoltages ; surge arresters ; compact transmission lines ; surge arresters selection

This study was performed for EDF (Électricité de France).