Figure 5.23 Typical effect of DC prestress (trapped charge) on the protective characteristic of a 15 kV-rated ZnO surge arrester
5.8.4.2 Mixed (DC+impulse) characteristics and effect of trapped charge
In AC power systems, under normal service conditions, surge arresters on isolated lines and cables can experience a direct voltage stress arising from trapped charge. If followed by a switching surge of reverse polarity, severe requirements will be imposed on the surge arresters. Combinations of direct and surge voltages can also occur in HVDC systems and due to circuit breaker restrikes during switching of capacitor banks or unloaded transmission lines where trapped charges are present. Circuit breaker restrikes can cause high rate-of-rise overvoltages of peak values of 2 to 3 p.u. following rapid polarity reversal, and may cause up to 8 p.u. especially for some types of oil circuit breakers [142]. This condition may be particularly onerous where the surge is opposite in polarity to the preexisting working voltage [143–146]. For a given level of peak discharge current through the arrester, a higher level of residual voltage of upto ten per cent is found to appear across an arrester when the polarity of the applied impulse opposes that of the direct voltage (Figure 5.23). This effect is especially marked in the peak residual voltage which is affected by the increased di/dt . xperimental tests have shown [147] that the effect increases with decreasing arrester protection level.
5.8.4.3 Mixed (AC+impulse) characteristics and effect of working voltage level
Transient high amplitude surges commonly occur in high voltage power systems, due to either external or internal influences, such as lightning strikes or switching operations. For most systems, these conditions are likely to occur when the system is already operating at its normal alternating working voltage. Laboratory experiments [148] revealed that for an impulse voltage superimposed on the power frequency