Direct voltage and trapped charge effects on the protective characteristic of ZnO surge arresters

In AC power systems, under normal service conditions, surge arresters on isolated lines and cables can experience a direct voltage stress arising from trapped charges. If this is followed on reconnecting by switching surges of reverse polarity, it can impose severe response requirements on the arresters. High-amplitude voltage reversal can also arise from restriking transients in circuit breakers. Direct voltage working stress with superimposed surges is also clearly present in HVDC systems. Despite this expected combination of direct voltage with superimposed surges, no analogous tests are specified in the relevant standards. New laboratory tests have been performed on ZnO surge arrester elements and arresters, where impulse voltages have been applied with and without the presence of pre-existing direct voltage or simulated trapped charge. These tests have been analyzed to ascertain the effect of prestressing on the current growth and protective characteristic of the arrester. In the laboratory source, the line/cable system has been represented by an equivalent capacitance which has been previously charged from a direct voltage. A single-stage capacitor bank is triggered to provide a lightning impulse test current. For a given level of peak discharge current, a higher level of residual voltage is found to appear across an arrester when the polarity of the applied impulse opposes that of the trapped charge voltage. This effect is especially marked in the peak residual voltage under this combination. Initial results suggest that the effect increases with decreasing protection voltage level.